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Jason DavisSeptember 21, 2015

How to Download Weather Satellite Images from Space

During the LightSail test mission, dozens of radio trackers around the world tuned in to our spacecraft's signal. Many of you sent us beacon packets, some of which were very helpful in telling us what our spacecraft was doing when it was out of range of our U.S.-based tracking stations. 

For our second flight—still tentatively scheduled to launch on a SpaceX Falcon Heavy in Sept. 2016—we'd like to better support and leverage the radio community. Our ultimate goal is building a software tool that will help LightSail trackers capture and submit data to our packet repository in real-time. Other missions, like the University of Michigan's GRIFEX CubeSat, have done this. 

We also want to make tracking LightSail an inclusive activity that anyone, including folks without any prior radio experience, can enjoy. (Providing you live within range of LightSail's 24-degree inclination orbit—we'll calculate a specific latitude range for that.) 

Since I'm also a radio newbie, we figured a good initial task would be seeing how hard it was for me to get my own Software Defined Radio (SDR) system up and running, at a relatively low cost, and perform a few basic exercises. In other words, if I can figure it out, I can write how-to guides for everyone else. 

So, how am I doing? 

Reasonably well, thanks to tech support from Justin Foley and John Bellardo, two radio experts at Cal Poly San Luis Obispo, LightSail's primary ground station. Early on, I found a guide by Matt Gray explaining how to download images from NOAA weather satellites. I figured this would be a good starting exercise, and now that I've had a few successes, I'm ready to post my first SDR activity guide. Here are a few caveats:

Let's get started!

The goal

Here is my best weather satellite image to date:

United States from NOAA 19

Jason Davis / WXtoImg

United States from NOAA 19

The software package we'll be using can generate approximate true-color versions of the above image and draw map boundaries, like this:

United States from NOAA 19 (colorized with map overlay)

Jason Davis / WXtoImg

United States from NOAA 19 (colorized with map overlay)

Shopping list

Hardware:

The assembly here is straightforward. Connect the Tram antenna to your USB stick using the cable adapter, and plug everything in to your laptop. 

Location:

You'll get better results if you do this from an area relatively clear of obstructions like buildings and trees, with good horizon-to-horizon vistas. For me, this is a nearby park. If you can drive to your location, try sticking the magnetic Tram antenna on the top of your vehicle. Your vehicle's metallic body will become a large ground plane and improve the antenna's reception.

Software:

SDR program

Windows: SDR Console
Mac/Windows: Cubic SDR

I like the simplicity of Cubic SDR, but found it used a lot of processing power and overheated my laptop a couple times. SDR Console requires more initial configuration, but it's feature-rich and uses less overhead, so I've switched to it entirely.

Weather satellite image processing

Mac/Windows: WXtoImg

Audio piping program

Windows: VB-Audio Virtual Cable
Mac: Soundflower

You'll use this software to pipe the satellite's audio signal from your SDR program into WXtoImg. 

Configuration

First, install your audio piping program. Both VB-Audio and Soundflower are easy to install. Here's what the end result should look like on either Mac or Windows. On Mac, it's a program that runs in the background; on Windows, you essentially get a new audio device under the recording tab. 

Audio piping software, Mac and Windows

Jason Davis

Audio piping software, Mac and Windows

We'll be telling the SDR software to output to either Soundflower or VB-Audio, while WXtoImg will use Soundflower or VB-Audio as its input.

Next, install WXtoImg. There are a few options to change:

Options > Recording Options: Change your soundcard to either Soundflower or VB-Audio.
File > Update Keplers: Do this every few days to download the latest weather satellite position data.
File > Satellite Pass List: This shows you the upcoming satellite passes for your location. Pay particular attention to MEL (Maximum Elevation). The higher the MEL, the stronger the signal (90 degrees means the satellite will pass directly overhead).

Finally, install your SDR program.

CubicSDR

I recommend you try CubicSDR first because it's the easiest to configure. And if it doesn't overheat your laptop, great! 

The lone important setting (besides the satellite frequency, which you'll get from WXtoImg) to change in CubicSDR is Audio Output. Change this to either Soundflower (2ch) or VB-Audio.

Beyond that, the program is easy to use. You set the frequency and center frequencies by clicking the top and bottom of each number to cycle up and down (it took me an embarrassingly long time to figure that out). Once the signal appears, click on it and fine-tune your focus using the closeup window in the upper-left. 

A good bandwidth to start with is 36000 (don't worry, I'll provide a final checklist for everything in a bit).

CubicSDR

Jason Davis

CubicSDR

SDR Console

If CubicSDR isn't working for you, give the harder-to-configure (but more feature-rich) SDR Console a try. There are many other programs out there, too.

Installing CubicSDR is straightforward, but you'll get error messages as soon as you try to select your USB port's SDR stick and antenna rig. Here's the fix:

Download Zadig, which allows you to install the latest drivers for your USB stick. 

In the Zadig console, click Options > List all devices. Select "Bulk-In, Interface (Interface 0)" from the drop-down and click Replace Driver.

Download RTL SDR, which contains a TCP server that allows the USB stick to pipe its signal into SDR Console. (More documentation available here.)  

Unzip RTL SDR on your desktop, and open either the x32 or x64 folder, depending on whether you have a 32-bit or 64-bit Windows installation. (Not sure?

Run rtl_tcp.exe (you may want to create a desktop shortcut, since you have to run this every time you open SDR Console). A small command prompt window will come up; you can minimize this.

Now, open CubicSDR. You'll be prompted to add a new radio. Search for your RTL SDR stick—it should now be showing after all of the work we did above—and click OK:

SDR Console radio selection

Jason Davis

SDR Console radio selection

Use the newly defined radio and get click Start. When I first started the program, the audio came through my speakers—loudly—so you may want to turn them down ahead of time. Then, immediately send that audio to the correct place:

Audio > Playback device > Soundflower or VB-Audio

SDR Console

Jason Davis

SDR Console

Now you're ready to download your first image!

Image download

Here's the general order of operations for downloading a satellite image. Make sure you have all of these steps completed at least a couple minutes before the pass begins:

WXtoImg will now wait patiently for the pass to start:
WXtoImg waiting on pass

Jason Davis

WXtoImg waiting on pass

WXtoImg will start recording right on time, but don't expect to see a strong signal on your SDR program yet. The signal will gradually fade in as the satellite moves farther into range. Click the waterfall plot to center on it, and in the zoom window, fine-tune your focus. For an example of what the signal looks like, see the above CubicSDR and SDR Console screenshots—both were taken during live passes. Notice how the audio stream you want is actually about a dozen data streams close together in the spectrum.

As the signal strength grows, a raw image will start to form from noise in WXtoImg. The image on the left is visible light; the one on the right is infrared. 

WXtoImg partial download

Jason Davis

WXtoImg partial download

Keep an eye on your SDR program throughout the pass to make sure it stays centered on the signal. Because of the Doppler shift, the signal will shift throughout the pass.

And that's it! By default, WXtoImg will process your image in a variety of different modes as soon as the pass is over. You can also click the Raw Images tab to see and reprocess all of your past attempts.

For a color image, try: Enhancements > MCIR map color IR

And to fiddle with your map overlay, go to: Options > Map Overlay Options 

Personally, I usually prefer the original black and white visible light image, with no map overlay. It gives me the feeling that I'm a passenger aboard the weather satellite, looking down at myself, the space nerd in the parking lot with a radio antenna on his car roof. 

Read more: LightSail

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Jason Davis

Journalist and Digital Editor for The Planetary Society
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