A joint experiment in amateur satellites / 2015-05-30

2015-05-30 A joint experiment in amateur satellites 5 years ago
Recently a number of satellites was launched including some cubesats with amateur radio functions. One of the interesting ones is ParkinsonSat named after Bradkin Parkinson who is famous for his contributions to the GPS system.

PSAT has a very interesting transponder: uplink at 28.120 USB psk31 and downlink at 435.350 MHz FM with telemetry in PSK31 included. PSK31 has a very narrow bandwith at 31 Hz, so the entire downlink signal can include 30 distinct PSK31 signals.

With my setup I can transmit to the satellite on HF or try to recieve it with the arrow antenna, but actually making a QSO through it without a rotor to aim the antenna sounds impossible to me. But, there is enough to experiment with. Recently on the amsat-bb mailing list the following post was made by Robert Bruninga from the PSAT team:
What we really need [..] is for everyone to give us feedback as to the minimum power they needed to maintain a minimum signal in the passband and of course, what kind of antenna they are using under those conditions. And this is best early on when there are only 1 or 2 usewrs at a time..

If you don't yet have PSK31, but want to get some data, team up with an HF friend in your area and ask him to just transmit for the next 10 minutes and you can then receie his downlink while commenting on his signal strength throughout the pass.
I decided to take the challenge and I asked another amateur interested in satellites, Jan van Gils PE0SAT to record the downlink while I would send out PSK31 signals in the uplink band. Today starting at 09:59 UTC we had a nice pass over the Netherlands and I started transmitting
PE0SAT DE PD4KH VIA PSAT AT 5 WATT 0959Z
adjusting output power on my radio and adjusting the power noted in the message for later decoding.

PE0SAT made a recording of the downlink in SDRsharp and gave it to me in the form of a I/Q wav file sampled at 192 kHz. Being a linux user I wanted to decode this in gqrx. It took me a bit to find out how to convert the .wav file to something gqrx would see as a recorded I/Q file. For those looking for it:
$ sox SDRSharp_20150530_095915Z_435402kHz_IQ.wav -t raw -r 192000 -c 2 -e float gqrx_20150530_095915_435402000_192000_fc.raw
The date/time/frequency/samplerate are included in the filename so gqrx reads the I/Q file correctly.

Playing this file in gqrx and routing the output audio to fldigi gave a nice waterfall display with PSK31 signals but the PSK31 wasn't decoding at all. I started on the telemetry signals and saw:
$ 44<DLE> !K <! <SI><<SI> <<SOH><SI>~ e Pyniol¦Y<SI>
which looks nothing like the PSAT telemetry specification. So I assumed I did something wrong in converting the I/Q file. I tried lots of ways to do this conversion but gave up and started using SDRsharp for Windows and used the option in SDRsharp to record the narrow-FM decoded audio. Playing this file in audacious and routing the audio to fldigi gave the same problem as before with exactly the same weird decodes of the telemetry data. I noticed the telemetry PSK31 was on 285 Hz where the specification says it should be on 312.5 Hz. I decided to speed up the audio file by a factor of 1.096491 and suddenly I was able to decode telemetry packets from the recorded audio. In total 31 packets, 29 looking good:
W3ADO-5 beacon B 002 09 23 862 268 +2

W3ADO-5 beacon B 003 40 23 874 269 +3

W3ADO-5 beacon B 004 31 23 828 258 +6

W3ADO-5 beacon B 005 37 23 863 268 +4

W3ADO-5 beacon B 006 34 23 879 271 +5

W3ADO-5 beacon B 007 21 24 818 255 +10

W3ADO-5 beacon B 008 99 30 815 255 +12

W3ADO-5 beacon B 009 12 23 816 257 +13

W3ADO-5 beacon B 010 50 23 816 255 +14

W3ADO-5 beacon B 011 06 23 814 256 +15

W3ADO-5 beacon B 012 34 23 820 256 +15

W3ADO-5 beacon B 013 15 23 815 259 +16

W3ADO-5 beacon B 014 78 23 813 254 +16

W3ADO-5 beacon B 015 34 24 827 259 +14

W3ADO-5 beacon B 016 53 23 814 257 +16

W3ADO-5 beacon B 017 37 23 856 266 +13

W3ADO-5 beacon B 018 40 23 813 257 +15

W3ADO-5 beacon B 019 43 23 815 257 +17

W3ADO-5 beacon B 020 43 27 811 253 +17

W3ADO-5 beacon B 021 18 27 811 256 +18

W3ADO-5 beacon B 022 34 26 809 255 +19

W3ADO-5 beacon B 023 12 23 811 255 +19

W3ADO-5 beacon B 024 59 28 809 254 +19

W3ADO-5 beacon B 025 31 23 809 254 +20

WDO-5 beacon B 026 31 43 807 255 +20

W3ADO-5 beacon B 027 81 34 808 254 +20

W3ADO-5 beacon B 028 37 32 830 262 +14

W3ADO-5 beacon B 029 62 29 810 2

W3ADO-5 beacon B 030 71 34 809 254 +18

W0O-•beacon B 031 71 36 809 256 +20

W3ADO-5 beacon B 032 75 34 806 255 +20

W3ADO-5 beacon B 033 75 44 808 254 +21
after looking for telemetry I browsed through the downlink signal looking for other PSK31 traffic, including my own. The results:
EA4CYQ

CQ vir
PSAT atde LZ1CWK LrCt

CQ DE IK80ZV

R3LW

RQ7K

E PD4KH VIA PSAT AT 20 WATT 1004Z
EA4CYQ, LZ1CWK, R3LW all mention an interest in amateur satellites on their QRZ page.

The conclusions sofar:
  • 20 watts on HF needed for me to make a readable signal
  • the automatic switch in the transponder does not recognize PSK31 from earth very well

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