GPS-Based Autonomous Navigation for the BIRD Satellite

The German micro-satellite BIRD will be equipped with an autonomous navigation system that provides real-time onboard orbit determination and prediction functions based on GPS position fixes. The navigation system design is presented together with a description of the algorithms required to achieve a position and velocity accuracy significantly better than the performance of the GPS position fixes. The BIRD onboard navigation system enhances the spacecraft autonomy as it enables for the first time a geocoding of satellite images on-the-flight. Accordingly, the post-processing and ground operations workload is reduced, that renders the system especially attractive for future highly autonomous satellite missions. An integral part of the BIRD Attitude and Orbit Control System is the Rockwell Collins GEM-S (GPS embedded Module) five channel C/A- and L1 P-code receiver. GEM-S provides instantaneous position fixes that are treated as measurements within the onboard orbit determination and pediction process. The GPS receiver is efficiently commanded making extensive use of macro commands, that significantly reduce the uplink data volume. In addition, an onboard GPS almanac storage will allow for a time-to- first-fix of about 20 s, even from a receiver cold start. The autonomous navigation system is furthermore equipped with a backup onboard orbit predictor based on NORAD twoline elements to guarantee the availability of orbit positions even for failures of the GPS receiver or the orbit determination process