25 September 2017

5400 KHz (cf) 2380Bd mix-mode burst waveform
(by: i56578, ANgazu, KarapuZ, Rapidbit)

This is a very strange burst waveform modem that can be heard on 5398.2 KHz/USB (the central tone is exactly at 4000.0 Khz) at different times and SNRs. As Karapuz sais, probably it is Chinese equipment, but ANgazu guess the transmitter is near Spain, perhaps in Algeria: it's worth noting that there are 3 Chinese made ships in Algerian Navy and probably they maintained the factory radio-equiment:
http://www.defenceweb.co.za...Sea&Itemid=106 are

The analysis of the bursts is not simple since the results exibith four different modulations in each burst (GFSK, MFSK-4, PSK-4, PSK-2), anyway all the transmissions have two fixed points (fig. 1):

-  constant manipulation rate of ~2380 symbols/sec 
-  all the bursts end with a PSK-2 segment

Fig. 1
Some good recordings can be download from here:

1. i56578-23-September recording
A detailed analysis of the bursts reveals a mix of GFSK 2380Bd/1500 and PSK-2 in both the two groups A and B, as shown in Figures 2-4.

Fig. 2 - group A
Fig. 3 - group B
Fig. 4 - modulation used

2. i56578-10-October & Rapidbit-rec05 recordings
The analysis of these recording reveals the use of QPSK and PSK-2 modulations in the group A and MFSK-4 and PSK-2 modulations in the group B (Figures 5-8).

Fig. 5 - group A
Fig. 6 - group B

Fig. 7 - QPSK and PSK-2 modulations used in group A

Fig. 8 - MFSK-4 and PSK-2 modulations used in  group B

3. observations & oddities
Tx windows are 4 min alternating short and long runs: the short transmissions are about 86 sec and the long ones last about 172 sec (twice the short). ANgazu recorded about 6 hours of monitoring period and processed the files using SA raster: as you can see, time window is 4 m. There are 13 transmissions and a no Tx period, building up groups of 14 frames starting by long Tx (Fig. 9)

Fig. 9

Each transmission consists of two groups of bursts (here termed "A" and "B"), possibly the group B is a repetition of A using different modulation to improve the reliability of the system. Every group starts using a longer carrier and a wider burst (about 2 sec). Unless some little variations, burts have a duration of ~720 msec and are spaced by a 500 msec unmodulated 1800Hz tone. It seems that if QPSK is used in the group A then group B will use MFSK; as seen, all the bursts end with BPSK segments.

In this signal, there are transients when changing modulation even if all modulations are BPSK, so no phase continuity is in use (Fig. 10)

Fig. 10

Phase vector rotates in one or other sense avoiding continous rotation in one sense. In this sample it can rotate both senses, mostly CCW or mostly CW (Fig. 11)

Fig. 11
In most systems, sync sequences are placed at signal start. In this one, after demodulated some BPSK bursts Sync seems to be 256 bits at the end (Fig. 12). Perhaps the burst shoul be reversed?

Fig. 12
(to be continued)

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