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How many satellites are required for GBAS to function correctly?
  • A
    4 satellites are required for GBAS operations.
  • B
    GBAS uses the signals from at least 3 geostationary satellites.
  • C
    GBAS requires at least 5 satellites to detect and exclude the possible faulty satellite.
  • D
    GBAS uses the exact same satellites as the receiver in the aircraft.

Refer to figures.
GBAS is a system that attempts to reduce the natural errors within the GNSS system by providing very localised augmentation to the usual GNSS satellite signals, via a VHF data broadcast (VDB) that a suitably equipped aircraft can receive and use to fix any position errors. It is also called "differential GPS", and can correct for errors induced by satellite clocks, ephemeris and ionospheric propagation to make the augmented GNSS signal very accurate. These are measurable errors, which are measured by multiple receiver antennas and interpreted by a ground station, usually located at an airport which can make use of this highly accurate GNSS. Errors in the receiver, multipath signals, and some small atmospheric propagation errors can still occur, but the intention is to get the accuracy below 1m for aircraft on final approach. The GBAS can also, like its satellite based cousin, SBAS, give integrity warnings about faulty satellites, as it should be able to detect this at the nearby antennas.

This question, however, is much simpler than upon initial inspection, as it is just asking about how many satellites we require for a 3D position, which is 4 satellites. Standard GNSS relies on satellites transmitting a timing signal (called the PRN signal) with exact timing data included, so that the receiver can measure this against the known time, and calculate a distance from all the satellites within view. Then a process called trilateration is used to determine the exact position of the receiver from multiple ranges from satellites of known locations (which creates multiple range spheres around the satellites, and the receiver is where the faces of all spheres meet [annex 2 above]).

The GBAS signal does not require any more or less satellite ranges than this, as 4 is sufficient to perform its job (RAIM and geostationary satellites are not required for GBAS). More satellites are usually more accurate, but with GBAS, the accuracy is extremely high anyway, so it does not make much difference.

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