Anomalous dispersion

Anomalous dispersion, caused by water vapour absorption lines in the near infrared affects the dispersion of the atmosphere. As there are no direct refraction measurements available, the effect has to be modeled by analysing absorption line data of the HITRAN database. Reginald Hill from the National Oceanic and Atmospheric Administration submitted a CD-ROM containing absoption line data as well as FORTRAN code for analysis of the data, for which we are very thankful. It is a remaining action item to test the code and make it available to the SLR community. There are preliminary estimates on the magnitude of this effect for certain wavelengths:

wavelength [$\mu m$]	deviation in $n_{Gr}$
0.85	$< 10^{-7}$
1.064	$< 10^{-7}$
1.570	$< 10^{-6}$

A deviation of $10^{-7}$ in the group refractive index $n_{Gr}$ causes a zenith path error of roughly 1 mm.
Moreover it is to note, that such calculations require a detailed knowledge of the absorption line shapes. It is objected to get a grant for a joint russian-german project providing these absorption line as well as refractive index data for selected wavelegnths.