Global Navigation Satellite System Based Coarse Attitude Determination on Small Satellites

The operation of attitude sensors on small spacecraft may be limited by unfavorable environmental characteristics or operating scenarios, reducing attitude estimate accuracy. To counter this, a single Global Positioning System (GPS) antenna on the spacecraft can be augmented to function as a coarse attitude sensor, providing additional measurements to supplement missing sensor data without adding mass, volume, or power requirements to the spacecraft. This technique is also interesting, as typical means of GPS attitude determination utilize multiple GPS antennas. In this work, carrier to noise density ration (C/N0)measurements from a GPS receiver are incorporated with commonly used attitude sensors within an extended Kalman filter to improve the accuracy of attitude estimates. This filter is evaluated using simulated three-axis magnetometer, sun sensors, and GPS measurements created from attitude telemetry from the Space Flight Laboratory (SFL) CanX-5 nanosatellite currently in orbit. Results show that under eclipse conditions where the estimator was denied sun sensor measurements, using GPS C/N0 measurements to supplement correction increased attitude estmiate accuracy by two to three times than when the spacecraft is limited to using magnetometer measurements alone. Further evaluation with flight data indicates estimate accuracy is dependent on the accuracy and precision of available C/N0 measurements