The GBAS ground subsystem makes uses of GNSS to provide precision approach and landing with benefits of enhancing safety and operational efficiency. In a typical GBAS installation, a VHF data broadcasting (VDB) radio shall be installed at the vicinity of the airport to broadcast the correction message to the aircraft. According to the ICAO SARPs, GBAS operates in the frequency band 108 – 117.975 MHz. The lowest assignable frequency is 108.025 MHz and the highest assignable frequency is 117.950 MHz. The separation between assignable frequencies (channel spacing) shall be 25 kHz.
With reference to ICAO Doc 9718 Vol. II, the channel spacing for a VDB should have minimum of 1 MHz channel spacing from any existing VOR, ILS LOC or other VDB within a defined range, which is the range where no interference is anticipated. Based on this range and the minimum channel spacing requirement, a VDB frequency must be assigned so that there will be no harmful interference from and to other VHF sources including VOR, ILS LOC, VHF COM, and FM broadcast. As such, States should take a holistic approach for frequency assignment to GBAS VDB, VOR, and ILS LOC. It should be noted that each State might have its own organisation that assigns the frequency, as well as its own conditions.
Two main aspects should be considered for the frequency assignment for a GBAS VDB:
The same‐airport compatibility refers to the compatibility with VHF facilities in the same airport where the GBAS is installed. It involves analysis on impact/interference among VHF facilities on aircraft, GBAS VDB facilities and other airport VHF facilities, including the consideration of minimum separation of VDB transmitter antenna from aircraft to avoid saturation of VHF receiver on aircraft. It would mainly be the matter of siting criteria for GBAS VDB ground facilities and is not relevant to frequency coordination.
The airport‐to‐airport compatibility is the frequency compatibility of VDB with other VHF facilities outside the airport where the GBAS is installed. The coordination must be done not only within the State, but also with other States, when the facility is close to a border, to ensure no adverse impact due to any interference from and to other VHF sources. Such consideration is described in the ICAO Handbook on Radio Frequency Spectrum Requirements for Civil Aviation (Doc 9718) Vol. II, with the following inputs and principles for the derivation of the minimum geographical separation distance:
- The EIRP of the undesired facility.
- Minimum filed strength of the desired GBAS VDB required throughout the designated operational coverage.
- The ratio of the desired‐to‐undesired signals must be greater than a threshold with respect to the relative separation distance between the GBAS VDB receiver and the undesired transmissions.
- The D/U ratio threshold is defined as a function of the frequency separation, but it is also a function of the power input to the receiver.
- The D/U ratio can be assessed at the edge of the designated operational coverage where the desired field strength will be minimum.
- The transmission loss along the radio path of the undesired transmitter can be estimated as per the ITU propagation curve (ITU‐R Recommendation P.528-5).
