Star tracker detector having a partial memory section

Television – Camera – system and detail – Solid-state image sensor

Reexamination Certificate

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Details

C348S314000

Reexamination Certificate

active

06252627

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to obtaining information using a charge coupled device (CCD) and, in particular, obtaining location or acquisition information using one or more stars in which the CCD has a memory section that has fewer lines and cells than an imaging section thereof.
BACKGROUND OF THE INVENTION
In many aerospace related systems having orbiting space based satellites, the satellites must be in particular orientations to perform their intended tasks. For example, telecommunication satellites may require that their antennas are positioned for appropriate transmitting and receiving of wireless communications. Additionally, space exploration satellites may require very precise satellite orientation for high resolution imaging of celestial bodies. All inertial guidance systems drift with time and their calibration needs to be periodically updated. Reckoning from stars with a tracker can provide this calibration with the required precision.
A satellite's position and/or orientation may be determined by various types of inertial guidance systems such as a laser gyroscope. However, such inertial guidance systems are only useful for positioning and orienting an orbiting satellite up to a certain degree of accuracy. When further accuracy is desired, other techniques may be required. In particular, it would be desirable to enhance satellite position accuracy by using a real time star tracking system that tracks the images of predetermined stars. That is, such a star tracking system may be utilized to fine tune the position and/or orientation (collectively, denoted “positioning information”) of an orbiting satellite, wherein a separate inertial guidance system supplies initial estimates of such positioning information. Moreover, when installed on a satellite, it would also be desirable for such a star tracking system to be of reduced size, weight and power consumption. Moreover, it would be also desirable that such a star tracking system be capable of tracking a plurality of stars simultaneously.
SUMMARY OF THE INVENTION
The present invention is a star tracker that includes a novel frame transfer CCD, wherein the memory section is substantially reduced in size, and further wherein there is only a minor increase in the parallel charge transfer inefficiency (CTI) over that of a full frame CCD. Moreover, the present invention allows high update rates (frame rates) with low actual pixel read rates and hence reduced bandwidth demands. Additionally, the present invention eliminates the perturbing influence of varying image smear on determining stellar image centroids by providing a substantially uniform smear under all star tracking scenarios. Also, the present invention reduces the image smear generating interval. Thus, the present invention provides high frame rates with low to moderate pixel read rates, and hence low bandwidths which translate to low noise levels, thereby making possible high signal-to-noise ratios. Further, the present invention generates only a small thermal load increase over that of a full frame CCD.
The tracking device of the present invention includes a novel charge frame transfer coupled device (CCD) having an imaging area for integrating (i.e., capturing) images thereon, and a reduced size memory section into which each such image is transferred and compacted therein. Subsequently, each image in the reduced memory section may be read out of the CCD and processed while a next image is being integrated in the imaging area. In particular, the novel CCD outputs the compacted image data from the reduced memory section to tracking modules for further processing so that stars may be accurately tracked by accurately generating centroids of their images.
The CCD included in the present invention is a frame transfer type having a split memory section. The split memory section has a first sequence of rows of charge collecting cells (CCD cells) for storing a first portion (e.g., an upper half) of an image obtained from a first part of the CCD imaging area. Note that this first sequence of rows are at a first (e.g., top) end of the CCD imaging area. Additionally, the memory section has a second sequence of rows of charge collecting cells at an opposite end of the CCD imaging area for storing a second portion (e.g. a lower half) of the image obtained from a second part (e.g., a lower half) of the CCD imaging area. In one embodiment, each of the memory section first and second portions contain half of the cell charges (i.e., pixels) collected in the CCD imaging area. Further, the rows of the CCD imaging area cells for the top half of the imaging area may be parallelly shifted into the first portion of the memory section, and the bottom half of the rows of the CCD imaging area may be parallelly shifted into the second portion of the memory section. Accordingly, it is an aspect of the present invention that by concurrently shifting each half of the imaging area into its corresponding portion of the memory section, the latency time for image transfers to the memory section is reduced from that of a CCD having a memory section at only a single end of the imaging area.
Although the total number of memory section charge cell rows for the present invention is substantially fewer in number than the number of such rows in the imaging area, the present invention is able to provide rapid transfer of an entire image from the imaging area to the reduced size memory section in a manner such that all the desired star tracking information within each image is preserved in the reduced size memory section. That is, all such desired star tracking information can be represented within a reduced size memory section since typically: (a) only a small number of stars are simultaneously tracked (e.g., less than or equal to 4), and (b) each star's image lies within, for example, a relatively small square (also denoted “track-box”) of charge collecting cells of the imaging area (i.e., such a track-box is “small” compared to the total size of the imaging area). Thus, by coalescing (or “binning” as it is referred to in the art) rows of image charges that do not intersect such track-boxes, a substantially reduced memory section becomes adequate for the star tracking task. That is, the present invention shifts images from the imaging area into the memory section while concurrently: (a) binning the rows of pixels (denoted also “pixel lines”) not intersecting any track-box, and (b) not binning image area pixel lines that intersect at least one track-box. Moreover, the reduction in the memory section has the added benefit that the charge transfer inefficiency (CTI) of the CCD is reduced in that each charge (i.e., pixel) is transferred a reduced number of times in comparison to a memory section that is substantially the same size as the imaging area.
Additionally, it is an aspect of the present invention to include modules for generating one or more “guard” lines of pixels between each binned pixel line and each non-binned pixel line in the memory section, wherein such guard lines are intended to have relatively little charge therein and therefore function as insulators for inhibiting the charge leakage between binned and non-binned lines in the memory section by providing a place to accumulate dark current and any other background signal charge.
It is a further aspect of the present invention that during the tracking of stars, there are one or more modules provided for both determining a most recent centroid of each star image being tracked, and predicting a subsequent most likely centroid for each such star image. In fact, such predicted centroids are used to determine (the centers of) subsequent track-box positions in a next image integration. Further, note that such track-box prediction modules may require position, orientation and angular rotation rate information from the satellite in which the present invention is incorporated to correctly predict where new star image centroids are likely to be.
When a star tracker according to the present invention is operating

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