Software-Defined Radio

Software-defined radios, or SDRs, are devices that allow computers to receive raw electromagnetic information at a programmable frequency. In their simplest form, software-defined radios take in the frequency and bandwidth the user wishes to sample, along with a sampling rate, and return to the user values from the antenna at that given frequency at the sampling rate specified. Figure 4-2 shows the software-defined radio used in this chapter.

A circuit inside of the software-defined radio is tunable such that only energy at the desired frequency is sampled; the rest of the electromagnetic spectrum exciting the antenna is ignored. Since this system is in fact tunable, one must also adjust the antenna length for maximum performance. The software-defined radio kit mentioned at the beginning of this book, which utilizes a device known as “RTL-SDR,” comes with a telescoping antenna, so the user can adjust its length for different frequencies. The RTL-SDR device is capable of receiving frequencies of around 20 MHz–2 GHz.

In summary, software-defined radios are computer-tunable devices that allow one to listen to a large swath of the electromagnetic spectrum.

The Satellites

Out of the tens of thousands of satellites whizzing by or remaining stationary overhead, we will try and receive communications from just three. These three satellites are NOAA-15, NOAA-18, and NOAA-19. NOAA 18 is shown in Figure 4-3.

NOAA, or the National Oceanic and Atmospheric Administration (of the United States), operates a fleet of satellites primarily for climate and meteorological monitoring. These satellites are equipped with a wide variety of sensors, but only the image data transmitted from the satellites is of interest to us. The three satellites mentioned above transmit images at a frequency that’s easy to receive and in a format that’s easy to decode.

These satellites move around the Earth in a configuration known as a sun-synchronous orbit . The mathematical details of such an orbit are out of the purview of this text, but the physical orbits themselves are not too difficult to understand. These satellites orbit roughly north to south (or south to north depending on where the satellite is) and move sideways across the globe roughly 1000 miles with each orbit. They are also at a height such that they can see a patch of the Earth roughly 3000 miles in diameter. The satellites take around 100 minutes to complete one orbit of the Earth.

The configurations of the orbits, and the fact that there are three satellites, mean that one should be visible in the sky (and thus able to receive transmissions from) every few hours. This will give you plenty of opportunities to try and get radio transmissions! The details of the orbit work out such that one of these NOAA satellites will take 10–20 minutes to cross your sky. If the satellite passes directly overhead, it will appear in the sky longer than if it were to pass by closer to the horizon.

Satellite passes such as these are characterized by their maximum elevation, or the maximum angle the satellite makes between it and a horizontal plane. This is roughly equivalent to the maximum angle between the satellite and the horizon. A satellite with a low maximum elevation will appear very low in the sky, the duration of the pass will be short, and the satellite will stay close to one cardinal direction during the pass. In contrast, passes with a high maximum elevation can last a long time; the satellite will fly almost directly overhead and will come above the horizon in one direction and go back below the horizon in the opposite direction. A diagram of an orbit is shown in Figure 4-4.

The middle plane in the figure represents the ground—everything above this is visible sky. The arc is the path of an orbiting object across the sky. This pass has a maximum elevation of roughly 40 degrees.

For this application, passes with a higher elevation are better for a number of reasons. Firstly, a longer duration means the received picture will be larger. These satellites transmit images of the ground as they pass by, so the longer you receive transmissions from the satellite, the more of the ground you will be able to see. Secondly, the quality of the signal will be much better for higher-elevation passes. When the satellite is higher in the sky, there’s less of a chance of its signal being attenuated by trees and buildings. Additionally, the satellite is closest to you when it’s directly overhead, so the received signal will be the strongest.

SDR Software: CubicSDR

The program that we will use to control the RTL-SDR device is CubicSDR. All SDR programs contain the same basic features:

• A waterfall plot that allows you to view the intensity of the electromagnetic spectrum across a section of frequencies

• A means of configuring the receive frequency and bandwidth of the radio

• A means of decoding the received information and outputting it in a certain format

CubicSDR’s main interface window is shown in Figure 4-5. FM radio stations in my area are shown. The station farthest to the right transmits digital audio (branded “HD Radio”) above and below the main analog signal, which is why it has those two strips on the side.

CubicSDR checks all these boxes in addition to being free, cross-platform, and well designed. Start by downloading CubicSDR from https://github.com/cjcliffe/CubicSDR/releases . You can find the build for your platform in the “Assets” section. Choose the proper version, and install the software on your computer. The download will be a .dmg for macOS and a .exe for Windows.

If you’re on Windows, you will need to follow the setup instructions at www.rtl-sdr.com/rtl-sdr-quick-start-guide/ to install the proper RTL-SDR drivers. Scroll down until you see instructions for using RTL-SDR with CubicSDR. If you’re on a Mac, your device should work out of the box. Plug your SDR into a free USB port, and start CubicSDR. You should plug the device in before starting CubicSDR. If all goes well, a window such as the one shown in Figure 4-6 should appear.

Upon startup, you should see an image similar to the one above. Select your SDR by clicking it—it will likely be labelled something like “Generic RTL …” if you’re using a Mac—and click “Start.” The main interface will then appear. The various portions of the interface are labelled in Figure 4-7.

This is CubicSDR’s main interface. The term “demodulator” is used throughout the interface—a demodulator is simply a region of the electromagnetic spectrum that CubicSDR samples and converts to audio. You can enable a demodulator by clicking region 6. The white-shaded area represents the bandwidth; CubicSDR will take all the received energy from this region and pass it through various demodulation filters. More will be said on modulation and demodulation later.

1. 1.

   Demodulator and Output Selection: Here you can select the demodulator type (leave this on FM for now) and the output channel of the audio.

    
2. 2.

   Magnified Waterfall Plot: When CubicSDR is actively demodulating a signal, a zoomed-in window of the main waterfall plot will appear here.

    
3. 3.
   Audio Output: This is the result of the demodulation process; it shows the waveform of the audio that CubicSDR is currently outputting. This region also contains numerical outputs of the frequency, bandwidth, and center frequency.

   • Frequency: This is the center frequency of the active demodulator. This value will be blank until you add a demodulator.

   • Bandwidth: This is the bandwidth of the demodulator.

   • Center Frequency: This is the center frequency of the large waterfall plot.

    
4. 4.

   Active Demodulators View: This screen shows you all the active demodulators and their frequencies.

    
5. 5.

   Instantaneous View: This region is an instantaneous view of a slice of the electromagnetic spectrum. Frequency is on the X-axis, and power (scaled logarithmically) is on the Y-axis.

    
6. 6.

   Waterfall Plot: This is the main waterfall plot showing the received power across a slice of the electromagnetic spectrum over time.

    

You can change the demodulation frequency, bandwidth, and center frequency of the window using the numbers in region 3. Moving your mouse over the numbers will reveal arrows to change them, but after changing a selection using the arrows, you can press the space key to enable keyboard input. You can enter a value like 144 MHz by typing “144 M” and pressing the enter key. Now that you’re familiar with the interface, let’s try and actually receive something interesting!

1. 1.

   Set the center frequency to 100 MHz, and you should see various bands of activity somewhere on the waterfall plot. These patches of activity are FM radio stations that transmit near 100 MHz.

    
2. 2.

   Click the center of one with your mouse, and drag the edge of the white-shaded region to enclose all of the greenish signal.

    
3. 3.

   Ensure that the output device in the top-left of your screen is set to the speakers on your computer, turn your volume up, and you should hear the radio station coming out of your computer!

    
4. 4.

   The preceding figure (labelled with numbers) shows what CubicSDR’s interface should look like after these steps.

    

Decoding Software: WXtoImg

WXtoImg is a rather archaic program that takes in audio from the NOAA satellites and converts it to an image. The software is no longer maintained, but a dedicated fan uploaded the last builds of WXtoImg to https://wxtoimgrestored.xyz . Navigate to this web site’s downloads page, and download the correct version of the software for your system. Open it up, and you should be presented with a mostly gray screen with a few tabs. An example of this is shown in Figure 4-8.

Before you’re ready to decode an image, you’ll need to configure a few things first:

1. 1.

   Go to the “Options” menu bar and select “Ground Station Location.” Enter your latitude and longitude, and select “Ok.”

    
2. 2.

   Next, go to “File” and then “Update Keplers.” This selection downloads tracking information for the three NOAA satellites of interest so the program knows when they’ll fly overhead.

    
3. 3.

   After this operation completes, you can select “Satellite Pass List” in the file menu, and WXtoImg will output a list of all the times the satellites will pass overhead. The important item here is “MEL,” or maximum elevation. If the maximum elevation is too low, you’ll get a very small and poor-quality image—only passes with elevations of around 30 degrees or more will produce good results. You can still practice on lower-elevation passes. WXtoImg even gives you the frequency at which each of the satellites transmits (measured in MHz).

    

Antenna Setup

The last thing you need to do to prepare is to configure your antenna. The provided telescoping antenna works decently well for receiving signals from satellites. For this application, we’ll use it in a “v-dipole” configuration. We need each side of the antenna to resonate at 137 MHz—a quick calculation reveals that the proper length for each side is around 54 cm (a quarter of the wavelength of 137 MHz radio waves). Extend your antenna segments to this length, and then fan out each side such that they’re separated by 120 degrees. When mounted horizontally, this configuration is great for receiving energy coming from the sky. The radiation pattern is shown in Figure 4-11.

I attached my antenna to a long piece of lumber (shown in Figure 4-12); it’s helpful to get your antenna off the ground, but if you can’t, you can use the tripod and rest your antenna on the ground. Orient in such that the center of the antenna points north. It’s also important to do this outside. A building will attenuate the already weak signals coming from the satellite almost 1000 km away.

Another interesting thing to investigate is placing a metal sheet below your antenna. The function of this is somewhat intuitive—by reflecting energy coming past the antenna back toward it, it will increase the antenna’s gain. The effect of placing a large metal sheet beneath the antenna is illustrated in Figure 4-13.

Note how energy no longer radiates below the antenna—all the gain in that direction has been “transferred” skyward. You can barely make out the v-dipole in the center of the sheet.

First Flyover

You’re now ready for the main event!

1. 1.

   Select a daytime flyover from the list in WXtoImg; pick one that has a maximum elevation of greater than 50 degrees.

    
2. 2.

   Set up your antenna using the preceding information.

    
3. 3.

   Configure CubicSDR like before. Ensure that your audio piping program is the output, add a demodulator with a frequency of the one listed in the satellite pass list, and set the bandwidth to 40 kHz.

    
4. 4.

   Go back to WXtoImg, and select “Record” from the file menu. Instead of clicking “Manual Test,” you should now click “Auto Record,” and WXtoImg will wait until the satellite is overhead to start decoding audio.

    
5. 5.

   As the satellite comes above the horizon, you should begin to see a faint signal appear on the waterfall plot in CubicSDR. This will look something like the waterfall shown in Figure 4-14.

    

Seeing the faint traces of the satellite’s signal appear out of noise for the first time is truly magical—there’s a satellite hundreds of miles above you hurtling across the sky, and it’s talking to you! You’ll notice that the signal appears to slide in frequency throughout the time you can see the signal. This is due to the Doppler shift. As the satellite approaches, its large forward velocity shifts the signal a few kilohertz up. As it recedes, the opposite effect occurs.

Going back to WXtoImg, a view of the sky should appear line by line. There will be two images side by side with some black and white bars. In the daytime, one of the images is visible light and the other is infrared. In the nighttime, both are infrared. The black bars help programs such as WXtoImg align the image later. The black bars also serve as a good indication that you’re receiving something. Some images might look like the one in Figure 4-15: almost all noise, with some barely intelligible markings.

After the flyover, you can mess around with some of WXtoImg’s image processing features. In the “Options” menu, you can enable, disable, and align the map overlay featured in my photos. In the “Enhancements” menu, you can have WXtoImg create a false-color combination of the visible light and infrared data. This is only available for some images, as the satellite will need to be transmitting the correct type of data at the time. An example of what you can do with WXtoImg’s advanced features is shown in Figure 4-16.

This map uses data from the infrared sensor in addition to location information to capture the surface temperature of bodies of water in the image. An interesting feature of note is how portions of the ocean (such as around New York City) are occluded from the temperature map. This is because there was a large cloud over the area at the time, and since water is opaque to infrared radiation, no thermal data from the ocean was able to reach the sensor. Also note how comparatively cooler the Great Lakes are than the ocean.

Summary

Congratulations! If all went well, you have now successfully communicated directly with a satellite. I highly recommend exploring WXtoImg further to see what else the program has to offer. Additionally, experimenting with your antenna setup to get the strongest signal possible is a rewarding experience. It’s also possible to construct a permanent outdoor antenna for satellite communications; you can find instructions for this online. In a later chapter, we will use an amateur radio satellite to talk with individuals thousands of miles away.