Sigma Estimation, Inflation, and Monitoring In the LAAS Ground System

Boris Pervan, Sam Pullen and Irfan Sayim

Abstract:	The prescribed Local Area Augmentation System (LAAS) algorithms for the generation of vertical and horizontal protection levels implicitly assume a zero-mean, normally distributed fault-free error model for the broadcast pseudorange corrections. While such a model appears to be consistent with reference receiver ranging errors due to thermal noise and diffuse multipath, remaining errors such as ground reflection multipath and systematic reference receiver/antenna errors may be non-gaussian, non-zero-mean, or both. To ensure navigation integrity, special care must therefore be taken on the ground in the definition of the broadcast correction error standard deviation (sigma_pr_gnd). In this regard, this paper defines a new approach toward the estimation and inflation of the broadcast value of sigma_pr_gnd to account for both ground reflection multipath errors as well as gaussian errors due to thermal noise and diffuse multipath. A sufficient condition for the acceptability of non-zero means is also presented. An additional requirement concerning the broadcast sigma_pr_gnd is that it be possible to monitor for unexpected violations (failure events where, during operation, the true sigma exceeds sigma_pr_gnd) with acceptable residual integrity risk. The use of a ‘real-time’ sigma estimate as a monitor statistic is undesirable because the real-time estimate is slow to respond to actual sigma violations due to the averaging required to reduce the estimate error. A better approach based on CUSUM statistical process control is described in this paper. Because the CUSUM method is quicker to detect sigma increases from nominal to levels above the inflated value of sigma_pr_gnd, it significantly reduces the residual risk posed by potential sigma_pr_gnd violations.
Published in:	Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000) September 19 - 22, 2000 Salt Palace Convention Center Salt Lake City, UT
Pages:	1234 - 1244
Cite this article:	Pervan, Boris, Pullen, Sam, Sayim, Irfan, "Sigma Estimation, Inflation, and Monitoring In the LAAS Ground System," Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000), Salt Lake City, UT, September 2000, pp. 1234-1244.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In