27 June 2016

Russian Air Force "Chayka", FSK 150Bd/500


This transmission was heard this morning on 14781.1 Khz (cf) at 0720 UTC sending encrypted bursts of ~21 secs duration. "Chayka" is the nick name for this commands/signaling FSK messaging system used  for  (military) aircrafts-ground communications.
The shift is costant 500 Hz while the manipulation speed may be 75, 150, 300 and 1200 Baud: in this sample the measured speed is 150 Baud (fig. 1).

fig. 1 - manipulation speed and shift
Each "message" consists of a preamble phase and the data phase; during the preamble phase the lower tone frequency is transmitted followed by the alternation of the two tone frequencies. The data phase may consist of separated segments and/or insertions of the 2-tones alternation, as clearly visible in figure 2 where the signal has been resampled to 4 KHz.

fig. 2
At least according to this sample, the messages can be continuously transmitted as 2 in-a-row or 3 in-a-row without manipulation breaks (figure 3) and this affects the lengths of the preambles.

fig. 3

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