31 December 2021

UK DHFCS St.Eval running the Aus 600Bd/850 FSK waveform

Since a few days ago, approximately from the fourth week of December, on 2815.0 KHz (CF) it's possible to receive a strong and continuous 600Bd/850 FSK transmissions which is likely a fleet broadcast (figure 1).

Fig. 1 - 600Bd/850 FSK waveform

Transmissions are secured by KW-46 crypto devices; indeed, as per STANAG-5065, the 7-bit stream is featured by the M-sequence generated by the polynomial x^31+x^3+1 which substitues the stop-bits and acts as sync sequence for the receive KW-46, the remaining 6 bits are encrypted (figure 2).

Fig. 2 - KW-46 sync sequence

Regarding these broadcasts, there are two aspects that are worth underlining: 

a) that particular FSK format (600 Baud, 850 Hz shift) was recently adopted by the Australian Defense Force (ADF) MHFCS (1) to replace their previous ISB system. As a further confirm, these signals exhibit a quite unique sign when inspecting their harmonics: the presence of several spectral lines in the 7^ power, as shown in figure 3. It's to notice the lack of the "paired" 50Bd/850 FSK channel (operating 4 KHz above);

Fig. 3 - the harmonics in 7^ power

b) that channel (2815.0 KHz CF) belongs to the UK Defence High Frequency Communications System (DHFCS) and until a few before it was possible to receive on that frequency the ordinary fleet broadcast consisting of STANAG-4481F (75Bd/850 FSK) transmissions encrypted with KG-84/KIV-7 devices.

Given the probable location of the transmitter, I chose to run the Direction Finding tool using 'test' receivers near England; furthermore, given the frequency, I preferred to carry out the tests (TDoA method) in the morning to avoid wave reflections and therefore try to obtain results based as much as possible on direct wave reception. As expected, all the several DF runs point to an area close to the UK RAF site of St.Eval in Cornwall (figure 4): an HF transmitting station, with Penhale Sands as the associated receiving station, which is operated by Babock Defence Communication (as part of the Defence High Frequency Communications System) on behalf of the Ministry of Defense.

Fig. 4 - Direction Finding (TDoA) results

Therefore it can be said that the previous NATO STANAG-4481F (KG-84 secured) broadcast has now been replaced by the ADF 600Bd/850 (KW-46 secured) broadcast, both transmitted from the UK DHFCS site in St.Eval: that's a really curious "paradigm shift" in terms of FSK waveform and encryption (75Bd/850 → 600Bd/850, KG-84 → KW-46), don't know if it has something to do with the trilateral security partnership between Australia, the United Kingdom, and the United States (AUKUS), or it's some training/exercise. Anyway, I checked the FSK frequencies of DHFCS (at least the ones I know) to understand if this mode was also used in other channels but it was not successful. Frequency monitoring will give us more information.


1) Australian Defence Force (ADF) Modernised High Frequency Communications System (MHFCS) is a managed, long-range strategic communications system that enables the secure exchange of information, such as voice, e-mail, facsimile, interactive data and organizational messages, between fixed and mobile stations using one integrated system. 

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