World Geodetic System (WGS).
GPS receivers operate on the World Geodetic System (WGS) global geocentric reference system, or datum. It is global because, unlike other datums that only apply to certain regions, WGS can be used over the entire Earth. It is geocentric because, unlike other datums that use arbitrary points within the Earth as their origin, the origin of WGS is at the actual center of the globe. Most military and commercial receivers allow the user the capability to select the reference datum, but the receiver will default to WGS if none is selected.
In other words WGS 84 can be explained as :
WGS 84 is an Earth-centered, Earth-fixed terrestrial reference system and geodetic datum. WGS 84 is based on a consistent set of constants and model parameters that describe the Earth’s size, shape, and gravity and geomagnetic fields. WGS 84 is the standard U.S. Department of Defense definition of a global reference system for geospatial information and is the reference system for the Global Positioning System (GPS). It is compatible with the International Terrestrial Reference System (ITRS).
Definition of Frame
Earth’s center of mass being defined for the whole Earth including oceans and atmosphere
Z-Axis = The direction of the IERS Reference Pole (IRP). This direction corresponds to the direction of the BIH Conventional Terrestrial Pole (CTP) (epoch 1984.0) with an uncertainty of 0.005″
X-Axis = Intersection of the IERS Reference Meridian (IRM) and the plane passing through the origin and normal to the Z-axis. The IRM is coincident with the BIH Zero Meridian (epoch 1984.0) with an uncertainty of 0.005″
Y-Axis = Completes a right-handed, Earth-Centered Earth-Fixed (ECEF) orthogonal coordinate system
Its scale is that of the local Earth frame, in the meaning of a relativistic theory of gravitation. Aligns with ITRS
Given by the Bureau International de l’Heure (BIH) orientation of 1984.0
Its time evolution in orientation will create no residual global rotation with regards to the crust
Cartesian Coordinates (X, Y, Z). WGS 84 (G1674) follows the criteria outlined in the International Earth Rotation Service (IERS) Technical Note 21. The WGS 84 Coordinate System origin also serves as the geometric center of the WGS 84 Ellipsoid and the Zaxis serves as the rotational axis of this ellipsoid of revolution. WGS 84 geodetic coordinates are generated by using its reference ellipsoid.
WGS 84 identifies four defining parameters.
- The semi-major axis of the WGS 84 ellipsoid,
- the flattening factor of the Earth,
- the nominal mean angular velocity of the Earth, and
- the geocentric gravitational constant as specified below.
- WGS84 is realised by adopting the coordinates of stations around the world surveyed by Doppler satellite surveying technique.
- The origin of WGS84 is located at the Earth center with an uncertainty of 1 to 2 meters.
ACCURACY OF WGS 84 COORDINATES
Numerous techniques now exist to establish WGS 84 coordinates for a given site. The accuracy and precision achieved by these various techniques vary significantly. The most common, currently-available techniques are listed below:
- General geodetic solution for station coordinates, orbits, and other parameters of interest.
- Direct geodetic point positioning at a stationary, solitary station using a “geodetic quality”, dual frequency GPS receiver and NIMA Precise Ephemerides and Satellite Clock states (note that the effects of Selective Availability (SA) must be removed).
- Same as above but using the Broadcast GPS Ephemerides and Clock States.
- GPS differential (baseline) processing from known WGS sites
- GPS Precise Positioning Service (PPS) navigation solutions
- Mean over some averaging interval
- Photogrammetrically-derived coordinates
- Map-derived coordinates from digital or paper products.
- By Local Geodetic Datum to WGS 84 Datum Transformation.
- Because geospatial information often originates from multiple sources and processes, the absolute accuracy of a given WGS 84 position becomes very important when information from these various sources is combined in “Geographic Information Systems” or “geospatial databases”. Because of their high fidelity, surveyed WGS 84 geodetic control points can often serve to improve or validate the accuracy of maps, image products or other geospatial information. Even GPS navigation solutions can serve a similar role, as long as the accuracy of these solutions is well-understood.