Cynthia B. Erickson and Janet W. Campbell

On dark clear nights, you can see satellites moving slowly through the sky. They appear like stars, but unlike the fixed constellations in the celestial sphere, these “stars” move from horizon to horizon in the time span of a few minutes. These tiny star-like specks of light are the result of a scientific revolution that has taken place over the last 40 years. They are not stars, of course, but satellites orbiting the Earth every 100 minutes or so carrying cameras and sensors. These satellites are studying the Earth’s spheres — her atmosphere (air), hydrosphere (water), cryosphere (ice), and biosphere (all living organisms) — using a technology known as remote sensing.

This primer presents some of the fundamental concepts of remote sensing which we hope will give you a basis for understanding satellite imagery. Perhaps the most famous image of the Earth from space is the “blue marble” — a photograph of the Earth taken by Apollo 17 astronauts on their journey to the Moon in December 1972.

Most remote sensing images, however, are not photographs but rather digital images produced by computers. Although some digital images might look like photographs, others look like abstract paintings. A digital image is made up of picture elements — called “pixels” — arranged horizontally and vertically on the computer screen or printed page. (Play movie to right.) In a satellite image each pixel corresponds to a particular location on the Earth which is actually a square area of finite dimensions rather than a point location. The pixel is colored or shaded according to the data acquired by the satellite for that particular location. The degree of realism involved in the final image and the selection of colors used to represent the data are controlled by the computer operator. In a sense, each image is an artistic rendering of scientific data.