UK DHFCS FSK 800Bd/850 & STANAG-4285 1200bps/L

Interesting and unknown (at least for me) FSK transmission in 800Bd/850Hz mode sent to me by my friend cryptomaster. These transmissions have been heard during the evening (UTC) on the frequencies 8014, 8180, 10008 and 12414 kHz.

Fig. 1 - FSK parameters

The signal has an ACF of approximately 1280 ms which corresponds to a period of 1024 bits (Figure 2).

Fig. 2 - ACF value and relative bitmap

  

The bitstream obtained after demodulation has a complex and very interesting structure (Figure 3).

Fig. 3 - demodulated bitstream

The most interesting thing is that after a few days the FSK signals have disappeared and been replaced by STANAG-4285 signals in two of the above-mentioned frequencies: 8013.20 and 10008.20 KHz/USB (Figure 4).

Fig. 4 - FSK & STANAG-4285 transmissions

The S-4285 transmissions - which have also disappeared - were recorded in 1200bps/L mode and once demodulated show a bistream period of 1536 bits which is very similar to the 1024 bits bitstream of the previous FSK transmissions: just 512 bits of difference, see Figs. 3 and 5 (below).

Fig. 5 - 1536 bits period bitstream of the STANAG-4285 transmissions

The format of the bitstreams (both FSK and STANAG-4285) suggests the use of a TDM (Time Division Multiplexer), probably the GA-205 model, or a modified version of it, as already analyzed in other posts relating to UK DHFCS (Defense High Frequency Communications Service) and Australian MHFCS (Modernized High Frequency Communications System) transmissions [1]. The STANAG-4285 1200bps broadcasts utilize time-division multiplexing to carry up to 12 KW-46 encrypted streams on a single frequency. Moreover, FSK/STANAG-4285 transmissions on the same channel have already been observed previously and attributed to DHFCS: Tx site of Crimond [2].  a single frequency.

Direction Finding tests (TDoA algorithm), although slightly inaccurate, indicate St. Eval (DHFCS) as the likely site of signal transmission (Figure 6).

Fig. 6 - some Direction Finding tests

It's difficult to determine the purpose of these broadcasts; perhaps they're tests or specific needs. Let's hope we have better luck and catch more similar signals.

https://disk.yandex.com/d/qIF5cmD797W_iQ (FSK signal)

https://disk.yandex.com/d/Y50xjENZ50TPAg (STANAG-4285 signal)

[1] http://i56578-swl.blogspot.com/search/label/1536-bit%20TDM
[2] http://i56578-swl.blogspot.com/2024/06/300-bps-fsk-stanag-4285-fleet-broadcast.html