296
H.R. Gordon
chartered the Athena II, a 165 ft decommissioned patrol boat, from the Vietnam
War, that was capable of speeds as high as 40 kt. The participating NET members
on the Athena II were E. Baker, D. Clark and myself. Dennis’ idea was to perform a hydrographic station at local noon, simultaneous with the satellite overpass.
Measurements of upwelling spectral radiance, downwelling spectral irradiance,
beam attenuation coefficient, phytoplankton pigments, TSM, particle size distribution, Secchi depth, and atmospheric transmittance were carried out at each station. I
measured the backscattering coefficients of water samples in the blue (436 nm) and
green (546 nm) with a light scattering photometer, and made hand-held atmospheric
transmittance measurements with a Volz-like sun photometer.
The speed of the Athena II enabled us to proceed to the nadir point of the next
day’s satellite overpass with plenty of time to prepare for the station. With a ship
proceeding at normal speeds (∼10 kt) the best one could hope to accomplish would
be to have the next day’s station located near the edge of the scan, where atmospheric
correction would be significantly more difficult because of the longer path through
the atmosphere. During that cruise we made several stations in which the weather
was sufficiently clear that excellent simultaneous CZCS imagery was obtained. In
the next 8 months Dennis Clark organized two more cruises (Gulf of California and
Middle Atlantic Bight) in support of the CZCS validation (and algorithm development). To underscore the difficulty of validating an ocean color sensor, one should
note that of the 55 stations made underneath the CZCS, only 9 were usable for
validation because of cloud contamination and the proximity of land.
At the time of launch, the proposed algorithms had yet to be implemented on the
CZCS Processing System at GSFC. Processing a CZCS scene at that time (1979)
was an enormous task, as the large mainframe computers were excruciatingly slow
even by standards that would be set within the next 5 years. Under pressure to finish
the validation, we had to scrounge computer time wherever we could find it. Dennis
Clark, Jim Mueller, and I (assisted by Dave Ball of Computer Sciences Corporation,
who worked with Jim) found an available computer coupled to an image display
device at the AOIPS (Atmospheric and Oceanic Image Processing System) facility
at GSFC. The computer was a PDP 1155 (minicomputer), and we were allowed
to use it from time to time between the hours of about 6 PM–6 AM. The room
was very cold and I always brought a hood from my parka to keep warm while
processing the data.
I had developed a program that could take the ephemeris for the orbit and compute the Rayleigh scattering component for each pixel along a scan line. I did this
computation using a UNIVAC 1106 mainframe computer for two overpasses coincident with our surface measurements, and stored the results on tape. I could also
take the latitude and longitude along ship tracks and determine the line and pixel
numbers along the track. We then took 512 × 512 pixel subscenes of CZCS images
and applied the atmospheric correction algorithm (assuming ε(λ i ,λ i ) = 1) to the
imagery to derive an estimate for L w (λ i ). This processing took several seconds per
scan line. The monitor displayed the original L t (λ i ), which was then replaced by
L w (λ i ) as each scan line was completed. The first image processed was from Orbit
130 (Fig. 17.3) near the Mississippi Delta.
H.R. Gordon
chartered the Athena II, a 165 ft decommissioned patrol boat, from the Vietnam
War, that was capable of speeds as high as 40 kt. The participating NET members
on the Athena II were E. Baker, D. Clark and myself. Dennis’ idea was to perform a hydrographic station at local noon, simultaneous with the satellite overpass.
Measurements of upwelling spectral radiance, downwelling spectral irradiance,
beam attenuation coefficient, phytoplankton pigments, TSM, particle size distribution, Secchi depth, and atmospheric transmittance were carried out at each station. I
measured the backscattering coefficients of water samples in the blue (436 nm) and
green (546 nm) with a light scattering photometer, and made hand-held atmospheric
transmittance measurements with a Volz-like sun photometer.
The speed of the Athena II enabled us to proceed to the nadir point of the next
day’s satellite overpass with plenty of time to prepare for the station. With a ship
proceeding at normal speeds (∼10 kt) the best one could hope to accomplish would
be to have the next day’s station located near the edge of the scan, where atmospheric
correction would be significantly more difficult because of the longer path through
the atmosphere. During that cruise we made several stations in which the weather
was sufficiently clear that excellent simultaneous CZCS imagery was obtained. In
the next 8 months Dennis Clark organized two more cruises (Gulf of California and
Middle Atlantic Bight) in support of the CZCS validation (and algorithm development). To underscore the difficulty of validating an ocean color sensor, one should
note that of the 55 stations made underneath the CZCS, only 9 were usable for
validation because of cloud contamination and the proximity of land.
At the time of launch, the proposed algorithms had yet to be implemented on the
CZCS Processing System at GSFC. Processing a CZCS scene at that time (1979)
was an enormous task, as the large mainframe computers were excruciatingly slow
even by standards that would be set within the next 5 years. Under pressure to finish
the validation, we had to scrounge computer time wherever we could find it. Dennis
Clark, Jim Mueller, and I (assisted by Dave Ball of Computer Sciences Corporation,
who worked with Jim) found an available computer coupled to an image display
device at the AOIPS (Atmospheric and Oceanic Image Processing System) facility
at GSFC. The computer was a PDP 1155 (minicomputer), and we were allowed
to use it from time to time between the hours of about 6 PM–6 AM. The room
was very cold and I always brought a hood from my parka to keep warm while
processing the data.
I had developed a program that could take the ephemeris for the orbit and compute the Rayleigh scattering component for each pixel along a scan line. I did this
computation using a UNIVAC 1106 mainframe computer for two overpasses coincident with our surface measurements, and stored the results on tape. I could also
take the latitude and longitude along ship tracks and determine the line and pixel
numbers along the track. We then took 512 × 512 pixel subscenes of CZCS images
and applied the atmospheric correction algorithm (assuming ε(λ i ,λ i ) = 1) to the
imagery to derive an estimate for L w (λ i ). This processing took several seconds per
scan line. The monitor displayed the original L t (λ i ), which was then replaced by
L w (λ i ) as each scan line was completed. The first image processed was from Orbit
130 (Fig. 17.3) near the Mississippi Delta.