The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B5. Beijing 2008 
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and forward intersection. Finally, the aerial triangulated high 
resolution images are also used for obtaining precise spatial 
information, such as building outlines and large scale mapping. 
2.2 Low altitude remote sensing system with unmanned 
airship 
With several years’ research in corporation with another group, 
low altitude remote sensing system based on unmanned airship 
is developed. The unmanned airship automatically flies 
according to predefined flight routes under controlment of 
autopilot system. Hardware of the remote sensing system is 
composed of unmanned airship, autopilot system, task payload, 
wireless communication equipment, ground control station and 
optional GPS reference station, as shown in figure 1. 
|“GPS Ref. ]_ _ 
I Station I 
Wireless 
Communication 
Figure 1. Hardware of the low altitude remote sensing system 
2.3 Data acquisition 
The unmanned airship can automatically fly according to the 
predefined flight routes under controlment of autopilot system. 
Low altitude aerial image sequences are taken by the 
aforenamed non-metric digital camera. The camera is fixed on 
three axes stabilising platform, which can keep the camera 
orientation more stable than the airship itself while flying. 
Low altitude image sequences for experiments are taken with 
the developed remote sensing system in Nov. 2007. Wind speed 
is about 7m/sec during image data acquisition. Ground coverage 
of the test area is about 700m*700m. The flying height of 
unmanned airship above the ground is about 150m. 24mm lens 
is mounted on the Kodak Pro SLR Camera. So ground coverage 
of each image is 150m*225m, and ground sample distance 
(GSD) of images is about 0.05m. Predefined distance between 
two exposure points is 28m, and distance between two strips is 
50m, i.e. the forward overlap and side overlap is 80% and 75% 
respectively. Totally 299 images of 13 strips are taken with the 
unmanned airship. Figure 2 shows the planar position of camera 
center of all images. Caused by wind sensitivity characteristics 
and imperfection of the three axes platform, the maximum bias 
from exposure point to predefined flying strip is about 5m, 
height maintaining accuracy when compared with predefined 
flying height is better than 10m, orientation variations between 
adjacent images are about 5 degrees. Five typical stereo images 
of one strip are shown in figure 3. As can be seen, the overlap 
and camera orientation are stable and fitted with that of the 
predefined ones. 
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The autopilot system is composed of computer, GPS receiver, 
MEMS gyroscope, barometric altimeter, airspeed indicator and 
flying control software. The autopilot system collects real time 
information such as position and speed of the unmanned airship 
and automatically controls the unmanned airship according to 
predefined flying routes. The task payload consists of digital 
camera, video camera and three axes stabilising platform on 
which the two cameras fixed on. The platform can automatically 
keep the predefined camera orientation against the swing or 
oscillation of unmanned airship. The dashed arrows in figure 1 
indicate control signals and information transformation from the 
ground station to autopilot system by wireless communication 
equipment, while bold arrows indicate the information and data 
transformation (e.g. real time video) from the autopilot system 
and task payload to ground station. 
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Figure 2. Planar position of exposure points 
Technical parameters of the low altitude remote sensing system 
are listed in the following: length of the airship is 13.2m, 
cruising speed is about 30~40km/h, maximum payload capacity 
is about 15kg, accuracy of flying tracks against the predefined 
routes is about 5m, posture maintaining accuracy of the three 
axes stabilising platform is about 3 degrees in roll and pitch and 
5 degrees in yaw. The non-metric camera fixed on the platform 
is Kodak Pro SLR with 4500pixels*3000pixels image format 
and 0.008mm pixel size. 
Figure 3. Stereo images taken by the remote sensing system 
To fully evaluate the precision of image sequences acquired by 
the low altitude remote sensing system, totally 64 ground points