sensor in several test flights [Ackermann, Lindenberger &
Schade 1992],[Ackermann, Englich & Kilian 1994].
Mirror
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Figure 1: Principle of a laser line-scanner
The technical development in the last years made a new
generation of laser sensors available. Those laser scanners
permit an area covering measurement of topographical ter-
rain surface with a high point density. To allow an area
covering data capture with a laser sensor the laser beam
has to be deflected periodically. Within the system used
to provide our test data sets, the deflection is performed
by a nutating mirror, which sends the deflected laser beam
through a linear fiber-optic array. The fiber-optic array
consists of different single optic channels with different
viewing directions. Therefore the laser scanner virtually
defines a push-broom-system, which -in combination with
the movement of the system in flight direction- results in
a strip-wise data acquisition. Figure 1 shows a schematic
picture of a laser line scanner. Table 1 summarizes the
main parameters of a laser scanner of this type [Lohr &
Eibert 1995]. The main advantage of the additional use of
a fiber optics, is the stability of the direction of the laser
beam in a highly kinematic environment like an aircraft.
The mechanical swinging mirror might be deflected addi-
tionally and uncontrolled by accelerations of the aircraft
during the flight. This results in a distorted direction of
the laser beam, which is corrected by the fiber optics.
sensor type
pulse modulated Laser Radar
scanning principle
fiber optic line scanner
range
< 1000 m
measurement principle | run-time measurement
scan frequency 300 Hz (adjustable)
field of view +7
pixels per scan 127
swath width 250 m
at 1000m flight height
accuracy of a single < 03m
distance measurement
laser classification class 1 by EN 60825 (eye-safe)
Table 1: Performance parameters of the used laser scanner
The laser rangefinder, used in an airborne laser sensor sys-
tem has to meet a few physical characteristics to be suitable
as sensor for the measurement of the topographical terrain
surface. One important aspect is the wavelength of the
laser rangefinder. To measure points on the earth surface,
the laser beam should not penetrate into the ground. This
can avoided by using a wavelength in the near infrared.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996
Another important requirement is the use of a pulsed laser
rangefinder, which is able to registrate the last reflected
parts of the emitted pulse. Even though some pulses are
completely reflected by leaves or branches of the veget-
ation, in many cases the reflected pulse will refer to the
terrain and can therefore be used to determine the terrain
surface.
2.2 DETERMINATION OF THE ABSO-
LUTE ORIENTATION
In order to perform the evaluation process the laser scanner
data has previously to be georeferenced. In this process the
exterior orientation of the laser sensor at the time of meas-
urement is determined. The determination of the absolute
orientation (position and attitude) of the laser sensor at
the time of measurement is necessary to derive the ground
coordinates of the laser points.
Different sensors can be used to measure directly the ab-
solute orientation of the laser sensor at the time of meas-
urement. Using either inertial navigation systems (INS) or
a multi-antenna GPS receiver or a setup of several GPS
receivers, the exterior orientation of the sensor can be ob-
tained directly.
Either the INS or the GPS can be used as a single sensor
to perform the positioning and the attitude determination
of the laser sensor. But due to the error characteristics of
both sensors, GPS is used to provide the positioning and
an INS is used for the attitude determination.
The positions, measured with GPS, and the attitudes,
measured with the INS, have to be interpolated to provide
a position and attitude for each laser measurement. The
GPS data are of a relatively low data rate (up to 10 Hz),
in comparison to the data rate of laser scanner (1000 Hz
and more). Therefore a model of the flight path of the air-
craft and an interpolation of the positioning parameters is
necessary. Another possibility to condense the positioning
information provided by GPS, is to use additionally rel-
ative positions from the INS, which are measured with a
higher data rate (e.g. 100 Hz or more).
3 EVALUATION OF LASER
SCANNER DATA
Combining the GPS/INS system for the determination of
the exterior orientation with an laser sensor for distance
measurement, a powerful system for the direct 3D meas-
urement of the topographical terrain surface is available.
To derive a Digital Terrain Model from the measured data
a lot of single evaluation steps are necessary. Roughly
spoken, four main evaluation steps can be distinguished
during the processing.
3.1 Preprocessing
During the preprocessing task, the ground coordinates of
the laser points are derived from the raw data provided by
the laser scanner and the orientation sensors. Therefore the
measurements of the different sensors (GPS, INS and laser
scanner) have to be synchronized, the positions and atti-
tudes have to be determined for each single laser measure-
ment. Then the synchronized measurements can be trans-
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