5 March 2021

Rus PSK4 1200Bd

Transmission heard on 8741.5 KHz/USB, thanks to the KiwiSDR owned by YO3IBZ (Bucharest, Romania), on march 3rd. Although I tuned it on 8741.5 KHz I think the real tuning frequency be 8742.0 KHz (usual 1800 Hz subcarrier). As shown in Figure 1 the waveform employs a QPSK modulation at a rate of 1200Bd, ACF  result is 425 ms, ie 511 dibit symbols @1200Bd speed.

Fig. 1

As expected from ACF (511 symbols), the demodulated bitstream consists of a continuous repetition of the same 1022-bit pattern (Figure 2) thus - from time to time - I tuned it waiting for traffic, but luckless until it went off. All I can tell is that the signal was on-air all the "monitoring" period (ie 12 hours, from 0700 to 1900 utc), but in the following days and until today (March 5th), the transmissions seem to have ceased, at least on the reported frequency.

Fig. 2
 TDoA attempts clearly indicate an area south-east from Moscow as the most probable site of the Tx (Figure 3)

Fig 3

https://disk.yandex.com/d/O6yCdhU29f0Y3w

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