Multipath effects in GNSS observations significantly reduce positioning accuracy and are challenging to model and mitigate. We propose three statistical tests for multipath detection using the carrier-to-noise power density ratio (C/N0). Each test has certain advantages and disadvantages: the first S1 based on the C/N0 is sensitive to an abnormal C/N0 but cannot distinguish multipath and NLOS (non-line-of-sight). The second S2 based on the differenced C/N0 between frequencies can detect multipath without being affected by NLOS, but at least two frequencies are needed. Additionally, the combination of S1 and S2 can be used to identify NLOS. The third S3 based on the epoch difference of C/N0 is preferred for kinematic mode. The best performance is achieved when all three tests are used in combination. For the static multipath detection experiment in a harsh environment, a function related to the azimuth is established to mitigate the impact of azimuthal dependence of C/N0 on multipath detection for low-cost receivers. The detection rates for a Trimble Alloy receiver with a geodetic antenna and a low-cost u-blox F9P receiver with a microstrip patch antenna were 83.67% and 73.40%, respectively, while their incorrect detection rates were 10.11% and 20.24%, respectively. For single-epoch static RTK positioning with the u-blox F9P, the proportion of horizontal position error within 0.2 m and 2 m were improved from 61.5% to 78.8% and 78.3% to 92.8%, respectively, once multipath observations were identified. For the kinematic experiment, 47.05% and 61.79% of the observations were detected as affected by multipath interference, and 70.95% and 85.27% of NLOS observations were detected via the combination of S1 and S2 for the Alloy and u-blox F9P receivers, respectively. However, due to the sensitivity of low-cost devices to multipath effects, the number of available satellites will significantly decrease during kinematic positioning. In such cases, a weighting method should be applied.

The long convergence time and non-robust positioning accuracy are the main factors limiting the application of precision single-point positioning (PPP) in kinematic vehicle navigation. Therefore, a dual/triple-frequency multi-constellation PPP-RTK method with atmospheric augmentation is proposed to achieve cm-level reliable kinematic positioning. The performance was assessed using a set of static station and kinematic vehicle positioning experiments conducted in Wuhan. In the static experiments, instantaneous convergence within 1 s and centimeter-level positioning accuracy were achieved for PPP-RTK using dual-frequency observation. For the kinematic experiments, instantaneous convergence was also achieved for dual-frequency PPP-RTK in open areas, with RMS of 2.6 cm, 2.6 cm, and 7.5 cm in the north, east, and up directions, respectively, with accuracy similar to short-baseline real-time kinematic positioning (RTK). Horizontal positioning errors of less than 0.1 m and 3D positional errors of less than 0.2 m were 99.54% and 98.46%, respectively. Additionally, after the outage of GNSS and during satellite reduction in obstructed environments, faster reconvergence and greater accuracy stability were realized compared with PPP without atmospheric enhancement. Triple-frequency PPP-RTK was able to further enhance the robustness and accuracy of positioning, with RMS of 2.2 cm, 2.0 cm, and 7.3 cm, respectively. In summary, a performance similar to RTK was achieved based on dual-frequency PPP-RTK, demonstrating that PPP-RTK has the potential for lane-level navigation.

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.

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.