Wideband transmission heard a few days ago on 19829.0 KHz (cf) around 1713Z, the recording was kindly sent to me by my friend linkz who also performed - successfully - the Direction Finding attempts (see below).
As from Figure 1, the signal occupies a 6 KHz bandwidth and is modulated using PSK8 at the symbol rate of 4800 Baud. Given that the subcarrier is about 6000 Hz, and it shall be 3300 Hz (300 + 1/2 BW, as usual), the signal should be -2700 Hz shifted (the tuning frequency should be around 19826.0 KHz/USB).
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| Fig. 1 - signal parameters |
The ACF value and its framing are quite interesting: as can be seen in Figure 2, the autocorrelation plot shows pronounced spykes at 892.4 ms (4284 symbols/12852 bits) due to the existence of a sort of "superframe" consisting of seven frames marked by less evident spikes. The latter have a value of 127.5 ms (612 symbols/1836 bits) consisting of 544 symbols of unknown data followed by 68 (known) channel mini-probe symbols.
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| Fig. 2 - 127.5 ms & 849.4 ms ACFs |
From the above results (bandwidth, modulation and framing) the signal belongs to MIL-STD 110D Appendix D (WBHF, WideBandHF), more precisely the Waveform Number 7: this appendix is a non-mandatory part of MIL-STD-188-110C; however, when data is to be communicated in single contiguous HF radio bandwidths greater than 3 kHz, up to 48 kHz, the waveforms employed shall be in accordance with this appendix. The PSK8 demodulated bitstream is shown in Figure 3.
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| Fig. 3 - bitstream after PSK8 demodulation |
It is worth noting (and verify) some features of this waveform.
As per Table D-XXI the mini-probes consists of a 36 symbol "base sequence" cyclically extended to the required length: in our case, 68 symbols. W/out going into the merits of the mini-probes formation, some resulting mini-probes are shown in Figure 4.
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| Fig. 4 - 68 symbols mini-probes |
In the zoomed bitstream in Figure 5, a characteristic pattern of the mini-probes is seen at intervals of 64 frames: this is because the mini-probes are also utilized to identify the long interleaver block boundary. Indeed, in our case the block length is just 64 frames. The boundary marker is accomplished by tansmitting a cyclically rotated version of the mini-probe (#D.5.2.2).
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| Fig. 5 - interleaver block boundary |
Figure 6 shows the mini-probe marking the long interleaver block boundary: in accordance with Table D-XXI, the mini-probe is formed of the 36 symbols base sequence after 18 cyclic rotations.
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| Fig. 6 |
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| Fig. 7 - the 7-frame groups that form the data block and that cause the 892.4 ms ACF |
As said above, my friend linkz did a great Direction Finding job and pinpointed Oxford Junction (IA) as Tx site location (see Figure 8 below).
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| Fig. 8 - DF runs (TDoA algorithm), thanks to linkz |
The Oxford Junction transmitter site was operated by Rockwell Collins (now part of Collins Aerospace): a paper that they presented at HFIA Meeting in San Diego (February 4, 2010) just confirms the assumption and also shows an aerial photo of the HF station (Figure 9), notice that both EarthExplorer and Google Earth obscured that site.
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| Fig. 9 |
Since the Tx location, probably the heard transmissions are WBHF over-the-air test by Collins Aerospace... but that's another guess.
