18 Field Radiometry and Ocean Color Remote Sensing
323
The superstructure perturbations largely vary with the illumination conditions,
the seawater inherent optical properties and the deployment geometry (Gordon,
1985; Doyle and Zibordi, 2002; Hooker and Morel, 2003; Piskozub, 2004; Hooker
and Zibordi, 2005b). Wave effects can produce quite large uncertainties as a function of sea state and seawater optical properties (Mobley, 1999; Hooker et al., 2002a;
Zibordi et al., 2004a; Zibordi et al., 2009a). Self-shading produces a decrease in the
in-water measurements of the upward light field (Gordon and Ding, 1992; Zibordi
and Ferrari, 1995; Aas and Korsbø, 1997). This effect scales with the seawater
absorption coefficient and the size of the instrument case.
18.6.1 Super-Structure Perturbations
In the early 1970s data produced with a photographic system measuring the inwater radiance distribution visually documented the effects of ship perturbations
in underwater light measurements (Smith, 1974). Approximately 10 years later
the ship-shading effects were quantitatively investigated through Monte-Carlo simulations (Gordon, 1985). This theoretical study led to the proposal of collecting
in-water radiometric data at distances larger than 10 m from the ship, to minimize
the superstructure perturbations. Later work (Voss et al., 1986; Helliwell et al., 1990;
Saruya et al., 1996; Weir et al., 1994; Piskozub, 2004) confirmed that in-water radiance and irradiance measurement uncertainties increase substantially when reducing
the deployment distance of the instrument from the ship.
In developing operational protocols for optical radiometric measurements to support satellite ocean color calibration and validation activities, Mueller and Austin
(1995) suggested the minimum ship distance from the instrument deployment point
as a function of the seawater diffuse attenuation coefficient. More recently the need
to accurately quantify uncertainties in measurements taken at the AAOT led to
extended investigations of tower perturbations in optical radiometric data collected
near its superstructure with in- and above-water radiometers (Zibordi et al., 1999;
Hooker and Zibordi, 2005b). Results from these analyses became the rationale for
the development and implementation of operational methods for the minimization
of superstructure perturbations. In the case of above-water radiometry, the minimization of perturbing effects due to deployment superstructures can be obtained
through the adoption of rigid measurement geometries (Hooker and Zibordi, 2005b).
For in-water radiometers the first choice is to make measurements sufficiently far
from superstructures. When this is not feasible, it may be alternatively possible to
remove the perturbation effects by simulating the radiance fields for each specific
measurement condition accounting for the deployment geometry, and the marine
and atmospheric optical properties (Doyle and Zibordi, 2002).
18.6.2 Wave Effects
The focusing and defocusing of sunrays refracted by surface waves produce large
light fluctuations in the upper sea layer. The origin, amplitude, frequency and depth
323
The superstructure perturbations largely vary with the illumination conditions,
the seawater inherent optical properties and the deployment geometry (Gordon,
1985; Doyle and Zibordi, 2002; Hooker and Morel, 2003; Piskozub, 2004; Hooker
and Zibordi, 2005b). Wave effects can produce quite large uncertainties as a function of sea state and seawater optical properties (Mobley, 1999; Hooker et al., 2002a;
Zibordi et al., 2004a; Zibordi et al., 2009a). Self-shading produces a decrease in the
in-water measurements of the upward light field (Gordon and Ding, 1992; Zibordi
and Ferrari, 1995; Aas and Korsbø, 1997). This effect scales with the seawater
absorption coefficient and the size of the instrument case.
18.6.1 Super-Structure Perturbations
In the early 1970s data produced with a photographic system measuring the inwater radiance distribution visually documented the effects of ship perturbations
in underwater light measurements (Smith, 1974). Approximately 10 years later
the ship-shading effects were quantitatively investigated through Monte-Carlo simulations (Gordon, 1985). This theoretical study led to the proposal of collecting
in-water radiometric data at distances larger than 10 m from the ship, to minimize
the superstructure perturbations. Later work (Voss et al., 1986; Helliwell et al., 1990;
Saruya et al., 1996; Weir et al., 1994; Piskozub, 2004) confirmed that in-water radiance and irradiance measurement uncertainties increase substantially when reducing
the deployment distance of the instrument from the ship.
In developing operational protocols for optical radiometric measurements to support satellite ocean color calibration and validation activities, Mueller and Austin
(1995) suggested the minimum ship distance from the instrument deployment point
as a function of the seawater diffuse attenuation coefficient. More recently the need
to accurately quantify uncertainties in measurements taken at the AAOT led to
extended investigations of tower perturbations in optical radiometric data collected
near its superstructure with in- and above-water radiometers (Zibordi et al., 1999;
Hooker and Zibordi, 2005b). Results from these analyses became the rationale for
the development and implementation of operational methods for the minimization
of superstructure perturbations. In the case of above-water radiometry, the minimization of perturbing effects due to deployment superstructures can be obtained
through the adoption of rigid measurement geometries (Hooker and Zibordi, 2005b).
For in-water radiometers the first choice is to make measurements sufficiently far
from superstructures. When this is not feasible, it may be alternatively possible to
remove the perturbation effects by simulating the radiance fields for each specific
measurement condition accounting for the deployment geometry, and the marine
and atmospheric optical properties (Doyle and Zibordi, 2002).
18.6.2 Wave Effects
The focusing and defocusing of sunrays refracted by surface waves produce large
light fluctuations in the upper sea layer. The origin, amplitude, frequency and depth