this transmission is composed of messages sent using standard MS188-110 Serial Tone waveform (the ones indicated as 3, 4 , 9, ...) and messages (as 9, 11, ...) transmitted using a MS188-110 like waveform that most likely is a proprietary variant. The transmission ends with a short op-chat, thatmay providesome cluesabout the originof the signals, and one interesting not-standard ALE message the closes the link and characterized by the presence of a LFM pulse waveform preamble.
Since it is the better recording of the not-standard MS188-110 signals, I report below the analysis of the message #11 only.
For what concerns the carrier, its modulation and symbols rate, the signal shares the same parameters of the MS188-110 modem: PSK-8 on a single 1800 Hz carrier frequency and a constant 2400 symbols per second output waveform (pic. 1)
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| pic. 1 |
The structure of the signal is indeed different: after a ~211ms sync preamble phase, the data phase consists of 51.25ms frames of alternating data and known symbols (pic. 2). After 57 data frames a symbol sequence (most likely a subset of the initial preamble) is reinserted possibly to facilitate late acquisition, Doppler shift removal, and sync adjustment as requested by MS188-110 standard (pic. 3).
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| pic.2 - sync preamble and data phases |
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| pic. 3 - preamble re-insertions |
The most peculiar aspect of this waveform is its data frame that counts 123 symbols or 369 bit (51.25ms, as indicated by the ACF function in pic 4). The data frame consists of 91 data symbols and a mini-probe of 32 symbols of known data (pic. 5).
The length of the mini-probe, 32 symbols, is quite common and is largely used in MS188-110 waveforms, including the appendix D and C. The oddity is the 91 symbols length of the data block. We will need additional recordings to indagate it.
The length of the mini-probe, 32 symbols, is quite common and is largely used in MS188-110 waveforms, including the appendix D and C. The oddity is the 91 symbols length of the data block. We will need additional recordings to indagate it.
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| pic. 4 - 51.25ms ACF |
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| pic. 5 - frame structure |
About the short op-caht in the final part of the transmission, a friend of mine suggests that the language may belong to the Iranian group (Dari, Pushto, Kurdish) and possibly the protocol itself is developed there: the recording is available for who wants to indagate, simply email me.
The ALE sequence the closes the link is shown enlarged (1/16) in pic. 6.
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| pic. 6 - the ending 2G-ALE sequence |
It consists of three messages, each consisting of a Linear Frequency Modulation (LFM) pulse preamble followed by the "common" MS188-141 2G-ALE waveform: MFSK-8 carriers, manipulation speed of 125 baud and 250Hz step between carriers (pics 7,8).
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| pic. 7 |
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| pic. 8 MFSK-8 grid |