Assessment of APL Error Model Components in Unfavorable Environments

Andreas Teuber

Abstract:	A GPS-only based navigation is not sufficient to accomplish the stringent integrity, availability and continuity requirements towards precise approach systems (CAT I-III). Therefore, Airport Pseudolites (APL) have been introduced as one option of a ground-based augmentation system (GBAS). However, some effects accompanying APL signals may even lead to distortions and finally to a deterioration of the positioning solution compared to a GPS-only solution. An error model taking these effects into account was developed [1]. The error sources considered in the error model comprise the tropospheric influence on differential APL measurements, a possible bias originated by extended dynamics of APL signals and a signal creeping error caused by ray propagation on the surface of conducting bodies in the case of a missing line-of-sight. During flight experiments at the airport of Braunschweig (Germany) an improvement in positioning could be proved using a wideband APL [1]. However, the basic conditions of the flight experiments were that “good” that the latter two error components - bias and signal creeping - could be neglected. Major objective of this work is to evaluate whether there is a necessity or not for using these two components in more unfavorable environments. Unfavorable environments mean that there may be some constraints to the GBAS site design, or rough weather conditions causing a turbulent approach, or unavailability of low-elevated satellites. Thus, simulated data have been generated. This simulation manipulates code and phase measurements like a real error would do. Finally, data samples of a real flight but with exactly predefined and thus arbitrary changes on the carrier and code observations are available. They are used as the input for the processing. Code-smoothed carrier solutions and - though not usual in aviation applications - carrier phase solutions of ten approaches were processed. First they are compared to a reference phase solution generated by a standard post processing software not capable of processing pseudolite data. Positioning accuracy of the simulated signal creeping error and the added bias is analyzed for carrier-smoothed solutions and for carrier phase solutions. The performance between both solutions is compared. For carrier-smoothed code solutions an assessment of the dependence between the number of available satellites and the deterioration of accuracy is attempted. Besides, the number of occurring cycle slips was counted and connected with the results of the positioning accuracy.
Published in:	Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004) September 21 - 24, 2004 Long Beach Convention Center Long Beach, CA
Pages:	1813 - 1821
Cite this article:	Teuber, Andreas, "Assessment of APL Error Model Components in Unfavorable Environments," Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004), Long Beach, CA, September 2004, pp. 1813-1821.
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