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(2) 
h, is our best estimate of the ideal sea surface. The dominant 
influence upon h is from h gawos. 
Well documented in spaceborne altimetry literature is the fact 
that this reference level cannot be directly used as a surface 
from which an absolute measure of sea ice freeboard can be 
made. Although this holds true in the study region, we use the 
relative values as a basis to de-trend the elevation bias. The 
hezey values are dominated by thin firstyear (FY) ice and open 
water. The modelled EGM2008 mean deviation from hg; gy with 
inclusion of tidal variation is 1.81 m, over representing mean 
sea level. 
  
    
  
-77.8 -77,6 77.4 „77.2 „77.0 
Latitude (degrees) 
Figure 3. Repeat track of ICESat track 381 across McMurdo 
Sound. 9 tracks are present in the repeat with 5 covering the full 
length of the study area. All five tracks exhibit an unexpected 
positive deviation in h at the northern end of the profile. 
The following steps of the method rely on the fact that the 
lowest elevation retrieval that can possibly be retrieved by 
ICESat is the sea surface, and that the sea surface is visible 
somewhere along the satellite track. The elevation profile h 
must therefore be sampled for its lowest elevation retrievals. 
The sea surface height (SSH) is artificially generated from the 
lowest 5 % of elevation measurements along each track. The 
selection of a percentage value for this purpose is case 
dependent. Zwally et al. (2008) used a 2 % value and reported 
that an optimised value will be presented as our knowledge of 
the distribution of leads in the Southern Ocean improves. 5 % 
has been selected here as the amount of data along-track was 
less than available in large basin-scale assessments. This 
permits tracks with minimal elevation retrievals to be kept 
rather than discarded if only a limited number of points are 
available. This methodology filters out any tracks which are 
unable to produce at least 3 lowest elevation retrievals i.e. 
tracks that have less than 60 individual elevation retrievals. 
Furthermore, unlike previous methodologies, the site specific 
requirements resulted in no segmentation of the groundtrack 
with regard to a running mean and computation of relative 
elevations. It was known that the southern end of the elevation 
profiles were ice covered areas, therefore splitting the 
groundtrack for further improvement on residual geoid errors 
and the influence of DOT and tide would result in 
underestimated freeboards. Open water was only expected in 
the northern and central areas of the study area and therefore 
each individual track was not segregated but treated as one. 
The lowest 5 % elevation retrievals are then averaged, this value 
(hg) is then subtracted from h giving a freeboard (Fb) value: 
Fb- h-h, (3) 
This is shown schematically in Figure 4. 
  
  
  
  
  
  
  
  
  
a6 
  
i RUE i ub i Das 
3*3 TIS TT TRÉ MWR 755 
Lana deus) 
Figure 4. Near coincident ICESat altimetry/reflectivity (T = 
16/11/2003 0900), Envisat ASAR (T -17 hours) and MODIS (T 
+5.5 hours) imagery. Black dots are shot locations. Metres axis, 
Fb (red), h (grey) and h, (blue) 
3. RESULTS 
From 2003-2009 the McMurdo Sound area (Figure 1) hosted 
two sea ice types. A MY sea ice area persisted in the southern 
portion of the Sound, fast to the McMurdo Ice shelf in the 
south, Hutt Peninsula in the east and the Antarctic continental 
coast in the west. A variable FY sea ice cover existed to the 
north and covered the remainder of the study area. The area was 
assessed during Austral Spring, during the months of 
September, October and November making seven annual 
investigations of sea ice freeboard from 2003 through 2009. The 
study area was assessed holistically (Table 1 and Figure 5), and 
for MY and FY alone (Figure 6).