Interesting and unid STANAG-4539 bursts recorded on 5270.0 KHz/usb. The user data rate is 3200 bps and is obtained using QPSK modulation, short interleaver is used. Notice that the quadrature phase-shift keying constellation is scrambled to appear, on-air, as a PSK8 constellation. The bursts have a duration of 1350 ms, each transports nine 256-symbols data blocks, and do not seem to obey to a particular timing.
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| Fig. 1 - S4539 3200bps waveform |
The bitstreams after demodulation have a common preamble consisting of three components (Figure 2):
a) idle sequences of '0's and '1's (depending on the polarity of the receive modem)
b) initial 334-bit (frame sync?) sequence
c) 128-bit length Initialization Vector, three times repeated (3x128-bit)
encrypted data block follow.
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| Fig. 2 - COMSEC preambles |
The most interesting component is the 334-bit sync sequence that has a particular 24-bit period and - as a mere attempt - I found that it could be generated by the polynomial x^21+x^10+1.
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| Fig. 3 - 334-bit sync sequence |
By the way, MIL 188-220D describes the three components which compose the initial COMSEC preamble: the Bit Synchronization subfield or the Phasing subfield (it may consists of a string of alternating ones and zeros), the Frame Synchronization subfield, and the Initialization Vector subfield (Figure 4); it must be said, anyway, that the Bit Synchronization patterns do not match. Notice that Figure D3 illustrates the case where the Robust Frame Synchronization is not used (see 188-220D #D.5.2.2)
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| Fig. 4 |
User, purposes of the transmissions, and Tx location(s) are unidentified; the only thing I can add is that the better reception is possible by using receivers located in the north Europe countries: by the way, I used two KiwiSDRs located in Denmark [1] and Norway [2].
https://disk.yandex.com/d/KgUT7aaESDD6hA
[1] http://85.191.81.117:8073/
[2] http://kiwi.wlansupport.no:8073/
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