Multi-GNSS Ambiguity Resolution as a Substitute to Obstructed Satellites in Precise Point Positioning Processing

Abstract

Global Navigation Satellite Systems (GNSS) have been undergoing major changes recently. While old constellations are getting modernized, new ones are being completed. These developments mean that more satellites than ever are in orbit and broadcasting GNSS signals. This ever-increasing number of satellites is naturally having major effects on the positioning performance of the Precise Point Positioning (PPP) measurement processing technique. Indeed, while PPP suffers from relatively long initial convergence times, it has been proved that making use of as many signals as possible helps in decreasing this PPP convergence time. On the other hand, Ambiguity Resolution (AR) has also proved to be very effective in reducing convergence time, especially when combined with multiple constellations and multiple frequencies. The purpose of this paper is to investigate the importance of AR in the current multi-GNSS context and vice versa using static, geodetic receiver measurements. At first, the effect of the number of used constellations and AR on the PPP performance is investigated. The results indicate that fixing ambiguities with fewer constellations can be more beneficial than processing more constellations without any AR. For instance, a typical fixed GPS + float GLONASS solution converges in 9.5 minutes compared to 11.5 minutes for a typical float GPS + GLONASS + Galileo + BeiDou solution. These results illustrate that AR is still relevant even with the myriad of satellites currently in orbit. The next step in the paper analyzes the relationship between the number of satellites and multi-GNSS float and fixed solutions through the simulation of obstructions. Results show an exponential decay in the convergence time as the number of satellites increases for both float and fixed solutions, following the law of diminishing returns, meaning that ever-increasing the number of satellites is not the answer to improving the PPP performance. In that case, AR is a good alternative to further reduce the PPP convergence time, as the fixed solution results in reduced (40% to 50%) convergence times for all the tested average number of processed satellites. Results also show that even with a 30° elevation mask, a multi-GNSS PPP-AR solution can be comparable and have an even shorter convergence time (17%) than a float multi-GNSS solution without any obstructions – despite the full float solution using more than 9 additional satellites compared to the obstructed solution.