Geoscience Reference
In-Depth Information
As a possible successor of the GOCE mission several projects using
atom inter-
ferometry
are under consideration (Yu et al.
2006
). Such atom interferometers make
it possible to measure the absolute acceleration value plus the complete gravity gra-
dient tensor at very high sensitivity, i.e. 10
−
15
ms
−
2
or well below 1mE in space.
Implementing such a device would allow to observe a broader frequency range at a
higher temporal resolution leading to a higher accuracy of the measurement.
3.2 Impact of the Atmosphere on Satellite Gravity Observations
Exploring the Earth gravity field through satellites requires the removal of short
term (sub-daily) mass variations in the system Earth, including all solid, liquid and
atmospheric particles. Due to the fluctuation of those masses at various temporal
and spatial scales, like high and low atmospheric pressure systems, as well as due
to a strong dependency on the sampling rate of the ground track of the satellite, a
long observation time does not guarantee that the introduced variations in the gravity
field are canceled out by the mean operator. De-aliasing then denotes incorporating
such instantaneous variations of atmospheric masses with respect to a static mean
state of the atmosphere, either during the preprocessing of observations or during
the estimation procedure of the gravity field solution. The same holds for all other
mass variation effects inside the system Earth; only that within the atmosphere also
the center of mass of the atmospheric column is varying, which interferes again with
the satellite observations (Flechtner
2007
; Gruber et al.
2009
).
In order tominimize the aliasing signals the determination of accurate
Atmospheric
Gravity field Coefficients
(AGC) is indispensable. For the determination of AGC it
has become state of the art to use high resolutionNumericalWeatherModels (NWM),
which take into account the three-dimensional distribution of the atmospheric mass.
By subtracting the gravity spherical harmonics of the instantaneous atmosphere from
the ones of the mean atmospheric field, the residual gravity spherical harmonic series
are obtained. These describe the deviation of the actual gravity field from the mean
gravity field due to atmospheric mass variations.
3.3 From Atmosphere to Gravity
The atmosphere is nearly in hydrostatic equilibrium, which means that the change
in atmospheric pressure on the surface is proportional to the change of mass in
the corresponding atmospheric column, including variations in water vapor mass as
well as in the dry air mass.
describes the density along the column which can be
expressed in terms of surface load
ρ
(Boy et al.
2001
; Flechtner
2007
) and which
is linked directly to the surface pressure variation
Δσ
Δ
p
.
