CN105352609B - A kind of Optical remote satellite absolute radiation calibration method based on space lambert's sphere - Google Patents

Abstract

An absolute radiometric calibration method for optical remote sensing satellites based on a space Lambertian sphere. By deploying a space Lambertian sphere in space, the solar radiation attenuated to a suitable range is directly introduced into the satellite remote sensor. Determine the background signal by imaging the deep space background through satellite attitude maneuvering; use satellite tool software to calculate the time period that meets the calibration conditions, and select the imaging time point, and determine the space Lambertian sphere reflection signal through satellite attitude maneuvering to image the space Lambertian sphere, From this, the irradiance at the entrance pupil of the remote sensor at the imaging moment is calculated, and the absolute radiation calibration coefficient of a single point is determined by acquiring multiple sets of data. The method of the invention avoids the influence of atmospheric conditions and ground target characteristics, establishes a unified space optical radiation reference, meets the radiation calibration requirements of full optical path, full aperture, and solar spectral distribution matching, and can realize normalized monitoring of satellite remote sensors .

Description

An Absolute Radiometric Calibration Method for Optical Remote Sensing Satellites Based on Space Lambertian Sphere

technical field

The invention belongs to the field of remote sensing satellite radiation calibration and relates to a method for normalized absolute calibration of the on-orbit radiation performance of space optical remote sensing satellites.

Background technique

With the deepening of remote sensing applications, quantitative remote sensing has become the focus of remote sensing applications, and the basis and premise of quantitative remote sensing is the radiometric calibration of remote sensors. The radiometric calibration of space remote sensors can be divided into laboratory calibration before launch and on-orbit calibration after launch. After the satellite is launched, with the influence of various factors such as the working environment and long-term working status, the calibration coefficient before launch may be changed. Therefore, the on-orbit calibration after launch is the key to ensure the reliability and accuracy of remote sensing data. The essential.

The on-orbit calibration methods after launch mainly include field calibration and on-board calibration. Site calibration relies on a large area of uniform and stable ground objects on the earth's surface as a calibration source to realize radiation calibration of satellites in orbit. This type of method is affected by the characteristics of ground objects and atmospheric conditions, and its calibration accuracy is low and the cost is relatively high. high. On-board calibration is achieved through an on-board calibration device. According to the different calibration light sources used, there are mainly methods such as built-in light, moon, star, and sun calibration. The built-in light is a commonly used method in the early calibration schemes, but the built-in light is an artificial light source added to the optical path, which has defects such as a non-full optical path and a large difference in spectral distribution from the sun. The moon emits light by relying on the sun, and its spectral characteristics are related to the solar spectrum and the absorption and reflection characteristics of the moon itself. Since the accurate lunar spectral distribution has not yet been obtained, its irradiance model has not yet reached the accuracy of practical application. Therefore, through the lunar calibration The method cannot be directly used for absolute calibration, but only for relative radiometric correction. As a kind of celestial object, stars also have a certain brightness, but except for the sun, the brightness of the stars in the current research cannot meet the requirements, and cannot be used for on-star radiation calibration. A commonly used calibration light source is the sun, which is a uniform and highly stable Lambertian light source. After attenuating the solar energy to an appropriate range through reflection or transmission, it can provide stable and accurate calibration for space remote sensors. way. Through the space-borne solar diffuser, the solar diffuse reflector covered with diffuse material has good uniformity and Lambertian properties. After the solar diffuser is opened, the solar calibration of the remote sensor can be realized. This method generally has The advantages of full aperture, full optical path, and good spectral matching are currently one of the main means of on-board calibration. However, limited by technical conditions, this method needs to install a separate solar diffuse reflector on the satellite itself. Its structural design is complex, it is easily polluted by satellite exhaust, and it increases the operating burden of the entire star system. In addition, this method has zero The failure of components will have a huge impact on the subsequent calibration work.

Contents of the invention

The technical problem solved by the present invention is: to overcome the deficiencies of the prior art, and to provide an absolute radiometric calibration method for optical remote sensing satellites based on a space Lambertian sphere, which can get rid of ground Influenced by target characteristics and atmospheric conditions, the sun is used as the calibration source to achieve high-precision absolute radiation calibration with full optical path, full aperture, and spectral matching.

The technical solution of the present invention is: a kind of optical remote sensing satellite absolute radiometric calibration method based on space Lambertian sphere, comprises the following steps:

(1) launch an artificial Lambertian sphere into space by means of carrier rocket launch or satellite release launch, the diameter and orbital position of the sphere meet the requirements of satellite calibration for signal energy; the artificial Lambertian sphere has a A homogeneous sphere close to Lambertian;

(2) Set the parameters of the satellite remote sensor to be calibrated so that the integration time, gain and series remain unchanged throughout the calibration process;

(3) Maneuver the attitude of the satellite, make the remote sensor of the satellite to be calibrated point to the deep space background, obtain the gray value of the imaging pixel of the remote sensor of the satellite to be calibrated, and perform a total of n measurements to obtain the gray value of the pixel k of each measurement Degree value DN _k0i , where n≥1, k is the position of the remote sensor pixel, the value range of k is from 1 to the total number of remote sensor pixels, i is the number of measurements, i=1,2,3.. ...., n; after n measurements are completed, calculate the average background signal DN _k0 of pixel k of the satellite remote sensor to be calibrated,

(4) set up the scene that comprises satellite, artificial Lambertian sphere and the sun, simulate the transmission of solar radiation energy, obtain the time period when the positional relationship between the satellite, artificial Lambertian sphere and the sun satisfies the calibration condition; Calibration conditions include geometric visible conditions and sensor visible conditions. The geometrically visible condition is that the line between the satellite and the artificial Lambertian sphere is not blocked by the earth. The sensor visible condition is that the artificial Lambertian sphere is outside the shadow of the earth. The Bo sphere can reflect solar radiation energy to achieve observation and imaging;

(5) Select an imaging time point from the time period that satisfies the calibration conditions, and perform attitude maneuver on the satellite, so that the entrance pupil normal of the remote sensor of the satellite to be calibrated points to the Lambertian sphere, and the remote sensor observes the artificial Lambertian sphere. The azimuth angle is denoted as φ, and the relative distance L _1(j) between the artificial Lambertian sphere and the sun, the relative distance L _2(j) between the artificial Lambertian sphere and the entrance pupil of the remote sensor of the satellite to be calibrated, and the relative distance between the sun and the artificial Lambertian sphere and the included angle θ _j of the line between the satellite remote sensor to be calibrated and the artificial Lambertian sphere, thus obtaining the irradiance E at which the artificial Lambertian sphere reflects solar radiation and reaches the entrance pupil of the satellite remote sensor _kj , the pixel position k corresponding to the artificial Lambertian sphere image point and the gray value DN _kj ;

(6) Under the condition of keeping φ constant, repeat step (5) to obtain the gray value DN _kj and entrance pupil irradiance E _kj corresponding to the n measurements of pixel k, combined with the background signal value DN of the pixel k _k0 , the linear response relationship is established by the least square method, and the absolute radiation calibration coefficient G _k of pixel k is obtained,

In the step (1), the satellite calibration requires that the signal energy is: the satellite remote sensor can receive the sunlight energy reflected by the artificial Lambertian sphere, and the signal voltage generated by the detector in the satellite remote sensor is at the saturation signal of the detector. Between 20% and 80% of the voltage value.

The artificial Lambertian sphere is an aluminum sphere, and the surface is coated with barium sulfate powder through an adhesive.

The advantage of the present invention compared with prior art is:

(1) The inventive method adopts the Lambertian sphere to indirectly introduce the solar radiation into the remote sensor as a calibration source. The calibration source is located outside the entire system, can calibrate the optical elements in the entire optical path, and can also fill the remote sensor aperture to meet Radiation calibration requirements for full optical path, full aperture, and solar spectral distribution matching;

(2) the inventive method adopts the Lambertian sphere on orbit to directly reflect solar radiation, gets rid of the limitation of atmospheric conditions and ground target characteristics, and avoids the uncertainty introduced by environmental factors such as atmosphere and ground targets in the solar radiation transfer link, Improved on-orbit absolute radiometric calibration accuracy;

(3) The method of the present invention establishes a unified space optical radiation reference by deploying Lambertian spheres in space, which can realize normalized monitoring of satellite remote sensors, correct the radiation response changes of satellite remote sensors in time, and improve the satellite remote sensors. Orbit radiation calibration response efficiency;

(4) The method of the present invention can carry out the calibration by using the Lambertian sphere in orbit, which is easy to implement, and does not need to install additional calibration equipment on the satellite itself, which is conducive to reducing the burden on the existing optical remote sensing satellite on-board system.

Description of drawings

Fig. 1 is a block flow diagram of the inventive method;

Fig. 2 is the schematic diagram of solar radiation energy transfer model of the present invention;

Fig. 3 is the characteristic parameter schematic diagram of the present invention;

Fig. 4 is the relationship between the Lambertian sphere reflection signal DN value and the deep space background signal DN value according to the present invention.

Detailed ways

As shown in Figure 1, it is a flow chart of the inventive method, and the main steps are as follows:

(1) launch an artificial Lambertian sphere into space by means of carrier rocket launch or satellite release launch, the diameter and orbital position of the sphere meet the requirements of satellite calibration for signal energy;

The signal energy requirement of satellite calibration is that the satellite remote sensor receives the solar energy reflected by the sphere, and the signal voltage generated by the detector in the remote sensor is between 20% and 80% of the saturation signal voltage value of the detector.

The artificial Lambertian sphere is a uniform sphere with a surface close to Lambertian, and one example thereof is a barium sulfate Lambertian sphere, which is made by spraying. First, polish the surface of the uniform aluminum sphere with sandpaper, mix barium sulfate powder with milky white glue to make a semi-paste and evenly spray it on the surface of the polished aluminum sphere, and obtain the artificial Lambertian sphere after drying.

(2) Set the parameters of the satellite remote sensor to be calibrated so that the integration time, gain and series remain unchanged throughout the calibration process.

(3) Maneuver the attitude of the satellite, point the remote sensor of the satellite to be calibrated to the deep space background, obtain the gray value of the imaging pixel of the remote sensor of the satellite to be calibrated, and perform n times (n≥1, the more times, the higher the accuracy) Measurement, to obtain the gray value DN _k0i of each measured pixel, where k is the position of the remote sensor pixel (the value range of k is from 1 to the total number of remote sensor pixels), i is the number of measurements, i=1,2,3...,n, after n measurements are completed, calculate the average value of the background signal DN _k0 of the pixel k of the satellite remote sensor to be calibrated

(4) Use the satellite tool software STK to simulate the transfer of solar radiation energy, establish a scene including satellites, Lambertian spheres and the sun, and obtain that when the positional relationship among the satellites, Lambertian spheres and the sun satisfies the calibration conditions The transfer model of solar radiant energy during the time period is shown in Figure 2. In Figure 2, the Lambertian sphere reflects the received solar radiant energy to the outside, and the remote sensor receives the radiant energy reflected by the Lambertian sphere.

Calibration conditions include geometric visible conditions and sensor visible conditions, and the two conditions must be satisfied at the same time. The geometrically visible condition is that the connection line between the satellite and the Lambertian sphere is not blocked by the earth. The visible condition of the sensor is that the Lambertian sphere is outside the earth's shadow, and the Lambertian sphere can reflect solar radiation energy to achieve observation and imaging.

(5) Randomly select an imaging time point from the time period that satisfies the calibration conditions, and perform attitude maneuver on the satellite so that the remote sensor of the satellite to be calibrated points to the Lambertian sphere, and the detector in the satellite remote sensor can obtain the Lambertian sphere The signal obtained by reflecting solar radiation. The azimuth angle of the Lambertian sphere observed by the remote sensor, that is, the angle between the line between the remote sensor and the Lambertian sphere and the direction of the entrance pupil of the remote sensor is recorded as φ.

(6) Obtain the positional relationship between the satellite, the Lambertian sphere and the sun at the moment of imaging. After the Lambertian sphere is launched into a specific orbit in space, it is necessary to estimate the on-orbit position of the Lambertian sphere through ground measurement, and obtain three characteristic parameters at the moment when the satellite images the Lambertian sphere, including the relative distance between the Lambertian sphere and the sun L _{1 (j)} , the relative distance L _2(j) between the Lambertian sphere and the entrance pupil of the satellite remote sensor to be calibrated, the connection line between the sun and the Lambertian sphere, and the connection line between the satellite remote sensor to be calibrated and the Lambertian sphere The included angle θ _j . The schematic diagram of the characteristic parameters is shown in Figure 3.

(7) Calculate the irradiance E _j where the solar radiation reaches the Lambertian sphere by the following formula.

in:

E(λ) is the spectral irradiance at the place where the solar radiation reaches the Lambertian sphere, and the unit is W·m ^-2 ·μm ^-1 ;

λ ₁ and λ ₂ are the lower limit and upper limit of the spectral band of the remote sensor to be calibrated respectively;

R is the radius of the sun, R=6.9599×10 ⁸ m;

j is the number of measurements.

M(λ) is the spectral radiance emission of the sun (in W·m ^-2 ·μm ^-1 ), the solar light source can be equivalent to a radiation black body with a temperature of 5900K, according to the Planck black body radiation formula, the spectral radiation of the sun The output degree can be expressed as In the formula, λ is the wavelength, c ₁ is the first black body radiation constant (c ₁ =3.741844×10 ⁴ W·m ^-2 ·μm ⁴ ), c ₂ is the second black body radiation constant (c ₂ =14388μm·K), T is the thermodynamic temperature (T=5900K).

(8) Calculate the irradiance E _kj at the entrance pupil of the satellite remote sensor by the following formula.

in:

ρ is the surface reflectance of the Lambertian sphere;

f(θ _j ) is the angle factor between the reflected energy and the incident energy, which is obtained by optical modeling of the space spherical target:

where d is the diameter of the Lambertian sphere;

(9) The Lambertian sphere is a point source target. After imaging the Lambertian sphere, obtain the position and gray value of the image point on the detector, which are denoted as k and DN _kj respectively. The relationship between the Lambertian sphere reflection signal DN value and the deep space background signal DN value is shown in Figure 4. In the situation shown in the figure, the image point position k=15, and the image point gray value DN _kj =B.

(10) Under the condition of keeping φ constant, repeat steps (5) to (9) to obtain the gray value DN _kj and entrance pupil irradiance E _kj corresponding to the n measurements of pixel k. Combined with the background signal value DN _k0 of the pixel, the linear response relationship is established by the least square method, and the absolute radiation calibration coefficient G _k of the pixel k is obtained. The specific formula is as follows:

The content that is not described in detail in the description of the present invention belongs to the well-known technology of those skilled in the art.

Claims (3)

1. a kind of Optical remote satellite absolute radiation calibration method based on space lambert's sphere, it is characterised in that including walking as follows Suddenly：

(1) radiation pattern is discharged by carrier rocket radiation pattern or satellite and emits artificial lambert's sphere into space, The diameter of sphere meets requirement of the satellite calibration to signal energy with orbital position；Artificial lambert's sphere has for surface Close to the uniform spherome of Lambertian characteristics；

(2) parameter of satellite remote sensor to be calibrated is set, the time of integration, gain and series is made to be kept not in entire calibration process Become；

(3) attitude maneuver is carried out to satellite, satellite remote sensor to be calibrated is made to be directed toward deep space background, obtains satellite remote sensor to be calibrated The gray value of pixel is imaged, carries out n times measurement, the gray value DN of the pixel k measured every time altogether_k0i, wherein n >=1, k are distant The position of sensor pixel, the value range of k are the sums from 1 to remote sensor pixel, and i is the number of measurement, i=1,2, 3......,n；After n times are measured, the background signal average value DN of satellite remote sensor pixel k to be calibrated is calculated_k0,

<mrow> <msub> <mi>DN</mi> <mrow> <mi>k</mi> <mn>0</mn> </mrow> </msub> <mo>=</mo> <mfrac> <mn>1</mn> <mi>n</mi> </mfrac> <munderover> <mo>&amp;Sigma;</mo> <mrow> <mi>i</mi> <mo>=</mo> <mn>1</mn> </mrow> <mrow> <mi>i</mi> <mo>=</mo> <mi>n</mi> </mrow> </munderover> <msub> <mi>DN</mi> <mrow> <mi>k</mi> <mn>0</mn> <mi>i</mi> </mrow> </msub> <mo>;</mo> </mrow>

(4) establish and include the scene of satellite, artificial lambert's sphere and the sun, emulate the transmission of solar radiation energy, obtain satellite, Position relationship between artificial lambert's sphere and sun three meets the period of calibration condition；The calibration condition includes several What visible condition and visible condition of sensor, the visible condition of geometry for satellite and the line of artificial lambert's sphere therebetween from The earth blocks, and the visible condition behaviour of sensor is made lambert's sphere and is in outside ground shadow, and artificial lambert's sphere can reflect sun spoke Penetrating can be imaged so as to fulfill observation；

(5) a certain imaging moment point is chosen from the period for meeting calibration condition, attitude maneuver is carried out to satellite, is made to be calibrated The entrance pupil normal of satellite remote sensor is directed toward lambert's sphere, and the azimuth that remote sensor is observed to artificial lambert's sphere is denoted as φ, obtains The relative distance L of artificial lambert's sphere and the sun_1(j), artificial lambert's sphere at satellite remote sensor entrance pupil to be calibrated it is opposite away from From L_2(j), line between the sun and artificial lambert's sphere and the company between satellite remote sensor to be calibrated and artificial lambert's sphere The angle theta of line_j, irradiation level E at satellite remote sensor entrance pupil can be reached by thus obtaining artificial lambert's sphere reflected solar radiation_kj, The corresponding pixel position k and gray value DN of artificial lambert's sphere picture point_kj, j is pendulous frequency；

(6) under conditions of keeping φ constant, step (5) is repeated, the n times for obtaining pixel k measure corresponding gray value DN_kjWith enter Pupil irradiation level E_kj, with reference to the background signal value DN of pixel_k0, linear response relationship is established by least square method, obtains pixel k Absolute Radiometric Calibration Coefficients G_k,

<mrow> <msub> <mi>G</mi> <mi>k</mi> </msub> <mo>=</mo> <mfrac> <mrow> <munderover> <mo>&amp;Sigma;</mo> <mrow> <mi>j</mi> <mo>=</mo> <mn>1</mn> </mrow> <mi>n</mi> </munderover> <mo>&amp;lsqb;</mo> <mrow> <mo>(</mo> <msub> <mi>DN</mi> <mrow> <mi>k</mi> <mi>j</mi> </mrow> </msub> <mo>-</mo> <msub> <mi>DN</mi> <mrow> <mi>k</mi> <mn>0</mn> </mrow> </msub> <mo>)</mo> </mrow> <mo>&amp;CenterDot;</mo> <msub> <mi>E</mi> <mrow> <mi>k</mi> <mi>j</mi> </mrow> </msub> <mo>&amp;rsqb;</mo> </mrow> <mrow> <munderover> <mo>&amp;Sigma;</mo> <mrow> <mi>j</mi> <mo>=</mo> <mn>1</mn> </mrow> <mi>n</mi> </munderover> <msubsup> <mi>E</mi> <mrow> <mi>k</mi> <mi>j</mi> </mrow> <mn>2</mn> </msubsup> </mrow> </mfrac> <mo>.</mo> </mrow>

2. a kind of Optical remote satellite absolute radiation calibration method based on space lambert's sphere according to claim 1, It is characterized in that：Requirement of step (1) Satellite calibration to signal energy be：Satellite remote sensor can receive artificial bright The solar energy of primary sphere reflection, and the signal voltage that the detector in satellite remote sensor generates is located at detector saturation signal Between the 20%~80% of voltage value.

3. a kind of Optical remote satellite absolute radiometric calibration side based on space lambert's sphere according to claim 1 or 2 Method, it is characterised in that：Artificial lambert's sphere is aluminum sphere, and surface is coated with blanc fixe by adhesive.