User's Manual

ROCKWELL COLLINS
COMPONENT MAINTENANCE MANUAL with IPL
TDR-94, PART NO 622-9352
Air Traffic Presentation on the ATC Radarscope
Figure 4/GRAPHIC 34-50-96-99B-071-A01
(2) SSR System Interrogation Description
(a) Refer to Figure 5/GRA PHIC 34-50-96-99B-072-A01 and Figure 6/GRAPHIC
34-50-96-99B-073-A01. The SSR uses a rotating directional antenna to transmit
two puls
es which are identified as P1 and P3. The spacing of these pulses
determines the mode. In mode A the pulses are spaced 8 μs while in mode C
the pulses are spaced 21 μs. The SSR also uses an omnidirectional antenna to
transm
it a third pulse designated P2. This pulse is transmitted 2 μs after the P1
pulse and provides a reference for side lobe suppression (S LS). The amplitude of
the P2 pulse is about the same as the peak side lobe of the directional antenna.
Ty p ic
ally, this is about 18 dB below the peak of the directional (main) beam.
(b) Refer to Figure 5/GRAPHIC 34-50-96-99B-072-A01. Notice the two airplanes on
the dr
awing. Air plane A is shown within the main lobe of the directional antenna.
The amplitude of the P1 and P3 pulses will be substantially greater here than the
P2 pulse radiated from the omnidirectional antenna. Therefore, the transponder in
this
air plane will inter pret this interrogation as valid.
(c) Airplane B, however, is outside the main lobe and within one of the side lobes.
The P
1 and P3 pulses detected here will be the result of side lobe radiation.
Recall that the P2 pulse is transmitted by an omnidirectional antenna and is about
34-50-96
Page 32
May 18/06