The Sentinel-6 mission aims to provide high-precision ocean altimetry measurements. Altimeter processing relies on highly accurate knowledge of satellite position and attitude. For precise orbit determination (POD), the Sentinel-6A spacecraft is equipped with a dual-constellation global navigation satellite system (GNSS) receiver and dual GNSS POD antennas. The satellite attitude is indispensable for connecting the satellite body reference frame and the inertial system. The actual satellite attitude is determined from the dual-antenna-based orbit differences and the antenna baseline information. We developed a yaw-attitude model for Sentinel-6A based on 75 days of data in 2021. Single-receiver ambiguity resolution is carried out to further explore the orbit determination accuracy. Different orbits, including the global positioning system (GPS)-only, Galileo-only, GPS/Galileo-combined orbits related to the GNSS POD antenna and orbit based on the data collected by the GNSS radio occultation (GNSS-RO) POD antenna (GNSS-RO orbit), are generated and their consistency is analyzed to investigate the impact of the attitude model and ambiguity resolution on POD. With the established model, the consistency between orbits determined using dual-antenna observations is improved by 41, 70 and 53% in along-track, cross-track and radial directions with respect to the nominal attitude. Compared with the GPS/Galileo-combined solution, the 3D RMS errors of the GPS-only and Galileo-only solutions are better than 4 and 3 mm, and a 12.6 mm RMS is obtained for the GNSS-RO orbit. The STD of satellite laser ranging (SLR) residuals for GPS/Galileo-combined solutions is improved from 14 mm with nominal attitude to 9 mm with the established attitude model. A 10 mm consistency of GNSS-RO, GPS-only and Galileo-only orbits with SLR observations is also achieved. © 2022, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.