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Karsten Jacobsen 
POTENTIAL AND LIMITATION OF DIRECT SENSOR ORIENTATION 
Karsten Jacobsen 
Institute for Photogrammetry and Surveying Engineering 
University of Hannover, Germany 
karsten @ipi.uni-hannover.de 
IC-11 Sensor orientation 
KEY WORDS: INS, GPS, Integration, Sensors, Orientation 
ABSTRACT 
Based on combined use of an inertial measurement unit (IMU) and GPS, sensor orientations can be determined directly. 
Problems are included in the determination of the misalignment between the camera and the IMU, the long and short 
term stability of the physical connection of the IMU to the camera lens cone, the separation of the GPS-shifts from the 
misalignment and the reliability of the sensor orientations. In addition, the focal length has to be checked under flight 
conditions if different flying heights above ground are used. 
The misalignment of the IMU from the camera axis can only be determined with sufficient accuracy by a reference 
flight strip flown in opposite directions and controlled with a standard bundle block adjustment. But also under this 
condition, some limitations are caused by the very high correlation of the sensor orientations, especially for normal or 
even smaller angle images. 
For an operational project the sensor orientations of approximately 3000 photos, flown at different days, have been 
determined by IMU + GPS. The misalignment was determined every day by means of a small reference strip, flown 
before and after the images of the main block. 
Limited, but still significant differences of the misalignment occurred. A check of the orientations against ground 
coordinates with 252 photos measured manually and 460 photos used in an automatic aero triangulation has shown 
sufficient results. Nevertheless larger discrepancies in the heading (kappa) have been shown — this also may be caused 
by the limited stability of the IMU mount in relation to the lens cone of the used LMK 2000. The lens cone is fixed only 
by one pin to the camera body. Also the other cameras do have similar mechanical problems. 
In the case of Kodak DCS images, supported by IMU and GPS, a component calibration (misalignment and GPS shift) 
was not possible because of the very small view angle. After a general shift of the GPS-data, these values have been 
fixed to allow a sufficient system calibration. For the resulting ground accuracy a separation of the different error 
sources was not necessary. 
A general problem of the directly determined sensor orientations is the missing reliability. Even simple errors like in the 
image numbers can only be seen by a model setup or the matching of created orthophotos. The model setup often is 
disturbed by not acceptable y-parallaxes. This can be solved by a combined bundle block adjustment. Based on the 
precise approximate image orientations an automatic aero triangulation can be handled much more easy, so the 
additional effort is limited. 
1 INTRODUCTION 
The image orientation for photogrammetric data acquisition is a very important, but time consuming procedure. With a 
block adjustment, the number of required control points has been 
reduced drastically. The next step was the combined bundle block 
adjustment with projection 
center coordinates 
determined by relative 
kinematic GPS-positioning. 
In the case of a real block 
structure, attitude data are 
not required, they can be 
determined by the combined 
block adjustment with GPS- Figure 2: block configuration of 
data (figure 1), if at least 2 linear objects - IMU-data not 
parallel flight strips are required 
available. The flight strips 
  
  
  
  
  
  
Figure 1: block configuration for combined 
adjustment with GPS - crossing flight strips 
every 20 — 30 base length or control points 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 429