As described previously, the offset of first-order (Bragg) peaks provides
information on the radial components of currents; thus two or more radar systems
(or multiple views from a single moving radar) can resolve ocean currents. This
gives information on connectivity between reefs and kinetic energy (mixing) at/
near reefs. Measurement of wind (first-order energy) and waves (second-order
energy) can also indicate levels of mixing around reefs. Examples of these
applications are provided in Chap. 13.
Studies using early SAR data from Seasat were initially hindered by the vast
quantity of data produced by the Seasat SAR instrument and the need to develop
digital SAR signal processing techniques. Also, observations were only available
for analysis when transmitted in real-time to ground stations, primarily due to the
lack of storage capacity onboard the satellite. While this onboard storage issue no
longer remains and digital SAR processing techniques are now very mature, the
large volume of SAR data generated by modern SAR satellite instruments still
leads to challenges in data acquisition, communication, and signal/product processing, particularly for near real-time applications. Some applications, such as
wind, also require accurate SAR calibration, necessitating use of active or passive
ground calibration targets and precise determination of antenna-pattern corrections
using distributed targets, such as the Amazon rain forest. Nonetheless, the highresolution information that can be extracted from these data is a worthy goal in
spite of the constraints.
11.3.4 Radar Validation
Validation of radar-measured currents has been undertaken using ocean drifter
position and in situ instrumentation (either direct measurement by a current meter
or remotely sensed by acoustic sounders, which employ the same Doppler mechanism as radar measurement but through sound waves, see Sect. 8.3.4). Direct
comparisons of current measurements from radar versus Acoustic Doppler Current
Profilers (ADCPs) have shown general agreement, with root-mean-square (RMS)
differences of only 4–20 cm s
-1 (Graber et al. 1997; Shay et al. 2007), but are
often limited by velocity shears between the surface radar measuring point and the
uppermost usable ADCP bin (Kohut et al. 2006). Currents inferred from ocean
drifter position (i.e., rate and direction of movement between locations) have shown
similarly strong correlation with radar-measured currents (Paduan 2006). The
technique of integrating radar currents to emulate path tracking has shown promise
(Ullman et al. 2006) and more recent advances in processing HF radar data have
shown improved comparisons with drifter tracks (Mantovanelli et al. 2010; refer
also to Sect. 13.2.2). This is significant for the application of particle tracking,
whether environmental (e.g., coral spawning) or for safety reasons (e.g., man
overboard). Radar wind measurements have been validated through comparison
with co-located meteorological buoys (Monaldo et al. 2001), dropwindsondes and
passive microwave wind measurements from aircraft (i.e., using the Stepped
11 Thermal and Radar Overview
309
information on the radial components of currents; thus two or more radar systems
(or multiple views from a single moving radar) can resolve ocean currents. This
gives information on connectivity between reefs and kinetic energy (mixing) at/
near reefs. Measurement of wind (first-order energy) and waves (second-order
energy) can also indicate levels of mixing around reefs. Examples of these
applications are provided in Chap. 13.
Studies using early SAR data from Seasat were initially hindered by the vast
quantity of data produced by the Seasat SAR instrument and the need to develop
digital SAR signal processing techniques. Also, observations were only available
for analysis when transmitted in real-time to ground stations, primarily due to the
lack of storage capacity onboard the satellite. While this onboard storage issue no
longer remains and digital SAR processing techniques are now very mature, the
large volume of SAR data generated by modern SAR satellite instruments still
leads to challenges in data acquisition, communication, and signal/product processing, particularly for near real-time applications. Some applications, such as
wind, also require accurate SAR calibration, necessitating use of active or passive
ground calibration targets and precise determination of antenna-pattern corrections
using distributed targets, such as the Amazon rain forest. Nonetheless, the highresolution information that can be extracted from these data is a worthy goal in
spite of the constraints.
11.3.4 Radar Validation
Validation of radar-measured currents has been undertaken using ocean drifter
position and in situ instrumentation (either direct measurement by a current meter
or remotely sensed by acoustic sounders, which employ the same Doppler mechanism as radar measurement but through sound waves, see Sect. 8.3.4). Direct
comparisons of current measurements from radar versus Acoustic Doppler Current
Profilers (ADCPs) have shown general agreement, with root-mean-square (RMS)
differences of only 4–20 cm s
-1 (Graber et al. 1997; Shay et al. 2007), but are
often limited by velocity shears between the surface radar measuring point and the
uppermost usable ADCP bin (Kohut et al. 2006). Currents inferred from ocean
drifter position (i.e., rate and direction of movement between locations) have shown
similarly strong correlation with radar-measured currents (Paduan 2006). The
technique of integrating radar currents to emulate path tracking has shown promise
(Ullman et al. 2006) and more recent advances in processing HF radar data have
shown improved comparisons with drifter tracks (Mantovanelli et al. 2010; refer
also to Sect. 13.2.2). This is significant for the application of particle tracking,
whether environmental (e.g., coral spawning) or for safety reasons (e.g., man
overboard). Radar wind measurements have been validated through comparison
with co-located meteorological buoys (Monaldo et al. 2001), dropwindsondes and
passive microwave wind measurements from aircraft (i.e., using the Stepped
11 Thermal and Radar Overview
309