Abstract: Objectives: The Sentinel-6A spacecraft is equipped with a GPS/Galileo dual-constellation global navigation satellite system (GNSS) receiver which provides an opportunity to investigate the precise orbit determination (POD) accuracy of low earth orbit (LEO) satellites based on multi-GNSS. Ambiguity resolution plays an important role in GNSS-based precise positioning and orbit determination. The single receiver ambiguity resolution is explored and the GPS/Galileo measurements are combined to further improve the kinematic orbit determination accuracy. Methods: Observation specific bias (OSB) product is employed to calibrate the satellite dependent phase delay, and single difference (SD) observation between GNSS satellites is applied to remove the phase delay of receiver. Combined with the related GNSS precise orbit and clock products, the wide lane and narrow lane ambiguities are fixed to integers. Then the SD ionosphere free (IF) ambiguities are recovered with the fixed ambiguities and are taken as pseudo observations to constrain the undifferenced IF ambiguities. The effect of GPS/Galileo combination and ambiguity resolution on kinematic orbit determination is analyzed with Sentinel-6A onboard data. GNSS products provided by the Center for Orbit Determination in Europe (CODE), Centre National d'Etudes Spatiales (CNES), German Research Centre for Geosciences (GFZ) and Wuhan University (WHU) are used for single receiver ambiguity resolution and POD. Different kinematic orbits including GPS only, Galileo only and GPS/Galileo combined solutions are generated. The reduced dynamic orbits with ambiguity resolution are also calculated to assess the accuracy of kinematic orbits. Results: Results show that the visible satellites and position dilution of precision (PDOP) are significantly improved in dual-GNSS solution. The three dimensional (3D) accuracy of the dual-constellation kinematic orbit with float ambiguity achieves 30 mm and shows an improvement of 20% when comparing with the GPS-only result. Fixing the ambiguity to integer significantly improves the POD accuracy. The 3D accuracy of the GPS/Galileo ambiguity fixed orbit is 20 mm, which is 30% better than that of the GPS-only result. With the products of CODE, CNES and GFZ, more than 93% of the GPS and 95% of the Galileo ambiguities are successfully fixed and is further improved to 97% in the case of dual-GNSS solution. The ambiguity fixing rate shows degraded performance when using the WHU product. Independent satellite laser ranging (SLR) observations are used to validate the kinematic orbits. The root mean square (RMS) of SLR residuals of GPS-only solution with fixed ambiguity is 13-15 mm while it is 12-14 mm for the orbits derived with dual-constellation observations and an average improvement of 10% is achieved by introducing the Galileo data. Conclusions: Fixing the ambiguity to integers improves the accuracy and stability of POD results. Compared with the GPS-only solution, GPS/Galileo combined solution improves the ambiguity fixing rate which then leads to an improvement of kinematic orbit determination accuracy.