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182 Fundamentals of Ocean Renewable Energy
7.3.2 HF Radar
The main difference between HF and X-band radar systems is that because
the former operates at much lower frequencies (3–30 MHz), the waves are
transmitted over much longer distances, and so the region that is sampled is
much larger, for example, up to 200 km [25]. However, because the wave length
is longer than X-band, the resolution of HF radar is coarser than X-band, with
typical resolutions of 300 m to 15 km achieved, depending on the transmission
frequency [26].
In contrast to X-band, HF radar systems require considerable infrastructure
installation in the form of radar antenna arrays, and so are generally considered
as investment in a long-term monitoring campaign. The advantages of HF
radars are that they can monitor large areas of the sea surface at high temporal
resolution, there is minimal risk of collision (in comparison to sea-based
measuring systems), and that they can be installed and maintained from land.
However, installation costs are high, and they suffer from interference from
other users of the broadcasting band, and a poor signal-to-noise ratio in high
winds and stormy seas [25]. Note that HF radar can also be used to measure the
vertical distribution of winds via a radar wind profiler.
HF radar stations can also estimate surface currents, including tidal currents.
They calculate the Doppler shift that is Bragg scattered at the surface. Because
the radial velocity (towards or away) from the radar station is estimated, two or
more radial stations are necessary to evaluate the components of water velocity.
NOAA has set up a network of HF radar systems around the US coast (ioos.noaa.
gov/project/hf-radar/) for real-time monitoring of coastal currents. The depth
that an HF radar signal can feel the currents is λ R /8π, where λ R is the radio
wave length. For a 12-MHz signal (suitable for current measurement of about
2 km resolution), the wave length is 25 m, and the effective depth is about 1 m.
An HF radar system with a frequency of 40–45 MHz can measure currents up to
300 m resolution. In terms of tidal energy resource assessment, a combination
of HF radar and ADCPs can provide a good assessment of the spatial (by HF
radar) and vertical (by ADCP) variability. Some applications of HDF radar in
tidal energy resource assessment can be found elsewhere [27].
7.3.3 Satellite and Airborne Remote Sensing
Satellites provide uniform global coverage of the sea surface, with no preference
for ports. However, the resolution of satellite data that is useful for resource
characterization tends to be coarse, for example, several kilometres resolution
global coverage in 1 year of data collection would not be uncommon. Airborne
remote-sensing platforms offer the advantage of improved resolution compared
with satellite platforms but, because over flights tend to be commissioned, at
increased cost to the end user compared with satellite data.