International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
One product of the QuickBird Imagery is the so called “Basic
Imagery" that is close to the original sensor image. The basic
imagery is a sensor corrected merged image taken by a
combination of shorter CCD-lines. DigitalGlobe is
commercializing it as level 1B and it can be compared with the
level 1A of Spot images. It is very close to the geometry taken
by a unique CCD-line of 27552 panchromatic and 6888
multispectral pixels without geometric distortion. The
information regarding the focal length differs for the scenes; it
is in the range of 8835 mm leading the 12 jm pixel size to
61cm ground pixel size in the nadir. Within the orbit direction
6900 lines/second are being exposed supported by a Transfer
Delay and Integration (TDI). The reflected energy is summed
up not only in one CCD-line but by shifting the generated
charge in correspondence to the image motion over a group of
CCD-elements. High frequency attitude motions of the platform
during image acquisition are removed from the basic imagery
and only low frequency disturbances remains. Along with the
images, the ephemeris and the attitude data are delivered. The
ephemeris data can be used for the orientation of the image by
making use of the ephemeris data included in the *.eph file with
respect to a geocentric system and the attitude data included in
the *.att file represented by four-element quaternions. These
four parameters describe the attitude of the camera with respect
to a Earth Centre Fixed (ECF) geocentric system, rotating with
the Earth.
DigitalGlobe distributes also two other image products. The so
called *Standard Imagery" is a projection of the image to the
rough Digital Elevation Model GTOPO 30 having a point
spacing of 30" or nearly 900 meters. The panchromatic image
has a ground pixel size of 61cm. The main disadvantage of the
GTOPO 30 is its low vertical accuracy. This can range between
10 to 450 m. Hence it is necessary to carry out a geometrical
improvement by using an acceptable DEM in addition to the
use of GCPs for a precise geo-location.
The other commercialized imagery product is the "Ortho
Ready". Like the Carterra Geo it is a projection of the image to
a plane with constant height, available in a cartographic
projection been selected by the customer. It has the same
ground pixel size like the Standard Imagery.
3. IMAGE ORIENTATION MODELS
Depending on the type of QuickBird image being handled,
different orientation models can be used, namely: For Basic
Imagery, orientation can be done using a) Bundle Approach and
b) Rational Polynomial Coefficients (RPCs). In the case of the
of handling the Standard Imagery and Ortho Ready Products to
improve their orientation and geo-referencing the affinity
transformation based on GCPs including corrections for relieve
displacement (through DTM) and improvements of the nominal
collection elevation and azimuth (when justifiable) is the
recommended procedure. Details regarding this last method are
explained in details in Büyüksalih G., et all (2003); Jacobsen,
K., Passini, R. (2003); Passini, R., Jacobsen, K. (2003); Passini,
R. (2003).
3.1 Bundle Orientation with Self Calibration
The Bundle Model is based on the widely known collinearity
equation in the CCD-line direction. The image position in the
orbit direction is expressing the change of the exterior
orientation as a function of the orbit. As aforementioned, the
exterior orientation parameters of each line image are different,
but the relationship of the exterior orientation to the satellite
orbit is only changing slightly. Hence for the classical CCD-line
cameras, the attitudes are not changing in relation to the
satellite orbit. Hence for an image it is possible to consider time
(space) dependent attitude parameters. Taking into
consideration the general information about the view direction
of the satellite, the “in track and across track view angles”
(included within the *.imd-file) and knowing that in a basic
imagery the effects of the high frequency movements have been
eliminated, then the effects of the low frequency motions of the
platform can be modelled by self calibration via additional
parameters.
The additional parameters been used by the Hannover
orientation program BLASPO are checked for numerical
stability, statistical significance and reliability in order to justify
their presence and to avoid over-parameterization. The program
automatically reduce the parameters specified by dialogue to
the required group by a statistical analysis based on a
combination of Student-test, the correlation and total
correlation. This guarantees that not over-parameterization
occurs. In that case an extrapolation outside the area covered by
control points does not become dangerous.
The elimination process is as follows:
1. For each additional parameter compute:
TAL M i t niu:
fi M On a digo fi | , reject if otherwise
Opi
2. Compute Cross-correlation coefficients for the
parameters
qi WO
R pui. Rj20.85 then eliminate the parameter with
ij
A Gigi
smaller f; value
3. Compute B = 1 — (diag N * diag N”), eliminate the
additional parameter that B;; > 0.85
3.2 Bundle Orientation using Ephemeris and Attitude
Quaternions
The Camera Sensor Model distributed by DigitalGlobe is such
that contains five coordinates systems, namely:
Earth Coordinates (E), Spacecraft Coordinates (S), Camera
Coordinates (C), Detector Coordinates (D), Image Coordinates
(1),. Definitions and details regarding these systems can be
found in DigitalGlobe QuickBird Imagery Products, Product
Guide.
The data contained in the Ephemeris File are sample mean and
covariance estimates of the position of the spacecraft system
relative to the ECF system. These files are produced for a
continuous image period, e.g., an image or strip, and span the
period from at least four seconds before the start of imaging
after the end of imaging.
The attitude file contains sample mean and covariance estimates
of the attitude space craft system relative to the ECF system.
The instantaneous spacecraft attitude is represented by four-
element quaternion. It describes a hypothetical 3D rotation of
the spacecraft frame with respect to the ECF frame. Any such a
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