International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B1. Istanbul 2004
The aerial photographs were scanned with a pixel size of 14
microns. At this photo scale, the nominal pixel ground
sample distance is 35 cm. The complete set of image scans
has a storage requirement of 150GB.
3. CALIBRATION SOFTWARE
The main software components that were used to perform the
geometric calibration of the OrbView-3 satellite are:
Alignment Kalman Filter, orbit determination, image
correlator, and multi-sensor triangulation. Each of these is
described in sections below. The image correlator and the
triangulation software were also used to build the Geometric
Calibration Range.
3.1 Alignment Kalman Filter
The exterior attitude orientation of the OV-3 satellite is
determined by using star trackers and gyroscopes. The star
trackers provide an absolute attitude reference at a discrete
sampling rate. The gyros provide relative attitude changes
at a fast sampling rate. The data from the star trackers and
gyros is blended in a Kalman filter to estimate the platform
attitude in an absolute attitude reference frame at a high
sample rate with good relative attitude changes.
In order to obtain accurate platform attitude estimates the
geometric calibration process needs to determine the
alignment angles between the star trackers and the gyros
coordinate axes. This is accomplished by carrying these
alignment angles as parameters to be estimated in an
algorithm called the Alignment Kalman Filter. Additional
parameters estimated by the Alignment Kalman Filter
include: gyro bias and scale factors. This filter can be
thought of as a self-calibration process. For the alignment
angles to be observable (estimable), the spacecraft has to
maneuver through a sufficient volume of 3D attitude space
and at different angular rates. The alignment angles between
the gyro and camera axes are determined in the triangulation
model.
3.2 Orbit Determination
The orbit determination software is Gipsy-Oasis and is
maintained by JPL (Jet Propulsion Laboratory). Gipsy-Oasis
contains sophisticated orbital models that include
components such as: gravity model, drag model, stochastic
force model, and a GPS receiver model. Precision orbit
determination uses the Rapid Product from the IGS
(International GPS Service) for post-processed GPS
ephemeredes.
3.3 Image Correlator
A flexible and efficient image correlator is key to the cost
effective use of controlled aerial photography for control
point generation. Since the image correlator needs to locate
common image points in both the aerial and satellite
imagery, it must work well with non-homogenous image
sets. The differences between the aerial and satellite imagery
an be caused by temporal effects, such as fields with
different crops, or by image scale and rotation. The image
correlator reduces scale and rotation differences by
rectifying both image sources to the same scale and
orientation. The rectification is performed on the fly. The
image correlation is performed on the rectified imagery and
the image coordinates of the match points are transformed
back into the coordinate systems of the original images.
3.4 Multi-Sensor Triangulation
Triangulation software is used to estimate the camera
calibration parameters. This software needs to be rigorous,
flexible and robust. The design of the triangulation software
follows an object oriented approach that includes a
framework structure, utilities and a Developers Took Kit
(DTK). The DTK is the used to rapidly bring in new sensor
models. The formal division between framework and sensor
factory allows the sensor developer to focus in on the sensor
I/O and math model in the DTK and the framework contains
the memory management and the least squares adjustment
engine [Mulawa 2000]. The OV-3 geometric camera
Calibration model contains parameters to model the interior
orientation, distortion and camera alignment to platform.
4. ON-ORBIT GEOMETRIC CALIBRATION OF OV-3
The on-orbit geometric calibration of a system having as
many sensors as a high resolution imaging satellite takes
place over a period of time and is accomplished by the
achievement of milestones events. lt is this method that is
used in this paper to describe the geometric calibration
process. The calibration process has many experts involved
in tuning and calibration of the sensor components that they
are responsible for. While a substantial amount of work is
done in parallel by the geometric calibration team members
on sensor components, there is also a sequential approach to
bringing the system into calibration. For example: camera
focus, orbit determination and the attitude determination
systems must be calibrated prior to completion of the camera
calibration. The camera calibration is the last step in the on-
orbit calibration of the satellite.
4.1 Milestone Events
OV-3 Launched 2003 July 26
First Image 2003 July 27
Initial Calibration of the Attitude 2003 July 27
Determination System
; ; : 2 22
Coarse Boresight Adjustment 2003 Aug 22
Final Camera Focus Adjustment 005 Sep 0
Orbit determination model tuned and | 2003 Sep 17
verified
^ an 2003 Sep 17
Coarse geometric camera calibration 2 oe
Refined calibration of the attitude 2003 Oct 03
determination system
2003 Nov 04
Initial geometric camera calibration
Table 1: OV-3 Geometric Calibration Milestone Events
4.2 OV-3 Launched: 2003 June 26
A
The launch vehicle performed well and placed OV-3 into its
nominal orbit. The satellite beacon was heard on the first
pass over the northern terminal. Command and control of
the satellite was established. During the next month, the
camera door remained closed while the satellite was out-
gassed and was raised to its final orbit. À series of tests were
performed to ensure the safe operation of the satellite.
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