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DO-253D Change 1
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Software development levels required should be determined based on a system
continuity assessment. Software level A is recommended for equipment expected
to support autoland.
VDB Receiver Equipment Classes
This standard defines two classes of VDB receivers, which are referred to as Class A and
Class B receivers. Both classes are fully compatible with the minimum and maximum
VDB signal field strength specified in ICAO Annex 10 as illustrated in the VDB Link
Budget given in Appendix K. Both classes are backward compatible with the VDB field
strength specified in all older amendments of the ICAO Annex 10 that included GBAS
VDB specifications. The two classes of receivers have been defined to accommodate
different ranges of Aircraft Implementation Loss and Antenna Gain Variation in the
horizontal plane. Their definition is transparent to the SARPs at the field strength level
except for the effects of the maximum allowed antenna gain variation in the horizontal
plane of 15 dB.
Note: Previous versions of this standard allowed up to 20 dB antenna gain variation in
the horizontal plane.
As is described in Section 3.10, Aircraft Implementation Loss (AIL) is defined as the RF
signal loss in dB including all loss and gain elements in the entire VDB RF receive chain
(e.g., RF cables, splitters, connectors) from receipt of the RF signal-in-space from either a
horizontally or vertically polarized antenna (referenced to an isotropic radiator) to the VDB
receiver antenna port.
The expressions “antenna gain in the horizontal plane” and “antenna gain variation in the
horizontal plane” are used to represent the nominal geometry for VDB signal reception in
which the transmitter is sufficiently far from the aircraft to consider that the signal is
received in the horizontal plane. The “antenna gain variation” always refers to the variation
in antenna gain at any given frequency in the VDB band. The expressions “horizontal
antenna gain” and “vertical antenna gain” refer to the gain of horizontally and vertically
polarized component of the signal, respectively.
In operationally relevant areas where there is significant reduction in the aircraft VDB
antenna gain with respect to the horizontal antenna gain such that the maximum aircraft
implementation loss is exceeded, it is assumed that the aircraft is close enough to the
transmit VDB antenna that excess field strength above the minimum field strength
requirement is sufficient to compensate for the reduced antenna gain. If there are any
severe aircraft VDB antenna nulls with elevation (e.g., directly below the aircraft), it is
assumed that these nulls are quickly passed through by the aircraft during approach or
departure operations. It is further assumed that the VDB antenna gain differences that
result from small aircraft attitude changes during the critical phases of operations (e.g.,
final stages of an approach and landing) are small and overcome by excess field strength.
Class A receivers are intended to accommodate installations on most of the aircraft fleet
which can utilize the standardized AIL and maximum antenna gain variation in the
horizontal plane as indicated in Section 18.104.22.168.1. Class B receivers are intended to
accommodate installation on aircraft that have AIL and/or maximum antenna gain
variations in the horizontal plane that are different than the Class A standardized values.
This allows manufacturers to specify the range of AIL and maximum antenna gain
variation that their equipment supports (see Section 22.214.171.124.2).
Both classes of receivers have sensitivity and signal saturation consistent with the full range
of field strength levels from the minimum field strength to the maximum field strength
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