PERFORMANCE OF AUTOMATIC TIE POINT EXTRACTION USING HRSC IMAGERY 
OF THE MARS EXPRESS MISSION 
C. Heipke®, R. Schmidt’, R. Brand*, J. Oberst’, G. Neukum® and the HRSC Co-Investigator Team 
"Institute of Photogrammetry and Geolnformation, University of Hannover, Nienburger Str. |, D-30167 Hannover, 
Germany, schmidt@ipi.uni-hannover.de 
"Institut für Planetenforschung, DLR Berlin, Rutherfordstr. 2, D-12489 Berlin, Germany, juergen.oberst@dlr.de 
“Institut für Geologie, Geophysik und Geoinformatik, Freie Universität Berlin, Malteserstr. 74-100, D-12249 Berlin, 
Germany, gneukum@zedat.fu-berlin.de 
Commission IV, WG IV/9 
KEY WORDS: Extraterrestrial, Planetary, Camera, Matching, Scanner, Multiline, High resolution, Three-Line 
ABSTRACT: 
In January 2004 the first European interplanetary spacecraft, Mars Express, was inserted successfully into a Martian orbit. The 
multiline scanner HRSC (High Resolution Stereo Camera) on board of Mars Express will image most of the surface of Mars in 
colour and stereoscopically in high resolution. The Institute of Photogrammetry and Geolnformation (IPI), University of Hannover, 
has created software for the automatic extraction of homologous image primitives from HRSC image data. With the obtained tie 
points a bundle block adjustment will be carried out with software developed at the Chair for Photogrammetry and Remote Sensing 
(LPF) of the Technische Universität München which will result in an improved exterior orientation of the camera. With these results 
high level products such as digital terrain models (DTMs), ortho photos and shaded reliefs can be derived from the imagery. 
This paper describes the used approach for the tie point matching. On the basis of some selected Mars Express orbits the results of 
the matching, the distribution of the tie points in the images and the achieved accuracy are presented and evaluated. 
I. INTRODUCTION 
In June 2003 the European Space Agency (ESA) launched the 
Mars Express spacecraft from the Baikonur launch pad in 
Kazakhstan. After a journey of about six months the orbiter was 
successfully inserted into a polar orbit around Mars. During its 
two years mission the HRSC on board of Mars Express images 
large parts of the Mars surface. The HRSC is a multisensor 
pushbroom camera consisting of nine charge coupled device 
(CCD) line sensors mounted in parallel for simultaneous high 
resolution sterco, multispectral, and multiphase imaging 
(Albertz et al., 1993). At pericenter about 300 km above the 
surface of Mars a ground resolution of about 12 m is attained. 
The Camera Unit (CU) of the HRSC additionally comprises a 
Super Resolution Channel (SRC) which captures frame images 
embedded in the basic HRSC swath at a ground resolution of up 
to 2.5 m. 
The three-dimensional position and attitude of the spacecraft is 
constantly determined by ESA by combining techniques of 
measuring Doppler shifts, acquiring ranging data, triangulation 
measurements and a star tracker camera. These measurements 
result in a three-dimensional position and attitude of the 
spacecraft over time which can be considered as approximate 
exterior orientation in classical photogrammetry. However, 
these values are not consistent enough for high accuracy 
photogrammetric point determination. Therefore, a bundle 
adjustment has to be performed using these values as direct 
observations for the unknown exterior orientation parameters. 
In addition to these observations automatically extracted tie 
points derived via digital image matching (DIM) are used as 
input for the bundle adjustment. For every orbit, where HRSC 
imagery exists, IPI and LPF are jointly processing the HRSC 
data. The tie points are obtained by software developed at IPI 
846 
(Schmidt, Brand, 2003) and the image coordinates of the tie 
points are sent to LPF as input for the bundle adjustment (Ebner 
et al, 2004). The mapping performance of the HRSC is 
analysed in (Oberst et al., 2004). 
The interior orientation of the HRSC has been calibrated in a 
laboratory at Dornier, Friedrichshafen and has been verified 
during the six months journey to Mars by means of star 
observations. So far no deviations from the calibration have 
been experienced so that the interior orientation of the HRSC is 
considered to be stable. 
In section two of this paper the approach used for the 
generation of tie points from HRSC imagery is presented. In 
section three the test data is described and the results of the tie 
point matching derived from some selected orbits are shown 
and discussed. The last section summarises the results and some 
conclusions are drawn. 
2. MATCHING APPROACH 
Our matching approach follows a coarse to fine strategy which 
means the matching result is refined step by step through image 
pyramids. As input data the HRSC imagery, the observed 
exterior orientation and the calibration data of the interior 
orientation are needed. As an optional input it is possible to use 
a DTM as approximate information. On Mars a high accuracy 
DTM derived from data of the MOLA instrument (Neumann et 
al., 2003) is available. 
At first point features are extracted using the Fórstner operator 
(Fórstner, 1986) and the images are matched pairwise in all 
combinations using the cross correlation coefficient as 
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