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

4 August 2023

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

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

Fig. 1

The used 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]: increasing the gain of the spectrum to 65 dB (Figure 2) is also possible to detect the message sent by the corresponding station so that we can see both the air & ground versions (...admitted that I have really 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 Research Institute developed the high-speed communication P-097M, that is the successor of P-099 Chaika: one of the key differences between the new system and its predecessors is the high 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 two frequencies are not constant and change after every switch: a single oscillator, maybe a VCO, is used (if two distinct generators were used we would see no phase changes). Given that Figure 5 shows the durations of two periods, it's possible to come back to the two tones frequency:

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

ie just 500 Hz shift.