Swiss Alpine Airborne SAR Experiment (SASARE) part II

Imaging of High Alpine Glaciers at the VHF-Band

Arnold Barmettler, Oliver Stebler, David Small, Erich Meier, Daniel Nüesch University of Zurich: Department of Geography, Remote Sensing Laboratories

Winterthurerstrasse 190 CH-8057 Zurich, Switzerland. barmettler@rsl.ch

Abstract—This paper describes first results from a low frequency sensor campaign in the high Alpine region of Switzerland. Several parallel tracks were flown with the Swedish radio wave sensor CARABAS-II in late autumn 2003. We rate the geometric quality of the automatic processing of the ultra-wideband (UWB) radar data, discuss its most characteristic features, as well as problems caused by highly variable topographic gradients. The relatively long wavelengths used should theoretically allow penetration into cold ice bodies. The first results did not show obvious evidence of subsurface echoes.

Keywords-SAR, VHF, UWB, ultra-wideband, Alpine glacier, mountains

I. INTRODUCTION Currently, most Alpine glaciers are undergoing dramatic

retreats that seem to be coupled with global climate change. Quantitative assessments of these changes and/or variations are important boundary conditions for climate models. Qualitative measurements – of e.g. new areas with crevasses, moisture content – can be important for skiing areas, and other recreational regions.

In order to understand the characteristics of radar remote sensing products to high Alpine surface features and properties, RSL conducted a multi-band SAR campaign in the heart of the Alps in late autumn 2003. The test site covered the upper part of the longest Alpine glacier, the Great Aletsch Glacier (survey starting at 2550 m a.s.l.), and a cold glacier at 4000 m a.s.l. At the time of the data takes, the snow and wet-snow lines were unfortunately outside the area. To assess the diurnal variation, the data takes were acquired over two missions.

In addition to microwave frequencybands (E-SAR sensor, see part I of this paper [1]), the Swedish radio wave sensor CARABAS-II (Coherent All RAdio BAnd Sensing) was flown concurrently over the test site. A multi-temporal and multi-baseline data set in the frequency range from 20-90 MHz was recorded. For calibration purposes, a 5 m trihedral reflector was mounted on the glacier. In addition to ‘real-time’ ground truth (e.g. in situ measurements of snow wetness and weather parameters, differential GPS), high-resolution true color aerial photographs were acquired almost simultaneously.

II. SENSOR

A. Swedish VHF/HF UWB-SAR CARABAS-II The Swedish FOI (Swedish Defence Research Agency)

flew the low frequency, ultra-wideband radar system CARABAS since the early 1990’s [2]. The current version, CARABAS-II, consists of dipole antennas mounted on both sides of the cockpit of a Sabreliner business jet with freestanding booms in front of the plane. The radar electronics and amplifiers are mounted in racks in the cabin compartment.

CARABAS-II can transmit and receive electromagnetic waves from 20 to 90 MHz, i.e. in the HF- and VHF-band. The relative bandwidth is 127%, making the radar a true ultra-wideband system (UWB). The wide bandwidth is numerically constructed from several 2.5 MHz wide pulses. Due to the long wavelengths (3-15 m) and the comparatively short antenna necessary to ensure airplane safety, the antennas have a nearly omnidirectional antenna pattern. This makes it feasible to have very long synthetic apertures – on the order of 20 km – and integration angles of over 90°.

III. ALPINE CAMPAIGN

A. Test Site Jungfraujoch-Aletsch Glacier In autumn 2003, CARABAS-II held a flight campaign in

Switzerland. The campaign took place in several missions over various test areas. Each area had its dedicated scientific objectives. This paper will only focus on the test site in the high Alpine region of Switzerland, the Jungfraujoch-Aletsch Glacier area.

CARABAS-II had not been flown over Alpine regions before the Swiss campaign. It was therefore not clear how the system would react to very rough topography. The applicability of CARABAS-II to producing radar images in such areas was the first goal of the two Alpine missions.

The SAR-Sensor flew at an altitude of almost 9000 m a.s.l., i.e. 6000 m about the reference height of 3000 m.

B. Jungfraujoch-Aletsch Glacier Tracks The test area was subdivided into two almost perpendicular

strips intersecting at the Konkordiaplatz, a glacial basin of ice almost 800 m thick. The first 10 km long strip, called JUJO

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(‘Jungfraujoch’) was mainly along the longest glacier in the Alps, the Great Aletsch Glacier. It covers the glacier continuously from 2550 m a.s.l. to 3600 m a.s.l. Two data takes from almost equal flight tracks were acquired within 4 hours. Furthermore, data from an interferometric repeat pass track has been collected. The test site is shown in Fig. 1.

Figure 1. Parallel interferometric synthetic apertures for CARABAS-II, and the two Alpine test sites JUJO and FISA in the Bernese/Valais Alps. Relief is

from the SRTM-3 DEM.

The temporal baseline allows for studies of the wet snow line. During the first flights, the zero degree freezing line was well below the lower end of the test site. In the course of the morning, the temperature rose significantly, but not enough to allow the snow and ice surface to become wet. In addition, another open question in this experiment was the ability to image crevasses.

C. Konkordiaplatz-Fiescherhorn Tracks Almost perpendicular to the flow direction of the Great

Aletsch Glacier are the repeat pass multi-baseline tracks from the strip FISA (‘Fischerhorn’) from Konkordiaplatz (2700 m a.s.l.) to Fiescherhorn (4049 m a.s.l.). The strips looks straight into sheer cliff rocks, and covers one of the few cold ice glaciers in Switzerland at an altitude of about 4000 m.

This track was chosen to minimize interference from radar signals reflected at the mirrored side of the test site (with respect to the sensor flight axis).

The primary objective was verification of the ability of CARABAS-II to work in the harsh environment with strong reflections and target areas with only weak radar returns. A more challenging goal was to look into the cold ice glaciers by detecting returns from below the ice surface.

D. Reference Target For assessing the geometric accuracy and to radiometrically

calibrate the radar images, a 5.5 m corner reflector was flown to flat area of the Great Aletsch Glacier, the Konkordiaplatz by

helicopter. Fig. 2 shows this reflector after being mounted on the snow covered glacier.

Figure 2. 5.5 m VHF trihedral corner reflector mounted on the Great Aletsch Glacier, viewing direction Southeast. Reflector courtesy FOI.

The position of the corner’s phase center was surveyed using differential GPS. Based on the displacement of the position between mounting and dismounting, one accurate velocity vector of the glacier has been calculated (for that place location: ~1 meter per day).

IV. RESULTS One data take produced up to 2 GB of range compressed

radar echo data. For azimuth focusing, RSL used its UWB-SAR processor [3]. This processor is a back projection processor, optimized for radar data focusing in rough terrain by working in such a way that it completely reconstructs the geometry among ground and sensor for each radar echo. This allows for near perfect motion compensation at the cost of significantly lengthening runtime.

Since the geometry is calculated on a pixel by pixel basis, the SAR image is directly processed onto a surface, in this case a high resolution DEM (digital elevation model). The projection of the DEM was chosen to be Swiss map coordinates. The output of the SAR processing is directly a geocoded SLC image. An advantage of this approach is that no further data interpolation is necessary, given that the target projection is known beforehand.

Back projection processing requires long runtimes. Hence the UWB-SAR processor was enhanced for parallel processing. The chosen approach distributes the calculation of smaller image blocks onto as many Linux/Unix workstations as available. As an additional advantage, the image can be inspected partially before the whole scene is completely processed. A processing example is shown in Fig. 3.

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Figure 3. RSL processed geocoded CARABAS-II UWB-SAR image from a FISA track. Image size corresponds to an area of 10 km x 10 km. In the

original image, the 5.5 m VHF trihedral corner reflector is clearly visible. Compare the sector with the full test site area (green) in Figure 1.

As expected from the rough topography, layover and foreshortening effects are dominant in the SAR image. Towards far range, there are also strong shadowing effects. Moreover, there are curved features present, which are not yet fully understood. They may either be superimposed signals from the unwanted, backlobe side of the synthetic aperture, or unfocused echoes from specular reflections e.g. from rock faces or crevasses, and/or double-bounce effects.

Seracs and groups of crevasses are visible: they return a distinct signal, as do the moraines, compared to the surrounding snow/ice.

The geometric accuracy of the processing chain was determined by comparing the positions of the 5.5 m corner reflector within the image and as interpolated from two differential GPS measurements. However, since the reflector was mounted on the glacier and the DEM used for the processing was already several years old, the heights were not the same. The area around the reflector was focused once again using the measured height as reference height. The horizontal position error was only 5 m.

Imaging of the cold ice glacier situated within the area was a scientific goal, and the flight geometry was optimized for this objective. Unfortunately, due to closed air space, the illumination direction had to be rotated by 180°. Instead of an optimal illumination geometry (regarding SNR) with a slight foreshortening of the slope of the cold ice glacier, the glacier was facing away from the synthetic aperture. Hence, only a small amount of the energy was returned to the sensor due to a shallow reflection angle. Additionally, the mountain Gross Fiescherhorn cast its shadow onto the main target area for parts of the synthetic aperture, leading to reduced resolution.

CONCLUSIONS This first data take of the CARABAS-II VHF-SAR in a

high Alpine region revealed the possibility of using ultra-wideband and wide-angle SAR in areas with high topography, the examples shown here exhibit with height variations of up to 1700 meters. Challenges are posed by areas with layover and strong interferences from specular reflections, which however may be suppressed to some extent.

To study the ability of UWB-SAR in the VHF-band to image and discriminate surface or subsurface details in the cryosphere, a test site with less interference and flatter topography would be preferable.

From the high Alpine data set, only one single flight track could be addressed so far. The opportunities available for repeat pass interferometric combinations will be exploited in the future.

ACKNOWLEDGMENT The authors wish to thank armasuisse (formerly known as

the Swiss Defence Procurement Agency), Swiss Air Force, FOI of Sweden, especially A. Gustavsson, P.-O. Frölind and L. Ulander, the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) of the Swiss Federal Institute of Technology Zurich (ETH) for their support.

REFERENCES [1] O. Stebler, A. Barmettler, D. Small, E. Meier, and D. Nüesch, „Swiss

Alpine airborne SAR experiment (SASARE) part I: multi-baseline polarimetric SAR interferometry studies at L- and P-band“, in Proc. IGARSS’04, 2004.

[2] B. Larrson, P.-O. Frölind, A. Gustavsson, and H. Hellsten, “Some results from the new CARABAS II VHF SAR system”, Proc. Third Internatinonal Airborne Remote Sensing Conference and Exhibition, Copenhaben, Denmark, pp. I-25-31, July 1997.

[3] A. Barmettler, E. Meier, and D. Nüesch, “Development of ultra-wideband SAR processor”, in Proc. of CEOS Workshop 2001, Tokyo, Japan, pp. 271–274, April 2001.

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