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

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. False color image
6. Layered image, boosts cloud visibility

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 and clang 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|c|t|l]
Output image type
Raw (r), Channel A (a), Channel B (b), False Color (c), Temperature (t), Layered (l)
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 [c|t|h]
Enhancements
Contrast calibration (c), Histogram equalise (h), Crop Telemetry (t)
Defaults: "ct"

-c <file>
Use configuration file for false color generation.
Default: Internal defaults

Output

Generated images are outputted in PNG, 8 bit greyscale for raw and channel A|B images, palleted images for temperature and GVI false color, 24 bit RGB for false color.

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

• r for raw images
• Sensor ID (1, 2, 3A, 3B, 4, 5) for channel A|B images
• c for false color.
• t for temperature calibrated images
• l for layered images

Currently there are 3 available enhancements:

• c for contrast equalise, on by default
• t for crop telemetry, on by default, only has effects on raw images
• h for histogram equalise, make the darkest color black and the brightest white

Example

aptdec -d images -i ab *.wav

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

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.