Obtaining NOAA Satellite Photos On The Cheap

You've seen the satellite weather photos on the evening weather reports. They're common things you don't think much about. Here's an example:

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The background image comes from an orbiting NOAA weather satellite using Automatic Picture Transmission or APT. Anybody, and I mean anybody can download the same images from NOAA for free, well, kinda free. You need some hardware and software.

This can all be done on a laptop running Windows but of course I prefer to do it the hard way and use Linux. Actually it may be the easier way.

So basically you need three bits of hardware:

-Computer
-RTL-SDR USB dongle
-Antenna

Basically any computer will do. If it starts and was made in the last 10 years and has USB ports it probably has enough horsepower to do this.

Then you need a Software Defined Radio which you can get cheaply here:

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Don't expect too much from that antenna though. Actually don't expect anything from that antenna. You can probably pull in local FM radio stations if they are nearby and that's a great start.

Now you need software. I'll be discussing how to do this in Linux because that is what I am using. Also all the Linux software required to do this is free, including the operating system.

I use LinuxMint 17.2 which is based on Ubuntu Linux. The software required for radio listening is called GQRX and can be found here to install under Ubuntu open a terminal and type:

sudo apt-get install gqrx

It may yank down some dependencies but should install without a hitch. Then to launch the program type:

gqrx

Workspace 1_002b

Sorry about the Pithos (Linux Freeware for Pandora) box in front of the GQRX screen. Anyway GQRX looks like a fancy radio receiver. As you can see in this instance it is tuned to 124.493 MHz and the place where the action is at is where the red vertical bar is. Also you can see there is no signal there. Just a demo screenshot.

Here is a photo of GQRX pulling down NOAA 18 APT transmission on 137.9125 MHz You can see there is a clearly defined signal here.

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That's almost it. GQRX and a USB dongle. You can listen to FM radio, emergency broadcasts, police, fire, aircraft, pagers, etc. If it transmits you can see the signal. May not be able to do much with it but you can know it is there.

Now satellites move. Fast. And NOAA satellites keep sweeping the earth so how do you know WHEN to try to catch the signal? They aren't always over your head. They only come by every so often. And there are a bunch of NOAA satellites but most of them are dead. As of this writing (Sep 2015) only NOAA 15, 18, and 19 are operational and 15 has a weak signal.

So you can probably google "NOAA 18" and find any manner of websites which depict where it is. I prefer to use a hunk of software called GPREDICT. It is depicted in the photo just above behind GQRX. GPREDICT can be installed by typing:

sudo apt-get install gpredict

Once in GPREDICT you set your base station in your preferences and in my case New Bern, NC was actually already listed so I didn't have to manually input latitude and longitude data. Once that is done you need to update the TLE files which change the satellite tracking from where it is supposed to be to where it actually is. This software doesn't actually connect to and track to the satellites, it just illustrates where they are SUPPOSED to be based on the TLE or Kepler data. If you look in the upper right hand corner of the photo above you can see that it tells you that NOAA 15 is next in 29 minutes, 15 seconds.

From GPREDICT you then select what satellites you want to track and it will graphically depict where it is. For example in my picture above if you look close you can see SO-50, NOAA 17, and FO-29 and NOAA 17 is right over New Bern NC. Too bad it isn't working.

Now once you lock on that nice solid NOAA downlink transmission it'll make a some unusual repeatable sounds, similar to a modem or fax machine. Not the same, but you get the idea. You simply record the sound by pushing the record button in GQRX.

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You'll need several minutes to get a good photo. Probably 10 or 12 minutes worth of audio. Maintaining this signal can be tough and the better the antenna you have, well………the better. You're not going to hear a NOAA satellite with the tiny antenna that came with the SDR USB stick. Lets say you get lucky and do hear it. You'll never extract useful data out of it because you'll never get 12 minutes worth of signal.

I opted for an Arrow II Satellite Antenna from here. Specifically the one on that page. It is tripod mounted and you can track the signal easier. That's one way. Another way is with a RadioShack Discone Antenna which is not optimum but will work. Also the best antenna for this is the Quadrifilar Helix (QFH) antenna. You can make one but it isn't a 5 minute, $5 project.

Anyway the old HAM Radio adage applies here. For every $10 of radio you have you need $100 worth of antenna. Fiscally that isn't really accurate because with the wonderful internet there are about a million super cheap, super great antenna plans and specifications. Just roll your own antenna from a trip to Lowes or Home Depot.

Now back to the project. So you've downloaded a big long wave file of click, beep, click, beep, click beep which seems pretty useless. Just like a fax machine there is a picture trapped in all that sound. Now you need to convert that wav file into 11.025 kHz. With linux you can use a terminal prompt to resample.

  sox input.wav output.wav rate 11025

So you navigate to the directory your wav file is in, type sox, followed by the name of the file that gqrx gave it, then the name of the file you want it named and then the sample rate of 11025.

Takes about a second or two to covert it.

Then you can get a program called WXtoImg to convert the resampled audio into a photo.

Once it is open just click, File, Open, and select the file you just resampled and click ok. Here is about 4 minutes of satellite telemetry from this morning. Also there are enhancements to make it look better.
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Pretty cool, huh? Free software, a $20 USB dongle and an antenna of some sort lets you talk to a satellite and decode its data hidden within the audio stream. Again, how cool is that? I'm not a weather junkie but I am a tech junkie and this project qualifies as Uber Cool.

Got the hang of this a little better. Updated photos.

First is NOAA 15 grab on 18 Sep and NOAA 18 on 19 Sep 2015

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