17 Some Reflections on Thirty-Five Years of Ocean Color Remote Sensing
301
where t(λ i ), the diffuse transmittance, can be computed with good accuracy by
simply ignoring aerosols. However, we still needed an independent method of estimating ε(λ i ,Red). This was supplied by what Dennis Clark and I referred to as
the “clear water radiance concept.” This was based on our observation that when
C ≤ 0.25 mg/m
3 , the normalized water leaving radiance, [L w (λ i )] N , defined through
L w = [L w ] N cos θ 0 t 0 /a 2
⊕ , where t 0 is the diffuse transmittance of the solar beam to
the sea surface, θ 0 is the solar zenith angle, and a ⊕ is the Earth-Sun distance in
astronomical units (the mean a ⊕ over 1 year is unity), at 520 and 550 nm were constant and known, and that at 670 nm was essentially zero (Gordon and Clark, 1981).
Thus, if clear water could be located in a scene containing the L w (λ i ) measurements, it would be possible to determine ε(520,Red) and ε(550,Red), and through
extrapolation (using a power law) ε(443,Red). Then, assuming ε(λ i ,Red) is independent of position in the image under consideration (a much weaker assumption than
assuming a constant aerosol concentration), and assuming the sensor calibration was
correct at 670 nm, we could estimate the water-leaving radiance in the other bands.
Note the assumptions required to perform this vicarious calibration assessment: (1)
the atmospheric correction algorithm is correct; (2) the ε-values are independent
of position and their variation with wavelength is given by a power (Ångström’s)
law; and (3) the calibration of the spectral band at 670 nm is correct. The procedure is then to fractionally change L t (λ i ), i.e., k(λ i ) for the fixed DC(λ i ), until the
measured and retrieved L w (λ i )’s are brought into confluence. Note that the computation of L r (λ i ) requires the extraterrestrial solar irradiance (Equation (17.2)), so any
error in this quantity will be interpreted as an error in sensor calibration, i.e., k(λ i ),
therefore the “calibration” will be dependent on the particular values used for the
solar irradiance. With these assumptions, we determined adjustments to the sensor
calibration that seemed to work well for imagery from all of the validation scenes
obtained during June of 1979. Unfortunately, when we used this vicarious calibration and examined data from earlier cruises, we found that the agreement between
the measured and retrieved L w (λ i ) values became increasingly poorer as we progressed backward in time. We interpreted this as a decrease in the sensitivity of the
instrument with time. This decreasing sensitivity with time was a major problem for
the analysis of CZCS data. It was apparently caused by residue accumulating on the
scan mirror due to out gassing of the instrument. This made it clear that ensuring
the stability of, or carefully monitoring the stability of, future ocean color sensors
was paramount. It is interesting to note that most of the validation data that were
obtained within a year of launch were also used to adjust the sensor calibration for
its variation with time. This is likely the origin of the term “cal-val” in reference to
such activities.
17.6 The NOSS Interlude
In the early 1980s, with the success of the CZCS, a proposal was made to include
an expanded instrument on a new platform, the National Ocean Satellite System
(NOSS). Armed with high quality CZCS imagery, and the IUCRM Water Color
301
where t(λ i ), the diffuse transmittance, can be computed with good accuracy by
simply ignoring aerosols. However, we still needed an independent method of estimating ε(λ i ,Red). This was supplied by what Dennis Clark and I referred to as
the “clear water radiance concept.” This was based on our observation that when
C ≤ 0.25 mg/m
3 , the normalized water leaving radiance, [L w (λ i )] N , defined through
L w = [L w ] N cos θ 0 t 0 /a 2
⊕ , where t 0 is the diffuse transmittance of the solar beam to
the sea surface, θ 0 is the solar zenith angle, and a ⊕ is the Earth-Sun distance in
astronomical units (the mean a ⊕ over 1 year is unity), at 520 and 550 nm were constant and known, and that at 670 nm was essentially zero (Gordon and Clark, 1981).
Thus, if clear water could be located in a scene containing the L w (λ i ) measurements, it would be possible to determine ε(520,Red) and ε(550,Red), and through
extrapolation (using a power law) ε(443,Red). Then, assuming ε(λ i ,Red) is independent of position in the image under consideration (a much weaker assumption than
assuming a constant aerosol concentration), and assuming the sensor calibration was
correct at 670 nm, we could estimate the water-leaving radiance in the other bands.
Note the assumptions required to perform this vicarious calibration assessment: (1)
the atmospheric correction algorithm is correct; (2) the ε-values are independent
of position and their variation with wavelength is given by a power (Ångström’s)
law; and (3) the calibration of the spectral band at 670 nm is correct. The procedure is then to fractionally change L t (λ i ), i.e., k(λ i ) for the fixed DC(λ i ), until the
measured and retrieved L w (λ i )’s are brought into confluence. Note that the computation of L r (λ i ) requires the extraterrestrial solar irradiance (Equation (17.2)), so any
error in this quantity will be interpreted as an error in sensor calibration, i.e., k(λ i ),
therefore the “calibration” will be dependent on the particular values used for the
solar irradiance. With these assumptions, we determined adjustments to the sensor
calibration that seemed to work well for imagery from all of the validation scenes
obtained during June of 1979. Unfortunately, when we used this vicarious calibration and examined data from earlier cruises, we found that the agreement between
the measured and retrieved L w (λ i ) values became increasingly poorer as we progressed backward in time. We interpreted this as a decrease in the sensitivity of the
instrument with time. This decreasing sensitivity with time was a major problem for
the analysis of CZCS data. It was apparently caused by residue accumulating on the
scan mirror due to out gassing of the instrument. This made it clear that ensuring
the stability of, or carefully monitoring the stability of, future ocean color sensors
was paramount. It is interesting to note that most of the validation data that were
obtained within a year of launch were also used to adjust the sensor calibration for
its variation with time. This is likely the origin of the term “cal-val” in reference to
such activities.
17.6 The NOSS Interlude
In the early 1980s, with the success of the CZCS, a proposal was made to include
an expanded instrument on a new platform, the National Ocean Satellite System
(NOSS). Armed with high quality CZCS imagery, and the IUCRM Water Color