Priyom Discovers Polish Intelligence Digital Transmissions

21 December 2014

On November 25th, 2014, we noticed a transmission made with an FSK digital mode used by Polish intelligence, which started an investigation of its internals. This system has been in use since at least 2009, and a decoder for it is already included in certain expensive software, but little of its operation habits have been documented in public sources.

This system uses the baud rate of 100 bd, the shift of 740 Hz, and a custom 16-bit alphabet.

POL FSK without a message (December 8th, 2014; 08:45 UTC; 9370 kHz)

POL FSK with a message (30 groups) (December 4th, 2014; 07:35 UTC; 16388 kHz)

We've managed to reverse-engineer the protocol used by this mode.

We learned that many of the schedules of E11, S11a and M03 are in fact repeats of the FSK broadcasts, almost all of which occur 15 minutes prior to the start time of voice and Morse code transmissions. As of posting this, there are 37 known analog schedules, 17 were found to have digital counterparts, many of which interestingly use completely different ID's, and none were found for 13. has compiled a list of the schedules and their links.

Interesting developments have occurred on this mode since we started the investigation. This system historically used the 625 Hz shift, and that was the configuration it used when we first found it. However, on December 4th, 2014, they started using the 740 Hz shift, which since December 10th is the mode's sole configuration.

POL FSK's old shift of 625 Hz (December 2nd, 2014; 06:30 UTC; 7772 kHz)

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