A New Spacecraft Attitude Determination Scheme Based bn the Use of GPS Line-Of-Sight Vectors

Pierluigi Silvestrin, Jesus Serrano, Jorge Potti, and Pelayo Bernedo

Abstract:	This paper presents the design and performances of a simple, fast, robust and reliable low-accuracy attitude determination concept based on the information pro- vided by a GPS receiver without using the carrier phase. The paper reviews different alternatives includ- ing GPS satellites visibility checking and signal level (in terms of signal to noise ratio, SNR) measurements. A detailed presentation of the selected concept is included: this uses two canted antennas and processes the SNR information provided by both antennas for the commonly tracked GPS satellites. This concept provides full attitude observability when at least two common satellites are tracked by the antennas, hence, the concept does not rely on some a-priori knowledge. The attitude determination scheme consists of a simple estimation algorithm which processes some GPS line- of-sight (LOS) vectors constructed by using the signal level measurements from the common GPS satellites tracked by two GPS antenna with a separation angle of about 10” to 20” and with a nominal orientation such that one of the antennas is pointing to the zenith in the nominal Earth pointing spacecraft attitude. The processing resources required by the designed attitude determination concept based on GPS LOS vectors are very small: the selected algorithm is very fast and the memory load is very small. The designed scheme based on GPS LOS vectors provides an estimate of the full spacecraft attitude even if the a-priori knowledge is very poor. The accuracy of the estimate mainly depends on the signal level mismodelling error level: few degrees accuracy is obtained when tenths of dB (30) SNR mismodelling errors are assumed and better than 18” accuracy is obtained for the worst case considered, namely, 3 dB SNR mismodelling error. Depending on the intended application, the concept might be improved (without increasing the complexity of the processing algorithm) with respect to the sensitivity to the SNR mismodelling errors. This can be performed by using a third antenna: with three sources of information one would be able to determine the two angles of the GPS LOS vector as well as determine the common SNR mismodelling term.
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:	1797 - 1806
Cite this article:	Silvestrin, Pierluigi, Serrano, Jesus, Potti, Jorge, Bernedo, Pelayo, "A New Spacecraft Attitude Determination Scheme Based bn the Use of GPS Line-Of-Sight Vectors," 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. 1797-1806.
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