Radar cross section measurement system and measurement method

Abstract

The invention discloses a radar cross section measuring system and method, and relates to the technical field of electromagnetic scattering measurement, and the system comprises a transmitting antenna which is used for transmitting a step frequency broadband signal; the spiral phase plate rotating disc is vertically and rotatably arranged in front of the transmitting port surface of the transmitting antenna through a rotating assembly, a plurality of spiral phase plates with different modalities are arranged on the spiral phase plate rotating disc, and the spiral phase plates modulate step frequency broadband signals, generate vortex electromagnetic waves with corresponding modalities and transmit the vortex electromagnetic waves to a target on the same horizontal plane; the receiving antenna is used for receiving a near-field echo signal reflected by a target on the same horizontal plane; and the controller is used for receiving the near-field echo signals of the vortex electromagnetic waves in different modes and processing the near-field echo signals to obtain the radar cross section of the target. The radar cross section of a large-size radar target can be quickly, accurately and effectively obtained through the measurement system and the measurement method which are used for near-field measurement and are converted into far-field conditions.

Description

Technical Field

[0001] The present invention relates to the technical field of electromagnetic scattering measurement, and in particular to a radar scattering cross section measurement system and a measurement method. Background Art

[0002] With the development of electronic information technology, the stealth technology of radar targets has gradually matured, posing new challenges to the accuracy of radar detection and recognition. At present, most radio frequency radars are based on plane electromagnetic waves, and use the frequency, polarization, amplitude and phase information modulation in the collected echo parameters to detect targets. The principle is mainly based on plane wave approximation. Most traditional methods improve the signal-to-noise ratio in the algorithm or use a multi-base station radar system that provides high power and multiple frequencies. However, when detecting weak radar cross section (RCS) targets, the improvement of its detection performance has reached its limit. It is an effective way to improve the ability to obtain information about weak scattering targets by introducing new electromagnetic waves. Vortex electromagnetic waves are different from traditional plane waves. They have a spiral phase wavefront structure. They carry orbital angular momentum characteristics and can form multi-modal waves at the same frequency, providing richer target information representation capabilities than plane waves. Multi-modal vortex electromagnetic waves can provide more degrees of freedom to control the shape and direction of the radar beam, thereby achieving high-resolution imaging. By synthesizing and processing vortex electromagnetic waves of different modes, more detailed image information of the target can be obtained, and the resolution and clarity of the imaging can be improved.

[0003] The RCS characteristics of the target can be obtained quickly, accurately and effectively through measurement. Since RCS measurement requires irradiating the target with plane waves or vortex electromagnetic waves, when the frequency and radar target size are large, it is difficult to meet the far-field conditions due to factors such as the range and geographical conditions of the test site. Summary of the invention

[0004] Based on the above-mentioned defects of the prior art, the present invention provides a radar cross section measurement system and a measurement method, which solves the problem that far-field conditions are difficult to meet for RCS measurement of large-sized radar targets.

[0005] The present invention adopts the following technical solutions: In a first aspect, the present invention provides a radar cross section measurement system, comprising: A signal source for generating a stepped frequency broadband signal; A transmitting antenna, electrically connected to the signal source, for transmitting a stepped frequency broadband signal; A spiral phase plate turntable is rotatably arranged in front of the transmitting aperture of the transmitting antenna, and a plurality of spiral phase plates of different modes are arranged on the spiral phase plate turntable. When the spiral phase plate turntable rotates until one of the spiral phase plates is aligned with the aperture of the transmitting antenna, the spiral phase plate modulates the stepped frequency broadband signal to generate a vortex electromagnetic wave of a corresponding mode and transmits it to a target on the same horizontal plane; A receiving antenna, used to receive near-field echo signals reflected by targets on the same horizontal plane; The controller is used to receive and process the near-field echo signals of vortex electromagnetic waves of different modes to obtain the radar scattering cross section of the target.

[0006] Preferably, the transmitting antenna and the receiving antenna are both rectangular horn antennas.

[0007] Preferably, the spiral phase plate rotating disk is rotatably arranged in front of the transmitting aperture of the transmitting antenna through a rotating assembly, and the rotating assembly comprises: A bracket is fixedly arranged in front of the transmitting aperture of the transmitting antenna, and the spiral phase plate turntable is rotatably arranged on one side of the bracket; A rotating shaft wheel is arranged on the other side of the bracket, and the rotating shaft wheel is fixedly connected to one side of the spiral phase plate rotating disk through a rotating shaft; The driving motor is electrically connected to the controller, and its output shaft is connected to the rotating shaft wheel through a belt.

[0008] Preferably, it further comprises a low-reflection foam bracket, the target is arranged on the low-reflection foam bracket, a turntable is arranged at the bottom of the low-reflection foam bracket, and the turntable is electrically connected to the controller.

[0009] Preferably, wave absorbing materials are provided around the transmitting antenna, the receiving antenna and the target.

[0010] Preferably, the frequency band of the stepped frequency broadband signal is 2 GHz to 18 GHz.

[0011] In a second aspect, the present invention provides a measurement method of a radar cross section measurement system, comprising the following steps: Establish a coordinate system with the transmitting antenna as the origin to determine the location of the target; Acquire multiple near-field echo signals of vortex electromagnetic waves of different modes based on the radar cross-section measurement system; Performing spectrum transformation on each near-field echo signal to obtain a first expression; Integrate the first expression to obtain the second expression; Perform inverse Fourier transform on the second expression to obtain a far-field echo signal; Obtain the radar cross section under each mode according to the far-field echo signal; The radar cross section of each mode is selectively combined to obtain the radar cross section of the target.

[0012] Preferably, the spectrum transformation is performed on each near-field echo signal to obtain a first expression, which is as follows: ; in, is the near-field echo signal, for R Plane wave spectrum vector on the plane, The target location point The position vector of and The wave propagation directions are of Direction Component Towards the weight, represents the exponential function, is an imaginary unit; The first expression is integrated to obtain a second expression, which is as follows: ; In the formula, is the near-field echo signal after spectrum vector transformation, and are the polar angle and azimuth corresponding to the target position direction, k is the wave number, r To test the distance, is the far-field scattering pattern; The second expression is subjected to inverse Fourier transform to obtain a far-field echo signal, which is shown as follows: ; In the formula, Expressed as Plane Spectrum Vector of Towards the weight, The target location y Towards the direction vector, for Plane Spectrum Vector of z Towards the weight, The target location z Towards direction vector.

[0013] Preferably, the radar cross section under each mode is obtained according to the far-field echo signal, as shown below: ; In the formula, is the modal number l The radar cross section measured under is the incident field, It is expressed as the target equivalent far-field echo signal distribution.

[0014] Preferably, the radar cross section under each mode is selectively combined to obtain the radar cross section of the target (9), which is specifically as follows: ; In the formula, To merge the results, Indicates the modal number The radar cross section below.

[0015] Compared with the prior art, at least one of the above technical solutions adopted by the present invention can achieve the following beneficial effects: The present invention proposes a radar cross-section measurement system, which transmits a signal to a spiral phase plate through a transmitting antenna, and the spiral phase plate modulates a stepped frequency broadband signal to generate a vortex electromagnetic wave and transmits it to a target on the same horizontal plane, and a controller receives the echo signal of the vortex electromagnetic wave to obtain a near-field echo signal of the target. A plurality of spiral phase plates are integrated on a spiral phase plate turntable, and the controller controls the rotation of the rotating assembly to drive the turntable to rotate. Therefore, the present invention can quickly switch to a suitable spiral phase plate according to different frequency requirements, realize the automatic switching of multi-mode vortex electromagnetic waves, and obtain the near-field echo signal of a large-sized radar target in the form of different mode vortex electromagnetic waves.

[0016] Based on the above radar cross-section measurement system, the present invention also provides a radar cross-section measurement method, which obtains corresponding far-field echo signals by performing spectral transformation, integration and inverse Fourier transformation on near-field echo data under multi-mode, and finally selectively merges multiple far-field echo signals to obtain the radar cross-section of large-size radar targets. The present invention can quickly, accurately and effectively obtain the radar cross-section of large-size radar targets through a measurement system and a measurement method that measure near-field conditions and transform them to far-field conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0018] Figure 1 A schematic diagram of the structure of a radar cross section measurement system according to an embodiment of the present invention; Figure 2 A schematic plan view of a continuous spiral phase plate according to an embodiment of the present invention; Figure 3 A schematic diagram of the structure of a radar cross section measurement method according to an embodiment of the present invention; Figure 4 This is a diagram of RCS measurement results according to an embodiment of the present invention; Figure 5 This is a relationship diagram between the detection probability of vortex electromagnetic waves and RCS thresholds for different mode numbers in an embodiment of the present invention.

[0019] In the figure: 1- vector network analyzer, 2- transmitting antenna, 3- receiving antenna, 4- spiral phase plate turntable, 5- spiral phase plate, 6- belt, 7- driving motor, 8- absorbing material, 9- target, 10- low reflection foam bracket, 11- turntable, 12- controller, 13- communication cable, 14- bracket. DETAILED DESCRIPTION

[0020] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0021] 1. Explanatory Examples In order to enable those skilled in the art to fully understand how to implement the present invention, this section provides an illustrative example that expands and describes the technical solution of the claims.

[0022] Figure 1 FIG. 1 is a schematic diagram of the structure of a radar cross section measurement system of the present invention. Figure 1 A radar cross section measurement system provided by an embodiment of the present invention is introduced in detail, including a signal source, a transmitting antenna 2, a receiving antenna 3, a spiral phase plate turntable 4, a spiral phase plate 5, a rotating component and a controller 12.

[0023] In this embodiment, both the transmitting antenna 2 and the receiving antenna 3 are ultra-wideband horn antennas, and their structures are formed by gradually unfolding a rectangular waveguide. In the present invention, two horn antennas with the same characteristics are used, one of which radiates the signal generated by the vector network analyzer 1, irradiates the target 9 after passing through the spiral phase plate 5, and the other antenna receives the reflected signal of the target 9. The controller 12 is a main control computer.

[0024] The signal source is a vector network analyzer 1, which is electrically connected to the transmitting antenna 2 and the receiving antenna 3 through a coaxial cable. The vector network analyzer 1 is electrically connected to the controller 12 through a communication cable 13. The driving motor 7 is connected to the controller 12 through the communication cable 13, and the speed of the driving motor 7 is controlled by the controller 12. The driving motor 7 is connected to the central rotating shaft wheel of the spiral phase plate rotating disk 4 through a belt 6 to realize the driving of the spiral phase plate rotating disk 4.

[0025] The vector network analyzer 1 selects parameters such as the frequency band, frequency step and transmission power of the transmitted microwave signal according to the test requirements. Generally, the frequency setting is selected from 2GHz to 18GHz. The vector network analyzer 1 is used as a scattering measurement instrument to output a stepped frequency broadband test signal and transmit it to the transmitting antenna 2. The front end of the transmitting antenna 2 is covered with a spiral phase plate 5 to modulate and generate a vortex electromagnetic wave. The vortex electromagnetic wave is used to illuminate the target 9. The receiving antenna 3 receives the backscattered signal (i.e., the near-field echo signal) from the target 9 and transmits it to the vector network analyzer 1 through a coaxial cable. The vector network analyzer 1 analyzes and processes the near-field echo signal, and feeds the processing result back to the controller 12 to output the measurement result.

[0026] In order to generate multi-mode vortex electromagnetic waves, spiral phase plates 5 of different modes are arranged on the aperture of the transmitting antenna 2. The spiral phase plate 5 adds a rotational phase factor to the electromagnetic waves generated by the transmitting antenna 2 to generate orbital angular momentum (OAM) electromagnetic waves, namely vortex electromagnetic waves.

[0027] The spiral phase plate 5 is used to modulate the stepped frequency broadband test signal emitted by the transmitting antenna 2 to generate vortex electromagnetic waves. The spiral phase plate 5 is distributed in a spiral shape. The present invention adopts a gradient spiral phase plate, and the phase change is continuous. The structural parameters and operating frequency of the spiral phase plate 5 are related to the mode of the vortex electromagnetic wave to efficiently generate vortex electromagnetic waves.

[0028] Reference Figure 2 , a continuous spiral phase plate is used in system construction and experimental measurement, for example, a spiral phase plate 5 that generates a vortex electromagnetic wave of mode 1, whose height is proportional to the rotation of the azimuth angle, and the azimuth angle rotates from 0 to 2 The height of the spiral phase plate 5 is increasing, and a step covers the entire surface 360 ​​degrees. When the height reaches the given size height.

[0029] The spiral phase plate turntable 4 is used to fix the spiral phase plate 5. A plurality of mounting positions are evenly distributed on the spiral phase plate turntable 4, and each mounting position is used to mount a spiral phase plate 5 of a different mode. The spiral phase plate turntable 4 is connected to the drive motor 7 through a high-precision rotating shaft wheel. The drive motor can accurately control the rotation angle of the spiral phase plate turntable 4, so as to realize fast and accurate switching of different spiral phase plates 5 to align with the transmitting antenna 2.

[0030] Furthermore, a positioning device, such as an electromagnet, can be provided at the connection between the spiral phase plate disk 4 and the transmitting antenna 2 to ensure that the spiral phase plate 5 is accurately docked and fixed to the transmitting antenna 2 to ensure the transmission efficiency and stability of the vortex electromagnetic wave.

[0031] The spiral phase plate rotating disk 4 is inlaid with spiral phase plates 5 with different mode numbers to generate different mode numbers. l The arrangement of the spiral phase plate 5 on the spiral phase plate rotating disk 4 needs to ensure that it is vertical in the vertical direction and aligned with the rectangular opening surface of the transmitting antenna 2.

[0032] The central rotating shaft wheel of the spiral phase plate turntable 4 is connected to the driving motor 7 through a belt 6, and the driving motor 7 is connected to the controller 12 through a communication cable 13. The controller 12 controls the rotational angular velocity and rotation angle of the driving motor 7 by sending instructions to achieve that the various types of spiral phase plates 5 on the spiral phase plate turntable 4 can be aligned with the rectangular opening of the horn antenna.

[0033] The controller 12 can control the spiral phase plate disk 4 to switch to the corresponding position according to the input electromagnetic wave mode number and angular velocity. At the same time, the controller 12 can also monitor the position state of the spiral phase plate disk 4 and the operation of the drive motor 7 in real time to ensure safe and reliable operation of the system.

[0034] The main control computer is used to store the drive motor control program. In order to accurately switch the spiral phase plate 5, the drive motor 7 needs to maintain a certain rotational angular velocity. The main control computer can realize the rotation angle control of the drive motor 7. At the same time, as the analyzer remote control machine and storage machine of the vector network analyzer 1, the scattering data can be saved in time.

[0035] The target 9 is placed on a turntable 11 in a dark room and supported by a low-reflection foam bracket 10. The transmitting antenna 2 and the receiving antenna 3 are installed on a stable support frame. The distance between the transmitting antenna 2 and the receiving antenna 3 is as close as possible, and the distance between the target 9 and the transmitting antenna 2 and the receiving antenna 3 can be recorded.

[0036] The turntable 11 is electrically connected to the controller 12 via a communication cable 13 , a low-reflection foam bracket 10 is placed on the turntable 11 , and a target 9 is placed directly above the low-reflection foam bracket 10 , while ensuring that the target 9 and the transmitting antenna 2 and the receiving antenna 3 are on the same horizontal plane.

[0037] An absorbing material 8 is laid between the receiving antenna 3 and the target 9 to ensure that the coupling between the probe aperture and the target 9 within the scanning range is minimized to reduce the error in the measurement process.

[0038] After determining the measurement parameters according to the test requirements, the measurement system is first started without a target. The vector network analyzer 1 uses a frequency sweep method to measure the scattering data in the empty environment through the transmitting antenna 2 and the receiving antenna 3, and transmits the measurement data back to the remote control machine for storage. At this time, the measurement environment is controlled unchanged, and the target 9 is placed on a low-reflection foam bracket 10 and measured in the same way. The measurement data under all modal numbers are compared with the measurement data under the empty target to obtain relatively "clean" target scattered echo data.

[0039] Based on the same concept, the present invention also provides a radar cross section measurement method, referring to Figure 3 , specifically including the following steps: S1: Spectral transformation. According to Huygens principle, the field at any position in space is equivalent to the superposition of the radiation fields of all points on any surface. To calculate the far-field RCS, it is necessary to solve the echo signal at the far-field position. The coordinate system is established at the position of the transmitting antenna 2, and the target 9 is at the position At the location, the vector network analyzer 1 measures the plane where the target 9 is located. The near field echo signal is Based on the spectrum expansion theory, the near-field echo signal Perform spectrum transformation, that is, transform the wave propagation direction of Direction Component and Direction Component Another expression of the near-field echo signal is obtained by integral solution: (1); In the formula, for Plane wave spectrum vector on the plane, The target location point The position vector of represents the exponential function, is an imaginary unit, Represents the dot product calculation of two elements.

[0040] S2: Solve the integral of the echo signal. Solve the integral of the near-field echo signal by the stationary phase method: (2); In the formula, and are the polar angle and azimuth corresponding to the target position direction, k is the wave number, r To test the distance, is the far-field scattering pattern.

[0041] S3: Calculation of far-field echo signal distribution. Perform inverse Fourier transform calculation on the echo signal integral obtained by the stationary phase method to obtain a two-dimensional spectrum vector: (3); (4); (5); In the formula, Expressed as Plane Spectrum Vector of Towards the weight, The target location y Towards the direction vector, The target location point The position vector of The target location z Towards the direction vector, is the polar angle direction of the target position after spectrum transformation, The target's position vector.

[0042] in, (6); (7); in, L y For sampling surface The length of the direction, L x For sampling surface The length of the direction, R The target location plane .

[0043] Further calculation yields: (8); (9).

[0044] use , the far-field echo signal distribution is calculated: (10); In the formula, For sampling surface The summation index of the sampling points in the direction, For sampling surface The summation index of the sampling points in the direction, The sampling surface The number of sampling points in the direction, The sampling surface The number of sampling points in the direction, The sampling surface The sampling interval of the direction, The sampling surface The sampling interval of the direction.

[0045] S4: Solve the RCS of a certain mode. Use the definition formula to calculate the RCS of a certain mode: (11); in, is the incident field, It is represented as the target equivalent far-field echo signal distribution, that is, the calculation result of step S3.

[0046] The RCS under a certain mode is calculated by formula (11).

[0047] S5, repeating steps S1 to S4 until the RCS under all modal numbers is solved; S6. Selective merging of RCS under different modal numbers. Since the target has different scattering responses to OAM electromagnetic waves with different modal numbers, the selective merging RCS method is used, that is, ,in Indicates the modal number In this way, the RCS measured under different modal numbers can be effectively and comprehensively utilized to improve the target RCS measurement results.

[0048] 2. Evidence of the effects of the embodiments The embodiments of the present invention have achieved some positive effects during the development or use process, and indeed have great advantages over the prior art, which are described below in conjunction with the process diagram of the test process.

[0049] Figure 4 is the modal number of the vortex electromagnetic wave (Plane wave), and From the RCS result diagram measured below, it can be seen that the RCS distribution of the target is significantly different under different modal illumination.

[0050] Figure 5 is the modal number and Compared with the detection probability diagram of RCS with different thresholds using the selective merging RCS method, it can be seen that the merging method can improve the detection probability of the target under ordinary plane wave illumination.

[0051] Although the preferred embodiments of the present invention have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0052] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A radar cross section measurement system, characterized in that: include: A signal source for generating a stepped frequency broadband signal; A transmitting antenna (2), electrically connected to a signal source, and used for transmitting a stepped frequency broadband signal; A spiral phase plate rotating disk (4) is rotatably arranged in front of the transmitting aperture of the transmitting antenna (2), and a plurality of spiral phase plates (5) of different modes are arranged on the spiral phase plate rotating disk (4). When the spiral phase plate rotating disk (4) rotates until one of the spiral phase plates (5) is aligned with the aperture of the transmitting antenna (2), the spiral phase plate (5) modulates the stepped frequency broadband signal to generate a vortex electromagnetic wave of a corresponding mode and transmits it to a target (9) on the same horizontal plane; A receiving antenna (3) for receiving a near-field echo signal reflected by a target (9) on the same horizontal plane; The controller (12) is used to receive and process near-field echo signals of vortex electromagnetic waves of different modes to obtain a radar cross section of the target (9).

2. A radar cross section measurement system as claimed in claim 1, characterized in that: The transmitting antenna (2) and the receiving antenna (3) are both rectangular horn antennas.

3. A radar cross section measurement system as claimed in claim 1, characterized in that: The spiral phase plate rotating disk (4) is rotatably arranged in front of the transmitting aperture of the transmitting antenna (2) through a rotating assembly, and the rotating assembly comprises: A bracket (14) is fixedly arranged in front of the transmitting aperture of the transmitting antenna (2), and the spiral phase plate rotating disk (4) is rotatably arranged on one side of the bracket (14); A rotating shaft wheel is arranged on the other side of the bracket (14), and the rotating shaft wheel is fixedly connected to one side of the spiral phase plate rotating disk (4) via a rotating shaft; The driving motor (7) is electrically connected to the controller (12), and its output shaft is connected to the rotating shaft wheel via a belt (6).

4. A radar cross section measurement system as claimed in claim 1, characterized in that: It also comprises a low-reflection foam bracket (10), the target (9) is arranged on the low-reflection foam bracket (10), a turntable (11) is provided at the bottom of the low-reflection foam bracket (10), and the turntable (11) is electrically connected to the controller (12).

5. A radar cross section measurement system as claimed in claim 1, characterized in that: Wave absorbing materials (8) are provided around the transmitting antenna (2), the receiving antenna (3) and the target (9).

6. A radar cross section measurement system as claimed in claim 1, characterized in that: The frequency band of the stepped frequency broadband signal is 2 GHz to 18 GHz.

7. A measurement method based on the radar cross section measurement system according to claim 1, characterized in that: The following steps are involved: A coordinate system is established with the transmitting antenna (2) as the coordinate origin to determine the position of the target (9); Acquire multiple near-field echo signals of vortex electromagnetic waves of different modes based on the radar cross-section measurement system; Performing spectrum transformation on each near-field echo signal to obtain a first expression; Integrate the first expression to obtain the second expression; Perform inverse Fourier transform on the second expression to obtain a far-field echo signal; Obtain the radar cross section under each mode according to the far-field echo signal; The radar cross section of each mode is selectively combined to obtain the radar cross section of the target (9).

8. The measurement method of a radar cross section measurement system according to claim 7, characterized in that: The spectrum transformation is performed on each near-field echo signal to obtain a first expression, which is as follows: ； in, is the near-field echo signal, for R Plane wave spectrum vector on the plane, The target location point The position vector of and The wave propagation directions are of Direction Component Towards the weight, represents the exponential function, is an imaginary unit; The first expression is integrated to obtain a second expression, which is as follows: ； In the formula, is the near-field echo signal after spectrum vector transformation, and are the polar angle and azimuth corresponding to the target position direction, k is the wave number, r To test the distance, is the far-field scattering pattern; The second expression is subjected to inverse Fourier transform to obtain a far-field echo signal, which is shown as follows: ； In the formula, Expressed as Plane Spectrum Vector of Towards the weight, The target location y Towards the direction vector, for Plane Spectrum Vector of z Towards the weight, The target location z Towards direction vector.

9. The measurement method of a radar cross section measurement system according to claim 8, characterized in that: The radar cross section under each mode is obtained according to the far-field echo signal, as shown below: ； In the formula, is the modal number l The radar cross section measured under is the incident field, It is expressed as the target equivalent far-field echo signal distribution.

10. The measurement method of a radar cross section measurement system according to claim 9, characterized in that: The radar cross section under each mode is selectively combined to obtain the radar cross section of the target (9), which is shown as follows: ； In the formula, To merge the results, Indicates the modal number The radar cross section below.

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