International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part Bl. Istanbul 2004
2. SYSTEM CALIBRATION
The system calibration is important task for direct sensor
orientation. In direct sensor orientation, the GPS/IMU measure
the true physical imaging sensor position, velocity and attitude
when imaging sensor recording the images. The exterior
orientation parameters are determined by interpolation based on
the ground control points in indirect method. In the case of
direct sensor orientation, the exterior orientation parameters are
measured directly and object points coordinates are
extrapolated from projection centers. Because of this, the
modelings of interior geometry of imaging sensor and the
relation between sensors have major importance.
The system calibration is the first steps of direct or integrated
sensor orientation. It includes the determination of the attitude
relation and shifts between the IMU and the imaging sensor
(boresight misalignment), GPS antenna offsets and time
synchronization errors as well as the interior orientation of
imaging sensor. The system calibration is cover calibration of
all sensors and calibration between sensors (Skaloud, 1999).
The calibration of sensors is include the calibration of imaging
sensor, IMU calibration for shift and drift parameters and GPS
antenna multipath calibration etc. The calibration between
sensors is contain the determination of GPS antenna offset,
positional and attitude offset between the imaging sensor frame
and IMU body frame.
The interior orientation parameters of imaging sensor are
determined by laboratory calibration but in flight condition
these parameters can be differs from actual parameters. GPS
and IMU calibration are performed after production. These
calibration parameters can be checked also in the integration
process of GPS and IMU measurement by Kalman Filtering
(see for detail Schwarz at. al., 1994). The offset between GPS
antenna and imaging sensor is measured with standard
surveying methods. The determination of the boresight
misalignment is a more difficult task. The coordinate axes of
imaging sensor are not parallel to the IMU body frame and the
attitude relation between the IMU body frame and the imaging
sensor frame can not be measured directly. Because of this, the
boresight misalignment, the relation between the IMU and the
imaging sensor, is determined by comparison of the GPS/IMU
derived sensor orientation parameters with the orientation of
bundle block adjustment. During system calibration, correct
mathematical model also important to obtain optimal solution.
2.1 Coordinate System
The national coordinate system is used for bundle block
adjustment and traditional photogrammetric data handling.
These coordinate systems are not orthogonal and do not
correspond to the correct mathematical model used in
photogrammetry. The difference between correct mathematical
model and curved earth cause vertical deformation. This
deformation is compensated by earth curvature correction of the
image coordinates in traditional approach.
The national coordinate systems are mixed coordinate systems.
The horizontal coordinates are belonging to map projection and
vertical coordinates are generally orthometric heights. The
horizontal coordinates of map projections have scale factor and
this scale factor causes affinity deformation (Jacobsen at al.,
1999). The image orientation in direct sensor orientation is
based on directly measured exterior orientation by GPS/IMU.
206
The scale factor of notational net has influence on to the flying
height and this influence has to be taken into account.
2.2 The Calibration of Imaging Sensor
The interior orientation parameters of imaging sensor are
determined in laboratories under constant and homogenous
temperature conditions. Under actual flight conditions, the
temperature is colder than laboratory condition. This
temperature change is cause a lens deformation. Meier (1978)
investigated the focal length change of Zeiss cameras as a result
of lens deformation depending upon flying height. The change
of the focal length corresponds to the change of scale factor for
the height. Because of this, the determination of interior
orientation parameters has mayor importance for direct sensor
orientation. The situation is similar also for the location of the
principal point.
2.3 Boresight Misalignment
Using GPS/IMU integrated system, position is measured by
GPS antenna and attitude is measured by IMU system during
image exposure by imaging sensor. For direct sensor
orientation, the relation between sensors has to be determined
precisely. GPS antenna offset is measured by conventional
survey method. The boresight misalignment, the relation
between IMU and imaging sensor can not be measured directly
(Figure 1). The attitude and shift relationship of IMU body
frame and imaging sensor frame is determined by comparison
of the GPS/IMU derived sensor orientation parameters with the
results of bundle block adjustment of reference block. The IMU
generates roll, pitch, and yaw as attitude information. The IMU
attitude information is related to geographic north while
photogrammetric orientation phi, omega and kappa are related
to grid north. The convergence of meridian has to be taken into
consideration for transformation from IMU orientation to
photogrammetric orientation (Jacobsen, 1999).
IMU body frame
Imaging sensor frame
Figure 1. The relation between IMU and imaging sensor
3. THE EFFECT OF SYSTEM CALIBRATION
The effect of system calibration on direct sensor orientation is
investigated using the data set of the OEEPE test “Integrated
Sensor Orientation" (Heipke et al., 2001). The test field in
Fredrikstad, Norway, is about 5 x 6 km? and has 51 well
distributed signalized control points with UTM/EUREF89
coordinates and ellipsoidal heights was used for the OEEPE
test. The accuracy of used signalized control points in test field
is better then 0.01 m.
The calibration flights in two different scales (1:5.000 and
1:10.000) were flown over reference area for system
Intern«
calibra
1:500(
Carriec
receive
Nilsen
flight
Figure
The
differ
calibr
adjust
(1:56
the c
appro
descr
