Home' RTCA Documents for Review : DO-253D_Change1_FRAC Contents Appendix K
DO-253D Change 1
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continuity requirement allocated to the VDB can be met. A message reception opportunity
is a transmitted signal that complies with all the transmit requirements and the receiver is
in an environment that has an associated message reception rate requirement, considering
the receiver recovery time associated with a short term VDB outage. Note, for example,
that there are specific environments where the receiver does not have an associated
message reception rate requirement (e.g., when interference is outside the required receive
environment to include interference power, D/U ratios, etc.).
Specifically, to meet the loss of continuity risk associated with the loss of VDB messages
in the presence of short duration outages, the following must be satisfied to maintain
continuity during an operation:
For GAST D as well as for GAST C systems that support the authentication protocols,
at least 3 opportunities to receive Message Type 2 need to be provided to the receiver
within each and every 60 second period (sliding window).
For GAST C, at least 3 opportunities to receive Message Type 1 need to be provided
to the receiver within each and every 3.5 second period (sliding window).
For GAST D systems where the aircraft position is 200 feet or more above the Landing
Threshold Point (LTP), at least 3 opportunities to receive Message Type 1 and Message
Type 11 (paired by the modified z-count) need to be provided to the receiver within
each and every 3.5 second period (sliding window).
For GAST D systems where the aircraft position is below 200 feet above the LTP, at
least 3 opportunities to receive Message Type 1 and Message Type 11 (paired by the
modified z-count) need to be provided within each and every 1.5 second period (sliding
o To ensure an aircraft on approach is provided at least 3 opportunities to receive a
set of Type 1 and Type 11 messages (paired by the modified z-count) when the
aircraft arrives at the 200 feet above the LTP, one will need to consider the impact
of any VDB outage that occurs during the 1.5 second period prior to the aircraft
reaching 200 feet above the LTP.
For both GAST C and GAST D services, the extent of a VDB outage cannot exceed
2.5 seconds (to comply with receiver requirements for recovery from a VDB outage).
o Note that while one outage duration cannot exceed 2.5 seconds, satisfying the other
items identified above for the supporting GAST C and/or GAST D services may
limit the maximum outage duration to be less than 2.5 seconds.
The “short duration” VDB outage time is defined to be the total time duration for one
outage period of the VDB whereby an aircraft conducting the operation experiences
interference at the VDB receiver that: a) has ILS localizer or VOR power above Sma x and
below Sma x + 9 dB, or b) the D/U is insufficient for required VDB receiver message
reception, or c) both “a” and “b”, plus any associated VDB receiver recovery time. Note
that an undesired VDB signal never exceeds the maximum allowed field strength of the
desired VDB signal within the service volume.
For the case of ILS localizer interference, this can happen, for example, when an aircraft
on the final stages of approach flies over the localizer prior to the runway threshold. For
the case of VOR interference, this can happen, for example, when an aircraft on the final
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