STAR SENSOR USING CCD

Abstract: The invention relates to a star sensor for use in the attitude control of satellites. The star sensor comprises a charge-coupled device (12) for detecting the light rays of a star, a filter circuit (22) for filtering the output of said charge-coupled device (12), a zero-cross detector (23), a reference voltage generator (16) and a voltage comperator (24) for detecting the points where the voltage of the output of the filter circuit (22) is equal to a predetermined level, and an A/D converter (19), a multiplexer circuit (20) and a data processor circuit (21) for providing signals representative of the brightness and position of the star. The star sensor requires a buffer memory of small capacity and is capable of accurately determining the central position of the star image.

Description: The present invention relates to a star sensor for use in the attitude control of satellites and, more particularly, to a star sensor using a charge-coupled device (CCD) in its star imaging array. In one of the conventional techniques of using a CCD, as disclosed in the article "Development of a star sensor using CCD" by K. Ninomiya et al., SANE 82-48, pp. 33-40, the output of the CCD, after being amplified, is applied to a sample-and-hold circuit for voltage retention. The sustained voltage is then compared with a reference voltage by a voltage comparator. This reference voltage corresponds to the star magnitude, and the sustained voltage, if found to be above the reference voltage, is subjected to analog-digital conversion and supplied to a data processing circuit At the same time, the picture element (pel) position information (i.e., the coordinates of the star) at this moment is supplied from a coordinate counter, which counts the clock pulses outputted from the CCD driving circuit, to the data processing circuit. The latter circuit stores in a buffer memory the CCD output voltage thereby obtained, which is not below the reference voltage, and its positional information with respect to every pel on the CCD. Then, it picks out the highest among the stored voltages and determines the central position of the star on that basis. Since all the data regarding the pels which exceed the reference value are stored, the conventional star sensor requires a large-capacity buffer memory, which is costly to manufacture and results in added weight. Furthermore, background light rays entering uniformly into the optical system tend to be sensed and affect star image signals adversely, depending on how high the reference voltage of the voltage comparator is set. Star sensors using CCDs are described in G. Borghi et al, "STAR DETECTION AND TRACKING USING CCDs", IFAC Automatic Control in Space , 1982, pp. 289 to 295. The proposed star sensor has a star search mode and a star track mode, and has a great advantage in accuracy, but it does have the following problems: it is complex in circuitry because it requires two sets of hardware, one for the search mode and the other for the track mode, and also involves the need for different software arrangements for the two modes. Furtheremore, this paper in no way touches on the resolution of background light. US-A-4 092 670 discloses a solid state star tracker in which a band-pass filter filters and passes therethrough each CCD output pulse. A level crossing detector senses each output pulse of the band-pass filter in accordance with a predetermined level to provide the predetermined level crossing timings. By averaging these timings, a horizontal or vertical position can be determined. Since the band-pass filter passes the output of each CCD output pulse, the band-pass filter must have a bandwidth which is wide enough to pass all components of the CCD output pulses, except for very low and very high frequency components. An object of the present invention, therefore, is to provide a CCD-based star sensor which requires a buffer memory of small capacity. Another object of the invention is to provide a CCD-based star sensor capable of accurately determining the central position of the star image even if background light rays overlap into the star image signals. These objects are achieved by the star sensor according to the claims. Brief Description of Drawings Other objects, features and advantages of the present invention will become more apparent from the detailed description hereunder taken in conjunction with the accompanying drawings, wherein: FIG. 1 is a schematic block diagram of a conventional star sensor; FIG. 2 outlines the composition of a CCD contained in the detector of the star sensor shown in FIG. 1; FIGS. 3A to 3K are waveform diagrams illustrating the operation of the CCD shown in FIG. 2; FIG. 4 is a block diagram illustrating one preferred embodiment of the star sensor of the invention; FIGS. 5A to 5D are waveform diagrams illustrating the operation of the star sensor shown in FIG. 4; FIG. 6 is a block diagram illustrating another preferred embodiment of the star sensor according to the invention; FIG. 7 is a mapping plan of the CCD shown to describe the star sensor of FIG. 6; FIGS. 8A to 8F are waveform diagrams showing the star sensor of FIG. 6; and FIGS. 9A and 9B are mapping plans of the CCD shown to describe the star sensor of FIG. 6. Detailed Description of Preferred Embodiments Referring to FIG. 1, an optical element 1 focuses rays of light from a star, and supplies them to a detector section 2. Detector section 2 concerts the focused light rays into a focused star image signal, and supplies it to a data processing circuit 3. On the basis of the supplied data, the circuit 3 calculates the brightness and position of the star. The result of this calculation enables the determination and control of the attitude of the satellite. To facilitate understanding of the present invention, the operation of a CCD will be briefly explained with reference to FIGS. 2 and 3A to 3K before describing the invention in more detail. The CCD illustrated in FIG. 2 comprises a photoelectric converter section 4 including a plurality of columns of photoelectric converter elements for converting light rays into an electric charge corresponding to the intensity of the light rays and for storing the charge. A transfer electrode 6 transfers the charge thus developed at section 4 to a vertical transfer register 5 in response to a transfer signal  TG . The charge stored in the transfer electrode 6 is then transferred to a horizontal transfer register 7 in response to transfer clock pulses  V1 and  V2 . The charge fed from the horizontal transfer register 7 is provided to an output gate 8 in response to transfer clock pulses  H1 and  H2 . An output amplifier section 9, which is composed of the diffusion area 10 and field-effect-transistor (FET) amplifier 11, amplifies the charge from the gate 8 and is reset by a reset signal  R . The output V OUT of amplifier section 9 is a PAM signal as shown in FIG. 3K. In the figure, V DR and V D represent drive voltages for FETs. FIG. 3A shows a transfer signal  TG ; FIGS. 3B and 3C show transfer clock pulses  V1 and  V2 ; FIGS. 3D and 3E, the transfer clock pulses  V1 and  V2 of FIGS. 3B and 3C on an enlarged scale; FIGS. 3F and 3G, the transfer clock pulses  H1 and  H2 ; FIGS. 3H and 3I, the transfer clock pulses  H1 and  H2 of FIGS. 3F and 3G on an enlarged scale; FIG. 3J, the reset signal  R ; and FIG. 3K, the output voltage V OUT . Referring to FIG. 4, the output signal V₁ of a CCD 12 (See FIG. 5A), after being amplified by an amplifier circuit 13, is supplied to a sample-and-hold circuit 14 to become a signal V₂ (FIG. 5B). The signal V₂ is then caused to pass through a fixed delay circuit 25 and is converted into a digital signal at an A/D converter 19 controlled by a voltage comparator 24, which will be described below. The digitized signal is supplied to a multiplexer circuit 20 together with the positional information of the pels at the time this signal was obtained, i.e., the coordinate (position relative to the axis of light beam) information of the star obtained by counting with a clock counter 18 clock pulses from a CCD driver circuit 17. The multiplexer circuit 20 provides in multiplexed form the positional information of pels supplied from the clock counter circuit 18 and the digital signal supplied from the A/D converter circuit 19. The multiplexed signal is then fed to an on-board data processor circuit 21 at the next stage, wherein prescribed processing is achieved. Next, a characteristic part of the present invention will be described. A filter circuit 22 comprises a low-pass filter (LPF) and a high-pass filter (HPF) (neither illustrated). From the output signal V₂ coming from the CCD 12 via the sample-and-hold circuit 14, first the LPF provides a signal V₂₁ shown in FIG. 5B. This signal is then applied to the HPF, which has a characteristic such as to produce the signal V₃ shown in FIG. 5C. The latter is supplied to a zero-crossing detector circuit 23, which detects the zero-crossing point of the output V₃ shown in FIG. 5C and supplies an output V₄ shown in FIG. 5D to the voltage comparator 24. As is evident from the figures, the zero-crossing point of V₃ is at the trailing edge of the pel manifesting the highest output. It is arranged that the amplitude of the output V₄ is determined by the output of CCD 12. The voltage comparator 24 for comparing the output V₄ with a reference voltage from a generator 16, supplies the result of comparison as a control signal to both the A/D converter 19 and the clock counter circuit 18. The reference voltage corresponds to the predetermined magnitude of a star, and the comparator 24 will output the control signal if a signal of a voltage above the reference voltage is inputted from the zero-crossing detector 23. Thus, the A/D converter 19 and the clock counter circuit 18 are triggered only when the output from the CCD 12 is above the reference voltage. Upon triggering, the output of the A/D converter 19 is a signal delayed by, for example, three bits by the delay circuit 25, i.e., has the peak value of the output V₂ (FIG. 5). Therefore, the data processor circuit 21 is supplied by the A/D converter 19 with the digitized peak value of one horizontal sequence of pels of the CCD image and by the clock counter circuit 18 with positional information behind the peak value by the delay effected by the delay circuit 25. This positional information can be made the information of the peak value by simply counting down or up by the delay the value supplied from the counter circuit 18. Receiving a peak value and its positional information from every one of the horizontal and vertical sequences of pels, the data processor circuit 21 accurately determines the brightness and position of the star. The quantity of data to be memorized need not be greater than one each of horizontal and vertical sequences around the star image on the CCD. In other words, it is sufficient to store, for instance, 10 pieces of data as in FIGS. 9A and 9B, which will be described later. Furthermore, the control signal from the comparator circuit 24 is generated on the basis of a signal having passed through the filter circuit 22. Therefore, the star image detection is not adversely affected by background light or the dark current fluctuation of the CCD. Background light rays, which are uniformly superimposed on the desired signal, for instance like the dotted lines in FIG. 5A, are removed by the function of the HPF in the filter circuit 22. The dark current fluctuation is also eliminated by the filter circuit 22. Referring now to FIG. 6, a clock counter circuit 118 and an A/D converter 119, unlike their respective counterparts in the embodiment illustrated in FIG. 4, sequentially supplies the star image data and the positional information from time to time, both received from a CCD 12, to a data processor circuit 121 by way of a multiplexer circuit 20. The data processor circuit 121 memorizes a column or a row from the horizontal or vertical sequences of pels of the CCD 12, executes on the basis of this data the same algorithm as the filter circuit 22 and the zero-crossing detector 23 in the embodiment shown in FIG. 4, and thereby detects the brightness and position of the star image. Next, the operation of the star sensor of FIG. 6 will be described in detail with reference to FIG. 7 and 8A to 8F. In FIG. 7, which illustrates the image of a star formed on the CCD 12, the addresses of pels in the horizontal (H) direction are designated as H L , H L+1 , H L+2 and so on, and those in the vertical (V) direction, as V K , V K+1 , V K+2 and so on. It is seen that the center of the star image has the address of (H L+2 , V K+2 ). The output level of each pel is represented by S K,L and so forth. The position and brightness of the star to be determined are the values at (H L+2 , V K+2 ) and S K+2, L+2 , respectively. They are obtained in the following flow: (1) The data are edited on the V K line. Thus, the addresses and outputs of pels in FIG. 7 are rearranged as in FIG. 8A. If the input has a uniform background, it is represented by an increase or a decrease of the offset from the "0" level shown in FIG. 8A. (2) Data are interpolated into the V K line. Thus, as shown in FIG. 8B, the rearranged pels are interpolated between them to provide a suitable function. (3) The function interpolated into the V K line is transformed into a frequency domain by fast Fourier transform (FFT). Thus, as shown in FIG. 8C, there are obtained in frequency domain a fundamental, harmonics and D.C. components corresponding to a uniform background. (4) Filtering is achieved in frequency domain. Thus, filtering is so effected as to pass only the fundamental frequency component shown in FIG. 8C. The frequency characteristic of filtering is shown in FIG. 8D. The output after the filtering is illustrated in FIG. 8E. (5) The output filtered with respect to the V K line is transformed into time domain by inverse fast Fourier transform (IFFT). Thus, as shown in FIG. 8F, the waveform in time domain is obtained. (6) The center of L is determined from the waveform transformed into the time domain. Thus, the zero-crossing point is determined from the waveform of FIG. 8F, and the central position of L is determined. The address L+2 and the level S K, L+2 are memorized. The processing of the V K line has been described above. The V K+1 , V K+2 .... lines are similarly processed. FIG. 9A shows the result of the processing of the V K , V K+1 .... lines. Next, the processing illustrated in FIGS. 8A to 8F is applied to the H L , H L+1 .... column in the same way as to the V lines to obtain the result shown in FIG. 9B. On the basis of the crossing points of the broken lines in the results thereby obtained, shown in FIGS. 9A and 9B, the center of the star image focused on the CCD is determined, as illustrated in FIG. 7. The level at this address is also read out.

Claims: A star sensor comprising CCD means (12) comprising an array for detecting the light rays of a star and for producing output signals representative of the intensity distribution of said light rays at various points along a column and/or row of said CCD means (12); means (18) for counting clock pulses with which said CCD means (12) is driven; means (22) for filtering said output signals of said CCD means (12) in the frequency domain to produce a signal representative of the envelope component of the pattern of the output signals along a column and/or row of said CCD means (12), said envelope component having a first peak value and crossing a zero volt level; means (23) for detecting the first peak value and the zero-crossing point of said envelope component to produce a detected signal representative of said first peak value at said zero-crossing point; and computing means (21, 19, 24, 25) responsive to said detected signal output and to the output of said counting means (18) for providing signals representative of the brightness and position of said star. A star sensor as claimed in Claim 1, wherein said filtering means (22) comprises a low-pass filter for providing a low frequency component of said output of said CCD means (12) and a high-pass filter for providing a high frequency component of said output of said CCD means (12). A star sensor as claimed claim 1, wherein said counting means comprises means (18) for counting said clock pulses to provide a count result responsive to a control signal, and wherein said computing means comprises: comparator means (24) for comparing the output of said filtering means (22) with a reference voltage to provide said control signal to said counter means (18) when the output of said detecting means (23) exceeds said reference voltage; peak value supplying means (25, 19) for digitizing the output of said CCD means (12) to provide a second peak value digitized output in response to the output of said detecting means (23); and data processor means (21) for calculating the brightness of said star from said second peak value and the position of said star from said count result of said counter means (18). A star sensor as claimed in Claim 3, wherein said peak value supplying means comprises means (25) for delaying the output of said CCD means (12) by a prescribed number of bits and analog-to-digital converter means (19) for digitizing the delayed output of said CCD means (12) and supplying the digitized output to said data processor means (21) in response to the output of said detecting means (23). A star sensor as claimed in Claim 4, further including, between said CCD means (12) and said delaying means (25), means (13) for amplifying the output of said CCD means (12) and sample-and-hold circuit means (14) for sampling and holding the output of said amplifying means (13) for supply to said delaying means (25) and to said filtering means (22).

Inventors: YOKOTA, TOYOHACHI
Applicants: NEC CORPORATION
Application year: 1984
Main classification: G01S3
IPC codes: G01C21, G01S3, H04N7, G01C1