11 March 2022

1200Bd/800 Russian tactical datalink (2)

 (revised with some fixes)

My friend @pir34 (from UDXF group) and I are monitoring different frequencies which are supposedly changed on a per day/night basis, but not just that. We have noticed that the transmissions take place from at least two different sites (which I prefer not to indicate) and - more importantly - with two different data formats, even if they have the same transmission mode (1200Bd/800), same bit length (69) and same type of contents (radar tracks). In particular, the transmissions that take place on 6123.5 KHz/usb and 6232.5 KHz/usb (figures 2,3):

Fig. 1 - 6232.5 KHz transmission: spectrogram and modulation

Fig. 2 - 6232.5 KHz transmission: 69-bit period stream

According to COMIN Consulting (thanks to @pir34 for the report), this signal is known as "PIRS PEARCE" [1]: I met such signal some years ago, as annoted here in the blog, on about 20 MHz: as you see in figure, the bitstream is very similar although its length is 165 bits:

Fig. 3 - 165-bit frame length datalink

An anymous reader commented the post talking about the 165-bit "Akkord SS-PD" frame format, that he expected to see in the bitstream, and about the other more frequent frames of 69 & 117 bit lengths... just as the ones discussed in the previous post.

I did some research on the web about the Akkord format/algorithm but getting very few results: the most interesting referts to 165-bit length frames (called "Akkord-165") of which a description of the framing is also provided [2]:

4-bit (Start Of Message) + 6 x 24-bit words + 1 sep + 16-bit CRC

That described framing corresponds to those used in the formats of 69 and 117 bits reported in the previous post: in those cases the messages shall consist of two and four 24-bit words respectively (ie 48 and 96 bit length messages). The only difference is the polynomial used for the formation of the 16-bit cyclic code (thus, not used as a scrambler polynomial): in the Akkord-165 format it results to be x^16+x^12+x^5+1.

A second interesting source seems to indicate a device operating with two sub-set, each consisting of an "Akkord SS-PD" data transmission equipment plus a "similar" one and also provides the block diagram reported below in figure 4 [3]:

Fig. 4

(the label "C2" in figure 4 stands for the junction circuits between DTE and DCE)
That said, we think that the datalink transmission sites use two data formats both sent with the same 1200Bd/800 waveform, although the exchanged data are perfectly "understood" and parsed by the receive nodes. It's not clear if the two formats are provided by the data transmission unit shown in figure 4 .

[2] https://motherhouse.ru/en/mortgage/kompleks-sredstv-avtomatizacii-ksa-sbora-obrabotki-i-vydachi/

[3] http://vniira-ovd.com/index.php/en/products/communication/interface-equipment/bapd

1 comment:

  1. x^16+x^12+x^5+1 => X + 1 and x15 + x14 + x13 + x12 + x4 + x3 + x2 + X + 1