1 Spatial Modelling of the Terrestrial Environment - Georeferencial

16 Spatial Modelling of the Terrestrial Environment
2.2.1 Satellite Radar Altimetry
A thorough review of the use of satellite radar altimeters (SRAs) over ice sheets can
be found elsewhere (Zwally and Brenner, 2001). Presented here is a brief overview of the
key issues and problems associated with the use of SRA data over ice sheets. SRAs are
active microwave instruments that transmit a microwave pulse to the ground and measure its
two-way travel time. With sufficiently accurate determination of orbit it is possible to
measure the elevation of the sea surface with an accuracy of a few centimetres. Although
SRAs were designed primarily for operation over ocean and non-ocean surfaces, such as
ice sheets, certain limitations apply. In particular, the current fleet of SRAs are unable to
range to surfaces that have a slope significantly greater than the antenna beamwidth of the
instrument (which is typically about 0.7 ◦ ). Thus their use is limited to slopes less than about
1 ◦ . Consequently, accurate height estimates can only be obtained from larger ice masses
with low regional slopes, over distance of more than ∼50 km, i.e., over Antarctica and
Greenland.
There are four satellite missions which satisfied the dual requirement of having accurateenough
orbit determination and an orbital inclination that provided substantive coverage of
the ice sheets. The first of these was Seasat, launched in 1978. This satellite had a latitudinal
limit of 72 ◦ providing coverage of the southern half of Greenland and about one fifth of
Antarctica (Figure 2.2). Although the mission only lasted 100 days, it clearly demonstrated
Figure 2.2 Plot of the coverage by past, present and future satellite altimeter missions over
the Greenland and Antarctic ice sheets