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GPS APPLICATIONS AND REQUIREMENTS 47 Current and Future Applications and Requirements Meteorology In meteorology, GPS can be used to measure atmospheric water vapor. Water vapor is the principal mechanism by which moisture and latent heat are transported in the atmosphere and is therefore closely linked to weather and climate. As discussed in Appendix C, GPS signals are delayed by the ionosphere and neutral atmosphere as they travel towards the surface of the Earth. This delay can be estimated by a receiver observing the two principal GPS transmission frequencies. When combined with surface pressure data, the estimated signal delay can provide a measurement of wet delay, which in turn, can be converted into precipitable water vapor. GPS sensing of precipitable water vapor with millimeter accuracy has been demonstrated successfully. The use of this technique for weather forecasting is being explored, and has been proposed for climate research. Another innovative use of GPS for meteorology is the new field of Earth-atmospheric occulation measurements. This technique uses a GPS receiver on a satellite in low-Earth orbit to track a GPS satellite as it sets behind the Earth. As the GPS signal passes through the edge of the atmosphere it is refracted, causing delay and Doppler shift, which is measured with millimeter accuracy by the spaceborne receiver. The index of refraction of the atmosphere can then be determined as a function of height. This index can then be analyzed to produce atmospheric temperature profiles and a measure of water vapor content. The first demonstration of this promising GPS application, which is also important to global change research, is scheduled to take place in 1995. Oceanography One importance of GPS to the field of oceanography is its potential ability to determine precise orbital parameters for the Topex/Poseidon satellite, which in turn, provides accurate radar altimetry of the ocean's surface. In general terms, Topex/Poseidon data improve in several ways as more precise orbital information becomes available. The issue is to separate orbital error from tides, general circulation, and gravity-field error. General circulation needs to be determined at the 1-centimeter level, a reasonably easy task with the GPS precise positioning service (PPS), but difficult, or perhaps even impossible, with other methods of orbit determination. Orbital error would no longer be a significant factor for all Topex/Poseidon data if orbits could be determined with an accuracy of Â± 1 millimeter. Using the GPS PPS, this is a distant, although not unobtainable goal. In the wider context of oceanography, one can assert that every time there has been a real improvement in navigation whole new fields of study have opened. GPS with SA set to zero provides a real improvement in navigation. Ocean-surface height measured by ships at sea, and the positioning of a tomographic lagrangian drifter also can be accomplished