Unteraargletscher

Particular attention in the Swiss Alps is paid to Unteraargletscher, Bernese Alps, because this glacier has been the focus of a concentrated glaciological process study over the last few years. As a consequence the flow dynamics of this glacier are known in considerable detail. In particular, the spatial pattern of annually averaged horizontal surface velocities has been mapped with a high resolution, and hourly temporal changes in flow velocities have been measured over periods of months. The methods used to determine surface geometry and velocities include the use of automized theodolite, an array of permanently installed GPS receivers, laser scanner measurements, and semi-automized extraction of surface velocities from annual aerial photographs. Despite extensive field measurements and intense use of traditional remote sensing methods on this glacier, the vertical component of the surface motion is relatively poorly known and direct measurements of the vertical velocity component are highly desirable.

Swiss 1:100000 scale topographic map of Unteraargletscher, Bernese Alps, Switzerland. Image size is 15.5 km x 10.0 km.

This missing information could be retrieved with ERS DINSAR because the incidence angle is nearly vertical (ca. 23). The line-of-sight surface displacement map of Unteraargletscher from DINSAR may be therefore useful in combination with high precision aerial photographs in the estimation of the surface mass-balance distribution using the kinematic boundary condition without the use of any ground measurements. The use of a high precision DEM determined with aerial photographs to subtract the topographic effect on the phase of the interferograms should be further investigated to avoid the assumption of similar displacement during two winter days and possibly improve the temporal sampling of the displacement values.

Ancillary Data Unteraargletscher

Velocity data

Unteraargletscher was considered as a test glacier for the methodological set-up of differential SAR interferometry, because over the last few years this glacier has been the focus of a concentrated glaciological process study (Bauder, 2001; Schuler, 2002) and therefore the flow dynamics are known in considerable detail. The figure below shows the annually averaged horizontal surface velocity field determined by Bauder (2001) from annual aerial photographs in 1997 and 1998.

Elevation data

A high accuracy DEM with a posting of 25 m (DHM25, ©2004 swisstopo therefore not shown) was used for investigation of 2-pass (versus 4-pass) differential interferometry. In this case, the critical issues are: the accuracy of the DEM and the date of acquisition of its source data. For the Swiss Alps the vertical accuracy is estimated to be 3 m, and over Unteraargletscher the contour lines for deriving the DEM were updated in 1994, relatively close in time to the SAR data acquisition.

SAR Data Unteraargletscher

A number of ERS-1/2 Tandem phase (1-day) pairs of SAR scenes were available to the INTEGRAL project, courtesy of ESA AO3-178 and CAT1-2338 projects, PI T. Strozzi.

ALT Data Unteraargletscher

N/A

InSAR-derived Products Unteraargletscher

Comparison of 4-pass and 2-pass Differential Interferometry techniques

In SAR interferometry, two complex SAR images acquired from slightly different orbit configurations and at different times are combined to exploit the phase difference of the signals. The interferometric phase is sensitive to both surface topography and coherent displacement along the look vector occurring between the acquisition times of the interferometric image pair. In the 4-pass differential interferometric approach the differential use of two interferograms with similar displacement allows calculation of a topographic-only phase interferogram, which is unwrapped and used subsequently in the removal of the topographic-related phase from the mixed-phase interferogram to derive a displacement map. In the 2-pass differential interferometric approach the topographic-related interferometric phase is simulated from an external Digital Elevation Model (DEM) and removed from the mixed-phase interferogram to isolate the displacement related phase.

Two winter, descending orbit ERS-1 / 2 Tandem pairs (one day acquisition time interval) were considered for investigation of 4-pass differential interferometry over Unteraargletscher. The two interferograms, acquired on the 7 and 8 March 1996 and on 11 and 12 April 1996, have different baselines and are suitable for differential analysis.

Unteraargletscher 4-pass example Line-of-sight surface displacement map of Unteraargletscher in map geometry for the 7/8 March 1996 from the 4-pass approach. One color cycle corresponds to 5 cm line-of-sight displacement. Look direction (descending mode) is indicated be the arrow (incidence angle ~23). A point at the end of the glacier terminus on the slope of the valley was selected as stable reference. The maximum velocity of more than 4 cm in one day was measured in the upper part of the glacier characterized by the steeper topography. As expected, the displacement velocity decreases towards the end of the glacier. The effects of the line-of-sight direction are very clear: where look and flow direction approximate perpendicular directions, the SAR measurements are not sensible to the displacement.

Unteraargletscher 2-pass example Line-of-sight surface displacement map of Unteraargletscher in map geometry for the 7/8 March 1996 from the 2-pass approach. One color cycle corresponds to 5 cm line-of-sight displacement. Look direction (descending mode) is indicated be the arrow (incidence angle ~23). The 2-pass approach has an advantage over the 4-pass approach, in that it avoids the need for phase unwrapping of the topographic-related phase. The overall pattern of surface displacement is very similar to the 4-pass-derived pattern, with variations in displacement smaller than 1 cm.

The 2-pass (with use of a high precision external DEM) and the 4-pass interferometric approaches produced overall similar results. The 2-pass approach was preferred for in-depth analysis of motion because it is more straightforward for areas of steep topography and not sensitive to areas of decorellation encountered in one of the two Tandem pairs considered for 4-pass interferometry. The main differences are found in the masked areas:

The 2-pass approach did not allow retrieval of information over the lowest part of the glacier, because of decorellation in April 1996.

For the steep slopes of the valley (because of phase unwrapping difficulties of the topographic related differential phase), the 4-pass pass approach has larger masked areas in layover. The 2-pass approach succeeded in deriving some displacement values even in layover regions.

Conversion to 3D velocity using DEM

The application of dual-azimuth SAR interferometry by combination of data in ascending and descending mode with the assumption of flowing parallel to the surface of the glacier was not successful, because over the European Alps the angle between observations in ascending and descending mode is 24. This is too different from the perpendicular direction required for a stable Cartesian coordinate system solution of dual-azimuth SAR interferometry. Therefore, temporal evolutions of surface displacement over Unteraargletscher were derived only in the line-of-sight directions and transformed to 3-dimensional velocity maps by assuming ice flow along the slope of the filtered DEM. The DEM was filtered with an averaged distance of about 600 m in order to reduce local effects. Areas where look direction and assumed flow direction were close to perpendicular were not considered. In comparison to the line-of-sight displacement maps, the 3-dimensional velocity maps show therefore a reduced amount of area with valuable information.

Temporal evolution of glacier velocity

Selected surface displacement maps of Unteraargletscher are shown below for line-of-sight, and for derived 3D. During the winter, surface displacement was computed over a larger area than in summer because of the generally higher degree of coherence. For comparison of the 1-day displacement rates from DINSAR with the annually averaged horizontal surface velocities from aerial photographs, summer and winter DINSAR displacement maps were averaged for images of ascending and descending orbits.

Over the central part of Unteraargletscher maximum surface velocities in one day in winter are on the order of 5 to 10 cm (about 20-30 m/year) at the confluence of Lauteraar- and Finsteraargletscher. For some days in summer, these velocities are roughly doubled (about 50 m/year). These values are consistent with a horizontal surface velocity field of around 30 m/year from annual aerial photographs in 1997 and 1998 (Bauder, 2001). The glacier velocity is dependent on the thickness of the glacier, the surface slope of the glacier, the basal sliding and the softness of the ice; hence the seasonal speed-up is due to increased availabilityof meltwater, which enables more basal sliding.