31 March 2016

Unid 2400Bd PSK-2 burst waveform (prob. SAT Telemetry downlink)


This signal was heard at 1245 UTC on 12168.0 KHz/USB on March 30th.
The burst waveform use a PSK-2 (prob. DBPSK) serial tone modulation of an 1800 Hz carrier at 2400 symbols per second (pics 1 and 2). Each transmission consists of 12 x 4000ms bursts, ending with a single 1500ms burst. 

pic. 1
pic. 2 - PSK-2 modulation of a 1800Hz carrier at 2400 Baud

Each 4000ms burst consists of 8 x 500ms frames (pic.3), for a total of 9600 bits; ACF = 500ms/1200 bits (of course). 

pic. 3 - frame structure
Each 1200-bit frame is structured as a preamble followed by a 67/134 bits period sub-frames, as shown in pictures 4 and 5 (the underlying sub-frames structure is also visible above in pic. 3).

pic. 4 - possible frame periods
pic. 5 - 1200 bit frame seen as a 134 bits period

Using a test-string such as "0011011000010101111" and synchronizing the stream on a 67-bit period, it's clearly visible that the burts send repetitive blocks of data
 
pic. 6 - 67-bit synchronized period

Such parameters (500 mS ACF, BPSK, 2400 Baud and repetitive blocks) are typical for SAT Telemetry downlink, my friend AngazU says. Searching the web I found that this mode is used by DELFI-N3XT cube-nano-picosat and cube-sat QB50. Downlink frequencies are planned on VHF 145 MHz so I don't know why 12168.0 KHz, maybe a relay for sporadic use... but who knows?

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