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In Situ and Remote Methods for Resource Characterization Chapter | 7 181
the use of satellite sensors. However, remote-sensing instruments can also be
carried by aircraft (airborne remote sensing), or installed on land- or sea-based
platforms (e.g. X-band and HF radar).
Remote sensing can be categorized as either passive or active. Passive
sensors gather radiation that has been emitted or reflected by the sea surface,
for example, reflected sunlight. By contrast, active sensors emit energy, then
detect the radiation that is reflected or backscattered from the target.
7.3.1 X-Band Radar
X-band radars are installed on most large research vessels, and on many
offshore installations. Their original use is for ship traffic control and navigation.
However, with the addition of some hardware and software components, X-band
radars can be used to measure waves and currents (e.g. [22]). X-band is a
segment of the microwave radio region of the electromagnetic spectrum, within
the frequency range 8–12 GHz.
The images produced by marine radars detect not only hard targets such
as ships and coastlines, but also reflections from the sea surface, known as
‘sea clutter’ [23]. Given a wind speed of more than approximately 3 m/s, the
backscatter from the sea surface becomes visible in radar images [24]. Such
reflections of waves are mostly due to resonance between the radar waves and
the features at the water surface (Bragg Scatter) [15]. Because the radar wave
length is in the centimetre range, only very short water waves reflect the radar
waves. However, the base signal (the very short waves) is modulated by longer
waves. For navigational purposes, this sea clutter is treated as a nuisance, and
is discarded; but it contains valuable information on the actual sea state. With
appropriate software, it is possible to analyse this signal to gain information
on wave height, wave length, wave period, and surface currents. X-band radar
systems scan the ocean surface in real time at high temporal (1–2 s) and spatial
(5–10 m) resolution. An area of sea surface of several square kilometres can
therefore be continuously monitored. X-band radar systems can be installed on
moving vessels or on fixed platforms offshore or at coastal sites.
The main limitations of X-band radar are the limited range, and the
requirement for sufficient sea clutter (and associated significant wave height)
to allow imaging of the waves. Due to the limited range of X-band radar, a fixed
platform would have to be installed reasonably close to the area of interest, or the
system would have to be mounted on a vessel. The need to install a fixed offshore
platform would sacrifice some of the inherent advantages of X-band radar as a
remote-sensing technique, such as easy installation and onshore maintenance,
and would represent considerably increased cost. X-band radar systems are also
sensitive to interference from rainfall, and indeed this radar frequency is used
by weather radars to monitor rainfall.