GPS-Enhanced Onboard Navigation System (GEONS)

While real-time positioning computed by standard GPS service is adequate for some onboard applications, inherent position discontinuities are not acceptable for high-precision instrument applications, such as view-period prediction and maneuver planning, both of which are computations that require a continuous prediction of the spacecraft state. Real-time positioning also requires simultaneous measurements from four GPS satellites, a mission-limiting factor that must be considered. GEONS processes data from standard GPS receivers, onboard communication equipment, and/or attitude sensors, producing accurate absolute and relative navigation solutions in real time. Other functions, including onboard maneuver control and relative navigation for keeping formations are also supported by GEONS. All information is quickly available to scientists when it is included in the down-linked telemetry stream. GEONS provides high-quality solutions with fewer than four visible GPS space vehicles by employing an extended Kalman filter (EKF) augmented with physically representative models for gravity, atmospheric drag, solar radiation pressure, clock bias, and drift to provide accurate state estimation and a realistic state error covariance. GEONS incorporates the information from all past measurements—carefully balanced with GEONS’ data of the physical models governing these measurements—to produce an optimal estimate of the user spacecraft’s orbit. GEONS’ high-fidelity state dynamics model reduces sensitivity to measurement errors and provides high-accuracy velocity estimates, permitting accurate state prediction during signal outages or degraded coverage. Autonomous navigation reduces total mission cost by eliminating the need for routine, ground-based orbit determination and special tracking services, and it is required for advanced mission concepts, such as satellite formation flying. GEONS was designed for autonomous operation within the very limited resources of an onboard computer. Autonomous initialization and enhanced fault detection capabilities are implemented using instantaneous geometric GPS solutions. By incorporating information from past measurements, GEONS provides highly accurate orbit estimates even with one visible GPS space vehicle—and even during signal outages or degraded coverage. This unprecedented accuracy and reliability reduces navigation errors and does it autonomously with minimal onboard computer resources. Additionally, GEONS processes Doppler measurement data from onboard attitude sensors. These different types of measurements are all incorporated into the GEONS software, producing a navigation system capable of handling multiple orbit regimes and navigation subsystems, while requiring no additional hardware. Fusion of these data types increases system stability and reliability and enables graceful degradation in the event that a component fails during orbit.