The Meteorological Role of the Global Positioning System in NOAA's Integrated Upper-Air Observing System

The U.S. Department of Defense developed the satellite-based Global Positioning System (GPS) to provide positioning, navigation and time-transfer information anywhere on Earth with high accuracy under all weather conditions. GPS is a “dual-use” system, in that its signals are available for both military and civilian applications free of user fees. When GPS was launched in the 1980's, the atmosphere was treated as a nuisance parameter and the ability to use GPS for atmospheric remote sensing was completely unanticipated. The use of GPS for weather forecasting, climate monitoring, and calibrating, validating and quality controlling other observing systems such as satellites and radiosondes was developed and demonstrated by NOAA's Forecast Systems Laboratory in collaboration with several universities and NOAA's National Weather Service, National Environmental Satellite, Data, and Information Service, and National Ocean Service. Ground-based GPS provides continuous total refractivity measurements of the atmosphere from which water vapor can be retrieved under all weather conditions with high accuracy and reliability at very low cost. GPS observations are comparable to total column measurements made by radiosondes and satellite sounders, but require no external calibration, and use different physics to make them. This makes GPS a completely independent way to verify the accuracy of global water vapor observations, and this is especially important in remote areas of the planet where there are no redundant observations available for observation verification and quality control. This paper describes how the Earth's atmosphere affects the GPS radio signals, and how continuous monitoring of the GPS signal with ground and space-based GPS receivers provides critical but previously unavailable information about water vapor and temperature variability in the lower atmosphere. The use of GPS with other sensors including satellites, radar and mesoscale networks of observing sensors to provide the more complete description of the atmosphere needed for breakthrough improvements in weather warnings and forecasts will be discussed. Examples illustrating the use of GPS in numerical weather prediction, including data assimilation, verification, quality control, and parameterization are presented. Ground-based GPS Meteorology is ready to transition from NOAA research to operations, where it is expected to make an immediate impact on NOAA weather and water, climate, and transportation services.