6.43 BBRC CT-401 Fixed-Head Star Tracker
As an example of the fixed-nead star tracker, we will describe the CT-401, manufactured by BBRC and shown in Fig. 6-42. The CT-4Q1 has flown on the SAS-3 and HEAO-1 missions and is scheduled for HEAO-C and MAGSAT. Schematically, the tracker is similar to the instrument shown in Fig. 6-39. The specified accuracy of the tracker over its 8- by 8-deg field of view is ±3 arc-minutes without calibration, or ± 10 arc-seconds after applying corrections for electro-optical distortion, temperature, ambient magnetic field, and star intensity obtained from preflight calibration. Because this instrument is capable of observing several stars within a relatively short period of time, it is frequently called a star camera.
The tracker has four commandable thresholds corresponding to selection of stars brighter than approximately +3.0, +4.0, +5.0, and +6.5 nv These settings add to the flexibility of the instrument, because it can be used both for coarse attitude determination or attitude acquisition when set at +3.0 n^ or for fine attitude determination when set at +6.5 nv (As discussed in Section 7.7, many fewer stars are measured at + 3.0 mv than at 6.5 m^, which permits star identification with a coarse a priori attitude.) A 9- by 9-arc-minute receiving aperture (1FOV) scans the 8- by 8-deg FOV using the search pattern shown in Fig. 6-43. The
FOV scan continues until a star is found which exceeds the threshold level. When this occurs, the current scan line is completed, and the track pattern shown in Fig. 6-44 begins. A feedback system in the electronics assembly centers the track pattern on the star image. Two-axis star position signals are determined by the electronics assembly from the rising and falling edges of the star image. These signals, u and v, determine the tangent plane coordinates of the star, as explained in Section 7.6. As a star moves in the FOV due to attitude changes, the track pattern follows and remains centered on the star image. Tracking of the same star continues until the tracker is commanded to return to the search mode, the star signal drops below the threshold, or the star leaves the FOV.
Figure 6-44 shows the electronic sampling procedure used in the track mode. As the IFOV is deflected past the star image, a video signal is produced. When this signal exceeds and then falls below the threshold level, two pulses for each axis are generated by the electronics. These pulses correspond to the position of the IFOV during the leading and trailing edge crossings. The electronics combines these signals to determine the position of the star image in the FOV. These star position signals are then used to keep the track pattern centered ovei the star image. Star coordinates on both axes are updated every 100 ms. The resulting data samples are filtered through an RC filter with a time constant of approximately 450 ms. If significant motion of the star image with respect to the previously sampled position occurs, major tracking problems will result. For example, the CT-401 may fail to track stars moving faster than approximately 0.6 deg/sec in the FOV.
When the search mode resumes, the o coordinate for the beginning of the new search line will be the o coordinate of the star which was last tracked, plus a small increment (0.4 deg) to avoid retracking the same star. If this would place the aperture beyond the edge of the FOV, the search pattern returns to the start position. The starting u coordinate is at the beginning of a new line.
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