28 March 2018

unid 3-of-6 multitone system (tentative)


On August 16, 2017 I copied an interesting multitone signal on 14642.0 KHz and 16114.0 KHz on USB. The signal uses 6 tones, 400Hz spaced, starting from 650Hz, and it sounds just like a frog. Tranmissions do not have a preamble, last for a few minutes (the longer I heard last up to 16 minutes) and consists of 820ms blocks: 500ms data block followed by 320ms interval (Fig. 1)

Fig. 1
Transmissions end with the sending of the 6 carriers in a special sequence (Fig. 2)

Fig. 2
Three of the six tones are used to form the code symbols, ie they are sent simultaneously (Fig. 3): since given six tones there are twenty combinations of three that can be drawn without repetitions, this system use a 20 symbols alphabet set. It's important to note in Fig. 3 that each data block always consists of the ordered sequence of all the possible symbols (!) that makes a speed of 40 symbols/sec that is - in some way - coherent with the used shift (40Bd/400hz).

Fig. 3

Many "exotic" coding could be derived from this set (as 123=A,124=B,... or 123=0,124=1,...) and a 6-bit representation could be one of these: e.g. using the lower tone as the LSB we get the sequences:

000111 001011 010011 100011 001101 010101 100101 011001 101001 110001 
001110 010110 100110 011010 101010 110010 011100 101100 110100 111000

(you may play around it inverting the order, changinB g polarity, differential decoding,...)

I don't know who they are and where the signals come from, anyway the fact of sending all the alphabet symbols leads to think that it could be a test of a new system, maybe aimed to Intel/Diplo services... but it's only a my supposition and - if that is the case - we should wait for further transmissions from the "production" frog-modem.

➤8 Feb / 7 Mar updates
The same system has been spotted on 10222.0 KHz/USB on February 8 (Fig. 1) and on 7674.0 KHz/USB on March 7 (Fig. 2): data blocks last respectively 250 msec and 200 msec

Fig. 5 - 250 msec blocks
Fig. 6 - 200 msec blocks
In my opinion, looking at the three intercepts, it seems that these are still test transmissions in which different data-formats are used:

➤28 Mar update
The MFSK-6 modem seems to be entered in production mode: spotted today on 10222.0 KHz/USB with no repetitions of the same pattern, as noted in the precedent intercepts.  Note as the system uses 3-tone symbols and a symbol rate of 100symbols/sec.

Will wait for further recordings to confirm my guess.


➤17 July update
several TDoA multilaterations on both 10222 and 14462 channels indicate Frankfurt as source of the signal, below a TDoA run on 10222/usb (July,14)


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