Journal Cover
Journal of Navigation
Journal Prestige (SJR): 0.493
Citation Impact (citeScore): 2
Number of Followers: 243  
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 0373-4633 - ISSN (Online) 1469-7785
Published by Cambridge University Press Homepage  [387 journals]
  • NAV volume 72 issue 6 Cover and Front matter
    • PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000717
      Issue No: Vol. 72, No. 6 (2019)
  • NAV volume 72 issue 6 Cover and Back matter
    • PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000729
      Issue No: Vol. 72, No. 6 (2019)
  • Verification of AIS Data by using Video Images taken by a UAV
    • Authors: Fan Zhou; Shengda Pan, Jingjing Jiang
      Pages: 1345 - 1358
      Abstract: Effective technical methods for verifying the authenticity and accuracy of Automatic Identification System (AIS) data, which are important for safe navigation and traffic regulation, are still lacking. In this study, we propose a new method to verify AIS data by using video images taken by an Unmanned Aerial Vehicle (UAV). An improved ViBe algorithm is used to extract the ship target image from the video images and the ship's spatial position is calculated using a monocular target-positioning algorithm. The positioning results are compared with the position, speed and course data of the same ship in AIS, and the authenticity and accuracy of the AIS data are verified. The results of the experiment conducted in the inland waterways of Huangpu River in Shanghai, China, show that AIS signals can be automatically checked and verified by a UAV in real time and can thus improve the supervision efficiency of maritime departments.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000262
      Issue No: Vol. 72, No. 6 (2019)
  • Inland Ship Trajectory Restoration by Recurrent Neural Network
    • Authors: Cheng Zhong; Zhonglian Jiang, Xiumin Chu, Lei Liu
      Pages: 1359 - 1377
      Abstract: The quality of Automatic Identification System (AIS) data is of fundamental importance for maritime situational awareness and navigation risk assessment. To improve operational efficiency, a deep learning method based on Bi-directional Long Short-Term Memory Recurrent Neural Networks (BLSTM-RNNs) is proposed and applied in AIS trajectory data restoration. Case studies have been conducted in two distinct reaches of the Yangtze River and the capability of the proposed method has been evaluated. Comparisons have been made between the BLSTM-RNNs-based method and the linear method and classic Artificial Neural Networks. Satisfactory results have been obtained by all methods in straight waterways while the BLSTM-RNNs-based method is superior in meandering waterways. Owing to the bi-directional prediction nature of the proposed method, ship trajectory restoration is favourable for complicated geometry and multiple missing points cases. The residual error of the proposed model is computed through Euclidean distance which decreases to an order of 10 m. It is considered that the present study could provide an alternative method for improving AIS data quality, thus ensuring its completeness and reliability.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000316
      Issue No: Vol. 72, No. 6 (2019)
  • Improved Integral LOS Guidance and Path-Following Control for an Unmanned
           Robot Sailboat via the Robust Neural Damping Technique
    • Authors: Guoqing Zhang; Jiqiang Li, Bo Li, Xianku Zhang
      Pages: 1378 - 1398
      Abstract: This paper introduces a scheme for waypoint-based path-following control for an Unmanned Robot Sailboat (URS) in the presence of actuator gain uncertainty and unknown environment disturbances. The proposed scheme has two components: intelligent guidance and an adaptive neural controller. Considering upwind and downwind navigation, an improved version of the integral Line-Of-Sight (LOS) guidance principle is developed to generate the appropriate heading reference for a URS. Associated with the integral LOS guidance law, a robust adaptive algorithm is proposed for a URS using Radial Basic Function Neural Networks (RBF-NNs) and a robust neural damping technique. In order to achieve a robust neural damping technique, one single adaptive parameter must be updated online to stabilise the effect of the gain uncertainty and the external disturbance. To ensure Semi-Global Uniform Ultimate Bounded (SGUUB) stability, the Lyapunov theory has been employed. Two simulated experiments have been conducted to illustrate that the control effects can achieve a satisfactory performance.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000353
      Issue No: Vol. 72, No. 6 (2019)
  • An Accurate Motion Compensation for SAR Imagery based on INS/GPS with
           Dual-filter Correction
    • Authors: Linzhouting Chen; Zhanchao Liu, Jiancheng Fang
      Pages: 1399 - 1416
      Abstract: Current Motion Compensation (MOCO) methods using Inertial Navigation System (INS)/Global Positioning System (GPS) integrated systems have provided an important advance in Synthetic Aperture Radar (SAR) imagery, but most of these methods only work well over a short imaging period. With the development of high-resolution SAR that provides image gathering over long periods, the need for higher levels of INS/GPS performance than normally available is desired. The higher requirement of INS/GPS for SAR MOCO is two-fold: (1) the accurate knowledge of location information, and (2) the smoothness of relative change in navigation error. In this paper, we design an INS/GPS architecture with dual-filter correction to obtain accurate absolute velocity and position measurement information with smooth low relative error noise over a long image gathering period. Real SAR data experimental results show that the proposed method effectively improves the MOCO performance of INS/GPS with long SAR imaging periods, in which the SAR azimuth resolution reaches 1·45 m, which is very close to the design value of 1 m.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000365
      Issue No: Vol. 72, No. 6 (2019)
  • Determination of Seasonality Patterns in the Transport of Cruise
           Travellers Through Clustering Techniques
    • Authors: Jeronimo Esteve-Perez; Antonio Garcia-Sanchez
      Pages: 1417 - 1434
      Abstract: The tourist attraction of a cruise itinerary is composed of the on board experience plus the shore-based experience. Due to the positive dynamism of the cruise industry since the beginning of the twenty-first century, cruise lines are driven to innovate to create new experiences that help maintain high demand rates. From the point of view of creating new experiences on land, cruise lines move their vessels from one destination region to another to maximise the vessel's occupancy and to offer itineraries with a wider variety of shore-based attractions. These new itinerary designs lead to alterations in the seasonality patterns of the neighbouring regions. In this work, the 17 most important cruise ports located in the northeast quadrant of the Atlantic Ocean are analysed to find groups of ports with homogeneous seasonality patterns using clustering techniques. The analysis showed two different seasonality patterns. Consequently, some implications to improve the use of the ports of both clusters are included.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000237
      Issue No: Vol. 72, No. 6 (2019)
  • Determining Restricted Fairway Additional Width due to Bank Effect for
           Fine Form Vessels
    • Authors: Mate Baric; Robert Mohovic, Djani Mohovic
      Pages: 1435 - 1448
      Abstract: In order to determine fairway width accurately and to ensure an adequate level of safety, it is necessary to fulfil different safety standards and user needs. There are a number of international recommendations for fairway design, but the recommendations by Permanent International Association of Navigation Congresses (PIANC), Puerto Del Estado (ROM 3·1) and the Japanese Ministry of Land, Infrastructure, Transport and Tourism (MLIT) are some of the best known. However, in all of these recommendations, additional width due to ship-bank effect is obsolete and needs to be improved. The objective of this paper is to use a navigational simulator with a predefined ship-bank interaction model and fine form ships models to determine the ship trajectory caused by ship-bank effect. The methodology of research used in the paper consisted of using ship trajectory data to propose a deterministic model for improving fairway additional width due to ship-bank effect. Results showed that such a method is not time-consuming and can improve fairway design in terms of ship-bank effect, increasing additional width accuracy.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000250
      Issue No: Vol. 72, No. 6 (2019)
  • Towards a Model of Regional Vessel Near-miss Collision Risk Assessment for
           Open Waters based on AIS Data
    • Authors: Weibin Zhang; Xinyu Feng, Yong Qi, Feng Shu, Yijin Zhang, Yinhai Wang
      Pages: 1449 - 1468
      Abstract: The absence of a regional, open water vessel collision risk assessment system endangers maritime traffic and hampers safety management. Most recent studies have analysed the risk of collision for a pair of vessels and propose micro-level risk models. This study proposes a new method that combines density complexity and a multi-vessel collision risk operator for assessing regional vessel collision risk. This regional model considers spatial and temporal features of vessel trajectory in an open water area and assesses multi-vessel near-miss collision risk through danger probabilities and possible consequences of collision risks via four types of possible relative striking positions. Finally, the clustering method of multi-vessel encountering risk, based on the proposed model, is used to identify high-risk collision areas, which allow reliable and accurate analysis to aid implementation of safety measures.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S037346331900033X
      Issue No: Vol. 72, No. 6 (2019)
  • Analysis of Speed Prediction Error on Oceanic Flights
    • Authors: Ryota Mori
      Pages: 1469 - 1480
      Abstract: The accuracy of speed prediction error to the next waypoint is a key factor in maintaining longitudinal separation on oceanic routes. This estimation is often used by air traffic control to estimate the future position of aircraft, and the estimation errors result in potential separation infringement. While most aircraft can calculate the estimated time at each future waypoint using the onboard Flight Management System, the factors affecting the inaccuracy of the estimation require more clarification. This paper investigates the accuracy of the speed prediction error on oceanic routes and examines the main factor of error using airline flight data. The results show that wind prediction error is a main source of speed prediction error, and significant differences of the speed prediction error among airlines and aircraft types were observed.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000432
      Issue No: Vol. 72, No. 6 (2019)
  • Sensability and Excitability Metrics Applied to Navigation Systems
    • Authors: Martín España; Juan Carrizo, Juan I. Giribet
      Pages: 1481 - 1495
      Abstract: To evaluate the aptness of a navigation system in a particular application, the designer needs to assess its performance over typical trajectories travelled by the vehicle itself. Moreover, he or she may be required to judge which components of the kinematics state may be better estimated (and which will not). The main contributions of this work are two novel and complementary performance measures that, in concert, allow for the assessment of a navigation system within the actual context of its application over specific trajectories. For a given on board instrumental configuration, the “excitability metric” permits the isolation of the contribution of the information conveyed by the vehicle's motion itself, while, the “sensability metric” measures the resultant overall quality of the kinematics state estimation. The same tools could help the designer planning appropriate vehicles manoeuvres in order to obtain a required precision for each estimated component. While emphasis is given on the mathematical justification of those metrics, their use is also illustrated with real flight data recorded from a sounding rocket.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000328
      Issue No: Vol. 72, No. 6 (2019)
  • Probabilities in a Gaussian Cocked Hat
    • Authors: Robin G. Stuart
      Pages: 1496 - 1512
      Abstract: A round of three celestial sights yields three lines of position along which the observer's true position could lie. Due to measurement errors, the lines of position do not intersect at a point but rather form a triangle called the “cocked hat”. The probability that this encloses the observer's true position is well known to be 25% which is the average over all possible cocked hats that could arise when the sights are made. It does not apply to any specific set of sights and in that case the probabilities depend on the statistical distribution of the measurement errors. With fixed azimuths for the observed celestial bodies and assuming a normal distribution for the errors in their measured altitudes, a closed form analytic expression is derived for the probability of the observer's position falling inside the cocked hat and this is related back to the global average. Probabilities for exterior regions bounded by the lines of position are also obtained. General results are given that apply for any number of lines of position.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000110
      Issue No: Vol. 72, No. 6 (2019)
  • Adaptive Anti-Disturbance Method for Magnetometer and INS Integration in a
           Road Vehicle
    • Authors: Zongkai Wu; Wei Wang
      Pages: 1513 - 1532
      Abstract: The integration of magnetometers and Inertial Navigation Systems (INS) is widely used in low-cost navigation systems. However, even if the system has been calibrated, random magnetic disturbances still appear in practical applications, which lead to large heading errors. To solve this problem, an adaptive anti-disturbance method to overcome random magnetic disturbance is proposed. First, disturbances are classified and analysed in detail based on actual road vehicle driving data. Then an Adaptive Robust Extend Kalman Filter (AREKF) is designed to resist sudden disturbances. However, an AREKF may accumulate errors slowly when a long-term disturbance exists. Considering this situation, this paper proposes that AREKF is used to maintain accuracy in the early stages, at the same time as the magnetometer is quickly calibrated with a Kalman filter. Then, the new magnetometer parameters are put into the AREKF to suppress long-term disturbances. Finally, cascading these two modules, not only the sudden disturbance can be overcome, but the situation of long-term disturbances can be suppressed. The results of simulation and an actual driving test show that the proposed method can effectively overcome random magnetic disturbances in both the short and long term.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000389
      Issue No: Vol. 72, No. 6 (2019)
  • An Integrity Monitoring Algorithm for GNSS Satellite Atomic Clocks
    • Authors: X.M. Huang; X. Zhao, J.Y. Li, X.W. Zhu, G. Ou
      Pages: 1533 - 1549
      Abstract: An algorithm for Global Navigation Satellite System satellite atomic clock integrity monitoring based on an extended measurement model is proposed. A detection statistic achieved by parity transformation is used to detect clock anomalies, and the concept of the optimal accumulation number, with a method to find it, is provided. Numerical simulations are adopted to verify the validity of detecting two typical anomalies.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000225
      Issue No: Vol. 72, No. 6 (2019)
  • A High-Sensitivity Acquisition Algorithm for BeiDou Signals with NH Code
    • Authors: Rongbing Li; Zhifeng Han, Jianye Liu, Yi Wang
      Pages: 1550 - 1564
      Abstract: BeiDou signals are modulated with a Neumann-Hofman (NH) code of 1 kbps. The frequent bit transitions lead to a sensitivity attenuation of classic acquisition algorithms. In order to increase acquisition sensitivity for weak BeiDou signals, a novel algorithm based on modified zero-padding and differential correlation is proposed. First, a zero-padding method is used to weaken the effect of NH code. Second, the differential coherent delay time is modified to 20 ms to remove the influence of data bit transitions. The integration time is extended to 10 ms to increase acquisition sensitivity. Finally, Monte Carlo simulations and real data tests are conducted to analyse the performance of the proposed algorithm. Simulated results show that the proposed acquisition algorithm outperforms traditional algorithms under a Carrier-to-Noise ratio (C/N0s) of 20~38 dB-Hz. The sensitivity of the proposed algorithm is about 10dB higher than traditional 6 ms repeated search algorithms. Real data test results show that the proposed algorithm outperforms the traditional method with weak signals. This algorithm can remove the effect of NH code and effectively increase the acquisition sensitivity. The proposed algorithm is suitable for acquisition of weak BeiDou signals.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000304
      Issue No: Vol. 72, No. 6 (2019)
  • Accuracy Analysis of Ionospheric Prediction Models for Repairing Cycle
           Slips for BeiDou Triple-Frequency Observations
    • Authors: Yao Yifei; Cao Xinyun, Chang Guobin, Geng Hongsuo
      Pages: 1565 - 1584
      Abstract: Both the code–phase combination and the Geometry-Free (GF) phase combination are widely employed to detect and repair cycle slips for BeiDou Navigation Satellite System (BDS) triple-frequency observations. However, the effect of residual ionospheric delay on Narrow-Lane (NL) or GF observations must be considered to avoid incorrect cycle–slip estimation. To improve the accuracy in repairing cycle slips, a corrective ionospheric delay value predicted from the previous ionosphere sequence is used to amend the NL or GF observations at the current epoch. The main purpose of the work reported here is to evaluate the efficacy of a three-step method proposed to detect and repair cycle slip using two extra-wide-lane code–phase and one GF phase combination observations. BDS triple-frequency data were processed in two stages: separate processing of geosynchronous Earth orbit satellites, and the division of inclined geosynchronous satellite orbit and medium Earth orbit satellites into two groups for processing at 30° elevation thresholds. Results revealed that using the prediction models to correct NL or GF observations could ensure a rounding success rate of cycle slip close to 100%, even under high ionospheric activity.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000456
      Issue No: Vol. 72, No. 6 (2019)
  • Investigation on GEO satellite orbit determination based on CEI
           measurements of short baselines
    • Authors: Zejun Liu; Lan Du, Yongxing Zhu, Zhihan Qian, Jinqing Wang, Shiguang Liang
      Pages: 1585 - 1601
      Abstract: Connected-Element Interferometry (CEI) is a technique for measuring the phase delay of difference of Time Of Arrival (TOA) of a downlink radio signal to two antennae on a short baseline. This technique can use an atomic clock for time-frequency transmission and achieve intermediate accuracy angular tracking. Owing to the relatively short length of the baseline, the passive reception mode, and near real-time operation, CEI can be used to continuously monitor the orbit variations of both cooperative and non-cooperative satellites. In this paper, a small-scale CEI system of two orthogonal baselines (75 m × 35 m) is investigated to track a Geostationary Earth Orbit (GEO) Television (TV) satellite at 110·5°E. The phases are extracted from correlation results. The results show that the Root Mean Square (RMS) of the phase fitting residuals, if not calibrated, is within 2° at night and up to 10° in the daytime. After applying the calibration signal, the RMS of the phase fitting residuals in the daytime decreases to the same level at night. Comparing the phase delay with the a priori phase delay using Two-Line-Element (TLE) data, the integer ambiguity is successfully resolved. Finally, a batch algorithm is used to estimate the orbit of the GEO satellite, and the orbit determination accuracy is evaluated using the precise orbits provided by the China National Time Service Centre (NTSC). The results show that the accuracies in the radial direction and the cross-track direction are less than 1 km, and the Three-Dimensional (3D) position accuracy reaches the 2 km order of magnitude.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000249
      Issue No: Vol. 72, No. 6 (2019)
  • AUV Bathymetric Simultaneous Localisation and Mapping Using Graph Method
    • Authors: Teng Ma; Ye Li, Yusen Gong, Rupeng Wang, Mingwei Sheng, Qiang Zhang
      Pages: 1602 - 1622
      Abstract: Although topographic mapping missions and geological surveys carried out by Autonomous Underwater Vehicles (AUVs) are becoming increasingly prevalent, the lack of precise navigation in these scenarios still limits their application. This paper deals with the problems of long-term underwater navigation for AUVs and provides new mapping techniques by developing a Bathymetric Simultaneous Localisation And Mapping (BSLAM) method based on graph SLAM technology. To considerably reduce the calculation cost, the trajectory of the AUV is divided into various submaps based on Differences of Normals (DoN). Loop closures between submaps are obtained by terrain matching; meanwhile, maximum likelihood terrain estimation is also introduced to build weak data association within the submap. Assisted by one weight voting method for loop closures, the global and local trajectory corrections work together to provide an accurate navigation solution for AUVs with weak data association and inaccurate loop closures. The viability, accuracy and real-time performance of the proposed algorithm are verified with data collected onboard, including an 8 km planned track recorded at a speed of 4 knots in Qingdao, China.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000286
      Issue No: Vol. 72, No. 6 (2019)
  • Precise Bathymetry as a Step Towards Producing Bathymetric Electronic
           Navigational Charts for Comparative (Terrain Reference) Navigation
    • Authors: Andrzej Stateczny; Daria Gronska-Sledz, Weronika Motyl
      Pages: 1623 - 1632
      Abstract: Bathymetric Electronic Navigational Charts (bENCs) contain only bathymetry data and can be used in applications such as underwater positioning, dredging and piloting. According to International Hydrographic Organization (IHO) standard S-57, Electronic Navigational Charts (ENCs) contain depth information with pure density of depth contours. Typical depth contours encoded by Hydrographic Offices are limited to 2, 5, 10 and 20 m. Availability of more depth contours in bENCs would allow the visualisation of a safety contour which is closer to users' specific needs, especially in restricted waters such as ports, lakes and rivers. Another problem is non – Global Navigation Satellite System (GNSS) Unmanned Underwater Vehicle (UUV) navigation. bENCs could be used as reference data for UUV comparative navigation. This is called terrain reference navigation. This article presents the results from bathymetric data processing that was performed to convert data contained in bENCs into a reference for underwater comparative navigation. We use data obtained using a multibeam echo sounder to produce depth data with a horizontal spacing of 0·10 m that is suitable for use in restricted waters. The experimental data was collected in and around the Port of Gdansk, Poland.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000377
      Issue No: Vol. 72, No. 6 (2019)
  • Evaluation and Application of the GPS Code Observable in Precise Point
    • Authors: Haojun Li; Jingxin Xiao, Bofeng Li
      Pages: 1633 - 1648
      Abstract: The accuracy of the Global Positioning System (GPS) observable, especially for the code observable, has improved with the development of Global Navigation Satellite System (GNSS) receiver technology. An evaluation of the GPS code observable is presented in this paper, together with a stochastic model for the code and phase observables in Precise Point Positioning (PPP), established using the evaluated results. The results show that the code observables of Leica GNSS receivers are generally better than those of some other brand receivers and the Root Mean Square (RMS) for the code observables of the Leica GRX1200PRO, which includes the multipath effect, reaches 0·71 m, although Coarse/Acquisition (C/A) code observables are tracked. The static positioning of the code observable can reach centimetre level and the convergence time for the JPLM station is just 2·5 hours. The positioning results show that it is difficult to converge the Up direction to the centimetre level, compared with the North and East directions. The results show that static positioning can be correlated with the accumulation characteristic of the error for the code observable, while that that of the kinematic mode can be correlated to the error value. The shortened PPP convergence times verify that the presented stochastic models are effective.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000274
      Issue No: Vol. 72, No. 6 (2019)
  • System Reliable Probability for Multi-AUV Cooperative Systems under the
           Influence of Current
    • Authors: Qingwei Liang; Tianyuan Sun, Junlin Ou
      Pages: 1649 - 1659
      Abstract: Real multi-Autonomous Underwater Vehicle (AUV) cooperative systems operate in complicated marine environments. The interaction between a multi-AUV cooperative system and its marine environment will affect the reliability of the system. Current is an important influencing factor of multi-AUV cooperative systems. A reliability index of multi-AUV cooperative systems known as System Reliable Probability (SRP) is proposed in this study. A method to calculate SRP is introduced, and the influence of current on SRP is discussed in detail. Current is considered an attack source, and the degree of its influence on SRP is calculated. As an example, the performance of this method is shown on two multi-AUV cooperative systems. Results show that the influence of the same current environment on different structures of the multi-AUV cooperative systems differs. This result provides a reference for the structure selection of multi-AUV systems. This study provides a practical method to estimate the reliability of multi-AUV cooperative systems.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000298
      Issue No: Vol. 72, No. 6 (2019)
  • GMT and Longitude by Lunar Distance: Two Methods Compared From a
           Practitioner's Point of View
    • Authors: Eric Romelczyk
      Pages: 1660 - 1664
      Abstract: This article discusses the technique of observing lunar distance - that is, angular distance between the moon and another celestial body - to establish universal time and longitude, from a practitioner's point of view. The article presents a brief overview of the principles underlying the lunar distance observation and its use in celestial navigation. A discussion follows of two different methods for finding universal time by observing lunar distance, Dr. Wendel Brunner's calculator-based method and the specialised inspection tables created by Bruce Stark. The article compares the two methods against each other for ease of use and accuracy. The author concludes that either method will provide satisfactory results, but that the technique of observing lunar distance is unlikely to regain relevance in the modern-day practice of navigation and is primarily useful as a skill-building exercise in making sextant observations.
      PubDate: 2019-11-01T00:00:00.000Z
      DOI: 10.1017/S0373463319000341
      Issue No: Vol. 72, No. 6 (2019)
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