How Satellites Are Used to Observe Earth

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After completing this lesson, you will be able to explain how satellites are used to observe the earth, including the ways that different parts of the electromagnetic spectrum are useful. A short quiz will follow.

What is a Satellite?

Have you ever looked at a particularly bright point in the night sky and wondered what it was? Points that are much brighter than most of the other stars usually aren't stars at all. More often than not, they're planets like Mars, Venus, or Jupiter. But now and again, you might be looking at a satellite.

A satellite is a body orbiting a planet such as the earth. That body can be made by humans, or it can be natural. The moon is earth's only natural satellite, but we have a lot of artificial ones. The brightest artificial satellite in the sky is the International Space Station.

An Artificial Satellite
An Artificial Satellite

How are Satellites Used?

Artificial satellites are used by humans in myriad ways. Some are used to send and receive signals without directly observing anything on the ground. This is the case with communication satellites for example.

But many satellites are used specifically to observe the earth's surface. Examples include satellites that are used for weather forecasting, spying, or conducting military operations. They are also used to take photographs for mapping purposes. It's these kinds of satellites we're going to talk about today.

How We Observe the Earth

Satellites observe the earth in many ways, depending on what their exact purpose is. In general, this process is called remote sensing. The most obvious kind of remote sensing is simply taking photographs of the ground. This can be used for all of the reasons we've already mentioned, from spying to observing weather patterns; photographs do at least show us how the clouds are positioned from one moment to the next.

But this isn't the only way that satellites observe the earth. Visible light is only one part of the electromagnetic spectrum (or EM). It's the part our eyes happen to see. But there are other parts of the spectrum we can sense, like infrared, radio, microwave, or ultraviolet waves. Materials reflect different parts of the spectrum in different ways. This can be used to see through certain layers (such as clouds) using microwaves: a microwave pulse is emitted, bounces off the ground, and is then detected again by the satellite. Similar techniques can be used to penetrate the oceans. Any hot object produces infrared, so infrared can be used to detect heat; humans light up like a Christmas tree on an infrared camera. This is incredibly useful in observing military operations. Every part of the electromagnetic spectrum gives you different information about the earth's surface.

A satellite using multiple parts of the EM spectrum
A satellite using multiple parts of the EM spectrum

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