## 1 Introduction
GRACE baseline accuracy is improved from 6 mm of float ambiguity solution to 2 mm of ambiguity fixed solution. Based on GPS orbit, clock, and WL bias products provided by Centre National d'Etudes Spatiales (CNES) [26], single receiver AR is also employed to generate the orbit of Sentinel-3A satellite and the root mean square (RMS) of SLR residuals decreases from 9 to 5 mm [15]. Using the observation specific bias (OSB) products provided by the Center for Orbit Determination in Europe (CODE), AR is applied to the POD of GRACE and Sentinel-3 satellites [33]. Ambiguity fixing improves the orbit quality with validation of KBR and SLR residuals, as well as the internal consistency between the reduced dynamic and kinematic orbits. With UPDs estimated via a global distributed network, kinematic orbits of Sentinel-3A and Swarm-A satellites are determined with AR [34]. SLR residualsshow an improvement of 20% for AR solution when compared with the FA solution. Different AR solutions, including DD AR, single difference (SD) AR, and integrated SD and DD AR solutions are investigated with GRACE data to access their effects on orbit accuracy [35]. Another factor affecting the orbit accuracy of the LEO satellite is the phase center position of the spaceborne GNSS antenna. Nominal antenna models obtained from ground calibration have been made available for the antennas deployed on several space missions, such as GRACE [36], GOCE [37], Swarm [38], and TerraSAR-X [39]. Such nominal antenna models, however, do not reflect the influence of error sources, which are additionally encountered in the actual spacecraft environment, e.g., the influence of near-field multipath [36]. In-flight calibration of the LEO antenna is necessary for the stringent orbit accuracy requirement, especially for the altimeter mission. Two different approaches, the residual approach and the direct approach, can be used to determine the empirical PCV correction of the LEO antenna. Using the residual approach, the PCV map of JASON-1 satellite is created. The mean post-fit phase residual is decreased from 8 to 5 mm, and 1 cm radial accuracy is demonstrated [40]. PCV correction of the GOCE satellite is determined with 154 days of data and the consistency of reduced dynamic and kinematic orbits is improved when applying the PCV map [37]. An error in the given phase center offset (PCO) of Sentinel-1A antenna has been found by comparing different PCVs and different POD approaches [14]. The influence of relative PCV on precise baseline determination (PBD) of formation flying satellites is also studied [41]. With application of the generated relative PCVs of GRACE and GRACE-FO satellites, the consistency of KBR measurements is improved by 30%. The effects of antenna PCV on orbit determination and clock estimation for CentiSpace-1 satellite, which served as a navigation satellite, were analyzed [42]. The twin GRACE-FO satellites (named GRCC and GRCD hereafter), designed as a successor of GRACE, and jointly developed by the National Aeronautics and Space Administration (NASA) and the German Research Centre for Geosciences (GFZ), were launched from California, USA, on May 22, 2018 [43]. The main goals of GRACE-FO are to continuously provide high resolution monthly solutions of the Earth's gravity field, surface mass change, and to measure the vertical temperature and humidity profiles of the Earth's atmosphere [44]. GRACE and GRACE-FO satellites adopt the same satellite appearance design, and are equipped with the KBR system, GPS receiver, SLR retroreflector, star camera assembly (SCA), and other scientific instruments [44,45]. The appearance and instrument installation position of GRACE-FO satellites are illustrated in Figure 1[46]. Effects of PCV correction on POD of GRACE-FO satellites have been explored [35,43]. However, impacts of different ambiguity resolution strategies and in-flight PCV calibration on POD and PBD still need more investigation. In this analysis, antenna PCV models of GRACE-FO satellites are developed to further exploit the POD accuracy.