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signal ratios for co-channel and adjacent channel rejection of the VDB were determined empirically. Table D-2
summarizes the assumed signal ratios based upon empirical performance of numerous VOR receivers designed for
50 kHz channel spacing;
e) for areas/regions of frequency congestion, a precise determination of separation may be required using the
appropriate criteria.
7.2.1.1.2 The nominal link budget for VDB is shown in Table D-3. The figures in the table assume a receiver height of
3 000 m (10 000 ft) MSL and a transmit antenna designed to suppress ground illumination in order to limit the fading losses
to a maximum of 10 dB at coverage edge. In the case of GBAS/E equipment, the 10 dB also includes any effects of signal
loss due to interference between the horizontal and vertical components.
7.2.1.2 FM immunity
7.2.1.2.1 Once a candidate frequency is identified for which the GBAS and VOR separation criteria are satisfied,
compatibility with FM transmissions must be determined. This is to be accomplished using the methodology applied when
determining FM compatibility with VOR. If FM broadcast violates this criterion, an alternative candidate frequency has to be
considered.
Table D-2. Assumed [D/U]required signal ratios to protect VOR from GBAS VDB
Frequency offset
[D/U]required ratio to
protect VOR receivers
(dB)
Co-channel 26
| fVOR – fVDB | = 25 kHz 0
| fVOR – fVDB | = 50 kHz –34
| fVOR – fVDB | = 75 kHz –46
| fVOR – fVDB | = 100 kHz –65
Table D-3. Nominal VDB link budget
VDB link elements
Vertical component link budget
at coverage edge
Horizontal component link budget
at coverage edge
Required receiver sensitivity (dBm) –87 –87
Maximum aircraft implementation loss (dB) 11 15
Power level after aircraft antenna (dBm) –76 –72
Operating margin (dB) 3 3
Fade margin (dB) 10 10
Free space path loss (dB) at 43 km (23 NM) 106 106
Nominal effective radiated power (dBm) 43 47
23/11/06 ATT D-18
Attachment D Annex 10 — Aeronautical Communications
7.2.1.2.2 The desensitization is not applied for FM carriers above 107.7 MHz and VDB channels at 108.050 MHz
because the off-channel component of such high-level emissions from FM stations above 107.7 MHz will interfere with
GBAS VDB operations on 108.025 and 108.050 MHz, hence those assignments will be precluded except for special
assignments in geographic areas where the number of FM broadcast stations in operation is small and would unlikely
generate interference in the VDB receiver.
7.2.1.2.3 The FM intermodulation immunity requirements are not applied to a VDB channel operating below
108.1 MHz, hence assignments below 108.1 MHz will be precluded except for special assignments in geographic areas where
the number of FM broadcast stations in operation is small and would unlikely generate intermodulation products in the VDB
receiver.
7.2.1.3 Geographic separation methodologies
7.2.1.3.1 The methodologies below may be used to determine the required GBAS-to-GBAS and GBAS-to-VOR
geographical separation. They rely on preserving the minimum desired-to-undesired signal ratio. [D/U]required is defined as the
signal ratio intended to protect the desired signal from co-channel or adjacent channel interference from an undesired
transmission. [D/U]required values required for protection of a GBAS receiver from undesired GBAS or VOR signals are
defined in Appendix B, 3.6.8.2.2.5 and 3.6.8.2.2.6. [D/U]required values intended for protection of a VOR receiver from GBAS
VDB transmissions as shown in Table D-2 are not defined in SARPs and represent the assumed values based on test results.
7.2.1.3.2 Geographic separation is constrained by preserving [D/U]required at the edge of the desired signal coverage
where the desired signal power is derived from the minimum field strength requirements in Chapter 3. This desired signal
level, converted to dBm, is denoted PD,min. The allowed signal power of the undesired signal (PU,allowed) is:
PUallowed(dBm) = (PD,min (dBm) – [D/U]required (dB))
The undesired signal power PU converted to dBm is:
PU(dBm) = (TxU (dBm) – L (dB))
where
TxU is the effective radiated power of the undesired transmitter; and
L is the transmission loss of the undesired transmitter, including free-space path loss, atmospheric and ground effects.
This loss depends upon the distance between the undesired transmitter and the edge of the desired signal coverage.
To ensure D/Urequired is satisfied, Pu ≤ DUallowed. The constraint for assigning a channel is therefore:
L(dB) ≥ ([D/U]required (dB) + TxU(dBm) – PD,min (dBm))
7.2.1.3.3 The transmission loss can be obtained from standard propagation models published in ITU-R
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