6234 Khz/USB STANAG-4285 600bpsL, very long idling periods (hours!) with sporadic asynchronous data transfers consisting of 15 bit and apparently nx32 bit (32,64,...,128) ACF blocks (Fig. 1). The 15-bit Block-A is clearly an async ITA2 5N1.5 stream (Fig. 2). By decoding the whole stream whit the HARRIS RF-5710A modem, also the Block-B seems to use ITA2 and 5N1.5 framing: indeed, the modem does not print bad framing errors. By the way, I do not think a framing change "on the fly" be possible. Looking at Block-B (Fig. 3), it seems arranged as 4 groups of 4 chars (the fourth as separator), but it's only a my guess.
In both blocks the 5-bit text appears as (off-line?) encrypted.
For a better understanding of the figures keep in mind that bit editors work with an integer number of bits therefore they cannot represent an half bit.
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| Fig. 1 - data transfer consists of two blocks |
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| Fig. 2 - 15-bit period Block-A |
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| Fig. 3 - 32/128-bit period Block-B |
The above images come from a 4285 demodulator output which returns the bits in air, the demodulated stream from 5710A modem (framing 5N1.5) is shown in Figure 4.
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| Fig. 4 |
For what concerns the user/source, I had the best SNR using a remote SDR in Greece (http://sdr.telcosol.gr:8073/), probably the transmitter is located in the eastern Mediterranean area: Greece, Cyprus Is. or Turkey. Some friends from radioscanner say it's the Turkish T-15 protocol.
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| Fig. 5 - monitoring the signal using remote SDRs in Greece and Spain (at same time) |
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