English 
Français 
العربية 
Abstract
GPS orbits are computed in WGS84 and ITRS reference frame- they are not computed in the frames of any geocentric datum. Positions determined using GPS refer to the frame of the orbits. In this paper the author developed a n Iraqi datum that relate the use of GPS to survey in a modern Iraqi geocentric datum instead of the old clark1880. The required modern geocentric datum is designed so that the GNSS surveyor seems to work directly in the datum. This consistency requires that the spatial reference relationship between the datum’s frame and the orbit ITRS frame be well known.
There are various types of mathematical approaches to transform positions between geocentric reference frames. Helmets transformation is quite widely used. In its full generality, Helmets transformation applies a translation, a rotation, and scale factor to transform an XYZ position from one frame to another, also accounting for time evolution of the frames with respect to each other and for tectonic plate motion. The author assumed the axes of both datum are parallel then a change of datum implies a change in any number of the five parameters (a, f, FYI, lambda, h) and hence a change in the 3 dimensional coordinates of all points represented on the original datum. This development will make it easy for all GIS users to get the exact position fixing for the whole country and work with unique coordinate system.
