With the development of the Low Earth Orbit (LEO) communication constellations, it has become a hot area of research to provide additional navigation augmentation services. Limited by volume, weight, power consumption, and running time, the in-flight performance of navigation augmentation payload remains to be investigated. In this paper, we analyze the data quality of on-board GNSS observation and evaluate the precision of short-arc dynamic Precise Orbit Determination (POD) performance based on the WangTong-01 (WT01) mission. Furthermore, the downlink navigation measurement data of WT01 satellites are analyzed and compared with the GNSS observations. The results show that the average multipath errors of the WT01 on-board GPS L1, L2 and BeiDou Satellite Navigation System (BDS) B1, B2 code observation are 0.54, 0.74, 0.65, and 0.58 m, respectively. The short-arc dynamic POD three-dimensional (3D) overlapping accuracy is 7.1 cm. The average multipath errors of downlink navigation signal Z1 and Z2 are 0.81 and 0.80 m, respectively, which at the same order of magnitude as GNSS signals. The maximum Carrier-to-Noise Ratio (C/N₀) value of WT01 downlink measurement data can reach 60 dB Hz, which is much stronger than GNSS and indicates the navigation signals of LEO satellites can meet the basic requirement of navigation augmentation.

Starting from February 2023, the International Laser Ranging Service (ILRS) began releasing satellite laser ranging (SLR) data for all BeiDou global navigation satellite system (BDS-3) medium earth orbit (MEO) satellites. SLR data serve as the best external reference for validating satellite orbits, providing a basis for comprehensive evaluation of the BDS-3 satellite orbit. We utilized the SLR data from February to May 2023 to comprehensively evaluate the orbits of BDS-3 MEO satellites from different analysis centers (ACs). The results show that, whether during the eclipse season or the yaw maneuver season, the accuracy was not significantly decreased in the BDS-3 MEO orbit products released from the Center for Orbit Determination in Europe (CODE), Wuhan University (WHU), and the Deutsches GeoForschungsZentrum (GFZ) ACs, and the STD (Standard Deviation) of SLR residuals of those three ACs are all less than 5 cm. Among these, CODE had the smallest SLR residuals, with 9% and 12% improvement over WHU and GFZ, respectively. Moreover, the WHU precise orbits exhibit the smallest systematic biases, whether during non-eclipse seasons, eclipse seasons, or satellite yaw maneuver seasons. Additionally, we found some BDS-3 satellites (C32, C33, C34, C35, C45, and C46) exhibit orbit errors related to the Sun elongation angle, which indicates that continued effort for the refinement of the non-conservative force model further to improve the orbit accuracy of BDS-3 MEO satellites are in need.

In recent years, the multi-GNSS positioning application is becoming more and more popular, same to the low Earth orbit (LEO) satellite precise orbit determination (POD) based on the onboard multi-GNSS measurements. The third-generation Beidou navigation satellite system (BDS-3) provides a new option to obtain the LEO satellite orbit solutions. However, the receiver intersystem bias (ISB) of different GNSS is unavoidable in multi-GNSS data processing. This paper’s main goal is absorption of the impact of the ISB between BDS-3 and BDS-2 on the LEO satellite POD. Taking GPS-based POD solutions for the reference orbit, this paper evaluates the orbit accuracy of BDS-2-based POD, BDS-3-based POD, BDS-2 and BDS-3 combined PODs with/without ISB. The BDS-3-based POD accuracy is 6.57 cm in the 3D direction, a 56% improvement over BDS-2-based POD. When the ISB between BDS-3 and BDS-2 is estimated, the BDS-2/3 combined POD accuracy of 5.37 cm in the 3D direction is better than that without ISB, which is a 64% improvement over BDS-2-based POD and 18% improvement over BDS-3-based POD. For GPS and BDS-2/3 combined POD, the GPS and BDS-3 joint POD solutions have the smallest RMS differences in overlapping consistency and smallest RMS differences compared to GPS-based POD. This study indicates that estimating the BDS-2/3 receiver ISB in BDS-2/3 joint POD could improve the orbit accuracy, and the GPS and BDS-3 joint POD solution is better than another combined POD. This paper will provide meaningful references for the LEO satellite multi-GNSS-based POD.

Data processing of the Global Navigation Satellite System using uncombined (UC) observations has been a hot topic in recent years. However, the computation complexity and time consumption of the UC approach is much higher than those of the linear combination approach due to there being more parameters of interest. Concerning this issue, we propose a three-step approach by combining the ionosphere-free (IF) observation model and the UC observation model in a network scheme to reduce computation complexity. The first two steps are based on the IF observation model to detect cycle slips and gross errors using preprocessing and post-fit residual screening algorithms, respectively, while the third step is based on the UC observation model in which the ambiguities estimated from the IF model are used for UC ambiguity resolution (IF-IF-UC). To validate this approach, a network of 140 globally distributed stations is chosen and 1 year of GPS/GALILEO/BDS-3 dual-frequency data in 2021 is processed. The 1D RMSs of satellite orbit difference and the average STDs of satellite clock differences with respect to ESA’s products for GPS, Galileo and BDS-3 are 1.8, 2.4 and 4.1 cm and 0.024, 0.019 and 0.028 ns, respectively. Compared with the GIM products provided by the Center for Orbit Determination in Europe, the mean RMSs of the VTEC differences for GPS, Galileo and BDS-3 are 2.38, 2.30 and 3.09 TECU. For comparison purposes, the approach where all three-steps are based on the UC model is also conducted (UC-UC-UC). Results show that satellite orbits, clocks and VTECs from the IF-IF-UC approach are almost the same as those from the UC-UC-UC approach. By introducing high-performance linear algebra libraries, the elapsed time of the proposed IF-IF-UC approach is about 1 h, which is an improvement of 36% when compared to the UC-UC-UC approach. Nevertheless, the elapsed time is nearly 6 d when the UC-UC-UC approach is conducted without introducing high-performance linear algebra libraries.

The BeiDou-3 Navigation Satellite System (BDS-3) transmit new frequency signals, B1C and B2A, achieving good interoperability with code division multiple access (CDMA) systems such as GPS and Galileo. These new signals feature significant improvements in modulation design and frequency planning, providing a more stable observation foundation for multi-system high-precision positioning. Focusing on resolving observation loss and observation noise amplification in traditional precise point positioning (PPP) and real-time kinematic (RTK) models, this study proposes an undifferenced and uncombined PPP-RTK models based on multi-frequency CDMA signals and analyses its rank-deficiency characteristics and optimization strategies. First we analyze the linear correlations among clock, phase bias, ionospheric delay ambiguity and other parameters, the S-basis linear transformation method is employed to eliminate rank deficiency, furthermore derive the full-rank equations of the multi-frequency CDMA-based PPP-RTK system and complete the product generation based on the new frequencies of the BDS-3 system. Utilizing data from 13 continuous observation stations in the Northern China region, this study conducts a comparative analysis of GPS, GALILEO, and the new (B1C/B2A) and legacy (B1I/B3I) frequencies of BDS-3 in terms of satellite clock offsets, phase biases, and ionospheric products. The results indicate that after convergence the GPS, GALILEO BDS-3 new frequency satellite clock standard deviation can reaches 0.03 ns; uncalibrated phase delays for the L1 frequency is approximately 0.02 cycles; the products of BDS-3 new frequency outperform the old ones. In terms of ionospheric products, data from 4 stations in the survey area over 5 d are selected, the slant delay derived from the geometry-free combination of phase observations is taken as the true value, by calculating the difference between the slant delay derived from model coefficients and the true value, the (root mean square) values of GPS, GAL (Galileo), BDS-3 (B1C & B2A), and BDS-3 (B1I & B3AI) are better than 0.232 m, 0.176 m, 0.164 m, and 0.536 m.