NOAA Satellite
APT Signal Decoder
Catches weather satellite passes and turns them into pictures of Earth.
A purpose-built receiver and quadrifilar helix antenna. It listens for APT downlinks from passing NOAA weather satellites, starts capturing the moment a pass begins, and decodes the audio into visible and infrared imagery.
01 Overview
NOAA's polar-orbiting weather satellites broadcast an analogue APT signal continuously as they pass overhead. Anyone with the right antenna can receive it. The hard parts are antenna polarisation, catching the pass at the right moment, and sampling cleanly enough that the decoder has something to work with.
The antenna is a quadrifilar helix, built for circular polarisation and a wide hemispherical pattern so a satellite stays in the beam horizon to horizon without steering. Behind it, a Raspberry Pi and an SDR sit in a 3D-printed enclosure that handles capture unattended.
On signal detect the receiver starts recording and processes at the correct sample rate. The audio goes to WXtoImg, which decodes it into imagery, overlays coastlines and boundaries, and combines sensor channels into visible and infrared composites.
- Role
- Design, fabrication, and software
- Antenna
- Custom quadrifilar helix, circular polarisation
- Receiver
- Software defined radio + Raspberry Pi
- Enclosure
- 3D printed, weather-tolerant
- Capture
- Automatic trigger on signal detect, correct-rate resampling
- Decoding
- WXtoImg, landmass overlay, visible / IR composites
- Stack
- C · Python · Embedded · Linux · Networking · Soldering