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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.
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