Page 195 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

In Situ and Remote Methods for Resource Characterization Chapter | 7 183

Satellite altimetry measures the time taken for a radar pulse to travel from
the satellite to the sea surface and back to the satellite. Radar altimeters map
the topography of the ocean surface with unprecedented accuracy. The extra
gravitational attraction of sea-bed features such as seamounts produces minor
variations in gravity, which in turn produce tiny variations in ocean surface
height. Combined with suitable algorithms, radar altimetry can therefore be used
to estimate bathymetry, and altimetry-derived data is integrated into popular
4
global bathymetry datasets such as GEBCO. Direct tidal analysis of altimetry
data can be used to derive estimates of the ocean tides [28], but altimetry data are
generally assimilated within hydrodynamic models to produce accurate global
tidal constituent databases such as FES2012 [29] that can be used for direct
(coarse spatial resolution) analysis or as model boundary conditions. Further, the
strength and shape of the returning radar signal from altimetry satellites provides
valuable information on wind speed and the height of ocean waves. Although a
single satellite track has high temporal resolution, repeat coverage of a region
is widely spaced in time, and so instantaneous altimetry data are best suited to
validating the spatial skill of a wave model, rather than directly characterizing
temporal properties. Finally, when a satellite altimeter approaches the coast,
land entering the radar footprint modifies the shape of the waveforms, making
the estimate of range and other derived quantities difficult in such regions. For
this reason, satellite altimetry data are usually discarded in the coastal zone, but
some recent work is beginning to make data in such regions useable (e.g. [30]).
Synthetic Aperture Radar (SAR) is based on the same principal as X-band
radar, but the instrument is carried by a satellite or aircraft. The problem in
observing waves from high altitude is that the antenna needs to be very large
in order to distinguish individual waves in the modulation pattern. In a SAR
system, the satellite or aircraft continuously illuminates the sea surface and
receives the scattered signal as it travels along its flight path. Knowledge of
this history permits later reconstruction of the reflected signals as if they were
received by a single ‘synthetic’ antenna occupying a physical space that is
defined by the movement of the satellite or aircraft along its flight path, even
though the signals were received by a much shorter antenna [31].
In any remote-sensing technology, it is important to obtain concurrent
ground truthing data (e.g. from a vessel or mooring) for either calibration of the
remote-sensing algorithms or validation of the remotely sensed data. Because it
is logistically difficult to organize such activities, for example, the concurrent
mobilization of an aircraft and a ship, it perhaps works best when a relatively
long-term mooring (e.g. a wave buoy) is already in place, and the remotely
sensed data primarily used to improve spatial coverage of the observed property.

4. See http://www.gebco.net.