As the name implies, radar (radio detection and ranging) is the underpinning principle of the system. The system transmits radio waves down to the ground and measures the time it takes them to be reflected back up to the aircraft. The altitude above the ground is calculated from the radio waves' travel time and the speed of light.[1] Radar altimeters required a simple system for measuring the time-of-flight that could be displayed using conventional instruments, as opposed to a cathode ray tube normally used on early radar systems.