Subsatellite Point

Fig. 6-29. SEASAT Scanwheel Configuration. (Compare with Fig. 4-3.)

attitude control properties are described in Section 18.3. Attitude accuracy and reliability is increased by dual scanner systems; in particular, and attitude accuracy about the velocity vector, or roll axis, is improved by using the difference between the two observed duty cycles, which is less sensitive to attitude variations than the individual scanner duty cycle measurements.

Slit Sensors. Slit sensors of a much different design than the narrow field-of-view detectors described earlier in this section were flown on the spinning spacecraft COS-B, ISEE-2, and SIRIO. The unit used on COS-B and ISEE-2 is called an attitude sensor unit, or ASU, and consists of two 120-deg by 1-deg slits [Massart, 1974]. The complete sensor system consists of two ASUs, mounted on COS-B as shown in Figure 6-30. On ISEE-2, both ASUs were mounted with the same orientation, similar to j that of the unit on the right-hand side of Fig. 6-30 [Wetmore, et al., 1976]. A cylindrical lens focuses radiation on a photodiode*,

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