Geog 280: Basic Geographic Techniques

Exercise 7: Satellite Imagery: Electromagnetic Energy and Bands

Now we turn to a different kind of remote sensing.  Satellites have been producing imagery since the 1960s, and satellites designed for mapping and GIS work have been orbiting Earth since 1972.  Our first order of business is to understand how satellites gather data.  Few satellites use conventional photography.  To understand their work, we first examine the characteristics of energy.  Then in the next pages we see how satellites use energy to gather images of Earth.  Finally, we will examine some specific satellites and the imagery they have produced.

Electromagnetic Energy

All remote sensing relies on energy that travels through space.  Within our atmosphere or in the oceans, we can detect objects with other means, such as by sound (as sonar does in water) or by motion (as seismometers do with earthquakes).  But satellites travel in space, and no sound or materials exist there -- at least not enough to matter.  So we must rely on energy that can travel through space.  All the energy that travels through the vacuum of space is known as electromagnetic energy.  Electromagnetic energy includes visible light such as we receive from the Sun, and also other energy such as x-rays, ultraviolet rays (which cause sunburns) and radio waves.

All electromagnetic energy is very similar.  In fact you can say exactly what kind of energy you are receiving if you know one trait of the energy:  its wavelength.  Electromagnetic energy can be described as traveling in waves, similar to how energy travels across the ocean's surface in waves of motion.  Energy can also be described as particles (photons), but we won't worry too much about these details of physics here.

The wavelength is simply the distance between crests of the waves.  The distance between wave troughs would be the same.  For electromagnetic energy, the wavelengths can range from microscopic to many feet or meters.  The range of wavelengths of energy is known as the electromagnetic spectrum.  The light that is visible to your eyes is a small part of the electromagnetic spectrum.  Visible light rays are approximately 1/1000th of a millimeter in wavelength -- very small!  A unit used for small distances like this is the micrometer, which is 1/1000th of a millimeter, or 1/1,000,000th (millionth) of a meter.  The micrometer is abbreviated with the symbol mm.  The m is the Greek letter mu and is the equivalent of our "m" letter; it may not display correctly on all browsers/computers.  The micrometer could have been "mm" but obviously that's already taken by millimeter.

Electromagnetic Energy Bands for Remote Sensing

Remote sensing of Earth most frequently uses energy in the visible, infrared, and microwave portions of the spectrum.  The Sun gives off most of its energy in visible and infrared, so that these are the "brightest" parts when we use naturally-occurring energy (it also helps explain why eyes are sensitive to what we call visible light -- there's lots of energy in those wavelengths to see).  Microwaves are also useful when we generate them ourselves and record the echo, which is what radar is all about.

A band is a specific range of wavelengths.  The visible band is the wavelengths our eyes normally can see, and it ranges from 0.4 mm (micrometers) to 0.7 mm.  The visible band can be subdivided into other bands, most often into the blue (0.4-0.5 mm), green (0.5-0.6 mm), and red (0.6-0.7 mm) bands.  Several parts of the infrared band are often recognized, including near infrared, mid-infrared and long infrared.  Because we sense longer infrared wavelengths as heat, this part of the infrared is often called thermal infrared.

Some bands are better than others for remote sensing.  The Sun provides lots of energy in the visible and infrared wavelengths, so these are used heavily.  Other wavelengths, such as x-rays and radio waves, have little energy from the Sun available so are not used from remote sensing.  Another problem with some wavelengths is that Earth's atmosphere absorbs them and blocks their transmission.  Ultraviolet, blue light, and parts of the infrared band are not as useful for remote sensing for this reason.  We discussed previously the scattering of blue light as a reason natural-color film is rarely used for aerial photography.

Multispectral Scanners

Satellites for Earth remote sensing most often record data about the Earth in one or more bands.  The earliest such satellite, Landsat, recorded four separate bands simultaneously:  red, green, and two near infrared bands.  Because the satellite sensor records several bands at a time, this kind of remote sensing is called multispectral scanning.

More recent satellites have expanded the number of bands they record.  A more recent satellite sensor records seven separate bands simultaneously, for example.  Some "hyperspectral" satellite sensors are under development now that may sense several hundred different bands at a time.  Each band would be a small slice of the visible or infrared spectrum.

Questions on this Page

7. What would the world look like if your eyes, instead of being sensitive to the visible wavelengths, were instead sensitive to X-rays or radio waves?  Would things in general be light or dark?

Bryan Baker, Sonoma State University, bryan.baker@sonoma.edu
Updated 17 February 1999