We are here concerned with local changes through time at four different spots along the downflow GPS profile. For that purpose, GPS receivers were dropped on the ground and left in place for a couple of days during which they were continuously measuring the ice surface height. By comparing the time-dependent surface height evolution with the corresponding tide, it is possible to confirm the presence or not of a tidal signal and its intensity when appropriate. Results over these points are in agreement with those from the GPS profiles. For instance point 1 down below exhibits a strong signal in phase with the tide denoting freely floating ice, as could already be deduced from its location along the GPS profile. Conversely, with point 4, which is situated in the supposedly grounded part of the profile, no tidal signal is visible.

Left: Time-dependant surface displacements during 2 days in january 2011 at Point 1. On the bottom panel are shown the tidal signal (black) compared to the vertical ice upper surface displacements obtained in RTK differential mode (red). GPS data were also post-processed so as to confirm the validity of the RTK method. Corresponding results are depicted as blue stars (upper panel) when ambiguities where fixed and as green stars otherwise.
Right: Comparison of the upper surface displacements and tides for Point 4. Black curves represent the tidal amplitude whereas the red ones stand for raw RTK GPS positions. Blue curves result from a 10-point smoothing of the raw data to which a vertical amplification has been applied (varying according to the point) in order to confirm or deny any correlation with the tides. Green curves just represent the smoothing of raw GPS data. (From Le Meur et al., 2013).

References

Le Meur, E., M. Sacchettini, S. Garambois, E. Berthier, A. S. Drouet, G. Durand, D. Young, et al. « Two independent methods for mapping the grounding line of an outlet glacier – example from the Astrolabe Glacier, Terre Adélie, Antarctica ». The Cryosphere Discussions 7, no 4 (8th august 2013): 3969-4014. doi:10.5194/tcd-7-3969-2013.

January 2011.