Eberhard Steinle 
  
airplane based on gyros. These are capable to determine a position with a higher temporal resolution (for more details | Am 
see (Haala et al., 1996)). brea 
the 
In Table 1 the performance parameters of this system are listed. It should be mentioned that the system is capable of | leve 
ws . 3 . . * + « - . "i | 
acquiring up to 5 points per m'. This results in data sets of high point density and suitable accuracy in position as well | com 
as in elevation. 
  
  
  
  
  
  
  
  
  
  
  
TE 
sensor type pulse modulated laser Radar range < 1000 m 
scanning principle fibre optic line scanner transmitter solid state at 1.5 um 
measurement principle run-time measurement scan frequency 300 Hz (adjustable) 
field of view +/- 7° number of pixels per scan 127 
swath width accuracy of a single distanc 
An 250m y B = 1.<03m 
(1000m flight height) measurement 
accuracies of point a resolution of a distance 
Seul P 203,03, 0.1 m Sfallignce <0.1m 
coordinates x,y,z measurement 
  
  
Table 1: Performance parameters of TopoSys laser system [Lohr & Eibert, 1995] 
2.3 Measurement Modes 
2.4 
Emitted laser pulses, reaching the terrain or objects on it, are not of infinite size but increase to illuminated regions (so- 
called footprints) with diameters of about 30 cm for typical measurement constellations. Within such footprints several a | 
different objects may be covered and the laser signal is split into different parts. bui 
proi 
  
Figure 2: Reflection of laser signal at different elevation levels 
In Fig. 2 an example is given to illustrate this phenomenon: A laser ray is covering a tree standing beside a house. The 
bright solid lines represent those parts of the laser signal that reach the ground, i.e. they penetrate the tree through gaps 
in the tree structure. Dashed lines indicate reflections at leaves and branches of the tree. These parts of the signal are 
reflected at a higher elevation level than the other ones and therefore reach the receiving unit earlier. Regarding the 
measurement principle (section 2.1) the system can register this first signal response at the receiving unit, i.e. operating | On 
in first pulse mode. In this case the elevation of the highest illuminated point inside the footprint (in this example the | Bu 
canopy of the tree) would be acquired. If the system is operating in last pulse mode, the last part of signal response is | det 
registered, i.e. the lowest elevation level inside the illuminated region is determined (in this example the ground level). | nt 
| are 
In the presented example an important effect is obvious too. Buildings which are partly covered by vegetation can be 
completely acquired in most cases in last pulse mode. As explained above, parts of the signal are capable to penetrate n 
the covering leaves (dark solid line) and are reflected at the roof. This effect can be observed with/at most deciduous P 
trees, but not in all cases of coniferous ones. If solid horizontal surfaces are illuminated by laser signals, there is no d C 
difference between first and last pulse mode due to the determined elevation. i 
Jat 
acc 
  
860 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000.