Yet another MS-110D sample [1] transmitted from Oxford Junction (IA) site, recorded on 19825.7 KHz/USB and sent me by my friend linkz: this signal too is PSK8 modulated at the symbol rate of 2400 Bd but occupies a 3 KHz bandwidth (Figure 1).
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| Fig. 1 - waveform main parameters |
The ACF results shown in Figure 2 formally show the same characteristics, that is, a sort of "superframe" lasting 840 ms (corresponding to 2016 PSK8 symbols, or 6048 bits) comprising seven frames each lasting 120 ms (corresponding to 288 PSK8 symbols, or 864 bits).
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| Fig. 2 - results from the Auto Correlation Function |
Bandwidth, modulation and framing match the Waveform Number 7 described in MIL-STD 110D Appendix D (WBHF, WideBandHF)
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The demodulated bitstreams conform to the bitmaps in Figure 2: in particular, the sequences circled in in Figs. 2, 3 are special mini-probes used to mark the interleaver boundaries. In this case they are transmitted every 64 frames, corresponding to the use of the "long interleaver" mode.
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| Fig. 3 - 864 bits (266+32 PSK8 symbols) period demodulated bitstream |
Just to verify compliance with the MS-110D standard, the mini-probes are made up of a repeated sequence of 16 symbols while the miniprobes used to mark the boundaries of the interleaver block are shifted by 8 steps (Figure 4).
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| Fig. 4 - the generic mini-probe and the interleaver marker mini-probe |
As expected, the 840 ms spikes resulting from ACF are due to the cyclic nature of the transmitted data: that is, the same block of data consisting of 7 frames (Figure 5).
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| Fig. 5 - data blocks after the removal of mini-probes |
As stated at the beginning, Direction Finding (TDoA algorithm) tests done by my friend linkz indicate Oxford-Junction as the site of the transmitting antenna (Figure 6); more over "It's interesting to note that this data seems to be sent always 22.5 kHz lower than the ALE slots. So far noticed on: 8000.7 USB (8023.2 - 22.5kHz), 18275.7 USB (18298.2 - 22.5kHz), 19825.7 USB (19848.2 - 22.5kHz)" linkz write.
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