C. C. Tscherning

SIMULATION OF REGIONAL GRAVITY FIELD RECOVERY FROM SATELLITE GRAVITY GRADIOMETER DATA USING COLLOCATION AND FFT

When planning a satellite gravity gradiometer (SGG) mission, it is important to know the quality of the quantities to be recovered at ground level as a function of e.g. satellite altitude, data type and sampling rate, and signal variance and noise. This kind of knowledge may be provided either using the formal error estimates of wanted quantities using least-squares collocation (LSC) or by comparing simulated data at ground level with results computed by methods like LSC or Fast Fourier Transform (FFT).Results of a regional gravity field recovery in a 10o×20o area surrounding the Alps using LSC and FFT are reported. Data used as observations in satellite altitude (202 or161 km) and for comparison at ground level were generated using theOSU86F coefficient set, complete to degree 360. These observations are referred to points across simulated orbits. The simulated quantities were computed for a 45 days mission period and 4 s sampling. A covariance function which also included terms above degree 360 was used for prediction and error estimation. This had the effect that the formal error standard deviation for gravity anomalies were considerably larger than the standard deviations of predicted minus simulated quantities. This shows the importance of using data with frequency content above degree 360 in simulation studies. Using data at202 km altitude the standard deviation of the predicted minus simulated data was equal to8.3 mgal for gravity and0.33 m for geoid heights.

Altimetric gravity anomalies in the Norwegian‐Greenland Sea ‐ Preliminary results from the ERS‐1 35 days repeat mission

New fast delivery 35 days repeat ERS-1 altimeter data have been used to recover the gravity field in the Norwegian-Greenland Sea. Furthermore the spectral characteristics of the gravity field have been investigated from 18 arcs of 3 days repeat period data. The recovered gravity field correlates exclusively with the major geological structures, which clearly demonstrates the potential of these new ERS-1 altimeter data.

Mass balance and surface movement of the Greenland Ice Sheet at Summit, central Greenland

During the GRIP deep drilling in Central Greenland, the ice sheet topography and surface movement at Summit has been mapped with GPS. Measurements of the surface velocity are presented for a strain net consisting of 13 poles at distances of 25–60 km from the GRIP site. Some results are: The GRIP site is located approximately 2 km NW of the topographic summit; the surface velocity at the GISP 2 site is 1.7 m/yr in the W direction. The present mass balance at Summit is calculated to be −0.03±0.04 m/yr, i.e. close to steady state. This result is the best now available for Summit. A small thinning rate might be a transient response of the Greenland Ice Sheet due to the temperature increase at the Wisconsin-Holocene transition.

The 1-cm geoid after GOCE

The new satellite gravity missions (CHAMP, GRACE and GOCE) will all bring substantial improvements to our knowledge of the gravity field and thereby of the (quasi-) geoid. One of the aims of the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) is to determine the geoid to within 1 cm at wavelengths down to 100 km.

External calibration of GOCE SGG data with terrestrial gravity data: A simulation study

Terrestrial gravity anomalies selected from three extended continental regions having a smooth gravity field were used in order to determine the appropriate size of the area for gravity data collection as well as the required data-sampling for calibration of the GOCE satellite gravity gradient (SGG) data. Using Least Square Collocation (LSC), prediction of gravity gradient components was carried out at points on a realistic orbit. Based on the mean error estimation it was shown that up to 80% of the signal of the gravity gradient components, as it is expressed through the covariance function of the terrestrial gravity data, can be recovered in the case of an optimal size of the collection area and of the optimum resolution of the data. These optimal conditions e.g. for the Australian gravity field, correspond to an 10° × 12° area extend and a 5′ data-sampling. It was also numerically demonstrated that it is possible to calibrate the GOCE SGG data for systematic errors such as bias and tilt.

MERGING OF AIRBORNE GRAVITY AND GRAVITY DERIVED FROM SATELLITE ALTIMETRY: TEST CASES ALONG THE COAST OF GREENLAND

The National Survey and Cadastre - Denmark (KMS) has for several years produced gravity anomaly maps over the oceans derived from satellite altimetry. During the last four years, KMS has also conducted airborne gravity surveys along the coast of Greenland dedicated to complement the existing onshore gravity coverage and fill in new data in the very-near coastal area, where altimetry data may contain gross errors. The airborne surveys extend from the coastline to approximately 100 km offshore, along 6000 km of coastline. An adequate merging of these different data sources is important for the use of gravity data especially, when computing geoid models in coastal regions.The presence of reliable marine gravity data for independent control offers an opportunity to study procedures for the merging of airborne and satellite data around Greenland. Two different merging techniques, both based on collocation, are investigated in this paper. Collocation offers a way of combining the individual airborne gravity observation with either the residual geoid observations derived from satellite altimetry or with gravity derived from these data using the inverse Stokes method implemented by Fast Fourier Transform (FFT).

Comparison of retracking algorithms for ERS-1 altimeter data over Greenland

Abstract Six months of processed ERS-1 full waveform altimetry data have been used to study the ERS-1 altimeter performance over the Greenland ice sheet. Surface profiles from repeated ground tracks have been studied to explain the character of the data and the performance of several waveform processing techniques. A cross-over analysis has been performed to assess the repeatability of retracked data. The RMS of ocean-mode cross-over differences is found to be around 0.5 m for most parts of the central ice sheet, with values larger than 2 m for areas with large variations in surface height. For these areas the ice mode cross-over differences are smaller. Average waveforms from the central part of the ice sheet show the presence of substantial sub-surface volume scattering.