Geoscience Reference
In-Depth Information
Global Navigation Satellite Systems (GNSS) and Very Long Baseline Interferometry
(VLBI), and those for optical techniques, like Satellite Laser Ranging (SLR), are dis-
cussed. Usually, residual tropospheric delays are estimated in the data analysis, and
the parameterization needed to do so is presented. Other possibilities of correcting
for the tropospheric delays are their calculation by ray-tracing through the fields of
numerical weather models and by utilizing water vapor radiometer measurements.
Finally, we shortly discuss how space geodetic techniques can be used in atmospheric
analysis in meteorology and climatology.
1 Introduction
After the signals of the space geodetic techniques have passed through the ionosphere
(see Part 2 (Alizadeh et al.
2013
) for more information about the ionospheric effects)
they also need to pass through the neutral atmosphere (primarly the troposphere)
before they are observed at the surface of the Earth. In the troposphere the sig-
nals experience propagation delays, just as they do in the ionosphere. However,
the frequency dependence of the delays is small. For microwave techniques like
Global Navigation Satellite Systems (GNSS) and Very Long Baseline Interferom-
etry (VLBI) there is practically no frequency dependence; thus it is impossible to
remove the tropospheric delay with a multi-frequency combination like it is in the
ionospheric case. For optical techniques like Satellite Lase Ranging (SLR) there
exists a small frequency dependence and thus it is possible in principle to remove
the tropospheric delays using two frequencies (see Sect.
4.3.2
); however due to the
amplification of the noise this is currently not practical. Thus the tropospheric delays
need to be corrected for by other means.
For this part of the topic it is assumed that the reader is familiar with the basic
properties of the atmosphere, i.e. what is described in Part 1 (Böhm et al.
2013
).
We begin here with the basic description of the refractivity of the air in the neutral
atmosphere (Sect.
2
). Expressions for calculating the refractivity from basic meteo-
rological measurements are presented, as well as the commonly used division of the
refractivity into a hydrostatic and a wet part. In Sect.
3
these results are used for cal-
culating the tropospheric path delay, and the properties of the hydrostatic (Sect.
3.1
)
and wet (Sect.
3.2
) delays are discussed. The modeling of the tropospheric delays
in the space geodetic data analysis is described in Sect.
4
. Either the tropospheric
delays are estimated in the data analysis or tropospheric delays obtained by external
measurements are used. Two possible sources of external tropospheric information
are considered: from ray-tracing through numerical weather models (Sect.
4.1
) and
inferred from microwave radiometer measurements (Sect.
4.4
). The models that are
commonly used when estimating the tropospheric delays in the data analysis are
given in Sects.
4.2
(microwaves) and
4.3
(optical). However, since the refractivity
of the atmosphere is varying randomly due to atmospheric turbulence these models
are not perfect. The effects of turbulence are described in Sect.
5
. This part of the
topic concludes with a discussion of the possible use of space geodetic techniques
for studies of the atmosphere (Sect.
6
).
