25 August 2018

DHFCS 1536-bit TDM protocol (1)

This afternoon I spotted a STANAG-4285 1200bps/L transmission on 7937.0 KHz/usb carrying the 1536-bit TDM data protocol already seen in some previous recordings discussed here. The most important thing is that the signal, with very a good reliability, comes from the RNAS (Royal Naval Air Station) near Crimond, Aberdeenshire (UK) and I remembered that a transmission that had the same characteristics (STANAG-4285 1200bps/L, 1536-bit protocol) was identified as coming from Cyprus Island (Figure 1).

Fig. 1
Well, both the stations belong to the Defence High Frequency Communications Service (DHFCS),  a British military beyond line-of-sight communication system operated by the Ministry of Defence (MOD) and used predominately by the Royal Air Force, Royal Navy and British Army, as well as other authorised users (Fig. 2).
Fig. 2 
This being said, it's likely to assume that the 1536-bit TDMA format is a proprietary protocol of used by DHFCS, maybe developed by Rockwell Collins who deployed the system? Some interesting information about DHFCS can be read from some presentations held in HFIA meetings. In particular, in the slides of Rockwell Collins - albeit a little dated - some screenshots related to the modem preset are shown where it is possible to see the setup of the waveform 4285 at 1200 bps/Long interleaver in async mode as well as the the GA-205 TDM multiplexer preset (Figs. 3,4). 

Fig. 3
Fig. 4
It is noteworthy that these slides date back to 2006.

27 August 2018 update
as expected, 14390.0 and 16287.0 transmissions (S4285 1200bps/L & 1536-bit TDM protocol) are from Ascension Island, overseas DHFCS stations:


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