World Geodetic System

The World Geodetic System 1984 ( WGS 84) is a geodetic reference system provide a uniform basis for positional information on Earth and in the near-Earth space.

Components

The system consists of:

  • A reference ellipsoid, which is adapted in its simplicity as possible the earth's surface, for locations by latitude and longitude;
  • A detailed model for the earth's shape deviates from this idealized form, called the geoid; currently the EGM96 is ( Earth Gravitational Model 1996);
  • A set of three-dimensional coordinates of the twelve distributed over the earth fundamental stations for anchoring the above models in the earth's crust.

Coordinate system

The coordinate system in which the reference ellipsoid, the geoid and the situation of fundamental stations are defined, is a Cartesian legal system (Z points towards the North Pole, X toward 0 ° longitude and latitude, Y by 90 ° East ). Its definition follows the requirements of the IERS:

  • It is centered on the center of gravity of the earth, including atmospheric mass.
  • Its length scale is local, in the relativistic curved by the mass of earth space.
  • The orientation of its axes relative to the Earth's crust ( reference pole and reference meridian) corresponded to the epoch 1984.0 of the IERS (then BIH).
  • Global The temporal evolution of its orientation follows the rotation of the earth's crust (the movement of the tectonic plates made ​​against each other, with no net rotation).

The measurement of the positions of the fundamental stations to each other and their connection to the International Terrestrial Reference Frame ( ITRF ) of the IERS are normally administered via GPS; is currently the station set G873 (epoch 1997.0 ).

Reference ellipsoid

The values ​​for the size and flattening of the earth and its gravitational field are specified consecutively. But that the ellipsoid forms a stable reference, its parameters in the WGS84 be set to:

  • Semi-major axis a = 6,378,137 meters,
  • Oblateness f = 1 / 298.257 223 563, corresponding to a semi-minor axis b = a (1 -f) of about 6356 meters 752.3142
  • Geocentric gravitational constant G · M ⊕ = 3.986 004 m3/s2 418.1014 (the product is more accurate to determine than the individual factors ),
  • Rotation speed = 7,292 115.10 -5 rad / s

Gravity model

Currently, the gravity model WGS84 EGM96 applies. The spatial dependence of the Erdschwerepotentials is described by spherical harmonics of the associated Legendre polynomials. The expansion coefficients Cnm and Snm are the series expansion up to degree n and order m of 360 surveyed, a total of more than 130,000 coefficients.

Short-term effects, such as tidal effects which cause a change in height of up to 50 cm, are not considered. Long-term changes, for example caused by the continental drift of a few centimeters per year are recorded as adjustments to the existing model.

Divergence with other geodetic data

The reference meridian of the IERS runs approximately 100 meters east of the Royal Observatory of Greenwich past ( at the equator is the deviation from the Greenwich prime meridian lower).

The negligible deviation of the defined flattening of the GRS originally used 80 ellipsoid ( f ≈ 1/298, 257 222) corresponds to 0.1 mm in the semi-minor axis and was created by rounding in the round-trip conversion between flattening and the corresponding coefficients of the spherical harmonic.

For G · M ⊕ 84 of imprecise GRS 80 value 3.986 005.1014 m3/s2 was used in an earlier version of WGS. This value is hard-coded in millions of GPS receivers and will therefore continue to be used by the GPS control segment to adapt to the predicted value with the newer satellite positions Kepler ellipses whose parameters are transmitted to the GPS receivers as ephemeris.

For epoch 1989.0 the ITRS and the ETRS were identical within the measurement accuracy. But the latter is connected via fundamental stations on the Eurasian plate and moves 2.5 cm per year against the global system.

Use of the system

Currently ( as of 2000) are 123 local data linked to the WGS- 84 ellipsoid, which the conversion of position information between the coordinate systems much easier. The default WGS 84 was adopted by resolution of the ICAO in 1989 for aviation.

Universe Oestrich-Winkel Rheingau Fundamentalstation Crust (geology) Earth mass International Civil Aviation Organization
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