Improved GRACE Level-1 and Level-2 Products and Their Validation by Ocean Bottom Pressure

The GRACE mission, launched in 2002, is the first realization of the satellite-to-satellite tracking (SST) concept in the low-low mode. These innovative observations, in combination with high-low GPS SST and accelerometer data, are the basis for dramatic improvements in the field of static and time-variable gravity field determination within the first years of the mission. Nevertheless, mid of 2005 the quality of these models was still significantly below that projected by simulation studies before launch (approximately a factor of 18 for monthly models). This has a significant effect on all higher-level scientific products derived from GRACE gravity field solutions such as ice melting in the Polar Regions, derivation of surface and deep ocean currents or estimation of ocean mass for sea level rise analysis. Therefore three principle causes have been investigated: the quality of the instrument data and the processing strategy to derive calibrated instrument from raw data, deficiencies in the background models, such as the short-term non-tidal atmosphere and ocean mass variations, and possibly wrong or insufficient parameterization of the K-band range-rate or accelerometer observations. Finally, a validation of the gravity models with globally distributed in-situ ocean bottom pressure (OBP) data was initiated. In this chapter the main results of this project are summarized which then were the basis for a consistent reprocessing of GRACE data leading to more consistent and accurate gravity field products.