The National Oceanic and Atmospheric Administration (NOAA) has been imaging Earth with satellites since 1975. These satellites are used to monitor weather patterns, climate change, and natural disasters. The data is transmitted to Earth in real time from these satellites, and can be received by anyone in the signal path.

In this post I will be collecting and displaying telemetry from the NOAA 18 satellite. The NOAA 18 satellite is in a polar orbit, meaning it orbits over the north and south pole. It takes images of the earth, and imediately transmits the image at a rate of 360 lines per minute. The data from this satellite is transmitted at 137.9125 MHz with Analog picture transmission APT.

Equipment⌗

For my collection I will be using a RTL-SDR, and the bunny ear antenna that came with it. I am using GQRX to receive the singal. Any SDR software that can demodulate and record FM will do. People have fancier antennas, but the bunny ears tuned to 137.9125 MHz work well enough for me.

Here is a link to this guy ADAM 9A4QV who made a V dipole antenna for NOAA sats. That inspired me to just use the bunny ears that came with my RTL-SDR. A leg length of 53.5 cm, and an angle of 120 degrees between the legs is required. The dipole should be pointed in the north-south direction, and parallel to the ground.

Find a pass⌗

I am using gpredict on my laptop and Heavens Above on my phone to find the next time the satellite is passing over me. I do not have great elevation, so usually only bother with 70 degrees and greater passes.

Receiving the signal⌗

When I had a pass coming up, I ran outside to the park with my laptop and RTL-SDR. I tuned GQRX to 137.9125 MHz, and waited for the power on the GQRX waterfall to start to rise. When the signal was coming in, I started to record. If you turn the sound on, the signal sounds like a constant beeping, a couple of times a second.

Decoding the signal⌗

I used a program noaa-apt to decode the signal. The program is available here. I selected the .wav file I recorded and clicked decode. Then in the processing step I selected histogram equalization.

I think the images came out well!

You can see Michigan the great lakes, and most of north east USA. There is a big cloud over New York, and a large string of clouds over the Atlantic.

NOAA-apt can overlay a map.

This was taken at night, so the system has changed over to use infrared imaging. During the day the images are visible light

Conclusion⌗

I hope you are inspired to go out and collect some satellite data! All you need is a computer, an SDR and an Antenna. I hope in the future to collect more data both from the NOAA sats, and maybe some of the Russian METEOR or the geostationary NOAA GOES sats which, require some special hardware. Drop me a line if you have any interesting satellite data you have collected!

-E

Update 2024-11-17⌗

I was able to capture a much better pass of the NOAA 15 satellite. This one is also at night, hope to get a day pass soon!