ISPRS Commission III, Vol.34, Part 3A „Photogrammetric Computer Vision“, Graz, 2002 
  
3. TEST FLIGHT CONFIGURATION 
The presented data from different calibration flights are part of 
a big production project in Saudi Arabia flown by Hansa 
Luftbild German Air Surveys. Within this project more than 
9000 images (scale 1:5500) were captured at 12 flight days 
from January, 29" — March, 25™ 2001, covering a time span of 
approximately 2 months. Parallel to the image data recording, 
GPS/inertial positions and attitude data were provided by the 
IGI AEROcontrol IId system, whose IMU was rigidly mounted 
at the camera body. For each mission day the same fully 
signalised flight line was normally flown twice with opposite 
flight directions for system calibration — typically once in the 
morning before and once in the evening after mission flight. 
This calibration strip consists of altogether 21 signalised ground 
control points (GCP) located in the standard or Gruber positions 
of each image resulting in 7 captured images per calibration 
line. Since almost all images were taken with the same Z/I- 
Imaging RMK Top30 — GPS/inertial installation (calibration 
flights 1-19, January, 29" — March, 15“), the results from the 
multiple calibration flight data allow for first investigations on 
the long term stability of system calibration. Only the last two 
missions were flown with a wide-angle RMK Topl5, therefore 
the inertial unit had to be demounted and fixed to the new 
camera body for this last two mission days (calibration flights 
20-23, March, 24™ and 25"). These wide-angle flights are non 
considered in more detail in the following. 
The input data for the system calibration were provided by IGI 
and Hansa Luftbild, respectively. The GPS/inertial data were 
processed using the AEROoffice software, afterwards the 
integrated GPS/inertial positions and attitudes are interpolated 
on the camera exposure times. The pre-surveyed translation 
offsets are already considered during GPS/inertial data 
integration. The image coordinates were obtained from 
MATCH-AT automatic aerial triangulation, where the GCP 
image coordinates were measured manually. 
4. TEST RESULTS 
Based on the integrated GPS/inertial-AT described in Section 2 
the calibration of system parameters was done for each 
calibration flight based on the given 21 GCPs and the exterior 
orientation results from the integrated GPS/inertial system. 
Since no quality measures for the GPS/inertial positions and 
attitudes were available from GPS/inertial data integration an 
assumed accuracy of 0.1m and 0.005gon was introduced for the 
stochastic model. This empirical accuracy should be expected 
from such high quality integrated GPS/inertial system if its 
accuracy potential is fully exploited. 
Within system calibration the inevitable angle offset and 
position shifts (if significantly present) are estimated in 
combination with the Ebner self-calibration parameters. In 
order to separate between global and strip-dependent shift 
parameters, the two flight lines per flight day were considered 
as one calibration block. Since the automatic AT was done for 
the different flight lines separately, the two strips are tied 
together only via the identical GCPs. For two of the normal- 
angle flight days only one complete calibration strip was flown 
due to weather conditions. These non-complete calibration 
flights are not considered in the further processing. Overall, 
eight complete calibration flight days are available for the 
GPS/inertial normal-angle camera configuration. 
4.1 Quality of GPS/inertial exterior orientations 
As one first result the directly measured exterior orientations 
from GPS/inertial are compared to the estimated values from 
AT. The remaining differences serve as first indication of the 
quality of GPS/inertial position and attitude determination. In 
Figures 3-6 the particular position and attitude differences are 
shown for the distinct camera stations from four representative 
calibration flight lines handled as two calibration blocks flown 
on January 29" and February 18. The statistical analysis from 
all considered normal-angle calibration flight blocks is given in 
Tables 2 and 3, respectively. 
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# Image of flight day 
Figure 4. Attitude variations 
(Flights 1+2, Jan 29). 
# d o of flight day 
Figure 3. Position variations 
(Flights 142, Jan 29). 
1.001 ——1L——1L——-1— 719—174 0.04 [——1——1————T————771—— 
Position [m] 
Attitude [gon] 
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E e-edEast | F À 
-0.75 +-+ dNorth -0.03r- 4 
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0 2 4 6 8106 12/14 18 0 2 4 6 8 
# Image of flight day 
Figure 5. Position variations 
(Flights 10+11, Feb 18). 
ee dOmega 
+--+ dPhi 
4--A dKappa 
10 128. 14 6 
# Image of flight day 
Figure 6. Attitude variations 
(Flights 10+11, Feb 18). 
  
  
  
As it can be seen from Table 2 the variations (STD) in the 
GPS/inertial positions are quite consistent and mostly in the 
range of 2dm which coincides with the typical GPS positioning 
performance after differential phase processing. Nevertheless, 
significant offsets or even drift effects are present, which can be 
clearly seen from Figures 3 and 4. Additional systematic errors 
are seen in attitude determination (Table 3). Although the mean 
variation in o— and q-angle is within the 15” level for the 
presented normal angle flights, the differences in « show larger 
systematic effects for some calibration blocks resulting in large 
STD values >0.01gon (>30”). As illustrated in Figure 6 for the 
calibration block flown on February 18" the x-angle shows a 
clear strip dependent systematic offset, which might be due to 
non optimal system alignment or insufficiently damping of 
inherent inertial error behaviour during GPS/inertial data 
processing. Errors in the estimated gyro scale factor will result 
in such a jump between two flight lines with opposite flight 
directions. Any uncompensated error will deteriorate the quality 
of object point determination after direct georeferencing. 
Former airborne tests using the AEROcontrol IId system have 
shown consistently higher quality results indicating that for 
some of the mission days the accuracy potential is not fully 
reached with the GPS/inertial data investigated so far. This fact 
reconfirms the high demands for careful processing of 
GPS/inertial data and well defined test flight conditions 
especially when data are used in system calibration and for later 
production projects.