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of the technology is achieved by taking profit of high frequency lasers, high precision airborne Inertial Navigation 
Systems (INS) and high precision scanners. These are combined with multi-channel GPS receivers to result in a 
complete topographic system. The technology was said to have advantages over photogrammetric methods in forest 
areas or in coastal zones where photogrammetric point measurements are problematic. 
Several companies offer laser scanning flights. The basic technology as described in the previous paragraph is the same. 
The differences are in general the different lasers used and the various scanning technology leading to different point 
distributions. Most companies are in addition offering derived products such as DEM or 3D city models, using their 
own software. For these products, the classification algorithms are of high importance. 
3 LASERSCANNING TECHNOLOGIES - STATE OF THE ART 
31 The Technology in General 
As already described, three important components determine the technology: the laser, INS and Differential GPS 
(DGPS). The basic idea is to calculate polar coordinates for the laser point. The laser rangefinder itself measures the 
distance between the aircraft and the laser point. The direction of the beam is calculated by combining the angle of the 
scanning system with the aircraft's angles, given by the INS. The position of the aircraft and thereby of the origin of the 
laser beam is known by using DGPS, and finally the coordinates of the laser points can be calculated in the European 
Terrestrial Reference System 1989 (ETRS89). 
As the laser beam can be extremely focussed, the diameter of the laser spot on the ground is only of some decimetres 
and varies from 0.3 to 1.5 m for different systems. The reflection coefficient of the ground determines which portion of 
the emitted signal returns to the laser. It is depending on the wavelength of the laser and differs especially for white and 
black surfaces. Most of the “second generation” scanners systems offer the possibility to register the reflectance value 
of the returned signal. As the reflectance depends on the surface material, the differentiation of the reflecting surface 
material is possible. “Natural surfaces” (vegetation) have a higher reflectance value than man made materials like 
asphalt or concrete. This possibility opens the field for extended or even new applications. 
In the laser scanner theme issue ISPRS journal of Photogrammetry and Remote Sensing, vol 54, July 1999, a thorough 
investigation of laser scanning systems and techniques can be found. 
32 Pulsed Laser 
Most systems are using a pulsed laser where the distance is given by measuring the time the laser emission, modulated 
by a distinct pulse of some nanoseconds, needs to run the double distance from the aircraft to the laser point. Even 
though the diameter of the laser beam is very small, it is not infinite. So, often the beam meets two or more obstacles 
partially, especially in forest areas. This results in two or more reflections of the pulsed beam returning to the laser 
rangefinder. Most of the systems are capable meanwhile to register all pulses returning to the laser rangefinder, at least 
the first and the last pulse (first pulse mode / last pulse mode). Based on these measurements, different applications are 
possible. 
33 Continuous Wave Laser 
Another system design is using a multi-frequency sidetone concept for the distance measurement. The laser beam is 
intensity modulated by a sinusoidal signal. The received sidetone phase is compared to the phase of the emitted 
sidetone. The distance is finally calculated from the phase shift which is proportional to the time the beam needs to run 
the distance between the aircraft and the ground. Two or more modulation frequencies are necessary to solve the 
ambiguity that occurs since the wavelength is normally smaller than twice the distance between the aircraft and the laser 
point. 
34 Scanning Technology 
While the laser rangefinder used for the distance measurements are more or less similar, the differences in the scanning 
technique are more important. Scanners are used to reach at a more or less wider strip of laser points. To achieve that, 
Mirrors or, for one system, fibre optics are used. 
In most of the Systems, a swinging or rotating mirror is mounted in the front of the pulsed laser deflecting the laser 
beam to the ground. The scanning angle of the mirror is registered or interpolated for each laser pulse. The technique 
results in z-line pattern or almost elliptical line pattern of the laser points on the ground. These systems are very flexible 
while the laser frequency, the scanning frequency and the scanning angle can be changed. 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 719