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Aptdec Logo

Copyright (c) 2004-2009 Thierry Leconte (F4DWV), Xerbo ( 2019-2022



Aptdec is a FOSS program that decodes images transmitted by NOAA weather satellites. These satellites transmit constantly (among other things) medium resolution (4km/px) images of the earth over a analog mode called APT. These transmissions can easily be received with a cheap SDR and simple antenna. Then the transmission can be decoded in narrow FM mode.

Aptdec can turn the audio recordings into PNG images and generate images such as:

  • Raw image: both channels with full telemetry included
  • Individual channel: one of the channels form the image
  • Temperature image: a temperature compensated image derived from the IR channel
  • Palleted image: a image where the color is derived from a palette (false color, etc)

The input audio format can be anything supported by libsndfile (although only tested with WAV and FLAC). Sample rate doesn’t matter, although lower samples rates will process faster.

Quick start

sudo apt install cmake git gcc libsndfile-dev libpng-dev
git clone --recursive && cd aptdec
cmake -B build
cmake --build build
# Resulting binary is build/aptdec


To create an image from gqrx_20200527_115730_137914960.wav (output filename will be gqrx_20200527_115730_137914960-r.png)

./aptdec gqrx_20200527_115730_137914960.wav

To manually set the output filename

./aptdec -o image.png gqrx_20200527_115730_137914960.wav

Decode all WAV files in the current directory and put them in images

mkdir images && ./aptdec -d images *.wav

Apply a denoise filter (see Post-Processing Effects for a full list of post-processing effects)

./aptdec -e d gqrx_20200527_115730_137914960.wav

Create a temperature compensated image for NOAA 18

./aptdec -i t -s 18 gqrx_20200527_115730_137914960.wav

Apply a falsecolor palette

./aptdec -i p -p palettes/WXtoImg-N18-HVC.png gqrx_20200527_115730_137914960.wav



-i [r|a|b|t|m|p] Output type (stackable)
-e [t|h|l|d|p|f] Effects (stackable)
-o <path>        Output filename
-d <path>        Destination directory
-s (15-19)       Satellite number
-p <path>        Path to palette
-r               Realtime decode
-g               Gamma adjustment (1.0 = off)

Image output types

  • r: Raw Image
  • a: Channel A
  • b: Channel B
  • t: Temperature
  • p: Palleted

Post-Processing Effects

  • t: Crop telemetry (only effects raw image)
  • h: Histogram equalise
  • l: Linear equalise
  • d: Denoise
  • p: Precipitation overlay
  • f: Flip image (for northbound passes)
  • c: Crop noise from ends of image

Realtime decoding

Aptdec even supports decoding in realtime. The following decodes the audio coming from the audio device pulseaudio alsa_output.pci-0000_00_1b.0.analog-stereo

mkfifo /tmp/aptaudio
aptdec -r /tmp/aptaudio
sox -t pulseaudio alsa_output.pci-0000_00_1b.0.analog-stereo.monitor -c 1 -t wav /tmp/aptaudio

To stop the decode and calibrate the image simply kill the sox process.

Palette formatting

Palettes are just simple PNG images, 256x256px in size with 24bit RGB color. The X axis represents the value of Channel A and the Y axis the value of Channel B.

Building for Windows

You can cross build for Windows from Linux with the script, you will need the following:

sudo apt install wget cmake make mingw-w64 git unzip

To build natively on Windows using MSVC, you will also need: git, ninja and cmake. Then run:


If you just wish to build libaptdec on Windows, libpng and libsndfile aren’t needed.

Building for Android

Requires at least API 23

Further Reading

User’s Guide for Building and Operating Environmental Satellite Receiving Stations

NOAA KLM coefficients

NOAA Satellite specifications and more information