ABSTRACT
This chapter discusses the use of the global positioning system (GPS) for monitoring of atmospheric water vapor. In this chapter, one can also find the history of the appearance of this system and the prospects of GPS used for global monitoring. Measurement of the concentration of water vapor in the atmosphere using the GPS is a variety of microvawe hygrometry. This ability to control the humidity of the air came after the deployment the satellite GPS constellation that filled the atmosphere with microwave signals based on atomic clocks. GPS is working under, in principle, all weather conditions and anywhere as long as there is an unobstructed line of sight to four or more GPS satellites. There are three components to the GPS system: (1) a space segment (the satellite constellation), (2) a control segment (the network of monitoring and tracking stations which are operated by the military), and (3) a user segment. For the user segment, GPS is designed to be a passive system. The key components of the International GPS Service (IGS) include a network of continuously operating GPS stations, data centers that collect and distribute data through the Internet, analysis centers who produce products based on data collected from the network, and a governing body to guide the development and progress of the IGS. Water vapors determined during the process of GPS observations can be represented as the integrated water vapor (IWV) or the total precipitable water (TPW). IWV is the total amount of water vapor present in a vertical atmospheric column. TPW is the amount of water in a column of the atmosphere, if all the water and water vapor in that column were condensed to liquid or precipitated as rain. IWV and TPW are equivalent parameters, but having different units of measurement.
