The World Geodetic System 1984 (WGS84) is the datum used by the Global Positioning System (GPS). The datum is defined and maintained by the United States National Geospatial-Intelligence Agency (NGA). Coordinates computed from GPS receivers are likely to be provided in terms of the WGS84 datum and the heights in terms of the WGS84 ellipsoid.
The difference between Hartebeesthoek 94 co-ordinates and WGS84 as used for the reference frame of GPS is approximately 0.2m and 0.3m in Lo y and x co-ordinates respectively. Therefore coordinates in these systems can normally be assumed to be coincident.for most GIS applications.
The current version of WGS84 is closely related to ITRF 2000 (epoch 2001.0). WGS84 is currently updated to GPS week G1150.
A technical description of the WGS84 datum is provided on the NGA website.
“ The International Terrestrial Reference System (ITRS) is a world spatial reference system co-rotating with the Earth in its diurnal motion in space. The International Earth Rotation and Reference Systems Service (IERS), in charge of providing global references to the astronomical, geodetic and geophysical communities, supervises the realization of the ITRS. Realizations of the ITRS are produced by the IERS ITRS Product Center (ITRS-PC) under the name International Terrestrial Reference Frames (ITRF). ITRF coordinates were obtained by combination of individual TRF solutions computed by IERS analysis centers using the observations of Space Geodesy techniques : GPS , VLBI , SLR, LLR and DORIS. They all use networks of stations located on sites covering the whole Earth.” (http://itrf.ensg.ign.fr/general.php)
There have been a number of realizations of ITRF with the most recent being ITRF 2008. The Hartebeesthoek 94 co-ordinate reference frame for South Africa is based on the ITRF91 (epoch 1994.0) co-ordinates of the Hartebeesthoek Radio Astronomy Observatory 36 m antenna. The co-ordinates of TrigNet stations, South Africa’s network of continuously operating GNSS reference stations, are based on ITRF 2005 (epoch 2010.02)
Whereas WGS 84 can be seen as the target, ITRF is seen as the bull’s eye in that target.