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DLR’s experience in radiometric calibration 
OF AIRBORNE AND SPACEBORNE SAR SYSTEMS 
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 
ABSTRACT: 
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 
1 - INTRODUCTION 
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 
sensors.