VHF Weather Satellites NOAA)
Now that I am getting more interested in VHF, I thought I would try my hand at decoding signals from the NOAA weather satellites that are on 137MHz. There are other weather satellites on 137MHz, the Meteor satellites, which produce higher quality images, but I have not tried to decode one yet. I will attempt to decode one soon, in the hope the procedure has been made easier, as the last time I looked it was rather involved.
To stand the best chance of getting good, reliable, decodes an antenna that has ‘Right Hand’ Circular Polarisation(RHCP) should be used. Something like a ‘turnstile’ (shown below) or a ‘QFH’ (QuadriFilar Helix) antenna would be ideal. The QFH comes in 2 main types: ‘short and fat’ or ‘tall and thin’, which describes the method of construction. A quick look on the web will reveal a whole host of DIY plans for QFH’s. If you possess the necessary skills and have the right equipment to construct it, then it should be fairly straight forward. If you, like me, are not great with plumbing and bending / brazing copper pipes, perhaps you may be able to get a friendly plumber to help with the mechanical side. Whichever type of antenna you choose, try to mount these high as possible and in the clear, so the antenna has an unobstructed view of the sky in all directions down to the visible horizon, or as close as it possible to get. That is the ideal, but as we live in a less than perfect world, just get it in the best position you can. You will get reasonable results from even an indoor / attic mounted antenna but not as good as an outside one. Both of these antennas are used by weather satellite enthusiasts. The FCD is a great receiver for this as the bandwidth can be set to the 40-50kHz necessary to decode the finer detail. Also, the Airspy is very well suited to this kind of task. A scanner, such as my Yupiteru MVT-7100 does cover these frequencies but their IF bandwidth is usually fixed at about 15kHz in Narrow FM mode (250kHz in Wide FM mode). 15kHz is too narrow and the decoding software will remind you of this fact! What you need is a tap/feed from the discriminator (this modification can be done to a lot of scanners) as this is taken before the IF filtering takes place so 40kHz (40kHz is ideal for the NOAA satellites) would be no problem.
I will attempt give a ”beginner’s guide” to decoding the NOAA satellite images. I say try because, I too, am a beginner, and have fumbled about until I managed to get a decent decode! Luckily it is not very difficult to get up and running, although there are a few traps that it is possible to fall into.
So, how do you go about capturing the images from these tiny boxes orbiting at 800+km above the earth?
2) An audio feed from the receiver to the decoding software - usually, this is via the computer soundcard (preferably using the ‘line in’ socket, or ‘mic in’ if that is not possible) or via a ‘virtual’ soundcard, which is software that routes the audio from one program to another. To complete the picture, you need some decoding software. All the images I have shown were produced using ‘WX2img’, a fantastic program that does all the hard work for you. The program is no longer being developed, but still works extremely well. The developer posted registration keys on his site to upgrade the software to the ‘professional’ version, but many were unable to get them to work. There are some working keys available, I think there are links on the WX2img site. You don’t need to upgrade, but as they have been made available by the developer, it seems silly not to take advantage of that.
When the images are downloaded, they are just a raw image file (as shown below). The software processes the telemetry and image data to generate a number of different images, depending on the settings. One of the very useful things the software does is to take account of the ‘Doppler shift’ - that is the change in frequency that occurs from an object as it approaches, and the recedes from you. As you can see, the raw image looks rather different to the processed image.
The things you need are:
- An antenna capable of hearing the NOAA satellites (tuned for 137MHz and Right Hand Circular Polarision (RHCP)
- A receiver capable of tuning to 137MHz in wide FM mode (a ‘normal’ scanner will work, but will give poor results due to the lack of necessary bandwidth).
You need a receiver that has an FM bandwidth of between 30 and 40kHz.
- A method of getting the audio output from the receiver into the decoder software. For a traditional receiver, you would use a shielded stereo jack lead from the headphone, external speaker or line out socket (if you have one) into the line/mic in on your computer soundcard.
The headphone / speaker outputs will also have a variable output level, set by the position of the volume control. The Line output is at a fixed level, regardless of volume setting. You would need to check, but I think any socket marked ‘AUX’ would also be suitable to use as a fixed level output of the correct impedance.
For SDR’s you need to use a virtual soundcard program (such as VAC or VB Cable) which mimics having a second (or more) soundcard and linking them output to input.
- Decoding software to process the audio into images.
Weather Satellite images can be downloaded directly from POES (Polar Orbiting Environmental Satellite) NOAA (National Oceanic and Atmospheric Administration) satellites on the 137MHz VHF band. With the advent of modern technology, the kit required to do this is now cheaper than ever. You could set yourself up to receive the weather images for about £40, or less if you make the antenna yourself.
So, how do you go about capturing the images from these tiny boxes orbiting at 800+km above the earth? You need a few things, you need an antenna, of the correct type and polarisation; you need a receiver capable of tuning to 137MHz in wide FM mode (a ‘normal’ scanner will work, but will give poor results due to the lack of necessary bandwidth. You need a receiver that has an FM bandwidth of between 40 and 50kHz. Next you need a way of getting the audio from your FM receiver into the computer software. Usually, this is via the computer soundcard (preferably using the ‘line in’ socket, or ‘mic in’ if that is not possible). To complete the picture, you need some software. All the images I have shown were produced using the free version of ‘WX2img’, a fantastic program that does all the hard work for you. When the images are downloaded, they are just a raw image file (as shown below). The software processes the telemetry and image data to generate a number of different images, depending on the settings. One of the very useful things the software does is to take account of the ‘Doppler shift’ - that is the change in frequency that occurs from an object as it approaches, and the recedes from you. As you can see, the raw image looks rather different to the processed image.
I have been downloading images for some time now, and have been pleasantly surprised by the quality of images. Not every image is good, and some are just noise. This is due to the location and type of antenna etc, and also the position of the satellite in relation to the antenna. Some passes produce great images, while some, particularly those that are only a few degrees above the horizon are quite often unusable. Below are some images captured from my location. You can download images like these from the internet, but where is the fun and challenge in that?