Home' RTCA Documents for Review : DO-253D_Change1_FRAC Contents Appendix K
DO-253D Change 1
© 2019, RTCA, Inc.
APPENDIX K: RATIONALE FOR THE VDB AIRBORNE REQUIREMENTS
The purpose of this appendix is to describe the rationale for the VDB airborne receiver
system and dynamic range requirements. In order to rationalize the basis of the receiver
requirements it is essential to consider the overall VDB system, its service volume, the
field strength limits that VDB ground stations are required to provide within that service
volume, and the Aircraft Implementation Loss (AIL) limitations that assure that the
airborne receiver receives signal levels within the dynamic range specified for its operating
VDB Coverage Volume
For the purpose of establishing a VDB link budget, the VDB coverage volume is defined
by a distance of 80 m minimum to 23 NM maximum from the VDB transmitter antenna.
This coverage volume is consistent with the minimum GBAS service volume as defined in
ICAO Annex 10. The use of this simplified definition of VDB coverage in no way implies
that service must be provided beyond the minimum requirements of ICAO Annex 10. It is
only a means to establish a link budget and demonstrate that the expected propagation
characteristics of the VDB signal are consistent with VDB transmitter and receiver
requirements within an airport environment. As stated in ICAO Annex 10, when
supporting approach services at airports with challenging VDB transmitter siting
constraints, it is acceptable to adjust the service volume when operational requirements
VDB RF Field Strength
GBAS ground stations 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. The field strength is measured as an average over the period of the
synchronization and ambiguity resolution field of the burst.
The VDB transmission provides a minimum field strength in its horizontal component of
215 microvolts per meter and a maximum of 0.879 V/m, and a minimum field strength in
its vertical component of 136 microvolts per meter and a maximum of 0.555 V/m within
the GBAS service volume for approach services as defined in ICAO Annex 10 Section
126.96.36.199.3.1 and RTCA/DO-246() Section 2.1.3.
Within the additional GBAS service volume for approach services supporting autoland and
guided takeoff as defined in ICAO Annex 10 Section 188.8.131.52.3.2 and RTCA/DO-246()
Section 2.1.3, the VDB transmission provides a minimum field strength in its horizontal
component of 215 microvolts per meter below 36 ft and down to 12 ft above the runway
surface and 650 microvolts per meter at 36 ft or more above the runway surface. The
vertically polarized component of an elliptically polarized broadcast signal is not required
to have increased field strength within the additional GBAS service volume for approach
services supporting autoland.
Note: The broadcast power of an installed VDB is constrained by many considerations,
only one of which is the desired field strength in the defined operational coverage.
Other constraints are adjacent and co-channel interference to neighboring
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