EVALUATION OF VERY LONG-BASE-LINE INTERFEROMETRY ATMOSPHERIC MODELINGIMPROVEMENTS

We determined the improvement in baseline length precision and accurac y using new atmospheric delay mapping functions (Ifadis (Ifadis, 1986) and MTT (Herring, 1992a)) by analyzing the NASA Crustal Dynamics Proj ect research and development (R&D) experiments and the International R adio Inter-ferometric Surveying (IRIS) A experiments. These mapping fu nctions reduce baseline length scatter by about 20 % below that using the CfA2.2 (Davis et al., 1985) dry and Chao (Chao, 1974) wet mapping functions. With the newer mapping functions, average station vertical scatter inferred from observed length precision (given by length repea tabilities) is 11.4 mm for the 1987-1990 monthly R&D series of experim ents and 5.6 mm for the 3-week-long ERDE series. The inferred monthly R&D station vertical scatter is reduced by 2 mm or by 7 mm in a root-s um-square (rss) sense. Length repeatabilities are optimum when observa tions below a 7-8-degrees elevation cutoff are removed from the geodet ic solution. Analyses of IRIS-A data from 1984 through 1991 and the mo nthly R&D experiments both yielded a nonatmospheric unmodeled station vertical error of about 8 mm. In addition, analysis of the IRIS-A expe riments revealed systematic effects in the evolution of some baseline length measurements. The length rate of change has an apparent acceler ation, and the length evolution has a quasi-annual signature. We show that the origin of these effects is unlikely to be related to atmosphe ric modeling errors. Rates of change of the transatlantic Westford-Wet tzell and Richmond-Wettzell baseline lengths calculated from 1988 thro ugh 1991 agree with the NUVEL-1 plate motion model (Argus and Gordon, 1991) to within 1 mm/yr. Short-term (less than 90 days) variations of IRIS-A baseline length measurements contribute more than 90% of the ob served scatter about a best fit line, and this short-term scatter has large variations on an annual time scale.