GPS Carrier-Phase RAIM

Duncan Cox Jr., William Michalson and Hua Hua

Abstract:	Carrier phase data from four or more GPS satellites can be used to determine the evolution of the trajectory of a vehicle from an arbitrary initial position with great precision and high update rate. The accuracy is limited primarily by incremental errors in satellite position and time, ionospheric and tropospheric delay, multipath, and SA. If estimates of the primary components of satellite and propagation errors are provided through LAAS or WAAS correction messages, then phase-based techniques can allow a user to accurately determine their trajectory evolution at the correction epochs. Carrier phase data from five or more GPS satellites can be used to determine the initial position associated with the above trajectory. The sensitivity of the position estimation errors to the measurement errors decreases as changes in satellite geometry evolve. Increasing the number of satellites generally improves the rate of sensitivity reduction (seven or more satellites results in rapid convergence). Once the initial position has been determined to a certain level of accuracy, the accuracy is essentially preserved as long as four or more satellites are available with suitable geometry. Carrier phase data from five or more GPS satellites can be used to provide redundant estimates of trajectory evolution and, hence, can be used to detect new (incremental, since the start of the trajectory) violations of integrity criteria. We call this “incremental RAIM.“[ l] Carrier phase data from six or more GPS satellites can be used to provide redundant estimates of the trajectory’s initial position and, hence, can be used in conjunction with incremental RAIM to provide absolute RAIM alarms, Once the initial integrity is established, absolute RAIM can be continued with incremental RAIM, requiring live or more satellites (not necessarily the same ones) with suitable geometry. Because of the enhanced resolution of the phase data, in comparison with that of pseudorange data, the carrier- phase RAIM system can be used to enhance the performance of conventional RAIM. The amount of improvement depends upon the availability of suitable error-correction data from whatever augmentation system is being utilized. The correction data from a WAAS that is designed for only pseudorange data may be sufficient for carrier-phase RAIM to be a useful augmentation. In this case, the carrier-phase-RAIM algorithm would incorporate accumulated incremental changes in the WAAS corrections, with the changes in ionospheric &lay reversed in sign to account for the ionospheric phase advances corresponding to the ionospheric group delays. However, modifying the WAAS or LAAS corrections may allow further improvements in carrier-phase RAIM performance. This paper is an extension of the discussion presented in [ 11. In contrast to this earlier discussion, here we assume that suitable corrections are received from either a WAAS or LAAS augmentation. In the following sections we present a description of the underlying theory for a carrier-phase RAIM system that is under development. In addition, some preliminary results obtained using the carrier-phase algorithm in the presence of satellite failures is presented. These results suggest that carrier-phase RAIM is possible for Category I approach. This concept is expected to be important for the successful evolution of future GPS WAAS and LAAS systems.
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:	1975 - 1984
Cite this article:	Cox, Duncan, Jr., Michalson, William, Hua, Hua, "GPS Carrier-Phase RAIM," 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. 1975-1984.
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