Home' RTCA Documents for Review : DO-253D_Change1_FRAC Contents Appendix K
DO-253D Change 1
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The use of multiple transmitter antennas transmitting in alternating time slots implies that
some bursts will be below the minimum field strength in some regions of the service
volume. The VDB MFR requirements specifically allow for a burst below minimum field
strength while maintaining full MFR performance on the burst within the field strength
limits as long as that low power burst is within 40 dB at the receiver input from the higher
power burst. Since this scenario is for cases where minimum field strength cannot be
provided with one transmitter antenna, it is required that the maximum field strength is
always respected in order to meet the MFR requirement.
The receiver MFR requirements also specify a degraded MFR (60%) when the 40 dB
constraint is violated. Again, the degraded MFR applies to signals within the field strength
requirements and allows for one of the signals (that is assumed to be missed 100% of the
time) to be below the minimum field strength. However, this requirement only covers the
scenario of “successive frames” to signify this alternating frame operation rather than
“successive bursts” (which allows empty frames) that is used in the full MFR requirement.
It is important to note that the MFR requirement considers the aggregate of all bursts within
the field strength requirement to establish the MFR performance. There is no implication
that grouping the bursts with similar power can be assumed to have any particular
allocation of the aggregate performance.
The MFR requirement includes the condition of the VDB field strengths in any and all
combinations of slots within a given frame varying by up to 72 dB spanning the full
dynamic range of the field strength combined with the maximum antenna gain variation in
the horizontal plane. The MFR does not include the case of power variations from Smin to
Sma x since it was specified under the assumption of one transmitter antenna. A multiple
transmitter antenna system must respect this constraint.
These MFR considerations resulted in guidance in ICAO Annex 10 Attachment D, Section
184.108.40.206 of multiple transmitter antenna ground subsystems summarized below:
Whenever multiple antenna systems are used, the antenna sequence and message
scheduling must be arranged to provide broadcasts at all points within the service volume,
that adhere to the specified minimum and maximum data broadcast rates considering the
receiver’s ability to adapt to transmission-to-transmission variations in signal strength in a
given slot. Exceeding the 40 dB signal power variation requirement at the receiver input
is acceptable for limited areas within the service volume, provided it can be shown based
on receiver behavior as specified in RTCA/DO-253() and the assumptions listed below,
that the resulting performance is acceptable. Allowance for the potential variation in
aircraft antenna gain of up to 15 dB must be made when converting field strength variations
to power variations at the receiver input.
Message transmission and reception rate requirements, and time to alert requirements
prevent Type 1 and Type 11 messages from being alternated between antennas in the same
slot from frame to frame. Only Message Type 2 and Message Type 4 (and Message Type
3 as a filler message) are candidates for being alternated between antennas in the slot from
frame to frame. Continuity is maintained as long as a Message Type 2 is received at least
once per minute. The receiver does not verify repeated reception of Message Type 4 during
the final stages of an approach.
If an area exists, where the signal power variation is excessive (considering the VDB
receiver 40 dB signal power variation requirement at the receiver input and the allowable
airborne antenna gain variations) when using alternating VDB transmit antennas in a
shared slot, such that it may cause one minute or more outage of Type 2 Message reception
as an approaching aircraft passes through it, then it may be necessary to address the
potential message loss from a probabilistic point of view. In these cases, the multiple VDB
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