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Slow-scan Television (SSTV)
SSTV is a way to transmit images over the air. The image is encoded into audio signals, then transmitted over radio signals, and finally decoded back into an image at the receiving station. First considered a actual radio communication by the FCC in 1964, it's a fairly simple way to get a contact in a very fun way.
Receiving SSTV From Space
Other than making full contacts between stations over terrestrial radio, the ISS occasionally holds events where they send images over SSTV. These images are related to space, often to commemorate astronauts or important space-related objects and events.
This is a good way to get started with SSTV, since it requires very little equipment and is very forgiving of mistakes.
Equipment & Preparation
Receiving SSTV from space requires minimal equipment:
- Satellite tracking website or application
- Handheld (HT) radio
- Audio recording device
- Image decoding software
Satellite tracking website and apps are in a pretty bad state unless you pay for apps or subscriptions. As of summer 2021, there are a couple ways to predict when the ISS will be overhead your location.
If you know your lat/long or Maidenhead grid, you can enter either of those into AMSAT’s ISS Predictor and get a list of upcoming ISS passes. A Maidenhead grid is sufficiently accurate and easier to copy/paste between the two websites.
Using a phone will work just as well, but gives you the added abilities of being portable and being able to see where the ISS will come over the horizon. The only free iPhone app with both capabilities is GoISSWatch. In order for it to work effectively you’ll need to allow location permissions when asked, and turn the “Visible Only” setting off. Then, when you go to Passes you’ll see the next times the ISS will be overhead.
Any VHF handheld radio will work using the stock (rubber ducky) antenna. Different antenna styles can improve the signal, like a directional yagi or egg-beater antenna, but a readable image can easily be received with even a $30 HT radio.
The simplest way to record audio is using the voice recording app on a cellphone, though anything that can record and playback audio will work. Unless you're in a very noisy environment (trucks driving by, etc.), then turning up the volume on the radio and holding the recorder nearby will work just fine.
Computer applications like MMSSTV (Windows), QSSTV (Linux), or Macrobot (Mac) offer more robust controls to fine tune and correct the image. But that involves the extra step of transferring files to the computer or a loss in audio quality from replaying the audio and recording it with the computer.
A simpler option is to use one of the many cellphone SSTV decoders, such as Black Cat SSTV (iPhone) and Robot36 SSTV (Android). Using those apps, the image is immediately decoded as the audio is received on the HT radio.
There are three things that can help with getting the best image possible.
- Be in a open, quiet space
- This helps eliminate background noise
- Bring your recording device right next to your HT speaker
- This will help further eliminate background noise and enhance the sound of the signal
- As the satellite drifts away, move 3 khz up during every 45 seconds
- This is because of the Doppler effect
Once you have the audio recorded, you’ll have a phone or computer decode (convert) that audio into images. For the most reliable results, with minimal chance of losing your image data, record an audio file and decode as a separate step. While a little riskier, there are fewer steps involved if you decode the image into a phone as it’s received.
As of summer 2021, there is only one reliable iPhone app for encoding and decoding SSTV, Black Cat System’s SSTV app. The app has sliders to correct the phase (correct image start) and skew (straighten), though it is common to accidentally mess up your phase and skew settings when you release your finger from the screen. The decoding itself seems to work reliably, but the auto start and auto save functions are likely to lose your data. Be sure to change your default settings (Setup button) so that Auto Start and Auto Save are turned off.
As the you hear the audio being received, press the Start button and simply hold the radio speaker up to the phone microphone (usually on the bottom) until the transmission is complete. Press stop on the app, adjust the Phase and Skew, and press the Save button when ready.
Using a Computer
If you choose to use one of the decoding apps on a cellphone, all you need to do is play the sound and it will automatically decode. For MMSSTV, first you must set the microphone to your headset, and then just play the signal and it will automatically decode.