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PHOTOGRAMMETRIC PROCESSING OF DIGITAL GALILEO SSI IMAGES
FROM ASTEROID IDA
T. Ohlhof!, M. Dorn!, R. Brand !, W. Zeitler !?
! Chair for Photogrammetry and Remote Sensing
Technical University Munich, Germany
Phone: +49-89-2892 3880, Fax: +49-89-280 95 73
E-Mail: timm@photo.verm.tu-muenchen.de
? Institute for Planetary Exploration
German Aerospace Research Establishment, Germany
Phone: +49-30-69545 398, Fax: +49-30-69545 402
E-Mail: Wolfgang.Zeitler@dlr.de
Commision IV, Working Group 6
KEY WORDS: Orientation, CCD, Extraterrestrial, Matching, DEM /DTM, Bundle Block Adjustment, Galileo, Ida
ABSTRACT
This paper deals with the photogrammetric processing of digital SSI images, which were acquired during the second
asteroid flyby in August 1993. The ground pixel size of the images varies between 25 m and 1,740 m. The results of
the photogrammetric processing comprise a ground control network and a DTM covering one hemisphere of Ida. The
control net consists of 96 points with an (interior) accuracy of 180 m. Based on the control net and on 7,300 ground
points derived from digital image matching a 1° x 1° DTM was obtained using a sphere as reference surface. The DTM
represents the global surface characteristics quite well, as verified by comparison with the images.
1 INTRODUCTION
79 images of the S-type asteroid 243 Ida and its satel-
lite Dactyl were obtained by the Solid-State Imaging (SSI)
camera in the course of the second Galileo asteroid flyby
in August 1993. During the 5h flyby the Galileo space-
craft (Figure 1) passed Ida at a minimum distance of 2,390
km. The ground pixel size of the images varies between
25 m and 1,740 m. These images provide a novel oppor-
tunity for direct measurements of the size, shape and mo-
tion as well as possibly also for mass and density of Ida.
However, precise estimates of size and shape cannot be
achieved easily, as the images have different ground reso-
lutions and show the odd-shaped asteroid rotating under
varying illumination conditions. Unfortunately, the avail-
®alileo
SPACECRAFT CONFIGURATION
PLASMA-WAVE
ANTENNA
MAGNETOMETER
SENSORS
BELOW: DESPUN SECTION
one ACA]
], ANTENNA À ÿ
t
Ww
+ PHOTOPOLARIMETER RADIOMETER
« NEARINFRARED MAPPING SPECTROMETER
* SOUO-STATE IMAGING CAMERA
+ ULTRAVIOLET SPECTROMETER
THERMO!
GENERATORS
Figure 1: Galileo spacecraft with the SSI camera mounted
on a scan platform
able orbit and attitude information of the spacecraft as
well as the ephemeris of Ida are of limited accuracy. Hence,
a combined evaluation of all images together with the nav-
igational information is required to extract the geometrical
and physical parameters of the asteroid.
The shape of asteroid Ida, which is a member of the Koro-
nis family, is more irregular than that of any solar system
object previously imaged by spacecraft. The radii from the
center of figure to the terrain surface range from 3.8 km
to 32 km, and the axes of a triaxial fit ellipsoid amount
to 29.9 km, 12.7 km and 9.3 km (Thomas et al. 1995).
A global 2? x 2? shape model was derived from limb, ter-
minator and shadow data and about 100 control points
(Thomas et al. 1995).
2 PHOTOGRAMMETRIC PROCESSING OF
SSI IMAGES
2.1 Data Flow
The data flow for the photogrammetric processing of SSI
imagery and additional navigation data is shown in Fig-
ure 2.
The input data for photogrammetric processing are SSI
image data and preprocessed navigation data. The images
of Ida were taken by the 2-D SSI CCD camera consisting of
800 x 800 sensor elements with 15x15 um? size each. The
spectral range extends from approximately 375 to 1,100
nm (Belton et al. 1992). Due to the very large focal length
of 1,500 mm the field-of-view of the camera is extremely
narrow (0.5? x 0.5?).
The image orientation procedure comprises the manual
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
