A Quick Look at Satellite Image Technology

The collection of satellite images to study the surface of the Earth is called Remote Sensing. Remote Sensing is detecting the qualities of something from a distance, without coming into contact with it – pretty much what our eyes do, or a camera. But unlike the images formed on our retinas, satellite images are digitized, meaning they're made up of pixels, each of which contains a number that represents a level of brightness. And unlike photographs, satellite images are georeferenced, meaning each pixel's location is precisely defined with reference to the Earth. This is what makes them so useful for studying processes on the Earth, from deforestation to melting glaciers to urban growth. It's also what makes "before and after" comparisons so precise.

There are many different satellites gathering data about the Earth and its processes. This site uses mainly Landsat, a series of satellites that have been in action since 1972. Landsat covers the entire Earth every 16 days. This means that if you wanted to see images of current events such as the eruption of a volcano or the spread of an oil spill, depending on the timing, you might be able to get the imagery immediately, or you might have to wait a couple of weeks.

The Electromagnetic (EM) Spectrum

Energy from the sun comes in many different wavelengths, which taken together are called the Electromagnetic (EM) Spectrum. The shorter the wavelength, the greater the energy.

NASA Goddard Space Flight Center and US Geological Survey

More of the sun's energy is given off in the visible light portion of the spectrum than in any other. Given the survival advantage that sight confers, it's not surprising this is the portion the eye gradually evolved to detect.


Stan Aronoff

The "signatures" of various surfaces, showing the proportion of energy reflected
(the scale on the left) in the various wavelengths (the scale on the bottom).

The various surfaces that this energy comes into contact with, such as seawater, huckleberry bushes, or asbestos shingles, will respond differently to different wavelengths, reflecting or absorbing more of some and less of others. Therefore each surface has its own "signature" made up of the proportions of energy it reflects in each wavelength. All of this can be measured by a sensor designed to capture that information. This is what makes it possible for a satellite to identify what's on the ground.

Each sensor is designed to collect information from particular wavelengths, depending on its mission. For Landsat, these include three segments of the visible light portion of the spectrum (blue, green, and red) and four from the infrared portion (one near infrared, two shortwave infrared, and one thermal or longwave infrared). Each of these segments of the spectrum is called a band.

NASA Goddard Space Flight Center and US Geological Survey

Landsat's seven bands, each of which collects reflected
light from one narrow segment of the EM spectrum.

From each band an image is produced in which the brightest pixels are the ones that reflect the most energy. Since each surface responds differently to the various wavelengths, the brightest pixels will not be the same ones in every case; compare, for instance, bands 3 and 4.

Band 1 – Blue

Band 2 – Green

Band 3 – Red

Band 4 – NIR

Band 5 – SWIR

Band 6 – Thermal

Band 7 - SWIR

The analyst then chooses which bands to work with, depending on the subject of interest. Our eyes are designed to see three primary colors (blue, green, and red) and their combinations. So when viewing the images with image processing software, the analyst will often view three bands at a time, and display them in those three colors so the content of each can be distinguished from the others. If the three visible light bands are selected, and displayed in their matching colors, the result looks much as the scene would to our eyes if we were viewing it from space. (See Image 1, a portion of the coast of Greece, below.) This is called a "true color" image.

Image 1

Image 2

Image 3

Other bands can be assigned to study other wavelengths. For a more complete explanation, see The Longer Version.

Canadian Centre for Remote Sensing

The "Invisible" Bands

Band 4, the Near Infrared (NIR) band, is the band most often used to study vegetation health. When plants absorb sunlight in photosynthesis, they actually use only certain wavelengths, primarily red and blue visible light. This is why they most often appear green to us, because much of the green light is reflected. However, far more of the NIR band is reflected, making it by far the most reflectant band for healthy vegetation. In fact, the entire infrared (IR) portion of the spectrum is highly reflected.

Band 6, the Thermal Infrared band, can be used to create thermal maps to show temperature differences, such as can be seen on the Rachel Carson page, where the sea surface temperatures in the Florida Keys have increased in recent years.

The upper portion of the Florida Keys in June 1985 and July 2005, demonstrating long-term warming of the sea surface.

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The three infrared bands are useful for studying a variety of surfaces, from vegetation to rocks and minerals. And while not quite in the thermal range, they are close enough to it to detect heat better than the visible bands, as you can see in these two images of Mt. Etna in Sicily, the first showing the three visible light bands, the second showing the three infrared bands. The information that can be derived from each image is quite different: The visible light image shows the plume of steam drifting from the crater, while the infrared image shows the heat of the magma below the surface.

NASA Goddard Space Flight Center and US Geological Survey

More Ways of Viewing Data

Since Remote Sensing gives us not only visual images but also the data that create them, we can learn far more about what's on the ground by analyzing the data in a variety of ways. Some techniques used on this website include Vegetation Indexes, which are used to study vegetation health, such as in the study of the Maine Woods on the Thoreau page; and Tasseled Cap, which is used to look at the loss of wetlands on the Twain page. These techniques are explained in greater detail in The Longer Version. Together they help make visible the environmental changes that are damaging and disrupting our literary landscapes, and aid the people who care about them in understanding what might be done to restore and protect them.