Argos Navigation and Attitude Determination System

Jon Petway

Abstract:	The ARGOS (Advanced Research Global Observation Satellite) vehicle carries eight DOD related experiments and is scheduled for launch into a nominally circular sun synchronous orbit in March 1997. A major objective of the navigation/attitude determination subsystem design has been to reduce cost by utilizing the GPS sensor in such a way that on-board hardware, software, and ground tracking requirements are simplified. In order to meet the attitude determination accuracy requirement, our primary concern, an approach comprising two mechanizations was used. The first of these utilizes GPS information (position and velocity) to determine the transformation matrix between the 52000 Earth Centered Inertial (ECI) frame and a set of axes referred to as Local Vertical Orbit Plane (LVOP). This latter set of axes would be a locally level set of axes for a spherical (non-oblate) earth. The second mechanization of the ARGOS attitude determination system is designed to hold the spacecraft to an earth geocenter pointing attitude such that vehicle BODY axes track the LVOP axes. This is accomplished by using orbital gyrocompassing and horizon sensor measurements to detect and control attitude deviations from LVOP alignment. Attitude errors result from imperfections in both of the mechanizations. In order to demonstrate compliance with accuracy requirements, a comprehensive error analysis was conducted. For the transformation matrix component, a covariance simulation was developed. This includes both the eight state Kalman filter embedded in the GPS receiver and a high fidelity simulation of the GPS constellation. Outputs of this simulation were used to derive the errors in the EC1 to LVOP transformation matrix. For the orbital gyrocompassing/horizon sensor component, horizon sensor noise and gyro drift as specified by the vendors were used to compute RMS errors in controlling the spacecraft to alignment with LVOP axes. A backup navigation capability was implemented based upon using GPS outputs to compute a set of orbital elements while GPS is active and reliable.
Published in:	Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995) September 12 - 15, 1995 Palm Springs, CA
Pages:	563 - 572
Cite this article:	Petway, Jon, "Argos Navigation and Attitude Determination System," Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995), Palm Springs, CA, September 1995, pp. 563-572.
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