The first GRACE gravity model EIGEN-GRACE01S was released on July 25, 2003. It is based on 39 days of preliminary GRACE flight instrument data gathered in August and November 2002. This model is about 5 times more accurate than the latest CHAMP field and about 50 times more accurate than pre-CHAMP satellite only gravity models (at 1000 km half wavelength).
|Fig.1: Gravity anomaly map derived from tracking data of 30 Earth orbiting satellites over more than 20 years (GRIM5-S1 model)||Fig.2: Gravity anomaly map derived from 16 months of CHAMP data only (EIGEN-CHAMP02S model)||Fig.3: Gravity anomaly map derived from 39 days of GRACE data only (EIGEN-GRACE01S model)|
Prior to GRACE and CHAMP, the long-wavelength part of the Earth's gravity field from space was determined from various tracking measurements of a greater number of Earth orbiting satellites. These measurements were of considerably varying vintage and quality, and of incomplete geographical coverage. Consequently the accuracy and resolution of the resulting Earth gravity field models were limited, with most of the satellite contributions limited to wavelengths of 1000 km or longer. At shorter wavelengths, the errors were too large to be useful. Only broad geological features of the Earth's structure could be detected. As a result, improvements to the Earth gravity models at medium and short wavelengths had to come from the use of measurements of terrestrial, marine- and air-gravimetry - also of varying vintage, quality and geographic coverage.
Fig.4: Cumulated geoid differences EGM96S, EIGEN-CHAMP02S and EIGEN-GRACE01S vs. resolution
Since the launch of CHAMP (GFZ�s CHAllenging Minisatellite Payload Mission) in July 2000 a new epoch in gravity modeling has begun. For the first time the Earth�s gravity field can be determined from instrument data of a single satellite. This improvement is due to the well-suited CHAMP instrumentation: High-low GPS measurements guarantee precise homogeneous satellite positioning on a global scale, an onboard accelerometer measures all non-gravitational forces acting on the satellite and a set of star-trackers determine the spacecraft attitude within the inertial reference frame.
GRACE, launched in March 2002 and being a tandem mission, is based for its high-low intersatellite tracking part on the CHAMP instrumentation with an improved accelerometer. Additionally the low-low inter-satellite range between the twin satellites is measured very precisely (mm level) with a K-band ranging system. Range variations between the pair of GRACE satellites - only about 220 km apart from each other - are a direct measure of medium to short wavelength features of the Earth�s gravity field and therefore EIGEN-GRACE01S gives more detail of the Earth's geological features. Future GRACE gravity models, derived from longer data spans and with further improved processing methods and models, are expected to increase the resolution and accuracy further.
Fig.5: Mantle gravity anomalies [mGal] calculated by subtracting from observed gravity (EIGEN-GRACE01S) the crustal gravity effect including topography (bathymetry) and crustal density variations down to the Moho boundary
This new gravity field will allow solid Earth scientists to more accurately infer the Earth's internal structure at finer resolution than was ever before possible from space. Ocean scientists can combine this gravity model with ocean height measurements from satellite altimeters to study global ocean circulation on a finer scale than has been previously possible. This will, in turn, enable a better understanding of the processes that drive the Earth's dynamic system (solid Earth, ocean and atmosphere), thus leading to better analysis and predictions of climate change & natural hazards (e.g., Earthquakes).