Introduction

On 25th of April at the ILRS meeting in Nice, the Refraction Study Group (RSG) was formed in order to complete the tasks mentioned in the following charter:
Considering that the current standard model developed by Marini and Murray for applying atmospheric propagation delay corrections to SLR observations is believed to be uncertain by up to 1 cm in the zenith delay term; and considering that it is tuned to particular wavelengths which do not encompass all the wavelengths which might be used in practice,

1. Assess the adequacy and accuracy of the Marini and Murray model for SLR at the 1 mm level, for all elevations above 15 degrees and all wavelengths from 355 nm to 1570 nm and perhaps beyond.
2. If necessary, define a new model and/or set of procedures which will satisfy these requirements, and verify it experimentally.
3. Define optimumschemes for the collection and pre-processing of meteorological and any other necessary data to be used as input to the model/procedure, and assess the impact of the absence of any such data which may be difficult or expensive to collect.
4. Prepare a formal recommendation to the ILRS, including a standard algorithm and set of recommended practices, for adoption by the stations and analysis centres.

The RSG constituted itself with the following members:

J.McK.Luck	JohnLuck@auslig.gov.au
P.Ciddor	Philip.Ciddor@tip.csiro.au
F.K.Brunner	brunner@aig.tu-graz.ac.at
R.Eanes	eanes@csr.utexas.edu
W.Gurtner	gurtner@ubeclu.unibe.ch
R.Haas	haas@oso.chalmers.se
J.Rueger	J.Rueger@unsw.edu.au
T.Otsubo	otsubo@crl.go.jp
P.J.Dunn	peter_j_dunn@raytheon.com
R.Govind	rameshgovind@auslig.gov.au
R.Noomen	ron.noomen@lr.tudelft.nl
T.Herring	tah@chandler.mit.edu
V.Mendes	vmendes@fc.ul.pt
H.J.Yan	yhj@center.shao.ac.cn
M.Becker	becker@ifag.de
Y.Galkin	galkin@mgul.ac.ru
S.Riepl	riepl@fs.wettzell.de

According to the proposed timetable of the charter, the present paper gives a report on charter item 1 and provides an overview on the results obtained so far. Since the envisaged topic is quite complex the report is cast into the follwing well seperated sections.