Developing an operational, surface-based, GPS, water vapor observing system for NOAA: Network design and results

The need for a reliable, low-cost observing system to measure water vapor i n the atmosphere is incontrovertible. Experiments have shown the potential for using Global Positioning System (GPS) receivers to measure total precip itable water vapor accurately at different locations and times of year and under all weather conditions. The National Oceanic and Atmospheric Administ rations's (NOAA) Forecast Systems Laboratory (FSL) and Environmental Techno logy Laboratory (ETL), in collaboration with the University NAVSTAR Consort ium, University of Hawaii, Scripps Institution of Oceanography, and NOAA's National Geodetic Survey (NGS) Laboratory, are addressing this need by deve loping a ground-based water vapor observing system based on the measurement of GPS signal delays caused by water vapor in the atmosphere. The NOAA GPS Integrated Precipitable Water Vapor (NOAA GPS-IPW) network currently has 3 5 continuously operating stations and is expected to expand into a 200-stat ion demonstration network by 2004. This paper describes the major accomplis hments of the project since its inception in 1994. Results from the analysi s of the effect of satellite orbit accuracies on IPW accuracy are discussed . Several comparisons with collocated remote and in situ measurements, incl uding radiosondes and ground- and space-based radiometers are shown. Result s from preliminary model runs using the FSL Forecast Research Division's Me soscale Analysis and Prediction System (MAPS) model are presented. This wor k shows the feasibility of an operational system using GPS to continuously monitor atmospheric water vapor in near-real time with accuracies (< 1.5 cm ) comparable to radiosondes and water vapor radiometers.