75Bd (actual 50Bd) 4481F

Last update: September, 6th 2021

It may happen to run across STANAG-4481F transmissions which have a weird raster pattern which in turn is represented by the bit editor as a 3-bit period stream. This page collects those "catches" (see the table below) and tries to make sense of this unexpected behavior, explaining the reasons.

Fig. 1

Discussing the signal with my friend cryptomaster, we tend to think that a 50 bps data flow is transmitted using a device which is designed to transmit only with a speed of 75 bps. Indeed, 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. Therefore the bits of the stream are actually allocated as follows:

Thus, after filtering out the column of the replicated bits it turns out the actual 50Bd speed. Resizing the new stream into a 7-bit pattern, the KW-46 sync sequence - generated by the polynomial x^31+x^3+1 - emerges (figure 2).
 
Fig. 2

For what concerns the speed of manipulation, a S-4481F transmission lasting for example 10 seconds generates 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 50 Baud transmission of the same duration (10 seconds).
 
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 modified quadrature amplitude detector of SA software: you can valuate the different results (figure 3).

Fig.3
 
Even more interesting: all the signals from Niscemi show the extra harmonics EXCEPT the signal on 6942 KHz which is correctly modulated (figure 4) and coincidentally does not shows the strange 3-bit pattern .

Fig. 4
 
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 (Figure 5). Thus, 50 Baud seems to be the right working speed! 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.  

Fig.5 
 
So, if the analysis is correct, it seems that they use the 75 bps STANAG-4481F waveform to send 50 bps streams(?!): usually in synchronous transmissions the DTE 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").
 
75Bd(50) STANAG-4481 heard transmissions:
  
Freq. (cf) Call
5120.5 NSY
5716.0 NAU
6383.0 NSY
6732.0 AJE
7545.5 NSY
8145.0 NSY
8145.0 AJE
8204.5 AJE
8564.0 -
9338.0 NAU
11980.6 AJE
13229.0 NAU

AJEUSAF, Barford St.John/Croughton UK
NAUUS Navy, Isabela PR
NPMUS Navy, Lualualei Hi
NSYUS Navy, NRTF Niscemi/Sigonella It

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