20 June 2024

300 bps (FSK & STANAG-4285) fleet broadcast, UK DHFCS

Fig. 1 - 300Bd fleet broadcast on 4505.0 KHz/USB

Uncommon 300 bps fleet broadcast heard on 4505.0 KHz/USB and using both FSK modulation (with a 850 Hz shift) and PSK modulation (STANAG-4285 300 bps/Long). I say here "uncommon" given that:
- usually (NATO) fleet broadcasts use FSK 50-75 bps or 600 bps, as per the standards STANAG-4481 & STANAG-4285;
- unlike the 50-75 bps, this FSK waveform uses a 1700 Hz offset from the suppressed carrier;
anyway, frequency offset and speed are still S-4481 compliant (1)(2)

Fig. 2 - FSK 300Bd/850 fleet broadcast

Both the broadcasts are secured by KW-46 encryption (or other emulating device such as the programmable KIV-7M) given the presence of the M-sequences generated by the polynomial x^31 + x^3 +1 (KW-46T uses that sequence to synch the KW-46R receive devices). Note in Figure 2 that the number of "positives" depends on the number of the analyzed bits.

Fig. 2 - KW-46 encryption in both FSK & S-4285 transmissions

Direction Finding (TDoA algorithm) pinpoints the Tx site of Crimond, former RNAS Rattray, belonging to UK DHFCS (Defense High Frequency Communications Service) which is  operated by Babcock International Group [1].

Fig. 3 - direction findings

I noticed that some (old) logs report the frequency of 5405.2 KHz for the operation of DHFCS STANAG-4285: the value I indicate here (4505.0 KHz) is currently the correct one and seems a "new" slot, although also used by E11. The 300 bps transmissions were on the air for a few days only, perhaps for testing purposes of some on-board (old) equipment or other needs.
Monitoring thanks to the Weston KiwiSDR (UK).



(1) Annex C to STANG-4481 "2. [...] It is recognised that equipment uses different offsets from the suppressed or reinserted carrier (i.e. 1700 and 2000 Hz) in order to achieve the mark and space frequencies. This will not cause an interoperability problem as long as the single channel wideshift of ± 425 Hz is used. 

(2) Annex C to STANG-4481 "5. [...] The minimum user data rate will be 75 bits per second (bps). Higher speeds, including 150 bps, 300 bps or higher may be implemented when necessary".

[1] ttps://en.wikipedia.org/wiki/Defence_High_Frequency_Communications_Service  

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