T/G Layout 1

Sensors onboard environmental satellites measure a vast array of information. In order

to understand how they work, and more generally, how/why remote sensing works, it

is important to understand electromagnetic radiation and the electromagnetic spectru m.

Electromagnetic radiation is composed of electric and magnetic fields that are generat-

ed by the oscillation of electrons in atoms or in a conducting material.

All matter, unless it has a temperature of absolute zero, emits electromagnetic radiant

energy or radiation. This radiation travels in electromagnetic waves—a coupled electric

and magnetic force field—that release energy when absorbed by another object. The

waves propagate through space at the speed of light. The full range of wave frequen-

cies—usually divided into seven spectral regions or bands—is the electromagnetic

spectrum. Note that the spectrum has rather that precise divisions. As shown in

the diagram, visible and ultraviolet are examples of electromagnetic radiation, differing

in their wavelengths (frequencies).

Most materials possess unique radiation pro p e rties or signatures, from which they can

be identified. In general, their emissivity (the rate at which they give out radiant energ y )

is determined by characteristics such as chemical composition, crystal structure, grain

size, surface roughness, etc.

In free space, electric and magnetic fields are at right angles to each other and main-

tain their relative positions during wave transmission. Radiation from some sources,

such as the Sun, doesn’t have any clearly defined polarization (electrical or magnetic

alignment)—meaning the electrical field assumes different directions at random.

However, waves can be polarized or aligned. Polarized paper, sunglasses, or camera

lens will demonstrate how effectively parallel slits filter light. Hold the filtering medium

horizontally up to a light source (light bulb, flashlight, projector) and note the passage

of light. Turn the medium vertically or add a second filter held vertically and see the

light disappear.

figure 43.

RE M O T E SE N S I N G A N D T H E

EL E C T R O M A G N E T I C SP E C T R U M

Radio
Microwave

Infrared

Visible

Ultraviolet

X-ray

Gamma ray