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M.-H. Rio
measurement is the difference of acceleration measured between two accelerometers in the direction joining them, which correspond to the second derivatives of the
gravitational potential.
The gradiometer will provide information for the short and medium wavelengths of the Earth Gravity field while longer wavelengths will be obtained by
high-accuracy Satellite-to Satellite measurements and star tracker information. The
primary objectives of the GOCE mission are to determine, at a spatial resolution
of 100 km, the Earth’s gravity field and the geoid height with unprecedented accuracy of respectively 1 mGal and 1 cm. A high number of scientific studies based on
altimetry uses gridded maps of Sea Level Anomalies computed from the merging of
data from different missions (Ducet et al., 2000).
The actual resolution of these gridded maps depends both on the number
of satellites and of their across-track distances. In a 2-satellite configuration
(Topex + ERS for instance), the maximum resolution achieved, for a 10 days
temporal resolution, is 250–300 km. In a 3-satellites configuration, the spatial
resolution is increased to 200 km. In the future, a configuration made of a
wide-swath altimeter like SWOT (Surface Water and Ocean Topography) and
2 nadir altimeters would further increase the spatial resolution to 100 km. At
that resolution, the accuracy of future GOCE geoids, and therefore of future
GOCE-based Mean Dynamic Topography will be sufficient to compute 100 km resolution maps of absolute dynamic topography, and associated geostrophic surface
currents.
However, nadir altimeters measure the sea level above the ellipsoid with a maximum frequency of 20 Hz (1 measurement every 350 m). In order to reduce the
measurement noise, single pulses are often averaged to a reduced frequency of 1 Hz
(1 measurement every 7 km). In the future, even higher spatial resolution will be
achieved thanks to the use of wide-swath altimeters (∼1 km). At this spatial resolution, the accuracy of future geoid models based on GOCE data will be insufficient
to directly apply Equation (10.1). Applying the repeat track method will therefore
be mandatory to compute along-track Sea Level Anomalies. In order to take major
benefit from the raw altimetric measurements, and extract the sea level anomalies
with maximum accuracy and resolution, high resolution mean profiles are needed
or, in the case of non repetitive orbit missions, high resolution Mean Sea Surfaces.
Then, to obtain absolute dynamic topography values from the sea level anomalies,
the computation of high resolution Mean Dynamic Topography estimates will be
mandatory. The required MDT resolution will be highly dependent on the ocean
region of interest (higher in western boundary currents, in semi-enclosed seas, in
short straits, gyres, where GOCE resolution will not be sufficient. . .lower in the center of suptropical gyres, where GOCE resolution may be sufficient. . .). To achieve
the higher resolution required, methods similar to those described in Section 10.4
will be applied, either using in-situ gravimetric data to enhance the geoid resolution,
or using in-situ oceanographic data to enhance the resolution of GOCE-based Mean
Dynamic Topography.
M.-H. Rio
measurement is the difference of acceleration measured between two accelerometers in the direction joining them, which correspond to the second derivatives of the
gravitational potential.
The gradiometer will provide information for the short and medium wavelengths of the Earth Gravity field while longer wavelengths will be obtained by
high-accuracy Satellite-to Satellite measurements and star tracker information. The
primary objectives of the GOCE mission are to determine, at a spatial resolution
of 100 km, the Earth’s gravity field and the geoid height with unprecedented accuracy of respectively 1 mGal and 1 cm. A high number of scientific studies based on
altimetry uses gridded maps of Sea Level Anomalies computed from the merging of
data from different missions (Ducet et al., 2000).
The actual resolution of these gridded maps depends both on the number
of satellites and of their across-track distances. In a 2-satellite configuration
(Topex + ERS for instance), the maximum resolution achieved, for a 10 days
temporal resolution, is 250–300 km. In a 3-satellites configuration, the spatial
resolution is increased to 200 km. In the future, a configuration made of a
wide-swath altimeter like SWOT (Surface Water and Ocean Topography) and
2 nadir altimeters would further increase the spatial resolution to 100 km. At
that resolution, the accuracy of future GOCE geoids, and therefore of future
GOCE-based Mean Dynamic Topography will be sufficient to compute 100 km resolution maps of absolute dynamic topography, and associated geostrophic surface
currents.
However, nadir altimeters measure the sea level above the ellipsoid with a maximum frequency of 20 Hz (1 measurement every 350 m). In order to reduce the
measurement noise, single pulses are often averaged to a reduced frequency of 1 Hz
(1 measurement every 7 km). In the future, even higher spatial resolution will be
achieved thanks to the use of wide-swath altimeters (∼1 km). At this spatial resolution, the accuracy of future geoid models based on GOCE data will be insufficient
to directly apply Equation (10.1). Applying the repeat track method will therefore
be mandatory to compute along-track Sea Level Anomalies. In order to take major
benefit from the raw altimetric measurements, and extract the sea level anomalies
with maximum accuracy and resolution, high resolution mean profiles are needed
or, in the case of non repetitive orbit missions, high resolution Mean Sea Surfaces.
Then, to obtain absolute dynamic topography values from the sea level anomalies,
the computation of high resolution Mean Dynamic Topography estimates will be
mandatory. The required MDT resolution will be highly dependent on the ocean
region of interest (higher in western boundary currents, in semi-enclosed seas, in
short straits, gyres, where GOCE resolution will not be sufficient. . .lower in the center of suptropical gyres, where GOCE resolution may be sufficient. . .). To achieve
the higher resolution required, methods similar to those described in Section 10.4
will be applied, either using in-situ gravimetric data to enhance the geoid resolution,
or using in-situ oceanographic data to enhance the resolution of GOCE-based Mean
Dynamic Topography.