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3.1.2.10.4.3 Antenna selection. Mode S transponders equipped for diversity operation shall have the capability to
evaluate a pulse sequence simultaneously received on both antenna channcls to determine individually for each channel
if the P, pulse and the P2 pulse of a Mode S interrogation preamble meet the requirements for a Mode S interrogation as
defined in 3.1.2.1 and if the P, pulse and the P, pulse of a Mode A, Mode C or internode interrogalion meet the
requirements for Mode A and Mode C interrogations as defined in 3.1.1.
Note.- Transponders equipped for diversity operation may optionally have the capability to evaluate additional
characteristics of the received pulses of tlze interrogations in making a diversity channel selection. The transponder may
as an option evaluate a complete Mode S interrogation simultaneously received on both channels to determine individuaIly
for each channel if the interrogation meets the requirements for Mode S interrogation acceptance as defined in
3.1.2.4.1.2.3.
3.1.2.10.4.3.1 If the two channels simultaneously receive at least a PI - P2 pulse pair that meets the requirements for
a Mode S interrogation, or a PI - P3 pulse pair that meets the requirements for a Mode A or Mode C interrogation, or if
the two channels simullaneously accept a complete interrogation, the antenna at which the signal strength is greater shall
be selected for the reception of the remainder (if any) of the interrogation and for the transmission of the reply.
3.1.2.10.4.3.2 If only one channel receives a pulse pair that meets the requirements for an interrogation, ox if only
one channel accepts an interrogation, the antenna associated with that channel shall be selected regardless of received
signal strength.
3.1.2.10.4.3.3 Selection threshold. If antenna selection is based on signal level, it shall be carried out at all signal
levels between MTL and -21 dBm.
Note.- Either antenrza may be selected if the dzfference in signal level is less than 3 dB.
3.1.2.10.4.3.4 Received signal delay tolerance. If an interrogation is received at one antenna 0.125 microsecond
or less in advance of reception at the other antenna, the interrogations shall be considered lo be simultaneous
interrogations, and the above antenna selection criteria applied. If an accepted interrogation is received at either antenna
0.375 microsecond or more in advance or reception at the othcr a.ntcnna, Lhe antenna selected for the reply shall be that
Annex 10 - Aeronautical Telecommunications Volume IV
which received the earlier interrogation. If the relative time of receipt is between 0.125 and 0.375 microsecond, the
transponder shall select the antenna for reply either on the basis of the simultaneous interrogation criteria or on the basis
of the earlier time of arrival.
3.1.2.10.4.4 Diversity transmission channel isolation. The peak RF power transmitted from the selected antenna shall
exceed the power transmitted from the non-selected antenna by at least 20 dB.
3.1.2.10.4.5 Reply delay of diversity transponders. The total two-way transmission difference in mean reply delay
between the two antenna channels (including the differential delay caused by transponder-to-antenna cables and the
horizontal distance along the aircraft centre line between the two antennas) shall not exceed 0.13 microsecond for
interrogations of equal amplitude. This requirement shall hold for interrogation signal strengths between MTL +3 dB and
-21 dI3m. The jitter requirements on each ind~vidual channel shall remain as specified for non-diversity transponders.
Note.- This requirement limits apparent jitter caused by antenna switching and by cable delay diflerences.
3.1.2.10.5 DATA PROCESSING AND INTERFACES
3.1.2.10.5.1 Direct data. Direct data shall be those which are required for the surveillance protocol of the Mode S
system.
3.1.2.10.5.1.1 Fixed direct data. Fixed direct data are data from the aircraft which do not change in flight and shall
be:
a) the aircraft address (3.1.2.4.1.2.3.1.1 and 3.1.2.5.2.2.2);
b) the maximum airspeed (3.1.2.8.2.2); and
c) the registration marking if used for flight identification (3.1.2.9.1.1).
3.1.2.10.5.1.2 Inte@aces formed direct data
Recommendation.- Interfaces from the transponder to the aircraft should be designed such that the values of the
fmed direct data become afunction of the aircraft installation rather than of the transponder configuration.
Note.- The intent of this recommendation is to encozrrage an interface technique which permits transponder exchange
without manipulation of the transponder itselffor setting the fixed direct data.
3.1.2.10.5.1.3 Variable direct data. Variable direct data arc data from the aircraft which may change in flight and
shall be:
a) the Mode C altitude code (3.1.2.6.5.4);
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