An altimeter or an altitude meter is an instrument used to measure the

altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...

of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term

bathymetry Bathymetry (; ) is the study of underwater depth of ocean floors ('' seabed topography''), river floors, or lake floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of wate ...

, the measurement of depth under water.

Types

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

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 Terrain-following radar (TFR) is a military aerospace technology that allows a very-low-flying aircraft to automatically maintain a relatively constant altitude above ground level and therefore make detection by enemy radar more difficult. It is ...

allowing

combat aircraft A military aircraft is any fixed-wing or rotary-wing aircraft that is operated by a legal or insurrectionary military of any type. Some military aircraft engage directly in aerial warfare, while others take on support roles: * 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 vehicle A ground-effect vehicle (GEV), also called a wing-in-ground-effect (WIGE or WIG), ground-effect craft/machine (GEM), wingship, flarecraft, surface effect vehicle or ekranoplan (), is a vehicle that is able to move over the surface by gaining su ...

s, as compared to laser, isotropic or ultrasonic altimeters.

Laser altimeter

Lidar Lidar (, also LIDAR, an acronym of "light detection and ranging" or "laser imaging, detection, and ranging") is a method for determining ranging, ranges by targeting an object or a surface with a laser and measuring the time for the reflected li ...

technology is used to help navigate the helicopter Ingenuity on its record-setting flights over the terrain of

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

by means of a downward-facing Lidar altimeter.

Global Positioning System

Global Positioning System The Global Positioning System (GPS) is a satellite-based hyperbolic navigation system owned by the United States Space Force and operated by Mission Delta 31. It is one of the global navigation satellite systems (GNSS) that provide ge ...

(GPS) receivers can also determine altitude by

trilateration Trilateration is the use of distances (or "ranges") for determining the unknown position coordinates of a point of interest, often around Earth ( geopositioning). When more than three distances are involved, it may be called multilateration, f ...

with four or more

satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...

s. 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.

References

External links

{{Aircraft components