Calibration and Error Assessment of GOCE Data

The GOCE level lb data products consist, among others, of time lines of calibrated and corrected gravity gradient data and SST data. The calibration for GOCE can be described as a multi-step procedure consisting of a pre-flight, on-ground calibration, an in-flight internal calibration and a so-called external (or absolute) calibration step. The latter consists of checking and correcting the data by comparison with external data, models or other external gravity field knowledge in order to improve absolute calibration parameters determined earlier and to correct for known or unknown errors in the data which have not been corrected for before. Obvious choices for external calibration are to compare the GOCE data with ground based gravity data, with existing global gravity field models or to intercompare different GOCE data products, like the accelerometer common mode observations with the GPS data. The latter two methods are studied in more detail here. It appears that for the gradiometer observations, the use of existing global gravity field models may help the calibration for the low frequencies below the measurement bandwidth. The calibration of the common mode observations using GPS data depends heavily on the quality of prior conservative force models. In addition, we consider here the problem of in flight performance monitoring of the GOCE measurements using a collinear track technique in analogy to satellite altimetry. We discuss the results of a new algorithm designed for a sun-synchronous frozen repeat orbit and treat two extreme scenarios. For the best case scenario our conclusion is that it is possible to monitor the stability of the instrument within the measurement band to around (MATH) whereas for our worst case scenario this is a factor of 10 less accurate.