GNSS-Based Precise Orbit Determination of LEO Satellites Using Un-Differenced and Double-Differenced Observations

One of important applications of the Global Navigation Satellite System (GNSS) in space is the precise orbit determination (POD) of satellites, particularly that of Low-Earth Orbit (LEO) satellites. Over time, progressively more LEO satellites equipped with GNSS onboard receivers will be in orbit. In order to satisfy both the absolute and relative accuracy of satellite mission requirements, efficient and precise determination of the LEO satellite orbits becomes a challenging problem. Currently, GNSS-based POD of LEO satellites can be performed using either un-differenced (UD) or double-differenced (DD) GNSS observations. Usually, precise absolute orbit accuracy of LEO satellites can be obtained using UD observations or high-low DD carrier-phases formed by two GNSS satellites, one ground station and one LEO satellite. The precise relative orbit accuracy can be obtained by using high-high DD carrier-phases formed by two GNSS satellites and two LEO satellites. If both UD and high-high DD observations are used for GNSS-based LEO POD, the orbits should be both absolutely and relatively well determined. Based on this motivation, we investigate how to use both UD and DD observations for GNSS-based LEO POD. For our investigation, we utilize real data from the GRACE (Gravity Recovery and Climate Experiment) satellites. The orbit accuracy is assessed using a number of tests, which include analysis of orbital fits, Satellite Laser Ranging (SLR) residuals, K-Band Ranging (KBR) range residuals and external orbit comparisons. Both the SLR residuals and external orbit comparison indicate that the 3-D orbit accuracies are about 2 cm. The external orbit comparison also shows that the radial and normal orbit accuracy is better than 1 cm. According to the KBR range residuals, the relative accuracy between the two GRACE satellites is about 2 mm. Therefore, based on the proposed approach, we conclude that the orbits for LEO satellites can be precisely determined -with both absolute and relative accuracy, without the use of global reference stations.