Precise point positioning ambiguity resolution (PPP-AR) is a key technique for high-precision global navigation satellite system (GNSS) observations, with phase bias products playing a critical role in its implementation. The multi-GNSS experiment analysis center at Wuhan University (WUM) has adopted the undifferenced ambiguity resolution (UDAR) approach to generate high-precision orbit, clock, and observable-specific bias (OSB) products to support PPP-AR since day 162 of 2023. This study presents the analysis strategy employed and assesses the impact of the transition to ambiguity resolution on the orbit precision, using metrics such as orbit boundary discontinuities (OBD) and satellite laser ranging (SLR) validation. Additionally, the stability of the OSB products and the overall performance of PPP-AR solutions are evaluated. The OBD demonstrates specific improvements of 7.1% and 9.5% for GPS and Galileo, respectively, when UDAR is applied. Notably, BDS-3 medium Earth orbit satellites show a remarkable 15.2% improvement compared to the double-differenced results. However, for the remaining constellations, the improvements are either minimal or result in degradation. Using GPS and GLONASS solutions from the International GNSS Service (IGS) and other solutions from the European Space Agency (ESA) as references, the orbit differences of WUM solutions based on UDAR exhibit a significant reduction. However, the improvements in SLR validation are limited, as the radial orbit precision is primarily influenced by the dynamic model. The narrow-lane ambiguity fixing rate for static PPP-AR, based on data from approximately 430 globally distributed stations, reaches 99.2%, 99.2%, 88.8%, and 98.6% for GPS, Galileo, BDS-2, and BDS-3, respectively. The daily repeatability of station coordinates is approximately 1.4 mm, 1.9 mm, and 3.9 mm in the east, north, and up directions, respectively. Overall, these results demonstrate the effectiveness and potential of WUM’s undifferenced ambiguity resolution approach in enhancing GNSS data processing and facilitating PPP-AR applications.

Traditional high-precision satellite orbits rely on globally dense and evenly distributed ground tracking stations, while the accuracy of precise orbit determination (POD) based on a regional network cannot compare with that of a global network. Low Earth orbit (LEO) satellites can serve as space-based monitoring stations to compensate for this. In response to the current regional integrated POD that only resolves the ambiguities of ground stations, this paper proposes an ambiguity resolution (AR) strategy related to LEO satellites to enhance GPS orbit accuracy. A joint observation network is established using seven International GNSS Service (IGS) stations within China and 10 LEO satellites, including GRACE-C/D, LuTan1-A/B, SWARM-A/B/C, Sentinel-3A/B, and Sentinel-6A. Experiments are conducted and analyzed from three aspects: independent baseline selection, the common view time, and the wide-lane (WL) threshold of double-differenced ambiguity. The ambiguity fixing strategy is determined to be a combination of inter-satellite and satellite–ground baselines, a common view time of 5 min, and a WL ambiguity threshold of 0.2 cycles. Taking the final products released by the IGS as the reference, the GPS orbit accuracy in the along-track, cross-track, radial, and 1D RMS is 3.23, 2.74, 2.36, and 2.89 cm, respectively, which represents improvements of 9.3%, 12.5%, 10.9%, and 10.8% compared with the solution that only fixes the ambiguities of ground stations. This result demonstrates that, in regional integrated POD, further implementation of LEO satellite-related ambiguity fixing significantly improves GPS orbit accuracy. Given the limitation that most LEO satellites can only receive GPS satellite signals, in the future, as more LEO satellites gain access to GNSS observations, the ambiguity fixing strategy presented in this paper can provide an effective and feasible approach.

A new antenna model for BeiDou-3 satellites in medium Earth orbit (MEO) and inclined geosynchronous orbit (IGSO) is presented. This model enables consistent processing of BeiDou-3 with other GNSSs in the IGS20 reference frame and has been adopted for use within the International GNSS Service (IGS) in February 2026. The model introduces estimated phase patterns for four different blocks of BeiDou-3 satellites and provides satellite- and frequency-specific phase center offsets (PCOs) for the B1, B2, and B3 bands. B1C/B2a phase patterns and PCOs are obtained as weighted averages of individual estimates from up to nine IGS analysis centers (ACs) to minimize the dependence on AC-specific models and processing standards. In order to support users of the B1I/B3I legacy signals, satellite-specific PCOs for this linear combination have been determined by two ACs. As part of the antenna model development, temporal variations of PCOs at the few decimeter level were observed over the 2021–2024 time frame for selected satellites built by the Shanghai Engineering Center for Microsatellites (SECM), which are handled as piece-wise constant PCOs in the new IGS antenna model for the affected satellites. The new phase center model is aligned to the IGS20 reference frame and ensures consistency between GPS, GLONASS, Galileo, and BeiDou-3 in multi-GNSS processing. A precise point positioning validation of the new antenna model with B1C/B2a observations confirms a 0.11 ppb scale consistency of BeiDou-3 with the IGS20 reference frame.

Information about satellite antenna phase center offsets (PCOs) is crucial for high-precision applications of global navigation satellite systems. Pre-launch manufacturer calibrations of the PCOs are available for all individual BDS satellites and three frequencies: B1, B2, and B3. With the imminent retirement of BDS-2 and the superior quality of B1C/B2a signals, the International GNSS Service (IGS) has launched a campaign for BDS-3 and QZSS phase center correction (PCC) calibration. However, only the IF combination of B1C/B2a is required. Considering the upcoming multi-GNSS and multi-frequency applications, the consistency of PCO values across different frequencies and GNSS is important.We present the estimation of BDS-3 satellite antenna PCCs consistent with the International Terrestrial Reference Frame (ITRF2020) from the ionosphere-free linear combinations of B1I/B3I and B1C/B2a. The results demonstrate that the nadir-angle-dependent phase center variations (PCVs) of the same satellite block type have fairly good consistency. The mean horizontal PCOs of the different frequencies agree at the millimeter level. The X-PCOs show a bias of about 1 cm compared to the manufacturer calibrations, whereas the Y-PCOs are free of such bias. For Z-PCOs, some SECM satellites exhibit unexpected long-term variations and/or abrupt jumps, while all CAST satellites remain stable. The mean estimated values of B1C/B2a and B1I/B3I for BDS-3M-CAST, BDS-3M-SECM, and BDS-3I-CAST are (-15.9, -13.5), (-17.8, -3.0), and (+26.9, +24.0) cm larger than the CSNO values, respectively.By applying the estimated PCCs instead of the CSNO values, the precise orbit precision can be improved by 3.6-12.2%, and the scale factors determined by BDS solutions exhibit good consistency with the IGS20 frame, with mean scale differences below 0.10 ppb for both frequency combinations.Furthermore, CSNO values for SECM satellites are excluded from delivering the TRF scale by BDS-3 as they still remain questionable. Although the scale delivered by GPS and Galileo shows good consistency, with +0.84 and +0.73 ppb compared to ITRF2020, the scale difference of BDS-3 with respect to ITRF2020 is -0.84 and -0.92 for B1C/B2a and B1I/B3I. These results demonstrate the consistency of different frequencies for BDS-3 but also highlight the discrepancy within GNSS, specifically between BDS-3 and GPS/Galileo.

Signal distortion bias (SDB) is a systematic pseudorange bias associated with receiver types. Its presence reduces the consistency of GNSS clock and signal bias estimated from inhomogeneous networks, significantly decreasing the ambiguity resolution rate and negatively affecting positioning and timing performance. Current research does not provide a comprehensive understanding of the error characteristics of SDB, and traditional SDB calibration methods exhibit certain limitations.This paper first identifies the correlation between SDB and factors such as receiver brand, model, as well as the dependencies on firmware versions, antennas, and radomes. In addition, we propose a geometry-free (GF)-aided multi-GNSS all-frequency SDB calibration method and strategy. The GF assistance addresses the ±0.5 cycle limitation associated with wide-lane (WL) ambiguity rounding, further improving the precision of SDB calibration. Based on this method and MGEX data, we calibrated and analyzed the SDB for all-frequency signals of GPS, Galileo, and BDS-3. Ultimately, we provide SDB corrections for all satellites per receiver group in SINEX BIAS format.We further investigated the impact of SDB on satellite clocks, code biases, phase biases, and PPP-AR both theoretically and experimentally. Results show that SDB correction significantly enhance the consistency of satellite clock and bias estimates across networks, with BDS-3 improving by over 90% and wide-lane ambiguity fixing rates for different receivers increasing by up to 20% at most. Moreover, an analysis of one-month data from 132 MGEX stations with hourly reset indicates that SDB correction reduces the multi-GNSS kinematic PPP convergence times for Septentrio, Leica, Javad, and Trimble receivers by an average of 0.43, 2.74, 2.63, and 4.28 epochs, respectively, with corresponding PPP-AR convergence improvements of 1.9%, 8.5%, 14.7%, and 17.7%. The average convergence performance across stations with different receiver types improved by 2.4%, 12.1%, 31.3%, and 25.2%, with maximum improvements of up to 70.8%. These analyses fully demonstrate the necessity of SDB modeling and correction, and it is recommended that the IGS Analysis Center adopt it.