IMPROVING THE EXTERIOR ORIENTATION OF MARS EXPRESS HRSC IMAGERY
Heinrich Ebner ', Michael Spiegel ', Albert Baumgartner ', Bernd Giese 2
Gerhard Neukum *, and the HRSC Co-Investigator Team
! Photogrammetry and Remote Sensing, Technische Universitaet, Muenchen, 80290 Muenchen, Germany
spiegel@bv.tum.de, www.remotesensing-tum.de
2 Institute of Planetary Research, German Aerospace Center (DLR), Rutherfordstr. 2, 12489 Berlin, Germany
3 Institute of Geosciences, Freie Universitit Berlin, Malteserstr. 74-100, 12249 Berlin, Germany
gneukum@zedat.fu-berlin.de
Commission IV, WG IV/9
KEY WORDS: Extra-terrestrial, Three-Line, Sensor, Orientation, Adjustment, DEM/DTM
ABSTRACT
This paper focusses on the estimation of exterior orientation parameters of the High Resolution Stereo Camera (1 IRSC) orbiting planet
Mars during the European Mars Express mission since January 2004. One of the challenges for the photogrammetric processing of
HRSC images is the low number of Ground Control Points (GCPs) on Mars which can be identified and measured in the images in the
usual way. Therefore, Mars Observer Laser Altimeter (MOLA)-data is employed as control information in the photogrammetric bundle
adjustment of HRSC images. A Mars Express specific concept for the photogrammetric point determination and the mathematical
model of bundle adjustment with the principle of three-line camera is described. Results are reported for the bundle adjustment with
and without MOLA-data as control information.
1 INTRODUCTION
The ESA mission Mars Express with the High Resolution Stereo
Camera (HRSC) on board was launched successfully at June 2,
2003 from the Baikonur Cosmodrome in Kazakhstan. In Decem-
ber 2003 the orbiter has arrived at Mars, the orbiting phase started
in January 2004 and the first images are acquired. During the first
two month of orbiting the HRSC acquired images of an area of
about 4 million square kilometers.
The primary goal of the Chair for Photogrammetry and Remote
Sensing (LPF) at the Technical University of Munich is to de-
termine the exterior orientation of HRSC orbiting planet Mars
during Mars Express mission. In general, the classical pho-
togrammetric point determination requires image coordinates of
tie points, interior and exterior orientation, and ground control
points (GCP). In case of HRSC on Mars Express tie points will be
measured automatically in the images by means of image match-
ing. Interior orientation is assumed to be known from calibra-
tion. Observations for the exterior orientation will be derived
from star observation, Inertial Measurement Unit (IMU) mea-
surements, and orbit analysis. Unfortunately, these observations
for the parameters of the exterior orientation will probably not
be precise enough for a consistent photogrammetric point deter-
mination on a global level. Nevertheless, they can serve as good
approximate values.
Additional control information is necessary in order to fit pho-
togrammetrically derived object points into the existing reference
system on Mars. On Mars there are only few precisely known
points which can serve as classical GCPs. But there is a large
number of ground points measured by MOLA. The special thing
about the laser points is, that they can not be identified in the
images in an easy way. Le., image coordinates of most of these
points can not be measured, and therefore, it is not possible to
treat them as normal GCPs in a bundle adjustment. As a remedy
852
it is proposed to use control surfaces derived from the MOLA
points.
In Section 2 the principle of nine-line HRSC, the acquisition of
the imagery, and the MOLA-data is described. The concept of
photogrammetric point determination with MOLA data as con-
trol information in the bundle adjustment is given in Section 3. In
Section 4 the first results of HRSC imagery processing are pre-
sented and analysed. Section 5 concludes the paper.
2 DATA SOURCES
2.4 High Resolution Stereo Camera (HRSC)
The HRSC (see Fig. 1) is a multi-sensor pushbroom camera con-
sisting of nine Charge Coupled Device (CCD) line sensors for si-
multaneous high resolution stereo, multispectral, and multi-phase
imaging. It has one panchromatic nadir channel, four panchro-
matic stereo channels, and four channels for color. The conver
gence angles between the nadir- and the stereo sensors arc 21 and
14 gon.
The sensor arrays with 5176 active pixels each are arranged per
pendicular to the direction of flight in one focal plane. The im-
ages are generated by catenating the continuously acquired line-
images. The result is one image per sensor and orbit. One image
strip includes all images of one orbit. The pixel size on ground of
12m will be reached at an altitude of 270 km at pericentre and in-
crease to 50 m at an altitude of 1000 km (Neukum and Hoffmann,
2000).
At pericentre one image strip covers an area of about 60km
across trajectory. In general, the strip has a length of about
300 km up to 4000 km. Figure 2 shows a part of an image from
orbit 68.
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