International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012 
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia 
  
707 
Simulated 
-— 
| Overlapping Waveform 
T [ss Echo 1 
mn Echo 2 
+33 Noise 
  
  
  
  
     
EW: Echo Width 
Distance: the 
range between 
two echoes. 
A1, A2: the 
maximum 
amplitude of the 
two echoes 
Amplitude 
  
  
10 15 20 
Time (NS) 
Figure 6. The factors related to the overlapped echo detection 
3.41 Echo width and distance: The echo width of laser 
signal changes according to variant LIDAR systems. Two 
different values of the echo width parameter corresponding to 
Leica ALS60 (5ns) and Optech ALTM3100 (8 ns) are utilized 
to test the influence on the detection results. Additionally when 
the distance between two echoes is close to the echo width, the 
synthesized echoes will approximate to single echo and are not 
easy to dissolve. To test how close distance of the two echoes 
can be dissolved by the detectors, some sets of the distance 
factor around the echo width value are used. As section 3.3.1, 
1000 waveforms are generated at each distance level. The 
detection rate would be: 
number of detecting 2 echo 
1000 
number of detecting 1 echo 
1000 
number of detecting more than 2 echo 
1000 
CR: x100% 
(8) 
MR2: x100% 
RR2: x100% 
  
3.4. Results of the influence of the echo width and 
distance on detecting overlapped echoes: Figure 7 shows the 
results of detecting the overlapped echoes with variant echo 
width and distance values. From Figure (a) and (c) the 
limitation of resolving overlapped echoes for zero-crossing 
detector is around an echo width. For wavelet-based detector, it 
can resolve two echoes which have distance less than an echo 
width if the echo width equals to 8 ns. Moreover the two echoes 
can be confidently detected by the wavelet-based detector in the 
condition that the distance is equal to the echo width. However 
some failures were produced by the zero-crossing detector. 
3.4.3 Relative intensity ratio: Another factor related to the 
overlapped echo detection is the relative intensity between two 
echoes. Considering the results of section 3.4.2, the echo width 
factor is fixed as 5 ns and the distance factor is set as 5 ns and 6 
ns respectively. The overlapped echoes with variant relative 
intensity ratio (A2/A1) were set up. And 1000 waveforms were 
generated at each relative intensity ratio level. The detection 
rate can be calculated by equation (8) as well. 
532 
Zero-crossing detector 
M2 echo 
E271 echo 
Bl more than 2 echo 
  
Echo width: 5 ns 
  
percentage of successful detection (%) 
5 
Distance (ns) 
(a) 
Wavelet-based detector 
  
percentage of successful detection (%) 
  
Distance (ns) 
(b) 
Zero-crossing detector 
  
INN 2 echo 
vetdth: 8 ns 
more than 2 echo 
  
  
   
  
percentage of successful detection (96) 
7 8 
Distance (ns) 
(c) 
9 10 
Echo width: 8 ns 
Wavelet-based detector 
F711 echo 
M more than 2 echo 
  
percentage of successful detection (%) 
  
7 8 
Distance (ns) 
(d) 
Figure 7. Results of overlapped echo detection with variant echo 
width and distance values, CR2: blue, MR2: green, RR2: red, 
(a)(c) zero-crossing, (b)(d) wavelet-based 
3.4.4 Results of the influence of the relative intensity ratio 
on detecting overlapped echoes: Figure 8 shows the results 
that how the relative intensity ratio factor influences the 
detection. It indicates that the wavelet-based detector has better 
detection accuracy than zero-crossing method especially when 
the distance is greater than the echo width. The wavelet-based 
detector can detect both of the echoes even the relative intensity 
ratio reach 4 if the distance between two echoes is greater than 
the range resolution of a LIDAR system.