A New Approach to RAIM and Receiver Position Location in WAAS

P. Laxminarayana, S.N. Narahari Pandit and D.C. Reddy

Abstract:	Bancroft's method is perhaps the best-known non-iterative method for Receiver Position Location (and hence for RAIM). However, there is no built-in integrity check on range measurements and hence feed back-computation of satellite distance is the only (integrity) check (with SSE threshold) which limits its usefulness in case of obtaining ‚fast™ fixes. Further it requires the knowledge of the approximate user position, which may not always be available. Therefore a new approach is suggested which is not only free from the above shortcomings but has the advantage that it gives user position, availability of RAIM and RAIM index. Further GDOP computations are not required to check the availability of RAIM. This new technique is based on the concept of dimensionality of the ihConstellation of the visible satellites and the Userll, which says that if there exist no errors in the range/ephemeris of the satellites and measured ranges then all of them lie in a three dimensional space. The approach exploits the fact that one can obtain a ‚Dispersion like™ matrix Sk x k from, Dk x k , the matrix of squared Euclidian distances between K points taken in pairs. Knowing the position of (K-1) satellites, one can find the user position from Sk x k if its rank (RAIM index) is 3. A rank greater than three implies significant errors in ranges/ephemeris, whereas a rank less than three indicates that satellites and the user are coplanar or collinear. Both of these conditions are not conducive for determining thuser position. Next availability of RAIM can be ascertained from the rank of matrix Sk-1 x k-1 obtained from Dk-1 x k-1 by omitting the row and column of satellite-to-user measured ranges. If the rank of the Sk-1 x k-1 is effectively less than '3', then RAIM is not available. Since Dk-1 x k-1 can be computed from the predicted position of satellites, it is possible to forecast the availability of RAIM which is another outstanding advantage of the proposed method.
Published in:	Proceedings of the 2001 National Technical Meeting of The Institute of Navigation January 22 - 24, 2001 Westin Long Beach Hotel Long Beach, CA
Pages:	465 - 472
Cite this article:	Laxminarayana, P., Pandit, S.N. Narahari, Reddy, D.C., "A New Approach to RAIM and Receiver Position Location in WAAS," Proceedings of the 2001 National Technical Meeting of The Institute of Navigation, Long Beach, CA, January 2001, pp. 465-472.
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