QPSK (Link-22) & PSK8 (unid) bursts

Since some days on 7907.0 KHz/USB, starting in the morning, it's possible to hear long sessions of 2400Bd burst signals which use different modulations and ways. Signal "A" in Figure 1 consists of 4-segment bursts, each segment lasting about 420 ms.

Fig. 1

Modulation used is PSK8 at the rate of 2400Bd. The demodulated bitstream has a period length 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.

Fig. 2 - demodulated bitstream of a PSK8 burst

Signal "B" in Figure 3 is the same as the above but in this case two 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 sort of schedule: in my opinion the time-slot paradigma (or Time Division Multiple Access mode, TDMA) is used (1).

Fig. 3

Signal "C" (Figure 4) use QPSK modulation instead (again 2400Bd) and 550 ms bursts with a "duty cycle" of 50%. 

Fig. 4

The demodulated bitstreams have a period length 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 eight "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).

Table I

Fig. 5

All my direction finding tries (TDoA algorithm with 5 receivers!) point to an area near Nuremberg (Nürnberg), Germany. Probably it's the USAG (U.S. Army Garrison) Ansbach base which is located in northern Bavaria, approximately 40 kilometers southwest of Nuremberg [1].

Fig. 6 - Direction Finding 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 lack of other 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/