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Mode S Individual Interrogation

Figure 1: Mode S - short interrogation (56 bits - 16.25 μs)

Figure 1: Mode S - short interrogation (56 bits - 16.25 μs)

Mode S Individual Interrogation

The Mode S up link interrogation format starts with two pulses, P1 and P2, which are solely for the purpose of suppressing existing Mode A/C only transponders so that they are not aware of the main Mode S information. The Mode S interrogation data contained in the P6 data block is phase modulated. The first phase reversal is the timing point for the subsequent bits (chips) of information. The Mode S interrogation may be of short (56 bits) or long (112 bits) format.

Figure 2: Mode S - long interrogation (112 bits - 30.25 μs)

Figure 2: Mode S - long interrogation (112 bits - 30.25 μs)

The Mode S side lobe suppression pulse P5 is transmitted from the control beam like the P2 ISLS in the Mode A/C system. If P5 is of more power than P6 it has the effect of overwhelming the sync phase reversal of P6 so that the Mode S transponder cannot read the subsequent information.