While an important goal in modern weather prediction is the improvement of short-term cloud and precipitation forecasts, our ability to do so is severely limited by the lack of timely and accurate water vapor data. Each of these systems has advantages and limitations. Surface-based radiometers Capable of high temporal resolution Costly, require frequent calibration, and do not function well in all weather conditions, especially rain. These systems make observations only during ascent and descent to major airports. Estimates using IR sensors are reliable only in cloud free areas.
Estimates using microwave sensors are valid in cloudy regions.
However, they are available only over the oceans and are generally less accurate than the IR-based estimates. Does not yet provide information about the vertical distribution of atmospheric water vapor, and since it only provides measurements of water vapor directly above the antenna, it has low spatial resolution. In this role, it will provide frequent water vapor observations to constrain numerical weather prediction models as well as independent calibration and validation of satellite measurements. Where, from a forecasting perspective, the need to have accurate water vapor data is greatest. This slowing delays the arrival time of the transmitted signal from that expected if there were no intervening media.