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1 POUNTAIN. t972 3 WETMORE. .1 ■!.. (974 3 RCA. 1975

4 SCOTT AND CARROLL. 1969

5 BBRC •• 8ALL BROTHERS RESEARCH CORPORATION

1 POUNTAIN. t972 3 WETMORE. .1 ■!.. (974 3 RCA. 1975

4 SCOTT AND CARROLL. 1969

5 BBRC •• 8ALL BROTHERS RESEARCH CORPORATION

small optical FOV (usually less than 1 deg). The gimbal mounts, however, give the sensor a much larger effective FOV. The coarse-alignment star trackers on OAO, for example, were gimbaled to cover an area with a 43-deg half-cone angle [NASA, 1970]. Gimbaled star trackers normally operate on a relatively small number of target stars (e.g., 38 for the OAO trackers).

Many different kinds of image definition devices are used in gimbaled star trackers to determine the position of the star with respect to the center, or null, position in the small FOV. The electronics assembly causes the gimbals to move so that the star image remains centered in the small FOV. The star's position is then given by the gimbal angle readout positions. Some image-definition devices employ an optical or electronic scan of the small FOV to provide star position information. For example, small FOV image dissector tubes may perform this function. Another type of scanning device is an optical wedge-slit system. A rotating optical wedge causes the star image to be deflected past an L-shaped slit. As the wedge rotates, the image of the star follows a circular path over the L slit. The optical wedge is designed so that the radius of the circle grows as the star image diverges from the null position. The electronics assembly determines the position of the star with respect to null by comparing the time difference between slit crossings and the rotation phase of the optical wedge.

One type of gimbaled star tracker does not use an image definition device at all, but rather reflects a defocused star image onto four photomultipliers in a square array. The star's position is determined by comparing the output signals of the four photomultipliers. This system has the advantage of simplicity. However, it suffers from disadvantages: temperature variations and changes in photomultiplier characteristics due to aging may introduce systematic biases; nonuniform background light or the presence of a second star within the small FOV causes serious errors.

Errors in determining the star position with respect to null and gimbal angle readout errors affect the overall gimbaled star tracker's accuracy. Typical accuracies range from 1 to 60 arc-seconds, excluding tracker misalignment. A major disadvantage of gimbaled star trackers is that the mechanical action of the gimbals reduces their long-term reliability. In addition, the gimbal mount assembly is frequently large and heavy.

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