Experiments in GPS Attitude Determination for Spinning Spacecraft with Non-aligned Antenna Arrays

John Carl Adams, Eric Olsen, Jonathan P. How

Abstract:	The space environment presents challenging operating conditions for a GPS attitude receiver. While spacecraft vehicle attitude dynamics may be at a lower rate than terrestrial vehicles or aircraft, they may undergo much more general attitude motions. As a result, for inertially fixed spacecraft or spinning spacecraft there is not preferred orientation of an array of antennas with aligned boresights that will give good GPS constellation visibility and ensure continuous attitude solution availability over an entire orbit [1]. For these cases of general vehicle attitude motion, continuous attitude solution availability can be achieved using an array of antennas pointing in different directions. However, a non-aligned antenna array requires modifications to the receiver signal processing algorithms to make attitude determination possible. Non-aligned antenna arrays introduce line of sight (LOS) dependent phase errors into the differential carrier phase measurements, the most significant of which are; the right hand circular polarization (RHCP) contribution to the carrier phase, antenna phase center variations, and multipath error. In previous work [2], the results of a 2-D attitude determination (in post processing) experiment using a passive spinning platform with only single degree of freedom rotation and a non-aligned antenna array were presented. This paper presents an extension of the previous demonstration to a 3-D experiment with a non-aligned antenna array. For this experiment, a spinning 3-D test platform was constructed to allow for full motion attitude sensing and future closed-loop control (see figures 1 and 2). The test platform has a non-aligned array of four antennas connected to a center node that is suspended from an overhead cable. The platform uses rate gyros as a reference attitude truth sensor. These tests are performed in a new laboratory environment with much larger transmitter distances. The experimental platform is used to demonstrate real-time attitude estimation, including real-time differential carrier phase bias estimation. A comparison of two techniques for compensating for the RHCP of the incoming GPS wave front is presented, one which explicitly includes the RHCP in the measurement model, at the cost of higher computational load, and another method that calibrates the predicted RHCP phase component out of the differential carrier phase measurements. A method for calibrating the antenna phase center variation out of the differential carrier phase measurement is demonstrated in simulation for the non-aligned antenna array.
Published in:	Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998) September 15 - 18, 1998 Nashville, TN
Pages:	1743 - 1752
Cite this article:	Adams, John Carl, Olsen, Eric, How, Jonathan P., "Experiments in GPS Attitude Determination for Spinning Spacecraft with Non-aligned Antenna Arrays," Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998), Nashville, TN, September 1998, pp. 1743-1752.
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