User Guide
Series III Avionics
Pilot's Guide
Page 26
TRS-42A Notes
1. Airborne ATC transponders are designed to operate in the Air Traffic Control Radar Beacon System
(ATCRBS) environment to support inflight aircraft identification and traffic control.
2. Airborne ATC transponders respond to signals from ATC secondary radar that scans the same
volume of airspace as, and in synchronization with, ATC primary radar. The ATC secondary radar
operates on 1030 MHz. On detecting a signal at this frequency, the airborne transponder responds
on a frequency of 1090 MHz.
3. Airborne ATC transponder responses consist of a combination of pulses determined by the position of
the CODE SELECT switches to the CD-422B Control Display Unit. Eight possible digits (0 through 7)
for each digit of the code allow a total of 4096 (8 x 8 x 8 x 8) code combinations.
4. An airborne ATC transponder transmits only when "swept" by the special ATC secondary radar. A
transmission lasts for a few millionths of a second. During this transmission, and for about a half
second afterwards, the letters ID are annunciated on the CD-422B. This informs the pilot that he is in
the ATCRBS environment and that his transponder is functioning normally.
5. Distance Measuring Equipment (DME) and airborne ATC transponders operate in the same
frequency band. To prevent interference, each system generates a special pulse during the time it is
transmitting. This pulse is called a suppression pulse and is wired between units to automatically
prevent simultaneous transmissions that might damage circuits of the receiving system that is
"listening" for very weak signals. This has no effect on DME or ATC Transponder instrumentation in
the cockpit.