Nw 373 x 105eT2

where P is pressure in millibars (mb), T is temperature in degrees Kelvin, and partial water vapor pressure, e, is in mb, all measured at the surface. Evaluation of Eq. (3.4.1) has been treated by Saastamoinen (1972).

In the zenith direction, the tropospheric delay is about 2 meters at sea level, but the effect is dependent on the elevation angle of the viewing direction through the atmosphere. For example, at 10° elevation, the delay is about 12 meters. Mapping functions are usually adopted to map the zenith effect to any elevation angle. These mapping functions are fundamentally dependent on the cosecant of the elevation angle, but the complexity of the mapping functions varies.

Various methods are used to generate the propagation delay, including ray-tracing. The signal delay is predominantly dependent on the cosecant of the satellite elevation seen at the station. A frequently used expression for the tropospheric delay, Apt, is

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