ey 
Figure 22. Sea- 
WiFS focal-plane 
assembly layouts. 
Specular sun reflection is avoided by tilting 
the telescope in the plane perpendicular to the 
scan plane (i.e., along track) to one of three posi- 
tions: +20°, 0°, or -20°. The continuous 360° 
scan allows reference sources to be viewed dur- 
ing the nonactive, or non-scene-viewing, part of 
the scan, as well as allowing a deep-space view 
for a zero reference just before the scan begins? 
There is also a solar diffuser that can be inserted 
into the field of view of the sensor for calibration 
against the input solar radiance. This concept 
has an active scan of 58.3°, providing a scan 
length of 2800 km (i.e., daily global coverage). 
After the scene energy leaves the 3X afocal 
telescope and fold mirrors, it is spectrally separat- 
ed into four beams. All separations are per- 
formed by dichroic beamsplitters to provide co- 
registration of the beams on the ground. The 
first dichroic splits the TIR (3 to 13 um) from the 
"The 360° scan would also permit calibration from a lunar 
view. However, provision of regular lunar radiance calibra- 
tion would require maneuvering the spacecraft. If deemed 
necessary, the occasions when a full moon happens to be 
within the field of view of the sensor could be calculated, and 
the sensor could be activated at these times. 
VNIR (0.4 to 1.0 um). The TIR travels straight 
through and is focused onto the detectors in the 
TIR aft assembly (consisting of focusing optics 
and two detectors). 
As mentioned above, one TIR detector per 
band minimizes the complexity of the instrument 
and allows a very simple radiative cooler to be 
used. The cooler envisioned, a standard prod- 
uct manufactured by Arthur D. Little Corp., pro- 
vides a detector operating temperature of about 
110K. The TIR detectors are discrete photocon- 
ductive HgCdTe elements. These detectors and 
the layout of their spectral filters are shown in 
Figure 22. The design of the aft assembly en- 
ables either two LWIR bands (e.g., 10.5 to 11.5 
um and 11.5 to 12.5 um), two mid-wavelength in- 
frared bands (e.g., 3.5 to 3.75 um and 3.75 to 
4.0 um), or even one band from each region 
(e.g., 3.5 to 4.0 um and 10.5 to 12.5 um) to be 
accommodated. However, in accordance with 
the users’ agreement discussed in Section 4, the 
baseline design consists of the two LWIR bands. 
The VNIR light reflected from the first dichro- 
ic IS spectrally separated by the second dichroic 
into a short-wavelength VNIR beam and a beam 
containing the two longest wavelength VNIR 
