Evaluation of the GNSS Multipath Environment in Space Proximity Operations: Experimental and Simulation Studies of Code Correlations in Hubble Servicing Mission 4

摘要

When two spacecraft are docking, Global Navigation Satellite System (GNSS) navigation of the controlled spacecraft is corrupted by GNSS signals reflected off the passive target spacecraft. Rather than treating these reflected signals, or multipath, as a source of error, they can be used as a source of information about the distance of the target from the receiver. Electromagnetic (EM) ray tracing software is used to simulate the reflected signals observed during Hubble Servicing Mission 4 (HSM4) and sampled GNSS data is simulated from the reflected signal properties. Tracking the simulated data with a custom GNSS software receiver shows the impact of signal reflections on the GNSS code correlation function. Errors in pseudorange measurements are compared to multipath-induced tracking errors expected from the mission geometry. Antenna properties and front-end filtering limit the ability to make the same multipath measurements from the Global Positioning System (GPS) data collected during HSM4, however. These limitations are discussed and an approach is proposed for validating the multipath model using feasible measurements from the flight data.