7 February 2017

a modified/proprietary MIL 188-110B/C Appendix C waveform

It may happen that manufacturers sometimes modify one or more parameters of standard HF waveforms and then obtain proprietary waveforms (and modems) as in the case of these signals, copied today on 8207.0 KHz/USB around 0940 UTC (Fig. 1).

Fig. 1
The different fadings that affect the bursts lead to think to two stations operating in half-duplex mode. Each burst has a ~2520 msec duration and consists of a 1500Hz single tone modulated with PSK-8 modulation at a costant symbol rate of 1800Bd (Fig. 2): that's a bit odd since we are used to observe PSK-8 modulations coupled with 2400Bd speed and 1800Hz sub-carrier.

Fig. 2
Things get more interesting analyzing preamble and data blocks structure. The first part of the preamble consists of seven blocks of 184 PSK-8 symbols and the data blocks are structured in frames consisting of 287 PSK-8 symbols (ie 861-bit period), as shown in Figure 3.

Fig. 3a
Fig. 3b

This is clearly a MIL 188-110B/C Appendix C (or STANAG-4539) modem, but what about the symbol-rate (1800Bd) and the sub-carrier (1500Hz)?
As said above, modem manufacturers may change one or more parameters of the HF waveforms: in this case we face a sort of underclocked STANAG-4539 modem, ie the modulation speed is reduced from 2400 to 1800 Baud as well as the sub-carrier frequency from 1800 to 1500 Hz.
Let's see a trick (thanks to KarapuZ) that prove that it's a S4539 modem: correcting the baud-rate and shifting the signal (Fig. 4) we get the right S4539 3200bps/short interleaving waveform which can be easily processed and demodulated (Fig. 5).

Fig. 4
Fig. 5
By the way, the signals transport STANAG-5066 data.

Another clue to identify the modem/manufacturer is the 1250Hz "roger beep" sometimes heard during voice-comms before the data transmissions. Helps are welcome.

A good "candidate" could be the CODAN 3212 HF-modem, this modem use a modified STANAG-4539 (188-110B App.C) to fit 2400 Hz channels (thanks to Johannes for the hint):

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