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.
