A
B
Interrogation --"-..'---,"~:---------
Reply --.l-.·~·-.L.-.:iI-.
Fig. 8-Mode-S elimination ofsynchronous garble.
The Lincoln Laboratory Journal. Volume 2. Number 3 (1989)
sition, an aircraft is locked out to further all-call interrogations. The process is repeated (and the probabilitymay be increased) until all aircraft in the garbling region are acquired. Take the sim-plest case. If two aircraft are contributing to a garbled condition, both vehicles could be in-structedto reply witha probabilityof0.5. Ifboth vehicles reply again or if neither of them does, the aircraft are reinterrogated until only one reply is received. The aircraft that replies is then discretelyinterrogated and subsequentlylocked out so that the other vehicle can then be easily acquired.
ATCRBS-to-Mode-S Compatibility
Compatibility between ATCRBS and Mode S is achieved through the following.
(1)
Mode-S transponders will respond to ATCRBS interrogations. However, a conventional ATCRBS sensor will detect a Mode-S-equipped aircraft as an ATCRBS aircraft.
(2)
ATCRBS-equipped aircraft will reply to Mode-S sensors.
(3)
Mode S operates on the same I,030-MHz uplink and I,090-MHz downlink that are used by ATCRBS. The use of the same frequencies greatly simplifies the construction of Mode-S sensors and transponders because the same trans-mitters and receivers will be able to handle both ATCRBS and Mode-S transmissions.
(4)
Mode-S waveforms were designed to prevent mutual interference with ATCRBS signals. This provision is de-scribed in greater detail in a following section.
ATCRBS Mode-S Time-Sharing
Through time-sharing, a Mode-S sensor can provide surveillance of both ATCRBS and Mode-S aircraft. Figure 9 shows the time line, drawn approximately to scale, of a typical Mode-S sensor. During the time of one beam dwell, the Mode-S sensor provides four Mode-A/C/S all-call periods. (A beam dwell is the
I+-Beam-Dwell Time---l /ATCRBS/Mode-s All-Call Periods
Time
'Mode-S Roll-Call Periods
Fig. 9-ATCRBS-Mode-S time sharing.
typical time, approximately 30 ms for a terminal sensor, that a beam is actually on an aircraft.) This scheduling provides the interrogation and listening intervals required by the mandatory two Mode-A and two Mode-C replies. All-call ac-quisition is also performed dUring this time.
Selective Mode-S interrogations are sched-uled during the Mode-S roll-call periods. Note that the use of monopulse for ATCRBS enables the sensor to devote most of the time line to Mode S.
Mode-S All-Call Interrogation
Figure 10 shows the all-call interrogation waveform that Mode S normally uses. As stated earlier, thewaveformiscomposedofthesamePI and P3 pulses used for ATCRBS interrogations. The P4 pulse identifies the interrogation as originating from a Mode-S sensor.
Ifthe Ppulse is 1.6 ).ls long, the interrogation
4
Mode A: 8.0 JJs Mode C: 21.0 JJs 2.0 JJs I' .I. .,
Interrogation ~~ 1-1 1-1 I-t 0.8 JJs 0.8 JJs *JJs
I~.O JJ~I
SLS Control .-J'P~'L...JP;1'---------
Transmission I-f
0.8 JJs
* Mode-AIC/S All-Call: 1.6 JJs Mode-AlC-Only All-Call: 0.8 JJs
Fig. 1Q-Mode-Sall-call interrogation waveform.
is a Mode-A/C/S all-call format. which elicits Mode-A/C replies from ATCRBS transponders and Mode-S all-call replies from unlocked Mode-S transponders. If the P4 pulse is 0.8 ).lS long, the interrogation is a Mode-A/C all-call format, which elicits Mode-A/C replies from ATCRBS transponders and no reply from Mode-S transponders. Mode-S sensors use this waveform in connection with special Mode-S-only all-call interrogations. The Traffic Alert and Collision Avoidance System (TCAS) also uses the waveform for surveillance of ATCRBS aircraft. TCAS acquires Mode-S aircraft pas-sively by listening for all-call replies (known as squitters), which are spontaneously generated approximately once per second by all Mode-S transponders. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：The Mode S Beacon Radar System(5)
