Limitations in Altimeter Aiding of the Navstar/GPS Constellation

Barry A. Stein, Douglas E. Giordano

Abstract:	The 18 and 21 satellite constellations envisaged for GPS yields occasional geometric configurations that result in poor navigation solutions. It has been suggested that altimeter aiding be used in these cases in order to improve the geometry. Analysis shows that under vertical, coplanar conditions the use of altimeter aiding is not feasible. A simulation model has been developed to include the orbital motion of the satellites and to determine when altimeter aiding cannot be utilized. Investigations have been carried out for cases where 4 visible satellites give poor GDOP values and for cases where only 3 satellites are available due to satellite failure, self-shadowing, or terrain masking. All possible cases of a single satellite failing have been studied. The masking and shadowing problems have been investigated parametrically. GDOP is calculated by a derived modification to the standard GnOP matrix elements by including an altimeter/range error ratio. The solution is obtained utilizing matrix manipulation techniques. Calculations have been included to determine coplanar convergence between the user and the satellites, as well as the verticality of the resultant plane in order to ascertain sensitivity to these parameters. Results indicate that altimeters can aid in improving the 21 satellite constellation degradation areas if appropriate satellite selection criteria are utilized. Exceptions to successful altimeter aiding occur when certain key satellites either physically fail or are shadowed, with the former being the more critical. The performance degradation occurs for time durations ranging from 2 to 26 minutes. Numeric data are presented in tabular and graphic formats. The latter are in the form of 3-n topographic maps which indicate failure location, size of the area involved, duration of the time the degradation persists, movement of the degradation areas during a 24 hour cycle, translation of the degradation areas when movement with the user occurs, and the severity of GDOP increase. This latter element is specifically depicted by the relative shapes on the topographic map.
Published in:	Proceedings of the 1985 National Technical Meeting of The Institute of Navigation January 15 - 17, 1985 San Diego, CA
Pages:	177 - 182
Cite this article:	Stein, Barry A., Giordano, Douglas E., "Limitations in Altimeter Aiding of the Navstar/GPS Constellation," Proceedings of the 1985 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 1985, pp. 177-182.
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