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White paper: A Ground-based Regional Augmentation System (GRAS) - The Australian Proposal
Abstract
Airservices Australia has proposed a blending of Space/Ground-based Augmentation System ( SBAS/GBAS ) concepts to enhance GPS/GNSS capabilities for supporting civil navigation needs in its national airspace. This approach, called the Ground-based Regional Augmentation System (GRAS), is SBAS-like in using a distributed network of reference stations for monitoring GPS, and a central processing facility for computing GPS integrity and differential correction information. But instead of transmitting this information to users via dedicated geostationary satellites (GEOs), GRAS delivers SBAS message data to a network of terrestrial stations for a local check and reformatting. Each site emits a GBAS-like, VHF Data Broadcast (VDB) signal in a TDMA-managed time slot. Users can employ a GPS/GBAS-capable receiver to obtain GPS augmentation data for both enroute as well as terminal area approach/departure operations depending on the VHF network coverage. The GRAS approach is beneficial in Australia where a GEO satellite is either not available or too costly to broadcast SBAS data. GRAS also allows for Sovereignty control while still providing unified corrections and integrity for enroute capability.
This paper presents further details of the GRAS concept envisioned for Australia, a description of the test bed developed for demonstrating GRAS technical capabilities, and the results of preliminary flight trials including en route and Non Precision Approach (NPA) operations. As a prelude to the development and validation of Standards and Required Practices (SARPs) for GRAS through ICAO, the paper also discusses potential message modifications that could be made for GRAS as an extension to the GBAS SARPs.