RADAR EXTENDS the range of the human eye. It can see through darkness, snow and cloud cover to locate objects and measure their distance. The principle is familiar. Dolphins and bats emit sound waves to gather information from their surroundings by sensing the echoes. Radar works the same way.

Electricity is converted into radio or microwaves which are beamed from an antenna at the speed of light. When the signal -- usually a group of pulses -- hits an object, the waves bounce back to the antenna, which has momentarily stopped transmitting so it can listen. Altered from bouncing off, say, an aircraft, the echoes are collected and processed by a receiver. Each microsecond (millionth of a second) of round-trip travel corresponds to a distance of about 500 feet. Additional information can be extracted by analyzing the alteration in each wave's shape.

Some radars do surveillance, scanning broad areas. Others are geared for targeting, where pencil-thin beams track a target continuously. And some, such as phased arrays, can search for new targets and track existing ones simultaneously.

Like the multiple frequencies of radio broadcasts, radar sensors can transmit on various channels. High frequencies (and thus short wavelengths, down to a few millimeters) are often preferred because the components, particularly the antennas, can be made smaller and the beam can be focused better. But transmissions in these frequencies degrade more rapidly in bad weather and the systems are often more vulnerable to countermeasures than systems using wavelengths of several meters. Many military radars use variable frequencies -- sometimes hopping across the spectrum bands randomly -- to foil enemy countermeasures.