APT stands for automatic picture transmission–a form of radio communication to send black and white images over an analog audio channel modulated in FM. Many weather satellites such as NOAA-15, NOAA-18, and NOAA-19 broadcast live images of Earth using the APT format. You can find out more about this type of transmission on the signal wiki.
APT Signal Information: https://www.sigidwiki.com/wiki/Automatic_Picture_Transmission_(APT)
Decoding an APT Image
The image is stored as transmittable audio saved in a wav file. Download and unzip the wav file.
Open apt.wav in an audio program such as Audacity. Listen to the signal. It sounds like an old telephone modem. That is because the image has indeed been modulated into an audible waveform. Lines of the encoded images are sent 2 per second. You should hear something that sounds like a metronome at 120 beats per minute which is the horizontal syncing of the transmitted image.
Now, download the software, noaa-apt, which we will use to decode the image.
Noaa-apt Download: https://noaa-apt.mbernardi.com.ar/download.html
Run noaa-apt, you should be greeted with this screen.
Click the “Input file:” box to open a “Open File” dialog box.
Select and open the apt.wav file you downloaded and extracted earlier.
Push the Decode button the main screen.
The program should now take a few seconds to decode the wav file. Once finished, flick the “Processing” tab.
From the “Processing” tab, make sure that the “Rotate image:” selection is “No.” If this image were captured by a weather satellite traveling South to North, then the image could be flipped to make North up or vice versa. The image encoded in apt.wav was created on a personal computer, so no rotation is necessary.
Now click the “Process” button.
Flag 1 is the text in the left-hand decoded image.