J Geod (2005) DOI 10.1007/s00190-005-0010-z ORIGINAL ARTICLE K. Snajdrova · J. Boehm · P. Willis · R. Haas H. Schuh Multi-technique comparison of tropospheric zenith delays derived during the CONT02 campaign Received: 3 May 2005 / Accepted: 17 October 2005 © Springer-Verlag 2005 Abstract In October 2002, 15 continuous days of Very Long Baseline Interferometry (VLBI) data were observed in the Continuous VLBI 2002 (CONT02) campaign. All eight ra- dio telescopes involved in CONT02 were co-located with at least one other space-geodetic technique, and three of them also with a Water Vapor Radiometer (WVR). The goal of this paper is to compare the tropospheric zenith delays ob- served during CONT02 by VLBI, Global Positioning System (GPS), Doppler Orbitography Radiopositioning Integrated by Satellite (DORIS) and WVR and to compare them also with operational pressure level data from the European Cen- tre for Medium-Range Weather Forecasts (ECMWF). We show that the tropospheric zenith delays from VLBI and GPS are in good agreement at the 3–7mm level. However, while only small biases can be found for most of the stations, at Kokee Park (Hawaii, USA) and Westford (Massachusetts, USA) the zenith delays derived by GPS are larger by more than 5 mm than those from VLBI. At three of the four DORIS K. Snajdrova · J. Boehm (B ) · H. Schuh Institute of Geodesy and Geophysics,Vienna University of Technology, Gusshausstrasse 27-29, 1040 Vienna, Austria E-mail: ksnajd@mars.hg.tuwien.ac.at E-mail: johannes.boehm@tuwien.ac.at E-mail: harald.schuh@tuwien.ac.at Tel.: +43-1-5880112864 Fax: +43-1-5880112896 K. Snajdrova Institute of Geodesy, Brno University of Technology,Veveri 95, 662 37 Brno, Czech Republic P. Willis Institut G´ eographique National, Direction technique, 2 avenue Pasteur, BP 68, 94160 Saint-Mande, France E-mail: pascal.willis@ign.fr P. Willis Jet Propulsion Laboratory, California Institute of Technology, MS 238-600, 4800 Oak Grove Drive, Pasadena, CA 91109, USA R. Haas Onsala Space Observatory, Department of Radio and Space Science, Chalmers University of Technology, SE-439 92 Onsala, Sweden E-mail: haas@oso.chalmers.se stations, there is also a fairly good agreement with GPS and VLBI (about 10 mm), but at Kokee Park the agreement is only at about 30 mm standard deviation, probably due to the much older installation and type of DORIS equipment. This comparison also allows testing of different DORIS analysis strategies with respect to their real impact on the precision of the derived tropospheric parameters. Ground truth informa- tion about the zenith delays can also be obtained from the EC- MWF numerical weather model and at three sites using WVR measurements, allowing for comparisons with results from the space-geodetic techniques. While there is a good agree- ment (with some problems mentioned above about DORIS) among the space-geodetic techniques, the comparison with WVR and ECMWF is at a lower accuracy level. The complete CONT02 data set is sufficient to derive a good estimate of the actual precision and accuracy of each geodetic technique for applications in meteorology. Keywords Troposphere · Zenith delay · GPS · VLBI · DORIS · Water Vapor Radiometer 1 Introduction Tropospheric parameters play an important role when ana- lyzing microwave space-geodetic measurements. In the stan- dard least-squares fit of these observations, there is a rel- atively high correlation of about −0.4 between the station height parameters and the tropospheric zenith delays (Boehm 2004), i.e., errors in the station heights can also be seen in the zenith delays and vice versa. The agreement between the tro- pospheric parameters derived from different space-geodetic techniques at co-located sites is an important indicator for their accuracy and reliability, since biases in the tropospheric parameters are mirrored as biases in estimated station heights. Furthermore, accurate absolute tropospheric estimates are important for atmospheric studies and — if obtained in near real-time — even for weather prediction models (Tomassini et al. 2002). It is also possible to derive atmospheric infor- mation from satellite-based measurements by applying the