Copyright (c) 2004-2009 Thierry Leconte (F4DWV), Xerbo (xerbo@protonmail.com) 2019-2022

Introduction

Aptdec is a FOSS library/program that decodes images transmitted by the NOAA POES 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, the transmission can be demodulated in narrow FM mode.

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

• Raw image: both channels (including telemetry)
• Individual channel: one channel (including telemetry)
• Visible image: a calibrated visible image of either channel 1 or 2
• Thermal image: a calibrated thermal image from channel B
• LUT image: a image where the color is derived from a LUT (used for false color, etc)

The input audio format can be anything supported by libsndfile (although only tested with WAV, FLAC and Ogg Vorbis). While sample rate doesn’t matter, it is recommended to use 16640 Hz (4x oversampling).

Quick start

Grab a release from Releases or compile from source:

sudo apt install cmake git gcc libsndfile-dev libpng-dev
git clone --recursive https://github.com/Xerbo/aptdec.git && cd aptdec
cmake -B build
cmake --build build
# Resulting binary is build/aptdec-cli

In place builds are not supported.

Examples

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

aptdec-cli gqrx_20200527_115730_137914960.wav

To manually specify the output filename

aptdec-cli -o image.png gqrx_20200527_115730_137914960.wav

Decode all WAV files in the current directory:

aptdec-cli *.wav

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

aptdec-cli -e denoise gqrx_20200527_115730_137914960.wav

Create a calibrated IR image from NOAA 18

aptdec-cli -i thermal gqrx_20200527_115730_137914960.wav

Apply a falsecolor LUT

aptdec-cli -i lut -l luts/WXtoImg-N18-HVC.png gqrx_20200527_115730_137914960.wav

Usage

Arguments

-h, --help                show a help message and exit
-i, --image=<str>         set output image type (see below)
-e, --effect=<str>        add an effect (see below)
-g, --gamma=<flt>         gamma adjustment (1.0 = off)
-s, --satellite=<int>     satellite ID, must be between 15, 18 or 19 or NORAD
-l, --lut=<str>           path to a LUT
-o, --output=<str>        path of output image
-r, --realtime            decode in realtime

Image output types

• raw: Raw Image
• a: Channel A (including telemetry)
• b: Channel B (including telemetry)
• thermal: Calibrated thermal (MWIR/LWIR) image
• visible: calibrated visible/NIR image
• lut: LUT image, see also -l/--lut

Post-Processing Effects

• strip: Strip telemetry (only effects raw/a/b images)
• equalize: Histogram equalise
• stretch: Linear equalise
• denoise: Denoise
• precipitation: Precipitation overlay
• flip: Flip image (for northbound passes)
• crop: Crop noise from ends of image

Realtime decoding

Aptdec supports decoding in realtime. The following captures and decodes audio from the pipewire interface:

arecord -f cd -D pipewire | aptdec -r -

or directly from an SDR:

rtl_fm -f 137.1M -g 40 -s 40k | sox -t raw -r 40k -e signed-integer -b 16 - -t wav - | aptdec -r -

Image data will be streamed to CURRENT_TIME.png (deleted when finished). To stop the decode and normalize the image simply Ctrl+C the process.

LUT format

LUT’s are just plain 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 (recommended) using with the following commands:

sudo apt install wget cmake make mingw-w64 git unzip
./build_windows.sh

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

.\build_windows.bat

If you only want to build libaptdec, libpng and libsndfile aren’t needed.

References

• User’s Guide for Building and Operating Environmental Satellite Receiving Stations
• NOAA KLM Users Guide

License

See LICENSE