Sunday, June 15, 2008

Decoding NOAA APT Weather Images

There are a lot of things in the radio communications hobby that are exciting... but for many monitoring satellite communications is at the top of the list. One of the more interesting satellite monitoring targets are the NOAA low earth orbit satellites, which fly in sun-synchronous polar orbits around the earth and continuously transmit satellite photos. These picture transmissions occur in the 137 MHz band and are easily monitored with a scanner and easily decoded with free software. In this post, I'll review how I was able setup a process to receive and decode these pictures using an AOR AR5000 Radio, a computer with a sound card, and free decoding software.

Currently, there are 3 production NOAA satellites, NOAA 15, 17 and 18. They transmit on the following frequencies:

NOAA 15 137.500
NOAA 17 137.620
NOAA 18 137.100

The transmission mode is FM, with a bandwidth of 30 KHz. There are very few scanners and communications receivers that have a IF bandwidth selection of 30 KHz, however you can still receive the signal using the standard mode selection of FM for your radio.

To get started, you will want to dedicate a receiver for satellite monitoring - the receiver should have a good outdoor antenna with a clear view of the sky. The easiest method is to use a discone antenna, or other roof mounted antenna. There are antennas that are designed specifically for the circular polarization of the NOAA Satellite signals, however a standard discone or other common outdoor scanner antenna should work fine.

Next, you will want to download and install the APTDecoder software package. APTDecoder is a free software package for recording and decoding signals transmitted by NOAA APT enabled weather satellites. It runs on Windows and is a very powerful tool for decoding the NOAA APT signals.

Your next step is to determine what your current station's latitude, longitude, and altitude is. This is important, since the NOAA satellites are in orbit around the earth, and you'll need to use APTDecoder to predict when the satellite passes overhead. Write these three pieces of important information down for input into APTDecoder.

Configuration of APTDecoder

After starting APTDecoder, launch the application and click Settings -> General Settings. You will want to enable the auto-recording and processing feature which starts the decoding process for you when a satellite comes into range.

Next, choose the Image Properties Tab and the Enhancements sub-tab and choose the Histogram equalize option. This will help to lighten the picture after decoding so it is more easily viewed.

There are a number of other features that can be enabled, such as auto radio control and more picture settings, but the above settings should get you started.

The next step is to define your ground station particulars. Choose Settings -> Ground Station, and enter your Latitude, Longitude, and Altitude. Then click Add, then scroll through the list to find your new entry and click Select.

Now that your ground station is configured, you'll need to download the latest Keplerian elements for the satellite orbits. Keplerian elements are 2 line text data sets which describe how a satellite is orbiting the earth. These are updated often and need to be refreshed to ensure accurate predictions on satellite passes. Click the Satellite -> Keplerian elements menu item, highlight the first URL entry, and click the Download button on the right. The modified timestamp should show an updated timestamp for the current date and time.

Now that you have configured the software, it's time to connect your receiver to the computer. If a satellite is actually overhead and in range, APTDecoder will start recording immediately. If this happens, just click cancel.

Connecting Your Scanner to the Computer

APTDecoder uses the sound card installed in your computer to decode the transmissions from the scanner. Connect an audio cable from the headphone or audio out jack on your scanner to the mic in jack on your sound card. Then, to set the audio level properly, choose the Tools -> Check audio level menu item in APTDecoder and follow instructions in the meter to set the audio level properly.

Decoding a Satellite Pass

With the application configured, APTDecoder is now tracking each of the 3 NOAA Satellites and is providing predication and tuning information to you in the interface. It is time to prepare for one of those passes and decode the transmitted data.

At the top of the APTDecoder window interface, you'll notice the phrase "Tracking NOAA-## on 137.620 MHz....." This top line is telling you the next satellite that is about to come into range (or is in range), on which frequency, and the specific time that the satellite will pass over your ground station. At the bottom, you'll see a line that says "Next: NOAA xx on 137.620 .........", this is the next satellite pass AFTER the one that is being tracked (sort of the "next, next" pass).

Reading the top line on the interface, tune your scanner to the frequency that APTDecoder says the next Satellite will be on. Remember, this line is telling you which satellite will be next, on which frequency, and when. In the following example below:

This tells us that NOAA 17 is next, transmitting on 137.620 MHz, and will come into range (AOS = Aquisition of Signal) at 17:06 UTC. The current time is 15:11 UTC, so we have about 2 hours before the next pass. To see where exactly each of the 3 NOAA Satellites are in relation to the earth, you can choose the Flat Earth Monitor tool (Tools -> Flat Earth Monitor).

To setup and decode the pass, tune your scanner to the proper frequency as indicated on the top line, plug in the audio cable, adjust the audio levels, and wait.

NOTE: Remember that NOAA APT transmissions on the 137 MHz band have a bandwidth of 30 KHz. If your radio has the capability to change the IF Bandwidth to 30 KHz, you should enable that setting. There are very few radios that have this capability (the Icom R9500, Icom R9000, and the AOR AR5000 are the ones I am aware that can), however it is reported that most scanners in standard FM mode (typically 15-20 KHz) will work OK, but don't expect a perfect decode unless you have a wider 30 MHz bandwidth.

When the satellite comes into range, APTDecoder will immediately start decoding, and you should begin to see each line decode and scroll. In the beginning of the pass, don't worry if there is a lot of noise, the satellite may need a few more minutes to get into range. The initial decode of the picture will be very dark looking, however after the entire pass finishes the decode will look just fine.

Each NOAA Satellite transmits two photos - a visible earth image on the left, and an infrared image on the right.

Start of decoding will look like the below. Don't forget, the initial picture will look very dark, however when the pass finishes the software will auto-process the image for proper light levels.

Still decoding...with the Flat Earth Monitor enabled showing the satellite transmit footprint around my ground station (you can enable this during the decode):

Finished. You can see both the visible image on the left, and the infrared image on the right.

That's it. Once that decode finishes the next upcoming satellite pass details will be displayed on the top line and you'll want to adjust the scanner frequency if needed.

I hope this was an enjoyable post, and that if you've never been able to decode NOAA Satellite transmissions this will be a good first step for you.