A Method for Selecting Terahertz Radiation Channels for Space Target Detection

Abstract

The present invention provides a method for selecting a terahertz radiation channel for space target detection. The method calculates the atmospheric transmittance and the atmospheric background brightness temperature in the absorption band within the range of 0.1 - 10 THz according to the real-time atmospheric profile data in the observation area, and further obtains the normalized Jacobian matrix of the atmospheric background radiation brightness temperature of different pressure layers at all absorption frequencies with respect to the water vapor volume mixing ratio; the absorption frequency corresponding to the maximum value in the Jacobian matrix and with the actual height at 15 - 25 km at the bottom of the stratosphere is selected as the radiation channel. These absorption frequencies are the most sensitive to the temperature and water vapor changes at the height of 15 - 25 km. Moreover, the temperature at the bottom of the stratosphere is relatively low, the water vapor content is scarce and evenly distributed, which can keep the atmospheric background brightness temperature observed by the detector low and stable, suppress the volatility caused by geographical environment factors, improve the radiation contrast, and thus achieve the precise positioning of the target.

Description

Technical Field

[0001] The present invention belongs to the technical field of terahertz passive detection, and specifically, relates to a method for selecting a terahertz radiation channel for space target detection. Background Art

[0002] Terahertz detectors are mainly used to passively receive the electromagnetic wave energy radiated from space targets and the Earth's surface, and their working center frequency is in the range of 0.1 - 10 THz. Due to the spectral characteristics of its working frequency band, terahertz detectors have the advantages of small volume, high detection spatial resolution, and the ability to penetrate rain and clouds, etc., so as to achieve all-weather and all-day observation, and have great research potential and application value in the field of space target detection.

[0003] When a terahertz detector conducts space target detection, it is often restricted by the orbital altitude, target flight trajectory, and detection angle, and often needs to detect with the Earth's atmosphere as the background. Whether a target can be detected depends on the radiation contrast between the target's own radiation brightness temperature and the surrounding atmospheric background radiation brightness temperature. However, the uncertainty of the atmospheric background radiation brightness temperature makes it difficult for the detector to obtain an ideal radiation contrast. On the one hand, due to the different penetration capabilities of terahertz waves in each frequency band, the proportion of the radiation contribution of each layer of the atmosphere in the total radiation is also completely different, which leads to the overall level of the atmospheric background brightness temperature observed by detectors in different frequency bands being very unstable, either high or low. On the other hand, terahertz waves will be attenuated by the absorption of atmospheric molecules and the scattering of suspended particles during the propagation in the atmosphere, and there will be great differences in different geographical locations and meteorological conditions. This difference makes the atmospheric background brightness temperature fluctuate in space and time, thus covering the target and making it impossible to effectively detect.

[0004] At present, the research on terahertz passive detection technology at home and abroad is still in the development stage, and the working frequencies of the spaceborne terahertz detectors already in use are all in the low-frequency band of 0.1 - 1 THz. These detectors mainly serve for applications such as meteorological remote sensing, so the radiation channel selection is near the absorption peaks of various gases and they do not have the ability to detect space targets. While the traditional radiation channel selection for space target detection in the far-infrared band mainly relies on the radiation contrast and ignores the water vapor factor that has the greatest impact in the terahertz band, which is not applicable to terahertz detectors. Summary of the Invention

[0005] To solve the problem that the instability and volatility of the atmospheric background radiation itself lead to a decrease in the radiation contrast and it is difficult to detect the target, the present invention provides a method for selecting a terahertz radiation channel for space target detection. By using this method, a lower and stable atmospheric background brightness temperature can be obtained, the volatility caused by geographical environmental factors can be suppressed, the radiation contrast can be improved, and thus the precise positioning of the target can be achieved.

[0006] To achieve the above object, the present invention proposes a method for selecting a terahertz radiation channel for space target detection, the method comprising: first, obtaining atmospheric profile data according to the longitude and latitude range of the observation area of the terahertz detector, and calculating the atmospheric transmittance and the atmospheric background radiation brightness temperature; then calculating the Jacobian matrix of the atmospheric background radiation brightness temperature with respect to the water vapor volume mixing ratio; and finally selecting alternative radiation channels according to the Jacobian matrix.

[0007] As an improvement of the above method, the method specifically comprises:

[0008] Step 1) Determine the longitude and latitude range of the observation area of the terahertz detector, and obtain the atmospheric profile data of this area in real time;

[0009] Step 2) According to the atmospheric profile data, use the line-by-line integration method to calculate the atmospheric transmittance in the range of 0.1-10 THz on the propagation path;

[0010] Step 3) Calculate the atmospheric background radiation brightness temperature of all absorption frequencies in the range of 0.1-10 THz according to the radiation transfer equation;

[0011] Step 4) Combine the atmospheric background radiation brightness temperature values in the area and the water vapor profile data, and calculate the Jacobian matrix of the atmospheric background radiation brightness temperature of different pressure layers with respect to the water vapor volume mixing ratio for all absorption frequencies;

[0012] Step 5) Perform normalization processing on the Jacobian matrix;

[0013] Step 6) Convert the divided pressure layers into actual heights, and select the absorption frequencies corresponding to the maximum values in the Jacobian matrix where the actual heights are 15-25 km at the bottom of the stratosphere as alternative radiation channels;

[0014] As an improvement of the above method, the calculation formula of the line-by-line integration method in step 2) is:

[0015] k e (v) = nSF(v)

[0016] In the formula, k e (v) represents the extinction coefficient with frequency v as the variable, n is the number density, S is the line intensity, and F(v) represents the line shape factor with frequency v as the variable.

[0017] As an improvement of the above method, the calculation formula of the atmospheric transmittance in step 2) is:

[0018]

[0019] In the formula, η v (S1, S2) represents the atmospheric transmittance from S1 to S2 on the propagation path, k e(v, s) represents the extinction coefficient at point s with frequency v.

[0020] As an improvement to the above method, the radiative transfer equation in step 3) is:

[0021]

[0022] In the formula, I v (s) represents the atmospheric radiance at point s, and J v (s) represents the radiation source function at point s.

[0023] As an improvement to the above method, the analytical expression of the Jacobian matrix in step 4) is:

[0024]

[0025] In the formula, T is the atmospheric background radiation brightness temperature matrix, P i is the i-th atmospheric pressure layer, S is the spectral line intensity, F is the line shape factor, and Φ is the polarization matrix.

[0026] As an improvement to the above method, the mathematical expression for the normalization process in step 5) is:

[0027]

[0028] In the formula, K norm is the normalized Jacobian matrix, K is the Jacobian matrix, W is the water vapor profile, represents the Hadamard product.

[0029] As an improvement to the above method, the conversion formula between the atmospheric pressure layer and the actual height in step 6) is:

[0030]

[0031] In the formula, h is the actual height, P is the atmospheric pressure layer, P0 is the standard atmospheric pressure, R is the ideal gas constant, T is the temperature of the atmospheric pressure layer, g is the acceleration due to gravity, and M is the molar mass of air.

[0032] The present invention also provides a computer device, including a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the computer program, the method described in any one of the above is implemented.

[0033] The present invention also provides a computer-readable storage medium. The computer-readable storage medium stores a computer program. When the computer program is executed by a processor, the processor is caused to execute the method described in any one of the above.

[0034] The advantages of a terahertz radiation channel selection method for space target detection provided by the present invention are as follows: The calculated absorption frequencies are used as alternative radiation channels, and these frequencies are most sensitive to temperature and water vapor changes at an altitude of 15 - 25 km. The temperature at the bottom of the stratosphere is low, and the water vapor content is scarce and evenly distributed, which can keep the brightness temperature of the atmospheric background observed by the detector low and stable, suppress the volatility caused by geographical environment factors, improve the radiation contrast, and thus achieve precise positioning of the target. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic diagram of the observation geometric model of the terahertz detector described in the present invention.

[0036] Figure 2 It is a schematic diagram of the observation field of view of the terahertz detector described in the present invention.

[0037] Figure 3 It is a flowchart of the terahertz radiation channel selection method for space target detection described in the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] The following further detailed description of the present invention is provided in conjunction with the accompanying drawings, so that those skilled in the art can implement it with reference to the text of the specification.

[0039] The present invention is based on the Figure 1 shown observation geometric model and proposes a terahertz radiation channel selection method for space target detection. When the detector detects with the earth's atmosphere as the background, whether the target can be detected depends on the radiation contrast between the radiation brightness temperature of the target itself and the radiation brightness temperature of the surrounding atmospheric background. The schematic diagram of the observation field of view is as Figure 2 shown. In order to obtain a high radiation contrast, this method selects absorption frequencies that are sensitive to temperature and water vapor changes in the atmosphere at the bottom of the stratosphere, so that the brightness temperature of the atmospheric background observed by the detector remains low and stable, and suppresses the volatility caused by geographical environment factors. The flowchart of the selection method is as Figure 3 shown, and the specific steps include:

[0040] Step 1) Determine the longitude and latitude range of the observation area of the terahertz detector, and obtain the atmospheric profile data of this area in real time;

[0041] According to the system parameters such as the orbital altitude, observation angle, and beam width of the terahertz detector, determine the longitude and latitude range of the observation area, and obtain the atmospheric profile data of gases such as temperature, water vapor, nitrogen, oxygen, and carbon dioxide within this range within a single day.

[0042] Step 2) According to the atmospheric profile data, use the line-by-line integration method to calculate the atmospheric transmittance in the range of 0.1 - 10 THz on the propagation path;

[0043] The extinction coefficient is calculated using the line-by-line integration method, and the specific calculation formula is as follows:

[0044] k e (v) = nSF(v)

[0045] In the formula, k e (v) represents the extinction coefficient with frequency v as the variable, n is the number density, S is the spectral line intensity, and F(v) represents the line shape factor with frequency v as the variable.

[0046] The relationship between the extinction coefficient and the atmospheric transmittance is as follows:

[0047]

[0048] In the formula, η v (S1, S2) represents the atmospheric transmittance from S1 to S2 along the propagation path, and k e (v, s) represents the extinction coefficient at frequency v at point s.

[0049] Step 3) Calculate the atmospheric background radiation brightness temperature at all absorption frequencies in the 0.1 - 10 THz region according to the radiative transfer equation;

[0050] The radiative transfer equation is solved using numerical calculation, and its formula is as follows:

[0051]

[0052] In the formula, I v (s) represents the atmospheric radiance at point s, and J v (s) represents the radiation source function at point s.

[0053] Step 4) Combine the atmospheric background radiation brightness temperature values in the region and the water vapor profile data to calculate the Jacobian matrix of the atmospheric background radiation brightness temperature at different pressure levels with respect to the water vapor volume mixing ratio;

[0054] The Jacobian matrix of the atmospheric background radiation brightness temperature at different pressure levels with respect to the water vapor volume mixing ratio is solved using numerical calculation, and its analytical expression is as follows:

[0055]

[0056] In the formula, T is the atmospheric background radiation brightness temperature matrix, P i is the i-th pressure level, S is the spectral line intensity, F is the line shape factor, and Φ is the polarization matrix.

[0057] Step 5) Normalize the Jacobian matrix;

[0058] Since the water vapor in the obtained Jacobian matrix is calculated according to the mixing ratio per unit volume, and there are significant differences in the water vapor volume mixing ratio of each layer of the atmosphere, it is necessary to normalize it using the water vapor profile.

[0059] The mathematical expression for normalization is:

[0060]

[0061] In the formula, K norm is the normalized Jacobian matrix, K is the Jacobian matrix, W is the water vapor profile, represents the Hadamard product.

[0062] Step 6) Convert the divided pressure layers into actual heights, and select the absorption frequencies corresponding to the maximum values in the Jacobian matrix with the actual heights in the range of 15 - 25 km at the bottom of the stratosphere as the alternative radiation channels;

[0063] Correspond the pressure layers with the actual heights according to the pressure - height conversion formula. The specific conversion formula is:

[0064]

[0065] In the formula, h is the actual height, P is the pressure layer, P0 is the standard atmospheric pressure, R is the ideal gas constant, T is the temperature of the pressure layer, g is the acceleration due to gravity, and M is the molar mass of air.

[0066] If the radiation contribution of a certain layer of the atmosphere accounts for a relatively high proportion in the total contribution, then the brightness temperature of the atmospheric background received by the detector mainly depends on this layer of the atmosphere. At the same time, the brightness temperature is also more sensitive to the changes in this layer of the atmosphere. Therefore, select the absorption frequencies at the bottom of the stratosphere at 15 - 25 km where the temperature is low and the water vapor is scarce as the alternative radiation channels.

[0067] After calculating the alternative radiation channels, then select the appropriate operating frequencies from the alternative radiation channels according to the instrument performance indicators.

[0068] The alternative radiation channels widely exist in the range of 0.1 - 10 THz, and the appropriate operating frequencies can be selected according to the parameters of the terahertz detector system.

[0069] The advantages of this method are as follows: The calculated absorption frequencies are used as the alternative radiation channels. These frequencies are the most sensitive to the temperature and water vapor changes at a height of 15 - 25 km. The temperature at the bottom of the stratosphere is low, the water vapor content is scarce and evenly distributed, which can keep the brightness temperature of the atmospheric background observed by the detector low and stable, suppress the fluctuations caused by geographical environmental factors, improve the radiation contrast, and thus achieve the precise positioning of the target.

[0070] The present invention can also provide a computer device, including: at least one processor, a memory, at least one network interface, and a user interface. Each component in the device is coupled together through a bus system. It can be understood that the bus system is used to realize the connection and communication between these components. In addition to the data bus, the bus system also includes a power bus, a control bus, and a status signal bus.

[0071] Among them, the user interface may include a display, a keyboard, or a pointing device (for example, a mouse, a trackball, a touchpad, or a touch screen, etc.).

[0072] It can be understood that the memory in the disclosed embodiments of the present application may be a volatile memory or a non-volatile memory, or may include both volatile and non-volatile memories. Among them, the non-volatile memory may be a read-only memory (ROM), a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), an electrically erasable programmable read-only memory (EEPROM), or a flash memory. The volatile memory may be a random access memory (RAM), which is used as an external cache. By way of example but not limitation, many forms of RAM are available, such as static random access memory (SRAM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), double data rate synchronous dynamic random access memory (DDR SDRAM), enhanced synchronous dynamic random access memory (ESDRAM), synchlink dynamic random access memory (SLDRAM), and direct rambus random access memory (DRRAM). The memory described herein is intended to include but not be limited to these and any other suitable types of memory.

[0073] In some embodiments, the memory stores the following elements, executable modules, or data structures, or subsets thereof, or extended sets thereof: an operating system and applications.

[0074] Among them, the operating system includes various system programs, such as the framework layer, the core library layer, the driver layer, etc., which are used to implement various basic services and handle hardware-based tasks. The application programs include various application programs, such as the Media Player, the Browser, etc., which are used to implement various application services. The program for implementing the method of the embodiments of the present disclosure may be included in the application programs.

[0075] In the above-mentioned embodiments, the program or instruction stored in the memory may also be called. Specifically, it may be the program or instruction stored in the application program. The processor is used for:

[0076] Execute the steps of the above method.

[0077] The above method may be applied to the processor or implemented by the processor. The processor may be an integrated circuit chip with signal processing capabilities. During the implementation process, each step of the above method may be completed by the integrated logic circuit in the hardware of the processor or the instruction in the form of software. The above-mentioned processor may be a general-purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components. It can implement or execute each method, step and logic block diagram disclosed above. The general-purpose processor may be a microprocessor or the processor may also be any conventional processor, etc. Combining the steps of the above-disclosed method may be directly embodied as being executed and completed by the hardware decoding processor, or executed and completed by the combination of the hardware and software modules in the decoding processor. The software module may be located in a mature storage medium in the art, such as a random access memory, a flash memory, a read-only memory, a programmable read-only memory, or an electrically erasable programmable memory, a register, etc. This storage medium is located in the memory, and the processor reads the information in the memory and combines its hardware to complete the steps of the above method.

[0078] It will be understood that the embodiments described in the present invention can be implemented using hardware, software, firmware, middleware, microcode, or a combination thereof. For hardware implementation, the processing unit can be implemented in one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field-programmable gate arrays (FPGAs), general-purpose processors, controllers, microcontrollers, microprocessors, other electronic units for performing the functions described in this application, or a combination thereof.

[0079] For software implementation, the techniques of the present invention can be implemented by executing the functional modules of the present invention (such as procedures, functions, etc.). The software code can be stored in a memory and executed by a processor. The memory can be implemented within the processor or outside the processor.

[0080] The present invention can also provide a non-volatile storage medium for storing a computer program. When the computer program is executed by a processor, the various steps in the above method embodiments can be implemented.

[0081] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the embodiments, those of ordinary skill in the art should understand that any modification or equivalent replacement of the technical solutions of the present invention does not depart from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A method for selecting a terahertz radiation channel for space target detection, which conducts earth exploration with the earth's atmosphere as the background; the method includes: First, obtain the atmospheric profile data according to the longitude and latitude range of the observation area of the terahertz detector, and calculate the atmospheric transmittance and the brightness temperature of the atmospheric background radiation; Then, calculate the Jacobian matrix of the brightness temperature of the atmospheric background radiation with respect to the volume mixing ratio of water vapor; finally, select the alternative radiation channels according to the Jacobian matrix; The specific method is as follows: Step 1) Determine the longitude and latitude range of the observation area of the terahertz detector, and obtain the atmospheric profile data of this area in real time; Step 2) According to the atmospheric profile data, use the line-by-line integration method to calculate the atmospheric transmittance in the range of 0.1 - 10 THz on the propagation path; Step 3) Calculate the brightness temperature of the atmospheric background radiation at all absorption frequencies in the range of 0.1 - 10 THz according to the radiation transfer equation; Step 4) Combine the brightness temperature value of the atmospheric background radiation in the area and the water vapor profile data, and calculate the Jacobian matrix of the brightness temperature of the atmospheric background radiation at different pressure layers with respect to the volume mixing ratio of water vapor at all absorption frequencies; Step 5) Perform normalization processing on the Jacobian matrix; Step 6) Convert the divided pressure layers into actual heights, and select the absorption frequencies corresponding to the maximum values in the Jacobian matrix with actual heights in the range of 15 - 25 km at the bottom of the stratosphere as the alternative radiation channels.

2. The method for selecting a terahertz radiation channel for space target detection according to claim 1, wherein The calculation formula of the line-by-line integration method in Step 2) is: k e (v) = nSF(v) where k e (v) represents the extinction coefficient as a function of frequency v, n is the number density, S is the spectral line intensity, and F(v) represents the line shape factor as a function of frequency v.

3. The method for selecting a terahertz radiation channel for space target detection according to claim 1, characterized in that, The calculation formula of the atmospheric transmittance in Step 2) is: where η v (S1, S2) represents the atmospheric transmittance from S1 to S2 on the propagation path, and k e (v, s) represents the extinction coefficient at point s with frequency v.

4. The terahertz radiation channel selection method for space target detection according to claim 1, characterized in that, The radiation transfer equation in Step 3) is: where, I v (s) represents the atmospheric radiance at point s, J v (s) represents the radiation source function at point s, η v (S1, S2) represents the atmospheric transmittance from S1 to S2 along the propagation path.

5. The method for selecting a terahertz radiation channel for space target detection according to claim 1, wherein The analytical expression of the Jacobian matrix in Step 4) is: where T is the brightness temperature matrix of the atmospheric background radiation, P i is the i-th pressure layer, S is the line intensity, F is the line shape factor, and Φ is the polarization matrix.

6. The method for selecting a terahertz radiation channel for space target detection according to claim 1, characterized in that The mathematical expression of the normalization processing in Step 5) is: where K norm is the normalized Jacobian matrix, K is the Jacobian matrix, W is the water vapor profile, denotes the Hadamard product.

7. The method for selecting a terahertz radiation channel for space target detection according to claim 1, wherein The conversion formula between the pressure layer and the actual height in Step 6) is: In the formula, h is the actual height, P is the pressure layer, P0 is the standard atmospheric pressure, R is the ideal gas constant, T is the temperature of the pressure layer, g is the acceleration due to gravity, and M is the molar mass of air.

8. A computer device, comprising a memory, a processor, and a computer program stored on the memory and executable on the processor, characterized in that, When the processor executes the computer program, it implements the method described in any one of claims 1 to 7.

9. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program, and when the computer program is executed by the processor, it causes the processor to execute the method described in any one of claims 1 to 7.

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