Operational Ground-Based GPS Water Vapor Observing System Strategies

Ground-based Global Positioning System (GPS) receivers are capable of making accurate measurements of atmospheric refractivity under all weather conditions. The first use of ground-based GPS as an atmospheric observing system was for the measurement of integrated (total column) precipitable water vapor (IPW) above a fixed site. GPS-IPW measurements are now made routinely on four continents using networks of GPS receivers that continuously monitor the GPS satellite constellation in high Earth orbit.

To make GPS-IPW measurements, signal delays calculated from all satellites in view are scaled to the vertical (zenith) and averaged. With a zenith-scaled measurement of the tropospheric signal delay, and a knowledge of the hydrostatic signal delay caused by the mass of the atmosphere, it is possible to objectively determine the signal delay caused by the water vapor in the atmosphere above a site with an error of about 5%. To accomplish this, it is necessary to make dual-frequency GPS range and carrier phase observations, and use improved GPS satellite orbits and Earth orientation parameters that are presently available about 13-h after the end of the current day. Since this degree of latency precludes the use of GPS-IPW data in modern numerical weather prediction models, methods are being developed to produce frequent orbit predictions and real-time orbit quality control parameters. Real-time GPS IPW calculated with quality controlled predicted orbits will be compared with IPW calculated using high quality (but 13-h latent) orbits to assess the relative accuracy and precision of this promising new technique.

Because higher spatial resolution water vapor observations are required for improved mesoscale weather forecasting, strategies for densifying the current network of ground-based GPS observing systems have been considered. Emphasis has been placed on minimizing the cost and implementation time for a national network by leveraging the investments that other federal agencies have and will make in the GPS infrastructure. The experiences of the NOAA Forecast Systems Laboratory (FSL) in implementing this strategy will be discussed.

Finally, it appears that the techniques being developed to quality control predicted orbits in real time can also be used to reduce systematic errors in line-of-sight (or slant-path) refractivity measurements to individual satellites. Recent results from observing system simulations at FSL indicate that it may be possible to recover the full three-dimensional structure of the moisture field from a densely spaced network of ground-based GPS observing systems. The hardware requirements and siting criteria for such a network will be described.