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0 500 1.205 2.005 
Figure 1. McMurdo Sound in relation to the Antarctic 
continent. 
2. DETERMINATION OF SEA ICE FREEBOARD 
The identification of the actual sea surface is the fundamental 
starting point for any freeboard investigation. The spaceborne 
retrieval of freeboard is hampered by the complexity of 
identifying a sea surface reference level over large areas. 
Multiple techniques can be used to establish tie-points at which 
additional information indicates that the sea surface is 
referenced by ICESat . They can be split into two groups, firstly 
by the use of the available ICESat product alone and secondly 
the additional use of auxiliary satellite imagery acquired in 
near-coincidence with the ICESat groundtrack. 
In small geographical regions, such as this under investigation, 
tidal analysis together with a known mean sea level is another 
alternative. 
An overview of ICESat's operational information is presented 
in Yi et al. (2011). The following analysis is based on GLA 13 
Records, Release 33 of the ICESat/GLAS data Product. 
The general equation for generating an ideal sea surface 
reference level which takes account of environmental variables 
is given by: 
Sea level = heLev =} hr + hig + hpor = hg (1) 
where hg; gy is the satellite measurement of surface height above 
the WGS-84 ellipsoid and hg is the geoidal height. By assuming 
that the geoid represents the mean sea level, the actual sea level 
is given by taking account of the tide level (hr) and by 
correcting for both the inverse barometric effect (hig) and the 
water level change due to dynamic ocean topography (DOT) 
(hpor). In general, the sum of hg, hr, hig, and hpor is a variable 
function in space and time and not known at the required 
accuracy. This can be circumvented by establishing the sea level 
at tie points along every single satellite track identifying the ice 
free sea surface. The steps used in this investigation to generate 
a sea surface reference level for the relative determination of 
freeboard height closely follows Zwally et al. (2008), who use 
open water leads in the sea ice for this purpose. However, some 
amendments need to be made considering the shape of the geoid 
and tide measurements due to lack of open water in the southern 
part of the study area. 
The sea ice area was first masked for land and ice shelf by 
comparison with Envisat Wide Swath Advanced Synthetic 
Aperture Radar (ASAR) imagery. As the resolution and 
georeferencing error of the imagery may have led to the 
inclusion of some ice shelf elevations an additional elevation 
mask was then applied to the data set allowing no positive 
deviation above the land fast multiyear (MY) ice at the southern 
end of the Sound. This area exhibits the maximum elevation of 
sea ice in the area as the positive geoid influence is at its 
greatest in the southern segments of the study area. Therefore, 
any value above the set -52.5 m WGS84 mask can be 
disregarded. The inclusion of large icebergs is also eradicated 
with this additional mask. Any tracks with land causing 
segregation of the track over greater than a 25 km distance were 
also removed. 
Finally, all laser shots which have reflectivity values lower than 
0.1 and higher than 0.9 are discarded as low and high 
reflectivity are indicative of forward scattering waveforms and 
saturated waveforms, respectively (Yi et al. 2011). 
The Earth Gravitational Model 2008 (EGM2008) depicts the 
general shape of the geoid upon hg; gy quite well (Figure 2). 
  
LICE TF PE nn neers Ep 
   
whi 
   
Dev. from WGS84 (m) 
  
  
  
-78.0 -77.8 -77.6 -77.4 -77.2 -77.0 -76.8 -76.6 
Latitude (degrees) 
Figure 2. Systematic sampling of every 100" shot in the record 
of all hg gy values (black) and EGM2008 (orange). The 
dominant trend in the hg; gy is clearly the geoid. The modelled 
trend is representative of the observations. 
The recorded ICESat surface elevation (hg gy) is the initial 
elevation retrieval used in this investigation. This surface 
elevation is referenced to the WGS84 ellipsoid. Values were 
then adjusted in order to remove any bias on the sea surface 
reference level related to saturation errors (hsar) and modelled 
tides (hr). Due to the small geographical area the influence of 
the inverse barometer effect (hg) was neglected. Any remaining 
bias on the sea surface reference level can then be attributed to 
the modelled geoid (hggmoes) tides and hpor. With insufficient 
information on the latter this is disregarded giving: 
h ~ hg gy + hgar + hr — hecmos (2) 
     
    
   
  
  
   
   
   
  
  
  
  
  
  
  
  
  
  
  
    
     
  
  
  
   
   
    
    
   
    
   
    
    
    
   
   
   
   
   
   
   
  
    
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