The Impact of Measurement Biases on Availability for CAT III LAAS

Jason Rife and Sam Pullen

Abstract:	"The original error description for the Local Area Augmentation System (LAAS) assumed unbiased measurement errors. In practice, however, satellite ranging errors may exhibit nonzero means. The LAAS error bound must account for these biases. This requirement places a particularly stringent demand on Category III (CAT III) LAAS, a future LAAS variant that will enable safe automated landing through rollout. This paper examines the availability impact of biases on CAT III LAAS. To provide an upper limit for tolerable LAAS biases, the paper first considers an improved LAAS messaging structure and asks, for this ideal case, what maximum level of bias is tolerable. Even in the ideal case, the maximum bias at worst-case CONUS airports is only 4-8 cm. This baseline case assumes a nominal LAAS error distribution and a relaxed Vertical Alert Limit (VAL) of 10 m. To tolerate larger biases, improvements to the nominal LAAS are required. For instance, a dual-frequency LAAS implementation can tolerate biases as high as 14-30 cm, even with a tighter VAL of 5.3 m. Tolerable biases for nonideal bounding algorithms are also considered. In order to achieve ideal performance, the LAAS message must be changed to transmit bias parameters to the airborne user. Preferably, the biasprotection algorithm would not require modification of the existing LAAS specification. Sigma inflation methods represent an alternative to bias-parameter transmission that requires no LAAS specification change but, consequently, that delivers somewhat lower bias tolerance. Performance varies widely for different classes of sigma inflation. Simulations indicate that the bestperforming sigma inflation method (real-time positiondomain inflation) nearly matches the bias tolerance possible with bias-parameter transmission. Unlike biasparameter transmission, however, the bias tolerance for sigma-inflation methods depends strongly on possessing precise knowledge of the airborne error, sair, at the LAAS ground facility (LGF)."
Published in:	Proceedings of the 61st Annual Meeting of The Institute of Navigation (2005) June 27 - 29, 2005 Royal Sonesta Hotel Cambridge, MA
Pages:	759 - 773
Cite this article:	Rife, Jason, Pullen, Sam, "The Impact of Measurement Biases on Availability for CAT III LAAS," Proceedings of the 61st Annual Meeting of The Institute of Navigation (2005), Cambridge, MA, June 2005, pp. 759-773.
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