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Ocean gliders have become ubiquitous observation platforms in the ocean inrecent years. They are also increasingly used in coastal environments. Thecoastal observatory system COSYNA has pioneered the use of gliders in theNorth Sea, a shallow tidally energetic shelf sea.  For operational reasons, the gliders operated in the North Sea are programmedto resurface every 3–5 h. The glider's dead-reckoning algorithm yields depth-averaged currents,  averaged in time over each subsurface interval. Underoperational conditions these averaged currents are a poor approximation ofthe instantaneous tidal current.  In this work an algorithm is developed that estimates the instantaneouscurrent (tidal and residual) from glider observations only. The algorithmuses a first-order Butterworth low pass filter to estimate the residualcurrent component, and a Kalman filter based on the linear shallow waterequations for the tidal component. A comparison of data from a gliderexperiment with current data from an acousticDoppler current profilers deployed nearby shows that thestandard deviations for the east and north current components are better than7 cm s<sup>−1</sup> in near-real-time mode and improve to better than6 cm s<sup>−1</sup> in delayed mode, where the filters can be run forward andbackward.  In the near-real-time mode the algorithm provides estimates of thecurrents that the glider is expected to encounter during its next fewdives. Combined with a behavioural and dynamic model of the glider,this yields predicted trajectories, the information of which isincorporated in warning messages issued to ships by the (German)authorities. In delayed mode the algorithm produces useful estimatesof the depth-averaged currents, which can be used in (process-based)analyses in case no other source of measured current information isavailable

Depth-averaged instantaneous currents in a tidally dominated shelf sea from glider observations