This paper describes the upgrade of the GOCE Level 1b gradiometer processing as part of ESA's Payload Data Segment (PDS). Four processing steps have been identified which can be improved: 1. The optimal determination of the angular rates of the satellite, based on a combination of star sensor and gradiometer data. This is the so-called angular rate reconstruction. 2. The optimal determination of the spacecraft's attitude, again based on a combination of star sensor and gradiometer data. 3. The combination of data of all simultaneously available star sensors. And, 4. the calibration of the measured accelerations is improved by taking the time dependence of selected calibration parameters into account. In this paper the complete upgrade of the gradiometer Level 1b processing is described, and the expected improvement of the gravity gradients as well as the gravity field solutions is quantified. The largest overall improvements are due to the new method for angular rate reconstruction, mainly at the low to medium frequencies and at the harmonics of the orbital revolution frequency. In addition, spurious artifacts in the gravity gradient V _yy , which are caused by non-perfect common mode rejection, can be reduced significantly by the improved calibration approach. We show that the standard deviation of the gravity gradient tensor trace can be reduced by about 90 percent for the frequencies below the gradiometer measurement band (i.e. below 5 mHz) and by about 4 percent within the measurement band (from 5 to 100 mHz). The geoid error of satellite gravity gradiometry solutions based on 61 days of data is reduced from 3.0 to 2.2 cm between spherical harmonic degree 20 and 150. The corresponding gravity anomaly error, between the same degrees, is reduced from 0.7 to 0.5 mGal.

The Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite, launched on 17 March 2009, is designed to measure the Earth's mean gravity field with unprecedented accuracy at spatial resolutions down to 100 km. The accurate calibration of the gravity gradiometer on-board GOCE is of utmost importance for achieving the mission goals. ESA's baseline method for the calibration uses star sensor and accelerometer data of a dedicated calibration procedure, which is executed every 2 months. In this paper, we describe a method for monitoring the evolution of calibration parameter during that time. The method works with star sensor and accelerometer data and does not require gravity field models, which distinguishes it from other existing methods. We present time series of calibration parameters estimated from GOCE data from 1 November 2009 to 17 May 2010. The time series confirm drifts in the calibration parameters that are present in the results of other methods, including ESA's baseline method. Although these drifts are very small, they degrade the gravity gradients, leading to the conclusion that the calibration parameters of the ESA's baseline method need to be linearly interpolated. Further, we find a correction of -36 × 10 ^-6 for one calibration parameter (in-line differential scale factor of the cross-track gradiometer arm), which improves the gravity gradient performance. The results are validated by investigating the trace of the calibrated gravity gradients and comparing calibrated gravity gradients with reference gradients computed along the GOCE orbit using the ITG-Grace-2010s gravity field model.