Determination of global geodetic parameters using low Earth orbit constellation: performance analysis and its potential enhancement to GNSS

As one of the four space geodetic techniques, the Global Navigation Satellite System (GNSS) has been playing an increasingly important role in determining high-quality geodetic parameters, including the Earth rotation parameters (ERPs) and geocenter coordinates (GCCs). In addition to GNSS observations from ground station networks, GNSS observations from low Earth Orbit (LEO) satellites can serve as an important supplement to improve the estimation of geodetic parameters. Over the past decade, with the proposal and validation of the concept of LEO constellation-enhanced GNSS, several LEO navigation-augmentation constellation projects are being planned or constructed. Unlike traditional LEO satellite missions, LEO constellations are not only equipped with onboard receivers but also broadcast downlink navigation signals, establishing a direct link between the LEO satellites/constellations and station fixed on the Earth’s surface. The emergence of LEO navigation-augmentation constellations provides a new technological means for geodetic parameters determination and performance enhancement.

In this study, we focus on the determination of global geodetic parameters using LEO constellation and its potential enhancement to GNSS. We begin with the effect of different orbital configuration on the estimation of ERP and GCC with LEO downlink observations. Different LEO constellations, with different orbital inclination, number of orbital planes, and altitude are designed, and their performance is comparatively analyzed. For LEO orbital inclination, Walker Delta (108/9/0) constellations are designed, with the orbital inclination varying from 45° to 90°. The results indicate that the performance of ERP and GCC estimation is optimal for inclinations 65°-75°. Meanwhile, the increase of the number of orbital planes is demonstrated to be beneficial for geodetic parameters estimation, under scenarios where either the number of satellites per plane or the total number of LEO satellites is fixed. What’s more, when the orbit altitude increases from 500 to 2000 km, the formal errors of ERP and GCC estimates decrease, which is mainly due to the increased number of satellites observed by the ground stations. Nevertheless, the Root Mean Square (RMS) values of length of day (LOD) and GCC reach the minimum at the altitude of approximately 1500 km.

Based on the better-performed LEO constellation (Walker: 144/12/0, orbital altitude: 1500 km), the potential enhancement to GNSS is further investigated. The results indicate that with downlink observations, GNSS and LEO constellation exhibit different capabilities in ERP and GCC estimation, i.e., GNSS performs better in ERP estimation, while LEO constellation is superior in GCC estimation. The GPS+LEO combined solution makes the smallest formal errors, which are 4.21 uas for polar motion, 9.94 uas/d for polar motion rate, 0.64 us/d for LOD, and (0.14, 0.34) mm for the X/Y and Z component of GCC, presenting improvement of 8.1%-75.5% over GPS-only solution. At the same time, the combined solution also improve the accuracy by 10.4%, 56.5%, 62.8%, and 84.7% for polar motion, polar motion rate, LOD, and GCC, respectively, compared with the GPS-only solution.