Editing Altimeter

{{Short description|Instrument used to determine the height of an object above a certain point}}
{{Not to be confused with|attitude indicator}}
[[File:3-Pointer Altimeter.svg|right|thumb|Diagram showing the face of the "three-pointer" sensitive aircraft altimeter displaying an [[altitude]] of {{cvt|10180|ft|m}}. Reference pressure of about 29.92 [[inHg]] (1013 hPa) is showing in the ''Kollsman window'']]
An '''altimeter''' or an '''altitude meter''' is an instrument used to measure the [[altitude]] of an object above a fixed level.<ref>{{cite book|title=A Dictionary of Aviation |first=David W. |last=Wragg |isbn=0-85045-163-9 |url=https://archive.org/details/dictionaryofavia0000wrag |publisher=Osprey |publication-place=Reading |date=1973 |edition=1st |page=33 }}</ref> The measurement of altitude is called '''altimetry''', which is related to the term [[bathymetry]], the measurement of depth under water. 

==Types==

===Pressure altimeter===
{{excerpt|Pressure altimeter}}

=== Sonic altimeter ===
In 1931, the US Army Air Corps and General Electric 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.<ref>[https://books.google.com/books?id=9ycDAAAAMBAJ&pg=PA35 "Meter Gives Elevation"], ''Popular Science'', March 1931</ref>

=== Radar altimeter ===
{{main|Radar altimeter}}
[[File:Cockpit of Piper PA-28-151 (G-BOYH) at Bristol Airport, England 15May2016 arp.jpg|thumb|The altimeter on this [[Piper PA-28]] 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 vehicle]]s, as compared to laser, isotropic or ultrasonic altimeters.<ref>
Nebylov, Prof. Alexander and Sharan Sukrit.
"Comparative Analysis Of Design Variants For Low Altitude Flight Parameters Measuring System".
''17th IFAC Symposium for Automatic Control.''
</ref>

=== Laser altimeter ===
[[Lidar]] technology is used to help navigate the [[Ingenuity (helicopter)|helicopter Ingenuity]] on its record-setting flights over the terrain of [[Mars]] by means of a downward-facing Lidar altimeter.<ref>{{cite news|url=https://spectrum.ieee.org/nasa-designed-perseverance-helicopter-rover-fly-autonomously-mars|title=How NASA Designed a Helicopter That Could Fly Autonomously on Mars|date=17 February 2021|work=IEEE Spectrum|access-date=19 February 2021 |archive-date=19 February 2021|archive-url=https://web.archive.org/web/20210219054558/https://spectrum.ieee.org/automaton/aerospace/robotic-exploration/nasa-designed-perseverance-helicopter-rover-fly-autonomously-mars|url-status=live}}</ref>

===Global Positioning System===
[[Global Positioning System]] (GPS) receivers can also determine altitude by [[trilateration]] with four or more [[satellite]]s. In aircraft, altitude determined using autonomous GPS is not reliable enough to supersede the pressure altimeter without using some method of [[GNSS Augmentation|augmentation]].<ref name="Albéri2017">{{cite journal|last1=Albéri|first1=Matteo|last2=Baldoncini|first2=Marica|last3=Bottardi|first3=Carlo|last4=Chiarelli|first4=Enrico|last5=Fiorentini|first5=Giovanni|last6=Raptis|first6=Kassandra Giulia Cristina|last7=Realini|first7=Eugenio|last8=Reguzzoni|first8=Mirko|last9=Rossi|first9=Lorenzo|last10=Sampietro|first10=Daniele|last11=Strati|first11=Virginia|last12=Mantovani|first12=Fabio|title=Accuracy of Flight Altitude Measured with Low-Cost GNSS, Radar and Barometer Sensors: Implications for Airborne Radiometric Surveys|journal=Sensors|date=16 August 2017|volume=17|issue=8|pages=1889|doi=10.3390/s17081889|pmid=28813023|pmc=5579878|arxiv=1802.00327|bibcode=2017Senso..17.1889A|doi-access=free}}</ref> In hiking and climbing, it is common to find that the altitude measured by GPS is off by as much as {{convert|400|ft|m|0|abbr=off}} depending on satellite orientation.<ref name="Garmin support">{{cite web|url=https://support.garmin.com/en-US/?faq=QPc5x3ZFUv1QyoxITW2vZ6|title=Understanding the Accuracy of the GPS Elevation Reading|website=Garmin|access-date=March 14, 2020|archive-url=https://web.archive.org/web/20200305121928/https://support.garmin.com/en-US/?faq=QPc5x3ZFUv1QyoxITW2vZ6|archive-date=March 5, 2020|url-status=dead}}</ref>

==See also==
{{div col|colwidth=35em}}
* [[Acronyms and abbreviations in avionics]]
* [[International Civil Aviation Organization#Use of the International System of Units|ICAO recommendations on use of the International System of Units]]
* [[Flight instruments]]
* [[Flight level]]
* [[Hypsometer]]
* [[Jason-1]] and [[Ocean Surface Topography Mission]] (Jason-2) are satellite missions that use altimeters to measure sea surface height
* [[Level sensor]]
* [[Lidar]]
* [[Pressure sensor]]
* [[Primary flight display]]
* [[Radar altimeter]]
* [[Satellite altimetry]]
* [[Turkish Airlines Flight 1951]], an accident attributed to a malfunctioning radio altimeter
* [[United Airlines Flight 389]], an accident attributed to misreading of an altimeter
* [[Variometer]], a gauge measuring the rate of change of altitude
{{div col end}}

==References==
{{reflist}}

==External links==
{{Commons}}

{{flight instruments}}
{{Aircraft components}}

[[Category:Altimeters| ]]