As of the beginning of 2019, nearly 5,000 satellites are orbiting our planet out of 8,300 objects launched into space since 1957. It takes careful planning and communication between these devices to keep them from crashing into one another, or back to the ground. How do satellites communicate with one another and how do they talk to home base here on Earth?
Radio Wave Communication
Turn on the radio. What you hear there are known as radio waves or frequencies. These signals can carry anything from data to phone calls to video and are a vital part of our global communication strategy. The problem with these signals is that you can only send them in a straight line, and they lose quality over time. That's why when you get further from a radio station, you start to lose the signal.
That's where satellites come in. Satellites are orbital radio relays. They capture a signal and send it either to another satellite or a receiver on the ground. This allows you to overcome the problem of line of sight, or only being able to send a radio signal in a straight line. Satellites are still affected by this limit, but there are so many of them orbiting the planet that they can bounce the data from one to the next — always in a straight line — until the last one can send it to the receiver.
The Problem of RFI
Have you ever heard a buzz in your car radio when you get an incoming phone call? Then you've experienced radio frequency interference or RFI. Since satellites rely on radio waves, this interference can affect their ability to receive and transmit signals, if the intervention is strong enough. Everything, from cellphones to microwaves, can give off RFI — and the more electronic devices there are in an area, the stronger the RFI concentration. With nearly 5,000 satellites circling our planet, the potential for RFI in orbit can become a huge problem.
There are some techniques available to minimise RFI, many of which are employed on new satellites. Most are equipped with RFI shielding, which serves to both protect the primary circuit boards and prevent the device from putting off any RFI of its own. The second is to include RFI filtering in the satellite design. These filters are available at both commercial and consumer levels, targeting RFI in the 150 kHz to 30 MHz range. This prevents the device from emitting or receiving RFI that could interfere with its functionality.
Frequency and Signal Strength
Getting in touch with a satellite isn't as easy as tuning a station on your radio dial. RF energy belongs to the electromagnetic spectrum, which means its frequency measures it in hertz (Hz) or wavelength. The two measurements are opposites of one another — as wavelength goes up, frequency drops and vice versa. You may already be familiar with a couple of RF bands. AM and FM radio, for example, are known as bands because they take up a specific portion of the RF spectrum. The amount of data a band can transmit is known as its bandwidth — a term you've probably heard about internet speed.
To send or receive RF energy, you need an antenna. You probably don't think twice about your car's antenna until it goes missing and you suddenly can't listen to your tunes anymore. Satellites have antenna too, designed to both receive and transmit data. To keep the satellite data bands free of clutter and interference, the International Telecommunications Union allocated a specific band of the RF spectrum specifically for satellite communication. In the U.S., if you want to communicate with a satellite, you have to get a special permit from the FCC to prevent your data from interfering with anyone else's use of it.
Calling a Satellite
In theory, all you need is a transmitter capable of sending information on the satellite's RF frequency to communicate with one of the thousands of devices orbiting our planet, but in reality, it's not that simple. You need special permission from your country's regulating body to reach out and contact one of these orbiting bodies.
Today, satellites handle everything from television and radio to cellphones and even secure scientific and military data, and we're launching more every year. If you look up at the night sky and see a star moving swiftly toward the horizon, you've probably spotted a satellite — and now you know how these pieces of advanced technology communicate with each other, and with us here on Earth.
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