Understanding SSTV
How an image becomes sound
SSTV scans a picture one horizontal line at a time — just like analogue television, but slowly enough to fit inside an audio channel. The transmitter sweeps a tone whose pitch encodes each pixel: darker areas produce lower frequencies and brighter areas higher ones, spanning 1500 Hz for black up to 2300 Hz for white. A 1200 Hz sync pulse separates the lines so the receiver stays aligned, and colour modes send the separate colour components one after another. Before the picture starts, a short digital header called a VIS code announces which mode is in use, letting decoding software lock on automatically. Because the entire signal sits between roughly 1200 and 2300 Hz, it passes through any ordinary single-sideband voice radio unchanged.
Common SSTV modes
Each SSTV mode is a trade-off between speed and detail. Faster modes finish in seconds but look coarse; slower, higher-resolution modes take minutes but produce sharp colour images. The ISS favours PD120 as a good balance for its brief overhead passes.
| Mode | Approx. time | Resolution | Typical use |
|---|---|---|---|
| Robot 36 | ~36 s | 320×240 | Fast colour, low detail |
| Martin M1 | ~114 s | 320×256 | Popular across Europe |
| Scottie S1 | ~110 s | 320×256 | Popular in North America |
| PD120 | ~126 s | 640×496 | Standard ISS downlink mode |
| PD180 | ~187 s | 640×496 | Higher quality, terrestrial HF |
Receiving SSTV from the International Space Station
The ISS is the easiest source of pictures from space, and reception needs no licence — you are only listening. During ARISS (Amateur Radio on the International Space Station) events, which often mark anniversaries or student projects, the station transmits a series of images in PD120 on 145.800 MHz FM, typically on a two-minutes-on, two-minutes-off cycle over several days. Point a VHF antenna skyward, tune a handheld radio or scanner to 145.800 MHz, and feed the audio into a free decoder such as Robot36 (Android), SSTV Slow Scan TV (iOS), MMSSTV (Windows) or QSSTV (Linux). Because the station sits in low Earth orbit, each pass lasts only about ten minutes, so check pass predictions for your location first. SSTV is just one of several amateur modes used by the ISS, sharing its 145.800 MHz downlink with crew voice contacts, while APRS packet data runs on a separate channel. Successful decoders can claim an official certificate by uploading their image and Maidenhead grid locator.
From Apollo to the ISS: a short history
The slow-scan idea predates amateur satellites entirely. American engineer Copthorne Macdonald built the first practical SSTV system in 1958, using a slow eight-second monochrome frame that fitted inside an ordinary radio channel. NASA adopted the same slow-scan principle for early crewed spaceflight: the television cameras on Apollo 7, 8, 9 and 11 sent slow-scan pictures that ground stations converted for broadcast. That conversion — filming a slow-scan monitor with a standard TV camera — is a large part of why the first Moon-walk footage from Apollo 11 in 1969 looked so grainy. Today's digital SSTV keeps the same line-by-line spirit but adds colour, sharper resolution and automatic mode detection, and it remains the simplest way for anyone to receive an image directly from orbit.