LION Navigator for Transfer to GEO Using Electric Propulsion

Abstract

GNSS space receivers are widely used for onboard auton-omous navigation of spacecraft platforms in low Earth orbit. Navigation by GNSS up to geosynchronous altitude was made possible through the introduction of a Space Service Volume which defines signal strength up to geo-synchronous altitude. For Galileo, similar definitions are under consideration. On this basis onboard autonomous navigation for commercial communication satellites be-came a realistic possibility, too. Transfer to geostationary orbit is still fully depending on classical RF tracking by ground station for orbit determination. With electrical propulsion, the transfer duration extends to several months. As a consequence onboard autonomous naviga-tion by satellite navigation has become of commercial interest. A GNSS navigation receiver on a spacecraft on transfer orbit has to cope with extreme signal conditions from very low (at perigee) to very high (at super-synchronous apogee) altitude, which is far above the constellation satellites. At this altitude only very rare and weak signals that spill over the limb of the earth can be used. An addi-tional difficulty is the varying spacecraft orientation which is not nadir pointing, as is commonly assumed, but is varying according to the demands of optimal attitude guidance laws and power requirements. By using both GPS and Galileo together the availability of navigation signals is increased. The paper describes the design process to determine basic parameters e.g. number and orientation of receive anten-nas, receiver parameters like C/N0 thresholds, and naviga-tion procedures. Detailed simulations are presented for selected parts of the transfer arc using verified models of the navigation receiver. Finally the geostationary transfer capabilities of the space-borne LION Navigator GNSS receiver are demon-strated in a closed-loop real time test environment under RF stimulation.