FSK 150Bd/500, prob. RusAF "Chayka" (telecode "Seagull")

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Interesting catch of any short FSK signals on 6885.5 KHz (cf) with modulation velocity of 150 bps and shift of 500 Hz. The FSK parameters and the receiver used, an AirSpy server in Ukraine, are good indications in favour of the Russian Air Force strategy called "Chayka" ("Чайка", Seagull): a command/signaling message system, encrypted, utilized for military aircraft-ground communications.

Fig. 1

The utilized equipment could be Р-095 or Р-099 (R-095 and R-099, if translated), that is "aviation on-board telecode communication equipment" [1][2]: expanding the gain of the spectrum to 65 dB (Figure 2) is besides possible to detect the message sent by the corresponding station so that we can see both the air & ground versions (...admitted that I have truly heard a Chayka signal).

Fig. 2

A "Chayka codegram" may consist of separated segments and/or insertions, as clearly visible in Figure 3 where the signal has been resampled to 3788 KHz.

Fig. 3

By the way, specialists of the Kaluga investigation Institute developed the high-speed communication P-097M, that is the successor of P-099 Chaika: 1 of the key differences between the fresh strategy and its predecessors is the advanced automation [3].

source: (http://wiki.airforce.ru/ - List of airborne radio communications: Data communication equipment)

As a final note, Figure 4 shows that the phases of the 2 frequencies are not constant and change after all switch: a single oscillator, possibly a VCO, is utilized (if 2 distinct generators were utilized we would see no phase changes). Given that Figure 5 shows the durations of 2 periods, it's possible to come back to the 2 tones frequency:

2: 0.001286 = 1555.20 Hz
2: 0.001896 = 1054.85 HZ

ie just 500 Hz shift.