What SSTV is
Slow-scan television sends still pictures over radio as audio tones. Each line of the picture is sent as a sweep of tones between 1500 Hz (black) and 2300 Hz (white), with a short 1200 Hz pulse to mark the start of each line. A picture takes from 36 seconds to three minutes, depending on the mode. Because it's just audio, you can receive SSTV with any radio and decode it from a recording made on a phone held near the speaker.
Decoding ISS SSTV
Several times a year the International Space Station transmits SSTV pictures on 145.800 MHz FM, usually in PD120 or PD180 mode, to mark anniversaries and events. A dual-band handheld is enough to receive them on a good pass. Check the satellite pass predictor for the ISS's times over you, set your radio to 145.800 MHz FM with the squelch open, and record the audio. A small Yagi pointed at the station gives a much cleaner picture. Then open the recording here. ARISS awards a certificate for each event to people who post their pictures.
How the decoder works
The decoder converts the audio to an instantaneous frequency, then looks for the VIS code that starts every transmission, which identifies the mode. If the start is missing, it works out the mode from the spacing of the sync pulses. It then measures every sync pulse it can find and fits a straight line through them. That corrects for the small difference between the sending and receiving sound cards' sample rates, which would otherwise make the picture slant. Weak or noisy lines are still drawn using that timing. Everything runs in your browser and the audio never leaves your device.
Supported modes are PD90, PD120 and PD180, which the ISS uses, Martin M1, popular in Europe, Scottie S1, popular in the US, and Robot 36, a quick colour mode.
Sending SSTV
The encoder makes a WAV file you can play into your transmitter's microphone input through an audio interface, or straight from a phone for a quick test. Find SSTV activity on 14.230 MHz USB on 20m, and on the image mode frequencies on other bands. Set the audio level so the transmitter isn't overdriven, and remember an SSTV picture keeps the transmitter on continuously for the whole picture, so use a power your radio can sustain. Include your callsign on the picture, and give it by voice or CW as your licence requires.
Questions
How do I decode SSTV from the ISS?
Record the audio on 145.800 MHz FM during an ISS SSTV event, then open the recording here. The ISS usually sends PD120 or PD180, and the mode is detected automatically.
Why is my SSTV picture slanted?
Slant comes from a small difference between the sample rates of the sending and receiving sound cards. This decoder measures the sync pulses and corrects for it automatically.
Which SSTV mode should I use?
Martin M1 and Scottie S1 are the most common on HF. PD120 gives a sharper 640 × 496 picture, and Robot 36 is quickest at 36 seconds.
Is my recording uploaded?
No. Decoding happens entirely in your browser.
What frequency is SSTV on 20m?
14.230 MHz USB is the main SSTV calling frequency, with activity a few kHz either side.