For series introduction, see GC8A2QP.
Radar was invented during World War II to enable the Allied forces to see enemy aircraft coming. Radar works on the principle of transmitted radio waves bouncing back to reveal the location of an object. The amount of time between the transmission of the signal and receiving the reflected signal can be used to calculate where the object is and how fast it is moving.
Radar can be either primary or secondary; each has its benefits. Primary radar detects any object that will reflect a signal, whether that is an airplane, a flock of birds, a rainshower, or even a truck driving down the highway. Secondary radar only picks up signals from aircraft transponders (so, it won't pick up birds, or planes without transponders). These transponders provide specific information, e.g. an aircraft's altitude and an assigned "squawk" code that is matched up to a specific aircraft on an air traffic controller's radar display.
Radar is one source that air traffic control displays use to display the location of aircraft. However, not every aircraft will actually appear on a radar display, because radar operates by line-of-sight. High-flying aircraft can be detected easily enough. However, if a plane is flying at a low altitude and/or far away from the radar antenna, it may be flying "under the radar," as illustrated by the fighter jet in the image below. In such a case, air traffic controllers rely on position reports from pilots to keep aircraft separated from each other.
N 48° 32.283 W 81° 19.991
