Radio Detection And Ranging, RADAR, is one of the primary methods of detection and navigation used today. Radio waves are sent out, and based on how they return bouncing off obstacles in the way, radar operators can take note of objects beyond the horizon. This is similar to how a bats echolocation operates.
At first, radar could only be mounted on large stations such as radar sites and ships, but eventually could be made small enough to be mounted on planes. Watch with a youtube video downloader the significance of radar in deciding the Battle For Britain in World War II. The German Luftwaffe often already had fuel limits just getting over the channel to attack Britain directly. The Royal Air Force, in being able to find them quickly, wasted much less fuel and could attack the enemy when it was most advantageous to them.
The radar in Radio Control Towers in airports direct aircraft landing and takeoff. Radar is also used in science for mapping and analysis of both terrestrial and astronomical events.
Today, radar is used not just to spot enemy aircraft but even directly as a guidance system for guns and missiles. One of the most important functions of stealth is to defeat or delay radar detection for planes, ships, and vehicles.