SDRplay as satellite downlink receiver?

Discussion in 'Satellite and Space Communications' started by WD9EWK, Aug 24, 2015.

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  1. WD9EWK

    WD9EWK Ham Member QRZ Page


    Another AO-73 pass this evening, another chance to use the SDRplay. The pass started around 0343 UTC, and - unlike the past couple of evenings - the satellite was back in its autonomous mode. This means that the transponder is only on when the satellite isn't receiving sunlight. I have heard the transponder switch off during similar passes, but this was the first time I could also see it with an SDR receiver. On this pass, the transponder switched off at 03:51:27 UTC. The satellite was just coming into sunlight, about 10 seconds before this happened. This is how SatPC32 showed the satellite at the moment the transponder switched off:


    This is how it looked on my HDSDR screen, around the moment the transponder switched off:

    Above the white horizontal line is the time before the switchoff. I was making a CQ call in SSB around 145.960 MHz USB on the downlink, and there was another station in SSB on the transponder above me, around the moment it switched off. Notice the vertical line around 145.935 MHz. It goes from a green/yellow line before the transponder switched off, to a red/orange line after the switchoff. This also indicates the signal strength increased, after the transponder switched off. Normally, the telemetry is transmitted at 30mW when the transponder is on, or at 300mW when the transponder is off.

    This RF recording is being uploaded to my Dropbox space as I write this. This will not only be an example of the power of using an SDR receiver to see all the activity on the satellite transponder, but also to let you see what happens as AO-73 passes back into sunlight, and its transponder switches off and the signal strength on the 145.935 MHz telemetry beacon increases by a factor of 10.

    Last edited: Sep 7, 2015
    K2NCC and W5SAT like this.
  2. WD9EWK

    WD9EWK Ham Member QRZ Page


    I tried another SO-50 pass earlier this morning, a 75-degree pass to my west, so I could try something KG7NXH suggested in HDSDR - making an IF recording, instead of an RF recording. The IF recording takes up less drive space than the RF recording, especially with the SDRplay and its minimum 200 kHz bandwidth. It worked, although there was a point in the recording where the downlink jumped up a few kHz. I don't know if this is the software or the satellite that did that, but I was able to work 5 stations up and down the west coast.

    This was a nice pass, after an earlier pass around 1400 UTC that covered the continental USA as SO-50 moved from east to west. This pass still covered much of the continental USA. Here's a screenshot from SatPC32 showing SO-50's footprint at 1545 UTC, near the midpoint of the pass:

    Other than XE2BHL in Tijuana/DM12, no other stations were heard from south of the USA/Mexico border - a shame.

    I tried the IF recording option in HDSDR instead of a full RF recording. The file size shrunk dramatically, down to just over 37 MB for almost 7 minutes. This would have been at least 300 MB, and probably near 400 MB, if I had made an RF recording. When playing back the recording after the pass, you can see that I am not able to see the full 200 kHz passband I have from the SDRplay. Here's the screen at 1542 UTC, early in the pass. I'm making a QSO with Leo W7JPI around this time:

    The red/orange line for the SO-50 downlink is still mostly up and down. Not much Doppler effect at this point. Now move ahead a minute, to 1543 UTC...

    still not a lot of Doppler. At 1544 UTC:

    you can see more Doppler in the downlink. Thirty seconds after this screenshot, it looked like this:

    This screenshot was taken at 15:44:30 UTC in the pass, as I am calling KH2FI in San Diego. The satellite is almost at its highest in the sky, and Doppler is definitely noticeable here. If I hadn't been using an SDR receiver - SDRplay, FUNcube Dongle Pro+, or something else - with HDSDR and its AFC function, I would be making frequency adjustments from 436.805 MHz to 436.800 MHz, and getting ready to tune down to 436.795 MHz real soon.

    The HDSDR IF recording from this SO-50 pass is in my Dropbox space, along with the screenshots from this post. My Dropbox space is at:

    The IF recording has a file name starting with HDSDR_20150907_154100 if you want to download it and try it for yourself.

    I will need to try the IF recording out with a linear transponder, to see if I can still record all - or most - of the transponder and not use as much space for the recording.

  3. WD9EWK

    WD9EWK Ham Member QRZ Page

    A correction to the following line from my previous post:

    SO-50 went from west to east during this pass. East-to-west movement would be an interesting thing, but I'll correct myself before someone decides I committed a major felony for this mistake.
  4. W5SAT

    W5SAT Ham Member QRZ Page

    Well if ever there was a thorough documenting of SDRPlay with the satellites this is it. Again, nice job!!!

    I was thinking about the RF spectrum shifting unusually in your last SO-50 pass where you recorded the IF with HDSDR. I just tested this to be sure how the IF recording works. Basically, it will record whatever bandwidth is selected under "Bandwidth" then Sampling rate Output column centered around what the tune frequency is set to. So, if you tune around while recording IF, the recording center frequency will follow the tuning. I'm guessing that's why frequency apparently drifted the way it did in that recording. So, I guess the caveat for IF recording is to not tune much while recording. The good thing is that the IF recording will already be somewhat doppler corrected...
    WD9EWK likes this.
  5. KA2CZU

    KA2CZU XML Subscriber QRZ Page

    great threads and fascinating stuff I'll catalog for use when I graduate from the satellite 101 remedial class :D

    are you running SatPC32 on the tablet as well? ( I assume no )
  6. WD9EWK

    WD9EWK Ham Member QRZ Page


    When I started this thread, I was hoping that someone else had some experience using the SDRplay with our satellites. When that didn't happen, I went through and did this by trial and error. Lots of trials, some errors, and more successes. I'm still learning about the SDR stuff, but I already knew SDR would be a powerful tool to have for working satellites. I was hoping to find another SDR receiver that could work with satellites, and work with the inexpensive Windows tablets that are now on the market. I have done that, and can switch between the SDRplay and FUNcube Dongle Pro+ for almost anything I'd do on satellites. The one exception, so far - I'm sticking with the FUNcube Dongle Pro+ when copying telemetry from a FUNcube satellite/payload. I'll let the FUNcube Dashboard software control the dongle and do its thing automagically.

    Maybe I need to play with the Output Bandwidth and widen that a bit more, so that I get more spectrum in the IF recording. Something I can try on another SO-50 pass. I should be able to try this on the SSB satellites, as long as HDSDR sees the entire width of the transponder. I didn't try this over the weekend, but I can give it a try some other time (if someone else doesn't beat me to this first). Receiving the AO-73 transponder, or AO-7's mode B transponder, should be easier for HDSDR to handle. There's less Doppler effect at 2m than 70cm, so I shouldn't see as sharp of an angle in the received signals at any point during a pass.

  7. WD9EWK

    WD9EWK Ham Member QRZ Page

    For me, working with SDR stuff is a way to learn and expand my knowledge. I want to do more than just talk on SO-50 with an HT and antenna. There's room for discussions on introductory topics, as well as more advanced topics, on forums like this one.

    I may not be able to set up a proper station at home, so I don't have to use a trash can in the back yard as my testing lab, but it is fun to do this with gear that works well for portable operation. I had fun over the 3-day holiday weekend on the radio, even with a bit of rain yesterday afternoon.

    No, but I could. SatPC32 runs fine on my tablets, with either 1 GB or 2 GB RAM.

    One issue with some tablets - like my 8-inch HP Stream 8 tablet - is the single micro-USB port. It can be used either to charge the tablet, or (with a USB On-The-Go cable) as a USB 2.0 port with an external device. Perhaps I could use a powered USB hub through the micro-USB port to have multiple USB ports at my disposal with this tablet, but I'm not looking to go in that direction. For one thing, I'd have to supply power to a USB hub. This is not an issue with other tablets, including the 10-inch WinBook TW100 tablet I have, as it has a full-sized USB port and a micro-USB port that - with an On-The-Go cable - would be the second USB port on the tablet. With two USB ports, I could connect an SDR receiver on one port, and a USB cable to control my transmit radio (FT-817ND), and then use SatPC32 to control the SDR receiver and the FT-817ND. HDSDR has an interface where SatPC32 can control it directly.

    Running HDSDR and SatPC32, along with having two USB devices run on the tablet, might be pushing the capabilities of these tablets. I have run both HDSDR and SatPC32 at the same time on these tablets, along with a FUNcube Dongle Pro+ connected to a USB port, without problems. Having the tablet control two USB devices, and run these two programs, could be an issue when all done at the same time. I'll leave that for others to try, as I don't normally use computer control of my radios when working satellite passes.

  8. KA2CZU

    KA2CZU XML Subscriber QRZ Page

    cool... I'm getting my connector / cable things fixed "soon", will give these homebrew antennas one more shot before breaking down for an arrow or elk. I tell you, sometimes I think I have early dimentia. Second, the SDR experiments you've (and Brad) been conducting give me a bad case of A.D.D. and make me want to run out and get going on them... gotta breath, take it slow.

    anyhow, really enjoy the threads on this.
    WD9EWK likes this.
  9. W5SAT

    W5SAT Ham Member QRZ Page

    Robert, if you want to get an SDR on the cheap this might be worth it for only $20. I actually just ordered this to have an extra, slightly better one around.

    I already have the older RTL SDR with the 820T tuner (this one has 820T2 tuner plus SMA connector and TCXO) and they are certainly not what you would call high end and won't compare to the better units like Patrick is experimenting with but for $20 they work amazing well. Main problem with the $20 dongles is IMD and front end overload but they receive quite well all things considered.

    If you're itching to play with SDR but don't want to spend much this is an option.
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  10. KA2CZU

    KA2CZU XML Subscriber QRZ Page

    done (now my A.D.D. will really kick in, once it arrives :D)

    I've got all sorts of cables and connectors on order. I'm determined to get and receive good sigs from the sats. Oh yes, had to order a bnc to sma connector along with this dongle!!
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