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).
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