WO2016076748A1

WO2016076748A1 - Method and device for increasing precision of orientation by the stars

Info

Publication number: WO2016076748A1
Application number: PCT/RU2014/000877
Authority: WO
Inventors: Марат Керимович АБУБЕКЕРОВ; Андрей Игоревич ЗАХАРОВ; Михаил Евгеньевич ПРОХОРОВ; Олег Юрьевич СТЕКОЛЬЩИКОВ
Original assignee: Общество С Ограниченной Ответственностью "Азмерит"
Priority date: 2014-11-14
Filing date: 2014-11-18
Publication date: 2016-05-19
Also published as: RU2585179C1; DE112014007166T5; EA026970B1; EA201401149A1

Abstract

The invention relates to space navigation. A method for increasing the precision of orientation by the stars consists in projecting an image of the stars, via an optical system, onto a matrix radiation receiver. Images of stars occupy an area of at least 2x2 pixels. Taking into consideration the individual characteristics of the pixels, the position of the weighted center of an image is determined. Data regarding individual pixel characteristics is updated periodically using a sensor, by means of performing a calibration which involves blocking light from the optical system using an opaque shutter controlled by a shutter control device, and uniformly illuminating the matrix radiation receiver using a calibrating illuminator. The opaque shutter is installed between the optical system and the matrix radiation receiver. The shutter consists of a rocker in the form of an aperture-shielding blade having a magnet, which is embedded in the rocker, and an actuating solenoid. The technical result consists in increasing the precision of determining orientation, and maintaining precision over a long period of time in the process of the functioning of the sensor.

Description

METHOD AND DEVICE FOR INCREASING ACCURACY ACCURACY OF STARS

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 developed and are used [1] (see Howell SB Handbook of CCD astronomy / 2-ed. // Cambridge observing handbooks for research astronomers, Vol. 5 Cambridge, UK : Cambridge University Press, 2006 ISBN 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 for determining the coordinates of point radiation sources (stars) obtained from matrix radiation detectors are developed with an error much smaller than the pixel size of a matrix radiation detector [2] (see Monet GD Introduction to CCD astrometry // ASP Conference Series, V. 23, P. 221-233, 1992), [3] (Kovalevsky Jean. Modern astrometry: transl. from English // Fryazino: Volume-2, 2004 - 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 for measuring the angular coordinates of stars [4] (see patent RU 2408849, IPC G01C21 / 24, published January 10, 2011), which includes projecting a portion of the starry sky onto an image sensor through an image lens, measuring the linear coordinates of image centers stars on the photodetector site, projection of calibration marks formed by the collimator of the geometric standard channel and rigidly connected to the base instrument coordinate system onto a matrix photodetector, measurement of linear coordinates atomic centers of the calibration image marks on the surface of the photodetector and converting the linear image centers coordinates star 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 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 system based on the celestial coordinates of the stars from the catalog and the coordinates of the images on the focal plane for images of stars identified with the stars in the catalog, while on the radiation matrix detector located in the focal plane of the optical system of the sensor, images of stars are created that occupy an area of at least 2><2 pixels, and the definition 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 radiation detector of the occupied image, which are previously determined and stored in an additional amount of permanent memory of the stellar orientation sensor.

Moreover, a method for long-term maintenance of increased accuracy in determining the orientation by stars in operational flight consists in regularly determining the individual characteristics of the pixels of the matrix detector of radiation and storing them in an additional amount of permanent memory of the sensor of stellar orientation, while data on the characteristics of the individual pixels of the matrix receiver of radiation, from time is updated using the sensor itself by taking measurements in modes in which the light from the optical system They are closed with an opaque shutter using a shutter control device, and also 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 amount 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; device s control of the opaque shutter and the 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 sensor for stellar orientation

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

FIG. 3 - view of the shutter from the side of the optical system

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

Disclosure of invention

Items shown in the drawings:

1 - radiation from stars (light from stars);

2 - optical system (lens);

3 - matrix radiation receiver;

4 - computing device (microprocessor);

5 - the amount of permanent memory containing the on-board catalog of navigation stars; 6 - an additional amount of permanent memory containing the characteristics of the individual pixels of the matrix radiation receiver;

7 - lightproof shutter (curtain);

8 - control device opaque shutter;

9 - built-in calibration illuminator (light source);

10 - solenoid;

1 - pixel on a matrix radiation detector;

12 - a rocking chair;

13 - 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 must exceed the pixel size (1 1). 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 setting up a stellar orientation sensor on the earth and using it in space, due to the influence of a stream of high-energy charged particles (ionizing radiation) in the matrix radiation receiver (3), individual pixel characteristics (1 1) change, i.e. the sensor is calibrated, which reduces the accuracy of the readings. The dark current increases approximately 100 times in 1/2 years 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, to preserve 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 calibration of the sensor in flight, and the resulting calibration new characteristics of the pixels of the matrix radiation detector (3) 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 when the temperature decreases by 5 ° C [5] (CCD47-20 Back Illuminated High Performance AIMO Back Illuminated CCD Sensor, e2v technologies inc., A1A-100041 Iss. 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 rockers 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, the magnet is attracted to the core of the solenoid and atvor always remained “normally open”.

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). A sensor with an open but not working shutter will be used continue to function, although after a while its accuracy will decrease due to the impossibility of 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 radiation matrix 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

Stellar orientation sensors function 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 sensor relative to the inertial coordinate system based on the celestial coordinates from the catalog and the coordinates of the 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 pushes away 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 operation even with the solenoid inoperative but without calibration.

Claims

CLAIM

1. A method of increasing the accuracy 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 sensor coordinate system relative to the inertial coordinate system based on the celestial coordinates from the catalog and the coordinates of the images on the focal plane for the image the magnification of stars identified with the stars in the catalog, characterized in that on the matrix radiation detector create images of stars occupying an area of at least 2x2 pixels, and determine 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 radiation detector of the occupied image, which determine preliminary and save in an additional amount of constant memory of the sensor stellar orientation.

2. The method according to p. 1, characterized in that the individual characteristics of the pixels are sensitivity, dark current in pixels, gain, read noise.

3. A method for long-term maintenance of increased accuracy in determining the orientation by stars in operational flight, which consists in regularly determining the individual characteristics of the pixels of the matrix detector of radiation and storing them in an additional amount of permanent memory of the sensor of stellar orientation, characterized in that the data on the characteristics of the individual pixels of the matrix receiver of radiation, from time to time they are updated using the sensor itself by measuring in modes in which the light from the optical The systems are covered with an opaque shutter using a shutter control device, and also under conditions in which the matrix radiation detector is uniformly illuminated by a calibration illuminator, 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.

4. The method according to p. 3, characterized in that for the purpose of calibration use a lightproof shutter consisting of a rocker in in the form of an aperture-shielding petal with at least one permanent magnet embedded in the rocker and at least one actuating solenoid interacting with the permanent rocking magnet in such a way 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 the shutter remains open all the time.

5. The method according to p. 3, characterized in that the individual characteristics of the pixels are sensitivity, dark current in pixels, gain, read noise.

6. The method according to p. 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 determining the orientation by stars, which implements a method for increasing the accuracy of determining the orientation by stars according to claim 1 and a method for long-term maintenance of an increased accuracy of determining the orientation by stars in an operational flight according to claim 3, comprising an optical system, a matrix radiation detector, and a control unit, characterized in the fact that the control unit of the device contains 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 read-only 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

Yu a calibration illuminator is installed to perform measurements in the lighting modes of the matrix radiation receiver by a calibration illuminator.

8. The device according to p. 6, characterized in that the lightproof shutter consists of a rocker mounted on the axis, in the form of an aperture shielding lobe, with at least one permanent magnet and at least one actuating solenoid mounted in the rocker, interacting with a 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 p. 6, characterized in that the optical system is made in the form of a lens.