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Chapter 5
Listening to Satellites

Being able to tune into NOAA satellites while they pass overhead and download the images they’re broadcasting is probably one of the coolest things about using an SDR setup. It’s old-school technology that’s been around since the 1960s but is now becoming easily accessible to hobbyists and enthusiasts, thanks to the SDR dongle and the software that you’re now familiar with.

NOAA currently has quite a few weather satellites orbiting the Earth, keeping track of data worldwide, but there are only three that are broadcasting signals that are easy to receive. They broadcast around 137 MHz, and the signal can only be received when the satellite is passing overhead. Due to their orbital parameters, you can download the satellite’s transmission and decode it about twice a day, depending on where you live.

The satellites—NOAA-15, 18, and 19—were launched in 1998, 2005, and 2009, respectively. They each broadcast a signal called Automatic Picture Transmission, or APT. APT was developed in the 1960s and is composed of two image channels, telemetry, and synchronization data, all transmitted as one horizontal scan line. You can use one of the programs we’ve already installed and used, GQRX or SDRSharp, to tune into each satellite’s particular frequency. You can then either record the transmission and send the audio recording to another application for decoding or you can decode the transmission on the fly and display it as it’s sent by the satellite overhead.

As it happens, just before this writing, NOAA released an official statement: “As of ~0000 UTC July 30, 2019 (DOY 211), the NOAA-15’s AVHRR motor current has once again started spiking, becoming saturated above 302mA at ~0600 UTC. The instrument is once again no longer producing data and may be stalled.” What this means for you is that if you happen to be trying to listen to 15’s pass overhead and come up with nothing, give the other two satellites a try before giving up. I was able to tune into the satellite’s signal, but I had no luck receiving an image, and I even got an error message I’d never seen before about missing telemetry data. So just be aware that NOAA-15 might be out of order when you read this.

As a weather satellite passes overhead, you’re dealing with the Doppler effect as it moves. What exactly is the Doppler effect?

The Doppler effect is the change in the frequency of a signal as you and the source move relative to each other. We experience it when we hear the siren on a moving ambulance rise in pitch as the vehicle speeds toward us, compressing the sound waves. If the ambulance doesn’t hit us, we hear the siren decrease in pitch as the vehicle moves away, stretching the sound waves of the siren.

The same principle applies to light—and, as it happens, radio waves. Astronomers were first able to determine that certain stars and galaxies were moving away from us by seeing that the color of their light was shifted red. Likewise, a satellite moving at only 0.0025% the speed of light can have a frequency shift of a few kilohertz as it moves from horizon to overhead to horizon again.

We can even calculate the Doppler shift of each satellite, based on some easily researched numbers. NOAA 15 has an orbital semi-major axis of 4464.32 miles and an orbital period of 101 minutes. That works out to (4464.32 × 2 × pi)/(101/60) = 16,663 miles per hour. Call it 16,666 to make the math easier. The speed of light is 186,282 miles per second, which multiplied by 60 seconds and by 60 minutes equals 670,615,200 miles per hour. 1/(670,615,200/16,666) = 0.000025, or 0.0025 percent the speed of light. The Doppler equation is (c/c+v) × f. In this case that works out to (1/1.000025) × 137 MHz = 137,003,425, or a shift of 3.4 kilohertz. Speaking from experience, that’s about the amount you’ll have to adjust the tuning over the course of the satellite’s pass overhead.

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Table of Contents for 5. Listening to Satellites

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Chapter 5Listening to Satellites

Table of Contents for
5. Listening to Satellites

Chapter 5
Listening to Satellites