Copyright (c) 2004-2009 Thierry Leconte (F4DWV), Xerbo (xerbo@protonmail.com) 2019-2020

Description

Aptdec is an FOSS program that decodes images transmitted by NOAA weather satellites. These satellites transmit continuously (among other things), medium resolution (1px/4km) images of the earth on 137 MHz.
These transmissions could be easily received with an simple antenna and cheap SDR. Then the transmission can easily be decoded in narrow band FM mode.

Aptdec can convert these audio files into png images.

For each audio file up to 6 images can be generated:

1. Raw image: contains the 2 transmitted channel images + telemetry and synchronization pulses.
2. Calibrated channel A image
3. Calibrated channel B image
4. Temperature compensated IR image
5. Palleted image
6. MCIR (Map Color InfraRed) image

The input audio file must be mono with a sample rate in the range of 4160-62400 Hz, lower samples rates will process faster. Aptdec uses libsndfile to read the input audio, so any format supported by libsndfile may be used (however it has only tested with .wav files).

Compilation

Aptdec is portable since it is written in standard C. It has successfully compiled and ran on Debian with both gcc, clang and tcc and will most likely work on any Unix platform. Just edit the Makefile and run make (no configure script as of right now).

Aptdec uses libsndfile, libpng and libm. The snd.h and png.h headers must be present on your system. If they are not on standard path, edit the include path in the Makefile.

Usage

To compile
make

To run without installing
./aptdec [options] audio files...

To install
sudo make install

To run once installed
aptdec [options] audio files...

To uninstall
sudo make uninstall

Options

-i [r|a|b|t|m|p]
Output image type
Raw (r), Channel A (a), Channel B (b), Palleted (p), Temperature (t) or MCIR (m)
Default: "ab"

-d <dir>
Images destination directory (optional)
Default: Current directory

-s [15|16|17|18|19]
Satellite number
For temperature calibration
Default: "19"

-e [r|a|b|c|t|m]
Effects
Histogram equalise (h), Crop Telemetry (t), Denoise (d), Precipitation (p) or Linear equalise (l)
Defaults: off

-m <file>
Map file generated by wxmap

-o <filename>
Output image filename

-p <filename>
Path to palette

-r
Realtime decode. When decoding in realtime it is highly recommended to choose a plain raw image.

Output

Generated images are outputted in PNG and are 24 bit RGB for all image types apart from pure greyscale images.

Image names are audiofile-x.png, where x is:

• r for raw images
• a channel A images
• b channel B images
• p for a paletted image
• t for temperature calibrated images
• m for MCIR images

Currently there are 6 available effects:

• t for crop telemetry, off by default, only has effects on raw images
• h for histogram equalise
• d for a median denoise filter
• p for a precipitation overlay
• f to flip the image (for southbound passes)
• l to linearly equalise the image, stretch the colors in the image to black and white

Examples

aptdec -d images -i ab *.wav

This will process all .wav files in the current directory, generate calibrated channel A and B images and put them in the images directory.

aptdec -e dh -i b audio.wav

Decode audio.wav with denoise and histogram equalization and save it into the current directory.

aptdec -e d -p palettes/N19-June-High-Vegetation.png -i p audio.wav

Create a false color image from the N19-June-High-Vegetation.png palette.

Realtime decoding

As of recently a realtime output was added allowing realtime decoding of images.

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

Perform a realtime decode with the audio being played out of alsa_output.pci-0000_00_1b.0.analog. To stop the decode kill the sox process.

Palette formatting

Palettes are PNG images that are 256x256 in size with 24bit RGB color. The X axis is the value of Channel A and the Y axis is the value of Channel B

Further reading

https://noaasis.noaa.gov/NOAASIS/pubs/Users_Guide-Building_Receive_Stations_March_2009.pdf
https://web.archive.org/web/20141220021557/https://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/tables.htm

License

See LICENSE.