not the indicated altitude of the standard barometric altimeter. A radar altimeter measures absolute altitude - the height Above Ground Level (AGL). Absolute altitude is sometimes referred to as height because it is the height above the underlying terrain. As of 2010, all commercial radar altimeters use linear frequency modulation - continuous wave (LFM-CW or FM-CW). As of 2010, about 25,000 aircraft in the US have at least one radio altimeter. History Original concept The underlying concept of the radar altimeter was developed independent of the wider radar field, and originates in a study of long-distance telephony at Bell Labs. During the 1910s, Bell Telephone was struggling with the reflection of signals caused by changes in impedance in telephone lines, typically where equipment connected to the wires. This was especially significant at repeater stations, where poorly matched impedances would reflect large amounts of the signal and made long-distance telephony difficult. Engineers noticed that the reflections appeared to have a "humpy" pattern to them; for any given signal frequency, the problem would only be significant if the devices were located at specific points in the line. This led to the idea of sending a test signal into the line and then changing its frequency until significant echos were seen, and then determining the distance to that device so it could be identified and fixed. Lloyd Espenschied was working at Bell Labs when he conceived using this same phenomenon to measure distances in a wire. One of his first developments in this field was a 1919 patent (granted 1924) on the idea of sending a signal into railway tracks and measuring the distance to discontinuities. These could be used to detect broken tracks, or if the distance was changing more rapidly than the speed of the train, other trains on the same line. Appleton's ionosphere measurements During this same period there was a great debate in physics over the nature of radio propagation. Guglielmo Marconi's successful trans-Atlantic transmissions appeared to be impossible. Studies of radio signals demonstrated they travelled in straight lines, at least over long distances, so the broadcast from Cornwall should have disappeared into space instead of being received in Newfoundland. In 1902, Oliver Heaviside in the UK and Arthur Kennelly in the USA independently postulated the existence of an ionized layer in the upper atmosphere that was bouncing the signal back to the ground so it could be received.