unid, possibly ARQ, system heard and tuned using the remote Twente-SDR on 5740.0 KHz/USB for a long standing time. The waveform used by S1 S2 stations is a 1800 Hz carrier which is modulated at constant rate of 2400 symbols per second (fig. 1): QPSK is probably used for the initial preamble and PSK-8 for the data blocks (fig. 2). Each transmission, both from S1 and S2, lasts about 1660 msec.
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| fig. 1 |
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| fig. 2 |
The use of QPSK modulation is confirmed by the four accentuaded positions in the constellation diagram (fig. 3), moreover the preamble seems to be sent at higher level than the data blocks (fig. 4).
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| fig. 3 |
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| fig. 4 |
The signal has an ACF of 112.5 msec which corresponds to a period length of 810 bit or 270 PSK-8 symbols @ 2400 Baud. However, the frame length is the half of the period i.e. 135 symbols: the ACF value may be due to the lenght of the scrambler or the length of the interleaver, i.e. 810 bit. Most likely the frame consists of 32 known symbols and 103 unknown data symbols (fig. 5).
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| fig. 5 |
https://yadi.sk/d/8sNOkDw6y4fap
An interesting comment from my friend ANgazu:
UNID PSK in last entry looks very interesting. Seems to be an hibrid modem.
Header is likely to use MSK. 32 symbols sync sequences use QPSK and are very similar to MIL-STD reinserts.
I agree, the strong PSK-4 positions in the constellation are just due to the repeated known symbols sequences.