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Satellite navigation systems enable long‐distance and high‐precision time transfers. The authors proposed an undifferenced precise time transfer method with information constraints in this study. A more accurate receiver clock offset can be obtained by constraining the station coordinate parameters and receiver clock offset parameters to achieve a precise time transfer. In this example, the research object is the time transfer link established by the Global Positioning System observation station using a caesium clock, rubidium clock, and hydrogen clock. The clock difference sequence of the time‐transfer link was calculated by combining the observation data and precise products. Following analysis of the examples, the following conclusions were reached. The undifferenced precise time transfer method based on information constraints can effectively be used to smooth the clock difference sequence of the receiver and the clock difference sequence of the time transfer link. To some extent, this method can also be used to reflect the performance of atomic clocks at Global Positioning System stations and is better suited for the precise time transfer of the equal‐clock long‐baseline time transfer link. Simultaneously, the frequency stability of this method is improved significantly in the short term compared to the traditional scheme without any constraints. The improvement of frequency stability is helpful to improve the accuracy of time transfer. [ABSTRACT FROM AUTHOR]

Specific emitter identification (SEI) is a professional technology to recognize different emitters by measuring the unique features of received signals. It has been widely used in both civilian and military fields. Recently, many SEI methods based on deep learning have been proposed, most of which assume that the training set and testing set have the same data distribution. However, in reality, the testing set is generally used later than the training set and lacks labels. The long time span may change the signal transmission environment and fingerprint features. These changes result in considerable differences in data distribution between the training and testing sets, thereby affecting the recognition and prediction abilities of the model. Therefore, the existing works cannot achieve satisfactory results for a long time span SEI. To address this challenge and obtain stable fingerprints, we transform the long time span SEI problem into a domain adaptive problem and propose an unsupervised domain adaptive method called LTS-SEI. Noteworthily, LTS-SEI uses a multilayer convolutional feature extractor to learn feature knowledge and confronts a domain discriminator to generate domain-invariant shallow fingerprints. The classifier of LTS-SEI applies feature matching to source domain samples and target domain samples to achieve the domain alignment of deep fingerprints. The classifier further reduces the intraclass diversity of deep features to alleviate the misclassification problem of edge samples in the target domain. To confirm the effectiveness and reliability of LTS-SEI, we collect multiple sets of real satellite navigation signals using two antennas with 13 m- and 40 m-large apertures, respectively, and construct two available datasets. Numerous experiments demonstrate that LTS–SEI can considerably increase the recognition accuracy of the long time span SEI and is superior to the other existing methods. [ABSTRACT FROM AUTHOR]

The Low Earth Orbit (LEO) satellites can be used to effectively speed up Precise Point Positioning (PPP) convergence. In this study, 180 LEO satellites with a global distribution are simulated to evaluate their contribution to the PPP convergence. LEO satellites can give more redundant observations and improve satellite geometric distributions, particularly for a single Global Navigation Satellite System (GNSS). The convergence speed of the PPP float solution using the Global Positioning System (GPS, G) or BeiDou Navigation Satellite System (BDS, C) single system as well as the G/C/Galileo navigation satellite system (Galileo, E)/GLObal NAvigation Satellite System (GLONASS, R) combined system with LEO satellites added is improved by 90.0%, 91.0%, and 90.7%, respectively, with respect to the system without LEO satellites added. We introduced LEO observations to assist GNSS in PPP-AR (Ambiguity Resolution) and PPP-RTK (Real Time Kinematic). The success fix rate of a single system is significantly improved, and the Time-To-First-Fix (TTFF) of G and G/C/E is reduced by 86.4% and 82.8%, respectively, for the PPP-AR solution. We analyzed the positioning performance of LEO satellite assisted G/C/E PPP-RTK in the reference networks of different scales, namely different atmospheric delay interpolation accuracies. The success fix rate of the G/C/E combined system is improved from 86.8 to 94.9%, and the TTFF is reduced by 36.8%, with the addition of LEO satellites in the 57 km reference network. In the 110 km reference network, the success fix rate of the G/C/E combined system is improved from 64.0 to 88.6%, and the TTFF is reduced by 32.1%. GNSS PPP-RTK with adding the LEO satellites in the reference networks of different scales shows obvious improvement because the atmospheric correlation decreases with increasing distance from the reference networks. [ABSTRACT FROM AUTHOR]

With the development of the Chinese BeiDou Navigation Satellite System (BDS), from a regional system BDS-2 to a global system BDS-3, more satellites can now be employed in the domain of globally precise time and frequency transfer. However, if the current BDS-3 and BDS-2, Global Positioning System (GPS), Galileo and GLObal NAvigation satellite system (GLONASS) are combined for multi-global navigation satellite system (multi-GNSS) time transfer, the characteristics of inter-system biases (ISBs) remain unclear. In this study, we analyzed the characteristics of ISBs between BDS-3 and BDS-2, GPS, Galileo and GLONASS, and revealed that their ISB series is not constant and that all the daily ISB series of the multi-GNSS show some systematic characteristics among different stations. Therefore, three stochastic models for ISBs of white noise process, random constant process and random walk process expressed as 'White', 'Hour' and 'Walk', respectively, were employed to assess the performance of the time and frequency transfer using the time link accuracy and frequency stability indicators. The results show that the models for the 'Hour' and 'Walk' schemes are notably better than that for the 'White' scheme by 10.4% and 16.8%, respectively, for the least noisy time link of WTZZ-BRCH. Furthermore, the improvements in the 'Hour' and 'Walk' schemes are 2.1% and 3.5%, respectively, for the TP01-BRCH time link, when compared with that of the 'White' scheme. With respect to frequency stability, the 'Hour' and 'Walk' schemes also perform better in terms of frequency stability than the 'White' scheme at different time intervals. Similarly, the improvements over the 'White' scheme are 5.05% and 4.97% for the TP01-BRCH time link and are 24.01% and 32.48% for the WTZZ-BRCH time link. [ABSTRACT FROM AUTHOR]

In order to analyse the contribution of inter-satellite ranging observation to the BeiDou regional navigation satellite system (BDS-2) satellite orbit determination under the condition of regional ground tracking stations, first, the methods of using the observation model, the dynamic parameters dilution of precision (DPDOP), and the satellite position dilution of precision (SPDOP) to describe the contribution of inter-satellite ranging observation to orbit determination accuracy are proposed. Then, based on the measured observation data of the regional ground tracking stations located in China, as well as simulated inter-satellite ranging observation data, the contribution of inter-satellite ranging observation to orbit determination accuracy for the three types of satellites used by BDS-2 is analysed. The results show that the inter-satellite ranging observation improves the solution strength of the dynamic parameters by adding redundant observations without adding new parameters to be estimated, which effectively improves the orbit determination accuracy of the along-track component of Geostationary Earth Orbit (GEO) satellite and the cross-track and radial components of Inclined Geosynchronous Orbit (IGSO) and Medium Earth Orbit (MEO) satellites. In addition, under the condition of the regional tracking stations, the inter-satellite ranging observation compensates for the disadvantage that the satellite cannot be tracked by the whole arc segment. The accuracy and continuous stability of the orbit determination solution and the geometry structure of satellites that can be observed by stations and other satellites are significant improved, for which the average DPDOP values of GEO, IGSO, and MEO satellites are improved 50.2%, 73.0%, and 72.1%, respectively. • Simulated ISL observations improves the orbit determination accuracy of BDS-2. • Simulated ISL observations compensates for the observed arc of satellites. • Simulated ISL observations improves the observed structure of satellites. [ABSTRACT FROM AUTHOR]

Abstract Solar radiation pressure is one of the main perturbations in satellite orbit determination. When a satellite enters the umbra or the penumbra area of an occulting body (e.g., the Earth or the Moon), a satellite shadow function model is used to estimate the occultation degree. Occultation of the Sun by the Earth and by the Moon is considered separately in the existing shadow function model, i.e., any area of overlap of the shadows of the Earth and the Moon is not considered. This directly affects the accuracy of the solar radiation pressure model, which then affects the accuracy of satellite orbit determination. In this study, the existing shadow function model was refined to consider the overlap of the shadows of the Earth and the Moon. Then, using the original and refined shadow function models, Global Positioning System (GPS) satellite orbits from 2008 to 2017 were analyzed and compared. The results show that there are 27 occurrences when GPS satellites enter an area of overlapping shadows of the Earth and the Moon. Among them, the longest duration of passage through the overlapping shadows by one satellite in one day is 474 s, and the maximum difference of the same epoch between the original and refined models is 0.329. In addition, Differences between the GPS observed orbits obtained separately using the original and refined models are small. However, differences between the GPS predicted orbits are substantial; the greatest difference is 46 mm. Furthermore, with the reference of overlapping arcs, when there are differences between the predicted orbits obtained separately using the original and refined models, 86.7% of the orbit determination solutions are improved. [ABSTRACT FROM AUTHOR]

• Real-time monitoring the dynamic variation of satellite orbital maneuvers. • Extending the usable observations for improving the user service. • Useful for precise orbit control and satellite healthy monitoring. The BeiDou Navigation Satellite System (BDS) frequently experiences orbital maneuvers. This study proposed a method for the real-time monitoring of the dynamic variation of satellite orbital maneuvers. Based on the epoch-differenced velocity estimation model, the epoch variation of the receiver clock error can be calculated for each station. The dynamic variation of the orbital maneuver can then be estimated real-time by the monitoring model, using observations from three or more BDS tracking stations. The feasibility and effectiveness of this method is validated by different datasets. The results show that the method can precisely detect orbital maneuvers, thereby extending the usable observations for about three hours. Thus, it is useful for improving the positioning, navigation, and timing service. The dynamic variations of orbital maneuvers can be monitored in real-time, which is useful for precise orbit control, and is helpful for keeping the satellite healthy and monitoring the navigation system operation. [ABSTRACT FROM AUTHOR]