Unid burst transmission recorded on 5330.50 KHz/USB (60 mt HAM band) using a remote KiwiSDR locate 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).
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| 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 use of PSK2 and PSK8 modulations, in particular PSK2 seems to be used for the initial preamble and for the inserts (miniprobes?) preceding the data blocks, the latter being modulated using PSK8. Obviously, PSK2 dibits are scrambled to appear on air as PSK8 tribit symbols.
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| Fig. 2 - PSK analysis |
Bitstream analysis of a single burst, after the removal of the preamble, reveals the use of six frames (as indeed visible in the previous Figure 2) each characterized by a length of 2160 bits or 720 symbols. More precisely, each frame consists of a first sequence of 72 known symbols, followed by 72 unknown symbols (use of 720/72 symbols is curious) and finally by another 576 unknown symbols (216+216+1728 bit). Figure 3 clearly shows this frame structure. By the way, the initial 72 symbols sequence can be descrambled by the polynomial x^9+x^6+x^3+1 (1).
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| Fig. 3 - bitstream analysis |
Symbols-oriented analysis of Figure 4 helps to better define the composition of the frames and particularly the used modulations. Indeed, looking at the 144 symbols diagram, the first 72 symbols are PSK8 modulated while the following 72 symbols are clearly modulated using PSK2. Thus, each 720 symbols frame consists of an initial 72 known symbols sequence with PSK8 modulation, 72 unknown symbols with PSK2 modulation and 576 unknown symbols with PSK8 modulation.
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| 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 total of 5212 symbols. Since each burst consists of six 720 symbols length frames, ie 4320 symbols, it follows that the initial 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 used (except for some initial "uncertainties" due the SA generic PSK demodulator I used). Preamble can be descrambled using the polynomial x^6+x^3+1 (1).
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| Fig. 5 - preamble analysis |
Evidence of Direction Finding (TDoA algorithm) indicate an area of the transmission site that could be compatible with Guam Island (Figure 6). However, in evaluating the goodness of such 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 quite close together).
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| Fig. 6 - some Direction Finding (TDoA algo) results |
https://disk.yandex.com/d/zu3AE6Ossjl_gQ
(1) SA is a signal analyzer and not a decoder, therefore its phase-plane demodulator does not sync any particular sequence, as it happens for "suited" decoders, and phase-offset errors are possible.
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