Life on planet Earth is dependent upon the atmospheric greenhouse effect to maintain a breathable atmosphere and a climate zone that is conducive to life. Ironically, it is this very greenhouse effect that has many scientists concerned. Is the planet warming or is it cooling? Ask different "experts" and you'll get different answers. Therefore, interest in climate studies has been increasing.
Water vapor is a greenhouse gas that can lead to global warming. It is both a symptom and a cause of the greenhouse effect. Water vapor continually cycles through evaporation and condensation, transporting heat energy around the Earth and between the surface and the atmosphere. Water vapor in the atmosphere allows the short wavelength radiation of the sun to pass through the atmosphere, but traps the long wavelength radiation emitted by the Earth's surface. This trapped radiation causes the temperatures to increase.
As the temperatures increase, the air can sustain a larger amount of water vapor, thereby increasing the greenhouse effect. This compounds the effect of other greenhouse gases that are controllable by man (CO2 for example). Thus, it is important that water vapor is understood for the formation of climate models.
The GPS techniques will better the analysis of global trends by providing an increased spatial and temporal resolution of atmospheric data.
Click here for more information on water vapor in the climate system.
Weather forecast models require three-dimensional temperature, moisture, pressure, and wind data (four dimensional in time). Typically this data is obtained through radiosondes and other techniques . These techniques are often limited spatially and temporally, thus limiting the effectiveness of the forecast models. By better understanding water vapor, as well as the other inputs to the models, more accurate forecast models can be developed and new observational techniques can be investigated.
The GPS techniques discussed here will provide additional atmospheric data to increase vertical resolution in the case of space based GPS receivers and horizontal resolution in the case of ground based GPS receivers. With both techniques, temporal resolution will be greatly improved.
As radio signals propagate through the atmosphere, they are refracted (bent and delayed) by the ionosphere and the neutral atmosphere (troposphere). The ionospheric effects can largely be removed through the application of a dual frequency signal. The hydrostatic component of the troposphere, often referred to as the "dry" component, is accurately modeled. The wet component, however, which is the region of the atmosphere below 8-10 km and contains significant levels of water vapor, is poorly modeled.
The effect of the wet troposphere is sometime mitigated by specifying a minimum elevation angle. this eliminates the bending effect, but not the delay. Sometimes the tropospheric delay is "solved for" in an orbit determination system. This affects orbit determination of satellites using doppler signals as well as positioning receivers on the surface of the Earth. This includes the use of TRANET, DORIS, VLBI, and GPS. Radar altimeters also encounter the tropospheric delay.
By solving for the wet troposphere independently with adequate spatial and temporal coverage, it may be possible to form empirical models (or even mathematical models) that will allow the delay to be removed without having to estimate it. Currently this is being done using data sets from water vapor radiometers.
For more information on Non-GPS techniques, click here.
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