23 February 2023

unid "mixed-mode" transmission

 updated

 Very interesting "mixed-mode" transmission cathced and sent me by by friend Michel (F1GOC)

Fig. 1

Segment 1 consists of an FSK modulation keyed at 200 Baud and 1000 Hz shift (Figure 2). The demodulated bitstream has a well defined period of 288 bit consisting of a data segment preceded/followed by a sync/probe sequence (Figure 3).

Fig. 2
 

Fig. 3

Segment 2 consists of an MFSK-7 modulation keyed at 30 Baud and 400 Hz separation between the tones. The raw demodulation of the seven tones (0-6;000-110) shows a repeated sequence of 940 bit length (Figures 4,5).

Fig. 4

Fig. 5

Segment 3 is the most interesting one. This segment consists of 7 subcarriers which coincide with the tones of the previous MFSK segment (see Figure 1), the modulation used for each subcarrier - in my opinion - seems to be ASK2/OOK. The "aggregate" speed of modulation is 200 Baud, ie 28.5 Baud per channel (Figure 6). The autocorrelation function of the signal shows a period of 10 seconds, corresponding to 2000 bit frames (Figure 7).

Fig. 6

Fig. 7

As a simple curiosity, the central frequency of the FSK segment does not coincide with the value of the center band of the following two segments (see Figure 1). Difficult to state the user and the purpose of the transmission, many "experimental" signals are in the air especially during this period, comments are welcome.

------- update  --------------------------------------------------------

As my friend cryptomaster commented (and my friend Nicola too in pvt), the FSK segment is the well-known Russian Intel "F06" waveform. Indeed, after the change of the bit-order is possible to clearly see the typical F06 32-bit sync sequence 7D12B0E6 (Figure 8). The final part of the decoding, thanks to the rivet_b90 tool, is shown below (Figure 9). 

And... yes, I'm a bit out of shape since the 288-bit period should have suggested the solution 😁

Fig .8 - F06 sync sequence
 

Fig. 9 - F06 decoding

https://disk.yandex.com/d/2hsIqrswtSwzHw

1 February 2023

unid ASK2/OOK transmissions

updated

Unid transmissions heard on 8120, 8130, 8140, and 8150 KHz (the latter moved to 8160 Khz) thanks to the KiwiSDR located at N4BUT Orlando, FL [1].

Fig. 1

At first sight the signal seemed a MPSK modulation, but working the signal along with my friend cryptomaster some other interesting features came out. Each transmission consists of four 100 Hz separated channels (a,b,c,d) each occupying a band of about 900 Hz, for a total bandwidth occupation of about 3900 Hz (Figure 2).

Fig. 2

In turn, each of the 4 channels consists of 4 sub-channels with a modulation rate of 49.6 Baud, the used modulation seems to be ASK2/OOK [2].

Fig. 3

Each of the four sub-channels shows strong ACF peaks of 1290 ms corresponding to a 64-bit length frame: the "aggregate" frame therefore has a length equal to exactly 1 Kb, ie 1024 bits or 128 bytes (64x4x4).

Fig. 4

Below in Figure 5 is a comparison of the four channels obtained from the analysis of my friend cryptomaster.

Fig. 5

Although I have kept an eye on that initial portion of the 8 MHz band (fixed/mobile band, shared with marine for simplex purposes), those transmissions have not appeared again (at least until today): difficult to define their purpose and user(s).

Below the interesting comment sent me by my friend Nicola, who I thanks for the collaboration:

"The interesting signal discussed in the blogpost “Unid ASK2/OOK transmissions” dated 1 February is probably a Frequency-Time Matrix (FTM) system, where a data symbol (bit string) is represented by FT-matrices, i.e. combinations of frequency and time domain positions. The 'secret' of this robust mode is that no frequency is repeated within each matrix. This characteristic is used to enhance the resilience against frequency domain broadband noise (or fading) affecting a broad range of frequencies and time domain narrowband noise (or fading).

https://disk.yandex.com/d/7u74pzY-bPntXg

[1] http://sdr.n4but.com:8173/?f=8120.00iqz10&pbw=10000
[2] https://en.wikipedia.org/wiki/Amplitude-shift_keying