GPS data processing using GAMIT/GLOBK

GAMIT, GLOBK, and TRACK form a comprehensive suite of programs for analyzing GPS measurements primarily to study crustal deformation. The software has been developed by MIT, Scripps Institution of Oceanography, and Harvard University with support from the National Science Foundation.GAMIT uses double differences between stations and satellites to cancel completely the effects of variations in the station clocks and incorporates a weighted least squares algorithm to estimate the relative positions of a set of stations, orbital and Earth-rotation parameters, zenith delays (tropospheric), and phase ambiguities. Since the functional model relating the observations and parameters is non-linear the least-squares fit for each session may need to be iterated until convergence. Despite the first solution of the parameters and elements of the covariance matrix obtained with GAMIT is not at all well constrained, it means that neither the coordinates of the sites nor the GPS satellites orbits are tightly constrained. Although baseline lengths are determined very precisely in the loosely constrained solutions, the entire GPS network and GPS constellation should be rotated and translated. We use GLOBK to combine each daily solution and estimate the 7 parameters transformation within a global reference frame (ITRF2005), enabling us to obtain very precise positions and velocities of sites as well as orbital and Earth-rotation parameters.