BS ISO 19161‑1:2020 Geographic information — Geodetic references Part 1: International terrestrial reference system (ITRS)

BS ISO 19161‑1:2020 Geographic information — Geodetic references Part 1: International terrestrial reference system (ITRS). 6 Realizations of ITRS 6.1 Description of a realization of ITRS 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 EartWs surface) and scale (based on the speed of light and the Earth’s gravitational constant) of the ITRS and its time evolution. EXAMPLE 1 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. 6.2 Classification of realizations 6.2.1 General 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. All 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). In this document, the following categories of realizations are distinguished: 6.2.2 Primary realization of ITRS It is a product created by the II3RS under the generic name ITRF. This document does not define these primary realizations nor give any requirements for them, as they are the sole responsibility of IERSll ll. Only a general explanation is provided (see Annex B). 6.2.3 Secondary realization of ITRS 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. For a TRF represented by a data set of estimated station positions and velocities, a 14-parameter similarity transformation is used, where the parameters represent the differences in the origin, orientation, scale and their time evolution with respect to an existing TRF at one or more given epoch(s). Secondary realizations...