Fringe 96

Glaciological Studies in the Alps and in Antarctica Using ERS Interferometric SAR

Abstract

Applications of repeat pass SAR interferometry and conditions for coherence were studied over Alpine glaciers and polar inland ice. In the test site Ötztal in the Austrian Alps field campaigns were carried out during ERS-1/ERS-2 tandem overflights. Five image pairs were available for interferometric analysis, each of which was acquired within 24 hours at specific dates between August 1995 and February 1996. Melting and, in case of dry snow, strong winds were identified as the main factors for phase decorrelation. For the region of the Heimefrontfjella mountain range in Dronning Maud Land, Antarctica, interferometric analysis was carried out for seven ERS-1 SLC quarter scenes from January and February 1994 acquired in 3 day repeat intervals. The map of ice motion, generated by means of differential interferometry, revealed a complex pattern of ice dynamic. The investigations of temporal and baseline-dependent decorrelation indicated significant differences between various snow and ice types, providing possibilities for target discrimination.

Keywords: Interferometry, SAR, coherence, ERS, glaciers, Antarctica

Introduction

Spaceborne radar interferometry has been recognized as a valuable tool for glaciological research, offering the possibility to monitor the ice motion and to map the surface topography over extended areas. Temporal decorrelation is a major problem for repeat pass interferometry over snow and ice. Furthermore, the penetration of microwaves into dry snow causes difficulties because the return signal originates from the volume and thus shows baseline-dependent decorrelation ( Gatelli et al., 1994). In addition to the relevance for interferometric analysis, investigations on signal coherence provide important insights into the interaction mechanisms of microwaves with snow and ice. In this paper we report on studies of coherence for various snow and ice types in the Alps and in Antarctica, based on interferometric SAR data from ERS-1 in 3-day repeat orbit and from the ERS-1/ERS-2 tandem campaign. The investigations are a contribution to the ERS-1/ERS-2 experiment AO2.A101 "Comparative Investigations of Climate Sensitivity and Dynamics of Glaciers in Antarctica, Patagonia, and the Alps".

Coherence Study on Alpine Glaciers

During the ERS-1/ERS-2 tandem operation 1995/96 four field campaigns were carried out on glaciers of the test site Ötztal, Austrian Alps, to learn about the conditions for signal coherence and temporal decorrelation. Over this area descending and ascending overflights of ERS with 35 day repeat orbits are separated only by 12 hours in time. Five tandem pairs from 23/24 August 1995, 27/28 September 1995, 6/7 December 1995, and 14/15 February 1996 were available for this study. Interferograms were generated for all of these image pairs. The complex degree of coherence between the radar signals S1 and S2 was calculated according to

where < > denotes ensemble averaging. For calculating coherence images 10 x 10 pixels averaging was applied, corresponding to about 70 independent looks, and the local fringe frequency was taken into account. Averaging is particularly important in regions of low coherence to obtain reliable numbers (Lee et al., 1994). The degree of coherence is the magnitude of and may assume values in the range from 0.0 to 1.0. As an example, Figure 1 shows the interferogram of 14/15 February 1995 for the ascending satellite pass. The effective baseline was Bn = 136 m, resulting in an elevation difference of 65 m for one fringe cycle. For this image pair the coherence was comparatively high throughout the scene. Only on steep foreslopes and in layover zones the fringe visibility is very poor or fringes are lost due to undersampling. The ice-free surfaces were covered with dry winter snow of 0.5 m to 1.5 m depth. The ice areas and the frozen firn on the glaciers were covered with 1 m to 2 m of winter snow. The arrows in the Figure indicate the flow direction of three main glaciers. The firn areas of Gepatschferner and Kesselwandferner form a comparatively gentle plateau of about 15 km2 in area at elevations between 3000 m and 3350 m. Because the ice on the plateau moves less than 5 cm per day, the fringes are primarily related to topography. Apart from the glacier plateau, the topography in the test site is quite steep, resulting in considerable loss of information due to layover and foreshortening (Rott and Nagler, 1994). The altitudes in Figure 1 are ranging from 2200 m to 3700 m above sea level.

Figure 1: Interferogram of the test site Ötztal, formed from an ERS-1 image of 14 February 1996 and an ERS-2 image of 15 February 1996, superimposed to an amplitude image. Glaciers: G - Gepatschferner, H - Hintereisferner, K - Kesselwandferner.

Figure 2 shows the degree of coherence for different surfaces in the test site Ötztal for 4 different tandem pairs of the ERS-1/ERS-2 mission. The baselines for the four image pairs ranged from Bn = 80 m (23/24 August) to Bn = 274 m (27/28 September), so that geometric decorrelation due to baseline effects is of little relevance. The meteorological and snow conditions for the 24 hour periods between the satellite overflights can be summarized as follows:

23/24 August 1995: Air temperature in 3000 m between +3 and +5°C, cloudy, part of the time fog and drizzle, little wind; water on the ice surfaces; in the accumulation areas coarse grained and wet snow with liquid water content of 5% by volume in the top layer.

27/28 September 1995: Air temperature in 3000 m dropping from -3 to -8 °C, overcast, fog, passing of a cold front with strong winds, snow drift, several centimeters of snowfall between the two overflights. The ice and firn areas of the glaciers were covered with 20 to 60 cm of snow from early September. The top snow layer (20 to 40 cm in 3000 m) was frozen, the thickness of the frozen layer varied with the altitude and increased between the two overflights.

6/7 December 1995: Air temperature in 3000 m -12°C, low wind velocities in the valleys, but gusty winds from the south on the mountains, no snowfall; 1m to 1.5m of homogeneous dry winter snow on the glaciers.

14/15 February 1996: Air temperature in 3000 m between -16 and -18°C, cloudy and some wind during the overflight of ERS-1, calm on 15 February; 1m to 2m of homogeneous dry winter snow on the glaciers.

Figure 2: Degree of coherence for 3 sites in firn areas and 2 sites in ice areas of glaciers, and for 2 sites in Alpine grassland of the test site Ötztal, from ERS-1/ERS-2 tandem data of 4 different dates.

As expected, the coherence of the ice areas on the glaciers is high for the two winter dates and low in August and September (Figure 2). However, in spite of strong melting in August, the degree of coherence reaches values up to 0.4 over parts of the ice and firn areas, thus being high enough to obtain fringes. For wet snow and ice the dominating backscatter mechanism is surface scattering. Apparently the geometric structure of the surface was preserved within the 24 hours between the image acquisitions because the overcast sky resulted in homogeneous melt conditions.

The coherence of the firn areas on the glaciers shows a more complex behaviour than of the ice areas. The coherence is in general quite low in the August and September interferograms. In September the coherence reaches values above 0.4 only on the high plateau where the freezing depth in the snowpack is larger than at lower elevations. In winter with dry snow, when the dominating part of the radar return originates from the volume, high coherence and stable conditions should be expected. However, between 6 and 7 December 1995 the signal decorrelated over parts of the firn areas, mainly on the high glacier plateau of Gepatschferner. On other glaciers the coherence was quite high. The only possible explanation for this are major changes of surface structure due to wind erosion and snow drift, effects which may vary significantly at regional and local scales in dependence orography.

In the unglaciated high Alpine areas the coherence is in general quite high throughout the year. A significant reduction is only observed for 27/28 September, related to major changes of meteorological conditions within 24 hours. The areas above the timberline are covered with low vegetation, mainly thin sedges and grasses, and some dwarf-shrubs. The surface is rough, and usually interleaved with rocks. In the sub-Alpine forests the degree of coherence is less than 0.4 throughout the year in the 24- hour repeat pass interferograms.

Ice Motion in the Heimefrontfjella Mountain Range, Antarctica

Investigations on coherence and ice motion were carried out in the Heimefrontfjella mountain range, Dronning Maud Land, East Antarctica. The area around Scharffenbergbotnen (74°35'S, 11°03'W) was selected, because measurements on microwave scattering and emission have been carried out in this region Rott et al., 1993 and because ice motion data are available for selected points (Stroeven and Pohjola, 1991). Seven ERS-1 SLC quarter scenes from January and February 1994 in 3 day repeat intervals were available for the analysis. With this data set it was possible to derive absolute ice motion because there is a number of stable targets (nunataks) in the scene which can be used as reference.

Figure 3: Ice motion in range direction in the region of Heimefrontfjella, Antarctica, derived by means of differential interferometry based on ERS-1 SAR data. Descrete steps of ice motion in false colors are superimposed to an amplitude image. A - Aubertisen, P - Pionerflaket, S - Scharffenbergbotnen, V - Veststraumen.

Figure 3 shows the results of the ice motion analysis for the full quarter scene, derived by means of differential interferometry from the image pairs 29 January/1 February (Bn = 167 m) and 1/4 February (Bn = 34 m). These two image pairs were selected for the motion analysis because of good coherence and short baselines. The Figure shows the motion in direction of the radar beam. The horizontal and vertical velocity components are not separated, but on the main ice streams, which show little inclination, the horizontal component is dominating. The ice plateau Pionerflaket, which rises to 2500 m a.s.l. and is visible at the right margin of Figure 3, blocks the iceflow from the inland. The ice on Pionerflaket, as well as the ice around the nunataks in the north-west of it (to the left in the Figure), shows very little motion. Ice motion data are available for Scharffenbergbotnen, which is an ice inlet at 1250 m above sea level surrounded by nunataks with peak altitudes between 1400 m and 2200 m a.s.l. Scharffenbergbotnen contains blue ice fields, which are areas of negative surface mass balance due to sublimation of ice as a result of strong katabatic winds. The ice moves towards the mountains with velocities of less than one meter per year (Stroeven and Pohjola, 1991). Also the interferometric analysis shows that this ice is stagnant.

The main transport of ice in the study area takes place through Aubertisen with the main flow direction towards north-west. A mountain ridge closely beyond the upper right corner of Figure 3 is an obstacle for the ice flow from above. Below this region of very low velocity two ice stream, from the south and from the west, are joining. The highest velocities with a maximum of 92 m per year are observed below this confluence. Below the slopes the ice flow turns towards west and joins the Veststraumen which is the major ice stream draining towards Riiser-Larsen Ice Shelf. On Veststraumen the magnitude of the velocity vector is about 30 % higher than the velocity component in range direction shown in Figure 3, because the angle between flow and look directions is about 45 degrees. The interferometric analysis provides remarkable detail on the complex ice motion pattern around the mountains.

Coherence Study in Antarctica

Target characteristics and temporal variations of coherence were investigated for the area of the motion analysis presented above. Figure 4 shows coherence images for the 3-day repeat interferogram with lowest coherence (23/26 January 1994) and with highest coherence (1/4 February 1994) among the available data. The baseline was small in both cases (54 m and 34 m). The area is almost completely covered with permanently dry firn which is strongly layered and has a one-way penetration depth of about 20 m at C-band Rott et al., 1993.

Figure 4: Interferometric correlation for ERS-1 SAR images pairs of 23/26 January 1994 (left) and 1/4 February 1994 (right). The grey scale corresponds to the degree of coherence from 0.0 to 1.0. Same area as Figure 3.

For the firn areas the degree of coherence is less than 0.4 in the 23/26 January interferogram and reaches values above 0.8 in the 1/4 February interferogram. From meteorological observations at the Neumayer station, which is located in a distance of 450 km from the study area, it is known that a low pressure system with strong winds was passing by between 23 and 26 January, and that the weather in early February was comparatively calm. Strong winds are usually strongly modifying the surface roughness of dry firn, as also known from field campaigns in the Heimefrontfjella region. Thus it can be concluded that wind is the main reason for decorrelation of cold polar firn.

In the February interferogram the coherence is low only on the steep backslopes of the ice plateau Pionerflaket where the signal to noise ratio is low. The noise signals are uncorrelated. High coherence is observed in all interferograms for the comparatively small areas of bare rock on nunataks, and for the blue ice fields and surface moraines which cover areas of several km2 size below the mountains.

Figure 5a: Degree of coherence for different time intervals of repeat pass interferograms (top) and degree of coherence in dependence of baseline in meters (bottom) for Surface moraine. The black dots in the top diagram indicate the effective baseline.

Figure 5b: As Figure 5a, Blue ice field.

Figure 5c: As Figure 5a, Firn area.

The temporal and baseline dependence of coherence for 3 selected sites is shown in Figure 5a , 5b , 5c . The coherence of the surface moraine, which covers a level area in Scharffenbergbotnen, is temporally stable at least up to the maximum investigated time difference of 18 days and decreases only slightly with increasing baseline. The baseline dependent decorrelation is small because surface scattering is dominating and spectral bandpass filtering according to the dominant fringe frequency was applied previous to the interferogram generation ( Gatelli et al., 1994). The blue ice field shows somewhat higher temporal variability. From surface observations it is known that sastrugi (snow dunes) may cover parts of the blue ice if the wind is not very strong, but the snow is blown away during storms. The baseline decorrelation is higher than for the moraine. Though surface scattering should be important for blue ice, a significant part of the backscattered signal originates from scattering at air bubbles within the volume.

The firn areas show a very pronounced decrease of coherence with increasing baseline and with time. The main reason for the temporal variability are changes in surface conditions, as explained above. Though the magnitude of the backscattering coefficient is stable in time, and more than 90 % of the signal originate from the snow volume, the volume-surface interaction term seems to be an important mechanism for phase changes. The importance of the volume scattering contribution and the large penetration depth are reflected in the observed decrease between coherence and baseline, which is in principal agreement with theoretical calculations ( Gatelli et al., 1994).

Conclusions

The investigations with interferometric ERS data in the Alps and in Antarctica were able to provide insights in the relations between coherence and target characteristics. Melting is the main factor for decorrelation of snow and ice, causing in general almost complete decorrelation within one day, though under certain conditions the coherence may stay high enough for obtaining fringes. For dry snow and polar firn the main factor for temporal decorrelation is wind. Another important factor for reduced coherence in dry polar firn is the baseline-dependent decorrelation. Coherence characteristics enable the separation of different snow and ice regimes, such as blue ice fields and firn. The motion analysis of the Antarctic region confirms the unique capabilities of differential interferometry for studying complex patterns of ice dynamics with remarkable detail.

Acknowledgement

The investigations were supported by the National Space Research Program of the Austrian Academy of Sciences. The ERS-1 SAR data were made available by the European Space Agency for ERS Experiment AO2.A101.

References

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Intensity and phase statistics of multilook polarimetric and interferometric SAR imagery. IEEE Trans. Geosci. Rem. Sens., 32 (5), 1017-1027.

Rott H., K. Sturm, and H. Miller, 1993:

Active and passive microwave signatures of Antarctic firn by means of field measurements and satellite data. Annals of Glaciology 17, 337-343.

Rott H. and T. Nagler, 1994:

Capabilities of ERS-1 SAR for snow and glacier monitoring in alpine areas. Proc. of Second ERS-1 Symposium ESA SP-361, 965-970.

Stroeven A.P. and V.A. Pohjola, 1991:

Glaciological studies in Scharffenbergbotnen. In: The Expedition Antarktis-VIII, Reports on Polar Research, 86, Alfred-Wegener-Institut, Bremerhaven,126-130.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry