Tuesday, February 26, 2008

Listening to satellites

Satellites are cool...

I have always wanted to pull real-time images off of weather satellites. I read articles of people doing this years ago, but never had the necessary equipment necessary to do it. Receiving these images is actually pretty easy... I had some horrible looking images within a few hours of setting up my system. So now that I have the necessary equipment and with it being February along with the fact that I'm getting really sick of winter, it's a good time to finally do this.

First received image: (very noisy since my antenna sucks.)

The United States currently has three transmitting operational polar orbiting weather satellites, NOAA-15, NOAA-17, and NOAA-18. These satellites travel in a sun-synchronous orbit around the earth at an altitude of about 850km. Because they are moving so fast making revolutions of the earth approximately every 90 minutes, each one passes over regions of the US multiple times each day.

Now what's interesting is these satellites are transmitting their imagery of the earth back to us on two separate frequencies. The first is a high resolution digital signal that transmits around 1.7Ghz, but the second is a lower resolution modulated signal that is around 137Mhz! This system is called APT (automatic picture transmission) and is not encrypted. The satellites are constantly transmitting this APT signal at about two lines of imaging per second and if you are able to receive this signal, you can view real-time weather satellite imaging.

Because of the speed of the satellites, you only have about a 12 minute window to capture the satellite imagery as it flies by overhead. This is plenty of time to get some good images.

The equipment:

The only equipment you need to receive these signals is a good scanner/receiver with wide bandwidth, a good antenna for 137Mhz, and a computer/software that can decode APT data.

Most consumer scanners / receivers on the market have two problems. First, they have the 137Mhz spectrum blocked so you can't receive it, and second they don't have a wide enough bandwidth at the receiving frequency to pull in all of the information. The receiver I'm using is a Kenwood RZ-1, which is sufficient for this task.

I have been using two pieces of software, Wxtrac and WxtoImg. Both work and are freeware. WxtoImg is definitely more powerful... but to unlock all of it's features you have to pay for licensing. JTrack is a powerful satellite tracking tool that is available to use free from NASA that I use to track passing satellites.

My results:

The above image is very low quality ( again from an inadequate antenna) but it does show the entire signal received. The left portion of the images in infrared data, while the right is visible data. The bending of the image is Doppler shift caused by the speed of the satellite moving across the sky. Once I receive a stronger signal, the software will compensate and correct this. The map of north america is applied by the software, it uses the known keplers of each satellite to keep track of the satellites exact location.

Here is another pass. I really need an antenna...

Improvements to make:

The antenna! I need a good antenna with a center frequency at 137.000Mhz. There are a lot of good designs out there, the quadrafilar looks to be the most popular, but also somewhat difficult to make. I will have to do more search into the best antenna...

The winter months are also the worst time to be receiving this satellite imagery. The low contrast from the clouds and snow covered ground both being a nice bright white color combined with the low amount of sunlight we receive this time of year makes decoding image data from signals that already have a very high signal to noise ratio extremely difficult.

I'm really hoping to get some good images soon.


  1. This sounds fantastic. I wish you luck with it. What kind of antenna are you currently using? Seems like a 137 MHz signal should be pretty easy to capture with a home brew J-pole antenna like we used for the balloon chasing. Any idea what your link budget is?