validations of their ability to measure various aspects of canopy 
structure and forest stand structure are presented. 
This description and validation of the SLICER (Scanning Lidar 
Imager of Canopies by Echo Recovery) canopy measurements 
provides a summary for the use of SLICER data in studies of 
forest canopy structure, including the work of Harding et al. 
(1994; 1995), Lefsky (1997), Lefsky et al., (1999a; 1999b), 
Harding (1998), Drake and Weishampel (1998), Means et al., 
(1999), and Rodriguez et al., (in review) and for future studies. 
The principles developed here also apply to canopy lidar data 
being acquired by the airborne Lidar Vegetation Imaging 
System (LVIS) and to be acquired by the spaceborne 
Vegetation Canopy Lidar (VCL). LVIS is a wide-swath, 
mapping system developed at GSFC that has superseded 
SLICER (Blair et al., 1999). VCL, scheduled for launch in 
2000, is expected to inventory canopy height and structure over 
approximately 5% of the Earth’s land surface between +68° 
during its 2 year mission (Dubayah et al., 1997). 
2. SLICER BACKGROUND 
The SLICER airborne lidar altimeter system consists of a 
ranging component and ancillary instrumentation for 
geolocation. The ranging component consists of a laser 
transmitter, scan mechanism, receiver telescope, detector, 
timing electronics, waveform digitizer, and an instrument 
contro] and data collection system. The ranging 
instrumentation is augmented by an Inertial Navigation System 
for precise determination of laser beam pointing, GPS receivers 
for differential, kinematic determination of aircraft position, 
and video equipment for image documentation of the ground 
track. Integration of the ranging data with laser beam pointing 
and aircraft position yields a position and elevation for each 
laser pulse return with respect to a geodetic reference frame. 
Key aspects of the SLICER system are reviewed below; 
complete documentation of the instrumentation and data 
products is provided in Harding et al., (In Review). SLICER 
data sets available for public distribution are described 
at http://denali.gsfc.nasa.gov/lapf. 
Several aspects of the SLICER design make it a powerful tool 
for characterizing canopy vertical structure. The combination 
ol a very narrow transmit pulse and a high-speed detector 
results in exceptional vertical resolution, allowing closely 
spaced canopy layers and the underlying ground within each 
footprint to be distinguished. Use of a very high-speed 
digitizer results in a non-aliased waveform record of 
backscatter energy that has extremely good vertical sampling, 
necessary for full analysis of waveform structure. SLICER 
evolved from a profiling lidar altimeter described by Blair et 
al., (1994) by the addition of a scanning mechanism. By 
scanning the laser footprints across the flight path a narrow 
swath results which provides both cross- and along-track 
information on canopy heterogeneity and ground slope beneath 
the canopy. SLICER employs a high power laser that enables a 
significantly higher flight altitude than is typical used by 
airborne laser altimeters, yielding larger footprints (nominally 
International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 3W14, La Jolla, CA, 9-11 Nov. 1999 
10 m but as large as 70 m) that are contiguous or even 
overlapped. The larger footprints thus fully illuminate the 
canopy, providing a measure of average canopy structure that 
avoids the sampling bias inherent to small footprint altimeters. 
The canopy in these large footprints typically contains some 
openings at nadir to the ground thus consistently yielding a 
ground return and enabling a measure of vegetation height for 
each laser pulse. In addition, the high flight altitude minimizes 
the variation in footprint size and received backscatter energy 
caused by changes in ranging distance due to topographic 
relief, thus simplifying data interpretation. Accurate pointing 
and position knowledge, and associated geolocation software, 
enable accurate determination of the location of each footprint 
so that the lidar data can be directly correlated with ground 
observations and remote sensing images. SLICER’s control 
systems and operational modes were designed to be flexible so 
that the effect of variations in footprint size, spacing and 
vertical sampling on characterization of canopy structure could 
be evaluated. 
Several implications of the instrument characteristics are 
significant for proper use of the SLICER data. First, the laser 
illumination across the swath is not uniform and thus canopy 
structure across the swath is sampled unequally. The pattern of 
circular, approximately contiguous footprints that each have a 
radial, Gaussian distribution of laser energy yields a swath 
illumination that is analogous to an inverted egg carton. 
Second, the backscatter amplitude recorded in the waveform is 
not an absolute measure of reflected laser energy. The transfer 
function between optical energy received by the instrument (i.e. 
backscattered photons) and the resulting digital count 
amplitude in the waveform is unknown due to uncalibrated 
instrument parameters. The transfer function varies spatially, as 
a function of beam position across the swath, and temporally on 
multiple time-scales, as a function of operating conditions. 
Thus, the amplitudes of waveforms can not be compared in an 
absolute sense. The waveform can only be used as a relative 
measure of the height distribution of backscattered energy 
within an individual footprint. 
Third, SLICER utilizes a threshold detection scheme to define 
the range to the first detected target within a footprint. 
Therefore, the detection of the canopy top requires that 
sufficient backscatter energy be received exceeding the 
detection threshold. The backscatter intensity depends on 
intercepted area and the near infrared (NIR) reflectance of the 
intercepted surfaces at 0° phase angle. Thus SLICER's ability 
to detect the canopy top, and the resulting derivation of canopy 
height, depends on the geometry of the outer canopy surface 
and the reflectivity of the components making up the outer 
surface. For example, narrow, erect conifer tips with NIR-dark 
needles are less easily detected than a concentration of NIR- 
bright deciduous leaves forming a well defined, umbrella-like 
crown top. Depending on these canopy characteristics, the 
SLICER measurement of canopy height can be biased low to 
varying degrees as compared to the outer-most canopy surface. 
     
  
   
   
   
   
    
     
   
    
     
   
   
     
   
   
    
    
   
   
    
    
  
        
   
  
  
   
  
    
    
    
  
     
   
    
     
   
     
    
   
  
    
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