Method for carrying out ground calibration on microwave radiometer by utilizing dielectric property of wave-transparent material

Abstract

The invention belongs to the technical field of atmospheric science remote sensing, and particularly relates to a method for carrying out ground calibration on a microwave radiometer by utilizing the dielectric property of a wave-transparent material, which comprises the following steps of: setting a distance between a dielectric plate and the microwave radiometer to be calibrated, and obliquely placing the dielectric plate and the horizontal direction at a fixed angle; placing a wave-absorbing material at a set distance from the microwave radiometer in the vertical direction of the same side of the dielectric plate; the dielectric plate is made of a wave-transparent material; respectively measuring the physical temperature of the dielectric plate and the radiation brightness temperature of the black body; the observation brightness temperature difference when the microwave radiometer observes the dielectric plate is obtained through calculation; and a calibration coefficient is obtained by observing radiation of a normal-temperature black body in combination with a microwave radiometer. The method provided by the invention is suitable for overall calibration of radiometer antennas of any aperture; calibration operation is convenient, and low-temperature equipment such as liquid nitrogen is not needed; the calibration dynamic range is large, the brightness temperature range of an observation scene is basically covered, and extrapolation is not needed.

Description

Technical Field

[0001] The invention belongs to the field of atmospheric science remote sensing technology, and in particular relates to a method for ground calibration of a microwave radiometer by utilizing the dielectric properties of a wave-transmitting material. Background Art

[0002] With advances in aerospace technology, satellite launch costs are gradually decreasing. Starlink has launched over 4,000 satellites, with plans to launch 42,000. Considering launch and manufacturing costs, mass production and miniaturization are inevitable trends in the development of this series of satellites. Against this backdrop, remote sensing satellites are also actively exploring lightweighting and miniaturization. As a key payload for remote sensing satellites and a crucial tool for remote sensing of sea surface wind vectors, the development of fully polarized microwave radiometers is also steadily progressing towards miniaturization.

[0003] With their small size, light weight, low power consumption, and low cost, small fully polarimetric microwave radiometers provide strong support for passive microwave remote sensing of sea surface wind vectors. They also offer broad prospects for the development and application of passive microwave remote sensing, making miniaturization a future trend in microwave remote sensing. The National Space Science Center of the Chinese Academy of Sciences has essentially completed the development of a small fully polarimetric microwave radiometer and is currently undergoing testing. Unlike traditional full-power microwave radiometers, the calibration process for fully polarimetric microwave radiometers is more complex. A suitable and lightweight calibration scheme is essential for fully polarimetric microwave radiometers to become satellite payloads and a prerequisite for accurate data inversion by future satellite-borne small fully polarimetric microwave radiometers. Summary of the Invention

[0004] The purpose of the present invention is to overcome the defects of the prior art and propose a method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transmitting material.

[0005] In view of this, the present invention proposes a method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transmitting material, comprising:

[0006] The dielectric plate is placed at a set distance from the microwave radiometer to be calibrated and tilted at a fixed angle to the horizontal direction; an absorbing material is placed at a set distance perpendicular to the microwave radiometer on the same side of the dielectric plate; the dielectric plate is made of a wave-transmitting material;

[0007] Measure the physical temperature of the dielectric plate and the radiation brightness temperature of the black body respectively;

[0008] The brightness temperature difference observed when the microwave radiometer observes the dielectric plate is obtained by calculation;

[0009] The calibration coefficient is obtained by observing the radiation of a black body at room temperature with a microwave radiometer.

[0010] Preferably, the fixed angle is 45 degrees.

[0011] Preferably, the observed brightness temperature difference ΔT when the microwave radiometer observes the dielectric plate is B for:

[0012] ΔT B =(τ e -1)T BD +e e T p +r e T BB

[0013] Among them, T BD is the radiation of the downwelling atmosphere or low temperature scene, T P is the physical temperature of the dielectric plate, T BB is the blackbody radiation brightness temperature of the absorbing material, r e is the equivalent reflectivity, τ e is the transmittance, e e is the emissivity.

[0014] Preferably, the τ e ,e e ,r e According to the following formula:

[0015]

[0016] Where r1 is the reflectivity of the dielectric plate and the air surface, d is the thickness of the dielectric plate, L2 is the loss factor, and β2′ is the component of the imaginary part of the propagation vector in the propagation direction;

[0017]

[0018] β2′=β2cosθ2

[0019] Where α2 and β2 are the real and imaginary parts of the propagation vector, respectively, and θ2 is the incident angle in the dielectric plate.

[0020] Preferably, when the wavelength of the spatial variation of the dielectric constant is much smaller than the wavelength of the electromagnetic wave in free space, according to the following formula:

[0021]

[0022] Get τ e ,e e ,r e ;

[0023] Where L2 is the loss factor, which satisfies the following formula:

[0024]

[0025] Where α2 is the real part of the propagation vector and θ2 is the incident angle in the dielectric plate.

[0026] Preferably, the calibration coefficients a and b are obtained according to the following formula:

[0027]

[0028] Among them, V in and V BD are the output voltages of the microwave radiometer when observing the dielectric plate and without the dielectric plate, respectively.

[0029] On the other hand, the present invention provides a method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transmitting material, comprising:

[0030] The dielectric plate and the metal plate are arranged in parallel, and are respectively placed at a set distance from the microwave radiometer to be calibrated, and are tilted at a fixed angle to the horizontal direction; the dielectric plate is made of a wave-transparent material;

[0031] Measure the physical temperature of the dielectric plate and the radiation brightness temperature of the black body respectively;

[0032] The brightness temperature difference observed when the microwave radiometer observes the dielectric plate is obtained by calculation;

[0033] The calibration coefficient is obtained by observing the radiation of a black body at room temperature with a microwave radiometer.

[0034] Preferably, the fixed angle is 45 degrees.

[0035] Preferably, the observed brightness temperature difference ΔT when the microwave radiometer observes the dielectric plate is B for:

[0036]

[0037] Among them, T BD is the radiation of the downwelling atmosphere or low temperature scene, T P is the physical temperature of the dielectric plate, T BB is the brightness temperature of the black body radiation, r e is the equivalent reflectivity, τ e is the transmittance, e e is the emissivity.

[0038] Preferably, the τ e ,e e ,r e According to the following formula:

[0039]

[0040] Where r1 is the reflectivity of the dielectric plate and the air surface, d is the thickness of the dielectric plate, L2 is the loss factor, and β2′ is the component of the imaginary part of the propagation vector in the propagation direction;

[0041]

[0042] β2′=β2cosθ2

[0043] Where α2 and β2 are the real and imaginary parts of the propagation vector, respectively, and θ2 is the incident angle in the dielectric plate.

[0044] Preferably, when the wavelength of the spatial variation of the dielectric constant is much smaller than the wavelength of the electromagnetic wave in free space, according to the following formula:

[0045]

[0046]

[0047] Get τ e ,e e ,r e ;

[0048] Where L2 is the loss factor, which satisfies the following formula:

[0049]

[0050] Where α2 is the real part of the propagation vector and θ2 is the incident angle in the dielectric plate.

[0051] Preferably, the calibration coefficients a and b are obtained according to the following formula:

[0052]

[0053] Among them, V in and V BD are the output voltages of the microwave radiometer when observing the dielectric plate and without the dielectric plate, respectively.

[0054] Compared with the prior art, the advantages of the present invention are:

[0055] 1. The method of the present invention is suitable for the overall calibration of radiometer antennas of any caliber;

[0056] 2. The calibration operation using the method of the present invention is convenient and does not require low-temperature equipment such as liquid nitrogen;

[0057] 3. The calibration dynamic range of the method of the present invention is large, basically covering the brightness temperature range of the observation scene, and no extrapolation is required. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 It is a schematic diagram of the transmission process of the dielectric plate in the air;

[0059] Figure 2 This is the calibration method of the external absorbing material of Example 1;

[0060] Figure 3 The calibration method of the metal plate combined with the dielectric plate of Example 2;

[0061] FIG4 is a performance comparison of two calibration methods for a 3 mm thick dielectric plate, FIG4(a) is the method of Example 1, and FIG4(b) is the method of Example 2;

[0062] FIG5 is a performance comparison of two calibration methods for a 5 mm thick dielectric plate, FIG5(a) is the method of Example 1, and FIG5(b) is the method of Example 2;

[0063] FIG6 is a performance comparison of two calibration methods for a 10 mm thick dielectric plate. FIG6(a) is the method using Example 1, and FIG6(b) is the method using Example 2. DETAILED DESCRIPTION

[0064] The present invention proposes a principle and method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transparent material, such as a high-density polyurethane foam commonly used in microwaves.

[0065] 1. Principle of electromagnetic wave radiation transmission of wave-transmitting dielectric plate materials

[0066] 1.1 Description of the transmission process of dielectric plates in air

[0067] The dielectric plate in the air can transmit and reflect the incident electromagnetic wave as follows: Figure 1 describe.

[0068] In air, the dielectric constant ε1 = ε1′. The dielectric constant ε2 of the medium is ε2′-jε2″, and the propagation vector γ2 = α2+jβ2.

[0069] Where α2 and β2 for any lossy medium are defined as follows:

[0070]

[0071] Where λ0 is the wavelength in air. The unit of α2 and β2 is Npm. If the medium is a weak attenuation medium, α2 and β2 can be approximately expressed as:

[0072]

[0073] When an electromagnetic wave is incident obliquely from the air onto the surface of a dielectric plate at an incident angle θ1, the law of refraction is satisfied. Therefore, we have:

[0074]

[0075] The above cosθ2 is an imaginary number, and the real angle χ2 is expressed as:

[0076]

[0077] in,

[0078] p=2α2β2

[0079]

[0080] 1.2 Calculation of reflection and transmission parameters caused by dielectric plate

[0081] The power reflectivity r1 of vertically polarized and horizontally polarized electromagnetic waves on the surface of the dielectric plate is expressed as:

[0082]

[0083] When considering the scattering of the dielectric plate, the equivalent reflectivity r of the dielectric plate can be solved using the coherent method. e , transmittance τ e and emissivity e e , the formulas are as follows:

[0084]

[0085] Where r1 is the reflectivity of the dielectric and air surfaces, L2 is the loss factor, and d is the dielectric plate thickness.

[0086]

[0087] β2′=β2 cosθ2 (11)

[0088] According to the principle of conservation of energy, effective emissivity + reflectivity + transmittance = 1, that is:

[0089] e e +r e +τ e =1 (12)

[0090] When the wavelength of the spatial variation of the dielectric constant When it is much smaller than the wavelength λ0 of the electromagnetic wave in free space, it can be considered that no scattering occurs. At this time, the incoherent method can be used to solve the three parameters as follows:

[0091]

[0092] The parameters are the same as those defined above.

[0093] The technical solution of the present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0094] Example 1

[0095] Embodiment 1 of the present invention provides a dielectric plate calibration method for a microwave radiometer.

[0096] The layout used for radiometer calibration is as follows Figure 2 As shown in the figure, the dielectric plate is tilted at 45°. The radiation leaking from the radiometer is absorbed by the absorbing material. At the same time, the blackbody radiation of the absorbing material is reflected by the dielectric plate and enters the antenna.

[0097] The radiation received by a microwave radiometer can be expressed as three parts: the radiance of the downlink radiation passing through the dielectric plate, the radiance of the dielectric plate itself, and the radiance of the blackbody radiation reflected by the dielectric plate. This can be expressed as:

[0098] T B,in =τ e T BD +e e T p +r e T BB (16)

[0099] Among them, T BD is the radiation of the downwelling atmosphere or low temperature scene, T P is the physical temperature of the dielectric plate, T BB is the radiation brightness temperature of the black body. In the experiment, T P and T BB The parameter τ of the dielectric plate can be measured or calculated. e ,e e ,r e The formula in Section 1 can be used to calculate the dielectric constant and the incident angle. The observed brightness temperature difference obtained with and without the dielectric plate is:

[0100] ΔT B =(τ e -1)T BD +e e T p +r e T BB (17)

[0101] Example 2

[0102] The second embodiment of the present invention proposes a calibration method using a combination of a metal plate and a dielectric plate, such as Figure 3 shown.

[0103] If the third layer is not air but metal, then when the transmitted electromagnetic wave encounters the metal, it is totally reflected (assuming the metal reflectivity is 1) and re-enters the medium. In this case, it is equivalent to a radiation source re-entering the medium at an exit angle and reaching the receiver after passing through the dielectric plate.

[0104] Radiation T BD The total energy that passes through the dielectric plate and reaches the third layer is:

[0105] T B2 =τ e T BD +e e T p (18)

[0106] This energy is reflected by the metal plate, enters the dielectric plate again, and transmits through the dielectric plate to reach the receiver. The transmittance is still τ e The total energy reaching the receiver is thus expressed as:

[0107] T B,in =τ e T B2 +e e T p +r e T BD (19)

[0108] Substituting (18) into (19) we can get:

[0109]

[0110] therefore,

[0111] It can be seen that the brightness temperature difference at this time has nothing to do with the ambient temperature, but only with the dielectric plate parameters and the background brightness temperature T BD related.

[0112] Radiometer calibration

[0113] When the radiometer observes the dielectric plate and the radiation background T without the dielectric plate BD When the influence of antenna pattern is ignored, the ideal observed brightness temperature can be expressed as:

[0114] T B,in =aV in +b (22)

[0115] T BD =aV BD +b (23)

[0116] ΔT B =T B,in -T BD =a(V in -V BD ) (twenty four)

[0117] If the radiometer also observes the brightness temperature of a normal temperature blackbody T BB , then we have:

[0118] T BB =aV BB +b (25)

[0119] Therefore, by combining the observed brightness temperature difference of the dielectric plate and the radiation of the black body, the calibration coefficients a and b can be obtained, namely:

[0120]

[0121] Using the measured value of the dielectric constant of the dielectric plate, ΔT can be evaluated B The size of , and then analyze the error caused by its use in calibration.

[0122] This solution can be implemented when T is not known accurately. BD Estimating ΔT in the case of B .

[0123] The following analysis of the incoherent method ΔT B With T BD The relationship between ΔT B For T BD Assuming a frequency of 37 GHz, ambient temperature and dielectric plate temperature of 280 K, dielectric plate thickness of 3 mm, 5 mm, and 10 mm, and dielectric constant of 2.6-0.008 j, two calibration methods are obtained. B With T BD 4, 5 and 6, wherein FIG4(a) is Example 1, FIG4(b) is Example 2, FIG5(a) is Example 1, FIG5(b) is Example 2, FIG6(a) is Example 1, FIG6(b) is Example 2.

[0124] From the results, we can see that as the thickness of the dielectric plate increases, the brightness temperature difference increases and the dependence on TBd increases. Therefore, the smaller the thickness of the dielectric plate, the better the brightness temperature difference will be for ΔT B The more accurate the estimation, the smaller the error in calibration. If the three parameters of the medium are known, then under the condition of real-time measurement of the ambient temperature and the temperature of the medium plate, T can be roughly given. BD , we can get a more accurate ΔT B Thus, the calibration accuracy is guaranteed. Taking the dielectric plate thickness of 3mm as an example, when the given atmospheric downlink brightness temperature error is 2k, the resulting ΔT B The brightness temperature error is less than 0.5 K. Therefore, both calibration methods can effectively improve the calibration accuracy of ground-based radiometers.

[0125] Finally, it should be noted that the above embodiments are intended only to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention and are intended to be encompassed by the claims of the present invention.

Claims

1. A method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transmitting material, comprising: The dielectric plate is placed at a set distance from the microwave radiometer to be calibrated and tilted at a fixed angle to the horizontal direction; A wave absorbing material is placed at a set distance perpendicular to the same side of the dielectric plate as the microwave radiometer; the dielectric plate is made of a wave-transmitting material; Measure the physical temperature of the dielectric plate and the radiation brightness temperature of the black body respectively; The brightness temperature difference observed when the microwave radiometer observes the dielectric plate is obtained by calculation; The calibration coefficient is obtained by observing the radiation of a black body at room temperature with a microwave radiometer.

2. The method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transmitting material according to claim 1, characterized in that: The fixed angle is 45 degrees.

3. The method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transmitting material according to claim 1, characterized in that: The observed brightness temperature difference ΔT when the microwave radiometer observes the dielectric plate B for: ΔT B =(τ e -1)T BD +e e T p +r e T BB Among them, T BD is the radiation of the downwelling atmosphere or low temperature scene, T P is the physical temperature of the dielectric plate, T BB is the blackbody radiation brightness temperature of the absorbing material, r e is the equivalent reflectivity, τ e is the transmittance, e e is the emissivity.

4. The method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transmitting material according to claim 3, wherein: The τ e ,e e ,r e According to the following formula: Where r1 is the reflectivity of the dielectric plate and the air surface, d is the thickness of the dielectric plate, L2 is the loss factor, and β2′ is the component of the imaginary part of the propagation vector in the propagation direction; β2′=β2cosθ2 Where α2 and β2 are the real and imaginary parts of the propagation vector, respectively, and θ2 is the incident angle in the dielectric plate.

5. The method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transmitting material according to claim 3, characterized in that: When the wavelength of the spatial variation of the dielectric constant is much smaller than the wavelength of the electromagnetic wave in free space, according to the following formula: Get τ e ,e e ,r e ; Where L2 is the loss factor, which satisfies the following formula: Where α2 is the real part of the propagation vector and θ2 is the incident angle in the dielectric plate.

6. The method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transmitting material according to claim 4 or 5, characterized in that: The calibration coefficients a and b are obtained according to the following formula: Among them, V in and V BD are the output voltages of the microwave radiometer when observing the dielectric plate and without the dielectric plate, respectively.

7. A method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transmitting material, comprising: The dielectric plate and the metal plate are placed in parallel, and are respectively at a set distance from the microwave radiometer to be calibrated, and are tilted at a fixed angle to the horizontal direction; The dielectric plate is made of wave-transparent material; Measure the physical temperature of the dielectric plate and the radiation brightness temperature of the black body respectively; The brightness temperature difference observed when the microwave radiometer observes the dielectric plate is obtained by calculation; The calibration coefficient is obtained by observing the radiation of a black body at room temperature with a microwave radiometer.

8. The method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transmitting material according to claim 7, wherein: The fixed angle is 45 degrees.

9. The method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transmitting material according to claim 7, wherein: The observed brightness temperature difference ΔT when the microwave radiometer observes the dielectric plate B for: Among them, T BD is the radiation of the downwelling atmosphere or low temperature scene, T P is the physical temperature of the dielectric plate, T BB is the brightness temperature of the black body radiation, r e is the equivalent reflectivity, τ e is the transmittance, e e is the emissivity.

10. The method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transmitting material according to claim 9, characterized in that: The τ e ,e e ,r e According to the following formula: Where r1 is the reflectivity of the dielectric plate and the air surface, d is the thickness of the dielectric plate, L2 is the loss factor, and β2′ is the component of the imaginary part of the propagation vector in the propagation direction; β2′=β2cosθ2 Where α2 and β2 are the real and imaginary parts of the propagation vector, respectively, and θ2 is the incident angle in the dielectric plate.

11. The method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transmitting material according to claim 9, wherein: When the wavelength of the spatial variation of the dielectric constant is much smaller than the wavelength of the electromagnetic wave in free space, according to the following formula: Get τ e ,e e ,r e ; Where L2 is the loss factor, which satisfies the following formula: Where α2 is the real part of the propagation vector and θ2 is the incident angle in the dielectric plate.

12. The method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transmitting material according to claim 10 or 11, characterized in that: The calibration coefficients a and b are obtained according to the following formula: Among them, V in and V BD are the output voltages of the microwave radiometer when observing the dielectric plate and without the dielectric plate, respectively.

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