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and latitude-dependent spatial variations as well as regular and non-regular temporal
variations in the ionosphere.
1 Setting the Stage: Geodetic Motivation
Space geodesy refers to observations that are transmitted or received by natural or
artificial objects outside the lower portions of the atmosphere, i.e., in space where
the density of the atmosphere is sufficiently small to allow stable satellite orbits.
We do not consider airborne instruments as space geodesy but we restrict the term
space geodesy
to extragalactic radio sources in the case of Very Long Baseline
Interferometry (VLBI) and to satellites. VLBI is a microwave-based technique that
measures the difference in arrival times at pairs of locations of signals from a radio
source by cross correlation (Schuh and Böhm
2012
). The observed radio sources are
principally extragalactic objects but signals from satellites can also be used. Geodetic
VLBI is the only technique for the realization of the Celestial Reference Frame (CRF)
and for the estimation of Universal Time (UT1) and nutation over longer time spans.
Furthermore, VLBI is a primary technique for the determination of the scale of the
terrestrial reference frame due to the accurate determination of very long baseline
lengths.
Global Navigation Satellite Systems (GNSS), including the United States Global
Positioning System (GPS), the Russian GLONASS (Globalnaja Nawigazionnaja
Sputnikowaja Sistema), the European Galileo, and others use satellites at orbital
heights of about 20000km that transmit microwave signals received by antennas
at the Earth surface or onboard other satellites like Low Earth Orbiters (LEO). The
strategy of the French system called Doppler Orbitography by Radiopositioning Inte-
grated on Satellite (DORIS) is different, with themicrowave signals being transmitted
by antennas on the ground and received onboard the satellites.
All microwave-based space geodetic techniques mentioned above (VLBI, GNSS,
DORIS) as well as altimeter satellite missions are subject to propagation effects in the
atmosphere. With respect to propagation, we divide the atmosphere into the neutral
atmosphere up to about 100km (see Sect.
2
in this part of the topic), and into the
ionosphere where the density of free electrons and ions is large enough to influence
the propagation of microwave signals, extending from about 60-2000km depending
on time and location (see Sect.
4
in this part). Microwave signals experience phase
and group delays of the same size in the neutral atmosphere, but they are subject to
phase advances and group delays in the ionosphere. The situation is different with
Satellite and Lunar Laser Ranging (SLR/LLR), which observe in the optical regime
and are only subject to delays in the neutral atmosphere.
Alizadeh et al. (
2013
) and Nilsson et al. (
2013
), both in this topic, deal with the
treatment of atmospheric delays in the analysis of space geodetic observations in the
neutral atmosphere and the ionosphere, respectively. While ionospheric delays can
be determined and eliminated by observing at more than one frequency, dedicated
