Packet radio from the ISS with GNU Radio and USRP

On July 10, 2010, I was listening to the linear transponder downlink of VO-52 when I suddenly noticed a very strong peak about 50 kHz below. I checked Gpredict and found out that it was the APRS downlink from the International Space Station RS0ISS on 145.825 MHz. Thanks to the flexible GNU Radio framework and the USRP, I could easily receive both satellites at the same time as shown on the video below.

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5.8 GHz Helical Feed for the 90cm dish

Yesterday was day 2 where we were repairing the broken Azimuth rotator and making a small 90cm dish ready to track UNITEC-1 on 5.84 GHz. Actually, we already fixed the rotator on Monday but we ended up mounting it 180° off and we decided to fix it properly instead of just correcting it in software.

Fixing the orientation of the Azimuth rotoator was very quick – it took only 17 minutes to get up to the mast, lift the antenna construction, change the orientation of the rotator and fasten the nuts and bolts again. We had the practice from yesterday.

Next item on the agenda was to make a small helix with two turns to feed the 90cm dish so that we can use this smaller dish for tracking UNITEC-1 in the beginning of the interplanetary cruise. We found some online helical antenna calculator to generate the design but that was more than 1 GHz off and it took a lot of tweaking and tuning to get it close to 5.8 GHz. Here are the results, photos and videos.

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SSTV Robot OZ9STV with USRP and WBX

I was fooling around with the USRP / WBX on the 2 meter band tonight when I suddenly stumbled upon some very strong SSTV transmission. It turned out to be the OZ9STV robot located only a few kilometers from me. That explained why I had more than 40 dB SNR even though I was only using a bad whip antenna indoors.

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Improved AGC for the simple SSB receiver

Few days ago I wrote about how I upgraded the simple double side band receiver implemented in the GNU Radio Companion to a simple single side band receiver. This initial implementation used some default values for the AGC attack and decay rates. These values were acceptable; however, I wanted to spend some time trying to find better values that would correspond to Fast, Medium and Slow AGC – just as we are used to in commercial radios.

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