15 March 2020

Again about the 3-bit format STANAG-4481F transmissions

(cryptomaster, I56578. KarapuZ)

STANAG-4481F on 18370 KHz from NPN US Navy, Guam
This is an update and just some remarks to a previous post post which I reference for background. All frequencies are CF (tuning + 2k).

1) the signal

Discussing the signal together, my friend cryptomaster had the suspect that a 50 bps data flow is transmitted using a device which is designed to transmit only with a speed of 75 bps: it could be correct.  The ratio 75/50 is equal to 1.5 thus each "original" bit is repeated 1.5 times. The bit editors work with an integer number of bits (they can't represent half bit) thus the 1.5 bit view is possible only by aggregating two consecutive frames and then getting an integer number of 3 consecutive bits (i.e. 1.5 x 2): thus the 3-bit structure that we see (it's the same of the async 5N1.5 framing which is represented as a 15-bit pattern, i.e. 7.5 x 2). Therefore the bits of the stream are allocated as follows:

A S-4481F transmission lasting 10 seconds produces 750 bits that can be arranged into a 3 x 250 bits pattern; by removing one column we get 2 x 250 = 500 bits that just match a 50bps transmission of the same duration (10 seconds).
But what about the M-sequence generated by the polynomial x^31+x^3+1 ? Notice that the Wagner(13,12) coding, which is used for example in STANAG-5065, replaces each second Fibonacci bit with the parity bit: well, the new Fibocaccci sequence bits (the half of the original one!) still belongs to the same polynomial x^31+x^3+1 (see this post).
Indeed, filtering out the replicated third bits from a 75bps demodulated stream  and resizing the resulting stream into a 7-bit pattern, it turns out that we get an usual KW-46 encrypted 7-bit stream (Fig. 1).

Fig. 1
In the light of the above, I analyzed again the signals in order to verify what we hypothesized and found above. I compared a signal from NSY Niscemi recorded on 6383 KHz (3-bit pattern S-4481F) and another one from NAU Isabela 12120 KHz (plain S-4481F) by using the the modified quadrature amplitude detector of SA software: you can valuate the different results (Fig. 2).

Fig. 2
Even more interesting: all the signals from Niscemi show the extra harmonics EXCEPT the signal on 6942 KHz which is correctly modulated (Fig. 3) and coincidentally does not has the 3-bit pattern (Fig. 8).

Fig. 3
Then I selected the 50 Hz clock from the NSY signal and subsequently I demodulate it by using the synch'ed FSK demodulator: the test was successful and replicated the same results that I found using the theory and manipulating the bitstreams (Fig. 4). So, 50 bps seems to be the right working speed.

If our analysis is correct and we are right, it seems that they use the 75 bps STANAG-4481F waveform to send 50 bps streams (?!). We do not know the reason but probably you can  do this. In synchronous transmissions the DTE usually provides the transmit clock to the modem but perhaps they could use a modem - e.g. like the Harris RF-5710A - which can recover the clock automatically from the incoming transmit data (transmit clock set to "DATA" or in "recovery mode").

As proved, decoding those signals using standards modes, or changing the speeds to 50 bps, unfortunately does not work: the only successful way is to sync the FSK demodulator to the 50 Hz clock of the signals. Since we are talking about shore-to-ship broadcast, I wonder how the receive ships may manage these transmissions.

2) the source

(monitoring was carried out according to a list of frequencies from logs and in any case not 24/7)
a) Using remote KiwiSDRs, and with the help of my friend Mike "mco", I checked several S-4481F transmissions  but - at present day - only those from NSY and AJE exhibit the odd 3-bit pattern we are talking about. Below the current list of the successful frequencies:

5120.5 NSY
6383.0 NSY
7545.5 NSY
8145.0 NSY, AJE
8204.5 NSY

It's to notice that most of the times the NSY frequencies are logged as "NSY Sigonella": well, NAVCOMTELSTA (U.S. NAVAL COMPUTER AND TELECOMMUNICATIONS STATION) Sicily, located in Naval Air Station Sigonella, manages the Naval Radio Transmitter Facility Niscemi, housing LF/HF transmitters [1][2]. Same story about AJE Barford St.John that probably is sometimes reported as Croughton, nearby (6 miles distant) [3][4]. 

b) Interestingly, 8145 KHz is shared by NSY and AJE; often I have been able to see contemporary broadcasts and same contents (Figs 5,6). The modified AM detector shows the same results as the ones of Fig. 3

The modified AM detector shows the same results as the ones of Fig. 3:

Fig. 7

 c) According to the Tx sites (NSY in Italy and AJE in UK) this type of traffic is beamed only by some European stations. 

d) As said above, I also spotted a S-4481F transmission on 6942 KHz that DF points to southern Sicily, thus it's again NSY. However, this signal does not have the expected 3-bit structure although it's contemporaneous to another S-4481F transmission beamed from NSY on 6383 Khz (Figs 8,9). So, it seems that most of the 3-bit structured signals come from NSY,  but not all those coming from NSY have that feature. Still not heard S-4881F transmissions on the other NSY frequencies 10974 and 15018 KHz.