GPS Networks for Atmospheric Sensing

Phase delays induced in GPS signals by the ionosphere and neutral atmosphere can be measured with high precision simultaneously along a dozen or so GPS ray paths in the field of view. These delays can be converted into total electron content (TEC) and integrated water vapor (if surface pressure data or estimates are available) along each GPS ray path. The resulting continuous, accurate, all-weather, real-time GPS moisture data help advance mesoscale modeling and data assimilation, severe weather, precipitation, cloud dynamics, regional climate and hydrology. Several networks are now being established for this purpose. They range from small (10 km) to global scale. We discuss the applications, data communication, and analysis techniques for three GPS networks. The first network consists of 14 low-cost single frequency GPS receivers in a small 8-km diameter area in Oklahoma. Single frequency L1 data are processed, under consideration of good ionospheric models, to determine accurate atmospheric delays and atmospheric water vapor in the directions of the GPS satellites. Use of similar low-cost L1 receivers in larger networks with hundreds of sites is under consideration. The second network is operated by the National Oceanographic and Atmospheric Administration Forecast Systems Laboratory (NOAA/FSL) to compute integrated zenith water vapor in near real time. The near real time zenith water vapor, daily slant water vapor and vertical ionospheric TEC results from this network are displayed at "gst.ucar.edu/gpsrg/realtime.html". We discuss the importance of near real time orbit improvements (as compared to the use of predicted GPS orbits) for the analysis of such a continental size network. Finally, we describe a global network of high-rate 1-sec GPS tracking stations that is being established in support of satellite missions that profile the atmosphere and ionosphere from low-Earth orbit. In summary, the proliferation of GPS networks for the primary purpose of atmospheric monitoring shows the importance and maturity that this relatively new application of the GPS has achieved in a very short time.