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Mesures physiques et signatures en télédétection

DLR’s experience in radiometric calibration
M. Zink, F. Heel and H. Öttl
DLR, German Aerospace Research Establishment
Institute of Radio Frequency Technology
82234 Oberpfaffenhofen, Germany
Tel.: +49-8153-28-2386, FAX: +49-8153-28-1449
The goal of the microwave remote sensing activities is to model the relationship between geophysical
or biophysical parameters and measured backscattering coefficients. Such quantitative analyses and
the development and improvement of models in different fields of application require calibrated data.
As the German Aerospace Research Establishment is holding the scientific leadership within the
X-SAR project and operates an own airborne SAR system, calibration has been an important topic
in our institute for more than five years. Also calibration experiments with spacebome systems
(ERS-1, JERS-1, SIR-C/X-SAR, PRIRODA) have been proposed and accepted.
KEY WORDS: Synthetic Apertur Radar, Radiometric Calibration
Monitoring Earth’s surface parameters using active microwave sensors requires reliable and re
peatable measurements of radar backscattering coefficients a°. Application scientists who want
to compare data from different sensors, extract geophysical parameters from backscatter measure
ments, carry out multitemporal studies, etc., can only do so using calibrated Synthetic Aperture
Radar (SAR) data products. Calibration means, that system related influences have to be recorded
or estimated and removed to obtain pure target signatures.
In the last years much effort has been invested by the microwave remote sensing community
in the radiometric calibration of SAR data. Airborne sensors like the DLR experimental airborne
SAR system (E-SAR) and the NASA/JPL multifrequency, multipolarization DC-8 SAR (AIRSAR)
have been used in dedicated calibration experiments to develop and test calibration procedures
and measurement techniques [1,2]. These airborne campaigns can be regarded as an important
step towards the operational calibration of data from present and future spacebome sensors. The
launch of the European Space Agency’s ERS-1 and the Japanese JERS-1 started a new age in radar
remote sensing. These sensors offer, together with the upcoming SIR-C/X-SAR mission (launch
April 1994), the Canadian Radarsat and NASA’s EOS SAR, the opportunity of obtaining good
quality, calibrated SAR image data from now on to the end of the century.
In this paper, we present highlights of our work on SAR calibration. We follow a logical
order. Internal calibration and monitoring of sensor instabilities allow to correct for transmit power
and receiver gain variations. Corresponding calibration loops for the X-SAR system are discussed
in Section 2 [3]. An important point is the knowledge of the inflight antenna pattern. Our approach
to measure the antenna pattern using ground receivers is presented in Section 3. Special attention
has been paid to radiometric corrections of data aquired over mountainous areas. This requires
precise knowledge of the scene topography. A new approach for topographic mapping is presented
m Section 4. Section 5 contains some results of absolute calibration and cross-calibration of different