New Test Results with a Ground-based Regional Augmentation System (GRAS) in Australia

Keith W. McPherson, Donna K. Kraus, William S. Ely, Kevin W. Bean, Graeme K. Crosby, Bryant D. Elrod and Joy M. Stewart

Abstract:	The ICAO Air Navigation Commission has tasked the ICAO GNSS Panel to develop Standards And Recommended Practices (SARPs) for the Ground-based Regional Augment-ation System (GRAS). The GRAS concept is a blend of the Space and Ground-based Augmentation System (SBAS/GBAS) concepts, which Airservices Australia is evaluating for enhancing GPS/GNSS to support civil navigation in its national airspace. Like SBAS, GRAS employs a distributed network of reference stations for monitoring GPS, and a central processing facility for computing GPS integrity and differential correction information. However, the delivery of this information to users is like GBAS, since it is transmitted by a local data link rather than dedicated geostationary satellites. In Australia, a network of terrestrial VHF stations provides this capability including a local check on the augmentation data before formatting it into GBAS-like messages. These messages are transmitted via a D8PSK VHF Data Broadcast (VDB) at 1 second intervals in a TDMA-managed 1/16th sec time slot. Users need only employ a GPS/GBAS-capable receiver to obtain GPS augmentation data for supporting enroute as well as terminal area approach/departure operations in all areas with VHF network coverage. Airservices Australia, with support from ITT Industries, is continuing its development and use of a test bed in eastern Australia to assess GRAS technical capabilities. The test bed is also being upgraded in preparation for validation of SARPs for GRAS that are expected to be considered by an ICAO GNSSP Working Group in late 2001. The test bed comprises five GRAS Reference Stations (GRS) distributed in eastern Australia, a GRAS Master Station (GMS) at Canberra, several GRAS VHF Stations (GVS), and a test aircraft equipped with prototype GRAS User Equipment (UE) and an independent GPS receiver for use in aircraft truth position determination. This paper describes the test environment for evaluating GPS+GRAS navigation performance (accuracy and integrity) and presents the latest results derived from various flight trials including enroute and approach operations. The results also include an assessment of the GRAS data link performance and the ability of a user to transition between the available GVS signals during enroute operations. The test results demonstrate the seamless delivery of GPS augmentation data in Australia using a single VHF frequency and multiple, TDMA-managed broadcast stations. The paper also presents navigation accuracy results achieved with GRAS augmentation data during typical flight operations in enroute through Non Precision Approach (NPA) phases of flight and precision approach testing based on Approach with Vertical Guidance (APVI) minima.
Published in:	Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001) September 11 - 14, 2001 Salt Palace Convention Center Salt Lake City, UT
Pages:	2386 - 2390
Cite this article:	McPherson, Keith W., Kraus, Donna K., Ely, William S., Bean, Kevin W., Crosby, Graeme K., Elrod, Bryant D., Stewart, Joy M., "New Test Results with a Ground-based Regional Augmentation System (GRAS) in Australia," Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001), Salt Lake City, UT, September 2001, pp. 2386-2390.
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