17 September 2021

SDPSK 40Bd 50Hz OFDM-60 (60-out-of-61 + pilot tone)

OFDM signal heard on 5767.0 KHz (cf) and consisting of 61 channels plus a pilot tone located at the lowest frequency (thus 62 channels in all). The modulator actually uses 60 of the 61 available channels (60-out-of-61) since the position #34 is empty (figure 1).

Fig. 1

For what concerns the OFDM formation, the channels have a 50 Hz spacing and are modulated using SDPSK - also called π/2-DPSK (1) - at the rate of 40 symbol/sec (2400Bd as "aggregate" speed).
Fig. 2

Fig. 3

The pilot tone at its lowest position, rather than at the usual highest frequency of 3300 Hz, leds to think of a Russian "Serdolik" waveform, indeed it's similar to the Serdolik OFDM-60 [1]: same speed (40Bd) and spacing (50 Hz) but PSK4 modulation. Some friends of mine (Karaputz, linkz,...) confirmed the idea, likely an enhanced waveform.
Reception and recording thanks to the "Tambov" KiwiSDR [2].
(1) In SA Phase-Plane using n-Ary = 4 and absolute mode (diff=0) the transitions between states are similar to QPSK but without diagonal paths (no "zero" crossings); in differential mode (diff=1) we see transitions between two states (Fig. 3) thus it's a Differential-PSK or DPSK. DPSK is called Conventional DPSK (or CDPSK) if the phase differences is in the set of [0,π] and Symmetrical DPSK (SDPSK, also called π/2-DPSK) if the phase difference is in the set of [π/2,-π/2].  As you see in Figure 3, the transitions in differential mode (diff=1) are in the set of [π/2,-π/2].

14 September 2021

2000Bd/3250 FSK on 26.9 MHz

Interesting FSK signal spotted on 26.9 MHz by friend killer258 from raduioscanner.ru: modulation speed is 2000 bps and 3250 Hz shift (figure 1).

Fig. 1

ACF results show a kind of "interleaved" values of about 161.7 ms that make a 322-bit length period; indeed, looking carefully at the pattern, the stream is actually formed of a 161-bit sequence wich is sent alternatively in positive and negative polarity (figures 2,3).

Fig. 2
Fig. 3 - 322/161 bit period

Also interesting are the oscillations during modulation in the first part of the transmission which are visible in figure 4. Tones do not preserve their phase (figure 5).

Fig. 4

Fig. 5

Thanks to killer258 who kindly allowed me to use his recordings.


6 September 2021

75(50)Bd 4481F: yet another channel

Yet another 75Bd/50Bd 4481F channel spotted on 9338.0 KHz (cf), most likely from NPM Lualualei, HI (thanks my friend Mike "mco"). After filtering out the column of the replicated bits from the demodulated stream, it turns out the actual 50Bd speed; then resizing the new stream into a 7-bit pattern, the KW-46 sync sequence emerges.


3 September 2021

async 5N1 STANAG-4481F, likely tests or training transmissions

5 September 2021 Update

Transmissions are now continuous and after the removal of the start/stop bits the text appears KG-84/KIV-7 secured. It's worth noting that the  128-bit Initialization Vector is splitted in two 64-bit groups and each group is repeated twice rather than four times (as instead it's used to do). That feature has already been osserved in other STANG-4481F transmissions from UK MoD [1] (spotted on 6245.20, 8056.7, 8127.0, and 10272.0 KHz all Cf).

Fig. 1

TDoA results now indicate definitively Crimond (figure 2).

Fig. 2
[1] https://i56578-swl.blogspot.com/2021/03/async-stanag-4481f-with-kg-84kiv-7.html

3 September 2021

Async (and episodic?) 5N1 STANAG-4481F "segments" spotted on 4539.7 KHz (cf): as a distinguishing feature, it's to notice the presence of a pilot tone at CF-700Hz (4539.0 Hz) preceding each segment and the slight diversity of the durations (both of the segments and of the pilot tone).

Fig. 1
I went casually on the signal at about 2150 (Sept. 1st) when it was already active and it lasted almost all night until about 0800 the following morning (Sept. 2nd) when it ceased, all times are UTC: after that I haven't heard it again, unless a sporadic carrier just on 4539.0 Khz. The effective duration of the transmission can be verified by looking at the 24-hour waterfalls as received by the WebSDR receiver in Twente (figure 2).
Fig. 2 - Twente WebSDR wtarefalls of 1 and 2 September
The 5-bit text after the removal of the start-stop bits seems to be encrypted, definitely not KG-84/KIV-7 or other encryption that I'm aware, or consisting of pseudo-random chars. I tend to think of training or test transmissions - that makes more sense - on a not "usual" frequency: indeed I did not find mentions of S4481/4539.7 KHz in the large collection of logs of UDXF group, only few S4285 logs from Italian-Ny.

Fig. 3

All the direction finding tests (figure 4) point to an area located in north UK, therefore the Tx site could be likely Crimond (Aberdeenshire, Scotland), belonging to UK MoD DHFCS [1]:  it must be noted - however - that the results could suffer from the non-continuity of the signal. By the way,  similar async STANAG-4481F transmissions have already been observed on last March and just from Crimond [2].

Fig. 4 - some TDoA results

[1] https://www.scottish-places.info/features/featurefirst94146.html
[2] https://i56578-swl.blogspot.com/2021/03/async-stanag-4481f-with-kg-84kiv-7.html