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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