Aperture Plate

\ APERTURE PLATE

Fig. 6-5. Two-Axis Mask Sun Detector [Schmidtbauer, et al., 1973]. The sunline is normal to the aperture plate if the output of all four solar cells is equal.

6.1.2 Sun Presence Detectors

Sun presence detectors are used to protect instrumentation, to activate hardware, and to position the spacecraft or experiments. Ideally, Sun presence detectors provide a step function response that indicates when the Sun is within the FOV of the detector. For example, the shadow bar detector shown in Fig. 6-6 has a steep output slope and, consequently, a limited FOV and a 1-arc-minute accuracy. The sensor mass is less than 200 g.

The critical angle prism illustrated in Fig. 6-7(a) is based on Snell's law, nsind = sind'. Consider radiation incident normal to the base of an isosceles triangular prism with index of refraction n, and base angle y, such that nsiny= I. The angle of the refracted radiation is 0' = 9O°, and the total output current from the photocells will be zero. Non-normal incidence will yield current in the detector for which 0' <90°. Figure 6-7(b) illustrates the total transmission for near normal incidence.

Another type of highly accurate null detector is illustrated in Fig. 6-8. The sensor optics are such that a null Sun angle will center the Sun image at the top of the wedge and mirrors on the side of the wedge will reflect radiation to yield a current balance in the photocells.

Spinning spacecraft frequently employ one or more Sun-presence detectors composed of two slits and a photocell, as shown in Fig. 6-9. Whenever the Sun lies

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