File : ITRF92.TXT DESCRIPTION OF THE ITRF92 SOLUTION The IERS Terrestrial Frame Section (ITFS) of the Central Bureau has computed a new solution of the IERS Terrestrial Reference Frame designated as ITRF92, and its associated combined velocity field. The main characteristics of the IERS Terrestrial Reference System (ITRS) are: a) geocentric, b) SI scale, in a local frame comoving with the Earth (in a relativistic modeling), c) orientation of the BIH Terrestrial System at 1984.0, d) time evolution following a no-net-rotation and translation with respect to the crust (Tisserand condition). For details, see the IERS Standards (1992), IERS Technical Note 13, Chap. 3. The ITRF92 global combined solution is divided into two parts: a set of station coordinates at epoch 1988.0 and a velocity field. Both of them are estimated, using least square adjustment, by combination of individual terrestrial reference frames provided by the analysis centers participating in the IERS activities. These individual terrestrial reference frames are derived from VLBI, LLR, GPS and SLR observations. THE ITRF92 STATION COORDINATES The determination of the ITRF92 station coordinates has been done by combination of 20 sets of station coordinates (SSC) received at the Central Bureau. In total, 287 stations located in 152 sites have been independently adjusted. The original formal errors given by the analysis centers in their individual solutions have been modified in order to obtain a realistic combined solution. Local ties were used as observations, with appropriate variances. They concern 40 colocation sites : 9 VLBI-SLR-GPS, 3 VLBI-SLR-LLR, 11 VLBI-SLR, 12 VLBI-GPS and 5 GPS-SLR. In this adjustment, 1988.0 positions have been adopted for input data and output solution. This means that before the adjustment, the individual data sets were reduced to the epoch 1988.0 using their respective site motion models. The origin and scale of the ITRF92 are defined by fixing to zero the three translations and the scale factor of the CSR SLR solution (SSC(CSR) 93 L 01). The orientation is defined such that no global rotation exists with regard to the ITRF91. The seven transformation parameters from the ITRF91 to the ITRF92 are: T1 = -1.1 cm, T2 = -1.4 cm, T3 = 0.6 cm, D = -0.14 X 10-8, R1 = 0.0, R2 = 0.0, R3 = 0.0. File ITRF92.TP gives transformation parameters from the ITRF92 to the individual terrestrial reference frames at epoch 1988.0, with scaled standard deviations. Please refer to the formula given at the end of the file, to avoid misuses of the signs of the parameters. File ITRF92.ssc gives the list of the ITRF92 station coordinates with scaled standard deviations. These coordinates are referred to the epoch 1988.0. For each station this file gives the DOMES number, name, CDP number (or the IGS 4-char abbreviation) and the individual solutions in which its coordinates are available. THE ITRF92 VELOCITY FIELD The ITRF92 velocity field has been obtained by combination of eight site velocity fields estimated by SLR and VLBI analysis centers. These eight velocity fields have been estimated together with eight SSCs among those incorporated in the ITRF92 station coordinates combined solution. The method for combining velocities is equivalent to and consistent with the method for combining SSCs into the ITRF92 station coordinates. In order to insure the condition of no-net-Rotation of the ITRS with respect to the crust, NNR-NUVEL1 was selected as the reference motion model as recommended by the IERS Standards (1992). A velocity field was so constructed using NNR-NUVEL1 for those sites which are available in the eight estimated velocity fields. The temporal variation (or rate of change) of the translation and rotation parameters of the constructed NNR-NUVEL1 velocity field were fixed to zero in the global adjustement. The scale factor is assumed to be invariant with time and so the corresponding parameters are fixed to zero for all velocity fields. NNR-NUVEL1 is a horizontal motion model. For the consistency of the three-dimensional combination, we used it as three-dimensional model with zero vertical velocity and assumed an error of 3 mm/year for the horizontal components and 1 cm/year for the vertical one. NNR-NUVEL1 velocities were deweighted in the combination (with 10 cm/year as a priori standard deviation) for sites whose motion may differ from NNR-NUVEL1, such as sites in California, Japan and the Mediterranean area. Consequently, the condition of no-net -rotation is in practice applied to those sites which are located on the rigid part of the tectonic plates and whose motion are consistent with that predicted by NNR-NUVEL1. File ITRF92.VTP gives the rate of change of the translation and rotation parameters. These values have to be considered as annual variations to the transformation parameters given in file ITRF92.TP. So for a given transformation parameter T at an epoch t in years, we have: . T(t) = T(1988.0) + T.(t - 1988.0) . . T and T are given in file ITRF92.TP and ITRF92.VTP respectively. When T . is not given one can assume T = 0. File ITRF92.SSV lists the ITRF92 combined velocity field. These velocities should be used to refer the ITRF92 coordinates from the 1988.0 epoch to another desired epoch. This file gives for each site its DOMES number, name, the individual solutions in which its velocities are available, the cartesian equatorial velocities and their standard deviations, the plate and a code : CN for Combined Velocity from VLBI and SLR estimates with NNR-NUVEL1 , C for Combined Velocity with NNR-NUVEL1 velocity deweighted, and N for NNR-NUVEL1 velocity.