Tilo Schöne (GFZ, Helmholtz-Centre Potsdam, Germany); Julia Illigner (GFZ, Helmholtz-Centre Potsdam, Germany); Robert Weiß (BfG, The German Federal Institute of Hydrology, Germany); Thomas Artz (BfG, The German Federal Institute of Hydrology, Germany); Xinge Huang (TU Berlin, Germany)

Consistent calibration and monitoring is a basic prerequisite for providing reliable time series of global and regional sea level variations from altimetry. The precision of sea level measurements and regional biases for eight altimeter missions (Jason-1/2/3, Sentinel-6MF, Envisat, Saral, Sentinel-3A/B) is assessed at eleven GNSS-controlled tide gauge stations in the German Bight (SE North Sea) for the period 2002 to 2021. The gauges are operated operationally by the German Waterway and Shipping Administration (WSV) and The German Federal Institute of Hydrology (BfG) and are partly located in open water, partly at the coast close to mudflats. The altimetry is extracted at virtual stations with distances from 2 to 24 km from the gauges. The processing is optimized for the region and adjusted for the comparison with instantaneous tide gauges readings. An empirical correction is applied to account for mean height gradients and slight differences of the tidal dynamics between gauge and altimetry. It improves the agreement between the two data sets considerably by 15-75%. The precision of the altimeters is depending on location and mission and is shown to be at least 1.8 to 3.7 cm based on an assumed precision of 2 cm for the gauges. The accuracy of the regional mission biases is strongly dependent on the knowledge of the sea surface height difference between the measurement locations. The most consistent biases are obtained based on the CLS2011 model with mission dependent accuracies from 1.3 to 3.4 cm. Hence, operational GNSS-controlled tide gauges, e.g. by WSV and BfG might complement the calibration and monitoring activities at dedicated Cal/Val stations.