Global Positioning System Reference
In-Depth Information
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C
HAPTER 6
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T
ROPOSPHERE AND IONOSPHERE
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9.8
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This chapter begins with a general overview of the troposphere and ionosphere and
a brief discussion of the relevancy of the atmosphere to GPS surveying. In Section
6.2 the tropospheric refraction is derived starting with the commonly used equation
that expresses the refractivity as a function of partial pressure of dry air, partial water
vapor pressure, and temperature. The equation for the zenith hydrostatic delay (ZHD)
by Saastamoinen (1972), the expression for the zenith wet delay (ZWD) by Mendes
and Langley (1999), and Niell's (1996) function for mapping the slant delays to the
zenith delays are given without derivation. The horizontal gradient method is briefly
discussed as a means to incorporate azimuth dependency of the refractivity. We then
establish the relationship between the zenith wet delay and precipitable water vapor
(PWV). Section 6.3 deals with tropospheric absorption and water vapor radiometers
(WVR) that measure the tropospheric wet delay. We present and discuss the radiative
transfer equation and the concept of brightness temperature. To demonstrate further
the principles of the water vapor radiometer, we discuss the relevant absorption
line profiles for water vapor, oxygen, and liquid water. This is followed by a brief
discussion of retrieval techniques to compute the wet delay and radiometer calibration
using tipping curves.
In Section 6.4 the causes of ionization are briefly discussed. The derivation of
the ionospheric refraction is sketched beginning with the Appleton-Hartree formula.
Section 6.5 gives expressions for the ionospheric delay of codes and ionospheric ad-
vances of carrier phases. Section 6.6 centers around the ionospheric-free and iono-
spheric functions for pseudoranges and carrier phases. The chapter concludes with
brief remarks on the global ionospheric model (GIM).
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