50 
TILT MECHANISM 
Figure 20. 
Initial Sea- 
WiFS design. 
DEROTATOR 
DICHROIC 
BEAMSPLITTERS 
ASSEMBLY 
This constraint left TDI as the primary tech- 
nique for increasing the signal-to-noise ratio 
(SNR) performance of the VNIR bands. Initial 
performance estimates lead to a requirement for 
five detectors in TDI along scan, thereby provid- 
ing a square-root-of-five improvement in SNR. 
Since sensor complexity (and cost and risk) in- 
crease rapidly with the number of detectors in 
TDI in the TIR region, those bands had to be limit- 
ed to one detector per band. 
Based on these parameters, summarized in 
Table 6, several different sensor configurations 
were evaluated. The initial design concept, de- 
veloped as an input to the SeaWiFS Working 
Group, utilized a continuously rotating scanner 
Table 6. 
Scan Rate 
GSD 
IFOV 
Detector Size 
Optics Focal Length 
Number of Detectors 
SCAN MECHANISM 
VNIR AFT ASSEMBLIES 
SOLAR DIFFUSER 
- fete 
)\ |_— RADIATIVE COOLER 
}] | 
J, 
th 
x, vA 
(90 degree fold), followed by an afocal telescope 
and an image derotator, as shown in Figure 20. 
This design was compact, lightweight, high in per- 
formance, and low in risk (i.¢., low in complexity). 
However, a polarization analysis showed that the 
design had a polarization sensitivity on the order 
of 5%, principally due to the large angles of inci- 
dence of the k-mirror derotator. Since, as dis- 
cussed in Section 4, low polarization sensitivity is 
one of the key requirements of SeaWiFS, an alter- 
nate concept was pursued. The alternate con- 
cept entailed a design with as few reflections as 
possible, kept as close to normal incidence as 
possible. The resulting concept is described 
below. 
Initial SeaWiFS Design Parameters 
lines per second 
km 
mrad 
um 
cm 
per TIR band 
per VNIR band, TDI along scan 
