A Doppler corrected SSTV decoder setup for Raspbian

Disclaimer: This is an experimental set of instructions and is NOT an official product. The purpose is to validate that the instructions work correctly and allow users to receive pictures from the ISS.

What is Slow Scan TV?

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.

The International Space Station has a long history 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.

Playing with SSTV on a mobile phone

To have a quick play, it is possible to install a mobile phone app that decodes SSTV through microphone input. Playing an SSTV test recording through the speakers of a computer with the phone placed near the speaker is usually good enough.

Android: Robot36
Apple IOS: CQ SSTV

Here's an MP3 test file you can download and play.

What you will need

Raspberry Pi 2B or later, USB keyboard, mouse and monitor
RTL-SDR USB dongle (search for RTL2832U, get one that comes with an antenna)

Initial setup and test

A guide for setting up your Raspberry Pi can be found here.
We presume you are running Rasbian Stretch Desktop edition available here

Install prerequisites.

Start > Accessories > Terminal

sudo apt-get update
sudo apt-get install rtl-sdr sox pulseaudio qsstv

Insert RTL-SDR dongle with antenna connected.

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.

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).

Start > Accessories > Terminal

rtl_fm -f 98.8M -M fm -s 170k -A fast -l 0 | play -r 170k -t raw -e s -b 16 -c 1 -V1 -

Expected output:

Found 1 device(s):
  0:  MAKE, MODEL, SN: 00000001

Using device 0: Generic RTL2832U OEM

-: (raw)

  Encoding: Signed PCM
  Channels: 1 @ 16-bit
Samplerate: 170000Hz
Replaygain: off
  Duration: unknown

Found MAKE MODEL tuner
Tuner gain set to automatic.
Tuned to 99055000 Hz.
oversampling input by: 6x.
Oversampling output by: 1x.
Buffer size: 8.03ms
Exact sample rate is: 1020000.026345 Hz
Create UDP thread
Created UDP thread
Main socket started! :-) Tuning enabled on UDP/6020
Sampling at 1020000 S/s.
Output at 170000 Hz.
In:0.00% 00:00:02.60 [00:00:00.00] Out:XXXk  [ -====|====- ]        Clip:0

Note that the Tuned to value is wrong. This is a display issue 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.

Check QSSTV settings are correct.
- Start > Internet > QSSTV
- Options > Configuration > Sound tab
- Audio Interface = PulseAudio
- Input and Output Audio Device = default -- Playback/recording through the PulseAudio sound server
- Sound Input = From sound card
- Sound Output = To sound card
- OK
Calibrate QSSTV.
- Options > Calibrate
- Go make a cup of tea/coffee.
- Click OK when both progress bars reach 99%.
Verify QSSTV decodes slow scan TV test file correctly.
- Download test file
- Start > Accessories > Terminal
```
wget https://raw.githubusercontent.com/davidhoness/sstv_decoder/master/sstv_test.mp3
```
- Select Receive tab in QSSTV.
- Use VIS = ON
- Auto Slant = ON
- Autosave = ON
- Signals = Normal
- Mode = Auto
- Click play ► button. Nothing will happen, but just click it to start the receiver.
- Open sstv_test.mp3 in Chromium browser. Let it play. Note that with VLC Media Player you get a skewed image sometimes.
- Observe the decoding in QSSTV. Notice the FFT and waterflall display activity on the right of the screen.
- Other SSTV test recordings can be found online such as: https://soundcloud.com/spacecomms/pd120-sstv-test-recording

Prepare to receive SSTV from the ISS

Note that the ISS is not always transmitting the SSTV signal. You can find out when it is here: http://ariss-sstv.blogspot.com/
- There is often a week-long SSTV event in April for the birthday of Yuri Gagarin.
- The usual SSTV frequency is 145.8 MHz.
Note regarding doppler shift: https://en.wikipedia.org/wiki/Doppler_effect
- A common example of Doppler shift 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.
- IMPORTANT: No Doppler shift compensation is performed on the ISS. All compensation must all be handled by individual ground stations.
- More information: https://www.qsl.net/ah6rh/am-radio/spacecomm/doppler-and-the-iss.html
Download 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. With QSSTV running in the background you should be able to receive pictures from the ISS.
- Start > Accessories > Terminal
```
wget https://raw.githubusercontent.com/davidhoness/sstv_decoder/master/doppler.py
chmod +x doppler.py
```
Modify the python script to set your location. Current location is set to ESTEC in the Netherlands.
- First look up the latitude and longitude of your location using Google Maps geocoder. Usually the postal code and country is sufficient.
- Open the file doppler.py for editing
```
nano doppler.py
```
- Edit the capitalised variables to specify your location, the altitude can be an approximation.
```
LATITUDE = "52.219308"
LONGITUDE = "4.419926"
ALTITUDE = 20
```
- Please also feel free to review the rest of the code at this time.
  - Note, writing this code could form a nice classroom activity however for this test the code is provided ready to go.
- Save the changes by pressing Ctrl-O, followed by Enter and then Ctrl-X to quit.

Final setup

Ensure the Raspberry Pi is online via Ethernet or WiFi. This is needed so doppler.py can get the ISS telemetry data and keep it updated.

Start rtl_fm in a Terminal window (keep this window open)

Start > Accessories > Terminal

rtl_fm -f 145.8M -M fm -s 170k -A fast -l 0 | play -r 170k -t raw -e s -b 16 -c 1 -V1 -

Start doppler.py in another Terminal window (keep this window open too)
- Start > Accessories > Terminal
```
./doppler.py
```
Start QSSTV
- Start > Internet > QSSTV
- Select Receive tab in QSSTV.
- Auto Slant = ON
- Autosave = ON
- Mode = Auto
- Click play ► button.
- FFT and waterflall display on the right should show noise coming from rtl_fm
Wait for the ISS to arrive. You may want to leave your ground station overnight or over the weekend to capture several passes.
- You can look up when the ISS will next pass your location on: https://www.heavens-above.com/
  - Click Unspecified in the top right to set your location
  - On the home page, under Satellites click ISS
  - Under Passes to include click all
OPTIONAL: Before you leave your ground station overnight. Run these commands to increase the process priority for the main four programs we're using in this setup.
- Start > Accessories > Terminal
```
sudo renice -1 $(pgrep rtl_fm)
sudo renice -1 $(pgrep play)
sudo renice -1 $(pgrep qsstv)
sudo renice -1 $(pgrep doppler)
```
OPTIONAL: If you want to, 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.
- Start > Accessories > Terminal
```
sudo date -s "YYYY-MM-DD HH:MM:SS"
```
- Ensure you return the system clock to the correct time before leaving the ground station running.
- When you return to your ground station select the Gallery tab in QSSTV to see what images were received.

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