EA026970B1 - Method of improving accuracy of determining celestial orientation and prolonged maintenance of high accuracy of determining orientation and apparatus therefor - Google Patents

Abstract

The invention relates to space navigation and can be used for prompt and accurate determination of a spacecraft in respect of the inertial coordinate system. A method for increasing the precision of orientation by stars consists in projecting an image of the stars, via an optical system, onto a matrix radiation receiver occupying an area of at least 2×2 pixels, and determining the position of the weighted center of an image with consideration for individual characteristics of the pixels in the area occupied by the image using a computer including an amount of read-only memory containing an on-board catalogue of navigation stars and an additional amount of read-only memory to store individual characteristics of the pixels of the matrix radiation receiver. Data regarding individual pixel characteristics are updated periodically using a sensor, by means of performing calibration which involves blocking the light from the optical system using an opaque shutter controlled by a shutter control device, and uniformly illuminating the matrix radiation receiver by a calibrating illuminator. The characteristics of individual pixels obtained are saved in the additional amount of the read-only memory. The opaque shutter installed between the optical system and the matrix radiation receiver consists of a rocker in the form of an aperture-shielding blade having at least one magnet and at least one actuating solenoid that are embedded in the rocker. The invention permits increasing the precision of determining orientation, and maintaining the precision over a long period of time in the process of the functioning of the sensor.

Description

(57) The invention relates to space navigation and can be used for prompt accurate determination of the orientation of the spacecraft relative to the inertial coordinate system. A way to increase the accuracy of determining the orientation by stars is to project the image of stars through an optical system onto a matrix radiation detector occupying a region of 2> <2 pixels and determine the position of the weighted center of the image of stars taking into account individual characteristics of pixels in the region occupied by the image using a computing device , including the amount of permanent memory containing the on-board catalog of navigation stars, as well as an additional amount of permanent memory for individual storage x characteristics of the pixels of the matrix radiation receiver. The data on the characteristics of the individual pixels of the matrix radiation detector are updated from time to time using the sensor itself by performing a calibration in which the light from the optical system is blocked by an opaque shutter using a shutter control device, as well as under conditions in which the matrix radiation detector is uniformly illuminated by a calibration illuminator. Moreover, the obtained characteristics of individual pixels are stored in an additional amount of read-only memory. The light-tight shutter installed between the optical system and the radiation matrix detector consists of a rocker in the form of an aperture-shielding lobe with at least one magnet embedded in the rocker and at least one actuating solenoid. The invention improves the accuracy of determining the orientation and maintain this accuracy for a long time in the process of functioning of the sensor.

FIELD OF THE INVENTION

The invention relates to space navigation and can be used for operational accurate determination of the orientation of the spacecraft relative to the inertial coordinate system.

State of the art

In astronomy, methods for processing scientific astronomical images obtained using matrix radiation detectors have been worked out and are used [1] (see But \\ e11 δ.Β. Naibook oG SSI a81goiot / 2-pb eOSatbpbde okegushd Laibookk Gog teektsy Arpotsp. Satbpds. IR: Satbbds iyuegyu Rg88, 20 06 ΙδΒΝ 0521852153). These methods take into account the pixel-by-pixel inhomogeneity of the dark (thermal) currents and the pixel-by-pixel inhomogeneity of the sensitivity of the radiation matrix detector.

In astrometry (the section of astronomy that deals with determining the coordinates of astronomical sources in the celestial sphere), methods have been worked out for determining the coordinates of point radiation sources (stars) obtained from matrix radiation detectors with an error much smaller than the pixel size of a matrix radiation detector [2] (see My! Ο .Ό. 1p1gobisyop 1o SSI a51gote1gu // LZR SoiGepese Zepe ^ V. 23, P. 221-233, 1992), [3] (Kovalevsky Jean. Modern astrometry: transl. From English. - 478 s).

The necessary parameters of the matrix radiation detector are determined by conducting its laboratory research or by obtaining special images (frames) before starting scientific observations, and image correction is not required, because with an error of the order of a pixel or slightly less, the position of the star is also determined by the uncorrected image of the sky.

The use of a high-precision method for determining the positions of images ([1] - [3]) on an unadjusted image will lead to the appearance of significant systematic errors in these positions (i.e., the shift of the measured image of the star from the true one). Ultimately, these errors reduce the accuracy of determining the orientation.

The closest analogue of the claimed invention is a method of measuring the angular coordinates of stars [4] (see patent KN 2408849, IPC С01С 21/24, published January 10, 2011), which includes projecting onto a matrix photodetector through an image lens a portion of the starry sky, measuring the linear coordinates of the centers images of stars on the site of the photodetector, projection of calibration marks formed by the collimator of the channel of the geometric standard and rigidly connected to the base instrument coordinate system onto the matrix photodetector, measurement of linear coordinates the center of the image of the calibration marks on the surface of the photodetector and the recalculation of the linear coordinates of the centers of the images of stars in angular coordinates. The method is implemented by a device for measuring angular coordinates, comprising a lens hood, a lens, an array photodetector, and a computing unit.

The disadvantage of these devices is the low accuracy of determining the orientation, caused primarily by the heterogeneity of the characteristics of the pixels of the matrix radiation detector, as well as by the subpixel sizes of the images of stars, moreover, image correction and high-precision determination of the centers of image of stars are not used here.

SUMMARY OF THE INVENTION

The problem solved by the claimed invention is to increase the accuracy of determining the orientation parameters by stellar orientation sensors and maintaining improved accuracy while reducing the parameters of the optical system and the matrix radiation detector of the star sensor.

The technical result of the invention is to increase the accuracy of determining the orientation and maintaining this accuracy for a long time in the process of functioning of the sensor.

The specified technical result is achieved by constructing on the focal plane images of stars occupying several pixels of the matrix image receiver. Such images can be obtained due to the design of the optical system (lens) of the sensor or by defocusing the images of stars. To maintain accuracy during the operation of the sensor, flight calibrations are performed. A way to improve the accuracy of determining the orientation by stars is to determine the coordinates of the centers of images of a group of stars on the focal plane, compare these stars with navigation stars from the on-board catalog and calculate the orientation parameters of the instrument coordinate system of the sensor relative to the inertial coordinate system based on the celestial coordinates of the stars from the catalog and coordinates images on the focal plane for images of stars identified with stars in the catalog, while on the matrix receiver images located in the focal plane of the optical system of the sensor, images of stars are created, occupying an area of at least 2x2 pixels, and determining the position of the weighted center of the image of the star, taking into account the individual characteristics of the pixels in the region of the matrix receiver of radiation occupied by the image, which are pre-determined and stored in an additional volume constant star sensor orientation.

Moreover, the method of long-term maintenance of increased accuracy of determining the orientation by

- 1,026,970 stars in the operational flight, consists in regularly determining the individual characteristics of the pixels of the matrix radiation receiver and storing them in an additional amount of permanent memory of the stellar orientation sensor, while the data on the characteristics of the individual pixels of the matrix radiation receiver are updated from time to time using the sensor itself taking measurements in modes in which the light from the optical system is blocked by an opaque shutter using a shutter control device m, as well as under conditions in which the matrix radiation detector is uniformly illuminated by a calibration illuminator (light source), while the obtained characteristics of the individual pixels of the radiation matrix detector are stored in an additional volume of the sensor’s permanent memory. The individual characteristics of the pixels are sensitivity, dark current in pixels, gain, read noise, etc.

The proposed device for determining the orientation by stars, which implements the above methods, contains an optical system, a matrix radiation receiver and a control unit, while the control unit is equipped with a computing device (microprocessor), the amount of read-only memory containing the on-board catalog of navigation stars, as well as an additional amount of read-only memory for storing individual characteristics of the pixels of the matrix radiation receiver; control devices for an opaque shutter and a calibration illuminator. The positions of the images of stars on the matrix radiation detector are defined as the weighted centers of the images of stars taking into account the individual characteristics of the pixels of the matrix radiation detector, which are stored in an additional amount of permanent memory of the device control unit, and an opaque shutter is installed between the optical system and the radiation matrix receiver for calibrations in the modes in which the light from the lens is blocked by an opaque shutter using a shutter control device, p Ed matrix radiation receiver installed calibration illuminator for illumination of a measurement modes matrix radiation receiver gauge illuminator.

For the purpose of calibration, a shutter is used, consisting of a rocker mounted on the axis, in the form of an aperture-shielding lobe with at least one permanent magnet embedded in the rocker and at least one actuating solenoid interacting with at least one a permanent rocking magnet so that when the voltage is applied to the solenoid, its polarity is opposite to the polarity of the permanent magnet, the magnet is repelled from the solenoid and the shutter overlaps the aperture, and when the solenoid is not energized magnet is attracted to the solenoid valve core and all remained in the open position.

Brief Description of the Drawings

FIG. 1 is a diagram of a stellar orientation sensor.

FIG. 2 - image of the dark current in pixels in the CMOS matrix.

FIG. 3 is a shutter view from the side of the optical system.

FIG. 4 is a view of the shutter from the side of the matrix radiation receiver.

Disclosure of invention

Items shown in the drawings:

- radiation from stars (light from stars);

- optical system (lens);

- matrix radiation receiver;

- computing device (microprocessor);

- the amount of permanent memory containing the on-board catalog of navigation stars;

- an additional amount of permanent memory containing the characteristics of the individual pixels of the matrix radiation receiver;

- lightproof shutter (curtain);

- control device opaque shutter;

- built-in calibration illuminator (light source);

- solenoid;

- pixel on a matrix radiation receiver;

- rocking chair;

- a magnet.

In this invention, a technique is used for correcting images obtained on a matrix radiation detector when projecting an image of stars through an optical system. For this, the image size of a point radiation source (star) on the radiation matrix detector should exceed the pixel size (11). The optimal situation is when a significant signal from a point source is contained in several pixels of a fragment of a matrix receiver with a size of 2x2 or 3x3 pixels. The positions of the weighted center of the image of the stars are determined taking into account the individual characteristics of the pixels in the region of the matrix radiation detector of the occupied image, which are previously determined and stored in an additional permanent memory of the stellar orientation sensor.

When tuning a stellar orientation sensor on the earth and using it in space due to the action of a stream of high-energy charged particles (ionizing radiation) in the matrix radiation detector (3), individual pixel characteristics (11) change, i.e. the sensor is calibrated, which reduces the accuracy of the readings. The dark current increases approximately 100 times in 1/2 year of the sensor being in the radiation belt. In this case, the map of dark currents completely changes under the influence of radiation. The sensitivity of the pixels also changes, but more weakly. In a long flight, in order to maintain the accuracy of determining the orientation, it is necessary to periodically update the map of the parameters of the pixels of the matrix radiation detector (3).

This disadvantage is eliminated in the claimed invention due to the calibration of the sensor in flight, and the new pixel characteristics of the matrix radiation receiver (3) obtained as a result of the calibration are stored in an additional memory (6).

The total increase in the dark current and related noise can be reduced by cooling the matrix radiation detector, since dark currents decrease by about 2 times with a decrease in temperature by 5 ° C [5] (SSE47-20 Vask 111sh1pa1ey Ηί§1ι RegTogshaise ΑΙΜΟ Vask ShshshpaUA SSE 8ey8og, e2u 1ess11pooshche5 1is., Α1Α-100041 Ι88. 6, 2006). The matrix radiation receiver can be cooled, for example, using a thermoelectric refrigerator (Peltier element) mounted on the matrix receiver from below.

During flight calibration, a light-tight shutter (7) is used, which blocks the radiation flux (1) from the lens (optical system) (2), incident on the radiation matrix detector (3), for the duration of the calibration measurements.

Performing calibrations with the lens open is impossible due to the influence of radiation from stars slightly weaker than the observation threshold. Therefore, a light-tight shutter (7) is provided in the star sensor for calibration.

To measure the sensitivity coefficients of pixels, the matrix radiation detector (3) is illuminated by a uniform radiation flux from an internal calibration light source (calibration illuminator) (9). To obtain a uniform radiation flux, its scattering on the inner surface of the closed gate can be used.

For calibration, a shutter was used, consisting of a rocker (12) on the axis in the form of an aperture-shielding lobe with at least one permanent magnet (13) embedded in the rocker and at least one actuating solenoid (10) interacting with the permanent magnet of the rocker in this way so that when the voltage is applied to the solenoid, its polarity is opposite to the polarity of the permanent magnet, the magnet repels from the solenoid and the shutter overlaps the aperture, and when the solenoid is not energized, the magnet is attracted to the core of the solenoid and then the thief remained normally open all the time.

An important additional requirement for the shutter is to return it to the open position if the sensor malfunctions (for example, when the power is turned off). The sensor with an open but not working shutter will continue to function, although after a while its accuracy will decrease due to the inability to conduct flight calibrations. With the shutter closed, the sensor cannot function.

In modern stellar sensors, the described methods for image correction and high-precision determination of the centers of image of stars are not applied.

An additional memory unit (6) is included in the sensor electronics unit for storing the pixel-by-pixel characteristics of a particular instance of a stellar sensor radiation detector array. These characteristics include dark (thermal) pixel currents and the ratio of the photosensitivity of the pixel to the average (nominal) value.

These data are used to correct the image obtained in the matrix radiation detector of the star sensor by subtracting from the raw image the average level of dark signals for each pixel reduced to the current temperature of the matrix radiation detector. After that, the individual pixel sensitivity is taken into account by multiplying by the coefficients from a map containing sensitivity inhomogeneities. Data on dark currents and light sensitivity of individual pixels (dark current map and sensitivity inhomogeneity coefficients) are stored in additional memory of the star sensor.

The implementation of the invention

The stellar orientation sensor operates as follows.

The images of stars are projected onto the site of the matrix radiation detector (3) through the optical system (2). Based on the images obtained, the control unit calculates the orientation parameters of the spacecraft. The control unit consists of a computing device (microprocessor) (4); the amount of permanent memory (5) containing the on-board catalog of navigation stars and an additional volume of permanent memory (6) for storing individual characteristics of the pixels of the matrix radiation receiver. To determine the orientation parameters of the spacecraft, the microprocessor determines the coordinates of the centers of the images of a group of stars on the focal plane, compares these stars with the navigation stars from the on-board catalog and calculates the orientation parameters of the instrument coordinate system of the sensor relative to the inertial coordinate system based on celestial

- 3,026,970 coordinates from the catalog and coordinates of images on the focal plane for images of stars identified with stars in the catalog. When projecting images of stars on a radiation matrix detector, images of stars are created that occupy an area of at least 2x2 pixels (i.e., they create a defocused image).

The position of the centers of the image of stars is determined taking into account the individual characteristics of the pixels (sensitivity, dark current in pixels, gain, read noise), which are updated from time to time using the sensor itself, and the data obtained is stored in an additional amount of read-only memory during calibration.

Calibration includes taking measurements in modes in which the light from the optical system (2) is blocked by an opaque shutter (7) using a shutter control device (8). In this case, the matrix radiation detector is uniformly illuminated by a calibration illuminator (9).

The calibration illuminator consists of a rocker (12) in the form of an aperture-shielding lobe with at least one permanent magnet (13) embedded in the rocker and at least one actuating solenoid (10). When voltage is applied to the solenoid, its polarity is opposite to the polarity of the permanent magnet, the magnet repels from the solenoid and the shutter closes the aperture, and when the solenoid is not energized, the magnet is attracted to the core of the solenoid and the shutter remains open all the time, due to which the sensor remains in working condition even with the idle solenoid but without calibration.

Claims (10)

1. The method of determining the orientation by stars, which consists in projecting the image of stars through the optical system onto a matrix radiation detector, determining the coordinates of the centers of the images of a group of stars on the focal plane, comparing these stars with navigation stars from the on-board catalog and calculating the orientation parameters of the instrument coordinate system of the sensor relative to the inertial coordinate systems based on celestial coordinates from the catalog and the coordinates of images on the focal plane for images of stars, after associated with the stars in the catalog, characterized in that for the images of stars occupying an area of at least 2x2 pixels on the matrix radiation detector, the positions of the weighted center of the star image are determined taking into account the individual characteristics of the pixels in the region of the matrix radiation detector of the occupied image, which are previously determined and stored in additional amount of permanent memory of the sensor of stellar orientation. 2. The method according to claim 1, characterized in that the individual characteristics of the pixels are sensitivity, dark current in pixels, gain, read noise. 3. The method according to claim 1 or 2, characterized in that the data on the characteristics of the individual pixels of the matrix radiation detector are periodically updated using the sensor itself by measuring in a mode in which the light from the optical system is blocked by an opaque shutter using a shutter control device, and in the mode in which the matrix radiation detector is uniformly illuminated by a calibration illuminator, while the obtained characteristics of the individual pixels of the matrix radiation detector are stored additional volume sensor permanent memory. 4. The method according to claim 3, characterized in that for the purpose of calibration, a lightproof shutter is used, consisting of a rocker in the form of an aperture-shielding lobe with at least one permanent magnet embedded in the rocker and at least one actuating solenoid interacting with the permanent magnet rocking in such a way that when voltage is applied to the solenoid, its polarity is opposite to the polarity of the permanent magnet, the magnet is repelled from the solenoid and the shutter closes the aperture, and when the solenoid is not energized a magnet is attracted to the solenoid core and the shutter stays open all the time. 5. The method according to claim 3, characterized in that the individual characteristics of the pixels are sensitivity, dark current in pixels, gain, read noise. 6. The method according to claim 3, characterized in that to reduce the average level of the dark signal, the matrix radiation detector is cooled using a thermoelectric refrigerator (Peltier element). 7. A device for implementing the method according to claims 3-6, comprising an optical system, a matrix radiation detector and a control unit, characterized in that the device control unit comprises a computing device (microprocessor); the amount of read-only memory containing the on-board catalog of navigation stars, as well as an additional amount of read-only memory for storing individual characteristics of the pixels of the matrix radiation receiver; a control device for an opaque shutter and a calibration illuminator, while the control unit is configured to determine the position of the weighted centers of the images of stars taking into account the individual characteristics of the pixels of the matrix radiation receiver and record these values in an additional permanent memory of the device control unit; a lightproof shutter located between the optical system and the matrix radiation detector for calibrations in modes in which the light from - 4 026970 lenses are blocked; a calibration illuminator located in front of the matrix radiation detector for performing measurements in the lighting regimes of the matrix radiation detector by a calibration illuminator. 8. The device according to claim 6, characterized in that the lightproof shutter consists of a rocker mounted on an axis, in the form of an aperture-shielding lobe, with at least one permanent magnet mounted in the rocker and at least one actuating solenoid interacting with the permanent magnet . 9. The device according to claim 6, characterized in that the individual characteristics of the pixels are sensitivity, dark current in pixels, gain, read noise. 10. The device according to claim 6, characterized in that the optical system is made in the form of a lens.