11 February 2021

weird, but intriguing, 75bps Baudot encoding with 3.8 stop bits (prob. ACP-127 msg)

That's a very interesting Baudot 75Bd/850 FSK signal recorded (and sent me) by my friend cryptomaster on 6721 KHz/USB: although its nominal parameters lead to an async STANAG-4481 transmission, the analysis (Figure 1) reveals an unexpected stop-bit length of 3.8 bits(!), i.e. an unusual 5N3.8 framing (it's to notice that, in case of lost sync, the stop-bit stretching allows faster resync and framing of the incoming data).

Fig. 1

Since the bit editor works using integer numbers (it can't draw half bit), the bitstream is shown with the 5N4 framing  (Figure 2, negative polarity): it means that parsing the 5-bit code will inevitably produce some errors.

Fig. 2 - the bitstream as it appears after demodulation

At first glance, the decoded text looks like a message with errors due to a noisy signal or having a poor SRN (Figure 3), but an in-depth analysis of one of the decoded text reveals interesting details that allow us to identify sender/recipient and the very nature of the message: in my opinion, a radar detection report sent in ACP-127 style message. Obviously, decoding suffers from the non standard framing so, after various decoding runs, I re-built the message using the parts taken here and there and that -in my opinion - make more sense.

Fig. 3 - Baudot decoded text

T-O  081506z Fet 21
that's the DATE-TIME-GROUP (DTG)  of the message; indeed, as you can see in Figure 1, the transmission was recorded on 2021-02-08t15_12_04z so we have 08 (day 8) 0815z (time) february 2021 (Fet 21). This way of coding the date also corresponds to what is indicated in the Code FM-13-x-SHIP (or shortly FM-13) [1].

is the originator's sign, the originator of this message is indicated by the designation immediately following. TIRPA  is the callsign adopted by the French Aviation Defense Service (ICAO DEF), TIRPA 29 could be an aircraft  [2]. TO is the "Action Addressee", ie the addressee(s) immediately following (CEFAE) is to take action on the message.

INFO is the Information addressee sign (as "CC"), in this message is the French Naval Aeronautical Base of Lann-Bihoué located in the Atlantic region, placed under the operational command of the Atlantic zone (AERONAV/AWLANT, see below) [3].

likely the Aéronautique Navale (AERONAV) operational command of the Atlantic zone (Air Wing Atlantic, AWLANT). Since the "scene", MCA could mean Maritime Cryptologic Architecture (here the term "cryptology" refers to the SIGINT functions of the maritime Services and incorporates Information Security or INFOSEC functions).

NMR/004 NP 0802 TXT LR 3/1
number of the message (NMR 04), date February 8th (NP 0802) and format of the message (TXT)

the term "VIENNE" could be misleading (city of Wien) but actually JEWNDE VIENNE stands for "JEANNE DE VIENNE", a F70 type anti-submarine frigate of the French Navy. DDGH is the acronym of Destroyer, Helo Capable, Guided Missile.

it's likely a (geo) position  (suffix is probably GP, GOLF PAPA ). Don't know the format the coordinates are expressed, it seems they do not exactly follow the FM-13 [1] specs even if the terms North and West are easily recognizable. Trying a parsing, the result is quite realistic (46°25'12", 2°23'24")

respectively course and speed (see below)

And here things get more interesting:

DRBV 15 / SN 65+53 / CONFIANCE 3

I guess this text refers to INTERCEPTION ESM, where ESM stands for Electronic Support Measures: i.e. something related to radars [4], indeed:

- DRBV-26A (Jupiter) is a Thomson-CSF early warning radar performing long-range air surveillance on board medium and heavy-tonnage ships. DRBV-26A operates in D-Band, and is available with a cost-effective fully solid-state transmitter [5][6]
- DRBV 15 (Sea Tiger) is operating in S-Band air and surface surveillance and target designation radar. The system is designed for medium to large ships and can be integrated with weapons systems [7] [8]
- CONFIANCE is the degree of the detection confidence, levels range is from 1 (low) to 3 (high).

 SN 40270 and SN 65+53 are probably some of the out measurements from the radars.

Break and End-of-Message Indicator (EOM)

Since the French Navy frigate "Jean de Vienne" is equipped with both the radars (DRBV-26A and DRBV 15) [9], I think that the recorded transmission could be an ACP-127 tactical message about a radar detection (or something very similar to it). It's not clear to me who the fields position-course/speed refer to (sender? ship? or maybe the radar target?).
Anyway, it's quite unusual to see such (prob.) ACP-127 messages in clear text as well as the 5N3.8 framing, probably derived from a STANAG-4481F modem: could they be trainings? Comments are welcome.

Frigate Jean De Vienne (CC BY-SA 3.0, https://commons.wikimedia.org/)

Update (thanks to Jean-Marc Liotier @liotier)
CEFAE Centre d'entraînement et de formation de l'aéronautique navale, navy school in Lann-Bihoué.
Looks like training is the reason for using this elderly format to report ESM sightings.


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