  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.
