During this quadrennium, very long baseline interferometry (VLBI) data acquisition and system development has focused on improving the accuracy of the system sufficiently to allow the determination of reliable estimates of height variations. Associated with this aim has been improvements in the determinations of horizontal velocity fields, monitoring water vapor delay using interferometric methods, and improvements to Earth rotation measurements. The primary aims of the improvements to height measurement accuracy have been to directly measure the contemporary magnitudes of post glacial rebound, and to determine a height reference system for measuring global sea level rise. High frequency Earth rotation studies have been carried out to better define the transformation parameters from an inertial coordinate system to an Earth fixed one, and to better understand the coupling between the components of the atmosphere-ocean-solid Earth system. Two major VLBI campaigns were carried out in support of these studies: (1) Epoch-92 in July 1992 and (2) Cont-94 in January 1994. Each of these campaigns lasted approximately two-weeks and involved multiple VLBI networks operating simultaneously in addition to other space geodetic systems operating during these periods.
Two major compilations of the VLBI results (and results from other space geodetic systems) have been published during this quadrennium. The proceedings from the Chapman conference on Geodetic VLBI Monitoring Global Change [ Carter, 1991] provides a summary of the status of the accuracy, expected improvements to, and the applications of VLBI to monitoring global change circa 1991. The other major compilation was the three volume monograph on contributions of space geodesy to geodynamics [ Smith and Turcotte, 1993]. These three volumes provide a comprehensive summary of the results obtained from the NASA Crustal Dynamics Project.