Analysis of the International Space Station multipath and masking environment for Automated Transfer Vehicle Relative GPS rendezvous manoeuvres

The observation of large GPS tracking errors in an International Space Station GPS receiver used in Relative GPS guided rendezvous manoeuvres, of the Automated Transfer Vehicle, led to the execution of flight campaigns to quantify and understand the GPS signal reception environment for different Solar Array configurations. The effect of masking caused by the presence of a Soyuz spacecraft in the vicinity of the GPS antenna was investigated and specific analysis of GPS pseudorange outliers was conducted. It was found that the outliers, of potential concern for rendezvous manoeuvres, could predominately be associated with single reflections from the International Space Station´s Solar Arrays, sometimes in combination with Solar Array masking. The receiver´s Delay Lock Loop was found to be captured by the reflected signal. Using the identified reflection/masking mechanism, a dynamic multipath prediction model was developed to predict the amplitude and duration of large multipath events for candidate Solar Array configurations for which flight data was not available so as to assist solar array configuration decisions for future dockings.