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A mode A/C transponder receives a P2 pulse from which the amplitude is larger than that of the P1 (and P3) pulse. What is the matter?
  • A
    The transponder is located in the direction of one of the side lobes of the interrogator antenna.
  • B
    The interrogation is only meant for mode S transponders.
  • C
    The transponder is located in the direction of the main lobe of the interrogator antenna.
  • D
    The transponder is beyond the usable range of the SSR ground station.

SIDE LOBE SUPPRESSION (SLS)

Aerial side lobes cause a problem since aircraft in the close proximity of the aerial may be interrogated outside the main beam. Consequently, a false response is triggered.

To overcome this issue, a P2 pulse is added to the signal. which is used between pairs of pulses as a control pulse to prove response validity. The spacing between P1 and P2 is fixed at at 2 μs. This causes the P2 polar to be superimposed onto the pulse pair signal. The only time the P2 signal strength is smaller than that of P1 and P3, the signal is received in the main lobe. Reception in the sidelobe results in a higher signal strength of P2. The receiver compares the magnitude of the pulses and if the P2 pulse is larger than the interrogation pulse, the response is denied. Therefore, it only replies to interrogation by the main beam - this is called Side Lobe Suppression (SLS).

  • If the combined P2 signal is stronger => aircraft in a side lobe.
  • If the combined P2 signal is smaller => aircraft in a main lobe.

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