SSTV is a picture transmission method for transmitting and receiving static pictures via radio. Similar to a fax machine, or a 90s dial up modem, SSTV is an analogue signal that resembles a high pitch cacophony of bleeps and screeches. It uses frequency modulation, where the signal frequency shifts up or down to designate pixel brightness and colour. A transmission consists of horizontal lines of pixels, scanned from left to right, encoded as audio. The audio is transmitted using radio and converted back into the picture at the other end using special software.
The International Space Station has a long [history](https://www.spaceflightsoftware.com/ARISS_SSTV/archive.php) of transmitting SSTV signals and these instructions show you how to receive them using just a Raspberry Pi computer and an RTL-SDR USB dongle.
Why use a Raspberry Pi? This could be done using a desktop PC or Mac however you often need to leave the receiver running overnight, waiting for the ISS to fly over your location, and it's usually easier to tie up a Raspberry Pi with this task than your main utilitarian computer that you use all the time.
To have a quick play, it is possible to install a mobile phone app that decodes SSTV through its microphone input. Playing the bleeps and screeches of an SSTV signal with the phone placed near the speaker is usually good enough.
This should work in a classroom provided there isn't too much background noise, if you ask everyone to install the app and then play the test recording at a reasonable volume all the phones will decode the picture. It will give everyone a good idea of how long it takes to obtain one image. Make sure you sneeze or cough half way through to show how this causes interference.
- [This RTL-SDR starter kit](https://www.amazon.com/RTL-SDR-Blog-RTL2832U-Software-Telescopic/dp/B011HVUEME/) is good and comes with a nice beginners dipole antenna that will pick up the ISS.
1. Verify RTL-SDR is working with `rtl_test` program.
- Start > Accessories > Terminal
```
rtl_test
```
Expected output:
```
Found 1 device(s):
0: MAKE, MODEL, SN: 00000001
Using device 0: Generic RTL2832U OEM
Found MAKE MODEL tuner
Supported gain values (XX): XX, XX, XX...
[R82XX] PLL not locked!
Sampling at 2048000 S/s.
Info: This tool will continuously read from the device, and report if
samples get lost. If you observe no further output, everything is fine.
Reading samples in async mode...
```
Leave for 30 seconds and look out for any messages about loss of samples.
Press `Ctrl-C` when 30 seconds as elapsed. The loss of 100 bytes or so is acceptable.
1. Verify RTL-SDR can tune to a commercial FM radio station with `rtl_fm` program. The command below pipes raw data from `rtl_fm` into to sox `play` which will then decode the raw data to produce audio output. Modify the value after the `-f` to specify your own FM station/frequency (98.8 is BBC Radio One in the UK).
Note that the `Tuned to` value is doesn't match. This is due to [DC spike](https://www.rtl-sdr.com/tag/dc-spike/) and can be ignored. You should now be able to hear audio from the commercial FM station. Move the antenna around or select a different FM radio station with a closer transmitter if the audio is noisy. Press `Ctrl-C` to quit from `rtl_fm`.
- Open `sstv_test.mp3` in the Chromium browser. You can either drag and drop the file from File Manager or just enter `file:///home/pi/sstv_test.mp3` into the address bar.
- Let it play. Note that with VLC Media Player you get a skewed image sometimes.
1. What is it? A common example of [Doppler shift](https://en.wikipedia.org/wiki/Doppler_effect) is the change of pitch heard when a police car or ambulance passes you. Compared to the emitted frequency of the siren, the frequency you hear is higher during the approach, identical at the instant of passing by, and lower during departure. The same thing happens with radio waves as with sound waves.
- The ISS is moving at ~27,600 km/h. This motion causes Doppler shift in the radio waves received at your location.
- To compensate for the effects of Doppler shift, ground stations must continually re-tune their receiver as the ISS approaches, passes overhead and flies away.
- As the ISS comes over the horizon (AOS or Acquisition of Signal) you would need to tune approx 3.5 kHz ABOVE 145.8 MHz.
- At the instant when the ISS is directly overhead the actual transmitting frequency of 145.8 MHz can be used.
- Just before ISS goes down over the horizon (LOS or Loss of Signal) you would need to tune approx 3.5 kHz BELOW 145.8 MHz.
- The amount of re-tuning is dependent on the elevation of the ISS above the horizon. For example, an overhead pass requires a lot of re-tuning because there's a huge change in distance and relative speed as the ISS passes by. A low elevation pass, where it just peeks above the horizon and goes down again, requires relatively little.
- Luckily **QSSTV can deal with Doppler shift itself** and so you don't have to do anything to compensate after tuning to 145.8 MHz.
1. If you want to do Doppler correction anyway it won't do any harm. It could, in fact, be a nice coding activity for a classroom.
1. [Here](https://github.com/davidhoness/sstv_decoder/blob/master/doppler.py) is a pre-made python script to compensate for Doppler shift. This program tracks the ISS using `ephem`, computes the Doppler corrected frequency for when the ISS is passing over and re-tunes `rtl_fm` via a UDP socket.
1. You will need to modify this python script to set your location. You can easily look up the latitude and longitude of your location using [Google Maps geocoder](https://google-developers.appspot.com/maps/documentation/utils/geocoder/). Usually the postal code and country is sufficient.
1. If you are using the RTL-SDR kit antenna, please read the getting started [guide](https://www.rtl-sdr.com/using-our-new-dipole-antenna-kit/) for it.
1. It's a good idea to adjust the length of the telescopic sections based on this dipole [calculator](http://www.csgnetwork.com/antennaedcalc.html) to match the antenna's resonant frequency to 145.8 MHz. This will help pick up weaker signals from low elevation passes.
- Remember there is about 2 cm of metal in each side of the antenna base. So take the calculated value and, before screwing the telescopic sections in, make them both this length minus 2 cm (excluding the screw threads).
1. Set the correct time on the Raspberry Pi. QSSTV saves the images with a UTC time filename and this is useful later when working out which ISS passes they are from.
- Note this command is different to the commercial radio station one. A commercial radio station uses wide band FM whereas the ISS transmission uses narrow band FM and so we have to set `rtl_fm` up differently.
1.**OPTIONAL:** If you are doing Doppler correction, you could manually set the system clock to one minute before an upcoming pass to test what the tuning will be like, although noting will be received of course.
