What is parameter ephemeris in gnss equation?
Answers
From 1987, GPS uses the World Geodetic System WGS-84, developed by the US Department of Defense (DoD), which is a unified terrestrial reference system for position and vector referencing [footnotes 1]. Indeed, the GPS broadcast ephemeris are linked to the position of the satellite antenna phase center in the WGS-84 reference frame. Thus, the user receiver coordinates will be expressed in the same ECEF frame.
The initial implementation of WGS-84 was realized from a set of more than a thousand terrestrial sites, which coordinates were derived from Transit observations [footnotes 2]. Successive refinements (which also leaded to some adjustments of the fundamental constants), using more accurate coordinates of the monitor stations, approximate to some ITRS realizations. For instance, realizations WGS84(G730) [footnotes 3] and WGS84(G873) correspond to ITRF92 and ITRF94, respectively. The refined frame WGS84(G1150) was introduced in 2002, which agrees with ITRF2000 at the centimeter level.
The parameters of the WGS-84 ellipsoid are given in the following table 1:
