BAYERNSAT – A REAL-TIME COMMUNICATION- ARCHITECTURE FOR INTERACTIVE EARTH OBSERVATION VIA GEOSTATIONARY INTER SATELLITE LINK FOR SMALL SATELLITE SYSTEMS Jürgen Letschnik, Matthias Raif, Kristian Pauly, Ulrich Walter Technische Universität München, Institute of Astronautics Boltzmannstr 15, D-85748 Garching - Germany Tel: +49(0)89-289-16007, e-mail: j.letschnik@lrt.mw.tum.de ABSTRACT In this article we propose the use of geostationary relay satellites for interactive Earth observation in real time for small satellite missions. This would provide interactive link times of at least 50 minutes during every orbit rather than those max. 7 minutes link times available for paths over ground stations. However, a drawback of such real-time communication via relay satellites is that the round-trip time of up- and downlink easily exceeds one second. This is because the large distances between spacecraft and ground segment to the relay satellite already make up a round-trip time of at least 0.5 seconds. In order to get as close as possible to this physical limit, we propose to split up the up- and downlink transmission to the relay satellite using two different ground segments. The crucial point of our concept is to minimize the time delay between the Downlink Ground Segment (DGS) and the User Observation Center (UOC). This is accomplished by physically locating the DGS as close as possible to the UOC. This may imply the Uplink Ground Segment (UGS) and the DGS to be several hundred kilometers apart. Furthermore we propose to split the transmitted payload data from the satellite into critical and non-critical data packets thereby eliminating redundancies. 1. INTRODUCTION Improved payload technology together with lower project costs have significantly ex- panded the fields of application for small satellites. For interactive Earth observation missions a conservative solution would require a network of ground stations across the globe. For a small satellite mission the cost of such a ground station network would easily amount to several times the cost of the satellite alone, thus exceeding the very limited financial budget of small satellites missions. However, an interactive observa- tion mission with small satellites could be accomplished quite easily by employing a geostationary satellite as relay. This concept is already been used for large spacecrafts (Shuttle) and satellites (Spot 4 or EnviSat [3]). A major drawback of current inter- satellite communication architectures is the delay time from the ground segment to the spacecraft and back again. This so-called round-trip time can add up to several sec- onds due to computation, hardware and protocols involved [2]. The communication architecture proposed here tries to achieve both long-duration links of a minimum of 50 minutes/orbit and round-trip times of less than one second. In the following we demonstrate how the round-trip time can be minimized in such a way that interactive real-time observation becomes possible. This concept has been developed