6 November 2016

unid 3KHz/6KHz channel waveforms in Maritime Band

These are unidentified signals heard by me and my friend KarapuZ in the Maritime Band segments, mainly 8 and 12 MHz, during daylight. Transmissions are a mix of 3 KHz and 6 KHz wide channels (BW) and use PSK modulation, symbols rate is dependent on the bandwidth:
3 KHz BW: 2400 symbols/sec, BPSK modulation (fig. 1)
6 KHz BW: 4800 symbols/sec, BPSK and QPSK modulation (figs. 2, 3)

fig. 1
fig. 2
fig. 3
The most interesting transmission is what seems an Independent Side Band ARQ system in which the sender use the upper band to convey messages and receiver use the lower band for ACKs, both use BPSK modulation and 2400Bd symbols rate (figs. 4,5)

fig. 4
fig. 5
In order to get better quality signals, at the same time I tuned the 12 MHz band using the remote SDR at Twente University: curiously such signals were not received at the remote site. I know that this test make a poor sense but it could be a little clue about the origin of these transmissions. 

Some friends of mine think these are transmission from Inmarsat HF for their maritime service (they acquired the former GW - Globe Wireless net).


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