A primary radar system sends a high-powered radio frequency from a rotating antenna and uses any reflected signals to determine the distance and speed of objects in the air or on water. The radio signal shows distance to an object from the time it took to make the round trip to the object. For radar used in aircraft control, the returning signal can also be used to determine the aircraft's approximate altitude, or height above ground. An antenna is a curved metal dish or structure that focuses a radio beam and transmits it in a specific direction.
Radar is an acronym, or shortened version, of the term "radio detection and ranging". First developed for aircraft detection in the 1930s, early radar had limited range due to the power limits for the antennas at that time. Although antenna power and software have improved, in the early 21st century the practical limit of air traffic primary radar is about 60 miles (100 kilometers).
The use of primary radar requires a great deal of signal power, because objects further from the antenna will reflect or send back a weak signal. At longer distances from the antenna, radar becomes unreliable as a way to determine aircraft position with only reflected signals. Increasing amounts of air traffic in the 20th century created a need for other aircraft positioning systems.
Beginning in the 1960s, aircraft began using transponders to assist in air traffic control. A transponder is both a receiver and a transmitter, which receives the radar signal from the primary radar and sends back a signal containing aircraft identification, altitude and speed information. This so-called secondary radar improves the aircraft position accuracy, because the transponder is powered by the aircraft and sends out a stronger signal than a primary radar signal.
Improved transponders starting in the late 20th century also provided additional information about the aircraft. Pilots could select settings that will tell an air traffic controller on the ground if the aircraft is hijacked, or under control of other persons, or if there is an emergency onboard. These active signals were sent to the secondary radar receiver located on the same antenna as the primary radar, and can be viewed on traffic control screens.
Boats on the water can also be detected with radar systems, with some limitations. High waves can mask or hide the radar returns of smaller boats, and the curvature or shape of the Earth make it impossible to see boats below the horizon. Large military ships may use radar-confusing shapes or coatings that absorb radar to make them appear as much smaller boats on radar screens.
Radar can also be used to detect weather. Water molecules in clouds can reflect some frequencies of radar signals, which will show rain-containing clouds. Early systems could only see moving raindrops, but systems since the late 20th century can detect moisture even without rain.
Doppler radar can detect the speed and direction of water droplets moving through air. The reflected signal is analyzed by software that shows whether the signal is moving toward or away from the antenna. It can show rotation indicating a possible tornado, even at night or when hidden by heavy rain.