User's Manual

ROCKWELL COLLINS
COMPONENT MAINTENANCE MA NUAL with IPL
TDR-94, PART NO 622-9352
SUBTASK 34-50-96-870-003-A01
C. ATCRBS Operation
(1) ATCRBS Interrogation Modes
(a) The SSR system interrogates the airplane transponder for air plane identity and
altitude. The interrogations are in the form of two modes: mode A for airplane
identity and mode C which is used to request altitude information (refer to the note
below for additional inform ation).
N
OTE: In the original definition of ATC modes, two other modes were defined;
mode B and mode D. These also differ only in pulse spacing with mode B
pulses spaced 17 μs, and mode D pulses spaced 25 μs. Mode D was
widely used in Great Br itain but recently the aviation industry
has settled on mode A for ATCRBS operation and mode C for
altitude reporting. Modes B and D have been largely abandoned.
The US Militar y uses a system similar to ATCRBS. It is known as IFF
(Identification - Friend or Foe). As this name implies, it is concerned
primarily w ith mission security. Three m odes are defined; modes 1, 2, and
3. Mode 3 is common to the civil mode A with a pulse spacing of 8 μs. This
allows air traffic control visibility of all airplanes, both civil and military.
(b) All pulses are 0.8 μs wide. The interrogations from the ground station are at
a frequency of 1030 MHz. The transponder replies at a frequency of 1090
MHz. The received signal from the airbor ne transponder is decoded by the
ground system and displayed on the ATC radar screen (see Figure 4/GRAPHIC
34-50-96-99B-071-A01). The replies produce either a single or double slash target
display on the controller screen. The controller can also elect to display the airplane
identification number (as selected by the aircrew) and altitude.
34-50-96
Page 31
May 18/06