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

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