GOCE gradients in various reference frames and their accuracies

The objective of GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) is the determination of the Earth’s gravity field with high spatial resolution. The main science sensor (the gradiometer) measures differential accelerations, from which the gravitational gradients, i.e. the matrix of the second derivatives of the gravitational potential, are derived. Some of them (the diagonal components of the gravitational tensor) are observed with highest accuracy, 4 mE/ √ Hz in a frequency range from 5 mHz to 100 mHz, whereas the off-diagonals are obtained less accurately. The gradients will be observed in the instrument frame, which approximates the along-track oriented, local orbital frame. For the transformation of the gradients in other frames (e.g. in the strictly earth-pointing frame or a local geodetic frame), the transformation parameters (orientation angles) and all components of the gravity tensor have to be known with sufficient accuracy. We show how the elements of the gravitational tensor and their accuracies look like in the various frames as well as their spectral behaviour, if only the GOCE observations are used for the transformation. Only V ′ zz keeps approximately its original accuracy in all frames discussed, except in the earth-fixed frame ITRF (International Terrestrial Reference Frame). Therefore we recommend to analyse the gradients as ‘close’ as possible in the observation frame.