Home' RTCA Documents for Review : DO-253D_Change1_FRAC Contents Appendix K
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DO-253D Change 1
D/U can no longer be lowered since the filtering of the Undesired signal power itself will
no longer be sufficient to meet the message failure rate; the minimum D/U is -60 dB.
Rationale for MFR Recovery
Note: Continuity is addressed in Section K.8
Rationale for MFR Recovery from Excess Undesired Power
When the VDB receiver is driven into compression due to excess undesired signal power,
the resulting MFR is not controlled. The recovery time of 187.5 milliseconds (3 VDB
slots) is short enough to allow continuity of the approach operation to be maintained if the
excess power duration is short enough. The recovery time was limited to an exposure to
excess power of 2.5 seconds since no added continuity benefit is obtained from a longer
Even though the recovery time requirement applies to both ILS localizer and VOR
undesired signals, excess undesired power is expected to occur only when overflying an
Rationale for MFR Recovery from Excess D/U
When the VDB receiver remains in linear mode (no excess power), it will comply with the
MFR requirement (Section 2.2.5) in the first slot that respects the D/U requirements
(Section 2.2.8). There is no explicit exposure time however this condition can lead to an
extended period where no messages are received. As stated in the notes to the MFR
requirement in Section 2.2.5, receiver performance is tested at the minimum rate of one
message each 10 seconds. Since this is well above the maximum of 2.5 seconds that is of
benefit to continuity, no explicit recovery requirement is necessary.
Conversion between Receiver Input Power and Signal Field Strength Requirements
All VDB receiver requirements in this standard are established at the receiver input
connector. Section references to the governing VDB receiver requirement are included in
the description below. VDB requirements are established in ICAO Annex 10 in terms of
signal field strength in V/m or signal power and D/U at the receiver input. Aircraft
Implementation Loss converts the power at the output of an isotropic antenna receiving the
specified field strength to the power at the receiver input. This standard does not enforce
a fixed range of AIL, it does however set constraints on the AIL that can be used along
with other aircraft installation constraints (refer to Section K.6) to convert requirements
specified as absolute or relative (D/U) signal power at the receiver input to field strength
The performance of the aircraft VDB antenna is specified only with respect to the antenna
gain in the horizontal plane while actual installed antenna gain can also vary with elevation.
Consensus has been reached, as described by the 3 assumptions listed in Section K.6.1.1,
on the impact of this for the desired signal level which allows the use of the horizontal
antenna gain to convert between receiver input and field strength.
For simplicity, this appendix will reference absolute power in dBm and relative power in
dB at the output of an isotropic receiving antenna. The term antenna power therefore refers
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