Pressure altimeter Sonic altimeter In 1931, the US Army Air corps and General Electric together tested a
sonic altimeter for aircraft, which was considered more reliable and accurate than one that relied on air pressure when heavy fog or rain was present. The new altimeter used a series of high-pitched sounds like those made by a bat to measure the distance from the aircraft to the surface, which on return to the aircraft was converted to feet shown on a gauge inside the aircraft cockpit.
Radar altimeter is seen on the top row of instruments, second from right. A
radar altimeter measures altitude more directly, using the time taken for a radio signal to reflect from the surface back to the aircraft. Alternatively, Frequency Modulated Continuous-wave radar can be used. The greater the frequency shift the further the distance travelled. This method can achieve much better accuracy than the pulsed radar for the same outlay and radar altimeters that use frequency modulation are industry standard. The radar altimeter is used to measure height above ground level during landing in commercial and military aircraft. Radar altimeters are also a component of terrain avoidance warning systems, warning the pilot if the aircraft is flying too low, or if there is rising terrain ahead. Radar altimeter technology is also used in
terrain-following radar allowing
combat aircraft to fly at very low height above the terrain. After extensive research and experimentation, it has been shown that "phase radio-altimeters" are most suitable for
ground effect vehicles, as compared to laser, isotropic or ultrasonic altimeters.
Laser altimeter Lidar technology is used to help navigate the
helicopter Ingenuity on its record-setting flights over the terrain of
Mars by means of a downward-facing Lidar altimeter.
Satellite navigation Satellite navigation receivers like those used with the
Global Positioning System (GPS) can also determine altitude by
trilateration with four or more
satellites. In aircraft, altitude determined using autonomous GPS is not reliable enough to supersede the pressure altimeter without using some method of
augmentation. In hiking and climbing, it is common to find that the altitude measured by GPS is off by as much as depending on satellite orientation. ==See also==