Abstract
The space segment of the European Global Navigation Satellite System (GNSS) Galileo consists of In-Orbit Validation (IOV) and Full Operational Capability (FOC) spacecraft. The first pair of FOC satellites was launched into an incorrect, highly eccentric orbital plane with a lower than nominal inclination angle. All Galileo satellites are equipped with satellite laser ranging (SLR) retroreflectors which allow, for example, for the assessment of the orbit quality or for the SLR–GNSS co-location in space. The number of SLR observations to Galileo satellites has been continuously increasing thanks to a series of intensive campaigns devoted to SLR tracking of GNSS satellites initiated by the International Laser Ranging Service. This paper assesses systematic effects and quality of Galileo orbits using SLR data with a main focus on Galileo satellites launched into incorrect orbits. We compare the SLR observations with respect to microwave-based Galileo orbits generated by the Center for Orbit Determination in Europe (CODE) in the framework of the International GNSS Service Multi-GNSS Experiment for the period 2014.0–2016.5. We analyze the SLR signature effect, which is characterized by the dependency of SLR residuals with respect to various incidence angles of laser beams for stations equipped with single-photon and multi-photon detectors. Surprisingly, the CODE orbit quality of satellites in the incorrect orbital planes is not worse than that of nominal FOC and IOV orbits. The RMS of SLR residuals is even lower by 5.0 and 1.5 mm for satellites in the incorrect orbital planes than for FOC and IOV satellites, respectively. The mean SLR offsets equal -44.9, -35.0, and -22.4 mm for IOV, FOC, and satellites in the incorrect orbital plane. Finally, we found that the empirical orbit models, which were originally designed for precise orbit determination of GNSS satellites in circular orbits, provide fully appropriate results also for highly eccentric orbits with variable linear and angular velocities.
Highlights
Galileo is the European Global Navigation Satellite System (GNSS) that is currently at the deployment stage
Tracking high-orbiting GNSS satellites poses a challenge for laser stations: on the one hand due to a weak returning signal in the presence of background noise especially during the daytime and on the other hand due to an increasing number of targets scheduled for satellite laser ranging (SLR) tracking
The opposite pattern is found for Galileo satellites when using ECOM1: The daytime tracking is associated with the positive residuals of about +100 mm, whereas the nighttime tracking is associated with negative residuals of −150 mm
Summary
Galileo is the European Global Navigation Satellite System (GNSS) that is currently at the deployment stage. Two early test satellites, called Galileo In-Orbit Validation Element-A (GIOVE-A) and GIOVE-B, were launched in 2005 and in 2008, respectively. These test satellites were used to validate critical technology components, as well as to secure the Galileo frequencies (Montenbruck et al 2015a). GIOVE-A and GIOVE-B were decommissioned in mid-2012 after the successful launch of their successors, i.e., the first pair of full-featured In-Orbit Validation (IOV) Galileo spacecraft (Steigenberger and Montenbruck 2017). Four IOV spacecraft were launched between 2011 and 2012. In 2014, the first pair of Full Operational Capability (FOC) Galileo satellites was launched on a Soyuz ST rocket, into wrong, highly eccentric orbits.
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