User's Manual

SYSTEM DESCRIPTION AND INSTALLATION MANUAL
T
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CAS/Part No.9005000
an ATCRBS interrogator (no P4), the Mode S Transponder
responds with a standard ATCRBS reply. An uplink
interrogation pulse group, which includes a P4 pulse, makes
the Mode S Transponder prepare to measure the P4 pulse
width. As soon as the Mode S Transponder detects the
rising edge of the P4 pulse, it disables its ATCRBS reply. It
then looks for a pulse width of 0.8 or 1.6 microseconds. A
pulse width of 0.8 microseconds is recognized by the Mode
S Transponder as an uplink ATCRBS-only All-Call, and it
does not respond at all. In this case, the ground station is
looking for all the aircraft in the vicinity that are equipped
with ATCRBS only. In the case of a 1.6-microsecond pulse
width, the Mode S interrogator wants the Mode S
Transponder to respond with the Mode S All-Call coded
reply. In this case, the interrogator receives the aircraft’s
identity, which is the unique number that is given to the
Mode S-equipped aircraft. The 1.6-microsecond pulse width
is in fact an All-Call interrogation for both ATCRBS and
Mode S Transponders, eliciting both ATCRBS and Mode S
replies. There is no interrogation addressing in this case as
each system responds within its own capability.
d
Mode S Interrogation
The final type of uplink interrogation is made up of P1 and
P2 pulses of equal amplitude followed by a long pulse of
constant amplitude called P6. This is a Mode S interrogation
pulse (P6) that occurs 1.5 microseconds after P2. When the
P1 and P2 pulses are of equal amplitude, the standard
ATCRBS transponders see the P2 pulse and do not
respond, since they interpret this as a side lobe
interrogation.
e Modes S Short/Long Interrogation Messages
The Mode S Transponder sees the P1 and P2 pulses and
prepares to receive P6 and the uplink Mode S message.
There are two types of messages that are defined in terms of
length. The first type of interrogation is a short message and
is 56 bits long; the second is a long message and is 112 bits
long.
4 Mode S Timing
For the ATCRBS/Mode S All-Calls the timing reference, or trigger
point, for the Mode S reply is from the leading edge of pulse P4.
From pulse P4, the response time for a Mode S reply is 128 ±0.5
microseconds. When responding to a standard ATCRBS
interrogation, the timing reference is the leading edge of pulse P3
and the ATCRBS response time is 3 microseconds.
A typical Mode S interrogation contains the equal amplitude P1 and
P2 pulses, in addition to the video pulse P6 that contains the Mode S
uplink message phase encoded information.
Pub. No. 8600200-001, Revision 004
34-45-29
1-114
04 Nov 2014
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