Bernese GPS Software Version 5.0 - Bernese GNSS Software

11. Troposphere Modeling and Estimation
11.1 Subdivision of the Atmosphere
The Earth’s atmosphere is usually subdivided into two main shells, the troposphere and the
ionosphere, since the signal propagation conditions are quite different in these two parts:
• The troposphere (neutral atmosphere), is the lower part of the atmosphere and extends
from the Earth’s surface up to an altitude of about 20 kilometers. The signal
propagation depends mainly on temperature, pressure, and water vapor content of the
atmospheric layers. At microwave wavelength the neutral atmosphere is not dispersive
while it is dispersive at optical wavelengths (SLR). For spaceborne receivers the
troposphere is irrelevant.
• The ionosphere is the upper part of the atmosphere. It is located approximately between
70 and 1000 kilometers above the Earth. The signal propagation is mainly
affected by free charged particles. The ionosphere is dispersive for microwave signals.
The ionosphere and relevant issues are described in detail in Chapter 12.
11.2 Motivation
Due to the availability of high accuracy orbits from IGS (see Section 2.2.1), orbit errors need
no longer be considered as an important error source. Propagation delays of the GNSS code
and phase signals caused by the neutral atmosphere (i.e., the troposphere) are probably the
ultimate accuracy-limiting factor for geodetic applications of the GNSS. The zenith path
delay (ZPD) due to tropospheric refraction is of the order of 2.3 m (or about 8 ns) for a
station at sea level and standard atmospheric conditions.
Let us first distinguish two kinds of troposphere biases:
• Relative troposphere biases caused by errors of (mismodeled) tropospheric refraction
at one endpoint of a baseline relative to the other endpoint.
• Absolute troposphere biases caused by errors of (mismodeled) tropospheric refraction
common to both endpoints of a baseline.
Both error sources are dealt with in detail in [Beutler et al., 1988]. It is remarkable that relative
troposphere biases cause primarily biased station heights whereas absolute troposphere
biases produce scale biases of the estimated baseline lengths.
Bernese GPS Software Version 5.0 Page 239