(2) changes in wave slope vary the amount of radar energy that is directly
reflected; and (3) wave orbital velocities lead to Doppler shifts that influence the
SAR image formation process, producing variations in image intensity called
‘‘velocity bunching.’’ Random movement of small waves, however, also produce
Doppler shifts that limit the azimuth (i.e., along-track) resolution of the image
spectrum (i.e., the ‘‘azimuth cut-off’’). Practically, this means that for polarorbiting satellites, wave information for short-wavelength wind waves is limited,
and accurate wave information can generally be obtained only for longer swell
waves. For these longer waves, one can measure dominant swell wavelength, wave
direction, wave spectra (Li et al. 2002), and significant wave height. Monitoring of
swell during bleaching events can provide information on upper-ocean mixing,
which may mitigate the severity of bleaching (Skirving et al. 2006).
From the two-dimensional SAR image spectrum, an inversion process utilizing
an appropriate transform function can be used to measure the corresponding twodimensional ocean surface wave spectrum. The wave spectrum can be obtained
directly from the SAR image spectrum with a linear transform in the case of longer
swell waves that are not very high and are travelling in the range direction.
However, swell waves travelling strictly in the azimuth direction, unless they have
wavelengths longer than about 300 m, are not imaged by typical space-based SAR
instruments. In general, when making wave measurements for shorter and steeper
waves using SAR instruments operating at altitudes typical for sun-synchronous
polar-orbiting satellites, non-linear transforms must be employed, although there
are still azimuth limitations (Vachon et al. 2004). In a comparison of swell wave
significant wave height derived from RADARSAT-1 Standard Mode imagery with
buoy wave measurements from NOAA buoy 46029 off the coast of Oregon
(46.144
o N, 124.51
o W) the root-mean-square difference in 168 significant wave
height comparisons made between 2001 and 2005 was 0.6 m (after eliminating
matches differing by more than 2 standard deviations). Wave heights for these
comparisons ranged from 0–4 m.
Vessel monitoring Vessel traffic in protected and use-restricted marine sanctuary areas is of interest to coral reef managers, and fishing vessel locations and
concentrations are of interest to fishery managers and fishing enforcement personnel. International vessels greater than 300 tons and all passenger ships are
required to carry Automatic Identification System (AIS) transmitters; however,
smaller vessels and non-compliant vessels cannot be monitored via AIS. SAR
instruments are the sensor-of-choice for monitoring such ships since SAR operates
day and night, cloudy or clear, and under a wide range of ocean conditions.
Vessels provide three different SAR signatures under the right ocean and wind
conditions: (1) direct returns; (2) wake patterns (Lyden et al. 1988); and (3) slicks
(Clemente-Colón et al. 1998). Since most ocean-going vessels have steel hulls,
they reflect the incoming SAR radar signal to provide a strong direct return (i.e.,
they are ‘‘hard targets’’), typically much more intense than the surrounding ocean
surface. Radar returns from vessels result from: (a) direct reflection of the radar
signal from portions of the ship oriented perpendicular to the incoming signal
(single bounce); (b) a double bounce from the hull and ocean surface; or (c)
13 Radar Applications
355
reflected; and (3) wave orbital velocities lead to Doppler shifts that influence the
SAR image formation process, producing variations in image intensity called
‘‘velocity bunching.’’ Random movement of small waves, however, also produce
Doppler shifts that limit the azimuth (i.e., along-track) resolution of the image
spectrum (i.e., the ‘‘azimuth cut-off’’). Practically, this means that for polarorbiting satellites, wave information for short-wavelength wind waves is limited,
and accurate wave information can generally be obtained only for longer swell
waves. For these longer waves, one can measure dominant swell wavelength, wave
direction, wave spectra (Li et al. 2002), and significant wave height. Monitoring of
swell during bleaching events can provide information on upper-ocean mixing,
which may mitigate the severity of bleaching (Skirving et al. 2006).
From the two-dimensional SAR image spectrum, an inversion process utilizing
an appropriate transform function can be used to measure the corresponding twodimensional ocean surface wave spectrum. The wave spectrum can be obtained
directly from the SAR image spectrum with a linear transform in the case of longer
swell waves that are not very high and are travelling in the range direction.
However, swell waves travelling strictly in the azimuth direction, unless they have
wavelengths longer than about 300 m, are not imaged by typical space-based SAR
instruments. In general, when making wave measurements for shorter and steeper
waves using SAR instruments operating at altitudes typical for sun-synchronous
polar-orbiting satellites, non-linear transforms must be employed, although there
are still azimuth limitations (Vachon et al. 2004). In a comparison of swell wave
significant wave height derived from RADARSAT-1 Standard Mode imagery with
buoy wave measurements from NOAA buoy 46029 off the coast of Oregon
(46.144
o N, 124.51
o W) the root-mean-square difference in 168 significant wave
height comparisons made between 2001 and 2005 was 0.6 m (after eliminating
matches differing by more than 2 standard deviations). Wave heights for these
comparisons ranged from 0–4 m.
Vessel monitoring Vessel traffic in protected and use-restricted marine sanctuary areas is of interest to coral reef managers, and fishing vessel locations and
concentrations are of interest to fishery managers and fishing enforcement personnel. International vessels greater than 300 tons and all passenger ships are
required to carry Automatic Identification System (AIS) transmitters; however,
smaller vessels and non-compliant vessels cannot be monitored via AIS. SAR
instruments are the sensor-of-choice for monitoring such ships since SAR operates
day and night, cloudy or clear, and under a wide range of ocean conditions.
Vessels provide three different SAR signatures under the right ocean and wind
conditions: (1) direct returns; (2) wake patterns (Lyden et al. 1988); and (3) slicks
(Clemente-Colón et al. 1998). Since most ocean-going vessels have steel hulls,
they reflect the incoming SAR radar signal to provide a strong direct return (i.e.,
they are ‘‘hard targets’’), typically much more intense than the surrounding ocean
surface. Radar returns from vessels result from: (a) direct reflection of the radar
signal from portions of the ship oriented perpendicular to the incoming signal
(single bounce); (b) a double bounce from the hull and ocean surface; or (c)
13 Radar Applications
355