1Gateway Antarctica, University of Canterbury, Private Bag 4800, Christchurch, New Zealand
2German Aerospace Centre (DLR), Oberpfaffenhofen, 82234 Weßling, Germany
3Antarctic Research Centre, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand
4GNS Science, Avalon, Lower Hutt 5011, New Zealand
5Department of Geography, University of Canterbury, Private Bag 4800, Christchurch, New Zealand
Abstract. Ocean tides close to the grounding line of outlet glaciers around Antarctica have been shown to directly influence ice velocity, in both linear and non-linear patterns. These fluctuations can be significant and have the potential to affect satellite measurements of ice discharge which assume displacement between satellite passes to be consistent and representative of annual means. Satellite observations of horizontal velocity variation in the grounding zone are also contaminated by vertical tidal effects, shown here to be present in speckle tracking measurements. Eight TerraSAR-X scenes from the grounding zone of the Beardmore Glacier are analysed in conjunction with GPS measurements to determine short-term and decadal trends in ice velocity. Diurnal tides produce horizontal velocity fluctuations of >50% on the ice shelf, recorded in the GPS data 4 km downstream of the grounding line. This decreases rapidly to <5% only 15 km upstream of the grounding line. Daily fluctuations are smoothed to <1% in the 11 day repeat pass TerraSAR-X imagery but fortnightly variations over this period are still visible and show that satellite-velocity measurements can be affected by tides over longer periods. The measured tidal displacement observed in radar look direction over floating ice also allows a~new method of grounding line identification to be demonstrated, using differential speckle tracking where phase coherence is too poor for SAR interferometry.