A. Niell, L. Petrov
Using a Numerical Weather Model to Improve Geodesy.

    The use of a Numerical Weather Model (NWM) to provide in situ atmosphere
information for mapping functions of atmosphere delay has been evaluated
using Very Long Baseline Interferometry (VLBI) data spanning eleven
years. Parameters required by the IMF mapping functions (Niell 2000, 2001)
have been calculated from the NWM of the National Centers for Environmental
Prediction (NCEP) and incorporated in the CALC/SOLVE VLBI data
analysis program. Compared with the use of the NMF mapping functions (Niell
1996) the application of IMF for global solutions demonstrates that the
hydrostatic mapping function, IMFh, provides both significant improvement in
baseline length repeatability and noticeable reduction in the amplitude of 
the residual harmonic site position variations at semidiurnal to long-period 
bands. For baseline length repeatability the reduction in the observed mean 
square deviations achieves 80% of the maximum that is expected for the
change from NMF to IMF. On the other hand, the use of the wet mapping 
function, IMFw, as implemented using the NCEP data, results in a slight 
degradation of baseline length repeatability, probably due to the large grid 
spacing of the NWM that is used.