A radar altimeter (RA), also called a radio altimeter (RALT), electronic altimeter, reflection altimeter, or low-range radio altimeter (LRRA), measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it takes a beam of radio waves to travel to ground, reflect, and return to the craft. This type of altimeter provides the distance between the antenna and the ground directly below it, in contrast to a barometric altimeter which provides the distance above a defined vertical datum, usually mean sea level. Principle 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. 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. To do this, the transmitter sends a frequency modulated signal that changes in frequency over time, ramping up and down between two frequency limits, Fmin and Fmax over a given time, T. In the first units, this was accomplished using an LC tank with a tuning capacitor driven by a small electric motor. The output is then mixed with the radio frequency carrier signal and sent out the transmission antenna. Since the signal takes some time to reach the ground and return, the frequency of the received signal is slightly delayed relative to the signal being sent out at that instant. The difference in these two frequencies can be extracted in a frequency mixer, and because the difference in the two signals is due to the delay reaching the ground and back, the resulting output frequency encodes the altitude. The output is typically on the order of hundreds of cycles per second, not megacycles, and can easily be displayed on analog instruments. This technique is known as Frequency Modulated Continuous-wave radar. Radar altimeters normally work in the E band, , or, for more advanced sea-level measurement, S band. Radar altimeters also provide a reliable and accurate method of measuring height above water, when flying long sea-tracks. These are critical for use when operating to and from oil rigs. The altitude specified by the device is