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3.7.3.5.3.1.2 Recommendation.— The data broadcast should be omnidirectional when required to support the
intended applications.
Note.— Guidance material concerning coverage for Category I precision approach and APV is provided in
Attachment D, 7.3.
3.7.3.5.3.2 GBAS positioning service. The GBAS positioning service area shall be that area where the data broadcast
can be received and the positioning service meets the requirements of 3.7.2.4 and supports the corresponding approved
operations.
Note.— Guidance material concerning the positioning service coverage is provided in Attachment D, 7.3.
3.7.3.5.4 Data broadcast characteristics
Note.— RF characteristics are specified in Appendix B, 3.6.2.
3.7.3.5.4.1 Carrier frequency. The data broadcast radio frequencies used shall be selected from the radio frequencies in
the band 108 to 117.975 MHz. The lowest assignable frequency shall be 108.025 MHz and the highest assignable frequency
shall be 117.950 MHz. The separation between assignable frequencies (channel spacing) shall be 25 kHz.
Note 1.— Guidance material on VOR/GBAS frequency assignments and geographical separation criteria is given in
Attachment D, 7.2.1.
Note 2.— ILS/GBAS geographical separation criteria and geographical separation criteria for GBAS and VHF
communication services operating in the 118 – 137 MHz band are under development. Until these criteria are defined and
included in SARPs, it is intended that frequencies in the band 112.050 – 117.900 MHz will be used.
3.7.3.5.4.2 Access technique. A time division multiple access (TDMA) technique shall be used with a fixed frame
structure. The data broadcast shall be assigned one to eight slots.
Note.— Two slots is the nominal assignment. Some GBAS facilities that use multiple VHF data broadcast (VDB)
transmit antennas to improve VDB coverage may require assignment of more than two time slots. Guidance on the use of
multiple antennas is given in Attachment D, 7.12.4; some GBAS broadcast stations in a GRAS may use one time slot.
3.7.3.5.4.3 Modulation. GBAS data shall be transmitted as 3-bit symbols, modulating the data broadcast carrier by
D8PSK, at a rate of 10 500 symbols per second.
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23/11/06 3-68
3.7.3.5.4.4 Data broadcast RF field strength and polarization
Note.— GBAS can provide a VHF data broadcast with either horizontal (GBAS/H) or elliptical (GBAS/E) polarization that
employs both horizontal polarization (HPOL) and vertical polarization (VPOL) components. Aircraft using a VPOL component
will not be able to conduct operations with GBAS/H equipment. Relevant guidance material is provided in Attachment D, 7.1.
3.7.3.5.4.4.1 GBAS/H
3.7.3.5.4.4.1.1 A horizontally polarized signal shall be broadcast.
3.7.3.5.4.4.1.2 The effective radiated power (ERP) shall provide for a horizontally polarized signal with a
minimum field strength of 215 microvolts per metre (–99 dBW/m2) and a maximum field strength of 0.350 volts per
metre (–35 dBW/m2) within the GBAS coverage volume. The field strength shall be measured as an average over the
period of the synchronization and ambiguity resolution field of the burst. The RF phase offset between the HPOL and
any VPOL components shall be such that the minimum signal power defined in Appendix B, 3.6.8.2.2.3 is achieved for
HPOL users throughout the coverage volume.
3.7.3.5.4.4.2 GBAS/E
3.7.3.5.4.4.2.1 Recommendation.— An elliptically polarized signal should be broadcast whenever practical.
3.7.3.5.4.4.2.2 When an elliptically polarized signal is broadcast, the horizontally polarized component shall meet
the requirements in 3.7.3.5.4.4.1.2, and the effective radiated power (ERP) shall provide for a vertically polarized signal
with a minimum field strength of 136 microvolts per metre (–103 dBW/m2) and a maximum field strength of 0.221 volts
per metre (–39 dBW/m2) within the GBAS coverage volume. The field strength shall be measured as an average over the
period of the synchronization and ambiguity resolution field of the burst. The RF phase offset between the HPOL and
VPOL components, shall be such that the minimum signal power defined in Appendix B, 3.6.8.2.2.3 is achieved for
HPOL and VPOL users throughout the coverage volume.
Note.— The minimum and maximum field strengths in 3.7.3.5.4.4.1.2 and 3.7.3.5.4.4.2.2 are consistent with a minimum
receiver sensitivity of –87 dBm and minimum distance of 200 m (660 ft) from the transmitter antenna for a coverage range of
43 km (23 NM).
3.7.3.5.4.5 Power transmitted in adjacent channels. The amount of power during transmission under all operating
conditions when measured over a 25 kHz bandwidth centred on the ith adjacent channel shall not exceed the values shown in
　
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