Flight Test Results of an Integrated Wideband Airport Pseudolite for the Local Area Augmentation System

Chris G. Bartone and Sai Kiran

Abstract:	The Local Area Augmented System (LAAS) is a differential GPS (DGPS) precision approach system that is based on a carrier-smoothed code architecture [1]. Current performance predictions of the LAAS do not satisfy the availability requirements for CATII/III precision approaches, without additional augmentation. Airport pseudolites (APLs) are being developed for LAAS CATII/II installations to increase the operational availability to meet overall LAAS requirements. Past developments have illustrated the feasibility of integrating a Coarse/Acquisition (C/A) code L1 APL in a real-time DGPS LAAS configuration for increased availability [2]. This paper demonstrates the feasibility of integrating a wideband APL (WBAPL) into the LAAS, and illustrates the benefits accrued from this integration. Flight test results from the real-time inclusion of a WBAPL in the differential position solution are presented. The primary error source for the APL link at the LAAS Ground Facility (LGF) is the ground-to-ground multipath [3]. Using a Multipath Limiting Antenna (MLA) for APL transmission, and two MLAs and two High Zenith Antennas (HZAs) for GPS and APL signal reception, Bias-Value (B- value) analysis is used to assess the performance of the ground WBAPL link at the LGF. Pseudorange corrections and B-values for the WBAPL signal received at both the HZAs as well as the MLAs were transmitted on a VHF Data Link (VDL) by the LGF, giving the airborne user the option to decide which of the measurements to use. Separate codes (34 for the HZA measurements, and 35 for the MLA measurements) were used to identify between the two WBAPL measurements. In order to verify the repeatability of the system performance, and to experiment with two potential WBAPL pulsing formats, two flight-tests were conducted: the first on the August 31, 2000 and the second on September 7, 2000. APL transmissions of both the C/A and WB codes of PRN 34 were implemented using the 1/11th Pulsing Format (~9.1% duty cycle) on August 31, 2000, and a Modified RTCA Pulsing Format (~9.8% duty cycle) on September 7, 2000. B-value averages for the WBAPL varied between 0.04 and 0.56 meters, and were generally higher than those obtained for GPS satellites. The standard deviations of the WBAPL B-values were, however, comparable to those of GPS. Despite the higher B-values, integration of the WBAPL into the differential solution resulted in little (<0.5 meters) differences in the final solution over the last 2.0 to 0.2 nmi of the approaches. In particular, for the September 7, 2000 flight, the average DGPS-only position solution showed a |µ| + 2ó value of 0.844 meters in the vertical, compared to a value of 1.233 meters with the inclusion of the WBAPL in the real-time differential solution. Inclusion of the WBAPL demonstrated an increase in availability, decrease in VDOP, and position accuracy comparable to that of DGPS-only performance. This paper details the first successful real- time integration of a WBAPL into a prototype LAAS.
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:	1172 - 1179
Cite this article:	Updated citation: Published in NAVIGATION: Journal of the Institute of Navigation
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