Ultra Wideband SAR: Processing of Low Frequency Radar Data

Contact Persons

Arnold. Barmettler

Keywords

syntetic aperture radar, wide beam, ultra wideband, UHF, VHF, HF, motion compensation

Quick Reference

Ultra-wideband synthetic aperture radar (UWB SAR) has a high potential for applications because it makes high-resolution low-frequency imaging radar feasible. In combination with other SAR frequency bands, topographic or even tomographic mapping will be possible and geophysical parameters determinable.

RSL developed its own SAR processor for frequency bands from P- to K-band for various space- and airborne SAR sensors. For the processing of VHF UWB data, modules for range and azimuth focusing were developed, due to the special system requirements at these wavelengths. Due to the nature of the relatively long wavelenghts involved (e.g. for the VHF SAR Sensor CARABAS-II 3-15m), the signal must be aquired along a flight track of over 20 km.

To study the interaction of VHF waves with different types of surfaces and to develop applications, a flight campaign was held in Switzerland with the Swedish VHF SAR CARABAS-II. In autumn 2003 data takes took place over several test sites. This includes a relatively flat area in the South of Zürich, extending to the foothills of the Alps, to very rough topography of the glaciated area of Jungfraujoch?Aletsch. This campaign was accompanied by other sensors, optical and radar.

Some Results

For geometric and radiometric calibration purpose of CARABAS-II VHF radar data, 5.1m trihedral corner reflectors like this one on the Great Aletsch glacier are used.

Image of the first high alpine experiment with VHF SAR CARABAS-II: amplitude image from the Jungfraujoch-Aletsch region (October 2003). Remark the small dot marked 'VHF Reflector' which is the detected signal of the large 5.1m corner reflector.
RSL-processed based on range compressed data courtesy FOI.

Focused CARABAS radar data for a 5 m trihedral corner reflector. The signal amplitude of the detected image is shown in false color and as a third dimension.

Methods

Processing Chain

Processing chain for UWB SAR image generation: single look complex (SLC) images are produced based upon data input from radar sensor, navigational measurements (from DGPS, IMU), and meteorological data.

Since the wide bandwidth is usually generated numerically from several narrow band channels (e.g. AeS, CARABAS), the broad bandwidth must be reconstructed from a set of sub-band echoes. This includes a range compression on a chirp-by-chirp basis, i.e. each channel is processed separately. Then, the decompressed echoes are stepped together in the frequency domain. The frequency bands UHF and VHF are heavily used for communication services. Hence the interference from these emitters on the SAR signal must be detected, and strong disturbances suppressed. Also, to reduce the impact of active low-frequency sensing to existing communication channels, the transmitted radar signal must be notched at the most important frequencies. The UWB SAR processor accounts for the essential proper motion compensation for low frequency (and hence wide beamwidth) radars. This step cannot be implemented as a linear filter. In order to account for this problem, an approach was chosen that combines motion compensation and azimuth compression. Instead of using a frequency-domain based algorithm (e.g. the w/k algorithm), the time-domain method filtered backprojection was implemented. Along with coping with long synthetic apertures, this approach has several advantages:

Easy and accurate integration of motion compensation
Differential round-trip delays of the radar echoes due to atmospheric influences within the troposphere can be corrected
Incorporation of the 2D-antenna beam pattern of the sensor
Correction for the influence of topography of the target area

Since time domain focussing is a very runtime consuming approach, effort has been taken to parallelize this task. Focussing of a CARABAS-II scene still takes several hours when using upto 30 processors.

Partnerships

Research has been conducted in cooperation with the following organizations:

Publications

BARMETTLER A., MEIER E., NÜESCH D. [2001]:
Development of an Ultra-Wideband SAR Processor (PDF, 1.9 Mb),
Proc. of CEOS SAR Workshop 2001, Tokyo, Japan, April 2-5, 2001.
BARMETTLER A., MEIER E., NÜESCH D. [2003]:
Tieffrequente abbildende Radarsysteme: ein neues Instrument in der Fernerkundung,
Geographica Helvetica, Heft 2-2003.
BARMETTLER A., MEIER E., NÜESCH D. [2004]:
Swiss Alpine Airborne SAR Experiment (SASARE) part II,
Proc. of IGARSS 2004, Anchorage, USA, September 20-24, 2004.

Department of Geography

Research Projects