Document Type


Publication Date

Spring 5-1-2004


Signals from the Global Positioning System (GPS) are used to retrieve the integrated amount of water vapor along the path between a transmitting satellite and a receiving station. This integrated quantity is called slant water vapor (SW). Measurements of SW allow for an improved assessment of the spatial distribution of water vapor within the atmosphere. This technique is developed and validated through simulations and comparisons to similar measurements from a pointing microwave water vapor radiometer. Absolute accuracy of zenith scaled SW is found to be 1.5 mm with a relative precision that is better than 0.5 mm. Dual and single frequency GPS stations are used to measure SW. Previously, only dual frequency GPS stations have been used for atmospheric remote sensing. The use of single frequency stations, which are significantly less expensive than dual frequency ones, allows for a denser placement of stations. The effects of the ionosphere on single frequency GPS observations are eliminated using global ionosphere models and double difference processing with short station separation. Networks of GPS stations are deployed in the Southern Great Plains of the United States. Combining SW measurements from all stations within a dense network allows for an estimation of the three dimensional distribution of water vapor above the network. This tomographic technique is improved by including vertical profiles from radiosondes. The retrieval of SW is utilized during the International H2O Project 2002 (IHOP_2002). Significant water vapor structure is observed within the atmospheric boundary layer, including dry line convergence and horizontal convective rolls. Tomography results computed during squall line passage indicate elevated levels of water vapor in the free troposphere prior to the onset of rainfall. A statistical analysis of the results obtained during IHOP_2002 show coherent water vapor structure across horizontal lengths ranging from less than 1 to almost 100 kilometers. A significant diurnal cycle of atmospheric water vapor variability is also found.