We have a point in the space, but we need a reference system, could be a Cartesian system or another. In this case we will collocate the point on ellipsoid, because the ellipsoid is the nearest geometric figure to the Earth form. The DATUM is a reference system. Exist vertical datum, marine datum etc. Our case is a geodetic DATUM.

The Datum has: a exactly reference surface with a fundamental point, in the fundamental point coincide the ellipsoid normal line and geoid normal line (vertical devation is null) and the function of this point will be to calculate the follow points in this Datum.

In this context, Exist Local datum nearly to earth form in a region. The satellite Datum is nearly to the total earth and to calculate this generally use the change in satellite orbits. But exist many local datum. Many agencies create many Datum’s with different purpose. For that rason we need to convert coordinates from a DATUM in a region to another DATUM in other region.

To define a local geodetic DATUM we need 8 parameters: 2 are from ellipsoid (Could be 3 if we define a ellipsoid with three axis), three: localization and center, and three: ellipsoid orientation.

To define a satellite DATUM is necessary the satellite parameters orbits and are based in earth reference stations coordinates. The orbit parameters are based in:

-       Geopotential models

-       Gravitation constant.

-       The earth rotation.

-       The light velocity.

-       Watch correction calculus from reference station.

Ellipsoid is unnecessary to satellite datum, No use to calculate the orbit, wherever an ellipsoid is associated to the satellite Datum.

The GPS coordinates are based in the WGS84 reference system and WGS84 ellipsoid. This is a satellite Datum.

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