11 January 2022

odd 16 times expanded 5N1 framing (UK DHFCS)

Rather curious pattern transmitted by the UK DHFCS site in Crimond on 4538.0 KHz/USB, reported to me by friend Linkz, using the STANAG-4285 waveform at 1200 bps/L submode: it's about a 16 times "expanded" asynchronous 5N1 stream (112-bit length framing, as shown in figure 1):

Fig. 1 - 16x 5N1 framing (112-bit period)

The source 5N1 framing can be obtained after removing the overhead bits: this can be easily done by manually editing the demodulated bitstream, for example with BEE, or by "downsampling" the bitstream by a factor of 16 using a simple script coded - for example - in Octave (figure 2).

function [a]=downTo16(fn)
  # pkg load signal before to run the script)
  # fn = filename containing a stream of ASCII 0's and 1's w/out spaces!
  a =[];
  k =7;
  fp =fopen(fn);
  fsk =fgetl(fp);
  bits = fsk=='1';
  y =downsample(bits,16);
  n =floor(length(y)/k);
  a =reshape(y(1:n*k),k,n)';
  dlmwrite("5N1.txt",a," ");

Fig. 2

After "downsampled" the bistream  (ie reduced to its "natural" 5N1 framing) and removing the initials reversals and start/stop bits, it's possible to detect the KG-84/KIV-7M sync sequence as well as the two 128-bit initialization vectors, here twice repeated (figure 3). 

Fig. 3

Now, given the adopted frame's structure and the 1200 bps speed, it turns out that the actual data rate is 1200÷16 = 75 bps (!) and therefore it matches the speed of STANAG-4481F (obviously bps=Baud in case of FSK): it's worthwhile to note that such STANAG-44881F transmissions from Crimond had already been reported [1].  

KG-84 devices talk to each other at 75 bps

I don't know why such a 112-bit pattern is used: definitely it's not an error detection and correction measure (too high ratio codeword/data, 93.75%). Judging by the bitstream, the 16x stuff sits between the crypto device and the STANAG-4285 modem, one could venture the hypothesis of  a failure of the FSK modem and its (temporary?) replacement with that workaround (1200bps is their usual STANAG-4285 speed)... but it's just a mere guess.

Fig. 4 - DF results (KiwiSDR TDoA algorithm)

(the zip file contains the wav recording, the 112-bit demodulated stream and the Octave scripts downT016.m and upTo16.m, so you can play with them. A quick howto for installing Octave is here)

[1] https://i56578-swl.blogspot.com/2021/03/async-stanag-4481f-with-kg-84kiv-7.html

5 January 2022

Yet another 75Bd (actual 50)/850 FSK

Yet another 75Bd (actual 50)/850 FSK bearing KW-46 encrypted broadcast spotted on 5189.50 KHz CF, this time probably(?!) sourced from some France AF base (figure 2). The reason of that "format" is still unclear, at least to me (1).

Fig. 1

It should be noted that not all the French sites in figure 2 host HF transmitters, as well as the possibility of some uncertainties of the current TDoA algorithm in finding the origin of FSK signals (as suggested by my friend Linkz); figure 3 shows the Direction Finding results got with the "old" TDoA algorithm: results point the coast of south UK.

Fig. 2 - Direction Finding TDoA results

Fig. 3 - Direction Finding TDoA results (old algorithm)


(1) https://i56578-swl.blogspot.com/p/75bd-actual-50bd-4481f.html