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Strong earthquakes have an impact on the regional thermal radiation background, which has been both observed and confirmed. This effect produces anomalies in the thermal radiation background (TRBA) and increases the difficulty of extracting a thermal radiation anomaly (TRA) that is associated with an earthquake occurring during the same time period. The extraction and identification of such anomalies has been ignored by previous studies. In this study, we investigate the time-frequency analysis (TFA) method, together with the wavelet filtering of the Daubechies method and the relative power spectrum analysis of the Fourier Welch method to extract and analyse the TRBA caused by the 2008 Wenchuan Ms 8.0 earthquake and the TRA of the 2013 Minxian-Zhangxian Ms 6.6 earthquake using data concerning the brightness temperature of a black body (TBB) from the Fengyun-2 series of geostationary meteorological satellites developed by China. The result showed that this method can effectively extract and analyse the TRBA caused by the Wenchuan earthquake and the TRA of the Minxian-Zhangxian earthquake form a complex background environment. Furthermore, we discussed the impact of the earthquake on the TRBA and segmented the process. The impact is mainly reflected by three aspects; the characteristic period of the TRBA changes, the TRBA occurs at the same time every year, which is identical to the time at which the earthquake anomaly occurred, and the impact process is in stages. We also summarized the correlation between the characteristic parameters of a TRA and the regional thermal radiation background, geography, and climatic factors. [ABSTRACT FROM AUTHOR]

The BeiDou navigation satellite system (BDS) comprises geostationary earth orbit (GEO) satellites as well as inclined geosynchronous orbit (IGSO) and medium earth orbit (MEO) satellites. Owing to their special orbital characteristics, GEO satellites require frequent orbital maneuvers to ensure that they operate in a specific orbital window. The availability of the entire system is affected during the maneuver period because service cannot be provided before the ephemeris is restored. In this study, based on the conventional dynamic orbit determination method for navigation satellites, multiple sets of instantaneous velocity pulses parameters which belong to one of pseudo-stochastic parameters were used to simulate the orbital maneuver process in the orbital maneuver arc and establish the observed and predicted orbits of the maneuvered and non-maneuvered satellites of BeiDou regional navigation satellite system (BDS-2) and BeiDou global navigation satellite system (BDS-3). Finally, the single point positioning (SPP) technology was used to verify the accuracy of the observed and predicted orbits. The orbit determination accuracy of maneuvered satellites can be greatly improved by using the orbit determination method proposed in this paper. The overlapping orbit determination accuracy of maneuvered GEO satellites of BDS-2 and BDS-3 can improve 2–3 orders of magnitude. Among them, the radial orbit determination accuracy of each maneuvered satellite is basically better than 1 m. simultaneously, the combined orbit determination of the maneuvered and non-maneuvered satellites does not have a great impact on the orbit determination accuracy of the non-maneuvered satellites. Compared with the multi GNSS products (indicated by GBM) from the German Research Centre for Geosciences (GFZ), the impact of adding the maneuvered satellites on the orbit determination accuracy of BDS-2 satellites is less than 9 %. Furthermore, the orbital recovery time and the service availability period are significantly improved. When the node of the predicted orbit is traversed approximately 3 h after the maneuver, the accuracy of the predicted orbit of the maneuvered satellite can reach that of the observed orbit. The SPP results for the BDS reached a normal level when the node of the predicted orbit was 2 h after the maneuver. [ABSTRACT FROM AUTHOR]

In addition to the orbital maneuver information, other satellite anomalies were also marked in the satellite's health status of broadcast ephemeris. For the signal-in-sp ace (SIS) performance evaluation, it is necessary to classify and evaluate the impact of a single fault, as well as the comprehensive impact. Therefore, it remains to identify the satellite orbital maneuver from the broadcast ephemeris and evaluate its impact on the signal-in-space performance. This study proposed an orbit maneuver detection method based on a combination of the square root of the major half-axis of the satellite orbit and the health status of the satellite recorded in broadcast ephemeris. The broadcast ephemerides provided by The Multi-GNSS Experiment (MGEX) from 2016 to 2020 were first used to detect the orbit maneuvers of BeiDou satellites; the orbit maneuvering and recovery periods of GEO and IGSO were detected next. The number of orbital maneuvers of the satellites made each year, and the interval of orbital maneuver of each satellite, was also obtained. The main findings of this study are as follows: (1) the proposed method can detect satellite orbit maneuvers and recovery periods recorded in the broadcast ephemeris; (2) the average orbit maneuvering interval of GEO and IGSO satellites were obtained and found to be approximately 27 days for GEO satellites except C04 and the IGSO, for which they were 45 days and approximately 178 days, respectively; (3) the average recovery time for the two types of satellite orbital maneuvers was about 6.6 h, and generally the same for the BeiDou-3 and BeiDou-2 satellites; and (4) using the detected orbital maneuver intervals and recovery periods, its impact on the signal-in-space availability and continuity of the BeiDou satellite was evaluated, which showed that the per-satellite availability of IGSO satellites were approximately 0.998 and 0.9998/h respectively. The per-satellite availability of each GEO satellite was about 0.986–0.996, and the continuity was about 0.9982/h-0.9992/h; therefore, the orbital maneuver reduced the per-satellite availability and continuity of the GEO and IGSO satellites by 0.002–0.014 and approximately 0.0002/h-0.0018/h, respectively. [ABSTRACT FROM AUTHOR]

• GTS distribution of orbit determination can be optimized by the proposed algorithm. • GTS saturated number for orbit determination can be got by the proposed algorithm. • ISL can weaken the influence of GTS distribution and number on orbit determination. A precise satellite orbit is the basis for high-precision services for users, and the determination of high-precision satellite orbits depends on the reasonable distribution and volume of ground tracking stations (GTSs). In July 2020, the BeiDou global navigation satellite system (BDS-3) was completed and began to be operated globally. Based on the principle of dynamic orbit determination, we first developed an algorithm to determine the optimal distribution and number of GTSs for accurate satellite orbit determination. Subsequently, based on 10 GTSs located in China, the optimal distribution and number of GTSs with and without inter-satellite link (ISL) ranging observations were determined. Finally, real measurements of GTS and ISL ranging observation data were used to validate the GTS results yielded by the algorithm. Using 18 BDS-3 satellites, and based on non-ISL observations and 10 Chinese regional GTSs, we found that GTSs distributed near the equator contributed significantly to the accuracy of orbit determination for these satellites compared to other GTSs. Orbit determination results obtained from the measured data of the optimal GTSs were more accurate than those obtained from the measured data of uniformly distributed global GTSs. Upon the addition of 55 uniformly distributed GTSs worldwide, the orbit determination accuracy for BDS-3 satellites tended to plateau. Moreover, following the addition of ISL, although the distribution and number of GTSs was optimized using the proposed algorithm, the orbit determination results from the measured data revealed that a change in the distribution and number of GTSs had a limited effect on the BDS-3 orbit determination results. [ABSTRACT FROM AUTHOR]

• Under regional GTS, ISL significantly improves BDS-3 orbit determination accuracy. • The initial weight ratio has little effect on the result when VCE algorithm is used. • VCE algorithm has contribute to fusion data processing of L-band and Ka-band. The BeiDou global navigation satellite system (BDS-3) has established the Ka-band inter-satellite link (ISL) to realize a two-way ranging function between satellites, which provides a new observation technology for the orbit determination of BDS-3 satellites. Therefore, this study presents a BDS satellite orbit determination model based on ground tracking station (GTS) observations and ISL ranging observations firstly to analyze the impact of the ISL ranging observations on the orbit determination of BDS-3 satellites. Subsequently, considering the data fusion processing, the variance component estimation (VCE) algorithm is applied to the parameter estimation process of the satellite orbit determination. Finally, using the measured data from China's regional GTS observations and BDS-3 ISL ranging observations, the effects of ISL ranging observations on the orbit determination accuracy of BDS-3 satellites are analyzed. Moreover, the impact of the VCE algorithm on the fusion data processing is evaluated from the aspects of orbit determination accuracy, Ka-band hardware delay parameter stability, and ISL ranging observation residuals. The results show that for China's regional GTSs, the addition of BDS-3 ISL ranging observations can significantly improve the orbit determination accuracy of BDS-3 satellites. The observed orbit determination accuracy of satellite radial component is improved from 48 cm to 4.1 cm. In addition, when the initial weight ratio between GTS observations and ISL ranging observations is not appropriate, the various indicators which include orbit determination accuracy, ISL hardware delay, and ISL observation residuals were observed to have improved after the adjustment of the VCE algorithm. These results validate the effectiveness of the VCE algorithm for the fusion data processing of the GTS observations and ISL ranging observations. [ABSTRACT FROM AUTHOR]

In this study, the double-differenced positioning bias of different frequency observations (PBDFO) in a code division multiple access (CDMA) signal system is analyzed based on real-time kinematic (RTK) positioning using raw observations at different frequencies. To test and verify the existence of the DD PBDFO, a set of ultra-short baseline (25 m) GNSS observation data were collected. The stations NT03 and NT04 were located in Xi'an City, Shaanxi Province. The observation time ranged from UTC 11:00 to 17:00 for 10 consecutive days from DOY 012 to DOY 021 in 2017. The results showed that there are mm–cm differences in the RTK solution results for the raw observations at different frequencies, the mean bias and the standard deviation (STD) of the positioning results of GPS L1 and L2 in the East, North, and Up components are (−0.595 mm, 1.35 mm, −0.133 mm) and (3.11 mm, 3.32 mm, 6.44 mm), respectively, but for BDS B1 and B2, the corresponding values are (2.16 mm, −4.24 mm, 9.48 mm) and (3.35 mm, 4.85 mm, 7.41 mm), respectively. The PBDFO between GPS L1 and L2 frequencies is lower in magnitude and more stable than the PBDFO between BDS B1 and B2 frequencies, the mean and the STD of the PBDFO between GPS L1 and L2 obtained from the parameter estimation method in the East, North, and Up components are (−0.606 mm, 1.03 mm, −0.217 mm) and (0.162 mm, 0.169 mm, 0.276 mm), respectively; but for BDS B1 and B2, the corresponding values are (3.00 mm, −3.83 mm, 8.59 mm) and (0.375 mm, 1.14 mm, 2.47 mm), respectively. Both PBDFO correction model and parameter estimation approach can achieve consistency in terms of the RTK positioning results at different frequencies. The double-differenced (DD) PBDFO obtained from the direct difference method and parameter estimation approach are consistent and relatively stable for the same period of continuous days. [ABSTRACT FROM AUTHOR]

This study proposes a combined precise point positioning (PPP) model, called the inter-system differenced PPP model, in which it formed the satellite-differenced observation between different satellite systems. Compared with the traditional combined PPP model, the inter-system differenced PPP model has the following characteristics: (1) The satellite difference between various systems can eliminate the receiver clock bias parameter, it enhances the strength of the PPP model. (2) Inter-satellite differences can eliminate some common model errors and reduce the impact of observation errors. (3) The system time difference can be calculated directly to provide the basis for time offset monitoring. In order to verify the positioning results of the inter-system differenced model, static PPP and dynamic PPP experiments were carried out to test the positioning accuracy and convergence time, and the results were compared with those of the traditional combination PPP model. In addition, the offset characteristics introduced by the inter-system differenced model were analysed, and the optimal estimation strategy was determined by dynamic PPP tests. Preliminary results are as follows: (1) For inter-system differenced static PPP, the convergence time of stations is less than 20 min. The standard deviation (STD) of the position bias for the East (E), North (N), and Up (U) components are better than 2.5 cm, 1.0 cm and 3.0 cm, respectively, and the corresponding root mean square deviation (RMS) are better than 3.0 cm, 1.5 cm and 6.0 cm, respectively. Compared with the traditional combined PPP model, the average convergence time of the inter-system differenced model is nearly the same, but its overall positioning accuracy is better. (2) For the inter-system differenced dynamic PPP model, the convergence time of stations is better than 30 min. The STD of the position bias for the E, N, and U components is better than 3.0 cm, 2.0 cm and 4.5 cm respectively, and the RMS is better than 4.0 cm, 2.5 cm and 7.0 cm respectively. Compared with the traditional combined PPP model, the convergence of the inter-system differenced model is slightly better, and the positioning accuracy does not differ significantly. Moreover, the inter-system differenced model is much better than the traditional model when fewer satellites are available. (3) The offset between GPS and BDS corresponding with the precise products of GFZ is related to the type of receiver, and the daily standard of offset is less than 0.5 ns. In addition, we determined optimum process noise of the offset parameter to be a 10 - 3 / 3 (m 2 / s) random walk after comparing several options. [ABSTRACT FROM AUTHOR]

As one of the major global navigation satellite systems (GNSSs), the performance of the Galileo Satellite Navigation System (Galileo) has been of pressing concern. By the end of June 2018, there were 17 operational satellites that provide navigation and positioning services. However, there is little research evaluating the performance of the Galileo single-system in terms of the average satellite coverage and precise positioning. To address this, this study firstly uses the number of visible satellites to analyze the global average satellite coverage of Galileo. Then different data processing strategies are used to evaluate the accuracy of the Galileo satellite orbit and clock. Finally, the performances of precise point positioning (PPP) and baseline solutions obtained using the Galileo single-system are analyzed. Results show that the number of visible Galileo satellites globally is between 6 and 7 and the time percentage where the number of visible Galileo satellites is greater than or equal to 4 is more than 90% globally. For the orbit determination of Galileo satellites, the Galileo/GPS combined solution is better than that of the Galileo single-system. Using the GeoForschungsZentrum Potsdam (GFZ) multi-GNSS orbit product (indicated by GBM) as a reference, the root mean squares (RMSs) of three-dimension and radial component of Galileo/GPS three-day combined solution are 4.4 cm and 1.8 cm, respectively. The accuracy of the Galileo/GPS single-day combined solution is almost the same as that of the Galileo/GPS three-day combined solution. For the Galileo satellite clock, the Galileo/GPS combined solution provides a better result than the Galileo single-system solution. The convergence times of the static and kinematic PPP solutions of the Galileo single-system are both about 2 h. The average the RMSs of the east (E), north (N), and up (U) components of the static Galileo-only PPP solution are 1.9 cm, 0.5 cm, and 2.0 cm, respectively. Those of the kinematic Galileo-only PPP solution are 2.1 cm, 1.4 cm, and 2.6 cm, respectively. Moreover, the baseline solution accuracy of the ultra-short baseline (48 m) of the Galileo single-system is comparable to GPS. For both static and kinematic solutions, the standard deviations (STDs) of the horizontal and vertical components are better than 6 mm and 8 mm, respectively. The accuracy of baseline solution results of the short baseline (1850 m) is poorer. For both static and kinematic solutions, the STDs of the horizontal and vertical components are better than 1.3 cm and 1.9 cm, respectively. [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]

Abstract A new BeiDou Navigation Satellite System (BDS) Real Time Kinematic (RTK) algorithm based on zero-combined observations and ionosphere constraints is developed in this study. When applying the zero-combined observations, the Double-Differenced (DD) ionospheric residuals are estimated as unknown parameters by adding the a priori information, i.e., the spatial and temporal ionosphere constraints. An experimental dataset was used to perform the validation and analysis. The results show that the proposed algorithm can precisely correct the DD atmosphere residuals, not only for the short baselines but also for the long baselines. Besides, the positioning results reveal much smaller noise and are more stable than the method of utilizing ionosphere-free combinations. A positioning accuracy of about 1–2 cm can be achieved with the BDS RTK for the baseline of 100 km. [ABSTRACT FROM AUTHOR]

Abstract This paper proposes a real-time kinematic (RTK) model that uses one common reference satellite for the Galileo system with four frequency observations. In the proposed model, the double-differenced (DD) pseudorange and carrier phase biases among the different frequencies are estimated as unknown parameters to recover the integer features of the DD ambiguities among the different frequencies for ambiguity resolution and precise positioning. Analysis results show that the E5a, E5b, and E5 frequencies have virtually the same performance in terms of the positioning accuracy, observation residuals, and ratio values of ambiguity resolution. However, the E1 frequency performs worse than the E5a, E5b, and E5 frequencies. The RTK results for the combination of multiple frequencies are much better than those for a single-frequency observation, the coordinates' standard deviation is improved about 20–30%, and the ambiguity fix time is improved about 10%. [ABSTRACT FROM AUTHOR]

A BeiDou Navigation Satellite System (BDS) Un-Differenced (UD) Network Real Time Kinematic (URTK) positioning algorithm, which is based on raw observations, is developed in this study. Given an integer ambiguity datum, the UD integer ambiguity can be recovered from Double-Differenced (DD) integer ambiguities, thus the UD observation corrections can be calculated and interpolated for the rover station to achieve the fast positioning. As this URTK model uses raw observations instead of the ionospheric-free combinations, it is applicable for both dual- and single-frequency users to realize the URTK service. The algorithm was validated with the experimental BDS data collected at four regional stations from day of year 080 to 083 in 2016. The achieved results confirmed the high efficiency of the proposed URTK for providing the rover users a rapid and precise positioning service compared to the standard NRTK. In our test, the BDS URTK can provide a positioning service with cm level accuracy, i.e., 1 cm in the horizontal components, and 2–3 cm in the vertical component. Within the regional network, the mean convergence time for the users to fix the UD ambiguities is 2.7 s for the dual-frequency observations and of 6.3 s for the single-frequency observations after the DD ambiguity resolution. Furthermore, due to the feature of realizing URTK technology under the UD processing mode, it is possible to integrate the global Precise Point Positioning (PPP) and the local NRTK into a seamless positioning service. [ABSTRACT FROM AUTHOR]

Strong-motion’s baseline shift error is very difficult to process precisely; it is mainly caused by the tilting, rotation of the ground and environment noises during the co-seismic period. In the study, we first studied how to effectively extract the strong-motion’s baseline shift error with GPS observation; this also provides a new way of correcting the baseline shift errors. Then we studied how to retrieve the ground tilting information of the station point with the collocated GPS and strong-motion observations, the information is an important input parameter of rotational seismology. In addition, both experimental result and seismic data show that the baseline shift error is mainly caused by the ground tiling and rotation during the co-seismic period. Also, there is a strong directly proportional relationship between the baseline shift error and ground tilting, of which the proportionality constant is approximately equal to the value of gravitational acceleration of the station. [ABSTRACT FROM AUTHOR]