SGPS ATTITUDE DETERMINATION

Spacecraft attitude systems usually consist of two components:

- vehicle orientation sensors: e.g. magnetometers, horizon sensors, sun sensors, gyroscopes, and star trackers; and

- active attitude control actuators: e.g. control momentum gyros, reaction wheels, offset thrusters, and magnetic torque rods.

GPS provides attitude and attitude rate data to actuators for real-time, autonomous attitude determination.

Benefits of GPS:

The elimination of different sensors and their interfaces. This in turn can reduce costs, power requirements, weight, and complexity, and increase system reliability.

Methodology:

An array of GPS antennas (e.g. four) are placed on the rigid structure of a spacecraft. Software controlled multiplexing allows for signals being received at all antennas to reach a single receiver, or signals from different antennas are processed by diiferent channels in the receiver. The carrier phase measurements are used to determine differential range between the "master" antenna and each of the other antennas in the direction of the line of sight from a GPS satellite to the master antenna. The components of baseline vectors between antennas in the spacecraft body-fixed frame and integer ambiguity resolution are required. Additional on-board code computes attitude from successive carrier phase measurements for real-time determinations. A Kalman filter can be used to incorporate data about the vehicle dynamics to improve the attitude estimates.

Mission examples:

- The CRISTA-SPAS mission, using a TANS Vector receiver, for a 24 hour period produced rms attitude variations around mean motion of 0.19 degrees in roll, 0.15 degrees in pitch and 0.26 degrees in azimuth.

- The RADCAL mission produced attitude determination accuracy estimated to be 0.5 degrees rms.

- The GADACS / SPARTAN OAST Flyer mission results have not been published yet; however, the attitude determination estimates from the systems two TANS Vectors is expected to be on the order of 0.1 degrees to 0.5 degrees.

- The planned Gravity Probe B mission will use a Tensor receiver as a back-up attitude sensor with an accuracy requirement of 0.1 degrees rms over one second.

References:

Bisnath, S. and R.B. Langley (1996) "Assessment of the GPS/MET TurboStar GPS receiver for orbit determination of a future CSA micro/small-satellite mission." Final report by the Department of Geodesy and Geomatics Engineering, University of New Brunswick, Frederiction, N.B. for the Directorate of Space Mechanics, Space Technology Branch of the Canadian Space Agency, St-Hubert, Que. under Public Works and Government Services Canada Contract No. 9F011-5-0651/001/XSD, July, 188 pp.

Lightsey, E.G. (1996) "Spacecraft attitude control using GPS carrier phase." In Global Positioning System: Theory and Applications Volume II. (Eds.) B.W. Parkinson, J.J. Spilker Jr., P. Axelrad, and P. Enge, AIAA, Washington, D.C., pp. 461-480.

Related Internet Sites:

CSR, University of Texas ICESat/GLAS Algorithm Theoretical Basis Documents (ATBD)

Due to the rapid developments in the field of spaceborne GPS and the aerospace industry in general, any comments, information or corrections pertaining to information on this site are welcome and encouraged.