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Eberhard Steinle 
  
2 LASER SCANNING PRINCIPLE 
In the context of building modelling in urban environment the investigations were restricted to airborne laser scanner 
systems. These are active scanners based on laser signals for measurement of slant distances to acquire information 
about the surface of the Earth and objects on it. As mentioned above, there are several laser scanning systems being 
operational. Concerning the measurement principle two different methods had been realized: runtime measurement 
using pulsed laser signals and phase difference measurement using continuous-wave lasers. Because most systems are 
based on the first mode, it will be described in more detail in the next section. A good overview on both topics can be 
found in (Wehr & Lohr, 1999). 
2. Measurement principle of pulsed laser scanners 
This type of laser scanner emits pulsed laser light in exactly determined time intervals. The system measures the 
runtime of these laser pulses, i.e. the elapsed time between emitting a signal and receiving it after reflection on the 
surface of the Earth or objects on it. Therefore, slant distances can be derived from these time differences by the well- 
known formula v = As / At or As =v / At. By means of the exterior orientation of the sensor (recorded by differential 
GPS (dGPS) and INS systems) 3D co-ordinates of the illuminated surface points can be determined. 
22 Design of pulsed laser scanning systems 
Laser systems need to be designed mainly regarding two components: the emitting and receiving unit and the 
positioning unit. Both will be described by means of the operational system TopoSys (Lohr & Eibert, 1995) which was 
used to acquire the data sets presented in this paper. 
Flight Segment f Mirror 
_ Laser 
  
  
Laser Scanner sui de 
dGPS equipment A T Motor | 
INS à FC ds. Glass Fibre 
Videocamera Optic 
     
  
  
    
    
I 
SL Pa 
ur um rf " 
ES 
= 
          
GES f Ground Segment 
S / 3 
GPS reference station(s) 
Hard - and Soft-ware 
for data processing 
Figure 1: Laser scanner system TopoSys 
In this system, the emitting and receiving unit is realized by means of a glass fibre optic. The laser light is emitted on a 
nutating mirror, i.e. a rotating mirror which deflects it on a glass fiber bunch. The ends of the glass fibres are connected 
to a row-shaped optic, so the resulting measuring pattern on the surface of the Earth is a single scanning line. In addition 
to the movement of the airplane this results in a strip-wise data acquisition as shown in Figure |. 
The positioning component of the system consists of several elements. As navigation principle differential Global 
Positioning System (dGPS) is chosen. Therefore, GPS antennas are mounted on the airplane, as well as on reference 
stations on the ground. Although this positioning strategy yields to good results concerning the accuracy of the obtained 
co-ordinates (in the range of some few centimeters), the measurement rate is lower than the one of the laser scanner. 
Therefore, additionally Inertial Navigation Systems (INS) are used, i.e. navigation units register the rotations of the 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 859