Gps/ins/star Tracker Navigation Using a Software Defined Radio

NAVSYS Corporation and Microcosm, Inc., have jointly developed a design for a flexible, high-performance, miniaturized, space-based Software GPS Receiver (SSGR) based on a Software Defined Radio (SDR) architecture that optimally combines GPS, INS, and star-tracker inputs to provide a flexible, integrated precision navigation and attitude determination solution for space applications. The SSGR is suitable for supporting multiple space missions including GPS metric tracking during launch, orbit determination during transfer to geostationary orbits, and high accuracy navigation, attitude control and timing. The flexibility of the SSGR design allows it to be re-programmed for use in launch and orbit entry, station-keeping and autonomous orbit estimation applications. In this paper we present a system design, analysis, and test results for the integrated SSGR navigation system. The filter design for the optimal integration of GPS, INS, and star-tracker measurements is presented along with simulation results that show predicted performance. In the design of a space-based GPS receiver, difficulty comes in testing, since the dynamics involved are radically different from anything achievable on the ground. Commercially available GPS simulators that are capable of simulating the space environment are very expensive, generally have high learning curves, and are limited in capability and flexibility. The multi-element Advanced GPS Hybrid Simulation (AGHS) capability available at NAVSYS addresses many of these concerns. As part of this effort, NAVSYS has modified its AGHS to support simulated star-tracker measurements for real-time receiver testing in space-based trajectories. Receiver test results using the AGHS are presented to validate performance predictions and demonstrate the benefits of the combined GPS, INS, and startracker approach.