Enhanced Orbit Determination of GNSSs with Optical Inter-satellite Links

Abstract

Optical Inter-Satellite Links (OISLs) connecting the Medium Earth Orbit (MEO) satellites of the next-generation GNSSs carry the potential to provide highly precise ranging at the mm to sub-mm level, synchronization of satellite clocks with ps-level accuracy and data communication at 50 Mbps as proposed in the context of the Kepler system. In this paper, we quantified the enhancement in Precise Orbit Determination (POD) and Signal-in-Space Range Errors (SiSREs) for different topologies of OISLs between MEO satellites. We studied the impact of OISL scheduler constraints, orbit modelling errors and observation biases with a full-scale simulation of the Galileo constellation in an scenario where each navigation satellite is additionally equipped with two Laser Communication and Ranging Terminals (LCRTs) for precise inter-satellite ranging and clock synchronization. The POD results and Signal-in-Space Range Errors (SiSREs) were compared to the baseline Galileo scenario with L-band data from ground network of 18 Galileo Sensor Stations (GSSs). We found that SiSRE is robust to OISL scheduler constraints such as the maximum link length and persistence, allowing optimization in terms of required transmit power. SiSRE values of 1-3 mm were obtained with perfect models, an order of magnitude smaller than 1.4 cm obtained with Galileo constellation. When considering large modeling errors in the POD process, we show that a SiSRE value of 1cm is still in principle attainable. This suggests that the OISLs between MEO and Low Earth Orbit satellites, originally considered in the Kepler system, are not that essential to achieve a good POD performance. The OISLs and synchronized clocks enable POD with ground network reduced to just one station under the assumption that station coordinates, the Earth's rotation parameters and the satellite antenna phase center offsets are not considered in the estimation. SiSRE values in this case are close to those obtained with 18 GSSs. We also studied the impact of OISL range biases and ionosphere-free pseudo-range group delay (GD) errors on POD, which showed a significant influence on SiSRE. However, we could demonstrate that these biases can be precisely estimated in the POD process. The OISL range biases could be estimated with a few mm accuracy or better, depending on the modelling errors. A few mm estimation precision was also obtained for the GDs in a POD scenario with one ground station, high-rate L-band data and the station clock synchronized to the satellite clocks. These biases are not expected to be a significant limiting factor for SiSRE in the proposed scenarios.