Use of a Ray-Tracing tool to predict the international space station multipath and masking environment

The Automated Transfer Vehicle (ATV) uses Relative Global Positioning System (RGPS) to navigate towards the International Space Station (ISS) while performing initial rendezvous manoeuvers. The RGPS solution includes a Global Positioning System (GPS) receiver in the ISS Zvezda module, whose observations are transmitted to, and applied by, the approaching ATV. The characterization of the GPS reception environment of the ISS is, therefore, a key element when planning rendezvous operations. The reception environment of the ISS is time-variant due to the relative motion between the ISS and the GPS satellites and to the sun-tracking orientation of the ISS Solar Array (SA) panels. In order to predict, for a given configuration, the most critical multipath and masking events, a Ray-Tracing (RT) tool has been developed at the European Space Research and Technology Centre (ESTEC) of the European Space Agency (ESA). The tool is based on Geometrical Optics (GO) and evaluates, for each multipath event, the associated effects on the pseudo-range error. The tool was validated using Flight Data Measurements (FDM) collected during two measurements campaigns which took place in 2010 and 2011, respectively. In particular, the validation activities show that the tool is able to timely predict the majority of the most critical multipath events. This paper presents the RT tool, the underlying validation activities and the prediction results which were prepared to support the rendezvous operations for the "Albert Einstein" ATV mission (June 2013). For this mission, three candidate configurations for the ISS were analyzed with the RT tool. Based on the prediction results, the configuration which minimizes the occurrence of the most critical events was also identified.