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Absolute altitude
The vertical distance of an aircraft from the air to the sea
absolute altitude is the vertical distance of an aircraft above the ground or sea, also known as "altitude". The vertical distance from standard atmospheric sea level, or
Mean sea level
Do the standard height. The height of the terrain and features marked on the navigation map is calculated according to the absolute height
[1-2]
.
- Chinese name
- Absolute altitude
- Foreign name
- absolute altitude
- spelling
- jue Duigo du
- connotative
- The vertical distance of an aircraft from the air to the sea
- alias
- altitude
- Single position
- km
Absolute height is also called"
Altitude
". Be higher than
Mean sea level
(also known as
geoid
The vertical height is called the absolute height. The heights shown on the map are all absolute heights. Since sea level is not a standard level, countries try to draw their maps as close to the standard level as possible. The absolute height of all parts of China is calculated from the origin of Qingdao Yellow Sea level (i.e. the average sea water level of the Yellow Sea).
Absolute height is generally measured by a barometric altimeter, and the measurement principle is as follows:
Barometric altimeter assumes that the law of atmospheric pressure decreasing with altitude is obtained according to the following formula:
Formula,
-- atmospheric pressure;
- the altitude of the plane;
- the mass density of air;
-- The acceleration of gravity. In the above formula, it is assumed that at altitudes below 11,000 meters, the temperature changes with height according to the following standard law:
Where, T is equivalent to the atmospheric temperature at the height of H;
- atmospheric temperature at the ground; H -- the height of the pair;
- Decrease rate of atmospheric temperature with altitude.
According to the above laws, the atmospheric pressure formula can be obtained as follows:
By solving the above formula, the formula of standard atmospheric pressure and altitude H can be obtained as follows:
The above formula assumes the rate of temperature lapse in the standard atmosphere
C/m, the temperature at sea level
Atmospheric pressure at sea level
760 millimeters of mercury. In the atmosphere above 11,000 meters, the temperature is assumed to remain a constant of 216.5K, so that:
Formula,
- 11,000 meters high;
- The equivalent of 11,000 meters of atmospheric pressure.
The altitude indicated by an ordinary barometric altimeter is only a function of atmospheric pressure and has nothing to do with atmospheric temperature change. Only at standard atmospheric temperature and standard sea level pressure can the altimeter give the correct number of absolute altitude
[3]
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Also known as gravity potential height, pressure height. According to the atmospheric pressure measured during the flight, the corresponding altitude is found from the standard atmospheric table. The altimeter in the aircraft is carved according to the correspondence between the atmospheric pressure value and the altitude value in the standard atmospheric table. If the pressure scale of the barometric altimeter is adjusted to the standard atmospheric state, the altitude indicated by the barometric altimeter at this time is called the standard barometric altitude. In order to prevent collisions, the standard air pressure height is used when the aircraft is flying long distance and in separate flights.
(Geometric height)
True height is also called geometric height or tape height. The height (distance) of an aircraft along the plumb line to the Earth's surface during flight. Usually, it can be measured by a radio altimeter, radar altimeter, laser altimeter, or photographic theodolite. Aircraft in the take-off, landing, ultra-low altitude flight, bombing, reconnaissance, search, rescue and agriculture and forestry operations and other tasks, as well as drones, aerial missiles in ultra-low altitude, sea skimping flight need to know the geometric height.
In many cases, not only the atmospheric pressure, temperature and other parameters of different sea levels are different, but also the atmospheric parameters of the same sea level are constantly changing in different seasons and different times. Therefore, barometric altimeters generally do not indicate the height relative to the actual sea level. In flight, the vehicle is relative to
Mean sea level
The height, that is, the vertical distance of the aircraft to the mean sea level is called the absolute height, and the absolute height is also called the altitude.
General barometric altimeters also do not reflect non-standard sea level conditions and altitudes relative to a specified location (e.g., airfield, missile launch site, test range). The altitude of an aircraft relative to a specified location expressed by the height of the gravitational potential or the height of the air pressure is called the relative altitude. Note that relative height is also different from true height. The prototype of the anti-ship missile, such as the Soviet P.-15 missile, used a pneumatic altimeter and its flight height was divided into three levels, that is, 100m, 200m, 300m. These heights are bound at the technical position or launch position before the missile is launched, so they are relative to the altitude there, that is, relative altitude. At this time, the output zero of the pneumatic altimeter should be pre-bound
[2]
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The altitude of an aircraft is its vertical distance in the air from a datum. The flight altitude obtained will be different depending on the datum level used to measure the altitude of the aircraft.
[4]
Relationships of various heights
As can be seen from the diagram, when the aircraft is flying flat, the relative height and absolute height remain unchanged, and the true height will change with the change of the landmark height directly below the aircraft. Because atmospheric pressure often changes, the standard air pressure plane will also change with the change in atmospheric pressure, so the standard air pressure height will change with the change in the standard air pressure plane position directly below the aircraft. If the standard air pressure plane coincides exactly with sea level, the standard air pressure height is equal to the absolute altitude. The vertical distance between the datum of absolute height and relative height is the height of the airport (or shooting range), and the relationship between them can be expressed as follows:
Absolute height = relative height + airfield (shooting range) elevation
The vertical distance between the standard air pressure altitude and the datum of the relative altitude is the standard air pressure altitude of the airport (or shooting range), and the relationship between them can be expressed as follows:
Standard air pressure altitude = relative altitude + airport (shooting range) standard air pressure altitude
The vertical distance between the datum plane of the absolute height and the true height is the elevation of the place, and the relationship between them can be expressed as follows:
Absolute height = true height + site elevation
From the above relationship, it can be seen that in flight, if the altitude of the airport (range), the altitude of the site and the standard air pressure of the airport (range) are known, another altitude can be calculated from the indication of the instrument. If the absolute and true altitude of the aircraft is measured by the barometric altimeter and the radio altimeter respectively, the local surface elevation (elevation) can be calculated according to the following formula, thus providing the original data of terrain matching:
Site elevation = absolute height - true height
