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ACCURACY (TWO AXES)

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The sensor lens assembly focuses the light from stars within its field on an opaque quartz reticle with a V-shaped slit etched in its surface (see Fig. 6-41). As the lens assembly sweeps past a star, the photomultiplier produces a pulse at the crossing of each leg of the V slit. The crossing time of the first leg (which is vertical) is proportional to the star's azimuth angle. The elapsed time between the crossing of the first leg and the second slanted leg is a function of the star's elevation in the spacecraft coordinate system. This procedure is essentially the same as that used in the V-slit Sun and Earth sensors described in Sections 6.1 and 6.2.

Fig. 6-41. V-Slit Sensor Reticle Configuration Showing Field of View With Respect to Optical Axis

The star scanner level detector receives the star pulses and excludes those whose magnitudes are dimmer than the selected threshold. This prevents overloading of the data handling system due to clusters of dim stars or background noise. The level detector may be commanded to any of eight detecting thresholds (from + 3.5 mv to +1.75 m, in 0.25-m, increments).

The electronic processor generates a 24-bit word for each star encountered. This 24-bit word consists of 14 azimuth bits denoting the time at which the leading slit was transited, 1 flag bit indicating if three or more pulses were encountered within the maximum azimuth time of 250 ms, and 9 bits representing the. time between the pulses from the vertical and slanted slits. At 6 rpm, the 250-ms time interval corresponds to an angular separation of approximately 9 deg between the two pulses. The transit times are counted with a 1600-Hz clock which is periodically resynchronized to the spacecraft clock.

The star sensor electronics generate a limited number of false sightings which do not correspond to valid star transits. These false sightings arise from photomultiplier and electronics noise. Because they occur in a random fashion, discrimination from valid star transmits may be readily accomplished.

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