Capturing NOAA Automatic Picture Transmissions

I was able to successfully download and decode the NOAA weather imagery from the NOAA-18 satellite. The Automatic Picture Transmissions (APT) has a downlink frequency of 137.9125 MHz.

Yagi antenna

What do you need?

VHF Yagi antenna – I used a hand held VHF/UHF yagi from Arrow Antennas that can cover most satellite frequencies and can also be used for talking over amateur radio satellites. I’ve read about people also using rabbit ear antennas as long as they are measured to the right length and angle. You may also be able to just use a discone antenna in the attic or on the roof.

SDR– The cheap and portable RTL-SDR that you can pick up for $20 bucks on amazon. It has an SMA female adapter that can use the male SMA coming off of the yagi.

Heavens Above

Satellite tracking software – There is the popular Gpredict that you and mobile Heaven’s Above app that will let you know when the satellite will be over head. You probably want it to be high off the horizon when it passes to avoid interference form buildings and trees.

Recording software – I recorded the signal with GQRX on Linux, but you can also use SDR#. The guide on the NOAA-APT software site has a good description of the specific configuration required. Most importantly, the signal is narrow so you may want to zoom in on the FFTs to get a better look. Also try to reduce the receive bandwidth to the size of the signal. GQRX and SDR# let you record a wav file of the transmission that you will use for decoding.

GQRX with configuration

Decoding software – I used NOAA-APT to decode the wav recording. It doesn’t have as many features as wxtoimg (like color images), but it is open source and user friendly. NOAA-APT will take GQRX recording and convert it into an image. You may see some artifacts like noise or missing sections depending on the quality of the recording.

NOAA-18 visible (left) and thermal (right) imagery

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