Angles of Arrival Tracking of GPS Signals With Digital Beamforming Monopulse For Attitude Determination

Chun Yang and David M. Lin

Abstract:	This paper presents the preliminary test results of tracking GPS signal’s angles of arrival (AOA) using the digital beamforming monopulse technique with an array antenna. The AOA estimates provide direct measurements of the line of sight (LOS) vectors in the array frame, which can be used for attitude determination. One advantage of this monopulse technique for GPS attitude determination is its ability to incorporate adaptive array processing so as to suppress interference that is difficult to realize with the conventional interferometry technique. In this paper, the prototype array antenna system is first described, which consists of a custom-made small linear array with four patch antennas, a custom-made four channel coherent RF front-end, a data acquisition system, and a GPS baseband signal processor. Key parameters of the digital beamforming monopulse technique are determined for the small array, together with the theoretical prediction of the angular accuracy as a function of signal to noise ratio (SNR) for a given beamwidth. Two approaches, namely, a conventional tracking loop and a Kalman filter, are then presented for the design of an angle-locked loop (ALL). Field test results are then presented to demonstrate the performance of the digital beamforming monopulse technique operating on real GPS signals. Amplitudecomparison monopulse and phase-comparison monopulse are compared for the antenna array with and without the channel biases compensated for. Angular tracking loops are then evaluated in terms of the updating rate, the loop filter parameters (order and bandwidth), and the design methods. Finally, the estimates of the off-boresight angle (OBA) by monopulse are compared with other three methods including the ephemeris-derived “reference.” Statistics calculated over time and across satellites indicate that the monopulse-estimated spatial angular accuracy agrees with the theoretical prediction for the SNR levels of our field tests.
Published in:	Proceedings of the 60th Annual Meeting of The Institute of Navigation (2004) June 7 - 9, 2004 Dayton Marriott Hotel Dayton, OH
Pages:	521 - 532
Cite this article:	Yang, Chun, Lin, David M., "Angles of Arrival Tracking of GPS Signals With Digital Beamforming Monopulse For Attitude Determination," Proceedings of the 60th Annual Meeting of The Institute of Navigation (2004), Dayton, OH, June 2004, pp. 521-532.
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