resolved. The wind direction can also be determined from the relative energy
magnitude of the two Bragg peaks.
Second-order processes (including double scattering and non-linearities in the
ocean waves) provide spectral energy at other frequency shifts from the incident
radar frequency. For example, Barrick (1977) derived a relationship between the
ratio of second-order to first-order energy, R, and the root-mean-square (rms) wave
height, h rms . Empirical testing of this relationship by Maresca and Georges (1980)
and Heron and Heron (1998) has verified the capability of monitoring of ocean
waves using radar. Further work by Wyatt (1991, 2011) has involved inverting the
radar backscatter spectrum to produce full wave spectra.
Useful observation of the ocean around and between reefs out to a range of
100–200 km is a unique feature of ground-wave HF technology (spatial resolution
of these observations is generally within 3–50 km). Radar systems with higher
frequencies also detect Bragg scatter; however, there are necessarily different ways
to interpret the echo spectrum, as the separation of first-order and second-order
spectral information is more difficult (Fig. 11.7b, c).
Very High Frequency (VHF) radar, with wavelengths 1–10 m (300–30 MHz),
exhibit a merging of second-order energy, due mainly to underlying swell, with
first-order backscatter from surface gravity waves, thereby broadening identifiable
Bragg peaks. The broadened Bragg peaks may nonetheless still be used to
determine surface currents and significant wave heights. VHF ground-wave radars
have spatial resolution down to about 25 m, producing detailed maps of surface
currents at a resolution unique to VHF radars, and for an operating range of
3–20 km. For coral reef applications, VHF ground-wave radar stations need to be
placed on a nearby atoll or island.
A significant consideration for the use of radars in coral reef regions is the
assumption of deep-water gravity waves in the theoretical formulation of Bragg
scatter. Gravity waves may be considered ‘‘deep’’ when water depth is on the order
of one-sixth of the wavelength of the Bragg waves. For example, a ground-wave
HF radar (15 MHz, wavelength 20 m) will have Bragg scatter from gravity waves
of wavelength 10 m, which require the water depth to be over about 2 m for the
deep-water assumption to hold. As such, retrievals over reef flats may be compromised. However, VHF radar does not have this issue due to the order-ofmagnitude shorter wavelength and is therefore effective at monitoring shallower
environs.
At radar frequencies in the C (4–8 GHz) and X (8–12 GHz) bands
(Table 11.3), Bragg scatter is from capillary waves, and modulation by underlying
gravity waves generally causes so much line broadening that the approaching and
receding Bragg waves are indistinguishable. In these bands, amplitude is used to
determine sea surface parameters, and it is possible to form very narrow beams
from large antennas to achieve high spatial resolutions. Satellite-borne scatterometers measure echo energy from different directions in order to determine wind
speed and direction at spatial resolutions of 12.5–50 km and, with special processing, experimentally down to 2.5 km (Plagge 2009). One application of X-band
11 Thermal and Radar Overview
301
magnitude of the two Bragg peaks.
Second-order processes (including double scattering and non-linearities in the
ocean waves) provide spectral energy at other frequency shifts from the incident
radar frequency. For example, Barrick (1977) derived a relationship between the
ratio of second-order to first-order energy, R, and the root-mean-square (rms) wave
height, h rms . Empirical testing of this relationship by Maresca and Georges (1980)
and Heron and Heron (1998) has verified the capability of monitoring of ocean
waves using radar. Further work by Wyatt (1991, 2011) has involved inverting the
radar backscatter spectrum to produce full wave spectra.
Useful observation of the ocean around and between reefs out to a range of
100–200 km is a unique feature of ground-wave HF technology (spatial resolution
of these observations is generally within 3–50 km). Radar systems with higher
frequencies also detect Bragg scatter; however, there are necessarily different ways
to interpret the echo spectrum, as the separation of first-order and second-order
spectral information is more difficult (Fig. 11.7b, c).
Very High Frequency (VHF) radar, with wavelengths 1–10 m (300–30 MHz),
exhibit a merging of second-order energy, due mainly to underlying swell, with
first-order backscatter from surface gravity waves, thereby broadening identifiable
Bragg peaks. The broadened Bragg peaks may nonetheless still be used to
determine surface currents and significant wave heights. VHF ground-wave radars
have spatial resolution down to about 25 m, producing detailed maps of surface
currents at a resolution unique to VHF radars, and for an operating range of
3–20 km. For coral reef applications, VHF ground-wave radar stations need to be
placed on a nearby atoll or island.
A significant consideration for the use of radars in coral reef regions is the
assumption of deep-water gravity waves in the theoretical formulation of Bragg
scatter. Gravity waves may be considered ‘‘deep’’ when water depth is on the order
of one-sixth of the wavelength of the Bragg waves. For example, a ground-wave
HF radar (15 MHz, wavelength 20 m) will have Bragg scatter from gravity waves
of wavelength 10 m, which require the water depth to be over about 2 m for the
deep-water assumption to hold. As such, retrievals over reef flats may be compromised. However, VHF radar does not have this issue due to the order-ofmagnitude shorter wavelength and is therefore effective at monitoring shallower
environs.
At radar frequencies in the C (4–8 GHz) and X (8–12 GHz) bands
(Table 11.3), Bragg scatter is from capillary waves, and modulation by underlying
gravity waves generally causes so much line broadening that the approaching and
receding Bragg waves are indistinguishable. In these bands, amplitude is used to
determine sea surface parameters, and it is possible to form very narrow beams
from large antennas to achieve high spatial resolutions. Satellite-borne scatterometers measure echo energy from different directions in order to determine wind
speed and direction at spatial resolutions of 12.5–50 km and, with special processing, experimentally down to 2.5 km (Plagge 2009). One application of X-band
11 Thermal and Radar Overview
301