3 
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2 
= 
7) 
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7) 
z2 
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ke 
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O 
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Figure 23. SeaWiFS 
estimated polarization 
sensitivity. 
01sec N 
-20 0 20 
OBJECT-SPACE SCAN ANGLE, DEG 
The polarization sensitivity depends on the 
scan angle, since the major contributor is the 
half-angle mirror. Figure 23 shows the SeaWiFS 
polarization sensitivity as a function of scan an- 
gle for fresh silver at a wavelength of 443 nm.'° 
The Thematic Mapper mirrors have measured 
polarization values very close to fresh silver, so 
this is a reasonable approximation to use. Also, 
calculating the polarization sensitivity at 443 nm 
is a worst case; the values are less at longer 
VNIR wavelengths. The figure shows that the 
polarization sensitivity is less than 1.4% and is 
significantly lower in the region around nadir. 
This allows those whose applications require 
even lower polarization sensitivities to still derive 
useful data from the SeaWiFS by using only the 
data acquired from near nadir. 
Calibration 
The high degree of calibration accuracy re- 
quired for SeaWiFS, described in Section 4, can 
be met only by using a multistep calibration pro- 
1Onssumes 100% polarized input. 
cess since the calibration accuracy and preci- 
sion called for probably cannot be achieved 
through preflight ground calibration alone. Dur- 
ing ground calibration, a calibration accuracy of 
3% will probably be acheivable for all bands. Af- 
ter launch, a data-gathering and calibration anal- 
ysis program will be required, as suggested in 
Section 3. Data gathered by ships coincident 
with SeaWiFS imagery will enable accurate cali- 
bration of the radiance measurements, probably 
to a calibration accuracy of about 1%. 
Calibration stability is actually more important 
than absolute radiometric precision. Because of 
this, a solar diffuser has been included in the in- 
strument, allowing periodic calibration of Sea- 
WiFS against the known exo-atmospheric solar il- 
lumination. Calibration against a lunar view was 
also requested by the users, since the moon acts 
as an almost ideal solar diffuser. Unfortunately, 
to provide this view on demand would require 
either an attitude adjustment of the spacecraft or 
the capability to point the instrument. Therefore, 
the moon will be considered a "target of opportu- 
nity," and a lunar calibration will take place only 
when the moon can be viewed during the 
space-view interval of the scan cycle. 
IMPLEMENTATION PANEL 
55 
