On-orbit data processing method for satellite-borne soft X-ray single-photon imaging camera

Abstract

The invention discloses an on-orbit data processing method for a satellite-borne soft X-ray single-photon imaging camera, and belongs to the technical field of X-ray pulsar detection imaging. The method comprises the following steps: carrying out dark field background correction on an original image of soft X-ray single photons received by a satellite-borne area array detector; performing line-by-line retrieval on the image by adopting a threshold value method, and when a DN value of a detected pixel exceeds a lower limit threshold value and does not exceed an upper limit threshold value, detecting the pixel and extracting a DN value of a peripheral n * n region and a pixel coordinate value as an effective single photon event; and after the full-image retrieval is finished, integrating the imaging auxiliary data with all the single-photon event data in the dynamic memory to form an event frame image. And finally, carrying out data transmission on the original image and a plurality of event frames to realize effective detection efficiency under the minimum bandwidth.

Description

Technical Field

[0001] The invention relates to an on-orbit data processing method for a satellite-borne soft X-ray single-photon imaging camera, belonging to the technical field of X-ray pulsar detection imaging. Background Art

[0002] Pulsars are relics of the evolution, collapse, and supernova explosions of massive stars. Pulsars generate signal radiation in the radio, infrared, visible light, ultraviolet, X-ray, and gamma-ray bands. Pulsars that radiate signals in the X-ray band are usually called X-ray pulsars. X-rays are high-energy photons that concentrate most of the radiation energy of pulsars. They are easy to detect and process with miniaturized equipment, but difficult to pass through the Earth's dense atmosphere. Therefore, they can only be observed in space outside the Earth's atmosphere. X-ray pulsar detection is mainly used in astrophysics research, pulsar navigation research, and space time scale benchmark technology research.

[0003] Starting with the ARGOS satellite in 1999, major economies around the world have successively carried out on-orbit X-ray pulsar detection activities. The latest eROSITA satellite was jointly developed by Germany and Russia and launched in 2019. Its observation energy range is 0.5keV~10keV, and its core sensor is pn-CCD. In July 2018, my country launched Nanjing University No. 1, which carries a lobster-eye X-ray camera for on-orbit X-ray detection.

[0004] The Crab pulsar is the pulsar with the largest radiation flux known so far, and it also requires the highest time resolution. The X-ray energy flux density of the pulsar PSR B0531+21 detected in low-Earth orbit is λ p =1.54×10 4 ph / m 2 / s, the background radiation intensity is about ten times the X-ray energy flux density of the pulsar radiation, λ n =1.54×10 5 ph / m 2 / s.

[0005] The detection area (optical aperture) is 0.1m 2 For an optical system with a detection efficiency of 10%, the number of received pulsar photons is 154 and the number of background photons is 1540. The photons reach the image plane and are received by n detectors, and their distribution conforms to the discrete uniform distribution. The number of photons received by each detector within 1 second is approximately For example, taking a 1K×1K pixel detector, when working at a frame rate of 1fps, due to the focusing effect, the photons are focused into the central 2K pixels, and each pixel receives an average of 0.85 photons per frame, so it is called X-ray single photon detection. A single X-ray photon will bombard an electron cloud in the silicon-based detector, which is collected and amplified by the detector to generate an imaging signal. One electron is generated for every 3.65eV of energy, so the number of photogenerated electrons generated by 0.2~10keV (6.2~0.124nm) is: 54~2739e-, that is, X-ray photons of different wavelengths appear as different DN values ​​on the image. The stronger the energy, the larger (brighter) the corresponding image DN value. However, due to the dark current noise of the silicon-based detector itself, the linear relationship between the incident photon energy and the image DN value does not pass through zero, and there is a positive intercept, that is, when there is no X-ray photon input, a DN value output will be generated. For CCD / CMOS detectors, the positive intercept of each pixel is different. In addition, X-ray detection systems usually use Wolter-type or lobster-eye optical systems, such as the attached Figure 1 , Attachment Figure 2 As shown, due to the focusing characteristics of the optical system, the X-ray photons are not incident vertically into the detector, and the absorption depth of X-ray photons in silicon-based detectors is about 10 to 120 um. The photogenerated electrons will diffuse to adjacent pixels. Therefore, there are errors in the measurement results of the optical system. Summary of the invention

[0006] The technical problem solved by the present invention is: to overcome the deficiencies of the prior art and propose an on-orbit data processing method for a satellite-borne soft X-ray single-photon imaging camera, which can effectively extract correct X-ray single-photon events.

[0007] The technical solution of the present invention is:

[0008] An on-orbit data processing method for a spaceborne soft X-ray single-photon imaging camera, comprising:

[0009] The satellite-borne array detector receives the original image and imaging auxiliary data of soft X-ray single photons and performs dark field background correction on the original image;

[0010] The image is searched line by line using the threshold method. When the DN value of the detected pixel exceeds the lower threshold and does not exceed the upper threshold, the pixel is detected and the DN value of the surrounding n×n area and the coordinate value of the pixel are extracted and put into the dynamic memory as a valid single photon event.

[0011] After the full image retrieval is completed, the imaging auxiliary data is integrated with the data in the dynamic memory, which is called the event frame image;

[0012] The original images at fixed intervals are combined with multi-frame event frame images and output to the entire satellite antenna system in sequence.

[0013] Furthermore, the original image is corrected for dark field background, and the correction formula is:

[0014] Y i,j =a i,j ×X i,j +b i,j +c

[0015] Among them, Y i,j is the image value of row i and column j after correction; X i,j is the image value of row i and column j before correction; a i,j is the linear correction coefficient of the dark field background correction of the pixel in the i-th row and j-th column; b i,j is the constant correction coefficient of the dark field background correction of the pixel in the ith row and jth column; c is the full-image bias correction coefficient of the dark field background correction.

[0016] Furthermore, the correction factor a i,j 、b i,j The initial values ​​of , c are obtained by ground calibration, stored in the onboard non-volatile memory, and updated on orbit.

[0017] Furthermore, the threshold method is used to search the image line by line to check whether the DN value of the detected pixel exceeds the lower threshold and does not exceed the upper threshold, where the upper threshold and the lower threshold are determined by the target spectral range. The specific method is:

[0018] The lower threshold is determined by the minimum detectable photon plus background noise, specifically:

[0019]

[0020] Among them, D_TH min is the lower threshold of the judgment, EG min is the minimum detectable X-ray photon energy, D_dark is the detector dark noise;

[0021] The upper threshold is determined by the maximum detectable photon plus background noise, and the specific formula is as follows:

[0022]

[0023] where D_TH max is the upper threshold of the judgment, EG min is the maximum detectable X-ray photon energy.

[0024] Furthermore, when the DN value of the detected pixel exceeds the lower threshold and does not exceed the upper threshold, the pixel is detected and the DN value of the surrounding n×n area and the pixel coordinate value are extracted and placed in a dynamic memory, and the n×n area is determined by the number of pixels that the X-ray single photon passes through in the detector in the maximum range.

[0025] Furthermore, the method for obtaining the value of the n×n region is:

[0026]

[0027] Among them, ceil() is the upward rounding function; S Xray is the maximum travel distance of a single X-ray photon in the detector; θ max is the single photon incident angle; S Pixel is the detector pixel size.

[0028] Further, extract the DN value of the 3×3 area around the pixel, the pixel is the central pixel; among the regional DN values, the DN values ​​located in the first row are the DN value of the first pixel, the DN value of the second pixel, and the DN value of the third pixel, respectively; the DN values ​​located in the second row are the DN value of the fourth pixel, the DN value of the central pixel, and the DN value of the sixth pixel, and the DN values ​​located in the third row are the DN value of the seventh pixel, the DN value of the eighth pixel, and the DN value of the ninth pixel;

[0029] When the central pixel is located in the first row of the whole image, the DN values ​​of the first, second, and third pixels are all 0;

[0030] When the central pixel is located in the last row of the whole image, the DN values ​​of the seventh, eighth, and ninth pixels are all 0;

[0031] When the central pixel is located in the first column of the whole image, the DN values ​​of the first, fourth, and seventh pixels are all 0;

[0032] When the central pixel is located in the last column of the entire image, the DN values ​​of the third, sixth, and ninth pixels are all 0.

[0033] Furthermore, the imaging auxiliary data is integrated with the data in the dynamic memory. The imaging auxiliary data includes the satellite attitude and orbit control information, imaging time information, optical system pointing, and exposure time at the moment of imaging. The data integration is specifically as follows: splicing the imaging auxiliary data and all valid single-photon events, and adding image frame headers and frame tails.

[0034] Furthermore, a frame of original image is transmitted every 3 to 5 minutes, and each single-frame original image is combined with multiple frames of event frame images and output to the entire satellite antenna system in sequence. The interval time is stored in the on-board non-volatile memory and updated on orbit.

[0035] Furthermore, the original image at fixed intervals is combined with multiple event frame images, and data is transmitted in a mode of superimposing one original image frame on 100 event frame frames, and output to the entire satellite antenna system.

[0036] The advantages of the present invention compared with the prior art are:

[0037] (1) Compared with the method of transmitting all original frame images, the present invention can effectively reduce the amount of on-orbit data. In view of the current situation of low transmission rate of ground-to-space digital transmission channels, it can realize large-scale effective data transmission, increase on-orbit observation time, and improve the efficiency of scientific mission completion.

[0038] (2) The present invention adopts a method for downloading the original frame images at fixed intervals, and can also retain the existing methods that can effectively identify the detector blind pixels, camera dark current characteristics and other features, thereby ensuring the effective acquisition of scientific data.

[0039] (3) The implementation process of the present invention is simple, the hardware consumption is low, and it can be implemented in a 2 million gate FPGA and a small capacity dynamic memory. The method has been verified to be effective and reliable through long-term on-orbit verification. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present invention. Moreover, the same reference symbols are used throughout the accompanying drawings to represent the same components. In the accompanying drawings:

[0041] Figure 1 Schematic diagram of an X-ray detection system using a Wolter-type optical system;

[0042] Figure 2 Schematic diagram of an X-ray detection system using a lobster-eye optical system;

[0043] Figure 3 The image after dark field background correction is performed in the embodiment of the present invention;

[0044] Figure 4 This is a flow chart of an on-orbit data processing method for a satellite-borne soft X-ray single-photon imaging camera according to an embodiment of the present invention;

[0045] Figure 5 Schematic diagram of a method for extracting a surrounding n×n area according to an embodiment of the present invention. DETAILED DESCRIPTION

[0046] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0047] The present invention proposes an on-orbit data processing method for a satellite-borne soft X-ray single-photon imaging camera. Aiming at the photoelectric characteristics of X-ray single photons in CCD / CMOS detectors, an event-oriented information extraction method is adopted. According to the threshold, a 3×3 area is retrieved and determined to be a single-photon event, and the event characteristics are judged to effectively eliminate pseudo-events such as cosmic high-energy rays and solar heavy particles, and the single-photon events of the entire image are stored and transmitted in a timely manner. The event extraction process includes: dark field background correction, event retrieval, event frame storage, and full-image frame-event frame mixed transmission. Among them, the dark field background correction parameters and event retrieval thresholds need to be corrected on-orbit according to the radiation accumulation effect of the detector.

[0048] The method specifically comprises the following steps:

[0049] S1: Perform dark field background correction on the input soft X-ray single photon image. The dark field background correction adopts a linear correction method to achieve the minimum resource deployment on orbit. The correction formula is:

[0050] Y i,j =a i,j ×X i,j +b i,j +c

[0051] Among them, Y i,j is the image value of row i and column j after correction; X i,j is the image value of row i and column j before correction; a i,j is the linear correction coefficient of the dark field background correction of the pixel in the i-th row and j-th column; b i,j is the constant correction coefficient of the dark field background correction of the pixel in the i-th row and j-th column; c is the full-image bias correction coefficient of the dark field background correction;

[0052] Correction coefficient a i,j 、b i,j The initial values ​​of , c are obtained by ground calibration, stored in the onboard non-volatile memory, and can be updated on orbit.

[0053] S2: Use threshold method to Figure 3 The image shown is retrieved line by line, and the threshold is divided into the lower limit D min and upper limit D max , less than the lower threshold D min The pixels are judged as noise and are greater than the upper threshold D max The pixels are judged as high-energy particle radiation pixels or bad pixels and are not considered as valid soft X-ray single photon events. min And D≤D max When the pixel is detected, the surrounding n×n area values ​​including the center point coordinates are extracted and placed in the dynamic memory as a valid single photon event.

[0054] Among them, the lower threshold and upper threshold are determined by the target spectral range, and the specific method is:

[0055] The lower threshold is determined by the minimum detectable photon plus background noise:

[0056]

[0057] where D_TH min is the lower threshold of the judgment, EG min It is the minimum detectable X-ray photon energy, which is determined by the task or optical system design and is usually in the range of 0.1keV to 0.5keV. 3.65eV is the bandgap width of silicon-based semiconductors. D_dark is the dark noise of the detector, which is quantified by the dark pixels of the detector.

[0058] The upper threshold is determined by the minimum detectable photon plus background noise:

[0059]

[0060] where D_TH max is the upper threshold of the judgment, EG min is the maximum detectable X-ray photon energy, which is determined by the task or optical system design and is usually in the range of 5keV to 10keV. 3.65eV is the bandgap width of silicon-based semiconductors. D_dark is the dark noise of the detector, which is quantified by the dark pixels of the detector.

[0061] The n×n region is determined by the number of pixels that a single X-ray photon travels through in the detector, as Figure 5 As shown, n is selected as:

[0062]

[0063] Among them, ceil() is the upward rounding function; S Xray The maximum travel distance of a single X-ray photon in the detector, such as the maximum travel distance of a 10keV X-ray single photon in a silicon-based detector is 120um; θ max is the single photon incident angle, which is usually determined by the optical system design; S Pixel is the detector pixel size.

[0064] In this embodiment, the 3×3 area value including the center point coordinates is extracted and stored in the dynamic memory as a valid single photon event. The information storage content should include: center pixel row value, center pixel column value, center pixel DN value, 1 pixel DN value, 2 pixel DN value, 3 pixel DN value, 4 pixel DN value, 6 pixel DN value, 7 pixel DN value, 8 pixel DN value, 9 pixel DN value.

[0065] S3: After the full image retrieval is completed, the satellite attitude and orbit control information at the imaging time, the imaging time information, the optical system pointing, the exposure time and other information are integrated with the data in the dynamic memory, which is called the event frame image. When storing, the false soft X-ray single photon events caused by bad pixels need to be eliminated. The coordinates of the bad pixels are stored in the onboard non-volatile memory. The coordinates of the bad pixels are obtained by long-term observation event statistics and then annotated to the onboard processing unit. According to the analysis of the on-orbit X-ray pulsar and cosmic background radiation dose, the data storage space of the dynamic memory should meet the information storage of at least 128 valid single photon events;

[0066] S4: Combine the fixed-interval single-frame original image with the multi-frame event frame image, splice the auxiliary data of an image and all valid single-photon events, and add the image frame header and frame footer. The auxiliary data should include the shooting time of this frame image, the whole star attitude orbit control data, the full image bias correction coefficient c, and the lower limit threshold D min , upper threshold D max and other camera working status parameters, and according to the on-orbit use requirements, a mixed transmission method of original images and event images is required, with one frame of original image transmitted every 3 to 5 minutes. The interval time is stored in the onboard non-volatile memory and can be updated on-orbit.

[0067] The present invention adopts a one-frame original image + several event frames (default 100 frames) mode for data transmission, wherein the original image is the original image of the detector imaging and the imaging auxiliary data. The event frame processing method is: dark field background correction is performed on the original image of the satellite-borne array detector receiving the soft X-ray single photon; then the threshold method is used to search the image line by line. When the DN value of the detected pixel exceeds the lower threshold and does not exceed the upper threshold, the pixel is detected and the DN value of the surrounding n×n area and the coordinate value of the pixel are extracted and put into the dynamic memory as a valid single photon event; after the full image search is completed, the satellite attitude and orbit control information, imaging time information, optical system pointing, exposure time and other information at the imaging time are integrated with all the single photon event data in the dynamic memory, which is called an event frame image; finally, the fixed-interval single-frame original image is combined with the multi-frame event frame image, and output to the whole satellite antenna system in sequence. The on-orbit data processing method proposed by the present invention has been verified on the Nanjing University No. 1 satellite.

[0068] The above-described embodiments are only preferred specific implementations of the present invention. Common changes and substitutions made by those skilled in the art within the scope of the technical solution of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for processing on-orbit data of a satellite-borne soft X-ray single-photon imaging camera, characterized in that: include: The satellite-borne array detector receives the original image and imaging auxiliary data of soft X-ray single photons and performs dark field background correction on the original image; The image is searched line by line using the threshold method. When the DN value of the detected pixel exceeds the lower threshold and does not exceed the upper threshold, the pixel is detected and the DN value of the surrounding n×n area and the coordinate value of the pixel are extracted and put into the dynamic memory as a valid single photon event. After the full image retrieval is completed, the imaging auxiliary data is integrated with the data in the dynamic memory, which is called the event frame image; The original images at fixed intervals are combined with multi-frame event frame images and output to the entire satellite antenna system in sequence.

2. The on-orbit data processing method of a satellite-borne soft X-ray single-photon imaging camera according to claim 1, characterized in that: Perform dark field background correction on the original image, and the correction formula is: Y i,j =a i,j ×X i,j +b i,j +c Among them, Y i,j is the image value of row i and column j after correction; X i,j is the image value of row i and column j before correction; a i,j is the linear correction coefficient of the dark field background correction of the pixel in the i-th row and j-th column; b i,j is the constant correction coefficient of the dark field background correction of the pixel in the ith row and jth column; c is the full-image bias correction coefficient of the dark field background correction.

3. The on-orbit data processing method for a satellite-borne soft X-ray single-photon imaging camera according to claim 2, characterized in that: Correction coefficient a i,j 、b i,j The initial values ​​of , c are obtained by ground calibration, stored in the onboard non-volatile memory, and updated on orbit.

4. The on-orbit data processing method for a satellite-borne soft X-ray single-photon imaging camera according to claim 1, characterized in that: The threshold method is used to search the image line by line to check whether the DN value of the detected pixel exceeds the lower threshold and does not exceed the upper threshold. The upper and lower thresholds are determined by the target spectral range. The specific method is as follows: The lower threshold is determined by the minimum detectable photon plus background noise, specifically: Among them, D_TH min is the lower threshold of the judgment, EG min is the minimum detectable X-ray photon energy, D_dark is the detector dark noise; The upper threshold is determined by the maximum detectable photon plus background noise, and the specific formula is as follows: where D_TH max is the upper threshold of the judgment, EG min is the maximum detectable X-ray photon energy.

5. The on-orbit data processing method for a satellite-borne soft X-ray single-photon imaging camera according to claim 1, characterized in that: When the DN value of the detected pixel exceeds the lower threshold but does not exceed the upper threshold, the pixel is detected and the DN value of the surrounding n×n area and the pixel coordinate value are extracted and put into the dynamic memory. The n×n area is determined by the number of pixels that the X-ray single photon passes through in the detector in the maximum distance.

6. The on-orbit data processing method for a satellite-borne soft X-ray single-photon imaging camera according to claim 5, characterized in that: The method for obtaining the value of n×n area is: Among them, ceil() is the upward rounding function; S Xray is the maximum travel distance of a single X-ray photon in the detector; θ max is the single photon incident angle; S Pixel is the detector pixel size.

7. The on-orbit data processing method for a satellite-borne soft X-ray single-photon imaging camera according to claim 1, characterized in that: Extract the DN value of the n×n area around the pixel, where the pixel is the central pixel, and when n is 3, among the regional DN values, the DN values ​​located in the first row are the DN value of the first pixel, the DN value of the second pixel, and the DN value of the third pixel, respectively; the DN values ​​located in the second row are the DN value of the fourth pixel, the DN value of the central pixel, and the DN value of the sixth pixel, and the DN values ​​located in the third row are the DN value of the seventh pixel, the DN value of the eighth pixel, and the DN value of the ninth pixel; When the central pixel is located in the first row of the whole image, the DN values ​​of the first, second, and third pixels are all 0; When the central pixel is located in the last row of the whole image, the DN values ​​of the seventh, eighth, and ninth pixels are all 0; When the central pixel is located in the first column of the whole image, the DN values ​​of the first, fourth, and seventh pixels are all 0; When the central pixel is located in the last column of the entire image, the DN values ​​of the third, sixth, and ninth pixels are all 0.

8. The on-orbit data processing method for a satellite-borne soft X-ray single-photon imaging camera according to claim 1, characterized in that: The imaging auxiliary data is integrated with the data in the dynamic memory. The imaging auxiliary data includes the satellite attitude and orbit control information, imaging time information, optical system pointing, and exposure time at the moment of imaging. The data integration is specifically as follows: splicing the imaging auxiliary data and all valid single-photon events, and adding image frame headers and frame tails.

9. The on-orbit data processing method for a satellite-borne soft X-ray single-photon imaging camera according to claim 1, characterized in that: A frame of original image is transmitted every 3 to 5 minutes. Each single frame of original image is combined with multiple frames of event frame images and output to the entire satellite antenna system in sequence. The interval time is stored in the onboard non-volatile memory and updated on orbit.

10. The on-orbit data processing method for a satellite-borne soft X-ray single-photon imaging camera according to claim 1, characterized in that: The original images at fixed intervals are combined with multi-frame event frame images, and data is transmitted in a mode of superimposing one original frame on 100 event frames, and output to the entire satellite antenna system.

Images