XW-2A is about to rise above the horizon, so I am going to try to decode it. Here are some basic details:
Downlink Frequency: 145.640MHz Callsign: BJ1SB Transmit Power: 100mW Launched: ?? Altitude: ?? Format: 19200bps/9600bps GMSK AX25 XW-2E and XW-2F are small daughter satellites that were piggy backed ontop of the spacecraft.
XW-2A is a micro satellite about 40cm by 40cm by 40cm. So quite large compared to other recent amateur spacecraft. It was previously known as CAS-2A and is part of a set of spacecraft called XW-2A, XW-2B, XW-2C and XW-2D. XW-2E and XW-2F seem to be small modules that are attached to XW-2A. I'm not sure if there was a primary mission other than Amateur Radio, but it has a CW beacon, a telemetry beacon and an inverting linear transponder.
The telemetry is AX25 at 19,200 bps or 9600 bps. It is GMSK, like CAS-4A, which as I noted there is just FSK filtered to be power efficient. So we don't use a special GMSK decoder, we can just use the hs_soundmodem "FSK G3RUH 19200bd" decoder, or the 9600bd version. Once the encoded bits are recovered it is classic AX25 frame encoding and KISS format.
I setup HDSDR to decode FM with a bandwidth of 40kHz. I'm using the Funcube Dongle tuned to 145640. There is a loud repeater at 145540 that produces images at 145880 and other spots, so I have to avoid that. I have hs_soundmodem set to FSK G3RUH 19200bd. With default settings. Looking at the waterfall I narrowed HDSDR to 36k bandwidth because 40 includes the CW beacon! But the signal looks too narrow and I see that it must be in 9k6 mode.
With that said the pass was likely too shallow. I never got a strong signal and nothing decoded.
I am going to try XW-2A again. There is a good pass soon:
I set the FCD center to 145660. That is really far from my normal monitoring frequency. I have HDSDR being tuned by SatPC32 to compensate for Doppler, but it is not exact. I need to tweak the tuning during the pass. it can be controlled in real time if you click the CAT menu. This dialog comes up:
From that you can pick which line in the Doppler.SQF file you want to use. You can add as many lines as you like for a spacecraft. I have picked the line for the telemetry downlink. This dialog will allow me to adjust the downlink if the calculation is slightly off. Which is typically the case.
I have a 4 element yagi pointed at the spacecraft with a mast head preamp. SatPC32 is steering the antenna. The SDR Dongle has a Low Noise Preamy (LNA) which is on and the Mixer gain is off. I will again record the pass. No need to hit the record button until the antenna start to move though. SatPC32 is in control of that..
With the pass started I start to get some data, very exciting! It looks like this from hs_soundmodem:
I'm using Mike's AGW Online Kiss program to decode the bytes, but it needs to have the right kelperian elements (KEPS) loaded in. I suppose it is for when people want to automate things. For a single decode it is a pain. Of course the KEPS I download from AMSAT are formatted with Unix line endings and crash the program. So I run them through unix2dos.exe to clean up the file and it works. But the bytes I am receiving look like garbage:
OK, the frames are way too short and I can decode similar frames from the noise. So that is not real data. Hmmm, why is it not decoding? The signal looks really strong in HDSDR when it is right overhead.
I tried the filter at various widths. As an FM signal it likely needs to be in the range of 10-20kHz wide. But no luck. It might be that I am too near the central spike and that is causing interference, but the filter is not overlapping it. I will have to try another day.
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