MWR

Jim Liljegren was invited by the Jet Propulsion Laboratory (JPL) to work with it on a proposal for extending global positioning system (GPS) water vapor measurement capabilities. Jim's role was to track the NASA EOS AM-1 GPS satellite with an microwave radiometer (MWR) to measure the water vapor along the line-of-sight between the satellite and a GPS receiver already installed at the SGP CART Site by JPL (e.g., JPL GPS Campaign) for comparison with the JPL GPS-determined result. Jim used the ARM CART spare MWR. This work began in June 1999 and extended through early September of 1999.

Scientific hypothesis: Measurements of precipitable water (PW) along lines-of-sight (LOS) to visible GPS satellites will be obtained under all weather conditions for the purpose of validating similar LOS measurements derived from a ground-based GPS receiver deployed by NASA/JPL. Once validated, similar GPS LOS measurements will be used to improve the calibration of the water vapor radiometer and radar altimeter on the TOPEX/Poseidon and Jason spacecraft.

Approach to test hypothesis: The spare ARM MWR was programmed to track the visible GPS satellites (i.e., those above the horizon) in rapid succession. Additional LOS measurements at/near the solar disk served as fiducial measurements to assure the pointing accuracy of the MWR and may also prove useful for comparison with PW measurements derived from MFRSR and other sun photometers.

Activity Summary

Results/Summary Report: Presented at the Fall 1999 AGU meeting, Geodesy Session G4 (Geodetic Measurement and Modeling Techniques)

Validating GPS-based Estimates of Line-of-Sight Precipitable Water

Yoaz Bar-Sever, Ragne Emardson, Jim Liljegren, and Francois Vandeberghe

We use a unique experimental configuration at the Cloud and Radiation Testbed (CART) site near Lamont, Oklahoma, to develop and validate the ability to extract line-of-sight (LOS) tropospheric delay and precipitable water (PW) content from the GPS data. Up to 12 signals arrive at the GPS antenna from distinct and rapidly varying directions. Due to the delay and bending caused by water vapor refractivity these signals carry substantial structural information about atmospheric PW distribution.

Accurately monitoring the distribution of water vapor in the atmosphere is of critical importance to forecasting convective activity, because of water vapor's profound impact on atmospheric stability. The small-scale variability of atmospheric water poses a challenge to observing systems and despite continuous enhancements to observation systems, the accuracy of forecasts is compromised by a lack of adequate observations of water vapor. Other applications include air quality modeling, SAR interferometry and radio science.

The main validation tools of the GPS LOS PW estimates are a pointed water vapor radiometer (WVR) and objective analysis fields that are generated on a local high-resolution grid by the fifth generation Penn State/NCAR mesoscale model (MM5) driven by local weather data. By validating and characterizing line-of-sight delay estimates we are establishing the foundations for tomographic resolution of vertical tropospheric profiles using dense GPS networks.

Timeline

Campaign Data Sets