PE&RS July 2016 Public - page 456

456
July 2016
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
S
ingle
P
hoton
L
idar
S
ystems
Sigma Space has developed three SPL systems so far. The first
one is the High Resolution Quantum Lidar System (HRQLS
pronounced “Hercules”). Besides HRQLS, two more advanced
SPL systems were also recently developed and fielded with
both higher operating AGLs (Above Ground Level) and higher
measurement rates. The High Altitude Laser (HAL) operates
up to 10,668 m (35,000 ft) at 3.2 Megapixels per
second, while the upgraded HRQLS-2 operates
at a nominal altitude of 3,810 m (12,500 ft) with
measurement rates up to six Megapixels per
second. The newer models are summarized and
compared to HRQLS in Table 1. The nominal
AGL is defined as the AGL where the probability
of receiving a return per pixel from a 10%
reflectance surface (eg., green vegetation) is 95%
and roughly 99% from a 15% reflectance surface
(eg., soil or dry vegetation). The systems can
be operated at substantially higher AGLs than
the nominal value (i.e., 5,486 m or 18,000 ft for
HRQLS-2 and 10,363 or 34,000 ft for HAL) to
achieve wider swaths and greater areal coverage
but with reduced detection probabilities per pixel.
All are designed to operate at aircraft speeds in
excess of 200 knots.  
The SPL data used in this study was acquired
by HRQLS, whose general parameters are
summarized in Table 2. It is designed to operate
at an AGL between 1,829 – 4,572 m (6,000 to
15,000 ft). It can operate with either a dual-
wedge or single-wedge optical scanner. The dual
wedge scanner permits a wide range of scan
patterns and swath widths in a single instrument
and allows the end user to vary the point density
at a given AGL.
D
ata
U
sed
The test data was collected in Maryland on
Nov. 21, 2014. The location of the areas and the
airplane trajectory are displayed in Figure 1.
Three datasets of diverse terrain were used
in this study: one forest, one urban, and one
bathymetric area. The close views of the three
dataset areas are shown in Figure 2 with
Google Earth images. The Forest dataset was
over Idylwild Natural Area in Caroline County.
It is part of the Marshyhope Creek watershed
and has many ancient sand ridges. A mixture
of pines, oaks and heath shrubs can be found
in the dry, sandy soils. It is mainly covered by
forest except that some bare ground is exposed
in the middle-bottom of the image. Several forest
roads are barely visible from the images. The Urban dataset
was collected at St. Aubins Heights in the northern part of
Easton County. The dataset has an irregular shape. Features
in the dataset include residential houses, shopping malls,
parking lots, grasses, streets and roads. The Bathy dataset
was collected at Eastwood Point in Easton County. The water
area in the dataset is part of the Tred Avon River, which flows
to the North Atlantic Ocean via the Chesapeake Bay.
Table 1. Performance summary of the three SPL systems.  The single pass ground
coverage is calculated without considering the side-lap which might be up to 30-
50% in practice. The mean measurements per square meter assume an aircraft flying
at 200 knots and operating in a standard clear atmosphere (15 km visibility) at the
nominal AGL. The swath, ground coverage, and mean measurements per square
meter vary over the range of scanner cone half angles available to the particular lidar. 
System
HRQLS
HRQLS-2
HAL
# Beamlets per pulse 
100 
100 
100 
Laser fire rate (kHz) 
25 
60 
32 
Measurement rate (Mpix/sec) 
2.5 
6.0 
3.2 
Nominal AGL (m) 
2,286
3,810
7,620
Scanner Cone Half Angle (deg) 
Variable: 0 to 20 10,15,20,or 30 
Swath at nominal AGL (m) 
0.5 – 1,664
1,344 - 4,399 
2,414
Max. single pass ground coverage
at 200 knots (km
2
/h) 
0.19 to 630 
498 to 1,630
894 
Mean measurements per sq. m
(>15% surface reflectance ) 
48,596 to 15 
43 to 13 
13 
Table 2. HRQLS parameters and specification.
Parameter
Specification
# Beamlets per pulse
100
Wavelength
532nm
Laser repetition rate
25kHz
Laser pulse width
700 psec
Laser output power
1.7W
Eye safety
Eye safe by ANSI standards
Multiple return capability
Yes
Pixel recovery time
1.6 nsec
RMS range precision
+/- 5 cm
Scan patterns
Linear, conical, spiral, etc
Scan width
0-40 degrees (selectable)
Operational altitude range (nominal), kft
6.5 – 10 (7.5)*
Swath vs AGL at max scan angle, km
1.4 - 2.2 (1.7)*
Ground coverage vs AGL, single pass at max. scan
angle, km
2
/hr
534 - 822 (616)*
Mean point density per sq. m, single pass at max
scan angle , >15% reflectance surface
17 – 11 (15)*
Size
19 W x 25 D x 33 H inches
Prime power
555 W
*min AGL– max AGL (nominal AGL)
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