  In this study we cross-correlate many IUE echellograms of a variety of stars
to evaluate systematic error sources in the wavelength zeropoint of all three
cameras. We first evaluated differences between the final archived ("NEWSIPS")
and the originally processed ("IUESIPS") spectra. These show a clear time
dependence in zeropoint for the SWP camera due to revisions in the IUESIPS
wavelength scale. Small IUESIPS - NEWSIPS differences are also found for the
LWR camera. We also examined wavelength zeropoint disparities between data
obtained both through the small and large entrance apertures and for
observations made by different target acquisition modes for faint and bright
stars. We found that velocities resulting from these alternative observing
modes are nil. For large-aperture observations the dominant error source is the
target position placement in the aperture. We searched for spurious trends with
time, and found only a suggestion of time trends for faint stars observed with
the SWP camera. We also discovered 1-day, +/-3 km/s sinusoidsal patterns in
intensive monitoring data which are ascribable to changes in telescope focus
resulting from thermal drifts. In the second part of the paper, we measured
mean zeropoint errors of NEWSIPS echellogram data against laboratory results by
using the GHRS spectral atlas of the 10 Lac. We find that the derived apparent
velocity difference for this star is -1 +/-3.5 km/s. Several less precise
comparisons lead to similar results. The zeropoints of the NEWSIPS-processed
LWP/LWR cameras are evaluated and are also found to be nearly zero (+/-5 km/s)
relative to HST atlases of Arcturus and Procyon atlas. These results do not
support result by Gonzalez-Riestra et al. that corrections should be introduced
to the wavelength scales of various NEWSIPS high-dispersion data products.
