IGS TROPOSPHERE WORKING GROUP CHARTER Recognizing the important contributions that GPS can make to meteorology, climatology and other environmental disciplines, due to the ability to estimate tropospheric quantities that affect the GPS signal, the IGS has sanctioned the creation of a Troposphere Working Group. The IGS Troposphere Working Group will promote, coordinate, facilitate, guide, and enhance the generation of useful tropospheric products based on data from the IGS ground network of GPS receivers. The primary tropospheric product that is currently generated from ground-based GPS data is the estimate of total zenith path delay. Ancillary measurements of surface pressure and temperature allow the extraction of precipitable water vapor from the total zenith path delay estimates. Water vapor is a key element in the hydrological cycle and is an important greenhouse gas in the atmosphere. Monitoring of long-term changes of its content and distribution is essential for estimating changes in climate. The very inhomogeneous and highly variable distribution of the atmospheric water vapor makes it to a crucial element in weather forecasting. Conventional observing systems such as radiosondes and remote sensing are insufficient for observing its high variability. Water vapor can also be estimated from space-born GPS receivers using ray tracing techniques. The ground-based and the space-based technique complement each other. The ground-based GPS provide continuous, high temporal resolved, vertical integrated water vapor at a site having a global coverage which is over the continents and limited over the oceans. The space-based GPS provide high vertical resolution (lateral integrated over few hundred kilometers) with good global coverage, but discontinuous in time and geographical location. Both techniques are dependent on ground-based data as well as on precise GPS orbits and its predictions. The Troposphere Working Group will: * Promote improvements in estimation techniques and data handling procedures through on-going collaboration and communication between the group members; and enhance the implementation of standards and technologies which will minimize the systematic differences in the estimates of the analysis groups. * Support the IGS Associate Analysis Center for tropospheric solution combination (currently JPL) in refining and assessing the combination algorithm, and in quality control assessment of the derived products. * Advertise and promote the utilization of the IGS tropospheric products by the widest possible user group. Provide feedback to the IGS from users of the tropospheric products. * Devise and evaluate the generation of new IGS tropospheric products in consultation with the potential user community. * Generate and maintain a prioritized list of sites that are candidate for deployment of meteorology sensors and/or GPS receivers based on their potential impact on atmospheric and environmental studies. * Gather, analyze and disseminate information to support the proper selection, installation and maintenance of instruments that affect the tropospheric products. * Organize and coordinate periodic campaigns for the calibration and assessment of the performance of instruments and products. Disseminate the results of these campaigns together with relevant recommendations for action. * Support a regular quality assessment of the derived products using permanently running collocated techniques. * Promote synergy between space-based and ground-based GPS techniques through interaction with active researchers in both fields. MEMBERS: Michael Bevis , Peng Fang , Godelieve Deblonde , Pedro Elosegui , William Kass , Carlos Garcia-Martinez , "Mireault, Yves" , , Seth Gutman , markus.rothacher@bv.tum.de, Sung.Byun@jpl.nasa.gov, Gerd Gendt , H.vanderMarel@geo.tudelft.nl, Tom Herring tah@chandler.mit.edu Members ex officio : Director of CB (R. Neilan) IGS Analysis Center Coordinator