GSP models > Atmospheric models

It is common in dealing with airborne research data to encounter many different altitude terms. These include geometric altitude, GPS altitude, INS altitude, pressure altitude, geopotential height, and so on. Despite the nomenclature, there are only two altitude scales involved: geometric altitude and geopotential altitude or height.

Geometric altitude

Geometric altitude is the scale we are most familiar with; it is what we would measure with a tape measure.

Geopotential altitude

Geopotential altitude is based on a scale that relates altitude to gravitational equipotentials, or surfaces of constant gravitational potential energy per unit mass. Although geopotential altitude approximates geometric height, they are not equal. An important type of geopotential height is pressure altitude, which is based on a standard atmospheric model for temperature as a function of pressure. One particular model, the International Standard Atmosphere (ISA), is what all aircraft altimeters use to relate static pressure measurements on an aircraft to a corresponding pressure altitude scale. There are also a number of additional altitude terms related to flying airplanes such as true altitude, indicated altitude, absolute altitude and density altitude.

Pressure Altitude

Although pressure altitude is a type of geopotential height, it is treated separately because of it's importance in atmospheric research. The pressure altitude scale is based on the International Civil Aviation Organization's (ICAO) International Standard Atmosphere (ISA) (see ref. [8]). It can be used to establish, using the hydrostatic equation and the ideal gas law, a relationship between pressure and pressure altitude, using geopotential height. It differs from "normal" geopotential height in that it is based on a model and it assumes that the humidity is zero. The model seldom looks like the actual atmosphere a plane is flying in, and real atmospheres never have zero humidity.

Pressure altitude is used so that aircraft, which use static pressure to determine altitude, can agree upon what "altitude" they are flying at without having to continually update their altimeters with local pressure corrections. Technically, this is only true above 18,000 feet (FL180). Below this altitude in North America aircraft make local altimeter corrections to ensure that they are flying at the correct altitude. Using this definition for pressure altitude, a pilot can say "I'm at Flight Level 330." (that is 33,000 feet), instead of "I'm at 262 hPa." Pressure just isn't very intuitive since it's logarithmic with altitude, and it also decreases with altitude. In addition to pressure altitude, there are five additional altitude scales relevant to aviation.

Relation between geometric altitude and geopotential altitude:

h = (Re * H) / (Re - H)

where Re = 6 356 766 m, the nominal radius of the earth
H = geopotential altitude, m
h = geometric altitude, m

The parameter name for geopotential altitude (H) or pressure altitude (since we use the ICAO ISA) in GSP is 'Zp'. The SAE ARP 755 standard parameter name is 'alt'.