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Mode S - Differential Phase shift Keying (DPSK)
Figure: Mode S - differential phase shift keying (DPSK)
Whether the interrogation is short or a long pulse, the total duration of the P6 pulse is either 16.25 µs or 32.25 µs. Mode S Uplink interrogations use in the P6 pulse Differential Phase Shift Keying (DPSK) to modulate the data in the uplink format. All subsequent information in the P6 pulse is coded as 180° phase reversals of the carrier frequency. Each reversal must have a duration of 0.08 µs. The P6 begins with an initial phase reversal at the start of the P6 pulse with a length of 1.25 µs. This is known as the sync phase reversal.
After the sync phase reversal all subsequent phase reversals indicate the 56 or 112 bit P6 information. All subsequent timing is taken from the point of the first phase reversal. Each phase has a duration of 0.25 µs and is known as a „chip”. The series of chips starts 0.5 µs after the sync reversal.
