Analysis of Differential Tropospheric Delay Resulting from Vertical and Horizontal Displacements for the LAAS MOPS

Christopher Varner

Abstract:	The LAAS concept includes a precision approach service that extends out 37 km (20 nm) from the landing threshold point of a runway serviced by a LAAS Ground Facility (LGF). The concept has been recently modified to provide a differential correction service (sometimes called the position, velocity, and time service or PVT). The distance between the LAAS reference point and operational boundary of the differential correction service could extend well beyond 37 km if data link repeaters or high power data link broadcasts are used. This change in the application of the LAAS concept could affect the method selected for modeling LAAS tropospheric errors. The tropospheric model for LAAS airborne applications corrects for vertical displacements between the aircraft and the reference station but ignores the effects of horizontal displacements. This paper analyzes the tropospheric error in terms of horizontal displacements. The work was completed for RTCA SC-159 Working Group 4 and the results are used to indicate the distance at which a correction for horizontal displacements may become necessary. A Saastamoinen model is used as a truth source for the differential analysis. The model’s assumptions and limitations are discussed and the results provide an estimate of the differential tropospheric error as a function of the horizontal displacement from the LAAS reference point. The impact of differential tropospheric errors resulting from horizontal displacements can limit the operational area of the LAAS differential correction service to a few hundred kilometers. Other non-tropospheric effects may be more limiting, but were not addressed.
Published in:	Proceedings of the 57th Annual Meeting of The Institute of Navigation (2001) June 11 - 13, 2001 Albuquerque, NM
Pages:	247 - 251
Cite this article:	Varner, Christopher, "Analysis of Differential Tropospheric Delay Resulting from Vertical and Horizontal Displacements for the LAAS MOPS," Proceedings of the 57th Annual Meeting of The Institute of Navigation (2001), Albuquerque, NM, June 2001, pp. 247-251.
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