31 December 2016

a STANAG-4285 variant

The STANAG-4285 waveform is well defined by looking at ACF duration and frame structure of the signal (Fig. 1), unless the initial 100msec tones which are not provided by the standard (Fig. 2).

Fig. 1
Fig. 2
Data signaling is 600 bps but the S-4285 standard decoders do not recognize the SOM/EOM sequences using both short or long interleaving.  Using the short interleaving it's possible to get  a 128-bit period for the datalink protocol: data seem to be arranged in blocks separated by 6x128 bit headers (Fig. 3).

Fig. 3
Traffic is quite constant on 6931.0 KHz/USB, is not preceeded by ALE or selcall procedures and the strengths and durations of the messages lead to think to an ARQ system so it is likely a well-known channel by the receive peer.  The same signal was heard on 5097.5 KHz/USB two years ago and reported here in radioscanner forum: R_V_S talk about a Croatian source and it could be possible since they have the internal resources to implement modified/proprietary protocols as their CROZ S-5066 mail gateway.


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