11.3.2 Radar Systems
Here we discuss in more detail the two types of radar systems that are presently
used for monitoring environmental conditions around coral reefs: ground-wave
radar; and air- or space-borne Synthetic Aperture Radar (SAR).
Ground-wave radar systems transmit vertically-polarized signals that graze the
ocean surface. Existing systems, either HF or VHF, are located at near-shore
terrestrial locations. There are two genres of ground-wave radar systems for
monitoring conditions around coral reefs: phased-array and direction-finding.
Phased-array radar systems use separate transmit (usually omni-directional) and
receive antenna arrays to extract ocean surface parameters across the domain of
interest. Setting the range to specific ocean target regions is achieved by considering the time delay between transmission and reception (range), and through
steering the receiver beam through prescribing the relative phase at each different
individual receive antenna (direction). In modern systems the time delay used for
ranging is coded in a frequency modulated chirp arrangement. Coverage of a broad
ocean region is undertaken by scanning across the region (i.e., modifying the
antenna phases) and acquiring echo spectra for each radial cell in the beam
direction. The Doppler shift of the Bragg peaks provides the radial-component of
the surface current at the target. Wave spectrum information is derived for each
ocean target using the second-order backscatter spectrum; however, the lessened
magnitude due to double-reflection reduces the range to which wave data can be
extracted to about half the range for currents. The radial resolution of the radar is
inversely proportional to the bandwidth of the transmitted radiation, while the
azimuthal resolution depends upon the length of the antenna array, which is
generally of the order of 6–8 wavelengths.
Direction-finding radars operate on the principle of transmitting the radar signal
in all directions and separating the backscatter after it is returned to three independent, orthogonal (x-y-z) receive antennas to determine the oceanic conditions.
Monitoring of a 360° ocean region is thus simultaneous, with ocean state
parameters extracted during post-analysis. As with the phased-array, the time
delay between transmission and reception provides the range to the ocean target
(coded as frequency chirp) and the surface current is determined from the Doppler
shift of the Bragg echoes. The bearing of the ocean target is determined using the
relative levels of backscatter energy acquired simultaneously at the three orthogonal receive antennas. As with phased-array radar, the radial resolution is
dependent upon the transmission bandwidth; however, the azimuthal resolution
depends upon the amplitude resolution of the orthogonal receive antennas. A
significant benefit of the direction-finding systems is their relatively small antenna
footprint (normally 2 poles with guy-ropes).
HF and VHF radar systems have been developed since the 1970s to the point
where commercial systems are now readily available (Table 11.4). Off-the-shelf
systems currently deployed over coral reef regions include the WERA (phased11 Thermal and Radar Overview
303
Here we discuss in more detail the two types of radar systems that are presently
used for monitoring environmental conditions around coral reefs: ground-wave
radar; and air- or space-borne Synthetic Aperture Radar (SAR).
Ground-wave radar systems transmit vertically-polarized signals that graze the
ocean surface. Existing systems, either HF or VHF, are located at near-shore
terrestrial locations. There are two genres of ground-wave radar systems for
monitoring conditions around coral reefs: phased-array and direction-finding.
Phased-array radar systems use separate transmit (usually omni-directional) and
receive antenna arrays to extract ocean surface parameters across the domain of
interest. Setting the range to specific ocean target regions is achieved by considering the time delay between transmission and reception (range), and through
steering the receiver beam through prescribing the relative phase at each different
individual receive antenna (direction). In modern systems the time delay used for
ranging is coded in a frequency modulated chirp arrangement. Coverage of a broad
ocean region is undertaken by scanning across the region (i.e., modifying the
antenna phases) and acquiring echo spectra for each radial cell in the beam
direction. The Doppler shift of the Bragg peaks provides the radial-component of
the surface current at the target. Wave spectrum information is derived for each
ocean target using the second-order backscatter spectrum; however, the lessened
magnitude due to double-reflection reduces the range to which wave data can be
extracted to about half the range for currents. The radial resolution of the radar is
inversely proportional to the bandwidth of the transmitted radiation, while the
azimuthal resolution depends upon the length of the antenna array, which is
generally of the order of 6–8 wavelengths.
Direction-finding radars operate on the principle of transmitting the radar signal
in all directions and separating the backscatter after it is returned to three independent, orthogonal (x-y-z) receive antennas to determine the oceanic conditions.
Monitoring of a 360° ocean region is thus simultaneous, with ocean state
parameters extracted during post-analysis. As with the phased-array, the time
delay between transmission and reception provides the range to the ocean target
(coded as frequency chirp) and the surface current is determined from the Doppler
shift of the Bragg echoes. The bearing of the ocean target is determined using the
relative levels of backscatter energy acquired simultaneously at the three orthogonal receive antennas. As with phased-array radar, the radial resolution is
dependent upon the transmission bandwidth; however, the azimuthal resolution
depends upon the amplitude resolution of the orthogonal receive antennas. A
significant benefit of the direction-finding systems is their relatively small antenna
footprint (normally 2 poles with guy-ropes).
HF and VHF radar systems have been developed since the 1970s to the point
where commercial systems are now readily available (Table 11.4). Off-the-shelf
systems currently deployed over coral reef regions include the WERA (phased11 Thermal and Radar Overview
303