2 March 2019

STANAG-5030/MIL-188-140 VLF/LF multichannel broadcast to submarines (2)

(this is a follow-up of the post published here)

The narrow 200Hz bandwidth for VLF/LF submarine broadcast and the low efficiency of the aerials are limiting factors, but the use of MSK (a form of QPSK) can allow optimum use of that narrow bandwidth. Indeed, using MSK it is possible to transmit two 100 Baud channels X and Y, each on a pair of phase, and each channel can consists of 2x50 Baud multiplexed channels. Thus, MSK can provide a TDM multi-channel broadcast of  up to 4x50 Baud within the 200Hz assigned band. These transmissions are easy to hear, either locally or, better, using remote SDRs such as the ones provided by Kiwi and thanks to the MSK demodulator coded by my friend Christoph [1] it is possible to study the bitstreams and verify their characteristics. 
The vast majority of users transmit four VALLOR channels (X1, X2, Y1, Y2), i.e. four 50 Baud channels which use KW-46 encryption system. In each channel, data are arrangend in the format defined by STANAG-5065 in which frames are delimited by the pseudo-random sequence generated by the polynomial x^31+x^3+1 ("Fibonacci bits") which also serves to sync the receive KW-46 devices. Error Correction And Detection (EDAC) is performed using (13,12) Wagner coding.

One of the examples of four VALLOR broadcast is the DHO38 station (Fig. 1): a VLF transmitter on 24.3 KHz used by the German Navy to transmit orders to submarines and navies of Germany and other NATO countries. Figure 2 shows the four X1, X2, Y1, and Y2 14-bit streams: the marked columns are the Fibonacci bits generated by x^31+x^3+1.

Fig. 1 - DHO38 constellation
Fig. 1 - the four 14-bit streams from DHO38

The most interesting subComm station is FUE French-Ny on 65.8 KHz from Kerlouan.

Fig. 3 - FUE constellation
As shown in Fig. 4, X1 and X2 channels use the same format of the French-Ny FSK 50/850 broadcast [2]. That format exhibits a characteristic 21-bit frame and, in a way similar to STANAG-5065, two/three sub-frames which are delimited by the bits of two LFSR markers M1 and M2 and a logical "1" value bit (1-bit). The sequences for the two markers are generated by the polynomials x^6+x^5+1 and x^7+x^6+1.
The other two channels Y1 and Y2 are sent using the 14-bit frames with KW-46 encryption.

Fig. 4 - the four streams from FUE
Don't know if it is their normal way to operate or it's just a coincidence, perhaps they use two channels for the shore-to-sub broadcasts (Y1 Y2) while the other twos (X1 X2) are connected to the shore-to-ship broadcast, maybe to forward these messages to subs, who knows?

[1] https://github.com/hcab14/signal-analysis/blob/master/m/demod_msk.m 
[2] http://i56578-swl.blogspot.com/2015/06/french-navy-broadcast-fsk-50bd850.html 

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