BS ISO 19161-1:2020 pdf free.Geographic information – Geodetic references.
The ITRS is defined jointly by the IAU and the IUGG It is the spatial three-dimensional part of a specific Geocentric Terrestrial Reference System (GTRS) for which the orientation is maintained in continuity with past international agreements (the so-called BIH orientation). Since 1988 both the monitoring of the definition of ITRS and the determination of its primary realizations, known as ITRF, have been the responsibility of the International Earth Rotation and Reference Systems Service (IERS]. A more detailed description of the ITRS, together with its effective origin, scale and orientation is available in Annex B (informative).A realization of ITRS is any TRF product containing the required numerical information (e.g. a set of static coordinates, or coordinates and velocities) satisfying the definition of origin [centre of mass of the Earth), orientation (no net rotation with respect to the motions of the Earth’s surface) and scale (based on the speed of light and the Earth’s gravitational constant) of the ITRS and its time evolution. EXAMPLE1 A realization of ITRS consists of a set of static coordinates or a set of coordinates and their time evolution, of physical points on the topographical surface of the earth.  EXAMPLE 2 The coordinate sets in a realization of ITRS can refer to each satellite in a constellation.Current ITRS realizations are obtained through processing and analysing datasets sourced from space geodetic techniques. Realizations may be determined using one or more of these techniques. AlI current realizations consist of a set of identifiers of physical points, with corresponding numerical coordinate information, expressed in a coordinate system (e.g. Cartesian, ellipsoidal).It is any of the other terrestrial reference frames that are aligned to the ItRF. Alignment to a TRF uses a 7- or 14-parameter similarity transformation in which all parameter values are zero to give a solution with the same origin, scale, orientation and time evolution as an existing TRF. For a TRF represented by a data set of estimated station positions, a 7-parameter similarity transformation is used, where the parameters represent the differences in the origin, orientation and scale with respect to an existing TRF.BS ISO 19161-1 pdf download.