The Onboard Navigation System for the Bird Small Satellite

Abstract

The BIRD mission is a small satellite project carried out by the German Aerospace Center (DLR). Launched on October 22, 2001 its major objectives are the test of a new generation of infrared array sensors as well as the detection and scientific investigation of hot spots, like forest fires or volcanic activities. As a technology satellite, BIRD is equipped with a GPS-based autonomous navigation system which provides real-time onboard orbit determination as well as orbit and event prediction capabilities. The Onboard Navigation System (ONS) supports the BIRD Attitude Control System with real-time attitude information to allow for a nadir pointing of the spacecraft during image sessions and high-rate data downlinks. In addition, precise ONS position data are merged onboard the spacecraft with BIRD image sensor data, thus enabling a geocoding of satellite images on-the-flight. Furthermore, the ONS derives precise timing information from the GPS receiver, which allows a proper synchronization of the BIRD onboard clock based on the measured bias and the determined drift rate. Finally, NORAD twoline elements are autonomously generated onboard from GPS position data, which are downlinked to a relocatable ground terminal for antenna pointing and pass scheduling. An example of the autonomous navigation capabilities of the ONS is the onboard computation of upcoming shadow transits and station contacts, that serves as a demonstration of more fundamental tasks like the support of onboard experiment scheduling or power management. Starting with an introduction to the BIRD satellite mission, a detailed mathematical description of the algorithms applied within the ONS is given. The concept and architectural design of the implemented system is presented with emphasis on the interaction of the ONS threads and their timing characteristics, followed by a description of the ONS software modules. A series of ONS-related software and hardware-in-theloop test cycles has been conducted in the preparation of the mission, which are presented. Based on more than two months of flight experience, the performance of the ONS is analyzed in detail, demonstrating the reliable performance of the system together with an excellent accuracy performance, which provides real-time position accuracy of a few meters on-board the BIRD satellite.