Since any days on 7907.0 KHz/USB, starting in the morning, it's possible to hear long sessions of 2400Bd burst signals which usage different modulations and ways. Signal "A" in Figure 1 consists of 4-segment bursts, each section lasting about 420 ms.
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| Fig. 1 |
Modulation utilized is PSK8 at the rate of 2400Bd. The demodulated bitstream has a period dimension of 18 bits (6 PSK8 symbols, Figure 2) that can be reduced to 3 bits or even 2 bits if the "1s" column is removed.
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| Fig. 2 - demodulated bitstream of a PSK8 burst |
Signal "B" in Figure 3 is the same as the above but in this case 2 station are involved, as it's easy to figure out looking at the different strengths of the bursts and their fading patterns. The "2-stations" mode starts randomly after a while but w/out a kind of schedule: in my opinion the time-slot paradigma (or Time Division Multiple Access mode, TDMA) is utilized (1).
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| Fig. 3 |
Signal "C" (Figure 4) usage QPSK modulation alternatively (again 2400Bd) and 550 ms bursts with a "duty cycle" of 50%.
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| Fig. 4 |
The demodulated bitstreams have a period dimension of 540 bits (270 PSK4 symbols, Figure 5) with a clearly visible framing consisting of 8 sections "Data-MiniProbe" of different durations. The 270-symbol frames and the durations of the 8 "Data-MP" sections are the same of the STANAG-4539 TDMA waveform WF2 (see Table I), thus the recording "C" is definitely a Link-22 transmission (2).
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| Table I |
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| Fig. 5 |
All my direction uncovering tries (TDoA algorithm with 5 receivers!) point to an area close Nuremberg (Nürnberg), Germany. most likely it's the USAG (U.S. Army Garrison) Ansbach base which is located in northern Bavaria, about 40 kilometers southwest of Nuremberg [1].
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| Fig. 6 - Direction uncovering tries |
For what concerns the PSK8 bursts, bitstreams and TDMA mode make me think about a Tactical Data Link (TDL), even if the analyzed waveforms do not match the standards Link-11 or Link-22 (atleast the ones I know). Anyway, given the deficiency of another information, it cannot be ruled out that it could be telemetry signals.
Monitoring has been made thanks to KiwiSDRs of OE3AKB (Landersdorf, AUSTRIA) e OZ1BFM (Vejby, Denmark) [2][3].
https://disk.yandex.com/d/b04EpZUQIO5GIw
(1) In TDMA mode each user is allowed to transmit only within specified time intervals named as "Time Slots" so that different users transmit in differents time slots.
(2) #2.3.2 Media Code Frame structure, Annex D to STANAG-4539
[1] https://installations.militaryonesource.mil/.../ansbach-united-states-army-garrison
[2] http://oe3akb.ddns.net:8073/
[3] http://oz1bfm.proxy.kiwisdr.com:8073/
