Software-Defined Radio
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
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.
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.
Initial Antenna Setup
Before messing around with any software, you’ll need to set up the SDR’s antenna first. To do this, first screw the two longer telescoping antennas into the antenna mount included with the kit. Attach one end of the long cable included in the kit to the output of the antenna mount, and then screw the other end into the exposed connector on the RTL-SDR device. Expand the antennas, and you’re good to go! Put this aside for now—we’ll use it later to verify that the software is working.
Necessary Software
SDR Software: We’ll need a program to control the software-defined radio and output the received audio. Programs of this type configure the SDR, take in the raw data, decode it, and output information as audio. More advanced programs can perform more complex decoding, but for this application, we only require a program that can decode frequency-modulated audio (more on this later).
Audio-to-Image Software: These NOAA satellites utilize an analog picture transmission (abbreviated APT) format. The SDR program outputs audio that gets turned into a real image by this program.
Audio Piping Software: We’ll need a link between the SDR program and the audio-to-image software. Basic audio piping programs imitate output devices like speakers for one program and input devices like microphones for other programs. This allows us to send audio from one program to another in a seamless manner.
SDR Software: CubicSDR
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 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.
- 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.
Magnified Waterfall Plot: When CubicSDR is actively demodulating a signal, a zoomed-in window of the main waterfall plot will appear here.
- 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.
Active Demodulators View: This screen shows you all the active demodulators and their frequencies.
- 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.
Waterfall Plot: This is the main waterfall plot showing the received power across a slice of the electromagnetic spectrum over time.
- 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.
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.
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.
The preceding figure (labelled with numbers) shows what CubicSDR’s interface should look like after these steps.
Decoding Software: WXtoImg
- 1.
Go to the “Options” menu bar and select “Ground Station Location.” Enter your latitude and longitude, and select “Ok.”
- 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.
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).
Piping Program
You now have an SDR program and a decoding program, but you need a way to link these two together. This is where the audio piping program comes in: it takes audio from CubicSDR and outputs it to WXtoImg. Audio piping programs imitate speakers for a program outputting sound and microphones for a program that takes sound as an input. Soundflower for macOS is my favorite; you can download it here: https://github.com/mattingalls/Soundflower . For Windows, VB-Cable ( www.vb-audio.com/Cable/ ) is a good, free option. Download the program of your choice, and install it.
First Test
- 1.
Repeat the same steps you used to make CubicSDR output sound from a local FM station.
- 2.In the upper-left corner of CubicSDR, change the output device from your computer’s speakers to the audio piping program. If everything was configured correctly, your audio piping program should appear as an output device in the list. This is shown in Figure 4-9.
- 3.
Now, open up WXtoImg and select “Recording Options” in the “Options” menu.
- 4.
Under “Common recording options,” select the dropdown next to “soundcard” and select your audio piping program.
- 5.
Close this window by clicking “Ok,” and then go to “Record” in the “File” menu. Make sure “Record and auto process” is selected, and finally click “Manual Test.”
- 6.If all goes well, static such as that shown in Figure 4-10 should start to fill the screen line by line.
WXtoImg processes the images from the satellites line by line, so you get to watch the image slowly appear during a real flyover. For now, WXtoImg is trying to decode the music coming from the FM station and turn it into an image. This music or voice won’t translate to an image, which is why static appears. If the image is black, this indicates that WXtoImg isn’t receiving any sound information. Try reconfiguring your audio piping program and checking that it is the output of CubicSDR and the input to WXtoImg using the mentioned dropdowns menus in both.
Antenna Setup
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
- 1.
Select a daytime flyover from the list in WXtoImg; pick one that has a maximum elevation of greater than 50 degrees.
- 2.
Set up your antenna using the preceding information.
- 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.
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.
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.
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.