11.3 water resOurces MOnitOrinG systeMs 11.3.1 real-ti M e Data -collection syste M technolo G ies Methods and systems for real-time data collection have been strongly influenced by the prolifera- tion of low-cost microprocessor data-collection platforms (DCPs). Community flash-flood warn- ing systems, statewide water-management information systems, and river-reservoir monitoring and operations systems are increasingly common due to improved capabilities of these DCPs, real-time reporting modes, and telecommunications techniques. A variety of telecommunications modes are used, including telephone and other dedicated landlines, Internet, radio, satellite, and meteorburst (i.e., VHF radio signals that are reflected at a steep angle off the ever-present band of ionized mete- orites existing from about 50 to 75 miles above the Earth). Hardware and software components of these systems are continually being refined to realize lower error and faster transmission rates. Remote-sensing technologies play a primary role in water resources monitoring. Satellites pro- vide visual and near-visual infrared (IR) imagery of the Earth’s surface and atmospheric reflec- tance. These images provide the basis for tracking storm systems as well as mapping crop types and status and flood extent. Communications satellites also provide the means for transmitting DCP data from the sensors to a central receiving station. Remote sensing also includes radar technolo- gies. The NEXRAD (next-generation radar) program of the National Weather Service comprises a nationwide network of Doppler radars that monitor storm dynamics. Global positioning system (GPS) technology has also impacted the collection of data for water resources monitoring. GPSs have wide application for real-time monitoring where personnel and equipment can be deployed to the field and their locations established. GPS signal processing has also been used to determine the state of the atmospheric water vapor. NOAA uses existing GPS sites to receive measurements for the total amount of water vapor above a GPS antenna. Water vapor refracts radio waves, including GPS signals. When this happens, the apparent distance between a GPS sat- ellite and a receiver on the ground is a little longer than it would be if the air were completely dry. Differences between the GPS signals at different times are correlated with the atmospheric water vapor, and a large GPS water-vapor-sensing system has been developed (http://www.esrl.noaa.gov/ media/2007/gps/).