12-element phased-array in the Great Barrier Reef, the width of the beam is 9.47°,
which means the native polar pixels are 4 9 4 km at a range of 24 km. Closer to
the radar the pixels retain the 4 km range resolution but are narrower in the
azimuthal direction; further away the pixels still retain the 4 km range resolution
but become extended in azimuth so that at a range of 100 km the pixels become
4 9 16 km. The phased-array data shown in Fig. 13.2 are not smoothed or
interpolated and are presented on a fixed rectangular grid with spacing 4 km. This
means the presentation does not capture the full spatial resolution at close ranges,
but over-samples at long ranges. The time resolution is 10 min between independent surface current maps.
The crossed-loop radar systems have fewer antennas but the trade-off is in
poorer resolution in both time and space. For the crossed-loop system on the
Turquoise Coast in Western Australia, the uncertainty in the azimuths is of the
order of 18°, which means the native polar pixels are 3 9 3 km at a range of
10 km. Closer to the radar the pixels are 3 km in range resolution but narrower in
the azimuthal direction; further away the pixels retain the 3 km in range resolution
but become extended in azimuth so that at a range of 100 km they are 3 9 31 km.
The crossed-loop data shown in Fig. 13.3 are smoothed as they are transformed to
a rectangular grid with spacing 3 km. Due to this smoothing process, the effective
spatial resolution becomes 20 km near to shore, increasing to over 50 km at the
most distant grid points. The time resolution is 80 min with output products issued
every 60 min.
The crossed-loop system is useful for measuring large scale oceanic currents,
such as in Western Australia where it is appropriate for following the meanders of
the Leeuwin current. The phased-array system performs well when assessing the
detailed flow near reefs and along the edge of the continental shelf. A typical
phased-array system has receiver antennas extending over a distance of *200 m,
while crossed-loop systems require only three (larger) antennas spaced about 60 m
apart. If space for the antennas is a limiting issue then the crossed-loop solution is
required, but, as shown in the case studies below, the phased-array system often
provides better performance.
13.2.3 Example Applications
Flow around ribbon reefs An early example of the use of HF ocean radar on coral
reefs is given by Young et al. (1994), where a radar station deployed on Lizard
Island, Australia, observed flow around the ribbon reefs about 18 km to the east.
The single radar station observed only radial currents, which included the flow
through the narrow channels and into the leeward side of the reefs on a flood tide
(Fig. 13.4; dashed lines). The solid lines show the modeled circulation, indicating
that oceanic water was brought directly on to the sheltered side of the reef, which
affects the distribution of nutrients on the reef and is expected to have significant
biological consequences.
13 Radar Applications
347
which means the native polar pixels are 4 9 4 km at a range of 24 km. Closer to
the radar the pixels retain the 4 km range resolution but are narrower in the
azimuthal direction; further away the pixels still retain the 4 km range resolution
but become extended in azimuth so that at a range of 100 km the pixels become
4 9 16 km. The phased-array data shown in Fig. 13.2 are not smoothed or
interpolated and are presented on a fixed rectangular grid with spacing 4 km. This
means the presentation does not capture the full spatial resolution at close ranges,
but over-samples at long ranges. The time resolution is 10 min between independent surface current maps.
The crossed-loop radar systems have fewer antennas but the trade-off is in
poorer resolution in both time and space. For the crossed-loop system on the
Turquoise Coast in Western Australia, the uncertainty in the azimuths is of the
order of 18°, which means the native polar pixels are 3 9 3 km at a range of
10 km. Closer to the radar the pixels are 3 km in range resolution but narrower in
the azimuthal direction; further away the pixels retain the 3 km in range resolution
but become extended in azimuth so that at a range of 100 km they are 3 9 31 km.
The crossed-loop data shown in Fig. 13.3 are smoothed as they are transformed to
a rectangular grid with spacing 3 km. Due to this smoothing process, the effective
spatial resolution becomes 20 km near to shore, increasing to over 50 km at the
most distant grid points. The time resolution is 80 min with output products issued
every 60 min.
The crossed-loop system is useful for measuring large scale oceanic currents,
such as in Western Australia where it is appropriate for following the meanders of
the Leeuwin current. The phased-array system performs well when assessing the
detailed flow near reefs and along the edge of the continental shelf. A typical
phased-array system has receiver antennas extending over a distance of *200 m,
while crossed-loop systems require only three (larger) antennas spaced about 60 m
apart. If space for the antennas is a limiting issue then the crossed-loop solution is
required, but, as shown in the case studies below, the phased-array system often
provides better performance.
13.2.3 Example Applications
Flow around ribbon reefs An early example of the use of HF ocean radar on coral
reefs is given by Young et al. (1994), where a radar station deployed on Lizard
Island, Australia, observed flow around the ribbon reefs about 18 km to the east.
The single radar station observed only radial currents, which included the flow
through the narrow channels and into the leeward side of the reefs on a flood tide
(Fig. 13.4; dashed lines). The solid lines show the modeled circulation, indicating
that oceanic water was brought directly on to the sheltered side of the reef, which
affects the distribution of nutrients on the reef and is expected to have significant
biological consequences.
13 Radar Applications
347