12 February 2020

Interesting MS-110D App.D (WBHF) traffic


Interesting traffic heard on 5750 KHz/USB and picked up thanks to the UK KiwiSDR owned by G8JNJ.
Most of the signals are definitely "110C Appendix D" 3 KHz BW waveform (WID 1 or 2, BPSK). The synchronization preamble has a framing of ~240ms length that makes 576 symbols @2400Bd speed (Fig. 1). From 188-110C App.D documentation, the orthogonal Walsh modulation is used in the synchronization section of the preamble and the length of the repeated super-frame is 18 channel-symbols, ie: 9 (fixed) + 4 (downcount) + 5 (waveform identification). Since in 3 KHz bandwidth waveforms the preamble channel-symbols are 32 symbol long, the length of each repeated superframe is: 18 (channel-symbols) x 32 (length of one channel-symbol) that just matches the measured 576 symbols length.
Data section has 40ms length frames (Fig. 2) i.e. each frame consists of 96 symbols: 48 unknown data + 48 known data (mini-probe). This framing meet the waveform IDs 1 and 2 of the 3KHz bandwidth set (BPSK modulated data).

Fig. 1 - Synchronization preamble superframes
Fig.2 - data section frames
Anyway, FLSU BW5 bursts and unid 2400Bd bursts are the most interesting aspects in this catch.
In my opinion the presence of (repeated) 3G-HF Fast Link SetUp (FLSU) BW5 bursts is rather strange also because the link seems to be terminated with a 188-141A 2G-ALE "TWS" sequence: a kind of "fall back" for 2G-ALE? Perhaps we're dealing with a STANAG-4538 "circuit-mode" service and I did not hear the BW5 PDUs sent by the other side of  the link, or perhaps BW5 PDUs are just used to signal the following traffic waveform.
The other 2400Bd bursts (Fig. 3) have a fixed duration of ~2840ms: unfortunately the poor SNR of the signals does not allow to get other significant parameters from their analysis.

Fig. 3
As said, the link was terminated using 2G-ALE: the TWS message was sent by the ALE callsign "AC7", It's to be noticed that during the monitoring period other ALE soundings from calls "AC7" and "AC9"  have been heard. According to recent UDXF logs, these calls refer to a unid Jordan network, although it sounds weird to me that they use WBHF technology. Maybe some WBHF trials... but it's just a guess.

southwest.ddns.net_2020-02-06T21_02_31Z_5750.00_usb.wav
southwest.ddns.net_2020-02-06T20_56_47Z_5750.00_usb.wav

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