###### Abstract
[7] investigated, for the first time, the size and stability of phase and code differential inter-system bias (ISB) between BDS-3/GPS/Galileo for tightly combined real-time kinematics (RTK). The characteristics of atmosphere and broadcast ephemeris errors in short baseline RTK were also systematically analyzed in [8]. In regard to code bias, the time-varying characteristic of the GPS differential code bias (DCB) was investigated using triple-frequency observations in [9]. As for the phase bias, [PERSON] et al. [10] constructed a new computationally efficient ambiguity acceptance test with a controllable success fix rate, namely, the fixed likelihood-ratio (FL) approach, which does not require Monte Carlo simulation. Focusing on the mathematical method of error modeling, [PERSON] et al. [11] employed independent component analysis (ICA) to remove the common mode error and select the optimal noise model for the GNSS coordinate time series by using observations from 79 GPS stations in Antarctica from 2010 to 2018. Precise orbit information for GNSS satellites is the basis for high-precision GNSS applications. In this special issue, several studies were carried out to improve GNSS POD accuracy. Aiming to shorten the interruption period of BDS broadcast ephemerides after satellite maneuverings and improve the ephemeris accuracy, [PERSON] et al. [12] proposed a two-step orbit recovery strategy based on a piecewise linear thrust model. In another contribution from [PERSON] et al. [13], the authors presented a three-step method for determining unhealthy time periods of GPS satellite orbit in broadcast ephemeris during POD to provide reliable information in near-real time. For the purpose of improving the ultra-rapid orbit accuracy for BDS-2/BDS-3, [PERSON] et al. [14] linked the predicted clock offsets into the POD as constraints to strengthen the estimation. By making use of SLR Normal Points (NPs) data only, [PERSON] et al. [15] performed the POD of BDS-2/BDS-3 and found 3D RMS values of 9-day overlaps of 0.49 and 1.89 m for BDS MEO and IGSO satellites, respectively. GNSS orbits are traditionally determined by observation data from ground stations, which usually need even global distribution to ensure adequate observation geometry strength. In order to overcome the weakness in the tracking geometry of ground stations, [PERSON] et al. [16] designed six different LEO constellations to enhance the GNSS POD and achieved a notable improvement in accuracy of over 70% compared with ground-based POD. As for the LEO POD, the potential of the use of inter-satellite link (ISL) was demonstrated in [17]. [PERSON] et al. [18] focused on the onboard real-time POD for low-cost single-frequency GNSS receivers and developed a series POD strategy. Along with up-to-date GNSS signals, equipment and precise orbit/clock products, the positioning accuracy of GNSS is undoubtedly a major concern. Relative positioning and PPP are two general positioning methods. [PERSON] et al. [19] demonstrated that the Galileo-only constellation is already able to achieve comparable PPP accuracy with GPS, and the accuracy can be further improved by combining GPS/Galileo observations or performed ambiguity resolution (AR). By making use of the fusion of three designed LEO constellation and GNSS, [PERSON] et al. [20] shortened the time to first fix (TTFF) of multi-frequency PPP AR to less than 5 minutes and over 60% accuracy improvement could be obtained with the LEO augmentation. As for relative positioning, [PERSON] et al. [21] developed an inter-system phase ambiguity fixing method based on un-difference (UD) observations by calculating the inter-system phase bias. In addition, the performance of GPS/Galileo relative positioning for bridge dynamic monitoring was demonstrated [22]. In particular, the use of GNSS in specific geoscience applications is discussed in this special issue.