Use of Post-processed GPS Data as a Truth Source for Long Baseline Flight Testing of the Ground-based Regional Augmentation System (GRAS)

William S. Ely

Abstract:	Since 1997, Airservices Australia has been developing a Ground-based Regional Augmentation System (GRAS) utilising a Very High Frequency (VHF) data link as a low cost option to providing wide area augmentation services similar to Satellite Based Augmentation Systems (SBAS) such as the American developed Wide Area Augmentation System (WAAS). In 2000, Airservices Australia, in conjunction with the Global Navigation Satellite Systems Panel (GNSSP) of the International Civil Aviation Organisation (ICAO), commenced work on Standards and Recommended Practices (SARPs) which describe the technical nature of GRAS. The validation of these SARPs requires the collection and assessment of data throughout the service volume of the GRAS system. To facilitate the assessment of both accuracy and integrity, it is necessary to provide a "Truth" system which also operates throughout the GRAS service volume. Since the implementation of GPS, the real time navigation accuracies achievable have proven problematic in identifying suitable measuring systems against which to measure the accuracy of GPS and its augmentations. This is particularly difficult for an aircraft in flight, which may operate over large distances, high altitudes above the ground and over a varying speed range. Traditional aircraft "Truth" systems have used inertial, optical, laser or radar techniques, none of which are capable of providing a "Truth" source to the accuracies required throughout the GRAS service volume. By necessity, the general principle of not using a system to measure itself has been largely ignored in a search to identify alternative ways of using GPS to measure GPS, with high levels of accuracy and integrity. Most recently, the use of post-processed, codeless dual frequency GPS data has become common-place in aircraft flight trials. In the case of GRAS, the flight test aircraft may be conducting approach and landing operations at the airfield where the ground component of the GPS "Truth" system is located, a short baseline situation, or alternatively the aircraft may be conducting enroute tests up to 400 nautical miles (NM) from the ground component of the "Truth" system. This paper analyses the results of a number of static trials and GRAS flight tests using both long and short baseline "Truth" data in a quest to verify the accuracies quoted by the GPS "Truth" system post-processing software.
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:	325 - 334
Cite this article:	Ely, William S., "Use of Post-processed GPS Data as a Truth Source for Long Baseline Flight Testing of the Ground-based Regional Augmentation System (GRAS)," 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. 325-334.
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