Unid burst transmission recorded on 5330.50 KHz/USB (60 mt HAM band) utilizing a distant KiwiSDR find at Oita, Japan [1]. The bursts have a duration of about 2606 ms with an interval of 3390 ms, the occupied bandwidth is 2200 Hz (Figure 1).
 |
| Fig. 1 |
The measured symbol rate is 2000 Bd on a 1274 Hz carrier: most likely 1200 Hz is the right value. The transitions diagram and the harmonic spectrum in Figure 2 show the usage of PSK2 and PSK8 modulations, in peculiar PSK2 seems to be utilized for the first preamble and for the inserts (miniprobes?) preceding the data blocks, the second being modulated utilizing PSK8. Obviously, PSK2 dibits are scrambled to appear on air as PSK8 tribit symbols.
 |
| Fig. 2 - PSK analysis |
Bitstream analysis of a single burst, after the removal of the preamble, reveals the usage of six frames (as indeed visible in the erstwhile Figure 2) each characterized by a dimension of 2160 bits or 720 symbols. More precisely, each frame consists of a first series of 72 known symbols, followed by 72 unknown symbols (use of 720/72 symbols is curious) and yet by another 576 unknown symbols (216+216+1728 bit). Figure 3 clearly shows this frame structure. By the way, the first 72 symbols series can be descrambled by the polynomial x^9+x^6+x^3+1 (1).
 |
| Fig. 3 - bitstream analysis |
Symbols-oriented analysis of Figure 4 helps to better specify the composition of the frames and peculiarly the utilized modulations. Indeed, looking at the 144 symbols diagram, the first 72 symbols are PSK8 modulated while the following 72 symbols are clearly modulated utilizing PSK2. Thus, each 720 symbols frame consists of an first 72 known symbols series with PSK8 modulation, 72 unknown symbols with PSK2 modulation and 576 unknown symbols with PSK8 modulation.
 |
| Fig. 4 - symbols analysis |
As from Figure 2, each burst has a duration of 2606 ms that - at the symbol rate of 2000 Bd - makes a full of 5212 symbols. Since each burst consists of six 720 symbols dimension frames, i.e. 4320 symbols, it follows that the first preamble is composed of (5212 - 4320) = 892 symbols. Figure 5 shows the preamble and its symbols analysis, from which it can be seen that PSK2 modulation is utilized (except for any first "uncertainties" due the SA generic PSK demodulator I used). Preamble can be descrambled utilizing the polynomial x^6+x^3+1 (1).
 |
| Fig. 5 - preamble analysis |
Evidence of Direction uncovering (TDoA algorithm) indicate an area of the transmission site that could be compatible with Guam Island (Figure 6). However, in evaluating the goodness of specified results it must be taken into account that the survey area is not densely populated with KiwiSDR receivers, especially in the East direction, and that the transmission was not continuous but - in fact - a train of bursts (although rather close together).
 | |
| Fig. 6 - any Direction uncovering (TDoA algo) results |
https://disk.yandex.com/d/zu3AE6Ossjl_gQ
(1) SA is simply a signal analyzer and not a decoder, so its phase-plane demodulator does not sync any peculiar sequence, as it happens for "suited" decoders, and phase-offset errors are possible.
