Global Positioning System (GPS) Revolutionizes Water Vapor Measurements


Results from an Atmospheric Radiation Measurement (ARM) study confirm the accuracy of using GPS for water vapor measurements. Simultaneous measurements from these multiple receivers are being used to construct high resolution, 3D maps of atmospheric water vapor. These 3D maps will be an important data source in the development of numerical weather prediction models. The experiment compared water vapor path measurements from GPS with simultaneous measurements obtained from a ground-based microwave radiometer (MWR) at the ARM central facility in Oklahoma. The MWR is self-calibrated based on fundamental physics and its measurements are an absolute standard. A comparison of over 100,000 measurements made over a span of two months showed a correlation between water vapor measurements made with the two technologies of 0.99 and a root-mean-square error of 1.3 mm or about 2-3%. These results clearly demonstrate the ability of networks of GPS receivers to accurately measure instantaneous water vapor along the transmission path. Thus the GPS units, at a one tenth the cost of the MWR, will provide an accurate, cost-effective technology for measuring the most abundant greenhouse gas, water vapor. Since water vapor absorbs energy in the atmosphere, having accurate measurements of its atmospheric concentration are critical for climate studies. The GPS receiver used in this experiment is one of 33 such receivers distributed around the ARM site in Oklahoma and one of hundreds in the continental U.S.