294
H.R. Gordon
ε(λ i ,λ j ) =
λ j
λ i
α
.
(17.4)
Under these conditions, if we knew L a at a single wavelength (and knew α) we
could determine L a at all other wavelengths. Knowing that L w (λ Red ) in the red is
often negligible compared to the blue and the green (Fig. 17.2),
L a (λ Red ) = L t (λ Red ) − L r (λ Red ), and L a (λ i ) =
F 0 (λ i )
F 0 (λ Red )
λ Red
λ i
α
L a (λ Red ),
(17.5)
essentially reducing the atmospheric correction problem to determining a single
parameter: α.
To test these ideas, in particular the formula for ε(λ i ,λ i ), Equation (17.4), we
attempted to use the Ocean Color Scanner (OCS, a CZCS simulator built by
NASA/GSFC and designed to fly on a U-2 aircraft) based at NASA Lewis Research
Center under the direction of Jack Saltzman. Data of L t (λ i ) were obtained by Jack
on a flight of altitude ∼ 15 km over the Gulf of Mexico south of the Mississippi
delta coincident with a CZCS NET prelaunch algorithm development cruise. I used
the data and Equation (17.1) to form
L a (λ i ) = L t (λ i ) − L r (λ i ) − t(λ i )L w (λ i ),
(17.6)
expecting the resulting spectral variation to be L a (λ i ) ∼ F 0 (λ i ) × (λ i ) −α , verifying
that we were on the right track. Indeed, the spectral variation did follow the expected
relationship, but with α = 8: completely impossible! Recall that for the Rayleigh
component L r (λ i ) ∝ F 0 (λ i ) × (λ i ) −4 , and the variation of the aerosol scattering
must be a weaker function of wavelength than molecular scattering. Thus, the results
made no sense whatsoever. I told Jack the results and he said he would get back to
me in a few days. Later he called and said I should take the measured L t (443) and
multiply it by 0.7, with corrections of a similar magnitude for the other spectral
bands. Thus, Jack believed the OCS calibration was in error by as much as 30%.
With errors of this magnitude, I saw no sense in trying to use the OCS to show that
our formulas were reasonable approximations to reality. That was the last time I
tried to use aircraft data to validate our atmospheric correction ideas, and there was
no prelaunch test of the algorithms or even their underlying assumptions.
17.3 IUCRM Colloquium: “Passive Radiometry of the Ocean”
During this same time period (1976–1978), I had tested these ideas using simulated data derived from multiple scattering solutions to the radiative transfer
problem in the ocean-atmosphere system (Gordon, 1978). They seemed to hold up
well, so Dennis Clark and I decided to combine his proposed algorithm (Clark,
1981) for estimating the water’s pigment concentration from radiance ratios, e.g.,
H.R. Gordon
ε(λ i ,λ j ) =
λ j
λ i
α
.
(17.4)
Under these conditions, if we knew L a at a single wavelength (and knew α) we
could determine L a at all other wavelengths. Knowing that L w (λ Red ) in the red is
often negligible compared to the blue and the green (Fig. 17.2),
L a (λ Red ) = L t (λ Red ) − L r (λ Red ), and L a (λ i ) =
F 0 (λ i )
F 0 (λ Red )
λ Red
λ i
α
L a (λ Red ),
(17.5)
essentially reducing the atmospheric correction problem to determining a single
parameter: α.
To test these ideas, in particular the formula for ε(λ i ,λ i ), Equation (17.4), we
attempted to use the Ocean Color Scanner (OCS, a CZCS simulator built by
NASA/GSFC and designed to fly on a U-2 aircraft) based at NASA Lewis Research
Center under the direction of Jack Saltzman. Data of L t (λ i ) were obtained by Jack
on a flight of altitude ∼ 15 km over the Gulf of Mexico south of the Mississippi
delta coincident with a CZCS NET prelaunch algorithm development cruise. I used
the data and Equation (17.1) to form
L a (λ i ) = L t (λ i ) − L r (λ i ) − t(λ i )L w (λ i ),
(17.6)
expecting the resulting spectral variation to be L a (λ i ) ∼ F 0 (λ i ) × (λ i ) −α , verifying
that we were on the right track. Indeed, the spectral variation did follow the expected
relationship, but with α = 8: completely impossible! Recall that for the Rayleigh
component L r (λ i ) ∝ F 0 (λ i ) × (λ i ) −4 , and the variation of the aerosol scattering
must be a weaker function of wavelength than molecular scattering. Thus, the results
made no sense whatsoever. I told Jack the results and he said he would get back to
me in a few days. Later he called and said I should take the measured L t (443) and
multiply it by 0.7, with corrections of a similar magnitude for the other spectral
bands. Thus, Jack believed the OCS calibration was in error by as much as 30%.
With errors of this magnitude, I saw no sense in trying to use the OCS to show that
our formulas were reasonable approximations to reality. That was the last time I
tried to use aircraft data to validate our atmospheric correction ideas, and there was
no prelaunch test of the algorithms or even their underlying assumptions.
17.3 IUCRM Colloquium: “Passive Radiometry of the Ocean”
During this same time period (1976–1978), I had tested these ideas using simulated data derived from multiple scattering solutions to the radiative transfer
problem in the ocean-atmosphere system (Gordon, 1978). They seemed to hold up
well, so Dennis Clark and I decided to combine his proposed algorithm (Clark,
1981) for estimating the water’s pigment concentration from radiance ratios, e.g.,