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channels include the frequency separation between the ILS and the VDB, the distance separation between the ILS coverage area
and the VDB, the VDB and ILS field strengths, and the VDB and ILS sensitivity. For GBAS equipment with transmitter power
of up to 150 W (GBAS/E, 100 W for horizontal component and 50 W for vertical component) or 100 W (GBAS/H), the 16th
channel (and beyond) will be below –106 dBm at a distance of 200 m from the VDB transmitter, including allowing for a +5 dB
positive reflection. This –106 dBm figure assumes a –86 dBm localizer signal at the ILS receiver input and a minimum 20 dB
signal-to-noise ratio.
7.2.4 Compatibility with VHF communications. For GBAS VDB assignments above 116.400 MHz, it is necessary to
consider VHF communications and GBAS VDB compatibility. Considerations for assignment of these VDB channels
include the frequency separation between the VHF communication and the VDB, the distance separation between the
transmitters and coverage areas, the field strengths, the polarization of the VDB signal, and the VDB and VHF sensitivity.
Both aircraft and ground VHF communication equipment are to be considered. For GBAS/E equipment with a transmitter
maximum power of up to 150 W (100 W for horizontal component and 50 W for vertical component), the 64th channel (and
beyond) will be below –120 dBm at a distance of 200 m from the VDB transmitter including allowing for a +5 dB positive
reflection. For GBAS/H equipment with a transmitter maximum power of 100 W, the 32nd channel (and beyond) will be
below –120 dBm at a distance of 200 m from the VDB transmitter including allowing for a +5 dB positive reflection, and a
10 dB polarization isolation. It must be noted that due to differences in the VDB and VDL transmitter masks, separate
analysis must be performed to ensure VDL does not interfere with the VDB.
7.2.5 For a GBAS ground subsystem that only transmits a horizontally-polarized signal, the requirement to achieve the
power associated with the minimum sensitivity is directly satisfied through the field strength requirement. For a GBAS
ground subsystem that transmits an elliptically-polarized component, the ideal phase offset between HPOL and VPOL
components is 90 degrees. In order to ensure that an appropriate received power is maintained throughout the GBAS
coverage volume during normal aircraft manoeuvres, transmitting equipment should be designed to radiate HPOL and VPOL
signal components with an RF phase offset of 90 degrees. This phase offset should be consistent over time and environmental
conditions. Deviations from the nominal 90 degrees must be accounted for in the system design and link budget, so that any
fading due to polarization loss does not jeopardize the minimum receiver sensitivity. System qualification and flight
inspection procedures will take into account an allowable variation in phase offset consistent with maintaining the
appropriate signal level throughout the GBAS coverage volume. One method of ensuring both horizontal and vertical field
strength is to use a single VDB antenna that transmits an elliptically-polarized signal, and flight inspect the effective field
strength of the vertical and horizontal signals in the coverage volume.
ATT D-21 23/11/06
Annex 10 — Aeronautical Communications Volume I
7.3 Coverage
7.3.1 The GBAS coverage to support approach services is depicted in Figure D-4. When the additional ephemeris error
position bound parameters are broadcast, differential corrections may only be used within the Maximum Use Distance (Dmax)
defined in the Type 2 message. Where practical, it is operationally advantageous to provide valid guidance along the visual
segment of an approach.
7.3.2 The coverage required to support the GBAS positioning service is dependent upon the specific operations
intended. The optimal coverage for this service is intended to be omnidirectional in order to support operations using the
GBAS positioning service that are performed outside of the precision approach coverage volume. Each State is responsible
for defining a service area for the GBAS positioning service and ensuring that the requirements of Chapter 3, 3.7.2.4 are
satisfied. When making this determination, the characteristics of the fault-free GNSS receiver should be considered,
including the reversion to ABAS-based integrity in the event of loss of GBAS positioning service.
7.3.3 The limit on the use of the GBAS positioning service information is given by the Maximum Use Distance (Dmax),
which defines the range within which the required integrity is assured and differential corrections can be used for either the
positioning service or precision approach. Dmax however does not delineate the coverage area where field strength
requirements specified in Chapter 3, 3.7.3.5.4.4 are met nor matches this area. Accordingly, operations based on the GBAS
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