Tropospheric Effects(cont The dry component, which is proportional to the density of the gas molecules in the atmosphere and changes with their distribution, represents about 90% of the total tropospheric refraction It can be modeled with an accuracy of about 2% that corresponds to 4 cm in the zenith direction using surface measurement of pressure and The wet refractivity is due to the polar nature of the water molecules and the electron cloud displacement Since the water vapor is less uniform both spatially and temporally, it cannot be modeled easily or predicted from the surface measurement As a phenomenon highly dependent on the turbulences in the lower atmosphere, the wet component is modeled less accurately than the dr The influence of the wet tropospheric zenith delay is about 5-30 cm that can be modeled with an accuracy of 2-5 cm Principles of the Global Positioning System 2005-41(1 Tropospheric Effects(cont The tropospheric refraction as a function of the satellites zenith distance is usually expressed as a product of a zenith delay and a mapping function A generic mapping function represents the relation between zenith effects at the observation site and at the spacecrafts elevation Several mapping functions have been developed(e.g, by Saastamoinen, Goad and Goodman, Chao, Lanyi), which are equivalent as long as the cutoff angle for the observations is at least 200 The tropospheric range co an be written as folle f(-)△+f(2) d(), fw()- mapping functions for dry and wet components, respectively A- vertical dry and wet components, respectively Gv Principles of the Global Positioning System 2005-4-1(18)9 Principles of the Global Positioning System 2005-4-1 17 Tropospheric Effects (cont.) ¾ The dry component, which is proportional to the density of the gas molecules in the atmosphere and changes with their distribution, represents about 90% of the total tropospheric refraction • It can be modeled with an accuracy of about 2% that corresponds to 4 cm in the zenith direction using surface measurement of pressure and temperature ¾ The wet refractivity is due to the polar nature of the water molecules and the electron cloud displacement • Since the water vapor is less uniform both spatially and temporally, it cannot be modeled easily or predicted from the surface measurements • As a phenomenon highly dependent on the turbulences in the lower atmosphere, the wet component is modeled less accurately than the dry • The influence of the wet tropospheric zenith delay is about 5-30 cm that can be modeled with an accuracy of 2-5 cm Principles of the Global Positioning System 2005-4-1 18 ∆ ∆∆ trop d d w w = + fz fz () () 0 0 ∆d w ∆ 0 0 , Tropospheric Effects (cont.) ¾ The tropospheric refraction as a function of the satellite’s zenith distance is usually expressed as a product of a zenith delay and a mapping function ¾ A generic mapping function represents the relation between zenith effects at the observation site and at the spacecraft’s elevation ¾ Several mapping functions have been developed (e.g., by Saastamoinen, Goad and Goodman, Chao, Lanyi), which are equivalent as long as the cutoff angle for the observations is at least 20o ¾ The tropospheric range correction can be written as follows: where fd(z), fw(z) - mapping functions for dry and wet components, respectively, - vertical dry and wet components, respectively