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Recommendation P.528-2 or from free-space attenuation until the radio horizon and then a constant 0.5 dB/NM attenuation
factor. These two methodologies result in slightly different geographical separation for co-channel and first adjacent channels,
and identical separation as soon as the second adjacent channel is considered. The free-space propagation approximation is
applied in this guidance material.
7.2.1.4 Example of GBAS/GBAS geographical separation criteria
7.2.1.4.1 For GBAS VDB co-channel transmissions assigned to the same time slot, the parameters for horizontal
polarization are:
ATT D-19 23/11/06
Annex 10 — Aeronautical Communications Volume I
D/U = 26 dB (Appendix B, 3.6.8.2.2.5.1);
PD,min = –72 dBm (equivalent to 215 microvolts per metre, Chapter 3, 3.7.3.5.4.4); and
TxU = 47 dBm (example link budget, Table D-3);
so
L ≥ (47 + 26 – (–72)) = 145 dB.
7.2.1.4.2 The geographic separation for co-channel, co-slot GBAS VDB assignments is obtained by determining the
distance at which the transmission loss equals 145 dB for receiver altitude of 3 000 m (10 000 ft) above that of the GBAS
VDB transmitter antenna. This distance is 318 km (172 NM) using the free-space attenuation approximation and assuming a
negligible transmitter antenna height. The minimum required geographical separation can then be determined by adding this
distance to the nominal distance between the edge of coverage and the GBAS transmitter 43 km (23 NM). This results in a
co-channel, co-slot reuse distance of 361 km (195 NM).
7.2.1.5 Guidelines on GBAS/GBAS geographical separation criteria. Using the methodology described above, typical
geographic separation criteria can be defined for GBAS to GBAS and GBAS to VOR. The resulting GBAS/GBAS minimum
required geographical separation criteria are summarized in Table D-4.
Note.— Geographical separation criteria between the GBAS transmitters providing the GBAS positioning service are
under development. A conservative value corresponding to the radiohorizon may be used as an interim value for separation
between co-frequency, adjacent time slot transmitters to ensure time slots do not overlap.
7.2.1.6 Guidelines on GBAS/VOR geographical separation criteria. The GBAS/VOR minimum geographical
separation criteria are summarized in Table D-5 based upon the same methodology and the nominal VOR coverage volumes
in Attachment C.
Note 1.— When determining the geographical separation between VOR and GBAS, VOR as the desired signal is
generally the constraining case due to the greater protected altitude of the VOR coverage region.
Note 2.— Reduced geographical separation requirements can be obtained using standard propagation models defined in
ITU-R Recommendation P.528-2.
Table D-4. Typical GBAS/GBAS frequency assignment criteria
Channel of undesired VDB in the same time slots
Path loss
(dB)
Minimum required geographical
separation for TxU = 47 dBm
and PD,min = –72 dBm in km (NM)
Cochannel 145 361 (195)
1st adjacent channel (±25 kHz) 101 67 (36)
2nd adjacent channel (±50 kHz) 76 44 (24)
3rd adjacent channel (±75 kHz) 73 No restriction
4th adjacent channel (±100 kHz) 73 No restriction
Note.— No geographic transmitter restrictions are expected between co-frequency, adjacent time slots provided the
undesired VDB transmitting antenna is located at least 200 m from areas where the desired signal is at minimum field
strength.
23/11/06 ATT D-20
Attachment D Annex 10 — Aeronautical Communications
Table D-5. Minimum required geographical separation for a VOR coverage
(12 000 m (40 000 ft) level)
VOR coverage radius
Channel of undesired GBAS VDB
Path loss
(dB) 342 km (185 NM) 300 km (162 NM) 167 km (90 NM)
Co-channel 152 892 km (481 NM) 850 km (458 NM) 717 km (386 NM)
| fDesired – fUndesired | = 25 kHz 126 774 km (418 NM) 732 km (395 NM) 599 km (323 NM)
| fDesired – fUndesired | = 50 kHz 92 351 km (189 NM) 309 km (166 NM) 176 km (94 NM)
| fDesired – fUndesired | = 75 kHz 80 344 km (186 NM) 302 km (163 NM) 169 km (91 NM)
| fDesired – fUndesired | = 100 kHz 61 No restriction No restriction No restriction
Note.— Calculations are based on reference frequency of 112 MHz and assume GBAS TxU = 47 dBm and VOR PD,min = –79 dBm.
7.2.2 The geographical separation criteria for GBAS/ILS and GBAS/VHF communications are under development.
7.2.3 Compatibility with ILS. Until compatibility criteria are developed for GBAS VDB and ILS, VDB cannot be assigned
to channels below 112.025 MHz. If there is an ILS with a high assigned frequency at the same airport as a VDB with a
frequency near 112 MHz, it is necessary to consider ILS and VDB compatibility. Considerations for assignment of VDB
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