The Asia-Pacific GNSS Test Bed is a regional collaborative programme that brings together aviation practitioners in the field of satellite navigation within the Asia-Pacific to study the performance of global navigation satellite systems (GNSS) and to develop a regional plan that will lead to a successful implementation of GNSS in the region. The program is a work program under the Asia-Pacific Economic Cooperation GNSS Implementation Team (APEC GIT), a working group under the APEC. The Test Bed has been in operation since August 2006. The system architecture reflects hybrid architecture between a satellite-based augmentation system (SBAS) and a ground-based augmentation system (GBAS). Test reference stations (TRSs), consisted of a dual-frequency GPS receiver, a communication interface, and a data archival hardware, have been installed in the participating economies. GPS data collected at the TRSs will be transmitted to the test master station (TMS) in Bangkok, Thailand. SBAS messages will be generated and the messages will be broadcasted through a test VHF station (TVS). A test user platform (TUP) will receive GPS signal and the SBAS messages broadcast from TVS and then calculate and archive the TUP’s positions.
This paper discusses performance analysis and flight test results conducted in Thailand in September 2007. During flight trials, noting that the Test Bed broadcasts SBAS messages through a VHF data link (VDL) similar to that of GBAS, the impacts of VHF broadcast, such as transmitting power, terrain effects, and impacts of different aircraft attitudes and positions are analysed. Preliminary analysis result indicates that, while the GBAS VDL coverage is very good through out the 30nm airport terminal area, during approach, and even during taxi, considerations should be given to levels of transmitting power to eliminate possible intermittent loss of VHF messages received by the aircraft receiver. This intermittent loss of VHF messages results in very strong fluctuation of horizontal protection levels (HPL) and vertical protection levels (VPL) calculated by the TUP. To yield with possible problem, windowing techniques are proposed and analysed.
