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aligned with the runway centre line as illustrated in Figure D-7. For approaches not aligned with the runway, the LTP/FTP
ATT D-27 23/11/06
Annex 10 — Aeronautical Communications Volume I
may or may not lie on the extended runway centre line. For this type of approach Δlength offset is not meaningful and should
be set to “not provided”.
7.11.5 SBAS service provider. A common format is used for FAS data blocks to be used by both GBAS and SBAS.
The SBAS service provider ID field identifies which SBAS system(s) may be used by an aircraft that is using the FAS data
during an approach. The GBAS service provider may inhibit use of the FAS data in conjunction with any SBAS service. For
precision approaches based on GBAS this field is not used, and it can be ignored by aircraft GBAS equipment.
7.11.6 Approach identifier. The service provider is responsible for assigning the approach identifier for each approach.
The approach identification should be unique within a large geographical area. Approach identifications for multiple runways
at a given airport should be chosen to reduce the potential for confusion and misidentification. The approach identification
should appear on the published charts that describe the approach.
7.12 Airport siting considerations
7.12.1 The installation of a GBAS ground subsystem involves special considerations in choosing prospective sites for
the reference receiver antennas and the VDB antenna(s). In planning antenna siting, Annex 14 obstacle limitation
requirements must be met.
7.12.2 Locating reference receiver antennas. The site should be selected in an area free of obstructions, so as to permit
the reception of satellite signals at elevation angles as low as possible. In general, anything masking GNSS satellites at
elevation angles higher than 5 degrees will degrade system availability.
7.12.2.1 The antennas for the reference receivers should be designed and sited to limit multipath signals that interfere
with the desired signal. Mounting antennas close to a ground plane reduces long-delay multipath resulting from reflections
below the antenna. Mounting height should be sufficient to prevent the antenna being covered by snow, or being interfered
with by maintenance personnel or ground traffic. The antenna should be sited so that any metal structures, such as air vents,
pipes and other antennas are outside the near-field effects of the antenna.
7.12.2.2 Besides the magnitude of the multipath error at each reference receiver antenna location, the degree of
correlation must also be considered. Reference receiver antennas should be located in places that provide independent
multipath environments.
7.12.2.3 The installation of each antenna should include a mounting that will not flex in winds or under ice loads.
Reference receiver antennas should be located in an area where access is controlled. Traffic may contribute to error due to
multipath or obstruct view of satellites from the antennas.
7.12.3 Locating the VDB antenna. The VDB antenna should be located so that an unobstructed line-of-sight exists
from the antenna to any point within the coverage volume for each supported FAS. Consideration should also be given to
ensuring the minimum transmitter-to-receiver separation so that the maximum field strength is not exceeded. In order to
provide the required coverage for multiple FASs at a given airport, and in order to allow flexibility in VBD antenna siting,
the actual coverage volume around the transmitter antenna may need to be considerably larger than that required for a single
FAS. The ability to provide this coverage is dependent on the VDB antenna location with respect to the runway and the
height of the VDB antenna. Generally speaking, increased antenna height may be needed to provide adequate signal strength
to users at low altitudes, but may also result in unacceptable multipath nulls within the desired coverage volume. A suitable
antenna height trade-off must be made based on analysis, to ensure the signal strength requirements are met within the entire
volume. Consideration should also be given to the effect of terrain features and buildings on the multipath environment.
7.12.4 Use of multiple transmit antennas to improve VDB coverage. For some GBAS installations, constraints on antenna
location, local terrain or obstacles may result in ground multipath and/or signal blockage that make it difficult to provide the
specified field strength at all points within the coverage area. Some GBAS ground facilities may make use of one or more
additional antenna systems, sited to provide signal path diversity such that collectively they meet the coverage requirements.
23/11/06 ATT D-28
Attachment D Annex 10 — Aeronautical Communications
7.12.4.1 Whenever multiple antenna systems are used, the antenna sequence and message scheduling must be arranged
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