Computation of Calibration Gradients and Methods for In-Orbit Validation of Gradiometric GOCE Data
Prof. Dr. Jürgen Müller(1), Dr Heiner Denker(1)
, Mr Focke Jarecki(1)
, and Mrs Karen Insa Wolf(1)
University of Hannover,
The accuracy planned for GOCE gravity gradiometry requires special
independent concepts for calibration and validation of the measured
Although the gradiometer will be calibrated internally on ground (pre-
flight) and prior to the measurement phase itself in orbit (in-
further external calibration is obligatory to establish the
relationship of the measurements to the earth's gravitational field at
the required accuracy level.
To keep the accuracy of the satellite data, a region with
correspondingly well known gravity field parameters has to be selected
for calibration. Central Europe has been identified as such a region
the Institut für Erdmessung has collected a database of 5' by 5'
gridded gravity anomalies.
For calibration and validation purposes, reference gravitational
gradients at GOCE altitude are computed from these terrestrial gravity
anomalies. The upward continuation is carried out by least squares
collocation and integral formulas.
The calibrated gravitational gradients have to pass a further quality
Even without the availability of independent data, some conclusions on
quality and consistency of the collected data can be drawn by
comparing measurements in the same satellite position, i.e. satellite
track cross-overs or repeat orbits.
Unfortunately, due to the orbit characteristics of the mission,
identical repeat positions are reached seldom throughout the mission
lifetime, so one has to take into account satellite ground track cross-
overs as well.
For those, a reduction concept has to be applied to consider the
differences caused by different satellite altitudes and orientations.
It is shown here, that present global gravity field models meet the
accuracy and resolution requirements of the reduction concept.
This relative validation procedure obviously just allows the
identification of parts of the possible errors.
So a further validation step has to be integrated to obtain absolute
differences between the calibrated measurements and the earth's real
If this comparison is done with terrestrial data in the same well
observed region mentioned above, it has to be performed in close
coordination with the calibration step to avoid misinterpretation.
But in contrast to the gradients' calibration, the absolute validation
step can also be carried out after further processing of the
measurements, e.g. based on new regional or global gravity field
models or the use of oceanographic data.
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,