Carrier Differential GPS for Real-Time
Control of Large Flexible Structures

E. Harrison Teague, Jonathan P. How, Bradford W. Parkinson

Department of Aeronautics and Astronautics
Stanford University

ABSTRACT

Current lab tests demonstrate vibration and orientation control of highly flexible vehicles using only the GPS carrier to measure motion. A 30-foot long test structure has been constructed that is suspended from above, and moves in an analogous way to a flexible orbiting platform. The test structure is outfitted with an array of GPS antennas for motion sensing, and an array of compressed air thrusters for control actuation. The sensor has shown better than 0.5 degree rotational accuracies as measured with respect to on board rate gyroscopes. We have shown simultaneous rigid-body orientation and elastic vibration control by closing a feedback loop from the GPS differential carrier phase (DCP) measurements to the thruster commands.

Central to the success of the GPS deformation sensing system is the sub-centimeter level differential position information available from GPS carrier tracking. However, it is challenging to use this accuracy due to inherent, and arbitrarily large, measurement biases. Bias resolution (akin to cycle ambiguity resolution) has been solved for vehicle navigation and attitude determination problems. We show bias estimation for an antenna array mounted on a structure whos relative antenna motions due to flexibility are on the same order of magnitude as those due to overall attitude changes.

This research is a significant step toward general distributed antenna array GPS carrier based sensing systems. Our techniques are applicable to systems that exhibit relative motions with frequencies (<10) Hz and deflections (>1 cm) that are detectable by current receivers.

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