Receiving Satellite Transmissions
Satellites transmit their signals back to Earth fairly indiscriminately, and anyone with the right equipment can receive the signals as they pass overhead. The complexity of the equipment needed depends on the satellite; but in this article, we'll look at the very basic setup required to receive weather images.
There are two main things that we need:
- An Aerial: we'll build our own using wire and various pieces of plumbing.
- An SDR Dongle: which needs to be bought. It converts radio signals into something your computer can understand (more later).
It also wouldn't be possible without some clever pieces of software. The ones we'll use are free and easily obtainable.
It is important to set the right expectations from the start. The image below shows what I've obtained using the setup described in this article. The country outlines are added by the software, so it's the view of the clouds that were received from the satellite.
It is possible to get much better results, such as these examples from the University of New South Wales and Dorset Live Weather. However, the image above shows a realistic starting point; and pictures of clouds from a satellite is still pretty amazing.
The satellites that we're interested in belong to the National Oceanic and Atmospheric Administration (NOAA). There is a group classed as 'polar orbiters' that scan the whole surface of the Earth, and so wherever you are they should fly over your head at some point. The most recently launched satellite, and the main one in use, is named NOAA-19. However, there are several older satellites still in orbit and continuing to transmit. These are NOAA-15 and NOAA-18.
Each satellite transmits on a slightly difference frequency, around 137MHz (megahertz). That might not mean much to you, but FM radio stations transmit in the 88MHz to 108MHz range. So, the signals that we want to detect aren't too far away from the standard FM stations that you listen to. You don't need to memorise the frequencies of each satellite; you'll be reminded by the software that we'll use.
Each satellite will 'appear' (you won't be able to visually see it obviously) at a point on the horizon, arc through the sky, and then 'disappear' at another point. The more sky that you can see the longer you can receive the signal for. Expect around 12 minutes if you have a good view of the horizon.
Software Defined Radio
Put simply "Software Defined Radio" (SDR) is a way of plugging an aerial into your computer and receiving radio waves. You will need to buy a small device that plugs into the USB port (they often look a bit like a USB flash drive). The one I have is shown in the image below and cost around £10.
There are a few different options available, with a review of some on the rtlsdr4everyone site, and are easily found on a site like Amazon. Most are in the same $15-$30 price point because the core component is essentially the same (a chip named R820T/2) - and you just pay for different levels of build quality. If you want to get a bit more serious then there is a big price jump to the next level, where you can expect to pay around $200.
Building an Antenna
I've lazily flipped between using 'antenna' and 'aerial' in this article; but although I've read that there might have been a difference historically, I think they're essentially the same thing now.
There are lots of different ways that you can shape and connect bits of metal to build an antenna, and it is a field of study in its own right. I just followed the suggestions of other people on the Internet and built a quadrifilar helix antenna, as shown below.
The supposed benefit of this type of antenna is that it can receive signals coming from all directions (omni-directional). This means that you shouldn't need to point the antenna at the satellite - so long as it is above the horizon you should receive the signal. That's the theory. My antenna actually turned out to be affected by direction; so as I turned it the signal would get stronger or weaker. I can't guarantee that would happen with one that you build, but it is something to look out for.
There are lots of plans on the Internet for how to build one. However, you might run into some initial confusion because I think that there are at least three different types:
- Metal loops: This is the type that I made. The jcoppens site has an excellent online calculator where you enter the frequency that you want to receive (keep the default 137.5 MHz), and it gives you the required dimensions. You can then enter the diameters of the bits of plastic pipe that you are using and print a template that shows you where to drill the holes. A guide is available from the House Dillon site. Most other examples that you'll find seem to use copper tubing.
- All coax: There are a few designs where coaxial cable is used instead of simple wire. They look essentially the same, but the connections between the loops seems a little more complicated.
- Half-loop coax: There is also a design where the loops are mostly simple wire, but half of one loop is coaxial cable. This seems to require two connections to be soldered, and so is even more complicated.
Just pick one of those guides that makes sense to you.
Nearly all of the designs seem to require soldering connections between the loops and coax cable, at least part-way down inside the plastic tube. That seemed like it was going to lead to a lot of frustration and more than a few expletives. So instead of holes at the top, I cut four slits. I could then get a good mechanical connection first by wrapping around some wire, before soldering and then sliding the wires into place. A few dabs from a hot glue gun holds everything in place. Of course, you want to be careful that sliding the wires into place does not cause any bending forces that will break the connections; but it worked better for me.[slideshow_deploy id='809']
An overview of all the different components that you need is shown in the diagram below. We've already covered the parts outside of the grey box.
It might look like a lot of steps, but you essentially need two pieces of free software:
- SDR#: included in the SDR software package from Airspy. This takes the signal from the SDR dongle, gives you an interface to choose the frequency that you are interested in, and outputs the audio. RTL-SDR.com lists some alternatives, but I haven't used any of them.
- WXtoImg: available from the WXtoImg site. This takes the audio signal from the satellite and converts it to an image.
You will probably need to set the correct drivers so that the dongle can be detected by the SDR# software. In the zip file that you download from Airspy will be a file named install-rtlsdr.bat. If you double-click on this, it will fetch another piece of software named Zadig.exe. If you then run Zadig.exe, you will be able to install the correct drivers. Once again RTL-SDR.com provides a complete guide on setting up SDR#.
SDR# will output the signal to your computer's speakers (or headphones). You can use the Stereo Mixer on Windows to allow WXtoImg to listen to this audio. Instructions of how to enable it are available from How-To Geek.
Listening to the Radio
A simple way to check that everything is working is to try listening to the radio. Even if you have built an antenna perfectly tuned to 137.5Mhz, it should still be able to receive FM radio. In fact, any bit of wire should allow you pick up the strongest stations.
Get used to switching between frequencies and adjusting the bandwidth to focus on a transmission. Also click on the cog icon to reach the settings and turn the RF Gain up to maximum for better results.
Location, location, location. Your antenna needs a good view of the horizon in all directions to maximise the time that it can receive the satellite's signal. You could mount the antenna on a roof or pole and run a long cable. However, if like me you kept a short cable then you are going to need to get a clear view yourself - which probably means the top of a hill or tall building.
WXtoImg includes a tool to list the satellite's passes over your location:
- If you didn't do it the first time that the application ran, then choose Options > Ground Station Location to set where you will be when you're listening to the satellite.
- Something else you probably did in the first run was to update the Keplers. If not, then choose the menu option File > Update Keplers to keep the expected locations of the satellites up to date.
- Choose File > Satellite Pass List to see the times when satellites will pass over - including the frequency to listen on.
Ideally look for a time when there will be two passes close together (say around 20-30 minutes apart). That way you can try to receive, process the result, and have time to make any adjustments before the next satellite.
While you're in WXtoImg, be sure to set it to use the Stereo Mixer:
- Choose the menu Options > Recording Options
- Select the Stereo Mix for the soundcard
When it's time, start WXtoImg recording by choosing File > Record, and choose Auto Record. This will start and stop the recording based on the predicted visibility of the satellites. The signal that you'll receive is quite distinctive (as shown in the image below). There should be several dashed lines, which matches the regular ticking sound that you'll hear.
Every second counts when it comes to receiving the signal. Any moments of poor reception will lead to noise in the image. You can expect the start and end to be of lower quality, because the satellite will be further away with more atmosphere to transmit through.
The motion of the satellite will also create a Doppler shift; which just means that the transmission frequency will seem to drift slightly from the right to the left in SDR#. To keep the best reception possible you might need to slowly reduce the frequency value to keep the peak in the middle.
The image above was received from NOAA-15, immediately before the image at the top of the article that was received from NOAA-18. I suspect that the newer satellites have better technology, so an image from NOAA-18 will be better than NOAA-15, and NOAA-19 best of all. Just a hypothesis at this point.
The RTL-SDR.com site has an excellent guide to receiving NOAA satellite images, with less waffle and more detail. However, I think there are a few things missing that I've learnt from experience and included here.