for applying similar technology to the Canadian ice caps. The 
intent is that that by studying these smaller ice masses, 
observed changes in the state of balance of Arctic ice, and their 
relationships to a changing climate can be more 
comprehensively understood. 
In addition, lessons learned and insight gained from the 
analysis of airborne laser altimetry will be incorporated in the 
future into the analysis of data from the Geoscience Laser 
Altimeter System (GLAS), to be flown on the Ice Cloud and 
land Elevation Satellite (ICESat), scheduled for launch in July, 
2001. 
2 INSTRUMENTATION AND MEASUREMENTS 
The laser altimetry system used in these studies is the Airborne 
Topographic Mapper (ATM), which combines high-pulse laser 
ranging with a scanning capability. The ATM instruments are 
operated with Spectra Physics TFR laser transmitters, which 
provide a 7 ns wide, 250 LJ pulse at a frequency-doubled 
wavelength of 523 nm. The laser beam is reflected toward the 
surface off a nutating mirror, which can be adjusted to angle 
settings of 5, 10, and 15 degrees off nadir. The scan mirror 
rotational frequency is 10-Hz, which provides a ground-track 
of overlapping elliptical spirals. For a nominal flight altitude 
of 400 meters, and an off nadir pointing of 10°, the scan swath 
is approximately 140 meters wide. At higher flying altitudes, 
and for the larger angle settings, the swath width is 
approximately 250 meters wide. The spot-density on the 
ground is a function of the sample rate, aircraft velocity, and 
swath width. For nominal conditions with the aircraft flying 
approximately 150 m/s, the spot density is roughly 1 every 6 
m”, but with a much denser sampling along the edges of the 
swath, and a more coarse sampling near the center. 
Topographic mapping surveys of ice are designed for optimum 
sampling, which varies with the size and shape of the ice-mass 
of interest, and the limitations of the aircraft platform. In the 
case of the Arctic Ice Mapping (AIM) program, these surveys 
have been made with the intent of repeating the exact flight 
tracks for thickness change measurements. In the initial 
surveys the aircraft is flown through a series of waypoints by 
means of an automated navigation system, which links the GPS 
information in real time, to the aicraft navigation system. After 
post-processing of the GPS data, and the correcting for attitude 
variations, the range measurement to the surface below can be 
converted to a footprint elevation. The accuracy of the 
elevation measurement is better than 10 cm rms (Krabill et al., 
1995). 
The GPS-based automated navigation capability is then used on 
the re-surveys to fly the aircraft as closely as possible along the 
original flight lines. Typically these re-flights are within 20 
meters of the original flight line, allowing sufficient overlap for 
comparison of elevation changes. The elevation of the footprint 
within each laser shot from the initial survey, is then compared 
to those from the re-survey that fall within a specified search 
radius (typically 1 to 5 meters for shot-to shot comparisons, and 
     
  
   
    
    
    
    
   
    
    
   
   
  
    
    
   
   
    
     
   
   
  
   
    
    
  
    
  
    
  
    
  
  
     
   
    
     
    
    
     
   
    
    
    
more with smoothed data), and the elevation change is 
determined. 
3 CANADIAN ICE CAPS 
The state of balance of the Canadian and other Arctic ice caps 
is of considerable interest because these ice masses are likely to 
be particularly sensitive to climate changes. Although their 
contribution to sea level rise is far less than that of Greenland 
or Antarctica, they are likely to respond to climate changes 
more rapidly than the large ice sheets; thus in some ways these 
final remnants of the Laurentide ice sheet may be more of an 
immediate concern. Moreover, their behavior in response to 
climate forcing may hold clues about the future of the 
Greenland ice sheet as well. 
In 1995, as a complement to the Greenland missions, airborne 
laser elevation surveys were made of ice caps on Baffin, 
Devon, Ellesmere, Axel Heiberg, and Meighen islands, as well 
as a few ice cap outlet glaciers. The Canadian survey lines are 
shown in Figure 1. While the surveys have provided valuable 
information on the topography of the ice caps, their greatest 
value is likely to come from repeat surveys for elevation 
changes. These measurements are planned for May of 2000. 
  
  
  
  
  
Figure 1. Flight trajectory of 1995 survey lines over several 
Canadian ice caps. These lines will be re-surveyed during the 
2000 field season, for the purposes of measuring elevation 
changes. The red + symbols indicate the locations of coastal 
weather stations, and the Green dots indicate the location of 
several ice core sites. 
  
Such elevation survey 
means of examining t 
mountain glaciers, wh 
advance and retreat o 
caps can occur with | 
research is intended t 
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from the Greenland n 
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Meaningful interpretat 
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interpretation requires 
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the time period that se 
assessing the significa 
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Toward that end, col 
various Canadian 
comprehensive set of r 
last 40 years in some « 
uses are as follows: 
Ice core data from 6 i 
accumulation history ( 
for six major ice caps. 
statistical significance 
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given as: 
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surface elevation a 
accumulation is greater 
of accumulation. Usii 
cores will be used to 
Observed elevation cha 
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2000 survey period has 
Up to 40 years of m 
information on the 
variability over time. 
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statistical significance 
the anomalous nature 
adapting Equation (1) 
accumulation.