Digitized images taken from surveillance aircraft, like the image that appears in this activity from the NOVA Web site, are not like traditional optical photographs. They are instead computer-generated interpretations of information contained in radio waves. Radar devices work by measuring the time it takes a radio wave to travel from a transmitter to an object and back to a receiver. Distance to the object is calculated based on the time interval measured. This is useful for detecting an object's position and assessing the direction and speed it's moving (if it is, in fact, moving). To create a detailed, three-dimensional image, more data is needed.

Synthetic aperture radar (SAR) devices are capable of capturing that additional data. From its on-board location, a SAR device sends pulses of high-frequency radio waves toward the ground. The pulses return to the device, but do so with varying amounts of energy depending on the characteristics of the objects off which they're bounced. SAR devices have short transmitting antennae. Operating over a long distance, however, they can effectively simulate a much longer, more powerful antenna.

The energy contained in a returned radar beam does not provide much useful information. It is only through powerful software and processing that we can combine multiple readings taken over time and distance to deduce what's down there. The more information processed, the higher the resolution of the image.

A digitized image is made up of pixels, which are very small squares that appear as dots on a computer monitor or ink on a printout. The resolution of a digital image is measured by counting the number of pixels across and down. With SAR processing, as with other powerful imaging systems, about a thousand calculations are performed for each pixel, producing an image containing several million pixels, like that of Washington, D.C., which appears in this activity.