Geodetic and Atmospheric Background - Atmospheric Effects in Space Geodesy - page 3

models like mapping functions and gradients need to be applied in the case of the
neutral atmosphere which is non-dispersive for microwaves.
Space geodetic techniques likeVLBI, GNSS, DORIS, or SLRare not only affected
by propagation delays or phase advances of the signals, but geodetic estimates are
also influenced by other atmospheric effects. Satellites at low orbits are subject to
atmospheric drag, which causes decreasing orbit altitudes and the destruction dur-
ing re-entry into the atmosphere starting at about 100km. In any case atmospheric
drag has to be modelled very carefully to achieve highest accuracy for satellite
ephemerides. Moreover, it is not just the direct interaction with atmospheric particles,
but also the gravitational effect on satellite orbits that need to be considered. Kar-
bon et al. ( 2013 ) in this topic describe the determination of atmospheric gravity from
data of numerical weather models, e.g., from the European Centre for Medium-range
Weather Forecasts (ECMWF). These gravity changes due to atmospheric variability
are also of great importance for gravity missions like the Gravity Recovery and
Climate Experiment (GRACE) or the Gravity Field and Steady-State Ocean Circula-
tion Explorer (GOCE) because the accurate correction of the observations prevents
aliased errors in static or time-varying gravity field models.
Rigorously, atmospheric gravity is determined from the three-dimensional density
distribution in the atmosphere, but it can also be approximated by calculations based
on surface pressure. Surface pressure is also the meteorological input parameter to
determine the displacement of the solid Earth due to atmospheric loading which
is described in detail by Wijaya et al. ( 2013 ) in this topic. Atmospheric loading
displacements can be as large as 2-3 cmwith respect to a mean state which goes along
with a mean reference pressure. This reference pressure, which has dependence on
the orography within an atmospheric analysis, is also of importance for hydrostatic
zenith delays (Nilsson et al. 2013 ) and atmospheric gravity variations (Karbon et al.
2013 ), and it will be discussed in Sect. 2.6 .
For the rigorous determination of atmospheric loading corrections, we need the
surface pressure not only at the site of interest, but—theoretically—over the whole
globe (see (Wijaya et al. 2013 )). The surface pressure variations (w.r.t. the reference
pressure) are weighted by the Green's functions which also account for geophysical
properties of the Earth via the load Love numbers. Things get complicated for sites
close to the ocean, because the ocean surface responds in a complex way to overlying
atmospheric pressure variations. In the lack of real ocean models, we need to apply
simple models like the inverted barometer hypothesis (IB) as described in Sect. 2.8 .
Other interesting phenomena are atmospheric tides, the most important of which
are thermally induced pressure variations at the 1-2hPa level occurring with periods
of 24h (S1) and 12h (S2). When using data from numerical weather models with
a 6-hour time resolution, it is not possible to properly account for S2 because it
is exactly at the Nyquist frequency of the underlying data. More information on
atmospheric tides is provided in Sect. 2.7 .
Space geodetic techniques like VLBI, GNSS, SLR, or DORIS are used to observe
Earth rotation variations, a large part of which is caused by atmospheric effects.
Schindelegger et al. ( 2013 ) in this topic focus on atmospheric excitation of Earth
rotation, e.g., how it can be determined from density variations (approximated by