RESULTS FROM TERRASAR-X
GEOMETRIC AND RADIOMETRIC CALIBRATION
B. Bräutigam, M. Schwerdt, M. Bachmann, B. Döring
German Aerospace Center (DLR), Microwaves and Radar Institute, Oberpfaffenhofen, D-82234 Wessling, Germany
Keywords: TerraSAR-X, Calibration, Antenna Model.
As TerraSAR-X, due for launch in June 2007, will be an
operational scientific mission with commercial potential,
product quality is of crucial importance. The success or
failure of the mission essentially depends on the calibration of
the TerraSAR-X system ensuring the product quality and the
correct in-orbit operation of the entire SAR system.
This paper describes the calibration procedures for
TerraSAR-X and the dedicated activities to be performed
during the five months commissioning phase. Results from
on-ground tests are discussed with respect to geometric and
radiometric calibration of the TerraSAR-X system.
The first German SAR satellite for commercial and scientific
applications, TerraSAR-X, will launch in June 2007.
TerraSAR-X is a flexible X-Band SAR built by EADS
Astrium and the German Aerospace Center (DLR) in a
public-private-partnership . The SAR instrument is
designed to cover a wide range of swath positions and to
operate in different operation modes by switching the
instrument over a multitude of antenna beams. For the various
antenna beams, an active phased array antenna electronically
forms antenna radiation patterns. The array consists of 384
radiating sub-arrays for horizontal and vertical polarisation
arranged in a matrix of 12 panels with 32 rows. Active
transmit/receive (T/R) modules individually adjust the array
elements in gain and phase for shaping and steering the
antenna pattern in azimuth and elevation direction.
The essential task of calibrating TerraSAR-X is to estimate
and correct systematic error contributions throughout the
complete SAR system and to tie-down image information
(magnitude and phase) to reference units in geophysical
terms. The quality of this calibration process depends on the
inherent stability of the radar system and the capability to
determine and monitor the radiometric and geometric
Due to the high degree of flexibility of TerraSAR-X
(StripMap, Spotlight, ScanSAR, right/left looking, etc.) and a
tight performance with an absolute radiometric accuracy
better than 1dB, it becomes clear that a conventional
calibration approach is not feasible, i.e. the real measurement
of all antenna beams in all operation modes, as performed for
the ASAR instrument of ENVISAT across the rainforest .
Hence, a new, more efficient, and affordable calibration
concept has been developed [3,4]. The key element of this
calibration concept is a novel antenna model approach [4,6].
The TerraSAR-X antenna model is utilised for generating all
reference antenna patterns and for beam optimisation.
To account for the restricted time of calibration campaigns
performed during commissioning phase of TerraSAR-X, the
number of passes and places of test sites is optimised versus
cost and time effort by calibrating only dedicated beams but
with the same test site.
The paper describes the in-orbit calibration procedure and the
different activities performed before and after launch of
TerraSAR-X. It shows the calibration status until satellite
launch and gives an overview of the planned activities during
the five months commissioning phase.
2 Scope of TerraSAR-X Calibration
Applying the novel antenna model approach for SAR system
calibration, the objective of in-orbit calibration can be sub-
divided into three major tasks:
Geometric Calibration: to assign the SAR system
to the geographic location on the earth surface.
Relative Radiometric Calibration: for radiometric
correction of SAR data within an illuminated scene.
measurement of the SAR system against standard
targets with well known geophysical characteristics.
The calibration steps rely on the antenna model which has
been validated on-ground. In addition to the tasks described
above, the verification of this antenna model has to be
executed in-orbit. This ensures the provision of the antenna
patterns of all operation modes and the gain offset between
The main goal after launch of the TerraSAR-X satellite is to
provide calibrated and verified SAR data products at the latest
by the end of the commissioning phase. Thus, a strategy for
an efficient but robust calibration approach has been