The Automated Transfer Vehicle (ATV) is part of the European contribution to the International Space Station (ISS) program. The main tasks of the ATV are to provide logistics supply, station re-boost and ISS waste retrieval. The rendezvous of the ATV and ISS is based on a real-time onboard relative navigation concept, using GPS data from receivers of ISS and ATV. The Navigation Support Office conducts in this context simulations before the flight and also post facto performance analysis of the relative orbit determination accuracy in order to support the ATV missions.