Home' RTCA Documents for Review : DO-253D_Change1_FRAC Contents Appendix K
© 2019, RTCA, Inc.
DO-253D Change 1
systems and minimum receiver input sensitivity. In the absence of other
transmitters (VOR, Localizer, or VDB), the minimum and maximum field strength
is consistent with the minimum receiver sensitivity, Smin, and maximum signal, Sma x ,
specified in Section 2.2.5 and minimum distance of 80 meters from the transmitter
antenna for a coverage range of 23 NM.
The VDB transmission minimum field strength is also constained by the adjacent channel
interference (undesired VDB, VOR, and ILS localizer transmitters). The field strength
must be within the D/U requirements of the VDB receiver. The D/U requirements apply
only when the undesired signal field strength is limited to the same maximum value as the
desired signal. Section K.5.2 addresses the conversion of receiver requirements specified
relative to the desired VDB signal power (D/U) from receiver input to signal in space.
Section K.6 describes the aircraft installation considerations and addresses the constraints
that support this D/U conversion.
Section K.7 describes the ground installation considerations for this D/U conversion and
gives examples on how to apply the aircraft installation constraints. Section K.8 addresses
the consequences on continuity of exceeding the maximum undesired signal field strength
or the receiver D/U. Throughout the operation where continuity is required including
during short duration VDB outages, as long as there are at least 3 message reception
opportunities within the message timeout periods for all messages being timed out by the
airborne receiver, the continuity requirement allocated to the VDB can be met.
VDB RF Field Strength Variation
The GBAS system was originally validated under the assumption that a time slot would
not be used by more than one VDB transmitter antenna. Full field strength variations
between slots allow the use of different transmitter antennas in separate time slots. The
VDB receiver Message Failure Rate (MFR) requirements allow for full dynamic range slot
to slot within a frame. The VDB receiver MFR requirements allow for frame to frame
power variations of 40 dB between successive bursts in a same time slot to account for
worst-case fading and maneuvers during the approach. The assumption was that all bursts
would meet the field strength requirements.
Successive bursts are defined as consecutive, scheduled bursts from a ground transmitter
subsystem. ICAO Annex 10 GBAS VDB ground subsystem transmitter broadcasts are
required to have a burst in at least every fifth consecutive frame for each assigned time slot
being used. The test procedures in this document conservatively test that the receiver will
recover from interruptions of reception of 10 seconds (i.e., 20 frames) or less; however the
test procedures do not attempt to cover all permutations of missing bursts in a sequence of
frames nor the effects of multiple transmitters. Note that the absence of a signal (i.e., no
burst transmitted) in a frame is not considered a violation of the 40 dB constraint.
Subsequent work on ground station siting has concluded that for large, complex airports,
it will be difficult or impossible to meet the VDB coverage requirements related to GAST
D with a single VDB transmitter antenna.
The authentication protocol, as defined in RTCA/DO-253C, supports the transmission of
Type 2 message only in a single designated time slot. This original protocol complicates
the use of antenna diversity to fill the field strength coverage holes. Thus the GBAS system
was changed to allow the Type 2 message to be transmitted in two consecutive time slots
and the sharing of one or more time slots in alternating frames between two transmitter
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