Field Guide · protocol

Also known as: APT, Automatic Picture Transmission

NOAA APT (Automatic Picture Transmission) is the long-running analog weather-image mode broadcast by NOAA’s polar-orbiting (POES) satellites near 137 MHz. It sends a continuous line-by-line raster: each video line amplitude-modulates a 2400 Hz subcarrier, and that subcarrier in turn frequency-modulates the VHF carrier — so the picture rides as AM inside an FM downlink.1 The result is a slow-scan grey-scale strip of the Earth beneath the satellite’s path.

one APT line (2 lines/second) → sync Aimage A (visible)sync Bimage B (IR) brightness = amplitude of the 2400 Hz subcarrier
Each APT line carries a sync burst and image data on a 2400 Hz AM subcarrier; two channels (visible and infrared) are interleaved per line.

Overview

APT descends from the earliest TIROS weather satellites of the 1960s and has flown on every NOAA polar orbiter since, chosen precisely because it can be received with the simplest possible ground station. Two 137 MHz APT-capable satellites remained active into the 2020s — NOAA-15, NOAA-18, and NOAA-19 (NOAA-18 was decommissioned in 2025) — each passing overhead a few times a day. The signal is unencrypted and low rate by design, so that anyone from a school to a fishing boat could pull down a fresh cloud image.2

Technical characteristics

Property Value
Band ~137 MHz VHF, ~34 kHz wide
Carrier modulation FM
Image modulation AM on a 2400 Hz subcarrier
Line rate 2 lines/second (120 lines/minute)
Word rate 4160 words/second
Content Two channels/line: visible + infrared, plus sync and telemetry

An APT frame interleaves two AVHRR imager channels (typically one visible and one infrared) with sync bars, minute markers, and a telemetry wedge used to calibrate the grey scale. Because the picture is amplitude-carried, receiver AGC and a clean FM demod matter for image quality.

History

The mode has been remarkably stable across six decades, outliving the analog technology of its origin because of its accessibility. Its digital successor, Meteor-M LRPT, and the geostationary GOES HRIT service deliver sharper, calibrated imagery, but APT’s simplicity kept it valuable to hobbyists and educators to the end of the legacy POES fleet.1

Deployment

APT is a receive-only public service — there is no user uplink. A ground station needs a right-hand-circularly-polarised antenna (typically a quadrifilar helix (QFH) or a turnstile) to match the satellite’s polarisation, a 137 MHz SDR, and free software to turn the audio into an image and apply map overlays.

Decoding it with GopherTrunk

GopherTrunk does not decode APT: it is a land-mobile trunking scanner, and APT is an analog image raster, not a voice or trunking protocol. That said, APT is one of the friendliest possible first satellite-reception projects — a $30 SDR dongle, a home-made QFH, and a pass prediction are enough to receive it — which is why it earns an entry in this guide even though it sits outside GopherTrunk’s decode chain. It is closely related to the amateur SSTV slow-scan modes in spirit, and to its digital replacement Meteor-M LRPT.

Sources

  1. Automatic picture transmission — Wikipedia, for the APT format, the 2400 Hz AM subcarrier, and the per-line channel structure. ↩ ↩2

  2. Polar Operational Environmental Satellites — Wikipedia, for the NOAA POES fleet that carries APT. ↩

See also