What Is SSTV? Slow-Scan Television from Space

📘 Definition

Slow-Scan Television (SSTV) is an amateur radio image transmission method that encodes still images as audio-frequency tones, typically between 1200 Hz and 2300 Hz. Unlike broadcast television which uses wide bandwidth, SSTV transmits within a standard voice channel (3 kHz bandwidth). Each horizontal line of the image is encoded as a swept tone — the frequency determines the brightness/colour of each pixel. A complete image takes 30–120 seconds depending on the mode (Martin, Scottie, Robot, PD modes). The ISS transmits SSTV images during special events (typically commemorating space milestones), and these transmissions on 145.800 MHz can be received and decoded with a handheld radio and a free smartphone app.

145.800 MHz

ISS Frequency

1200–2300 Hz

Audio Range

30–120 seconds

Image Time

PD120 (120 sec)

Common Mode

Understanding SSTV

How SSTV Encodes Images

An SSTV transmission begins with a calibration header that identifies the mode (PD120, Martin M1, Scottie S1, etc.). Then each line of the image is transmitted as an audio sweep — the instantaneous frequency maps to pixel brightness, with separate sweeps for red, green, and blue colour channels. A sync pulse at the start of each line keeps the receiver aligned. The result sounds like a series of sweeping musical tones — quite distinctive and immediately recognisable.

Receiving ISS SSTV

During SSTV events, tune a VHF radio to 145.800 MHz FM and hold it near your phone running an SSTV decoder app (Robot36 for Android, SSTV Slow Scan TV for iOS). The app decodes the audio in real time and builds the image line by line. A directional antenna improves results, but high-elevation ISS passes can be received with just a handheld and whip. Check the pass predictor for ISS timing.

SSTV in the Audio Archive

The satellite audio archive contains recordings of SSTV transmissions from the ISS and other satellites. Listen for the characteristic sweeping tones — each audible sweep represents one line of the image being transmitted.

Frequently Asked Questions

When does the ISS transmit SSTV? ▼

The ISS transmits SSTV images during special events — typically commemorating cosmonautics milestones, space anniversaries, and educational outreach events. These are announced on the ARISS (Amateur Radio on the ISS) website and social media. Events usually run for 2–3 days with repeated image transmissions.

What does SSTV sound like? ▼

SSTV has a very distinctive sound — a series of sweeping, warbling tones that rise and fall in pitch. Each sweep encodes one line of the image. The overall effect sounds almost musical. Listen to examples in the audio archive.

Can I decode SSTV without amateur equipment? ▼

Yes — for receive-only, you can use an RTL-SDR dongle (~£25) with a simple VHF antenna and free software (MMSSTV or QSSTV on desktop). No amateur radio licence is required for listening.