Abstract: In order to evaluate the performance of maritime single point positioning (SPP) performance, taking into account the fact that the broadcast satellite clock correction corresponds to the ionosphere-free (IF) combination or single-frequency B3, a multi-system single-frequency and dual-frequency IF SPP model is derived. The IF and single-frequency SPP positioning accuracy of GPS, GPS/Galileo, GPS/BDS and GPS/Galileo/BDS were evaluated using a set of offshore multi-system GNSS data measured by the unmanned surface vehicle (USV). The results show that compared with the GPS-only system, the SPP positioning performance after the system combination is improved with the plane accuracy of the GPS/Galileo/BDS IF SPP being better than 0.5 m, and the vertical direction accuracy being better than 0.6 m. Compared with the single-frequency SPP, the IF SPP has an improved positioning accuracy of about 1.0 m and 1.3 m in the horizontal and vertical directions, and the IF SPP has better positioning performance.