When the International Space Station passes overhead it is still possible to view the SSTV (Slow Scan Television) images they are sending on 145.800Mhz using your Linux computer and a Realtek SDR USB Dongle. I did this on Ubuntu 20.20 but it should work on any Linux running pulse audio.
NOTE 1: 12-31-2020 is the last day the ISS will be transmitting images (for now). This tutorial can still help you get GQRX sending audio to QSSTV or any other “recording” app.
Step 1: Install and configure GQRX
First, get GQRX running using this How To: https://gqrx.dk/download/install-ubuntu
NOTE: When I installed GQRX on Ubuntu 20.20 only the myriadrf repos worked. Your mileage may vary.
You now need to run GQRX and configure it to work with your dongle. This is out of the scope of this howto and much info is available via Google. Once you can listen to radio via GQRX it’s time to move to the next steps.
Here is what my settings for the ISS look like:
Step 2: Install and configure QSSTV
I followed this guide to get QSSTV working, however, I found that howto a bit over complicated and many of the steps unnecessary if you already have GQRX installed. I was simply able to install QSSTV like this:
sudo apt install qsstv
Run QSSTV and in the configuration for sound set input and output to Pulse Audio.
Step 3: Install Pulse Audio Volume Control
Now that you have both your apps, you need to install some stuff to get pulse to make them play together. First install pavucontrol:
sudo apt install pavucontrol
Step 4: Invoke the NULL Sink
Finally it’s time to make a dummy sink. This is the glue that puts it all together. Using pavucontrol, any app can be outputted to the null sink and then any other app can record from this null sink.
pactl load-module module-null-sink sink_name=virtual-cable
Step 5: Connect the Programs
Once this null sink is created open up Pulse Audio Volume Control, GQRX and QSSTV. In the recording tab, set the input for QSSTV to Monitor of Null Output.
Finally in the output devices tab, set GQRX to Null Output.
That should be about it.
Find out when the space station is passing overhead and then tune GQRX to 145.800… QSSTV will do the rest. Feel free to post any questions you may have.
NOTE: You can use this technique to pipe audio from any program to another.