NOAA APT reception with Gqrx and rtlsdr

About a year ago I posted a step-by-step guide about how to receive NOAA APT weather satellite images using Gqrx SDR and the Funcube Dongle. Today, I tried to repeat the procedure using my ezcap eztv666 rtlsdr dongle with very good results.

Lets start with a video showing Gqrx receiving the APT transmission from the NOAA-15 polar orbiting weather satellite during orbit 73676 over western Europe:

Watch video on YouTube.

As you can see both the baseband and the audio SNR is rather good. I suspect the recent gain improvements in the driver have something to do with it. During this experiment I have used my Arrow II satellite antenna; however, for a permanent installation a Quadrifilar Helix or a Turnstile antenna would be better.

As written in the Funcube Dongle article, the procedure is to:

  1. Use the proper FM deviation (17 kHz) and filter width (45 kHz).
  2. Use proper gain setting for max SNR. With the latest rtl-sdr I had to use max gain setting.
  3. Use a sample rate that works well with your device on your computer (I used 1024 ksps).
  4. Record to audio to a WAV file.
  5. Resample the audio to 11.025 kHz.
  6. Decode the image from the audio using atpdec or wxtoimg.

The easiest and fastest way to resample the audio is using sox:

  sox input.wav output.wav rate 11025

Atpdec (probably misspelled version of aptdec) is an open source APT decoder which is no longer maintained. It can decode the image but I have problems getting false color and temperature to work. Here is what I got from the recording of the NOAA-15 pass:

Weather satellite image received with RTL-SDR

The image was captured during a late afternoon pass, which is a good time for APT reception, because we can often see shadows beneath the clouds, giving the image a sense of 3D.

Wxtoimg is a commercial APT decoder, however, it comes with a limited free version. If you can make it work on your Linux box, it will provide you with many options for processing and enhancing the received APT images. This is the image decoded by wxtoimg with the “contrast enhancement” option:

Weather satellite image received with RTL-SDR

APT transmissions from the NOAA weather satellites contain two channels that are basically two images from two different instruments. Often, one channel will contain a visible light image and the other will contain and infrared image. On the image above we see the infrared image on the left and visible light on the right. Wxtoimg can combine the different images to create various false color images. Below I have a “HVC false color” image where I have also applied histogram equalization:

Weather satellite image received with RTL-SDR

You can now clearly see the landmass and the clouds, as well as the reflection of the Sun on the surface of the ocean.

I hope this demonstrates that RTL SDR devices do have practical applications. At $20 the rtlsdr is pretty much the cheapest NOAA APT / satellite communication receiver I have ever had.

Author: Alexandru Csete

Embedded software engineer in the satcom industry during the day. Radio amateur and SDR hacker during the night.