Method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transparent material

Abstract

The present application belongs to the field of atmospheric science remote sensing technology, and particularly relates to a method for ground calibration of a microwave radiometer by using dielectric properties of a wave-transparent material, comprising: setting a distance between a dielectric plate and a microwave radiometer to be calibrated, and placing the dielectric plate at a fixed angle with the horizontal direction; placing a wave-absorbing material at a set distance in the vertical direction on the same side of the dielectric plate as the microwave radiometer; the dielectric plate is made of a wave-transparent material; measuring the physical temperature of the dielectric plate and the radiation brightness temperature of a black body, respectively; obtaining the observed brightness temperature difference when the microwave radiometer observes the dielectric plate by calculation; and obtaining a calibration coefficient by combining the radiation of the microwave radiometer observing a normal-temperature black body. The method of the present application is suitable for overall calibration of radiometers of any caliber; the calibration operation is convenient, and does not require low-temperature equipment such as liquid nitrogen; the calibration dynamic range is large, basically covering the brightness temperature range of the observation scene, and does not require extrapolation.

Description

Technical Field

[0001] This invention belongs to the field of atmospheric science remote sensing technology, and in particular relates to a method for ground calibration of microwave radiometers using the dielectric properties of transparent materials. Background Technology

[0002] With advancements in aerospace technology, satellite launch costs have gradually decreased. Currently, Starlink has launched over 4,000 satellites, with a planned launch count of 42,000. Considering launch and manufacturing costs, mass production and miniaturization have become inevitable trends in the development of this satellite series. Against this backdrop, remote sensing satellites are also actively exploring lightweighting and miniaturization. As an important payload for remote sensing satellites and a crucial means of sea surface wind vector remote sensing, the development of the fully polarimetric microwave radiometer is also steadily progressing towards miniaturization.

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

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and to propose a method for ground calibration of microwave radiometers using the dielectric properties of wave-transparent materials.

[0005] In view of this, the present invention proposes a method for ground calibration of microwave radiometers using the dielectric properties of microwave-transparent materials, comprising:

[0006] The dielectric plate is placed at a set distance from the microwave radiometer to be calibrated and at a fixed angle to the horizontal direction; a microwave 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 microwave transparent material.

[0007] The physical temperature of the dielectric substrate and the radiative brightness temperature of the blackbody were measured separately.

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

[0009] The calibration coefficients were obtained by combining microwave radiometer observations of the radiation of a blackbody at room temperature.

[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 For radiation in the down-current atmosphere or low-temperature scenarios, T P T represents the physical temperature of the dielectric substrate. BB r is the blackbody radiation brightness temperature of the absorbing material. e For the equivalent reflectivity, τ e For transmittance, e e Emission rate.

[0014] Preferably, the τ e ,e e ,r e We obtain 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 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] τ e ,e e ,r e ;

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

[0024]

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

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

[0027]

[0028] Among them, V in and V BD These represent the output voltages of the microwave radiometer under the conditions of observing the dielectric plate and without the dielectric plate, respectively.

[0029] On the other hand, this invention proposes a method for ground calibration of microwave radiometers using the dielectric properties of wave-transparent materials, comprising:

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

[0031] The physical temperature of the dielectric substrate and the radiative brightness temperature of the blackbody were measured separately.

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

[0033] The calibration coefficients were obtained by combining microwave radiometer observations of the radiation of a blackbody at room temperature.

[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 For radiation in the down-current atmosphere or low-temperature scenarios, T P T represents the physical temperature of the dielectric substrate. BB r is the brightness temperature of the blackbody's radiation. e For the equivalent reflectivity, τ e For transmittance, e e Emission rate.

[0038] Preferably, the τ e ,e e ,r e We obtain 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 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] τ e ,e e ,r e ;

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

[0049]

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

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

[0052]

[0053] Among them, V in and V BD These represent the output voltages of the microwave radiometer under the conditions of 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 aperture;

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

[0057] 3. The method of the present invention has a large calibration dynamic range, basically covering the brightness temperature range of the observation scene, and does not require extrapolation. Attached Figure Description

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

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

[0060] Figure 3 This is the calibration method for the metal plate combined with the dielectric plate in Example 2;

[0061] Figure 4 compares the performance of two calibration methods for a 3mm thick dielectric substrate. Figure 4(a) shows the method used in Example 1, and Figure 4(b) shows the method used in Example 2.

[0062] Figure 5 shows a performance comparison of two calibration methods for a 5mm thick dielectric substrate. Figure 5(a) shows the method used in Example 1, and Figure 5(b) shows the method used in Example 2.

[0063] Figure 6 compares the performance of two calibration methods for a 10mm thick dielectric substrate. Figure 6(a) shows the method used in Example 1, and Figure 6(b) shows the method used in Example 2. Detailed Implementation

[0064] This invention proposes a principle and method for ground calibration of microwave radiometers using the dielectric properties of microwave-transparent materials. The microwave-transparent materials include high-density polyurethane foam and other materials commonly used in microwave applications.

[0065] 1. Electromagnetic wave radiation transmission principle of wave-transparent dielectric plate materials

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

[0067] A dielectric plate placed in air can perform the following operations regarding the transmission and reflection of incident electromagnetic waves: Figure 1 describe.

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

[0069] The α2 and β2 of any lossy medium are defined as follows:

[0070]

[0071] Where λ0 is the wavelength in air. The units of α2 and β2 are both Npm. If the medium is a weakly attenuating medium, α2 and β2 can be approximated as:

[0072]

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

[0074]

[0075] The above cosθ2 is an imaginary number, and its 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 substrate is expressed as follows:

[0082]

[0083] When considering scattering from the dielectric substrate, the equivalent reflectivity r of the dielectric substrate can be solved using a 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 thickness of the dielectric substrate.

[0086]

[0087] β2′=β2 cosθ2 (11)

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

[0089] e e +r e +τ e =1 (12)

[0090] When the spatial variation of the dielectric constant is at wavelength When the wavelength is much smaller than λ0 of the electromagnetic wave in free space, it can be assumed that no scattering occurs. In this case, the three parameters can be solved using an incoherent method as follows:

[0091]

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

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

[0094] Example 1

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

[0096] Layout for radiometer calibration, as shown Figure 2 As shown, the dielectric substrate is placed at a 45° angle, and the radiation leaking from the radiometer is absorbed by the absorbing material. Simultaneously, the blackbody radiation from the absorbing material is reflected by the dielectric substrate into the antenna.

[0097] The radiation received by a microwave radiometer can be expressed as three parts: the radiance of the downward radiation transmitted 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 by the formula:

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

[0099] Among them, T BD For radiation in the down-current atmosphere or low-temperature scenarios, T P T represents the physical temperature of the dielectric substrate. BB T is the brightness temperature of the blackbody's radiation. In the experiment, T... P and T BB It can be actually measured or calculated. The parameter τ of the dielectric substrate. e ,e e ,r e The temperature difference can be calculated using the formulas in Section 1 based on the dielectric constant and the angle of incidence. Thus, the observed brightness temperature difference obtained with and without a dielectric substrate is:

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

[0101] Example 2

[0102] Embodiment 2 of the present invention proposes a calibration method utilizing a combination of a metal plate and a dielectric plate, such as... Figure 3 As shown.

[0103] If the third layer is metal instead of air for transmitting parameters, then when the transmitted electromagnetic wave encounters the metal, it is totally reflected (assuming the metal's reflectivity is 1) and re-enters the medium. In this case, it is equivalent to a radiation source re-entering the medium at the exit angle, passing through the medium plate, and reaching the receiver.

[0104] Radiation T BD The total energy reaching the third layer through the dielectric plate is:

[0105] T B2 =τ e T BD +e e T p (18)

[0106] This energy is reflected by the metal plate, re-enters the dielectric plate, and then passes through the dielectric plate to reach the receiver. The transmittance remains τ. e Therefore, the total energy reaching the receiver is expressed as:

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

[0108] Substituting (18) into (19) and rearranging, we get:

[0109]

[0110] therefore,

[0111] It is evident that the brightness temperature difference at this point is independent of the ambient temperature, and depends only on the substrate parameters and the background brightness temperature T. BD related.

[0112] Radiometer Calibration

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

[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 simultaneously observes a blackbody with a brightness temperature of T at room temperature... BB Then we have:

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

[0119] Therefore, by combining the observed temperature difference of the dielectric plate and the radiation of the blackbody, the calibration coefficients a and b can be obtained, that is:

[0120]

[0121] ΔT can be evaluated using the measured value of the dielectric constant of the dielectric substrate. B The magnitude of the value is then analyzed to determine the error caused by its use in calibration.

[0122] This solution can be implemented when T cannot be accurately known. BD Estimate ΔT under the following circumstances B .

[0123] The following analysis focuses on the incoherent method ΔT. B With T BD The relationship, evaluating ΔT B For T BD The magnitude of the dependence. Assuming a frequency of 37 GHz, ambient and dielectric substrate temperatures of 280 K, dielectric substrate thicknesses of 3 mm, 5 mm, and 10 mm, and dielectric constants of 2.6–0.008 J, two calibration methods ΔT are obtained. B With T BD The relationships are shown in Figures 4, 5 and 6, where Figure 4(a) is Example 1, Figure 4(b) is Example 2, Figure 5(a) is Example 1, Figure 5(b) is Example 2, Figure 6(a) is Example 1 and Figure 6(b) is Example 2.

[0124] The results show that as the dielectric substrate thickness increases, the brightness temperature difference increases, and the dependence on TBd increases. Therefore, the smaller the dielectric substrate thickness, the less dependence on TBd. B The more accurate the estimate, the smaller the error in calibration. If the three parameters of the medium are known, then under the condition of real-time measurement of ambient temperature and medium plate temperature, a rough estimate of T can be given. BD This allows us to obtain a more accurate ΔT. B This ensures the accuracy of the calibration. Taking a dielectric substrate thickness of 3mm as an example, when the atmospheric downlink brightness temperature error is 2K, the resulting ΔT B The brightness temperature error is less than 0.5K. 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 only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand 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 all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transparent 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 microwave 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 microwave transparent material. The physical temperature of the dielectric substrate and the radiative brightness temperature of the blackbody were measured separately. The observed brightness temperature difference of the microwave radiometer when observing the dielectric plate was calculated. for: ； in, For radiation in the downdraft atmosphere or low-temperature scenarios, The physical temperature of the dielectric substrate. The blackbody radiation brightness temperature of the absorbing material. For equivalent reflectivity, For transmittance, For emissivity; The We obtain the following formula: ； ； ； in, The reflectivity of the dielectric substrate and the air surface. d For the thickness of the dielectric substrate, As the loss factor, This is the imaginary component of the propagation vector in the direction of propagation; ； ； in, and These are the real and imaginary parts of the propagation vector, respectively. The angle of incidence in the dielectric substrate; By combining microwave radiometer observations of the radiation from a room-temperature blackbody, the calibration coefficients can be obtained according to the following formula. a and b ; ； ； in, and These represent the output voltages of the microwave radiometer under the conditions of observing the dielectric plate and without the dielectric plate, respectively.

2. The method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transparent 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-transparent material according to claim 1, 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 ; in, The loss factor satisfies the following formula: ； in, Let be the real part of the propagation vector. is the incident angle in the dielectric substrate.

4. A method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transparent material, comprising: The dielectric plate and the metal plate are set parallel to each other and at a set distance from the microwave radiometer to be calibrated, and are placed at a fixed angle to the horizontal direction. The dielectric plate is made of a wave-transparent material; The physical temperature of the dielectric substrate and the radiative brightness temperature of the blackbody were measured separately. The observed brightness temperature difference of the microwave radiometer when observing the dielectric plate was calculated. for: ； in, For radiation in the downdraft atmosphere or low-temperature scenarios, The physical temperature of the dielectric substrate. The brightness temperature of a blackbody. For equivalent reflectivity, For transmittance, For emissivity; The We obtain the following formula: ； ； ； in, The reflectivity of the dielectric substrate and the air surface. d For the thickness of the dielectric substrate, As the loss factor, This is the imaginary component of the propagation vector in the direction of propagation; ； ； in, and These are the real and imaginary parts of the propagation vector, respectively. The angle of incidence in the dielectric substrate; By combining microwave radiometer observations of the radiation of a room-temperature blackbody, calibration coefficients were obtained. a and b : ； ； in, and These represent the output voltages of the microwave radiometer under the conditions of observing the dielectric plate and without the dielectric plate, respectively.

5. The method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transparent material according to claim 4, characterized in that, The fixed angle is 45 degrees.

6. The method for ground calibration of a microwave radiometer using the dielectric properties of a wave-transparent material according to claim 4, 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 ; in, The loss factor satisfies the following formula: ； in, Let be the real part of the propagation vector. is the incident angle in the dielectric substrate.

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