al
o m
=
On
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
Table 1: HRSC /SRC parameters
HRSC SRC
Focal Length, mm 175.0 988.6 +)
f/ratio f/5.6 9.2
IFOV, rad/pixel 0.000040 0.0000092
Field of View (rad)
Cross-track 0.2000 0.0094
Along-track 0.6597 ++) 0.0094
Pixel Size, microns 7.0 9.0
Detector Array Size 5184 *) 1024 x 1024 **)
*) Actual laboratory-calibrated flight model; the nominal
value is 975.0 mm.
++) The HRSC along-track FOV size is assumed to be equal
to the angular separation between the first and last line
detector. It is nominally equal to 2 x 18.9° = 37.8°, where
18.9° is the nominal stereo angle.
*) The HRSC sensor lines have 5272 "physical" pixels but
only 5184 of them are used for image reconstruction and
processing
**) The SRC has 1032 pixels and 1024 lines; however, only
1008 lines and 1008 pixels are available after calibration.
Table 2: Band width of HRSC filters
Detector Band Center Bandwidth
# [nanometers]
S1 675 180
RED 750 40
P2 675 180
BLUE 440 90
NADIR 675 180
GREEN 530 90
P1 675 180
IR 970 90
S1 675 180
3. SYSTEMATIC AND STANDARD
PHOTOGRAMMETRIC PROCESSING
The data processing essentially follows procedures, as they
were described earlier at an ISPRS conference in 1995
(Oberst et al., 1995). Following receipt on the ground, the
data are decompressed and radiometrically calibrated
resulting in “Level-2” products. Further systematic
processing includes geometric corrections. The nominal
orbit, pointing, and geometric calibration data are used to
project the images onto a 5 km/pix-MOLA Digital Planetary
Radii Model as a reference surface (“Level-3” products).
The five panchromatic channels in the Level-2 image data are
subjected to image matching for the production of DEMs
(Digital Elevation Models). The data are typically smoothed
to remove data compression artifacts beforehand. Gaps in the
matching are filled by interpolation to warrant smooth
terrain models for the further processing. The DEMs are
typically produced with a grid resolution of 200 m. Efforts
are under way to derive terrain models of higher resolutions
(50 m/DEM pixel) in selected areas, surface textures,
illumination geometry, and atmospheric conditions
permitting. The "effective" resolution of details in the
terrain models remains to be studied (see discussion further
below). By combination of the Level-2 data and the DEMs,
orthoimages are produced. Various additional products
include anaglyphs, oblique views, and movies. The
processing ultimately results in topographic color image
maps (Albertz et al., 2004).
SRC
Raster imaging —————7
peu Tee
SRC
Da i d
Contiguous imaging
Fig. 2: SRC imaging schemes. The images are not to scale:
SRC covers only 4% of the nominal HRSC sensor swath
4. INTERNAL CONSISTENCY
In order to make the topographic mapping with high
accuracy possible, the viewing rays from conjugate points
identified in the three stereo sensors (or five, when the
panchromatic camera sensors are included), ideally, must
intersect. We wish to examine, how well this condition is
met. We first of all collected a large number of such
conjugate points using a digital image matcher (Fig. 3). The
HWMATCHI program (developed for the HRSC project,
Heipke et al., 2004), which identifies large numbers of
"points of interest" and measures their line/sample
coordinates, is ideally suited for this task. Normally, » 1000
such points of interest are found within one typical HRSC
orbit strip.
Using the nominal orbit and pointing data, as delivered from
the Mars Express navigation team, we determined the
viewing ray vectors and the "intersection point" of the
viewing rays, i.e. the point where the three rays have their
closest approach. A plane perpendicular to the surface
normal was then placed through this intersection point. The
actual ray intersection points were plotted with respect to
this plane (Fig. 4).
First tests revealed an internal time-tag error of 8 ms,
associated with the nadir channel, consistent with
communications from the camera manufacturing team. This
error was removed to first order for this further study.
Inspection of the corrected data suggests that > 99% of the
stereo rays intersect within a sphere of radius 20m, which is
an impressive result, considering that the pixel resolution is
only 12.5 m for the nadir sensor and 25 m for the two stereo
channels. We conclude that the internal consistency of the
HRSC stereo mapping experiment, even with nominal orbit
and pointing data, is excellent.
1319
