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How does the GBAS precision approach work?

  • A

    A GBAS ground station sends correction data to the aircraft, which uses the data to determine the correct azimuth and glidepath.

  • B

    A GBAS ground station sends correction signals to a ground transmitter, which sends azimuth and glidepath information to the aircraft.

  • C
    A GBAS satellite sends azimuth and glidepath information to the aircraft.
  • D

    A GBAS station sends approach path information directly to the aircraft.

Refer to figure.
GBAS - GROUND BASED AUGMENTATION SYSTEM
GBAS is a system that attempts to reduce the natural errors within the GNSS system by providing very localised augmentation to the 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, then interpreted and distributed by a ground station via a VDB antenna, usually located at an airport which can make use of this highly accurate GNSS. There are still some errors that cannot be accounted for, but these are small. GBAS also provides integrity information about the GNSS satellites.

The operation of GBAS allows it to work in the same way as an ILS approach, down to 200ft minimums currently, and this is called the GBAS Landing System or GLS, and is a precision approach. It is tuned using a 5 digit channel number that picks up the correct VHF datalink signals for that approach. The VHF datalink (called the VDB) sends a signal that corrects the GNSS position, so that the onboard navigation system can guide the aircraft accurately on the approach, in both azimuth and glidepath.


Note: For this question, the examiner is mostly asking "what signals are sent in the VHF data broadcast?". The correct answer is "correction data", as this is exactly what the GBAS system does, it corrects the GNSS position. The GBAS system does not send glidepath and azimuth data to the aircraft, like an ILS or MLS would.This is the primary objective of this question.

The GBAS ground station does not tell the aircraft that it is 2 m right and 4 ft above the correct position, it tells it to adjust its GNSS position by a certain amount, then the aircraft makes sure it follows the approach correctly.

It also does not matter whether the VDB antenna is classed as part of the ground station or not, that is not the defining difference in this question. The ground station transmits VDB signals via the antenna to the aircraft, which still means that the ground station sends signals to the aircraft.

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