Uploaded by harshnycus

Altimeter - Wikipedia

advertisement
5/18/2021
Altimeter - Wikipedia
Altimeter
An altimeter or an altitude meter is an instrument used to
measure the altitude of an object above a fixed level. The
measurement of altitude is called altimetry, which is related to the
term bathymetry, the measurement of depth under water. The most
common unit for altimeter calibration worldwide is hectopascals
(hPa), except for North America and Japan where inches of mercury
(inHg) are used.[1] To obtain an accurate altitude reading in either
feet or meters, the local barometric pressure must be calibrated
correctly.
Contents
Pressure altimeter
Use in hiking, climbing and skiing
Skydiving
Use in aircraft
Diagram showing the face of the
"three-pointer" sensitive aircraft
altimeter displaying an altitude of
10,180 ft (3,100 m) and a pressure
reading of about 29.92 inHg (1013
hPa).
Use in ground effect vehicle
Sonic altimeter
Radar altimeter
Global Positioning System
Other modes of transport
See also
References
External links
Pressure altimeter
Altitude can be determined based on the measurement of atmospheric pressure. The greater the altitude,
the lower the pressure. When a barometer is supplied with a nonlinear calibration so as to indicate
altitude, the instrument is called a pressure altimeter or barometric altimeter. A pressure altimeter is the
altimeter found in most aircraft, and skydivers use wrist-mounted versions for similar purposes. Hikers
and mountain climbers use wrist-mounted or hand-held altimeters, in addition to other navigational
tools such as a map, magnetic compass, or GPS receiver.
The calibration of an altimeter follows the equation
[2]
https://en.wikipedia.org/wiki/Altimeter
1/7
5/18/2021
Altimeter - Wikipedia
where c is a constant, T is the absolute temperature, P is the
pressure at altitude z, and Po is the pressure at sea level. The
constant c depends on the acceleration of gravity and the molar
mass of the air. However, one must be aware that this type of
altimeter relies on "density altitude" and its readings can vary by
hundreds of feet owing to a sudden change in air pressure, such as
from a cold front, without any actual change in altitude.[3]
Use in hiking, climbing and skiing
Digital barometric pressure sensor
A barometric altimeter, used along with a topographic map, can help
for altitude measurement in
to verify one's location. It is more reliable, and often more accurate,
consumer electronic applications
than a GPS receiver for measuring altitude; the GPS signal may be
unavailable, for example, when one is deep in a canyon, or it may
give wildly inaccurate altitudes when all available satellites are near the horizon. Because barometric
pressure changes with the weather, hikers must periodically re-calibrate their altimeters when they reach
a known altitude, such as a trail junction or peak marked on a topographical map.
Skydiving
An altimeter is the most important piece of skydiving equipment,
after the parachute itself. Altitude awareness is crucial at all times
during the jump, and determines the appropriate response to
maintain safety.
Since altitude awareness is so important in skydiving, there is a wide
variety of altimeter designs made specifically for use in the sport,
and a non-student skydiver will typically use two or more altimeters
in a single jump:[4]
Hand, wrist or chest-mounted mechanical analogue visual
altimeters. This is the most basic and common type, and is
used by (and commonly mandated for) virtually all student
skydivers. The common design has a face marked from 0 to
4000 m (or 0 to 12000 ft, mimicking the clock face), on which an
arrow points to the current altitude. The face plate sports
sections prominently marked with yellow and red respectively,
signifying the recommended deployment altitude, as well as
emergency procedure decision altitude (commonly known as
"hard deck"). A mechanical altimeter has a knob that needs to be
manually adjusted to make it point to 0 on the ground before
jump, and if the landing spot is not at the same altitude as the
takeoff spot, the user needs to adjust it appropriately. Some
advanced electronic altimeters are also available which make
use of the familiar analogue display, despite internally operating
digitally.
Digital visual altimeters, mounted on the wrist or hand. This
type always operates electronically, and conveys the altitude as
a number, rather than a pointer on a dial. Since these altimeters
already contain all the electronic circuitry necessary for altitude
https://en.wikipedia.org/wiki/Altimeter
Digital wrist-mounted skydiving
altimeter in logbook mode,
displaying the last recorded jump
profile.
Skydiver in free fall, making use of a
hand-mounted altimeter. The
analogue face is visible, showing
colour-coded decision altitudes. The
depicted altimeter is electronic,
despite using an analogue display.
2/7
5/18/2021
Altimeter - Wikipedia
calculation, they are commonly equipped with auxiliary functions such as electronic logbook, realtime jump profile replay, speed indication, simulator mode for use in ground training, etc. An
electronic altimeter is activated on the ground before the jump, and calibrates automatically to point
to 0. It is thus essential that the user not turn it on earlier than necessary to avoid, for example, the
drive to a dropzone located at a different altitude than one's home which could cause a potentially
fatal false reading. If the intended landing zone is at a different elevation than the takeoff point, the
user needs to input the appropriate offset by using a designated function.
Audible altimeters (also known as "dytters", a genericised trademark of the first such product on the
market). These are inserted into one's helmet, and emit a warning tone at a predefined altitude.
Contemporary audibles have evolved significantly from their crude beginnings, and sport a vast array
of functions, such as multiple tones at different altitudes, multiple saved profiles that can be switched
quickly, electronic logbook with data transfer to a PC for later analysis, distinct free fall and canopy
modes with different warning altitudes, swoop approach guiding tones, etc. Audibles are strictly
auxiliary devices, and do not replace, but complement a visual altimeter which remains the primary
tool for maintaining altitude awareness. The advent of modern skydiving disciplines such as
freeflying, in which the ground might not be in one's field of view for long periods of time, has made
the use of audibles nearly universal, and virtually all skydiving helmets come with one or more builtin ports in which an audible might be placed. Audibles are not recommended and often banned from
use by student skydivers, who need to build up a proper altitude awareness regime for themselves.
Auxiliary visual altimeters. These do not show the precise altitude, but rather help maintain a
general indicator in one's peripheral vision. They might either operate in tandem with an audible
equipped with an appropriate port, in which case they emit warning flashes complementing the
audible tones, or be standalone and use another display mode, such as showing either green or red
light depending on the altitude.
Speaking altimeters (also known as voice altimeters). Another
type of altimeter that combines both audible and visual altimeter
functions. Unit has all necessary altitudes used in Skydiving and
announces it as a number in Skydiver's native language. These
are also inserted into helmet (the same pocket size as for
audibles), but emit voice with automatic volume adjustment
depend on the speed to clear hearing. Speaking altimeters
usually has software configuration via mobile application. Main
goal of this type of altimeter is strong security feature for
experienced skydivers, so they always know own current
position that very useful for FS load organizers or AFF
instructors as well.
Speaking Altimeter with helmet for
skydiving
The exact choice of altimeters depends heavily on the individual skydiver's preferences, experience level,
primary disciplines, as well as the type of the jump.[5] On one end of the spectrum, a low-altitude
demonstration jump with water landing and no free fall might waive the mandated use of altimeters and
use none at all. In contrast, a jumper doing freeflying jumps and flying a high performance canopy might
use a mechanical analogue altimeter for easy reference in free fall, an in-helmet audible for breakaway
altitude warning, additionally programmed with swoop guide tones for canopy flying, as well as a digital
altimeter on an armband for quickly glancing the precise altitude on approach. Another skydiver doing
similar types of jumps might wear a digital altimeter for their primary visual one, preferring the direct
altitude readout of a numeric display.
Use in aircraft
https://en.wikipedia.org/wiki/Altimeter
3/7
5/18/2021
Altimeter - Wikipedia
In aircraft, an aneroid barometer measures the atmospheric
pressure from a static port outside the aircraft. Air pressure
decreases with an increase of altitude—approximately 100
hectopascals per 800 meters or one inch of mercury per 1000 feet
near sea level.
The aneroid altimeter is calibrated to show the pressure directly as
an altitude above mean sea level, in accordance with a mathematical
model atmosphere defined by the International Standard
Atmosphere (ISA). Older aircraft used a simple aneroid barometer
where the needle made less than one revolution around the face
from zero to full scale. This design evolved to altimeters with a
primary needle and one or more secondary needles that show the
number of revolutions, similar to a clock face. In other words, each
needle points to a different digit of the current altitude
measurement. However this design has fallen out of favor due to the
risk of misreading in stressful situations. The design evolved further
to drum-type altimeters, the final step in analogue instrumentation,
where each revolution of a single needle accounted for 1,000 feet
(300 metres), with thousand foot increments recorded on a
numerical odometer-type drum. To determine altitude, a pilot had
first to read the drum to determine the thousands of feet, then look
at the needle for the hundreds of feet. Modern analogue altimeters
in transport aircraft are typically drum-type. The latest development
in clarity is an Electronic flight instrument system with integrated
digital altimeter displays. This technology has trickled down from
airliners and military planes until it is now standard in many general
aviation aircraft.
A drum-type aircraft altimeter,
showing the small Kollsman
windows at the bottom left
(hectopascals) and bottom right
(inches of mercury) of the face.
An old Aircraft Altimeter
Modern aircraft use a "sensitive altimeter". On a sensitive altimeter,
the sea-level reference pressure can be adjusted with a setting knob. The reference pressure, in inches of
mercury in Canada and the United States, and hectopascals (previously millibars) elsewhere, is displayed
in the small Kollsman window,[6] on the face of the aircraft altimeter. This is necessary, since sea level
reference atmospheric pressure at a given location varies over time with temperature and the movement
of pressure systems in the atmosphere.
In aviation terminology, the regional or local air pressure at mean
sea level (MSL) is called the QNH or "altimeter setting", and the
pressure that will calibrate the altimeter to show the height above
ground at a given airfield is called the QFE of the field. An altimeter
cannot, however, be adjusted for variations in air temperature.
Differences in temperature from the ISA model will accordingly
cause errors in indicated altitude.
In aerospace, the mechanical stand-alone altimeters which are based
on diaphragm bellows were replaced by integrated measurement
Diagram showing the internal
components of the sensitive aircraft
systems which are called air data computers (ADC). This module
altimeter.
measures altitude, speed of flight and outside temperature to
provide more precise output data allowing automatic flight control
and flight level division. Multiple altimeters can be used to design a
pressure reference system to provide information about the airplane's position angles to further support
inertial navigation system calculations.
https://en.wikipedia.org/wiki/Altimeter
4/7
5/18/2021
Altimeter - Wikipedia
Use in ground effect vehicle
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.[7]
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.[8]
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 traveled. 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 fighter aircraft to fly at very low
altitude.
Global Positioning System
Global Positioning System (GPS) receivers 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.[9] In hiking and climbing, it is common to find that
the altitude measured by GPS is off by as much as 400 feet (122
metres) depending on satellite orientation.[10]
Other modes of transport
The altimeter on this Piper PA-28 is
seen on the top row of instruments,
second from right
The altimeter is an instrument optional in off-road vehicles to aid in navigation. Some high-performance
luxury cars that were never intended to leave paved roads, such as the Duesenberg in the 1930s, have
also been equipped with altimeters.
Hikers and mountaineers use hand-held or wrist-mounted barometric altimeters, as do skydivers.
See also
Acronyms and abbreviations in avionics
https://en.wikipedia.org/wiki/Altimeter
5/7
5/18/2021
Altimeter - Wikipedia
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 geodesy
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
References
1. Aviation's Crazy, Mixed Up Units of Measure - AeroSavvy (https://aerosavvy.com/metric-imperial/)
2. Crocker, Graham Jackson, Chris. "The use of altimeters in height measurement" (http://www.hills-dat
abase.co.uk/altim.html). www.hills-database.co.uk. Archived (https://web.archive.org/web/201710250
75434/http://www.hills-database.co.uk/altim.html) from the original on 25 October 2017. Retrieved
29 April 2018.
3. "How Aircraft Instruments Work." (https://books.google.com/books?id=0SkDAAAAMBAJ&pg=PA118)
Popular Science, March 1944, p. 118.
4. "What's a Skydiving Altimeter (and How Does It Work?)" (http://skydivethewasatch.com/about/article
s/whats-a-skydiving-altimeter-and-how-does-it-work/). Skydive The Wasatch. Archived (https://web.a
rchive.org/web/20150423000923/http://skydivethewasatch.com/about/articles/whats-a-skydiving-alti
meter-and-how-does-it-work) from the original on 23 April 2015. Retrieved 2 February 2015.
5. Hawke, John. "Digital or Analog Altimeter" (http://www.dropzone.com/safety/Gear_and_Equipment/D
igital_or_Analog_Altimeter_1042.html). Dropzone.com. Archived (https://web.archive.org/web/20150
206181256/http://www.dropzone.com/safety/Gear_and_Equipment/Digital_or_Analog_Altimeter_104
2.html) from the original on 6 February 2015. Retrieved 2 February 2015.
6. "Archived copy" (http://www.stolaf.edu/people/hansonr/soaring/altimetr.htm). Archived (https://web.ar
chive.org/web/20060625001617/http://www.stolaf.edu/people/hansonr/soaring/altimetr.htm) from the
original on 2006-06-25. Retrieved 2006-06-15.
7. Nebylov, Prof. Alexander and Sharan Sukrit. "Comparative Analysis Of Design Variants For Low
Altitude Flight Parameters Measuring System". 17th IFAC Symposium for Automatic Control.
8. "Meter Gives Elevation" (https://books.google.com/books?id=9ycDAAAAMBAJ&pg=PA35), Popular
Science, March 1931
9. Albéri, Matteo; Baldoncini, Marica; Bottardi, Carlo; Chiarelli, Enrico; Fiorentini, Giovanni; Raptis,
Kassandra Giulia Cristina; Realini, Eugenio; Reguzzoni, Mirko; Rossi, Lorenzo; Sampietro, Daniele;
Strati, Virginia; Mantovani, Fabio (16 August 2017). "Accuracy of Flight Altitude Measured with LowCost GNSS, Radar and Barometer Sensors: Implications for Airborne Radiometric Surveys".
Sensors. 17 (8): 1889. arXiv:1802.00327 (https://arxiv.org/abs/1802.00327). doi:10.3390/s17081889
(https://doi.org/10.3390%2Fs17081889).
10. "Understanding the Accuracy of the GPS Elevation Reading" (https://support.garmin.com/en-US/?faq
=QPc5x3ZFUv1QyoxITW2vZ6). Garmin. Archived (https://web.archive.org/web/20200305121928/htt
https://en.wikipedia.org/wiki/Altimeter
6/7
5/18/2021
Altimeter - Wikipedia
ps://support.garmin.com/en-US/?faq=QPc5x3ZFUv1QyoxITW2vZ6) from the original on March 5,
2020. Retrieved March 14, 2020.
External links
Retrieved from "https://en.wikipedia.org/w/index.php?title=Altimeter&oldid=1002543384"
This page was last edited on 24 January 2021, at 23:24 (UTC).
Text is available under the Creative Commons Attribution-ShareAlike License; additional terms may apply. By using this site,
you agree to the Terms of Use and Privacy Policy. Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a
non-profit organization.
https://en.wikipedia.org/wiki/Altimeter
7/7
Download