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Advances in Space Research
Journal Prestige (SJR): 0.569
Citation Impact (citeScore): 2
Number of Followers: 395  
 
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ISSN (Print) 0273-1177
Published by Elsevier Homepage  [3159 journals]
  • Polar Traveling Ionospheric Disturbances inferred with the B-Spline Method
           and Associated Scintillations in the Southern Hemisphere
    • Abstract: Publication date: Available online 16 August 2018Source: Advances in Space ResearchAuthor(s): S. Priyadarshi, Q.H. Zhang, E.G. Thomas, L. Spogli, C. CesaroniAbstractA new method for analyzing travelling ionospheric disturbances (TIDs) is developed by using two B-spline basis functions of degree 4 on the Total Electron Content (TEC) data from the ground-based Global Positioning System (GPS) receivers. This method enhances the spatial resolution to about 0.1° (geographic latitude) x 0.1° (geographic longitude), which is useful in studying all scales (small, medium and large) TIDs. Using this method, we investigated TIDs and their associated scintillation on 18-19 July 2013 at Southern Hemisphere and found phase scintillation is more sensitive than amplitude scintillation to the TIDs at South Pole. To see the full impact of TIDs on scintillation, we have used a proxy phase scintillation index, calculated using geodetic GPS receivers over Antarctica. We have verified the presence of TIDs during these two days by using a Global Navigation Satellite System (GNSS)-TEC single station approach and SuperDARN slant range signals. Our results show the TEC fluctuations are associated with ionospheric scintillation. The shape of TIDs, their elongation and flattening along/across the geographic latitude/longitude, seems to be related to the magnitude and occurrence of ionospheric scintillations. Magnetospheric particle precipitation boost TEC gradients and generate stronger amplitude scintillation, however, large-scale plasma irregularities cause overall enhancement in magnitude of the phase scintillation index. Due to the high turbulence in the polar ionosphere, TIDs change their shapes quite quickly and/or may disappear in the background ionosphere. B-spline TIDs analysis method is very useful in identifying the visible as well as hidden TIDs parts in the polar ionosphere. For the first time, ionospheric scintillation has been investigated in the vicinity of TIDs at high- latitude in the southern hemisphere. Further, the presented B-spline TIDs analysis method is unique and simple in itself as it uses GPS receiver processed TEC data as the primary input. Our results show that at polar latitude it is not necessary that TIDs always appear near the high TEC regions. Usefulness of the B-spline TIDs detection method has been demonstrated in analyzing TIDs at all geographic locations and different solar activity conditions by comparing B-spline TIDs method produced results with the previous case studies.
       
  • Suggestion for search of silanone (H2SiO) in interstellar
           medium
    • Abstract: Publication date: Available online 15 August 2018Source: Advances in Space ResearchAuthor(s): M.K. Sharma, Monika Sharma, Suresh ChandraAbstractThirteen silicon bearing molecules are detected in the cosmic objects. Carbon (C) and silicon (Si) elements have similar chemical properties and the formaldehyde (H2CO) is identified in a large number of cosmic objects. Hence, there is a possibility of Silanone (H2SiO) being present in the ISM. For each of the ortho and para species of H2SiO, we have calculated energies of 100 lower rotational levels (up to 284 cm−1) and the Einstein coefficients for radiative transitions between the levels. We have solved a set of 100 statistical equilibrium equations coupled with the equations of radiative transfer for each specie where the collisional rate coefficients are taken from a scaling law. For the ortho-H2SiO, five transitions, 110-111, 211-212,312-313,413-414 and 514-515 are found to show the anomalous absorption, and one transition 212-111 is found to show the emission feature. For the para-H2SiO, four transitions, 101-000,202-101,660-514 and 661-542 are found to show the emission feature. These lines may help in the identification of H2SiO in a cosmic object, e.g., in IRC+10216.
       
  • Re-entry Trajectory Optimization using Pigeon Inspired Optimization Based
           Control Profiles
    • Abstract: Publication date: Available online 15 August 2018Source: Advances in Space ResearchAuthor(s): Gangireddy Sushnigdha, Ashok JoshiAbstractIn this paper, the entry trajectory optimization problem of lifting type re-entry vehicle with path constraints is solved using Pigeon Inspired Optimization (PIO). Entry trajectory optimization problem involves finding the control profiles, bank angle, and angle of attack to guide the vehicle safely to the destination. The proposed approach, parametrizes the bank angle to be a linear function of energy while the angle of attack is considered to be a monotonic function of Mach number. Thus, the problem of finding control profiles is transformed to three parameter search problem. The PIO algorithm is used to find the values of these parameters that minimizes the objective function. The terminal heading angle offset is minimized using traditional bank reversal logic. A new approach is proposed in which the bank angle is modulated to eliminate the oscillations observed in the altitude profile of an entry vehicle with high lift to drag ratio (L/D). A methodology to satisfy the given load factor constraint is also proposed, as an alternative to traditional penalty factor approach used for incorporating path constraints in PIO algorithm. The proposed approach is further validated by considering sub-cases with different load factor limits and bank angle as the only control variable. The angle of attack profile obtained from the previous case is considered as the nominal profile. The proposed trajectory optimization strategy using PIO algorithm is simulated for Common Aero Vehicle with high L/D ratio (CAV-H) with different load factor constraint limits. The results show that the obtained angle of attack profile minimizes the peak heat rate experienced by the vehicle and bank angle modulation eliminates the oscillations in the altitude profile as well as makes the entry trajectory satisfy the load factor constraint.
       
  • Parameters and Bifurcations of Equilibrium Points in the Gravitational
           Potential of Irregular-shaped Bodies Subjected to a Varying External Shape
           
    • Abstract: Publication date: Available online 14 August 2018Source: Advances in Space ResearchAuthor(s): Yu Jiang, Hexi BaoyinAbstractIn this work, we investigate variations in the positions, eigenvalues, Jacobi integrals, topological cases, as well as the stability of equilibrium points around the asteroid 243 Ida when varying the external shape of the body. First, we employ a polyhedral shape model to calculate the surface height, gravitational force acceleration on the surface, and effective potential on the surface of 243 Ida. We then adopt the homotopy analysis method to generate variations in the external shape of a generic body within a continuum between the modeled external shape of 243 Ida and that of a sphere while maintaining a constant volume. Then, we calculated the positions, eigenvalues, Jacobi integrals, as well as the Hessian matrices of the equilibrium points in the gravitational potential of the generic body. We analyzed the topological cases and stability of the equilibrium points based on the obtained eigenvalues and Hessian matrices. It is concluded that the positions, eigenvalues, and Jacobi integrals change when varying the external shape of the body. For the four external equilibrium points E1–E4, the norm of the position vectors increases; however, the norm of the position vector for the internal equilibrium point E5 decreases. Equilibrium points E2 and E4 are subject to Hopf bifurcation when varying the external shape of the body, and the topological cases and stability of E2 and E4 are correspondingly changed. In contrast, the topological cases and stability of equilibrium points E1, E3, and E5 remain unchanged when varying the external shape of the body.
       
  • Mechanism of error propagation from the subdaily Universal Time model into
           the celestial pole offsets estimated by VLBI
    • Abstract: Publication date: Available online 14 August 2018Source: Advances in Space ResearchAuthor(s): N. Panafidina, U. Hugentobler, H. Krásná, R. Schmid, M. SeitzAbstractWithin the analysis of space geodetic observations, errors of the applied subdaily Earth rotation model can induce systematic effects in different estimated parameters. In this paper, we focus on the impact of the subdaily Universal Time (UT1) model on the celestial pole offsets (CPO) estimated from very long baseline interferometry (VLBI) observations. We provide a mechanism that describes the error propagation from the subdaily UT1 into the daily CPO.In typical 24-hours VLBI sessions the observed quasars are well distributed over the sky. But the observations, if looked at from the Earth-fixed frame, are not homogeneously distributed. The amount of observations performed in different terrestrial directions shows an irregularity which can be roughly compared to the case where the observations are collected in only one Earth-fixed direction. This pecularity leads to artefacts in VLBI solutions, producing a correlation between the subdaily variations in UT1 and the position of the celestial pole. As a result errors in diurnal terms of the subdaily UT1 model are partly compensated by the estimated CPO. We compute for each 24-hours VLBI session from 1990 until 2011 the theoretical response of the CPO to an error in the subdaily UT1 by setting up a least-squares adjustment model and using as input the coordinates of the observed quasars and observation epochs. Then real observed response of the estimated CPO derived from the VLBI session solutions is compared to the predicted one. A very good agreement between the CPO values estimated from VLBI and the predicted values was achieved. The presented model of error propagation from the subdaily UT1 into the daily CPO allows to predict and explain the behaviour of CPO estimates of VLBI solutions computed with different subdaily Earth rotation models, what can be helpful for testing the accuracy of different subdaily tidal models.
       
  • An improved algorithm based on combination observations for real time
           cycle slip processing in triple frequency BDS measurements
    • Abstract: Publication date: Available online 14 August 2018Source: Advances in Space ResearchAuthor(s): Renhu Pu, Yongliang XiongAbstractThis research presents a new algorithm to resolve real-time cycle slip for undifferenced triple frequency observations of BeiDou navigation satellite system (BDS). The method of cycle slip (CS) detection is based on three linearly independent combinations: GIF (geometry free and ionosphere free) combination, GF (geometry free) phase combination and code-phase combination. First order differenced ionospheric delay is predicted by a back moving window filter. The difference between predicted and calculate first order differenced ionospheric delay is adopted to detect the insensitive cycle slips and to judge whether the detected cycle slips are repaired correctly. With respect to cycle slip determination, modified LAMBDA (MLAMBDA) method is adopted. The proposed method has been verified by real BDS observations adding various artificial cycle slips at every ten epochs. The results show that the proposed algorithm is reliable to detect and repair small and large cycle slips even under low sampling interval (30 s). The fix rate of CS can reach 100% for all the selected satellites. For the most insensitive cycle slip group (1, 1, 1), the fixing rate can reach 99.82% without considering the first order differenced ionospheric delay condition. The fixing rate can also reach 100% when ionospheric delay condition is considered. In terms of the comparative method, the fixing rate is about 91% in according to the IGSO satellites. These results demonstrate that the proposed algorithm can detect and repair all added cycle slips even under low sampling rate at 30 s.
       
  • Time-varying solar radiation pressure on Ajisai in comparison with LAGEOS
           satellites
    • Abstract: Publication date: Available online 14 August 2018Source: Advances in Space ResearchAuthor(s): Akihisa Hattori, Toshimichi OtsuboAbstractThis study aims to investigate solar radiation pressure acting on the spherical geodetic satellites, Ajisai, LAGEOS-1, and LAGEOS-2. The solar radiation pressure coefficients (CR) are derived in two independent ways: a) through precise orbit determination (POD) using satellite laser ranging (SLR) data, and b) through modeling using the optical properties of the satellite surface material. The average CR value of Ajisai (1.039), as calculated from the time series of CR POD estimates every 15 days, is consistently smaller than those of LAGEOS-1 (1.140) and LAGEOS-2 (1.103). This difference can be explained by the fact that the surface of Ajisai is mostly covered by mirrors. The Ajisai CR values estimated by POD show remarkable semi-annual variation, which disagrees with the results of a previous study (Sengoku et al., 1995) predicting that the CR of Ajisai varies almost annually. We attribute this semi-annual variation to two physical reasons: the non-spherical additional cross-sectional area due to the “attached fitting ring” and the low reflectivity of the surface material in the polar regions. Furthermore, the solar radiation pressure acting on Ajisai varies annually in a direction perpendicular to the sun-satellite vector. Finally, the two independent CR values of Ajisai agree more when we assume a total solar irradiance of 1361 W/m2, whereas the value 1367 W/m2 has been commonly used in POD.
       
  • Prospects of Probing the Radio Emission of Lunar UHECRv Events
    • Abstract: Publication date: Available online 13 August 2018Source: Advances in Space ResearchAuthor(s): A. Aminaei, L. Chen, H. Pourshaghaghi, S. Buitink, M. Klein-Wolt, L.V.E. Koopmans, H. FalckeAbstractRadio detection of Ultra High Energetic Cosmic Rays and Neutrinos (UHECRv) which hit the Moon has been investigated in recent years. In preparation for near-future lunar science missions, we discuss technical requirements for radio experiments onboard lunar orbiters or on a lunar lander. We also develop an analysis of UHECRv aperture by including UHECv events occurring in the sub-layers of lunar regolith. It is verified that even using a single antenna onboard lunar orbiters or a few meters above the Moon’s surface, dozens of lunar UHECRv events are detectable for one-year of observation at energy levels of 1018eV to 1023eV. Furthermore, it is shown that an antenna 3 meters above the Moon’s surface could detect lower energy lunar UHECR events at the level of 1015eV to 1018eV which might not be detectable from lunar orbiters or ground-based observations.
       
  • Analytical hierarchy process (AHP) based landslide susceptibility mapping
           of Lish river basin of Eastern Darjeeling Himalaya, India
    • Abstract: Publication date: Available online 13 August 2018Source: Advances in Space ResearchAuthor(s): Biplab Mandal, Sujit MandalAbstractLandslide is one of the most destructive natural hazards of Darjeeling Himalaya. In the present study, analytical hierarchy process (AHP) has been applied to develop landslide susceptibility map of the Lish river basin of Eastern Darjiling Himalaya based on geospatial tools. A total of 188 landslide location points of very small to large polygon coverage were identified with the help of GPS study, SOI (Survey of India) topo-sheets and Digital Globe Quick Bird image to prepare a landslide inventory map. 70 % training data sets of the landslide inventory was applied for training analytical hierarchy process (AHP) model and remaining 30% data sets were used for validation of the landslide susceptibility map. ERDAS IMAGINE, ASTER GDEM image, Arc GIS and PCI Geomatica software, topographical map, Landsat TM and 8 OLI satellite image, google earth image, soil map (NBSS and LUP Regional Centre), geomorphology and lithology (NATMO and GSI) were used and processed to prepare seventeen causative factors i.e. slope angle, slope aspect, slope curvature, altitude, relative relief, geomorphology, geology, distance to lineaments, lineament density, soil, LAND USE & land cover, NDVI, drainage density, distance o drainage, stream power index, topographic wetness index, and rainfall. Factor weights and class weights of all the data layers were derived using AHP and then weighted data layers were integrated to generate the landslide susceptibility map. To validate the result, success rate curve was made with the help of landslide susceptibility and cumulative percentage of landslide occurrence which showed the accuracy level of 89.72 %.
       
  • Quantum-Behaved Particle Swarm Optimization for Far-Distance Rapid
           Cooperative Rendezvous between Two Spacecraft
    • Abstract: Publication date: Available online 13 August 2018Source: Advances in Space ResearchAuthor(s): Kun Yang, Weiming Feng, Gang Liu, Junfeng Zhao, Piaoyi SuAbstractFocused on far-distance rapid cooperative rendezvous between two spacecraft under continuous large thrust, this paper presents a series of artificial intelligence algorithms for fuel and time optimization. The process of far-distance rapid cooperative rendezvous was optimized by a type of hybrid algorithm-integrated Quantum-behaved Particle Swarm Optimization (QPSO) and Sequential Quadratic Programming (SQP). The convergent co-state vectors were obtained by QPSO and subsequently set as the initial values of SQP to search for the exact solutions in a smaller area. Applications of non-coplanar cooperative rendezvous are provided to demonstrate that the QPSO-SQP algorithm has better performance than other popular algorithms in less time consumption, faster convergence rate and highly stable solutions.
       
  • The Characteristics of Valley Phase as Predictor of the Forthcoming Solar
           Cycle
    • Abstract: Publication date: Available online 13 August 2018Source: Advances in Space ResearchAuthor(s): Baolin TanAbstractIs Solar Cycle 24 anomalous' How do we predict the main features of a forthcoming cycle' In order to reply such questions, this work partitions quantitatively each cycle into valley, ascend, peak, and descend phases, statistically investigate the correlations between valley phase and the forthcoming cycle. We find that the preceding valley phase may dominate and can be predictor of the forthcoming cycle: (1) The growth rate in ascend phase strongly negatively correlates to valley length and strongly positively correlates to cycle maximum. (2) The cycle maximum strongly negatively correlates to valley length, and strongly positively correlates to cycle minimum. (3) The cycle period strongly negatively correlates to the valley variation. Based on these correlations, we conclude that the solar cycle 24 is a relatively weak and long cycle which is obviously weaker than Cycle 23. The similarity analysis also presents the similar result. The Cycle 25 is also inferred possibly to be a weak cycle. These results can help us understanding the physical processes of solar cycles.
       
  • Reconciling Space Object Observed and Solar Pressure Albedo-Areas Via
           Astrometric and Photometric Data Fusion
    • Abstract: Publication date: Available online 12 August 2018Source: Advances in Space ResearchAuthor(s): Vishnuu Mallik, Moriba K. JahAbstractThere are many Resident Space Objects (RSOs) in the Geostationary Earth Orbit (GEO) regime, both operational and debris. The primary non-gravitational force acting on these RSOs is Solar Radiation Pressure (SRP), which is sensitive to the RSO’s area-to-mass ratio. Sparse observation data and mismodeling of non-gravitational forces has constrained the state of practice in tracking and characterizing RSOs. Accurate identification, characterization, tracking, and motion prediction of RSOs is a high priority research issue as it shall aid in assessing collision probabilities in the GEO regime, and orbital safety writ large. Previous work in characterizing RSOs has taken a preliminary step in exploiting fused astrometric and photometric data toestimate the RSO mass, shape, attitude, and size. This works, in theory, since angles data are sensitive to SRP albedo-area-to-mass ratio, and photometric data are sensitive to shape, attitude, and observed albedo-area. By fusing these two data types, mass and albedo-area both become observable parameters and can be estimated as independent quantities. However, previous work in mass and albedo-area estimation has not quantified and assessed the fundamental physical link between SRP albedo-area and observed albedo-area. The observed albedo-area is always a function of the SRP albedo-area along the line of sight of the observer. This is the physical relationship that this current research exploits. It is shown through simulation that due to this physical link, and through the fusion of astrometric and photometric data, it is possible to observe the mass of a space object when the area is not known. Results for data from 100 trajectories generated from randomly sampled initial conditions are shown. It is seen that even when the area of the object is not known, the uncertainty in mass can be lowered from an initial value of 800 kg to the range 500 – 700 kg for 72% of the samples, 200 – 500 kg for 13% of the samples, and 0 – 200 kg for 15% of the samples. It is further shown that although the uncertainties are large, the actual errors in mass are much lower, with the error RMS being less than 100 kg for 30% of the samples, between 100 – 200 kg for another 30%, and between 200 – 300 kg for 24% of the samples.
       
  • Potential function based robust safety control for spacecraft rendezvous
           and proximity operations under path constraint
    • Abstract: Publication date: Available online 12 August 2018Source: Advances in Space ResearchAuthor(s): Qi Li, Bo Zhang, Jianping Yuan, Huan WangAbstractThis paper deals with the safety control problem for spacecraft rendezvous and docking with coupled position and attitude dynamics under external disturbances and unknown model parameters. Given the path constraint, a semi-cubical parabola based curve is employed to restrict the motion area of the chaser spacecraft during the process of rendezvous and proximity operations. By combining the sliding mode technique with the artificial potential function, a robust adaptive control strategy is presented for driving the chaser spacecraft to rendezvous and dock with a space target without violating the path constraint. The stability of the closed-loop system is then proved within the Lyapunov framework. Numerical simulations are carried out to illustrate the effectiveness of the proposed control strategy.
       
  • List of Referees
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s):
       
  • Lead detection using Cryosat-2 delay-doppler processing and Sentinel-1 SAR
           images
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s): Marcello Passaro, Felix L. Müller, Denise DettmeringAbstractIn the Arctic and Antarctic Ocean, where part of the sea surface is seasonally or continuously covered by sea ice, the sea level monitoring from satellite altimetry relies on the localisation of open water areas, especially on the detection of leads: long and narrow fractures in the sea ice, which dominate the radar echoes even if hundreds of meters away from nadir.The Cryosat-2 altimetry mission is based on the Delay-Doppler processing, in which the averaged waveform is formed by summing up several looks acquired at different look angles and stacked together. This imaging technique and the resulting improved along-track resolution are here exploited to evaluate different lead identification schemes.In particular, stack and power statistics of Cryosat-2 waveforms are used to classify leads on a subset of 12 tracks in which the altimetry-based classification is compared to a classification based on Sentinel-1A SAR images. For this scope, a dedicated SAR-image automated processing is proposed to avoid the manual classification.Results show that the adoption of a single new stack parameter (the Stack Peakiness) can perform equally well as the use of multiple stack parameters currently available. Moreover, a multi-waveform analysis of the Stack Peakiness helps to isolate the point where narrow leads cross the tracks at nadir.For all the tested strategies, the number of altimetry-detected leads that are unidentified by SAR is comparable to the number of detections from both sensors. This could be due to presence of narrow leads, not detected by SAR due to resolution limits, but still dominant in the radar altimeter return due to the high backscatter.
       
  • CryoSat-2 range, datation and interferometer calibration with Svalbard
           transponder
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s): Albert Garcia-Mondéjar, Marco Fornari, Jerome Bouffard, Pierre Féménias, Mònica RocaAbstractTransponders are commonly used to calibrate absolute range from conventional altimeter waveforms because of their characteristic point target radar reflection. The waveforms corresponding to the transponder distinguish themselves from the other waveforms resulting from natural targets, in power and shape. ESA deployed a transponder available for the CryoSat mission (a refurbished ESA transponder developed for the ERS-1 altimeter calibration). It is deployed at the KSAT Svalbard station: SvalSAT. The transponder is used to calibrate SIRAL’s range, datation, and interferometric phase (or angle of arrival) to meet the mission requirements. In these calibrations, 3 different types of data have been used: the raw Full Bit Rate data, the stack beams before they are multi-looked (stack data) in the Level 1b processor, and the Level 1b data itself. Ideally, the comparison between the theoretical values provided by the well-known target, and the measurement by the instrument to be calibrated provides us with the error that the instrument is introducing when performing its measurement. When this error can be assumed to be constant regardless the conditions, it will provide the bias of the instrument. If the measurements can be repeated after a certain period of time, it can also provide an indication of the instrument drift. This paper presents the method to estimate the range, datation and angle-of-arrival errors from the transponder data and applies it to six years of Baseline C data to derive potential biases and drifts.
       
  • Spectral windows for satellite radar altimeters
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s): Walter H.F. SmithAbstractA satellite altimeter’s waveform is a power spectral density (PSD) estimate that displays backscattered power as a function of range, and in the Delay/Doppler or multi-looked SAR (D/D-SAR) algorithm, also of along-track position. Earth surfaces reflect radar power at a continuum of ranges and along-track positions, and so waveforms inevitably suffer from spectral leakage. Leakage may be mitigated, and the PSD resolution (“point target response”, PTR) shaped, by employing a spectral window, in either or both of the range and along-track dimensions. This paper demonstrates the sampling, symmetry and zero-padding required to ensure that the window does not introduce any distortion of the waveform. Because the PTR shapes the waveform through convolution, this paper characterizes waveform resolution in terms of PTR integrals. Expressing these as matrix-vector quadratic forms shows that ideal windows may be built from eigenvectors of appropriate matrices. A new approach taken here is to seek windows that make the PTR as nearly Gaussian as possible, since a Gaussian PTR is assumed in theoretical models for the statistical expectation of waveforms from uniformly rough surfaces. Up to now, altimeters have used either rectangular or Hamming windows, but this paper proposes other windows that provide a narrower and more Gaussian PTR with adequate leakage suppression. CryoSat-2 SAR mode data over transponders and leads in sea ice are processed with various windows to demonstrate applications and results.
       
  • From conventional to Delay Doppler altimetry: A demonstration of
           continuity and improvements with the Cryosat-2 mission
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s): M. Raynal, S. Labroue, T. Moreau, F. Boy, N. PicotAbstractCryosat-2 is the first satellite that embarks an altimeter able to operate in several modes. Among them, the Synthetic Aperture Radar mode (SARM) also referred to as Delay Doppler mode is expected to improve the measurement resolution and precision. Based on a two year time series, this paper presents the SARM assessment over ocean. It demonstrates the seamless transition, below the centimeter level, between conventional pulse-limited and Delay Doppler altimetry at long wavelength for the sea level. Nevertheless, the SARM retrieved Significant Wave Height (SWH) is biased by 20 cm and depends on the sea state. This paper also confirms that, thanks to the better signal to noise ratio and along track resolution brought by the SARM, the small scales observability is significantly improved with respect to the Low Resolution Mode (LRM). The SARM Sea Level reveals a spectral slope never observed for wavelength below 30 km that is not yet explained. Further analyses at global scales using the Sentinel-3A dataset will certainly help to characterize the source of this signal.
       
  • The CryoSat interferometer: End-to-end calibration and achievable
           performance
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s): Michele Scagliola, Marco Fornari, Jerome Bouffard, Tommaso ParrinelloAbstractExploiting the interferometric capability of CryoSat, the interferometric phase related to the first arrival of the echo is used to retrieve the angle of arrival of the scattering in the across-track direction. This paper presents an assessment of the achievable end-to-end performance for the across-track angle that is retrieved by the CryoSat SARin acquisitions. In orbit calibration campaigns are periodically performed to monitor the performance and to calibrate the interferometer. We started from the analysis of the data acquired during the interferometer calibrations campaigns to verify if the current performance of CryoSat is in line with the system requirements. The analysis revealed that the interferometric measurements of CryoSat are exceeding the expected performance and that, by proper processing, it is possible to achieve further improvements for the accuracy on the retrieved across-track angle.
       
  • CryoSat-2 Full Bit Rate Level 1A processing and validation for inland
           water applications
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s): P. Moore, S.J. Birkinshaw, A. Ambrózio, M. Restano, J. BenvenisteAbstractThis study uniquely processes Cryosat-2 Full Bit Rate (FBR) SAR Level 1A data to recover inland water heights. The processing methodology involves an azimuthal Fast Fourier Transform (FFT) for the burst echo data followed by beam formation directed towards equi-angular ground points, stacking, slant range correction, multi-looking and finally retracking. It is seen that speckle in the burst echo data affects the recovered heights with precise heights recovered only through stacking and forming multi-look waveforms. Also investigated is the effect of different numbers of multi-looks in the stack to form the final waveform for retracking. A number of empirical retrackers are utilized over inland waters and compared against the oceanic SAMOSA2 and the OCOG/Threshold retrackers. Use of the SAMOSA2 retracker is shown to be inappropriate for inland waters. The use of 81 multi-looks from the stack centred on the nadir direction is shown to be preferred across Tonlé Sap with the RMS of height residuals in the range 4–6 cm. External validation across Tonlé Sap using gauge data shows that CryoSat-2 heights (RMS 42.1 cm) are comparable to OSTM (RMS 42.6 cm) despite the CryoSat-2 non-repeating orbit which precludes the use of a mean profile. Validation against gauge data at Kratie on the Mekong gives an RMS of 59.9 cm for Cryosat-2 against an RMS of 35.5 cm and 52.2 cm derived from Envisat. The CryoSat-2 results utilize an approximate correction for river slope as the river crossings span 5 km upstream to 80 km downstream of the gauge while the repeat pass crossings of Envisat are at 7 km and 43 km from the gauge. Validation of Amazon altimetric Surface Water Elevation (SWE) showed RMS agreement of 27.3 cm with Obidos gauge data and 56.3 cm at Manacapuru 650 km upstream of Obidos. Overall validations showed that CryoSat-2 altimetric river heights are more accurate than those from TOPEX/Poseidon, OSTM and Envisat for relatively large water bodies but less accurate than the Ka band SARAL (Satellite with ARgos and ALtiKa).
       
  • Analysis and retrieval of tropospheric corrections for CryoSat-2 over
           inland waters
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s): Telmo Vieira, M. Joana Fernandes, Clara LázaroAbstractThe application of satellite altimetry over inland waters requires a proper modelling of the various error sources involved in the determination of precise surface water heights above a reference ellipsoid or above the geoid.The objectives of this study are firstly the analysis of the errors present on the dry tropospheric correction (DTC) and on the wet tropospheric correction (WTC) provided in the CryoSat-2 (CS-2) products and secondly the development of methodologies to derive improved corrections, aiming at getting improved products for CS-2. This study is conducted on selected regions of interest, such as the Amazon and Danube rivers, Titicaca and Vanern lakes and the Caspian Sea. Since CS-2 has a geodetic orbit, its ground tracks allow the retrieval of precise surface water heights over regions not covered by any other satellite.The DTC and WTC present in the CS-2 products have been compared against corrections computed from the European Centre for Medium-Range Weather Forecasts (ECMWF) operational model at various levels: (i) the level of ECMWF model orography; (ii) the level of the Altimetry Corrected Elevations 2 (ACE2) digital elevation model and (iii) the level of mean lake/sea or river profile.An independent assessment of the corrections has also been performed by comparison with DTC derived from in situ surface pressure measurements and WTC retrieved from Global Navigation Satellite Systems (GNSS) data.Results show that the model-derived corrections present on CS-2 products seem to be referred to the model orography, except for the Caspian Sea where corrections seem to be referred to mean sea level (zero level). Model orography can depart from the mean river profile or mean lake/sea heights by hundreds of meters. Overall, ACE2 DEM is a better altimetric surface than ECMWF orography, however height errors up to hundreds of meters exist in ACE2. Height errors induce DTC errors that can reach several centimetres (11 cm in the Danube River) and WTC errors up to 2–3 cm. These errors are systematic, having always the same sign and magnitude for a given location, thus affecting the retrieval of the absolute water level.For rivers, the mean profile is the best representation of the surface height in the river basin and is also the best reference surface for use in the DTC and WTC estimations from an atmospheric model. The same happens with lakes or closed seas, where the corrections should be referred to the mean lake/sea level.Results show that, once computed at the correct mean river profile or mean lake/sea level, the DTC has a small variation, with a standard deviation going from 0.5 cm in the Amazon River to 3.0 cm in the Danube River. The DTC absolute values go from 1.48 m in Lake Titicaca to 2.32 m in the Caspian Sea. With a larger variability, once computed at mean river profile or mean lake/sea level, the standard deviation of the WTC goes from 2.7 cm in Lake Titicaca to 5–6 cm in all other regions and absolute values from only 6 cm in Lake Titicaca to 31 cm in the Amazon River.Once computed at the correct surface elevation the corresponding errors are expected to be less than 1 cm for the DTC and less than 2 cm for the WTC.
       
  • Evaluation of the precision of different Delay-Doppler Processor (DDP)
           algorithms using CryoSat-2 data over open ocean
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s): Eduardo Makhoul, Mònica Roca, Chris Ray, Roger Escolà, Albert Garcia-MondéjarAbstractThis article explores the modification and inclusion of different algorithms within an in-house Delay-Doppler Processor (DDP) chain in order to understand their potentiality and the related impact in terms of geophysical retrievals’ precision, when operating the synthetic aperture radar (SAR) mode of CryoSat-2 mission over open ocean. To do so, a flexible and tunable processing chain has been developed by isardSAT, being able to accommodate the different algorithms/options. Two different levels of analysis have been carried out. On one hand, the CryoSat-2 nominal SAR processing baseline is compared against the one considered in the state-of-the-art Sentinel-3 ocean topographic mission, and then, against a modified version of CS-2, where the samples set to zero by the processor on the stack are not included in the incoherent averaging. On the other, the integration of an advanced processing at stack level and based on the amplitude compensation and dilation compensation (ACDC) is evaluated within the CryoSat-2 DDP. A comparative study of the different DDP configurations is presented exploiting SAR full bit rate (FBR) data from CryoSat-2 over two regions in the central Pacific Ocean and the Agulhas box. Ad-hoc SAR ocean re-trackers are exploited to derive the geophysical parameters (sea surface height-SSH and significant wave height-SWH), such that the performance of the different baselines can be quantified in terms of precision, i.e., estimation noise.
       
  • A fast convolution based waveform model for conventional and unfocused SAR
           altimetry
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s): C. Buchhaupt, L. Fenoglio-Marc, S. Dinardo, R. Scharroo, M. BeckerAbstractIn this paper we derive a closed-form solution to generate a backscatter power representation for both unfocused synthetic aperture radar altimetry and conventional altimetry signals in the frequency/slow-time domain. This new approach involves fast convolutions and leads to a fast numerical computational model which uses very few approximations. The transform back to the range-time/doppler frequency domain is then made numerically.Two retrackers based on this Signal Model Involving Numerical Convolution (SINC) are constructed for conventional and synthetic aperture radar altimetry and are named SINC2 and SINCS respectively. Output from retracking are the epoch t0 with respect to the tracking reference point, the significant wave height and the backscattering cross section at normal incidence σ0.Main benefits of the SINC model are its flexibility, the possibility to use the real point target response (PTR) or more complex representations of the height probability density function of scattering sea surface elements (PDF), and its fast computation speed to retrack the waveforms.The retrackers are applied to CryoSat-2 reduced SAR (RDSAR) and unfocused SAR waveforms. The RDSAR SINC2 retracker is only ten percent slower than the commonly used MLE3 retracker, which is a minor slowdown compared to the benefits of the SINC model. The SAR processing including the SINCS retracking procedure is three times slower than the RDSAR processing including retracking with SINC2.Model and algorithms are validated on both synthetic data generated by Monte-Carlo simulations and on real datasets. SAR SAMOSA+ results accessible through the ESA G-POD SARvatore service serve as a reference. Cross-validation shows higher agreement between SAR SAMOSA+ and SAR SINCS sea level anomaly and lower agreement between SAR SAMOSA+ and RDSAR SINC2, with accuracy of −0.1 cm for SAR SINCS and 1.7 cm for RDSAR SINC2. Instead, the accuracy of significant wave height, 4.9 cm and 3.9 cm for SAR and RDSAR and of backscatter coefficient, 0.16 and 0.12 dB in SAR and RDSAR, are comparable.Cross-validation results shows for sea level anomaly and significant wave height a higher precision of SAR SINCS and a lower precision of RDSAR SINC2, with precision of 0.92 cm and 6.6 cm for SAR SINCS and 1.96 cm and 12.6 cm for RDSAR SINC2 in sea level anomaly and significant wave height respectively. Instead, the precision of the backscatter coefficient is higher in RDSAR SINC2 than in SAR SINCS, and is 0.014 and 0.016 dB.In summary the new open ocean retrackers are a viable alternative recommended for both reduced and unfocused SAR processing in open ocean.
       
  • Impact of long ocean waves on wave height retrieval from SAR altimetry
           data
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s): T. Moreau, N. Tran, J. Aublanc, C. Tison, S. Le Gac, F. BoyAbstractSAR altimetry is a new high-resolution operation mode exploited in new-generation altimeter missions, such as Sentinel-3. It takes advantage of its enhanced along-track resolution to make measurements of sea surface height variations in much greater detail than what can be achieved with conventional low resolution instruments (e.g. the Jason-3 altimeter). However, contrary to what is observed for conventional altimetry, long-wavelength ocean waves of a few hundred meters (swell and extreme wind waves) are no longer fully imaged in the instrument ground cells, and SAR waveforms have distorted shapes in such wave conditions. This affects the final retrieval of significant wave height (SWH). This paper analyzes the impact of long ocean waves on SAR-mode data by using both Cryosat-2 measurements and simulated data. Results from these two approaches are in good agreement and show that the estimated parameters from SAR-altimetry waveforms are particularly noisy under long-wave conditions and also biased when compared with conventional altimetry data. Additionally, we found evidence that these impacts are different between the two directions (along and cross-track directions) due to the asymmetry of the SAR-altimetry footprint. Simulations indicate that statistics of sea surface elevations within the SAR-altimetry footprint deviate from Gaussian behavior. The assumption commonly used for ocean retracking algorithms is therefore inaccurate. The sensitivity of SAR-mode altimetry data to long waves is a key issue for the ocean altimetry community, which is concerned to ensure the continuity of high-quality time series of the global sea-surface topography in future years. This is not only an issue for these new-generation radar altimeters (Sentinel-3 and Sentinel-6) but also for all innovative techniques or processing methodologies capable of providing higher spatial resolution of the ocean surface.
       
  • SARin mode, and a window delay approach, for coastal altimetry
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s): Pablo García, Cristina Martin-Puig, Mònica RocaAbstractThe CryoSat-2 (CS-2) mission is an altimetric mission designed to meet the measurement requirements for ice-sheet elevation and sea-ice ‘freeboard’. In addition to Cryosphere applications, this mission is of great interest to the Hydrosphere and Oceanographic communities since it allows for quantitative assessment of expected enhanced altimetric capabilities in coastal monitoring, ocean floor topography, gravity field and inland water monitoring. The CS-2 main payload is a Synthetic aperture Interferometric Radar ALtimeter (SIRAL) operating in three different modes, including the SAR interferometric mode (SARin). In the work described in this paper, dedicated algorithms are developed for exploiting SARin mode for enhancing the sea level mapping at coastal regions. When operating in SARin mode the altimeter measurements allow for the derivation of the across-track Angle of Arrival (AoA), and so the reflection points at any range bin can be geolocated. In coastal altimetry, land contamination highly degrades the backscattered echo. This is sometimes, overcome with the SARM when tracks are perpendicular to the coast, since this mode provides an along-track resolution enhancement with respect to LRM mode due to the different footprints shapes (LRM is circular, while SARM is a stripe of the same wide across track but only around 300 m along track). However, across-track interferences still remain an issue in coastal altimetry and SARin mode can provide a solution for their mitigation. This paper proposes two different solutions for the improvement of SSH retrievals in coastal zones starting from L1b SARin products, developing post-L1b algorithms. The first solution is based in the use of the AoA. The second solution goes beyond SARin mode, and provides an algorithm for all operational modes, based in the historical window delay within a coastal track section. For both options, a sub-waveform approach is considered for the waveforms cleaning and retracking. In addition, a SAR L2 retracker inherited from the SAMOSA model is adapted to SARin. The results are analyzed and the potential improvements are assessed with respect to SARin data retracker by the CS-2 ground processor. A dedicated CS-2 SARin area (mask) covering the Cuban archipelago has been created for this investigation. A land-sea mask has been created from the Open Street Map high resolution catalogue for this study. A dataset has been produced including both ESA CS2 IPF Baseline B SSH and the SSH series coming from this study.
       
  • Coastal sea level from CryoSat-2 SARIn altimetry in Norway
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s): Martina Idžanović, Vegard Ophaug, Ole Baltazar AndersenAbstractConventional (pulse-limited) altimeters determine the sea surface height with an accuracy of a few centimeters over the open ocean. Sea surface heights and tide-gauge sea level serve as each other’s buddy check. However, in coastal areas, altimetry suffers from numerous effects, which degrade its quality. The Norwegian coast adds further challenges due to its complex coastline with many islands, mountains, and deep, narrow fjords.The European Space Agency CryoSat-2 satellite carries a synthetic aperture interferometric radar altimeter, which is able to observe sea level closer to the coast than conventional altimeters. In this study, we explore the potential of CryoSat-2 to provide valid observations in the Norwegian coastal zone. We do this by comparing time series of CryoSat-2 sea level anomalies with time series of in situ sea level at 22 tide gauges, where the CryoSat-2 sea level anomalies are averaged in a 45-km area around each tide gauge. For all tide gauges, CryoSat-2 shows standard deviations of differences and correlations of 16 cm and 61%, respectively. We further identify the ocean tide and inverted barometer geophysical corrections as the most crucial, and note that a large amount of observations at land-confined tide gauges are not assigned an ocean tide value. With the availability of local air pressure observations and ocean tide predictions, we substitute the standard inverted barometric and ocean tide corrections with local corrections. This gives an improvement of 24% (to 12.2 cm) and 12% (to 68%) in terms of standard deviations of differences and correlations, respectively.Finally, we perform the same in situ analysis using data from three conventional altimetry missions, Envisat, SARAL/AltiKa, and Jason-2. For all tide gauges, the conventional altimetry missions show an average agreement of 11 cm and 60% in terms of standard deviations of differences and correlations, respectively. There is a tendency that results improve with decreasing distance to the tide gauge and a smaller footprint, underlining the potential of SAR altimetry in coastal zones.
       
  • SAR Retracking in the Arctic: Development of a year-round retrackers
           system
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s): Ann-Theres Schulz, Marc NaeijeAbstractGiven its polar orbit, CryoSat-2 provides frequent, high-resolution SAR altimeter measurements of the Arctic region over changing terrain (e.g. ice, water and sea ice). The purpose of this research is to improve SAR waveform retracking in the Arctic region by analysing different retrackers on their performance in varying (Arctic) conditions and combine the positive behaviours into one optimal retrackers system.Based on the accuracy and precision performance, four retrackers are evaluated in order to determine this retrackers system: the empirical primary peak centre of gravity, the primary peak threshold and the ESA retracker, as well as the physical SAMOSA3 retracker. Empirical retrackers determine the retracking point depending on the retrieved waveform statistics, while physical retrackers take the system characteristics, the geometry and the surface properties into account.Three sea surface types are considered in this research: ocean, sea ice, and ice leads. For the majority of the months, SAMOSA3 has the highest accuracy for ocean waveforms as its physical full-analytic approach provides a good fit for the predictable ocean waveforms. The primary peak retrackers have the best precision performance for irregular waveforms like sea ice and ice leads.The year-round retrackers system includes per month the most accurate retracker for ocean waveforms and the best precision retracker for sea ice and leads. To remove the bias caused by combining different retracking algorithms, four bias removal strategies are developed and evaluated on their accuracy and precision performance. The retrackers system with the primary peak centre of gravity as a basis retracker and a mean bias removal approach performed best, as it has a precision improvement of 47.1% with respect to retracking all waveform classes with the primary peak COG retracker solely. By applying the developed optimal retrackers system and using the bias removal strategy, the mean standard deviation of the altimeter measurements in the Kara Sea study area is reduced from 6.7 cm to 3.6 cm.
       
  • Assessment of CryoSat-2 interferometric and non-interferometric SAR
           altimetry over ice sheets
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s): Malcolm McMillan, Andrew Shepherd, Alan Muir, Julia Gaudelli, Anna E. Hogg, Robert CullenAbstractThe launch of CryoSat-2 heralded a new era of interferometric Synthetic Aperture Radar altimetry over the Polar Ice Sheets. The mission’s novel SAR interferometric (SARIn) mode of operation has enabled monitoring of rapidly changing coastal regions, which had been challenging for previous low resolution altimeters. Given the growing requirement to continue the 25-year altimeter record, there is now a need to assess the differences between existing SAR and SARIn altimeter datasets, with a view to understanding the impact on ice sheet retrievals of the different radar hardware and processing methodologies. Uniquely, CryoSat-2 data can be processed both with and without interferometric information, offering the opportunity to directly compare the SAR and SARIn products generated by the current ground segment. Here, we provide a first comparison of these Level-2 datasets, and evaluate their capacity to measure ice sheet elevation and elevation change. We find that the current interferometric product has substantially improved precision, accuracy and coverage compared to its non-interferometric counterpart, yielding a ∼35% improvement in the root-mean-square-difference (RMSD) of elevations recorded at orbital cross-overs, and a ∼30% lower RMSD of elevation rates relative to Operation IceBridge airborne altimeter measurements. This analysis demonstrates the value that the interferometer adds to the current CryoSat-2 configuration, and highlights the importance for non-interferometric SAR Level-2 processing of the auxiliary data used to identify the location of the echoing point. These results provide a benchmark of the relative performance of the Level-2 interferometric and non-interferometric products currently produced by the ground segment, which will help to inform the design and implementation of a future polar radar altimeter mission.
       
  • CryoSat-2 swath interferometric altimetry for mapping ice elevation and
           elevation change
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s): N. Gourmelen, M.J. Escorihuela, A. Shepherd, L. Foresta, A. Muir, A. Garcia-Mondéjar, M. Roca, S.G. Baker, M.R. DrinkwaterAbstractFor more than 25 years, satellite radar altimetry has provided continuous information on the state of the cryosphere and on its contribution to global sea-level rise. The technique typically delivers maps of ice-sheet elevation and elevation change with 3–10 km spatial resolution and seasonal to monthly temporal resolution. Here we show how the interferometric mode of CryoSat-2 can be used to map broad (5 km-wide) swaths of surface elevation with fine (500 m) spatial resolution from each satellite pass, providing a step-change in the capability of satellite altimetry for glaciology. These swaths of elevation data contain up to two orders of magnitude more surface elevation measurements than standard altimeter products, which provide single elevation measurements based on the range to the Point-Of-Closest-Approach (POCA) in the vicinity of the sub-satellite ground track. The swath elevations allow a more dense, statistically robust time series of elevation change to be formed with temporal resolution of a factor 5 higher than for POCA. The mean differences between airborne altimeter and CryoSat-2 derived ice sheet elevations and elevation rates range from −0.93 ± 1.17 m and 0.29 ± 1.25 m a−1, respectively, at the POCA, to −1.50 ± 1.73 m and 0.04 ± 1.04 m a−1, respectively, across the entire swath. We demonstrate the potential of these data by creating and evaluating elevation models of: (i) the Austfonna Ice Cap (Svalbard), (ii) western Greenland, and (iii) Law Dome (East Antarctica); and maps of ice elevation change of: (iv) the Amundsen Sea sector (West Antarctica), (v) Icelandic ice caps, and (vi) above an active subglacial lake system at Thwaites Glacier (Antarctica), each at 500 m spatial posting – around 10 times finer than possible using traditional approaches based on standard altimetry products.
       
  • Estimating Arctic sea ice thickness and volume using CryoSat-2 radar
           altimeter data
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s): Rachel L. Tilling, Andy Ridout, Andrew ShepherdAbstractArctic sea ice is a major element of the Earth’s climate system. It acts to regulate regional heat and freshwater budgets and subsequent atmospheric and oceanic circulation across the Arctic and at lower latitudes. Satellites have observed a decline in Arctic sea ice extent for all months since 1979. However, to fully understand how changes in the Arctic sea ice cover impact on our global weather and climate, long-term and accurate observations of its thickness distribution are also required. Such observations were made possible with the launch of the European Space Agency’s (ESA’s) CryoSat-2 satellite in April 2010, which provides unparalleled coverage of the Arctic Ocean up to 88°N. Here we provide an end-to-end, comprehensive description of the data processing steps employed to estimate Northern Hemisphere sea ice thickness and subsequent volume using CryoSat-2 radar altimeter data and complementary observations. This is a sea ice processor that has been under constant development at the Centre for Polar Observation and Modelling (CPOM) since the early 1990s. We show that there is no significant bias in our satellite sea ice thickness retrievals when compared with independent measurements. We also provide a detailed analysis of the uncertainties associated with our sea ice thickness and volume estimates by considering the independent sources of error in the retrieval. Each month, the main contributors to the uncertainty are snow depth and snow density, which suggests that a crucial next step in Arctic sea ice research is to develop improved estimates of snow loading. In this paper we apply our theory and methods solely to CryoSat-2 data in the Northern Hemisphere. However, they may act as a guide to developing a sea ice processing system for satellite radar altimeter data over the Southern Hemisphere, and from other Polar orbiting missions.
       
  • CryoSat: ESA’s ice mission – Eight years in space
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s): T. Parrinello, A. Shepherd, J. Bouffard, S. Badessi, T. Casal, M. Davidson, M. Fornari, E. Maestroni, M. ScagliolaAbstractCryoSat-2 is a satellite of the European Space Agency that was launched in April 2010. It is intended to monitor changes in the thickness of the marine ice, floating in the polar oceans, and to measure variations in the thickness of the vast ice sheets that overlie Greenland and Antarctica.The CryoSat-2 satellite has replaced the original CryoSat, which became lost due to a launch failure in October 2005.CryoSat-2 carries an innovative radar altimeter called SIRAL (Synthetic Aperture Interferometric Altimeter). It has two radar antennas, which met the measurement requirements for ice-sheet elevation and sea-ice freeboard with unprecedented accuracy. The satellite orbits the planet at an altitude of around 720 km with a retrograde drifting orbit inclination of 92° and a “quasi” repeat cycle of 369 days (30 days sub-cycle). CryoSat-2 is therefore able to reach latitudes up to 88° covering more than 4.6 million km2 of unexplored areas over the poles when compared with previous polar missions carrying an altimetry.The mission has achieved its prime objectives and is delivering high quality data, providing unique contributions to several Earth Science and applications domains, including ice, ocean, geodesy and hydrology, both at global and regional scales.The purpose of this paper is to provide a general overview of the mission and of its main scientific achievements after eight years in space.
       
  • Visibility of the CryoSat mission in the scientific and technical
           literature: A bibliometric perspective
    • Abstract: Publication date: 15 September 2018Source: Advances in Space Research, Volume 62, Issue 6Author(s): Ricardo Eito-BrunAbstractCryoSat is the first Earth observation mission launched by ESA with the purpose of observing the Earth’s polar ice masses. Six years after the launching of the CryoSat-2 satellite, its outcomes have surpassed the objectives initially planned, and the mission has provided additional insights on aspects like sea level measurement. This paper makes an analysis of the visibility of the CryoSat mission in the scientific and technical literature. The analysis covers two aspects: (a) the quantitative description of the literature generated as a result of the mission’s engineering and exploitation, and (b) the influence of this literature on the generation of additional scientific and technical knowledge. Although the generation of explicit, formal knowledge disseminated through journals, conferences and repositories is just one of the multiple benefits of space missions, the quantification of these outputs is relevant to assess the visibility of the mission in the scientific literature.
       
  • Motion Estimation of Uncooperative Space Objects: A case of Multi-Platform
           Fusion
    • Abstract: Publication date: Available online 9 August 2018Source: Advances in Space ResearchAuthor(s): Mahboubeh Zarei-Jalalabadi, Seyed Mohammad-Bagher MalaekAbstractThis work describes an efficient technique to sequentially combine estimates resulting from individual sets of measurements provided by a network of satellites. The prescribed method is especially effective to estimate motion states of an uncooperative space object using range image data. The technique, which is fast and suitable for on-line applications, could also be effective to capture stray objects or those satellites that require periodic servicing. Such missions call for high degree of precision and reliable estimation methods. In fact, the proposed estimation architecture consists of a network of synchronized platforms, i.e., Observer Satellites (OS), each with processing power and transmission capability, that are observing a common Target Space Object (TSO). All OSs are expected to have suitable measuring devices, such as active vision sensor, that provide sensory range image data. Each platform could also independently estimate its objective based on its own observations. The estimates are then transmitted to a fusion center to assimilate the fused estimate that is more accurate than any individual estimates. As a specific example, we show exploiting efficient algorithms in processing of range image data, filtering, and fusion of estimates enables the proposed method to be especially effective for active debris removal. Different case studies confirm that the method is capable of processing measured data fairly quickly and producing fused estimates with a tangible decrease in estimation error.
       
  • Apparent magnitude calculation method for complex shaped space objects
    • Abstract: Publication date: Available online 7 August 2018Source: Advances in Space ResearchAuthor(s): Yang Wang, Xiaoping Du, Jiguang Zhao, Ruixin GouAbstractThe apparent magnitude of a complex shaped space object is difficult to calculate since its visible triangular facets are hard to be determined. The most common-used visible facets determination method is to judge the incident and emergent angle, but this method doesn't apply to a space object which is a combination of convex bodies or has a concave surface. In this paper, a new quadtree-based hidden vertex removal method is presented, and it is composed of four steps: coordinate transformation, hidden vertex removal, quadtree decompositions and visible triangular facets determination. This method can determine visible triangular facets of a space object of any shapes. Once a space object triangular facet model, the observed direction of a telescope and the incident direction of the sunlight is given, the visible triangular facets of the space object can be obtained. Two experiments are carried out to verify the visible triangular facet determination performance of this method qualitatively and quantitatively. Experiment results show the visible triangular facet calculated by the proposed method is accurate and reasonable.
       
  • GPS/GLONASS Carrier Phase Elevation-Dependent Stochastic Modelling
           Estimation and Its Application in Bridge Monitoring
    • Abstract: Publication date: Available online 7 August 2018Source: Advances in Space ResearchAuthor(s): Ruijie Xi, Xiaolin Meng, Weiping Jiang, Xiangdong An, Qusen ChenAbstractThe Global Positioning System (GPS) based monitoring technology has been recognised as an essential tool in the long-span bridge health monitoring throughout the world in recent years. However, the high observation noise is still a big problem that limits the high precision displacement extraction and vibration response detection. To solve this problem, GPS double-difference model and many other specific function models have been developed to eliminate systematic errors e.g. unmodeled atmospheric delays, multipath effect and hardware delays. However, relatively less attention has been given to the noise reduction in the deformation monitoring area. In this paper, we first proposed a new carrier phase elevation-dependent precision estimation method with Geometry-Free (GF) and Melbourne-Wübbena (MW) linear combinations, which is appropriate to regardless of Code Division Multiple Access (CDMA) system (GPS) or Frequency Division Multiple Access (FDMA) system (GLONASS). Then, the method is used to estimate the receiver internal noise and the realistic GNSS stochastic model with a group of zero-baselines and short-baselines (served for the GNSS and Earth Observation for Structural Health Monitoring of Bridges (GeoSHM) project), and to demonstrate their impacts on the positioning. At last, the contribution of integration of GPS and GLONASS is introduced to see the performance of noise reduction with multi-GNSS. The results show that the higher level receiver internal noise in cost effective receivers has less influences on the short-baseline data processing. The high noise effects introduced by the low elevation satellite and the geometry variation caused by rising and dropping satellites, can be reduced by 10%-20% with the refined carrier phase elevation-dependent stochastic model. Furthermore, based on observations from GPS and GLONASS with the refined stochastic model, the noise can be reduced by 30%-40%, and the spurious signals in the real-life bridge displacements tend to be completely eliminated.
       
  • Estimation of Orbital Parameters of Broken-up Objects from In-situ Debris
           Measurements
    • Abstract: Publication date: Available online 7 August 2018Source: Advances in Space ResearchAuthor(s): Yutaka Kodama, Masahiro Furumoto, Yasuhiro Yoshimura, Koki Fujita, Toshiya HanadaAbstractEven sub-millimeter-size debris could cause a fatal damage on a spacecraft. Such tiny debris cannot be followed up or tracked from the ground. Therefore, Kyushu University has initiated IDEA the project for In-situ Debris Environmental Awareness, which conducts in-situ measurements of sub-millimeter-size debris. One of the objectives is to estimate the location of on-orbit satellite fragmentations from in-situ measurements. The previous studies revealed that it is important to find out the right nodal precession rate to estimate the orbital parameters of a broken-up object properly. Therefore, this study derives a constraint equation that applies to the nodal precession rate of the broken-up object. This study also establishes an effective procedure to estimate properly the orbital parameters of a broken-up object with the constraint equation.
       
  • Odometer and MEMS IMU enhancing PPP under weak satellite observability
           environments
    • Abstract: Publication date: Available online 7 August 2018Source: Advances in Space ResearchAuthor(s): Zhouzheng Gao, Maorong GeAbstractAccuracy of dynamic precise point positioning (PPP) degrades significantly under users’ challenging environments because of the limited or unavailable GPS observations. To overcome such weakness, a tight-integration system of micro-electromechanical systems (MEMS) inertial measurement unit (IMU), odometer, and GPS ionospheric delay constrained PPP is investigated to provide users positions and attitudes with higher accuracy, reliability, and continuity under the weak GPS observability environments. In this approach, the data from different sensors are integrated in two joint extend Kalman filters (EKF). One is for PPP/INS tight-integration and the other one is for odometer aiding the solutions from INS or PPP/INS tight-integration modes. Here, the advantages of slow time-varying ionospheric delay, high-accuracy in short time of inertial navigation system (INS), and hardly-disturbed odometer are utilized effectively to enhance PPP solutions. The corresponding mathematical models are provided and are evaluated by a set of one-hour real land-borne test data collected from a MEMS IMU, an odometer, and a dual-frequency GPS receiver in suburbs of Wuhan city, China, and also a set of simulated weak satellites availability data which is to imitate the GPS availability under unban canyons. Results indicate that the fusion system can improve navigation accuracy of GPS or GPS/INS significantly. That are, more than fifty percentages position enhancements in north-east-down components, and about forty percentages attitude enhancements in roll-pitch-yaw direction under the weak GPS availability.
       
  • The semi-analytical analysis of orbital evolution around an asteroid under
           the effects of the C20 term, the solar radiation pressure and the
           asteroid’s orbital eccentricity
    • Abstract: Publication date: Available online 2 August 2018Source: Advances in Space ResearchAuthor(s): Jinglang Feng, Xiyun HouAbstractThis paper aims to develop a semi-analytical method of propagating orbital motion near the equatorial plane of an asteroid, considering the combined effects of the asteroid’s oblateness (the C20 term), the solar radiation pressure (SRP) and the asteroid’s orbital eccentricity around the Sun (es). In the asteroid-centered frame, the Hamiltonian of the orbital motion is derived with Poincaré variables. It is firstly averaged over the orbital motion around the asteroid and then averaged over the asteroid’s eccentric orbital motion around the Sun. Time-explicit analytical solutions of the orbital eccentricity and inclination are obtained. The Lie transformation is applied to recover the eliminated oscillating terms of the inclination during the second average for a complete solution. We analyze the validity of these solutions for different semi-major axes and different values of area-to-mass ratios (A/m). We demonstrate the importance of considering the asteroid’s orbital eccentricity around the Sun and its role in enlarging the amplitude of orbital eccentricity for different A/m and orbital geometries. The solutions developed in this paper improve the knowledge of secular orbital evolution around asteroid. They can be applied to fast prediction of long-term orbital evolutions around near Earth asteroids (NEAs).
       
  • Extraction and analysis of geological lineaments combining a DEM and
           remote sensing images from the northern Baoji loess area
    • Abstract: Publication date: Available online 1 August 2018Source: Advances in Space ResearchAuthor(s): Ling Han, Zhiheng Liu, Yuming Ning, Zhongyang ZhaoAbstractGeological lineaments are important reactant of geological structure on the surface, and control the distribution of regional groundwater, geohazards, geothermal and earthquakes. The technology of geological lineaments extraction is of great significance for the analysis of regional plate movement and ore forming prognosis. However, the traditional methods are mainly based on the semi-automatic or manual visual interpretation, which consuming time and labor for the dependence on the rich professional experience and knowledge of the interpreter experts, and involving less in the special geomorphological regions. Taking Loess areas in northern Baoji as an example, this paper proposed a lineaments extraction algorithm based on tensor voting coupled Hough Transform, with the help of DEM and Landsat 8 OLI remote sensing images. Firstly, the best independent band combination of Landsat 8 OLI images for lineaments extraction were selected by using principal component analysis. Secondly, Gaussian high-pass filter was applied to sharpen the edge in the DEM and composite Landsat 8 OLI images. Linear boundary was extracted by tensor voting according to the conspicuousness of the vector sum superposition feature based on edge points. Finally, the Hough Transform was employed to search the edges and extracted the geological lineaments in this region. The experimental results showed that the orientation of lineaments was dominated by NW-SE and NE- SW, supplemented by NNW-SSE. Under the influence of the uplift of the eastern foot of Liupan Mountains and the southern margin of Ordos, the lineaments were mainly distributed over linear landforms, which were better consistent with previous studies about the direction of tectonics in this region. Compared with the segment tracing algorithm, this method has more applicability, efficiency, high practical value and scientific significance in the analysis of tectonic movement and evolution in special landform area.
       
  • Robust fault tolerant nonfragile H ∞ attitude control for spacecraft via
           stochastically intermediate observer
    • Abstract: Publication date: Available online 1 August 2018Source: Advances in Space ResearchAuthor(s): Chuang Liu, George Vukovich, Keke Shi, Zhaowei SunAbstractThe observer-based fault tolerant attitude control problem for spacecraft is addressed, wherein the constraints of H∞ performance, quadratic stability, model parameter uncertainty, measurement errors, external disturbances, controller perturbation, and actuator fault and saturation are considered simultaneously. A novel fault estimation approach is proposed, where a stochastically intermediate variable is introduced. Based on the exactly available expectation of such a variable, a new kind of intermediate observer is proposed to estimate the attitude information and fault signals simultaneously. Compared with the traditional fault estimation methods, the approach proposed in this paper is less conservative for it requires neither equation constraints nor observer matching condition and can tolerate stochastic failure. To achieve fault tolerant and nonfragile control performance, the estimation of fault signals and explicit controller perturbation are also exploited in the controller design. Based on Lyapunov stability theory, sufficient conditions for the existence of the observer-based fault tolerant nonfragile H∞ controller are given based on linear matrix inequalities (LMIs) in terms of additive perturbation and multiplicative perturbation. Numerical simulations are performed to demonstrate the effectiveness of the proposed method and also illustrate the advantage over the existing state feedback control method in terms of great reduction in energy consumption.
       
  • A suboptimal excitation torque for parameter estimation of a 5-DOF
           spacecraft simulator
    • Abstract: Publication date: Available online 1 August 2018Source: Advances in Space ResearchAuthor(s): Xu Zheyao, Chen Yukun, Xu ZhexuanAbstractFive degrees of freedom spacecraft simulators are designed to verify spacecraft control strategies, rendezvous and docking techniques. The accurate knowledge of simulator inertia parameters which can be calculated by parameter estimation is of vital importance in these experiments. However, the rotation limits of the simulator must be considered during estimation process. This paper presents an approach to determine a suboptimal excitation torque for the system identification of the inertia parameters. The continuous optimization problem is transcribed into a discrete nonlinear programming problem by integral gauss pseudospectral method. To reduce the effect of noise, the states and inertia parameters are estimated simultaneously by joint filter within Extended Kalman Filter (EKF) or Unscented Kalman Filter (UKF). The proposed method is validated by simulations and the results indicate that this method not only satisfies the constraints of simulator rotation angles, but also improves the efficiency of parameter estimation.
       
  • GLONASS real-time wide-lane ambiguity resolution with an enhanced
           geometry-based model for medium-range baselines
    • Abstract: Publication date: Available online 31 July 2018Source: Advances in Space ResearchAuthor(s): Longwei Xu, Hui Liu, Bao Shu, Fu Zheng, Ming Zhang, Chuang Qian, Yingzi DuanAbstractDouble-difference wide-lane integer ambiguities are determined first to assist narrow-lane ambiguity resolution in real-time medium-range baseline resolution. The Hatch-Melbourne-Wübbena (HMW) is the conventional strategy for wide-lane ambiguity resolution, which works well for GPS/Galileo/BDS. However, the un-canceled inter frequency bias (IFB) on double-difference measurement causes that the HMW combination is invalid on GLONASS wide-lane ambiguity resolution. The residual IFB in double-difference measurement may be several meters, especially between inhomogeneous stations. There is no effective method to model or tabulate ranging-codes IFB. In this paper, we propose a device-independent geometry-based model to achieve GLONASS real-time wide-lane ambiguity resolution for medium-range baselines. Since zenith tropospheric delay and slant ionospheric delay of satellites are estimated as unknown parameters, we utilize atmosphere-weighted algorithm to enhance the model strength and decrease the dependence on pseudorange measurement. For medium-range baseline, GLONASS wide-lane ambiguity float solutions can converge quickly by the geometry-based model. The cumulative frequency of WL AR can exceed 99% during 10 epochs. Benefiting from rapid and reliable wide-lane ambiguity resolution, the positioning accuracy of GPS/GLONASS RTK fixed solution are greater than 3 cm in level direction and 5 cm in upward direction for 40–100 km baselines. An approximately 35% improvement rate is observed in each direction compared with that of single GPS RTK. Since less available satellites for single-GLONASS mode, the convergence of wide-lane ambiguity becomes slower. For base stations with known coordinates, the enhanced geometry-based model can be developed to the enhanced geometry-fixed model and are applied to longer baselines. Assisting with the geometry-fixed model, more than 90% of GLONASS narrow-lane ambiguities can be fixed quickly for 80-160 km baselines. It should be noted that the accuracy of external atmospheric delay is critical to the performance of the geometry-based method. Its benefit may weaken with the increasing of baseline length, when external atmospheric delays are set as 0 directly.
       
  • Empirical Values of Branching Ratios in the Three-Body Recombination
           Reaction for O(aS) and O2(0,0) Airglow Chemistry
    • Abstract: Publication date: Available online 27 July 2018Source: Advances in Space ResearchAuthor(s): Yolián Amaro-Rivera, Tai-Yin Huang, Julio Urbina, Fabio VargasAbstractThe branching ratios ε and α in the three-body recombination reaction for O(1S) greenline and O2(0,0) atmospheric band airglow chemistry represent the fraction of O2 that branch into the b1∑g+ and c1∑u- electronic states, respectively. In the present work, the empirical values of these branching ratios have been deduced using a numerical optimization approach. They were obtained using the optimization scheme known as the Covariance Matrix Adaptation Evolution Strategy (CMA-ES) with our MACD-00 model and simultaneous volume emission rate (VER) measurements of the O(1S) greenline and O2(0,0) atmospheric band emissions. The CMA-ES was employed as the optimization algorithm that would match the O(1S) and O2(0,0) VER profiles simulated by the MACD-00 model to observations made by OXYGEN/S35, S310.10, NASA Flight 4.339, ETON flights P229H and P230H, OASIS, SOAP/WINE, MULTIFOT, and WINDII. We found that most of the values deduced for ε were in the [0.1, 0.3] range, while most of the values of α were in the [0.01, 0.03] range. Excluding the outliers, the average branching ratio values involving the production of O2(b1∑g+) and O2(c1∑u-) were determined to be ε = 0.15 ± 0.02 and α = 0.018 ± 0.004, respectively. Overall, the simulations showed good agreement with the observations albeit with some discrepancies in the peak altitudes and shape of the profiles, possibly due to small perturbations in the observed VER profiles that are not considered in our simulations.
       
  • Multi-spacecraft attitude cooperative control using model-based
           event-triggered methodology
    • Abstract: Publication date: Available online 27 July 2018Source: Advances in Space ResearchAuthor(s): Chengxi Zhang, Jihe Wang, Ran Sun, Dexin Zhang, Xiaowei ShaoAbstractThe attitude cooperative control of multi-spacecraft under undirected information flow is studied in this paper based on a novel state-irrelevant event-triggered control (ETC) strategy. In the proposed algorithm, the control updating and data-transfer among spacecraft need only be executed when certain conditions are triggered. Then, the system model is utilized to predict the future state based on the last transmission and to calculate the time of the next transmission event. Firstly, the proposed algorithm reduces the control updating frequency remarkably and avoids continuous communication; secondly, each spacecraft updates its controller independently, without requiring all members to update simultaneously; thirdly, it is still effective in the presence of input limitation. The consensus can be guaranteed under such control strategy. It has proved that there exists a lower bound for the update interval, avoiding a phenomenon that the infinite number of events may appear in a finite time interval, also called the Zeno phenomenon. The efficacy of the proposed algorithm is verified via simulations.
       
  • Finding the suitable drag-free acceleration noise level for future low-low
           satellite-to-satellite tracking geodesy missions
    • Abstract: Publication date: Available online 27 July 2018Source: Advances in Space ResearchAuthor(s): Seong Hyeon Hong, John W. ConklinAbstractThis paper evaluates the impact of residual acceleration noise on the estimation of the Earth’s time-varying gravity field for future low-low satellite-to-satellite tracking missions. The goal is to determine the maximum level of residual acceleration noise that does not adversely affect the estimation error. The Gravity Recovery And Climate Experiment (GRACE) has provided monthly average gravity field solutions in spherical harmonic coefficients for more than a decade. It provides information about land and ocean mass variations with a spatial resolution of ∼350 km and with an accuracy within 2 cm throughout the entire Earth. GRACE Follow-on was launched in May, 2018 to advance the work of GRACE and to test a new laser ranging interferometer, which measures the range between the two satellites with higher precision than the K-Band ranging system used in GRACE. Moreover, there have been simulation studies that show, an additional pair of satellites in an inclined orbit increases the sampling frequency and reduces temporal aliasing errors. Given the fact that future missions will likely continue to use the low-low satellite-to-satellite tracking formation with laser ranging interferometry, it is expected that the residual acceleration noise will become one of the largest error contributor for the time-variable gravity field solution. We evaluate three different levels of residual acceleration noise based on demonstrated drag-free systems to find a suitable drag-free performance target for upcoming geodesy missions. We analyze both a single collinear polar pair and the optimal double collinear pair of drag-free satellites and assume the use of a laser ranging interferometer. A partitioned best linear unbiased estimator that was developed, incorporating several novel features from the ground up is used to compute the solutions in terms of spherical harmonics. It was found that the suitable residual acceleration noise level is around 2 × 10-12 ms-2Hz-1/2. Decreasing the acceleration noise below this level did not result in more accurate gravity field solutions for the chosen mission architecture.
       
  • An Experimental Setup for Hollow Cathode Independent Life Test Simulating
           Hall Thruster Discharge Current Oscillations
    • Abstract: Publication date: Available online 27 July 2018Source: Advances in Space ResearchAuthor(s): Wenbo Li, Hong Li, Yongjie Ding, Liqiu Wei, Haifeng Lu, Qian Gao, Zhongxi Ning, Daren YuAbstractThe reliability of cathode is critical to electric propulsion systems. Although researchers have carried out many life tests on hollow cathode, the discharge current is almost steady when the cathode is independently tested for life. Actually, when it is coupled with the thruster, the discharge current is with large amplitude low-frequency oscillation. Therefore, we suggest a new external circuit for a cathode-independent life test, which could simulate the characteristics of discharge current oscillation when the cathode is coupled with Hall thruster. We carried out 160 hours of life test using the normal external circuit and the new external circuit, and the variation characteristics of the cathode orifice area are also studied during the life test. The results show that under the normal external circuit, the growth rate of the cathode orifice area initially increases and then decreases gradually with the increase of the time of life test, whilst the cathode orifice area in the new external circuit linearly increases. The new external circuit can provide a new method to simulate the discharge current oscillation when the cathode is coupled with Hall thruster during the independent life test of the cathode.
       
  • Use of Landsat 8 data for characterizing dynamic changes in physical and
           acoustical properties of coastal lagoon and estuarine waters
    • Abstract: Publication date: Available online 26 July 2018Source: Advances in Space ResearchAuthor(s): Theenathayalan Varunan, Palanisamy ShanmugamAbstractThis study intends to develop methodologies that use high resolution satellite data from Landsat 8 (Operational Land Imager) OLI and (Thermal Infrared Sensor) TIRS sensors for characterizing spatial and temporal changes in physical and acoustical properties of coastal lagoon and estuarine waters. It employs multiple steps to achieve this possibility: a novel atmospheric correction algorithm is applied to OLI spectral data to retrieve water-leaving radiances which are key inputs for the applied models; appropriate parameterizations are developed for the OLI bands and used in conjugation with a hybrid model to produce the spectral absorption coefficients of coloured dissolved organic matter (aCDOM) and to derive surface salinity fields which inversely correspond with the aCDOM values; an efficient algorithm is employed to estimate surface water temperature using thermal infrared bands, and well-known models are employed with the satellite-derived products to determine the acoustical properties (sound attenuation and speed). Results from the above methodology were evaluated using in-situ data and Landsat 8 OLI matchup data acquired over the coastal lagoon systems (e.g., Chilika Lagoon on the coast of Bay of Bengal) during monsoon and non-monsoon seasons. The uncertainties associated with the derived products such as CDOM, salinity, temperature, sound attenuation and speed were found to be within the desirable mission goal. Recognizing the importance of the salinity gradient that plays a unique and fundamental role in defining a transitional ecosystem, spatial and temporal patterns in the structure of the salinity gradient were examined together with the CDOM patterns. High resolution OLI products exhibited a general horizontal gradient with salinity decreasing from the lagoon mouth in the eastern and central sectors to the river mouth in the northern sector and a near uniform gradient with moderate salinity in the adjacent locations (southern sector) of the lagoon. The time-series of OLI products further showed that spatial and temporal structures of the salinity are modulated by the terrestrially delivered freshwater inputs, tidal forcing at the lagoon mouth, mixing of these two waters sources, and local geomorphology. Surface water temperature products derived from the TIRS sensor for the lagoon and its adjoining locations depicted a well pronounced seasonal cycle with warmer temperatures modulated by reduced mixing and increased solar heating and stratification during non-monsoon, summer months and cooler temperatures during monsoon, winter months. The effect of salinity and temperature on the sound attenuation and sound speed was prominent in the locations of the freshwater discharge and tidal mixing regimes, where the salinity exerted a greater influence on both sound attenuation and speed despite the opposing effects of surface water temperatures. In areas of surface heating and stratification, both salinity and temperature increased causing an increase in sound attenuation and speed over the ranges found in the lagoon. These results are important for sonar performance modelling and operation of acoustic devices in such shallow water environments impacted by the terrestrial and ocean forcing factors.
       
  • Contemporary Sea Level Changes from Satellite Altimetry: What have we
           learned' What are the new challenges'
    • Abstract: Publication date: Available online 25 July 2018Source: Advances in Space ResearchAuthor(s): Anny Cazenave, Hindumathi Palanisamy, Michael AblainAbstractSince the early 1990s, high-precision multi-mission satellite altimetry has provided a 25-year-long sea level record from which global mean sea level rise and superimposed interannual and regional variability can be derived. Most recent results show that the global mean sea level is rising at a mean rate of 3.1 +/- 0.3 mm/yr since January 1993. A clear acceleration is also visible on this 25-year time span, estimated to 0.10 mm/yr2. Mapping of spatial trend patterns continue to show deviation from the global mean rise in a number of regions. However, as the altimetry record lengthens, the ratio of regional trends to the global mean rise tends to decrease, with a factor of amplification of only 2, compared to 3 to 4 some years ago. Estimates of thermal expansion from Argo and ocean mass change from GRACE show that over the GRACE and Argo time span (since 2005) the sea level budget is almost closed. Assessment of the sea level budget over the entire altimetry era (since 1993) based on estimates of individual mass components for the glaciers and the ice sheets provides some upper bound for the still poorly known contribution from water storage on land. At regional scale, ocean thermal expansion is still the main cause of the spatial trend patterns observed by satellite altimetry. However, removing the steric component reveals residual signal that still needs interpretation. In the remaining of this review, we briefly discuss future sea level changes and associated coastal impacts. Finally, we address the issue of remaining gaps in sea level studies, in particular the need for producing coastal sea level products from dedicated satellite altimetry processing of sea level data in global coastal zones.
       
  • Simulating arbitrary hyperspectral bandsets from multispectral
           observations via a generic Earth Observation-Land Data Assimilation System
           (EO-LDAS)
    • Abstract: Publication date: Available online 23 July 2018Source: Advances in Space ResearchAuthor(s): M. Chernetskiy, N. Gobron, J. Gomez-Dans, O. Morgan, M. Disney, P. Lewis, C. SchmulliusAbstractThis paper presents results of using multi-sensor and multi-angular constraints in the generic Earth Observation-Land Data Assimilation System (EO-LDAS) for reproducing arbitrary bandsets of hyperspectral reflectance at the top-of-canopy (TOC) level by merging observations from multispectral sensors with different spectral characteristics. This is demonstrated by combining Multi-angle Imaging Spectroradiometer (MISR) and Landsat Enhanced Thematic Mapper Plus (ETM+) data to simulate the Compact High Resolution Imaging Spectrometer CHRIS/PROBA hyperspectral signal over an agricultural test site, in Barrax, Spain. However, the method can be more generally applied to any combination of spectral data, providing a tool for merging EO data to any arbitrary hyperspectral bandset.Comparisons are presented using both synthetic and observed MISR and Landsat data, and retrieving surface biophysical properties. We find that when using simulated MISR and Landsat data, the CHRIS/PROBA hyperspectral signal is reproduced with RMSE 0.0001 - 0.04. LAI is retrieved with r2 from 0.97 to 0.99 and RMSE of from 0.21 to 0.38. The results based on observed MISR and Landsat data have lower performances, with RMSE for the reproduced CHRIS/PROBA hyperspectral signal varying from 0.007 to 0.2. LAI is retrievedwith r2 from 0.7 to 0.9 and RMSE from 0.7 to 1.4. We found that for the data considered here the main spectral variations in the visible and near infrared regions can be described by a limited number of parameters (3-4) that can be estimated from multispectral information. Results show that the method can be used to simulate arbitrary bandsets, which will be of importance to any application which requires combining new and existing streams of new EO data in the optical domain, particularly intercalibration of EO satellites in order to get continuous time series of surface reflectance, across programmes and sensors of different designs.
       
  • Satellite ocean colour algorithm for Prochlorococcus, Synechococcus, and
           picoeukaryotes concentration retrieval in the South China Sea
    • Abstract: Publication date: Available online 23 July 2018Source: Advances in Space ResearchAuthor(s): Evgeny Morozov, DanLing TangAbstractAn algorithm for retrieval of surface waters cell concentrations (in cell/ml) for three picophytoplankton components, Prochlorococcus (Pro), Synechococcus (Syn), and picoeukaryotes (Peuk) in the South China Sea (SCS), from ocean colour satellite data was developed and tested. Level 3 merged multisensor Ocean Colour Climate Change Initiative satellite data is used. Training is performed using in situ data on abundances of the three phytoplankton components. Several predictors derived from satellite reflectance data were tested. The regression form that assures the highest accuracy of the algorithm was chosen based on cross-validation (CV). According to the CV on test data subset, the algorithm performance is characterized by the r value 0.89, 0.72, and 0.73 and MAPD 38, 71 and 51% for Peuk, Pro, and Syn respectively. This is one of the few studies aimed at the Peuk, Pro, and Syn distribution research in the northern SCS using ocean colour satellite data. This is the only research providing algorithm with accuracy estimates of the Peuk, Pro, and Syn concentrations retrieval from the ocean colour data. Analysis of the developed algorithm allows us to conclude that both mechanisms (specific spectral features caused by pigments composition and spectrum features sensitive to general primary productivity, e.g. band ratios in 443–510 nm range and spectrum absolute values) are important for getting accurate information on the picophytoplankton composition.
       
  • List of Referees
    • Abstract: Publication date: 1 September 2018Source: Advances in Space Research, Volume 62, Issue 5Author(s):
       
  • Aliasing Effect due to Convective Rain in Doppler Spectrum Observed by
           Micro Rain Radar at a Tropical Location
    • Abstract: Publication date: Available online 21 July 2018Source: Advances in Space ResearchAuthor(s): Soumyajyoti Jana, Gargi Rakshit, Animesh MaitraAbstractThe spectral reflectivity in terms of Doppler velocity obtained by micro rain radar (MRR) at a tropical location can reveal the splitting of Doppler spectrum of falling rain drops caused by strong downdraft. The phenomenon, known as aliasing, occurs in Doppler spectrum of MRR during intense convective events. In this case, the rain drop velocity exceeds the unambiguous Doppler velocity range that can be sensed by MRR. The downdraft affecting the raindrop velocity significantly causes an ambiguity in the Doppler spectrum of the radar signal scattered from raindrops. The aliasing effect is most prominent near the boundary layer height (0.8-2 km) for convective rain. Also at this altitude range, the resultant height gradient obtained from ECMWF vertical velocity of air mass data and drop terminal velocity, is maximum. The importance of the present study lies in the fact that the split in Doppler spectrum can be utilized to estimate downdraft velocity during rain. The de-aliasing technique has been applied to the raw Doppler spectrum of MRR to retrieve the rain drop size distribution conforming to ground based measurements.
       
  • Investigation of microwave optical constants of bulk iron oxides
    • Abstract: Publication date: Available online 21 July 2018Source: Advances in Space ResearchAuthor(s): V.V. Tikhonov, D.A. Boyarskii, O.N. PolyakovaAbstractThe paper presents an analysis of microwave optical constants of iron oxides (magnetite and hematite) by means of the Kramers - Kronig method. Spectral reflectivities of magnetite and hematite in a wide frequency range (from extreme ultraviolet to microwave) were derived based on multiple experimental data found in the literature. The obtained dependencies were used to calculate optical constants of the minerals at 12-145 GHz and compare the results with other reported theoretical and laboratory studies.
       
  • Satellite constellation design algorithm for remote sensing of diurnal
           cycles phenomena
    • Abstract: Publication date: Available online 21 July 2018Source: Advances in Space ResearchAuthor(s): Sung Wook Paek, Luzius G. Kronig, Anton B. Ivanov, Olivier L. de WeckAbstractThis paper proposes an algorithm to find the smallest satellite constellation satisfying a given set of Earth observation requirements. This methodology is exemplified with the Satellites Observing Lakes and Vegetation Environments (SOLVE) study, which aims at deploying a fleet of small satellites carrying miniaturized hyperspectral spectrometers. A key requirement of this mission is a high temporal resolution through which the ground target can be observed several times a day. Hourly observations are required in this mission in order to capture diurnal changes in water quality and vegetation environments. Given sensor specifications and observation requirements, the proposed algorithm determines orbital parameters of an optimal constellation design via a semi-analytical approach. This approach reveals trade-offs amongst performance metrics and deployment cost, providing better physical intuition for decision-making compared to stochastic optimization.
       
  • High-fidelity geometry models for improving the consistency of CHAMP,
           GRACE, GOCE and Swarm thermospheric density data sets
    • Abstract: Publication date: Available online 20 July 2018Source: Advances in Space ResearchAuthor(s): G. March, E.N. Doornbos, P.N.A.M. VisserAbstractDuring the last two decades, accelerometers on board of the CHAMP, GRACE, GOCE and Swarm satellites have provided high-resolution thermosphere density data to improve our knowledge on atmospheric dynamics and coupling processes in the thermosphere-ionosphere region. Most users of the data have focused on relative density variations. Scale differences between datasets and models have been largely neglected or removed using ad hoc scale factors. The origin of these scale differences arises from errors in the aerodynamic modelling, specifically in the modelling of the satellite outer surface geometry and of the gas-surface interactions. Therefore, the first step to remove the scale differences is to enhance the geometry modelling. This work forms the foundation for the future improvement of characterization of satellite aerodynamics and gas-surface interactions models at TU Delft, as well as for extending the use of sideways and angular accelerations in the aerodynamic analysis of accelerations and derivation of thermosphere datasets. Although work to improve geometry and aerodynamic force models by other authors has focused on CHAMP and GRACE, this paper includes the GOCE and Swarm satellites as well. In addition, it uses a density determination algorithm that is valid for arbitrary attitude orientations, enabling a validation making use of attitude manoeuvres. The results show an improvement in the consistency of density data between these four missions, and of data obtained before, during and after attitude manoeuvres of CHAMP and Swarm. The new models result in larger densities, compared to the previously used panel method. The largest average rescaling of density, by switching to the new geometry models is reached for Swarm at 32%, the smallest for GRACE at 5%. For CHAMP and GOCE, mean differences of 11% and 9% are obtained respectively. In this paper, an overview of the improvements and comparisons of data sets is provided together with an introduction to the next research phase on the gas-surface interactions.
       
  • The CryoSat Satellite Altimetry Mission: Eight years of Scientific
           Exploitation
    • Abstract: Publication date: Available online 20 July 2018Source: Advances in Space ResearchAuthor(s): M.A. Shea
       
  • Indoor and Outdoor Positioning System Based on Navigation Signal Simulator
           and Pseudolites
    • Abstract: Publication date: Available online 19 July 2018Source: Advances in Space ResearchAuthor(s): Chao Ma, Jun Yang, Jianyun Chen, Yinyin TangAbstractIn positioning technology, indoor positioning is known as the “last kilometer” problem because the global navigation satellite system (GNSS) cannot work indoors. A wide range of indoor positioning technologies have been developed, notably Bluetooth and Wi-Fi, and others using LED and ultra-wideband light sources. Although these technologies have had good success indoors, the indoor use of GNSS is still being pursued, and an indoor and outdoor joint location system remains a development aim. In this paper, we propose a new indoor positioning scheme that adopts pseudo-satellite (pseudolite) technology combined with a navigation signal simulator. Positioning is achieved by indoor pseudolite antennas that transmit the ‘actual’ satellite signals in space handled by the navigation signal simulator to an indoor user. However, the ‘actual’ satellite ephemeris stored in the pseudolites will bring false pseudoranges, which makes it necessary to adopt map matching technology to determine the real position. The results of our computer simulation showed that when the measurement error mainly multipath error in the room was within 1 m, the positioning results were better than 2 m in about 94% of instances. The proposed method provides a feasible solution for indoor and outdoor joint positioning. The advantages of this system include a better dilution-of-precision (DOP) than an independent pseudolite system indoors, no singular matrix, and initial point selection without limitation in the positioning equation. In addition, the introduction of a navigation signal simulator makes the system more flexible.
       
  • Diurnal variations in seasonal precipitation in Iran from TRMM
           measurements
    • Abstract: Publication date: Available online 19 July 2018Source: Advances in Space ResearchAuthor(s): Mohammad Sadegh Keikhosravi Kiany, Robert C. Balling, Randall S. Cerveny, Daniel S. KrahenbuhlAbstractWe analyzed three-hourly TRMM precipitation data for Iran over the period 1998–2013. During the winter season when cyclonic storms dominate the precipitation, 66% of the country does not display a significant diurnal cycle in precipitation; however, the more mountainous portions of the country display a diurnal cycle with the time of maximum occurring near 12.50 LST. During the spring season when convective precipitation dominates, 55% of the country has a significant diurnal cycle in precipitation with a time of maximum near 15.50 LST; the result clearly shows the convective nature of the precipitation in this season. In summer season, only the northern and southern regions of the country receive much precipitation with most of it occurring between 15.50 and 18.50 LST, with the pattern being strongest in the southern areas. In fall season 42% of the country displays a significant diurnal cycle in precipitation. In this season, south regions of the country have their maximum precipitation frequency between 12.50 and 15.50 LST. Nearly identical patterns exist for precipitation amounts when compared to frequency.
       
  • Torque model verification for the GOCE satellite
    • Abstract: Publication date: Available online 5 July 2018Source: Advances in Space ResearchAuthor(s): Tim Visser, Eelco N. Doornbos, Coen C. de Visser, Pieter N.A.M. Visser, Bent FritscheAbstractThe modeling of torques acting on satellites is essential for the design of satellite attitude control systems. The GOCE satellite, equipped with accurate accelerometers, star trackers and GPS receivers, presents an opportunity to validate these models. Although the forces on GOCE and other accelerometer-carrying missions have been extensively analyzed in the past, a similar analysis has so far not yet been made for the torques.In this paper, we present a set of torque models for the GOCE satellite. It consists of six main parts: (1) magnetic torquer actuators, (2) aerodynamic torque, (3) gravity gradient torque, (4) solar radiation pressure torque, (5) thruster torque, and (6) passive magnetic torque. The magnetic properties of the payload are approximated using a parametrization, of which the parameters are estimated from the observation data.Based on data recorded during selected spacecraft events, the model for the control torques can be validated and error sources are identified in the other models. The models perform best in roll and pitch, where the standard deviation is reduced to 15.2% and 2.1% of the standard deviation of the control torque around those axes respectively. In yaw the standard deviation is significantly larger at 30.5%. The remaining differences between models and observations show magnetic signatures due to electric currents and signatures of aerodynamic model errors. The latter correspond well with an increase in thermosphere density and wind speed with increased geomagnetic activity. The pitch torque is found to be a potential source of vertical wind data.
       
  • Graphene synthesized as by-product of gas purification in long-term space
           missions and its lithium-ion battery application
    • Abstract: Publication date: Available online 30 June 2018Source: Advances in Space ResearchAuthor(s): Yao Nie, Clayton Kacica, Marit E. Meyer, Robert D. Green, Pratim BiswasAbstractTo support long-term missions in space, it is important to recycle valuable consumables, such as oxygen (O2) and water. In a Sabatier reactor, hydrogen is employed to reduce carbon dioxide (CO2) to methane (CH4) and O2, then an integrated system is designed to recycle hydrogen from CH4, which includes a methane purification assembly (MePA), a plasma pyrolysis assembly (PPA), an acetylene separation assembly (ASepA). In the PPA reactor, carbon particulates were observed to form as a by-product. In this study, this carbon material was characterized and a significant fraction is graphene with a low oxygen content. HRTEM images clearly showed a partially crystalline hexagonal structure, which is a characteristic graphene signature. The specific surface area of the graphene was measured to be 258.5 m2/g, one tenth of the theoretical value of single layer graphene, which can be attributed to stacking of the few layers and partial crumpling of this material. To explore potential uses of this material, the crumpled graphene was used to synthesize anodes of lithium-ion batteries (LIBs), which were then tested for their electrochemical performance. In long-term cycle tests, LIBs made with crumpled graphene demonstrated a high retention rate after the first cycle, indicating very little additional degradation of the electrode. The high stability of the cells can be attributed to the greater variety and higher number of lithium ion storage sites in the crumpled graphene, compared with bulk graphite. Considering the crumpled graphene as a by-product and the high electrochemical performance of the crumpled graphene LIBs, crumpled graphene synthesized in the NASA plasma reactor has great promise in better supporting the long-duration space missions.
       
  • An efficient surrogate-based framework for aerodynamic database
           development of manned reentry vehicles
    • Abstract: Publication date: Available online 27 June 2018Source: Advances in Space ResearchAuthor(s): Meysam Mohammadi-Amin, Moein M. Entezari, Alireza AlikhaniAbstractIn this study, an efficient framework is developed via surrogate modeling for aerodynamic database generation and management of reentry vehicles. For reentry vehicles with the wide range of flight envelope, a large number of coefficients are required to fulfill the aerodynamic multi-dimensional tables. To reduce the number of high-fidelity analyses without considerable accuracy loss, a proper combination of sampling, interpolation, and data fusion methods are required. The proposed framework includes a multi-dimensional nonlinear interpolation (Kriging), a data fusion (co-Kriging) and a sampling method (Latin Hypercube Sampling) in an integrated structure coupled with aerodynamic solvers. The main idea is applying Kriging interpolation method on cheap data points to estimate the aerodynamic coefficients' trends over the entire space of variables, and refining the trends with accurate sample points and data fusion. Latin hypercube sampling method is used for optimal distribution of cheap samples and initial accurate sample points. After a few high-fidelity analyses, co-Kriging data fusion method is applied for the improving aerodynamic database fidelity via augmentation of trends with the accurate data. The process iterates using new accurate sample points (located on maximum mean squared error) until the mean squared error criteria is met. Cheap data are produced by a variety of low-fidelity solvers e.g. potential and Euler solvers and high-fidelity data are calculated by full Navier-Stokes solvers (CFD). For each regime of the flight envelope, i.e. subsonic, transonic, supersonic and hypersonic and each type of reentry configurations, e.g. Apollo-type, grid studies are done separately and the optimum grid and solver settings are implemented into the framework to facilitate the automatic aerodynamic database generation and management. All parts of the presented framework are validated independently in compare to some reference test cases. To show the capabilities of the developed framework, Orion reentry capsule with complete flight envelope is assumed as a sample. Orion aerodynamic database is generated efficiently and the obtained results are in good agreement in comparison with experimental data. In conclusion, the framework accuracy, flexibility, and efficiency are demonstrated.
       
  • A refined regional empirical pressure and temperature model over China
    • Abstract: Publication date: Available online 22 June 2018Source: Advances in Space ResearchAuthor(s): Weixing Zhang, Yidong Lou, Jinfang Huang, Wenxuan LiuAbstractAccurate pressure and temperature are indispensable in the GNSS water vapor retrievals. A refined regional empirical model, WHU_CPT, for estimating pressure, temperature and water vapor weighted mean temperature (Tm) with a horizontal resolution of 0.75° over China was developed. We found that the surface level reanalysis products are more reliable in reproducing surface temperature than pressure level products in the model construction. The model form was then determined individually for each meteorological variable by comparing performances of different models and by analyzing the Power Spectra Density of time series. Average RMS of pressure and temperature errors over China in WHU_CPT is about 4.07 hPa and 3.76 K, compared to 5.95 hPa and 5.92 K for GPT, 4.25 hPa and 5.14 K for GPT2, and 4.14 hPa and 5.14 K for GPT2w. Two Tm models were also developed in WHU_CPT. One is an empirical model, WHU_CPT (Tm1), which takes user location and time as inputs and the other one, WHU_CPT (Tm2), which additionally makes use of the measured temperature, is more suitable for users with surface temperature available. The mean RMS of Tm errors over China are about 4.45, 4.19, 3.81 and 2.97 K for the Bevis equation, GPT2w, WHU_CPT (Tm1) and WHU_CPT (Tm2).
       
  • Integrated simulations of Mars flights on the ISS
    • Abstract: Publication date: Available online 21 June 2018Source: Advances in Space ResearchAuthor(s): L. Narici, G. Reitz, C. LobascioAbstractThe human quest towards the exploration of the solar system and beyond will likely continue to grow in the next few decades. Many scientific and technological challenges still need to be approached and solved to enable long deep space human exploration. Once answers to these challenges are available, they will be integrated in the flight plans as ad hoc operational strategies.For the solution of specific scientific and technological problems, experiments using ground analogues may provide optimal responses; however, only the International Space Station (ISS) can play the role of integrated analogue, where the impact of micro-gravity, radiation, living and psychological conditions that astronauts will face during a deep space cruise, can be mimicked at the same time, in part or in whole.Today the ISS is a unique technological and scientific platform that enables researchers from all over the world to work on innovative experiments that could not be performed anywhere else. However, it is conceivable to use the deep space analogue features of the ISS to perform integrated tests of those operational strategies needed to allow for deep space voyages, towards a potential final integrated in-space test of the entire voyage to Mars, with a fidelity as high as achievable. This utilization strategy for the ISS would help focusing research and technology on open questions for deep space exploration, also enabling further tests of the journey to Mars using other advanced platforms that will be available on Moon orbit or surface in the near future. It will also provide the ISS with the rank of “springboard” towards deep space for the general public, increasing the awareness for human space exploration. In this paper we present the ISS4Mars idea, underlining its major goals and challenges.
       
  • Study of aerosol types and seasonal sources using wavelength dependent
           Ångström exponent over North-East India: Ground-based measurement and
           satellite remote sensing
    • Abstract: Publication date: Available online 19 June 2018Source: Advances in Space ResearchAuthor(s): Pranab Dhar, Trisanu Banik, Barin Kumar De, Mukunda M. Gogoi, S. Suresh Babu, Anirban GuhaAbstractThe spectral estimates of Aerosol Optical Depth (AOD, τ) were made by operating a Microtops-II sun photometer in the spectral range 0.380–0.870 μm over Tripura in north-eastern India and analyzed to infer the aerosol types and source characteristics in different seasons. The Ångström exponent (α) derived from spectral AOD in different wavelength (λ) range and subsequent second order derivative of Ångström exponent (i.e., α′) and their curvature analysis in the ln τ versus ln λ relationship has revealed the crucial information related to the dominance of different aerosol types and their characteristics in different seasons. The average AOD (mean ± standard deviation) at 0.5 μm in winter, pre-monsoon, monsoon and post-monsoon seasons are observed to be 0.70 ± 0.28, 0.74 ± 0.18, 0.55 ± 0.20, 0.44 ± 0.19 respectively; while the corresponding seasonal mean values of α (over 0.380–0.870 μm spectral range) are found to be 1.09 ± 0.17, 0.92 ± 0.24, 0.51 ± 0.27, 0.89 ± 0.38 respectively. Examination of the Ångström exponents derived from satellite retrieved AOD by Moderate Resolution Imaging Spectroradiometer (MODIS) instrument operating onboard Terra satellite, along with MODIS fine mode aerosol fraction and aerosol types indicates broad seasonal features of aerosol size spectrum over the study region similar to those observed from ground-based measurements. The estimation of the values of α at different spectral ranges indicate that winter season is mainly influenced by the fine-mode aerosols having fine-mode fraction (FMF) ∼ 0.7; whereas coarse-mode aerosols dominate in the monsoon season having FMF ∼ 0.3. The pre-monsoon and post-monsoon seasons exhibit the presence of mixed type of aerosols, with slightly greater fraction of fine-mode aerosol in pre-monsoon. Curvature analyses of Ångström exponent put insight into the consistency of observed features of seasonal aerosol types.
       
  • 3D radar wavefield migration of comet interiors
    • Abstract: Publication date: Available online 19 June 2018Source: Advances in Space ResearchAuthor(s): Paul Sava, Erik AsphaugAbstractImaging the interior structure of small planetary bodies facilitates a deep understanding of their origin and evolution, thus addressing fundamental questions about the formation of the Solar System. We show that high resolution 3D imaging of their interior structure is possible using radar waves that reflect from internal discontinuities of dielectric properties. A radar imaging mission at a comet nucleus would have the benefit of orbiting all around a finite and transparent body, collecting echoes that derive only from the target, and processing them collectively in phase.As is the case in the medical field, imaging a comet nucleus requires its illumination from multiple directions, which can be accomplished with a spacecraft in slow polar orbit around the studied object. Long acquisition time leads to a dense acquisition array resembling that of conventional synthetic aperture radar systems, but completely surrounding the nucleus (4π steradians). Acquisition with an orbiter at large distance from the comet nucleus (5× the mean diameter) results in relatively coarse data sampling relative to the radar wavelength, thus enabling 3D imaging with a short (
       
  • Rapid initialization method in real-time deformation monitoring of bridges
           with triple-frequency BDS and GPS measurements
    • Abstract: Publication date: Available online 19 June 2018Source: Advances in Space ResearchAuthor(s): Ruijie Xi, Weiping Jiang, Xiaolin Meng, Xiaohui Zhou, Qiyi HeAbstractRapid initialization with ambiguity fixed resolution plays a key role in real-time Global Navigation Satellite Systems (GNSS) displacement monitoring of bridges. In this study, we propose a rapid initialization method with triple-frequency (TF) BeiDou Navigation Satellite System (BDS) and Global Positioning System (GPS) observations. The key step of this method is to form extra-wide-lane (EWL) ambiguity with TF BDS and GPS carrier-phase to reduce the ambiguity search space. In order to improve the speed of ambiguity resolution (AR) further, the strategies of fixing ambiguities in groups sequentially from long wavelength to the short and partial AR (PAR) with elevation and signal-to-noise ratio (SNR) criterions were proposed. Based on the real monitoring data on the Baishazhou Yangtze River Bridge, the AR efficiency was tested. The results illustrate that over 90% EWL ambiguities can be fixed by only single-epoch with TF BDS + GPS. After EWL ambiguity is fixed, the wide-lane (WL) ambiguity success rate could improve from 20% with dual-frequency (DF) BDS + GPS data to 70% with TF BDS + GPS data, 50–80% and 80–90% for the three stations respectively. Since the area of ambiguity search space is reduced by TF observations, the ambiguity search time decrased significantly compared with the DF BDS + GPS. In the Time-to-first-fix (TTFF) test, over 85% of epochs can complete the initialization process by only one epoch with one hour of 10 Hz TF BDS + GPS data. However, DF BDS + GPS has only 25%. As for the initialization time longer than two epochs, the counts of TTFFs of TF BDS + GPS are significantly less than that of DF BDS + GPS. Meanwhile, the integrated BDS and GPS can further enhance the geometry of satellites, which will be better for the precision improvement than the increase of signal frequencies.
       
  • DEM thermal simulation of bit and object in drilling of lunar soil
           simulant
    • Abstract: Publication date: Available online 18 June 2018Source: Advances in Space ResearchAuthor(s): Jinsheng Cui, Xuyan Hou, Guilin Wen, Zhongwei LiangAbstractThe bit and object thermal problem was investigated in this study with particular regard to the drilling of lunar soil simulant. The discrete element method (DEM) was used to establish a thermal model of the simulant that considered convection, radiation, and geometry. Parameter calibration of the DEM heat transfer model was undertaken, and drilling experiments were performed under normal pressure to verify the model. Thermal simulations of the drilling of the lunar soil simulant under normal pressure and in a vacuum were also conducted, and very good agreement was observed between the simulation and experimental results. The temperature rise of the lunar soil simulant was found to be much less during drilling in a vacuum compared to drilling under normal pressure for comparable temperatures of the drill bit. And in a vacuum, lunar soil simulant had little effect on heat dissipation from the drilling tool.
       
  • Direct and indirect charged aerodynamic mechanisms in the ionosphere
    • Abstract: Publication date: Available online 18 June 2018Source: Advances in Space ResearchAuthor(s): C.J. Capon, M. Brown, R.R. BoyceAbstractObjects in a flowing plasma may exchange momentum both directly, through gas-surface interactions, and indirectly, through field effects. Applying a control surface approach developed in the study of dusty (complex) plasmas, this work outlines a framework for investigating the relative contribution of direct and indirect charged aerodynamic forces that may arise from the aerodynamic interaction of LEO objects with the ionosphere i.e. ionospheric aerodynamics. In particular, this work focuses on plasma interaction phenomena described by the scaling parameters αk and χ, which describe the ratio of body potential energy to ion kinetic energy and the ratio of body size to sheath thickness respectively. Ion kinetic dominated flows (αk≪1) were found to be well approximated by neutral aerodynamic treatments, the charged drag coefficient (CD,C) of a long (2D) cylinder with a diffusely reflecting surface with complete thermal accommodation to a 500 K wall calculated as 2.232. Body potential dominated systems (αk≫1), however, were dominated by indirect charged aerodynamic mechanisms, an example being the indirect thrust required to balance the sheath driven acceleration of ions through the fore-body sheath, which caused an 82% reduction in net charged drag for the αk=16.9 case. This drag reduction was partially offset by indirect drag forces caused by the deflection of non-colliding ions, the net CD,C being 7.741 for the αk=16.9. A similar study of the relative sheath thickness, described by χ, saw asymptotic behaviours given a constant αk of 1.072; direct charged drag forces becoming either Orbital Motion Limit (OML) for thick-sheath systems (χ→0) or sheath-limited (χ≫1). These results demonstrate the importance of accounting for both direct and indirect charged aerodynamic forces when studying ionospheric aerodynamics, while also providing new physical insights into physical mechanisms momentum exchange processes between a flowing plasma and immersed object.
       
  • Multi-objective trajectory optimization for a hybrid propulsion system
    • Abstract: Publication date: Available online 18 June 2018Source: Advances in Space ResearchAuthor(s): Taibo Li, Zhaokui Wang, Yulin ZhangAbstractIt is attractive to use a hybrid propulsion system (HPS) consisting of solar electric propulsion (SEP) and solar radiation pressure (SRP) in interplanetary and near-Earth missions. Compared with the equivalent pure sail and pure SEP trajectories, HPS can reduce fuel consumption and transfer time. A multi-objective optimization model for the three-dimensional rendezvous mission of hybrid propulsion system is established in this paper. The optimization index is defined as a weighted sum of transfer time and the fuel consumption. Solutions with the bang-bang thrust profile are obtained under the specific mission constraints of time and fuel consumption. The influence of weights on the optimization results is discussed, and a reasonable weight range is given based on the magnitude analysis. By combining the homotopy approach with time-optimal solar sail trajectory, the initial value of the covariates are well estimated. Numerical simulations are performed both on Earth–2000SG344 and Earth–Apophis rendezvous. The results indicate that the proposed method is advantageous to obtain solutions with different mission time and fuel consumption flexibility.
       
  • Uniaxial mechanical properties of multi-layer thin films in use for
           scientific balloons
    • Abstract: Publication date: Available online 15 June 2018Source: Advances in Space ResearchAuthor(s): Jianhui Hu, Yipo Li, Wujun Chen, Tengfei Zhang, Chengjun Gao, Taibai Shi, Deqing YangAbstractEssential and accurate mechanical parameters of multi-layer thin films for scientific balloons are indispensable for performing structural analysis. This study focuses on the determination of uniaxial mechanical properties of a new multi-layer thin film and the understanding of the effects of mechanical properties on structural behavior and altitude with three-dimensional numerical models. For uniaxial monotonic mechanical properties, a method to determine yield stress is proposed in combination of geometrical and energetic principles. It is found that average yield stresses are 20.5 MPa and 15.0 MPa for machine and transverse directions. The cyclic mechanical properties in terms of elastic modulus and ratcheting strain tend to be stable after certain cycles. Moreover, the Poisson’s ratios of 0.31 and 0.15 in machine and transverse directions are determined with digital image correlation (DIC) technique. Furthermore, these mechanical properties are utilized to analyze a 12 m diameter spherical superpressure balloons floating at the altitude of 20 km. A corresponding three-dimensional numerical model is developed to understand the effects of mechanical properties on structural behavior and altitude. The numerical results in terms of elastic moduli show that the effects of mechanical properties on structural behavior and altitude are relatively apparent and slight, respectively.
       
  • Thermal performance of liquid hydrogen tank in reduced gravity
    • Abstract: Publication date: Available online 14 June 2018Source: Advances in Space ResearchAuthor(s): Zhan Liu, Guoqing Zhou, Yanzhong Li, Penghui GaoAbstractFluid temperature stratification is significant to the safe operation of space storage tanks. A calculation model, accounting for the liquid–vapor phase change, is developed to investigate the thermal physical process in a liquid hydrogen (LH2) tank in reduced gravity. Viscous flow is considered in calculation model with Ra ranging from 0.1 to 105 to ensure the continuity of natural convection. The stratified layer parameters, the tank pressure rise, and the interface phase change are studied respectively. Influences of the initial liquid height, the initial ullage temperature and the tank wall heat flux on the development of thermal stratification are estimated. The results show that the stratified layer thickness rises with the initial liquid height. While the initial liquid height is large, it costs more time for the wholly development of fluid thermal stratification. Largely influenced by the temperature of the stratified layer, both tank pressure and phase change capacity increase with the initial liquid height. It seems that the initial ullage temperature has a weak effect on the development of fluid thermal stratification. Both the tank pressure and the phase change quality increase with the initial ullage temperature. The external tank wall heat flux promotes the development of thermal stratification. The stratified layer has a larger thickness and develops faster for the larger heat flux. Both tank pressure and phase change capacity increase with the external heat flux. Meanwhile, the intersection point exists between any two profiles.
       
  • Investigation of gravity effect on penetration resistance in Tongji-1
           lunar regolith simulant by centrifuge tests
    • Abstract: Publication date: Available online 12 June 2018Source: Advances in Space ResearchAuthor(s): Mingjing Jiang, Banglu Xi, Huayang LeiAbstractCone Penetration Tests (CPTs) are usually performed on the lunar regolith simulant on the Earth to study the geotechnical characteristics of lunar soil, which will be then used for the possible human activities on the Moon in the future. However, the gravity effect should be fully investigated before applying these experimental results to the Moon exploration, because the gravity level on the Moon is much lower than that on the Earth. For this aim, a series of CPTs was carried out in a centrifuge with a carefully designed testbed made of Tongji-1 lunar regolith simulant (TJ-1 simulant). The measured results were employed to examine the applicability of two traditional solutions and linear equation in describing the gravity effect along with some other existing experimental data. In addition, the linear equation fitted by the experimental data was used to predict the values of normalized tip resistances of lunar soil on the Moon. The results show that the two traditional solutions cannot describe the gravity effect well while the linear relationship between the peak (stable) normalized cone tip resistance and the reciprocal of gravity level can quantitatively describe the gravity effect on the penetration resistance. The predicted normalized peak (stable) tip resistances are larger than those of lunar soil on the Moon, which may be probably due to the unique in-situ conditions on the Moon.
       
 
 
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