A polarization remote sensor on-orbit polarization calibration method and device

Abstract

The present invention discloses an on-orbit polarization calibration method and device for a polarization remote sensor in the field of satellite calibration and verification technology. The on-orbit polarization calibration method and device include: equipping the polarization remote sensor with a zero polarization calibrator and a linear polarization calibrator; setting a threshold value of ground object target conditions that meet the polarization calibration, and screening calibration pixels according to the calibration threshold value. The on-orbit polarization calibration method and device of the present invention is an on-orbit calibration method for a polarization remote sensor equipped with an on-board polarization calibrator, which has better calibration accuracy and reliability. Observation and calibration are automatically switched by rotating a scanning mirror, and no complex motion device is required. The reliability is high. The device can perform a calibration once per scanning circle, which broadens the calibration data source and can easily obtain a large sample of massive calibration data. It can also significantly shorten the calibration cycle, increase the calibration frequency, and eliminate the need for field testing, effectively saving manpower and material resources.

Description

Technical Field

[0001] The present invention relates to the technical field of satellite calibration and verification, and in particular to an on-orbit polarization calibration method and device for a polarization remote sensor. Background Art

[0002] Compared with traditional intensity (scalar) observations, polarization detection is more sensitive to the microphysical and optical properties of atmospheric aerosols, and is relatively insensitive to the polarization contribution of the surface. Therefore, high-precision detection of comprehensive parameters of atmospheric aerosols can be achieved through polarization detection. At present, polarization remote sensing has become an international research hotspot. Countries and regions such as the United States and Europe have successively launched or plan to launch various types of polarization payloads. For example, the new generation of multi-angle, multi-spectral, and multi-polarization imagers (3MI) developed by the European Space Agency (ESA) inherits the design concept of POLDER and is planned to be carried on the second-generation European meteorological satellite (Metop-SG). In addition, there are also the APS that the United States has not successfully launched into orbit, as well as the HARP and MSPI that are about to be launched, and the SPEX that the Netherlands is about to launch, all of which are classic polarization remote sensors developed in recent years. The polarization remote sensors in orbit in China mainly include the Cloud and Aerosol Instrument (CAPI) developed by the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, the Multi-angle Polarimeter (MAI) developed by the Shanghai Institute of Technical Physics, Chinese Academy of Sciences, and the Multi-angle Polarization Imager (DPC) and Polarization Scanning Atmospheric Corrector (PSAC) developed by the Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences.

[0003] Polarization accuracy is a key factor in polarimetric measurements. It is generally accepted internationally that a spaceborne polarimetric remote sensor with a polarimetric measurement accuracy better than 0.005 exhibits high sensitivity and can significantly improve aerosol inversion accuracy. Polarization calibration is fundamental to the quantitative application of polarization information and effectively guarantees the instrument's polarimetric measurement accuracy. Polarization calibration establishes a quantitative relationship between the polarimetric remote sensor output and the known polarization state of the incident light, and also determines the physical parameters of the polarimetric remote sensor's vectorial radiation transmission. Due to the complex environments encountered during satellite launch and on-orbit operation, performance degradation of spaceborne instruments is inevitable. For most polarimetric remote sensors, such as POLDER, which do not have an onboard polarimetric calibrator, post-launch polarimetric calibration relies primarily on calculating the polarimetric radiation characteristics of natural scenes. This calibration or verification accuracy is difficult to meet the requirements of high-precision polarimetric remote sensing. PSAC, however, is the first polarimetric remote sensor equipped with an onboard polarimetric calibrator and successfully operated in orbit. As a novel polarimetric payload, existing polarimetric calibration methods and devices are not well suited for on-board polarimetric calibration of PSAC. Therefore, a method and device for on-orbit polarization calibration of a polarization remote sensor are needed to specifically solve the above problems. Summary of the Invention

[0004] The object of the present invention is to provide an on-orbit polarization calibration method and device for a polarization remote sensor, so as to solve the problems raised in the above background technology.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A polarization remote sensor on-orbit polarization calibration method, the calibration method comprising the following steps:

[0007] Step 1: Equip the polarization remote sensor with a zero polarization calibrator and a linear polarization calibrator;

[0008] Step 2: Setting a threshold value of ground object target conditions that meet polarization calibration, and selecting calibration pixels according to the calibration threshold value;

[0009] Step 3: For the calibration pixels selected in step 2, the relative gain calibration coefficient k is calculated using the zero polarization calibrator observation signal combined with the instrument polarization measurement model. 1λ 、k 2λ ;

[0010] Step 4: For the calibration pixels selected in step 2, use the linear polarization calibrator observation data combined with the instrument polarization measurement model to calculate the instrument bidirectional attenuation correlation coefficient αq λ 、αu λ ;

[0011] Step 5: Based on the on-orbit calibration coefficients calculated in steps 3 and 4, and the laboratory calibration coefficients, the polarization information of the target is calculated according to the remote sensor measurement model.

[0012] Step 6: Based on the long-term on-orbit calibration coefficients calculated in steps 3 and 4, monitor the stability of the on-orbit polarization radiation performance of the remote sensor.

[0013] Preferably, the remote sensor can obtain the zero polarization calibrator signal and the linear polarization calibrator signal each time it scans the sub-satellite point.

[0014] Preferably, the zero polarization calibrator in step 1 processes the earth observation target light into zero polarization light with equal radiance value.

[0015] Preferably, the linear polarization calibrator in step 1 processes the earth observation target signal into completely linearly polarized light with a reduced radiation brightness value and a determined polarization azimuth angle.

[0016] Preferably, the method for determining the ground object conditions that meet the polarization calibration and the screening method for the calibration pixels in step 2 is:

[0017] To avoid single-particle anomaly signals, based on the principle of equal radiance between two barrels in the same wavelength band, the single-particle anomaly threshold is defined as |(S0 + S90) / (S45 + S135) - 1| < threshold_indPart.

[0018] To avoid differences between the linear polarizer and zero polarizer and the targets observed at the sub-satellite point during payload operation, it is necessary to ensure the uniformity of the targets along the orbit direction. The uniformity of the targets along the orbit direction is defined as:

[0019] std(Sx_pre, Sx, Sx_next) / average(Sx_pre, Sx, Sx_nex) < threshold_unif, where x ∈ {0, 90, 45, 135}, and Sx is the response of the detector at the sub-satellite point.

[0020] To avoid the influence of noise on polarization calibration, the signal-to-noise ratio threshold is defined as: Sx > threshold_DN, where x ∈ {0, 90, 45, 135}, and Sx is the response of the detector at the sub-satellite point.

[0021] To eliminate the influence of the non-ideality of the calibrator, the degree of polarization of the ground object target needs to satisfy:

[0022] |sqrt(((s0 - s90) / (s0 + s90))^2 + ((s45 - s135) / (s45 + s135))^2)| < threshold_DOLP.

[0023] Preferably, the instrument polarization measurement model in step three is:

[0024]

[0025]

[0026]

[0027]

[0028] In the formula, DN aλ is the DN value detected by the channel with the polarization analysis direction of a in a certain wavelength band, A λ is the absolute radiometric calibration coefficient, IEp λ is the total intensity of the incident light in a certain wavelength band affected by the instrument polarization effect, Eq λ is the Q polarization component of the incident light in a certain wavelength band affected by the instrument polarization effect, Eu λ is the U polarization component of the incident light in a certain wavelength band affected by the instrument polarization effect, ε1 and ε2 are the deviation angles of the polarization analyzers of the two polarization prisms in the same wavelength band of the instrument, αq λ 、αu λare the two-way attenuation correlation coefficients of the two mirror tubes in the same band of the instrument, k 1λ 、k 2λ are the relative gain coefficients of the two mirror tubes in the same band of the instrument, C 12λ is the gain coefficient between the mirror tubes in a certain band of the instrument, qinst λ 、uinst λ is the residual polarization of the instrument in a certain band.

[0029] Preferably, the laboratory calibration coefficient in step 5 includes: the inter-tube gain coefficient C of a certain wavelength band of the instrument 12λ ; Residual polarization coefficient qinst of a certain band instrument λ 、uinst λ ; The polarization azimuth angle deviations ε1 and ε2 of the two polarizing prisms in the same band of the instrument.

[0030] An on-orbit polarization calibration device for a polarization remote sensor includes a remote sensor, a zero polarization calibrator, and a linear polarization calibrator. The zero polarization calibrator is equipped with a polarization scrambler array consistent with the remote sensor's wavelength band to convert qualified ground target signal light into zero polarization light within an aperture. The linear polarization calibrator is equipped with a polarizer consistent with the remote sensor's wavelength band to convert qualified ground target signal light into fully linearly polarized light within the aperture.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] 1. The on-orbit polarization calibration method and device for a polarization remote sensor of the present invention is an on-orbit calibration method equipped with an on-board polarization calibrator, which has better calibration accuracy and reliability.

[0033] 2. The present invention provides an on-orbit polarization calibration method and device for a polarization remote sensor. The on-orbit polarization calibration method automatically switches between observation and calibration by rotating a scanning mirror, does not require a complex motion device, and has high reliability.

[0034] 3. The on-orbit polarization calibration method and device for a polarization remote sensor of the present invention can perform a calibration once per scanning circle, thus broadening the calibration data source and making it easy to obtain a large sample of massive calibration data;

[0035] 4. The on-orbit polarization calibration method and device of the present invention can significantly shorten the calibration period and increase the calibration frequency;

[0036] 5. The polarization remote sensor calibration method of the on-orbit polarization calibration method and device of the present invention does not require field testing, effectively saving manpower and material resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0038] Figure 1 This is a diagram showing the installation position of the calibrator of the on-orbit polarization calibration method and device of the present invention;

[0039] Figure 2 It is a flowchart of the steps of the on-orbit polarization calibration method and device of the present invention. DETAILED DESCRIPTION

[0040] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0041] See also Figures 1 to 2 As shown, a polarization remote sensor on-orbit polarization calibration method and device, the on-orbit polarization calibration device includes a remote sensor, a zero polarization calibrator, and a linear polarization calibrator.

[0042] The zero polarization calibrator is equipped with a polarization scrambler array that is consistent with the remote sensor band, which can convert qualified ground target signal light into zero polarization light within a certain aperture range.

[0043] The linear polarization calibrator is equipped with a polarizer that is consistent with the wavelength band of the remote sensor, which can convert the qualified ground target signal light into completely linearly polarized light within a certain aperture range.

[0044] Each scan of the remote sensor can not only obtain a wide range of ground observation data, but also obtain the optical signals emitted by the zero polarization calibrator and the linear polarization calibrator that are consistent with the brightness of the ground target.

[0045] The screened calibrator optical signal can be used as a polarization reference to perform polarization calibration on the remote sensor, and a new polarization calibration coefficient can be obtained in real time.

[0046] A method for on-orbit polarization calibration of a polarization remote sensor comprises the following steps:

[0047] Step 1: Equip the polarization remote sensor with a zero polarization calibrator and a linear polarization calibrator;

[0048] Step 2: Set the threshold of the ground object target optical signal condition for polarization calibration, and screen the calibration pixels according to the calibration threshold;

[0049] Step 3: For the calibration pixels screened in Step 2, calculate the relative gain calibration coefficient k using the observation signal of the zero polarization calibrator combined with the instrument polarization measurement model 1λ 、k 2λ ;

[0050] Step 4: For the calibration pixels screened in Step 2, calculate the instrument bidirectional attenuation correlation coefficients αq λ 、αu λ ;

[0051] Step 5: According to the on-orbit calibration coefficients calculated in Step 3 and Step 4, and read the laboratory calibration coefficients, and solve the polarization information of the target according to the remote sensor polarization measurement model;

[0052] Step 6: According to the on-orbit calibration coefficients calculated in Step 3 and Step 4 for a long time, effectively monitor the stability of the on-orbit polarization radiation performance of the remote sensor.

[0053] The method for setting and screening the optical signal of the polarization calibrator is as follows:

[0054] Single particle anomaly: The detection value of the calibrator will randomly show single particle anomalies, manifested as a significant increase in the DN value of a single channel, and its relative standard deviation can reach more than 20%. Using the principle of equal radiance between the same band mirror tubes, the single particle anomaly threshold is defined as: |(S0 + S90) / (S45 + S135) - 1| < threshold_indPart. According to the fact that the normal values of a large amount of data are generally below 0.05 and the detected abnormal values are above 0.2, it is tentatively determined that threshold_indPart = 0.1.

[0055] Along-track direction uniformity: When the payload is operating, there is a half-pixel delay between the linear polarization calibrator and the zero polarization calibrator and the sub-satellite point, resulting in between the position before and the position after the sub-satellite point when the linear polarization calibrator and the zero polarization calibrator collect data. Therefore, it is necessary to ensure the uniformity of the scenes at these three positions. The along-track direction scene uniformity is defined as: std(Sx_pre, Sx, Sx_next) / average(Sx_pre, Sx, Sx_nex) < threshold_unif, x ∈ {0, 90, 45, 135}, and Sx is the detector response at the sub-satellite point. According to a large amount of measured data, it is tentatively determined that threshold_unif = 0.1.

[0056] Signal-to-noise ratio requirements: Define the signal-to-noise ratio threshold: Sx > threshold_DN, where x∈{0,90,45,135}, and Sx represents the sub-satellite detector response. dB represents the ratio of two quantities and has no unit. When converting voltage or current ratios, (A / B)dB = 20lg(A / B); when converting power ratios, (A / B)dB = 10lg(A / B). The DN value of a polarization remote sensor represents the digitized voltage and should be (SNRDB)dB = 20lg(SNR). Simulations show a signal-to-noise ratio of 35dB, which meets 0.0005 polarization accuracy, corresponding to a 50x multiplier. Assuming a noise factor of 40, a provisional threshold_DN value of 2000DN is set.

[0057] Polarization requirement: The optical components in the onboard calibrator include a Glan prism (99.99% polarization) and a depolarizer (98% depolarization). To eliminate the effects of calibrator non-idealities, optimal performance is achieved when the incident light is natural light. The polarization requirement threshold is defined as: |sqrt(((s0-s90) / (s0+s90))^2+((s45-s135) / (s45+s135))^2)| <threshold_DOLP。

[0058] When threshold_DOLP is tentatively set to 0.025, the depolarizer output light can reach an accuracy of 0.0005, and the number of on-orbit samples is also considerable.

[0059] The polarization measurement model of the polarization remote sensor is:

[0060]

[0061]

[0062]

[0063]

[0064] Where, DN aλ is the DN value obtained by channel detection with polarization resolution direction a in a certain band, A λ is the absolute radiation calibration coefficient, IEp λ It is the total intensity of the incident light in a certain wavelength band affected by the polarization effect of the instrument, Eq λ It is the Q polarization component of the incident light in a certain band after being affected by the polarization effect of the instrument, Eu λ It is the U polarization component of the incident light in a certain band affected by the polarization effect of the instrument. ε1 and ε2 are the polarization azimuth deviations of the two polarizing prisms in the same band of the instrument, respectively. λ 、αu λ are the two-way attenuation correlation coefficients of the two mirror tubes in the same band of the instrument, k1λ 、k 2λ are the relative gain coefficients of the two mirror tubes in the same band of the instrument, C 12λ is the gain coefficient between the mirror tubes in a certain band of the instrument, qinst λ 、uinst λ is the residual polarization of the instrument in a certain band.

[0065] When calculating polarization information, the laboratory calibration coefficients that need to be read include: the inter-tube gain coefficient C of a certain band of the instrument 12λ ; Residual polarization coefficient qinst of a certain band instrument λ 、uinst λ ; The polarization azimuth angle deviations ε1 and ε2 of the two polarizing prisms in the same band of the instrument.

[0066] Throughout this specification, references to terms such as "one embodiment," "example," or "specific example" indicate that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0067] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention, and such changes and modifications fall within the scope of the invention as claimed.

Claims

1. A polarization remote sensor on-orbit polarization calibration method, characterized in that: The calibration method includes the following steps: Step 1: Equip the polarization remote sensor with a zero-polarization calibrator and a linear-polarization calibrator; Step 2: Set the threshold of the ground object target conditions for polarization calibration, and screen the calibration pixels according to the ground object target conditions threshold; Step 3: For the calibration pixels selected in step 2, the relative gain calibration coefficient is calculated using the zero polarization calibrator observation signal combined with the instrument polarization measurement model. 、 ; Step 4: For the calibration pixels selected in step 2, use the linear polarization calibrator observation data combined with the instrument polarization measurement model to calculate the instrument bidirectional attenuation correlation coefficient 、 ; Step 5: According to the on-orbit calibration coefficients calculated in Step 3 and Step 4, and read the laboratory calibration coefficients, and solve the polarization information of the target according to the remote sensor measurement model; Step 6: Monitor the stability of the on-orbit polarization radiation performance of the remote sensor according to the on-orbit calibration coefficients calculated in Step 3 and Step 4 for a long time; The instrument polarization measurement model in Step 3 is: Where, is the DN value obtained by channel detection with polarization resolution direction a in a certain band, is the absolute radiation calibration coefficient, It is the total intensity of the incident light in a certain wavelength band affected by the polarization effect of the instrument. It is the Q polarization component of the incident light in a certain wavelength band affected by the polarization effect of the instrument. It is the U polarization component of the incident light in a certain band affected by the polarization effect of the instrument. 、 are the analysis azimuth deviations of the two polarizing prisms in the same band of the instrument, 、 are the two-way attenuation correlation coefficients of the two mirror tubes in the same band of the instrument, 、 are the relative gain coefficients of the two lens barrels in the same band of the instrument, is the inter-tube gain coefficient of a certain band of the instrument.

2. The on-orbit polarization calibration method for a polarization remote sensor according to claim 1, characterized in that: Each time the remote sensor scans the sub-satellite point, it can obtain the signals of the zero-polarization calibrator and the linear-polarization calibrator.

3. The on-orbit polarization calibration method for a polarization remote sensor according to claim 1, characterized in that: The zero-polarization calibrator in Step 1 processes the light of the ground observation target into zero-polarized light with equal radiance values.

4. The on-orbit polarization calibration method for a polarization remote sensor according to claim 1, characterized in that: The linear-polarization calibrator in Step 1 processes the ground observation target signal into completely linearly polarized light with reduced radiance value and determined polarization azimuth angle.

5. The on-orbit calibration method for a polarization remote sensor according to claim 1, characterized in that: The screening method for determining the ground object conditions and calibration pixels that meet the polarization calibration in Step 2 is: To avoid single-particle anomaly signals, define the single-particle anomaly threshold as |(S0 + S90) / (S45 + S135) - 1| < threshold_indPart; To avoid the difference between the linear-polarization calibrator and the zero-polarization calibrator and the target observed at the sub-satellite point during the operation of the payload, define the uniformity of the target in the along-track direction as: std(Sx_pre, Sx, Sx_next) / average(Sx_pre, Sx, Sx_next) < threshold_unif, x ∈ {0, 90, 45, 135}, Sx is the response of the sub-satellite point detector; To avoid the influence of noise on polarization calibration, define the signal-to-noise ratio threshold: Sx > threshold_DN, x ∈ {0, 90, 45, 135}, Sx is the response of the sub-satellite point detector; To eliminate the influence of the non-ideality of the calibrator, the degree of polarization of the ground object target needs to satisfy: |sqrt(((s0 - s90) / (s0 + s90))^2 + ((s45 - s135) / (s45 + s135))^2)| < threshold_DOLP.

6. The on-orbit polarization calibration method for a polarization remote sensor according to claim 1, characterized in that: The laboratory calibration coefficients in step 5 include: the inter-tube gain coefficient of a certain wavelength band of the instrument ; Residual polarization coefficient of a certain band instrument 、 ; The deviation of the polarization azimuth of the two polarization prisms in the same band of the instrument 、 .

7. An on-orbit polarization calibration device for a polarization remote sensor, applied to the on-orbit polarization calibration method for a polarization remote sensor according to claim 1, characterized in that: The on-orbit polarization calibration device includes a remote sensor, a zero-polarization calibrator, and a linear-polarization calibrator. A depolarizer array consistent with the remote sensor band is installed on the zero-polarization calibrator to convert the ground target signal light into zero-polarized light within the aperture. A polarizer consistent with the remote sensor band is installed on the linear-polarization calibrator to convert the ground target signal light into completely linearly polarized light within the aperture.

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