A distance compensation based RCS near-far field transformation calculation method

Abstract

The application aims to provide a distance compensation based RCS near far field transformation calculation method, and belongs to the technical field of RCS testing. The calculation method uses a large enough metal plate for calibration when measuring the near field echo signal, can shorten the overall test end face, improve the dynamic range of the test, and effectively reduce the loss interference of the intermediate path; then by measuring the echo signal received by the near field, and considering the actual situation that the distance from the test surface to the target will change with the rotation angle when the target rotates, a distance compensation function is introduced, the traditional NFFFT calculation formula is extended to the NFFFRT calculation formula based on distance compensation, and a more accurate RCS calculation method is provided for the stealth performance of the test target.

Description

Technical Field

[0001] This invention belongs to the field of RCS testing technology, specifically relating to a method for calculating near-field and far-field transformation of RCS based on distance compensation. Background Technology

[0002] Radar cross section (RCS) testing is a crucial method for studying stealth technology and target characteristics in the field of microwave radar stealth, with wide applications in radar system design, target detection and identification, and stealth technology. Based on stealth performance characteristics and application environments, commonly used RCS testing methods fall into three main categories: far-field RCS testing, indoor compact-field RCS testing, and near-field RCS testing. Far-field and indoor compact-field testing require significant financial investment and space constraints, especially for electrically large targets, where deployment mobility is limited, making on-site RCS diagnosis of stealth targets impossible. In contrast, near-field testing is highly efficient and low-cost, thus becoming a hot topic in the field of electromagnetic measurement in recent years, with many countries developing near-field RCS testing systems suitable for on-site measurement and diagnosis of stealth equipment.

[0003] Far-field radar cross-section (RCS) is a key indicator for evaluating a target's stealth performance. For near-field RCS test results, it's often necessary to convert the scattered waves measured in the near field into far-field data; this technique is called Near-Field to Far-Field Transform (NFFFT). Traditional NFFFT calculations (S. Omi, M. Hirose, M. Ameya, and S. Kurokawa, "Plane-wave synthesis employing propagating plane-wave expansion for 3-D and 2-D RCS prediction including the multiple scattering effects,"; T. Watanabe and H. Yamada, "Far-Field Radar Cross Section Determination From Near-Field 3-D Synthetic Aperture Imaging With Arbitrary Antenna-Scanning Surfaces") suffer from significant calculation errors. This is primarily because in near-field testing, the distance from the test surface to the target changes with the rotation angle; however, in traditional NFFFT calculations, the test distance remains constant. This leads to a greater error compared to the theoretical value as the rotation angle deviates further from 0°, making it difficult to achieve the desired accuracy.

[0004] Therefore, how to design compensation for near-field RCS test results so that accurate RCS results can be obtained in the near-field to far-field transformation calculation has become the research focus. Summary of the Invention

[0005] To address the problems existing in the background technology, the purpose of this invention is to provide a distance-compensated RCS near-field and far-field transformation calculation method. Based on the traditional NFFFT calculation formula, this method first measures the echo signal received in the near field, and then compensates for the test distance in the near-field and far-field transformation, thereby achieving accurate RCS calculation results.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] A method for calculating near-field and far-field RCS based on distance compensation includes the following steps:

[0008] Step 1: Adjust the polarization state of the transmitting antenna according to the required electromagnetic wave incident polarization requirements;

[0009] Step 2: Extract the test echo signal received by the target in the near field. Specifically as follows:

[0010] Step 2.1. Adjust the polarization of the receiving antenna to match that of the transmitting antenna. The transmitting and receiving antennas are set at a distance R0 from the center of rotation of the turntable.

[0011] Step 2.2. Do not place the target under test on the rotating platform. Place a metal plate directly in front of the rotating platform, rotate the platform and perform measurements. Measure the distance between the transmitting antenna and the receiving antenna at this time. parameter;

[0012] Step 2.3. Without placing the target on the turntable, remove the metal plate, rotate the turntable, and perform the measurement. Measure the distance between the transmitting and receiving antennas at this point. parameter;

[0013] Step 2.4. Place the target to be tested on the turntable, rotate the turntable and perform measurements, measuring the distance between the transmitting antenna and the receiving antenna at this time. parameter;

[0014] Step 2.5. Calculate the echo signal received by the target in the near field. The specific formula is as follows:

[0015]

[0016] Step 3: Based on the distance R0 between the antenna and the rotation center of the turntable, and the rotation angle of the target under test... and the echo signal obtained in step 2 The NFFFT formula for the traditional near-field and far-field transform is obtained, and the specific formula is as follows:

[0017]

[0018] in, Let P be the vector pointing from any scattering point P on the target to the rotation center O of the turntable. The coordinates of a certain measurement position of the target in polar coordinates are... k is the free-space wavenumber of electromagnetic waves, N0 = kr' max +10, r' max The maximum size of the target to be measured. Here, j is the Hankel function, and j is the imaginary unit.

[0019] Step 4: Perform range compensation on the traditional near-field and far-field transform NFFFT formula obtained in Step 3, using the shortest distance from the antenna end to the target as the compensation principle, to obtain the final NFFFT formula, specifically:

[0020]

[0021] in,

[0022] Furthermore, the test system in step 1 specifically includes a transmitting antenna 1, a receiving antenna 2, and a turntable 4; wherein the transmitting antenna 1 and the receiving antenna 2 are placed horizontally side by side and facing the target 3 to be tested, the center height of the antennas is consistent with the center height of the target 3 to be tested, and the target 3 to be tested is placed directly above the turntable 4 and can rotate with the turntable.

[0023] Furthermore, in step 1, the polarization state of the transmitting antenna is either vertical polarization or horizontal polarization.

[0024] Furthermore, in step 2, parameter, Parameters and The parameters are used after time-domain gating. calculate.

[0025] Furthermore, in step 2, the size of the metal plate must be larger than the size of the target to be tested, and it must be able to completely cover the target to be tested when placed.

[0026] Furthermore, in step 2, the isolation between the receiving antenna and the transmitting antenna should be less than -40dB to minimize the impact of antenna mutual coupling.

[0027] Furthermore, in step 2, the rotation angle of the turntable satisfies a rotation range of -180° to +180°.

[0028] Furthermore, the specific process of step 3 is as follows:

[0029] (1) If the scattered field is equivalent to a secondary radiation field with each scattering center as the source, then the test echo signal received in the near field can be expressed as:

[0030]

[0031] Let P be the vector pointing from any scattering point P on the target to the center of rotation O of the turntable. The vector pointing from the midpoint of the antenna to the center of the target being measured. The inherent characteristics of the scattering center of the target under test;

[0032] (2) Rearranging equation (4) into the weighted integral of the Green's function in two-dimensional free space with respect to the objective function, we get:

[0033]

[0034] Since the test distance satisfies R0 >> r' max The above formula can be simplified to:

[0035]

[0036] in, It characterizes the inherent scattering properties of the target, regardless of the test conditions;

[0037] (3) The relationship between the target's echo signal in the far field and the target's inherent scattering characteristics is as follows:

[0038]

[0039] (4) Combining the near-field test relations (6) and (7), we can obtain the following near-field and far-field transformation relations:

[0040]

[0041] Where, N0=kr' max +10.

[0042] Furthermore, in step 3, the distance R0 between the antenna position and the rotation center of the turntable should satisfy the near-field test condition, i.e., R0 < 2D. 2 / λ, where D is the maximum size of the antenna or the target under test, and λ is the wavelength corresponding to the operating frequency.

[0043] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0044] This invention provides a range-compensated RCS near-field and far-field transformation calculation method. When measuring near-field echo signals, a sufficiently large metal plate is used for calibration, which can bring the overall test surface closer, improve the dynamic range of the test, and effectively reduce the loss and interference of the intermediate path. Then, by measuring the echo signal received in the near field and considering the actual situation that the distance from the test surface to the target changes with the rotation angle when the target rotates, a range compensation function is introduced. The traditional NFFFT calculation formula is extended to the range-compensated NFFFRT calculation formula, providing a more accurate RCS calculation method for testing the stealth performance of the target. Attached Figure Description

[0045] Figure 1 This is a schematic diagram of the testing device of the present invention.

[0046] Figure 2 This is a schematic diagram illustrating the principle of the change in distance between the target and the antenna in this invention.

[0047] Figure 3 In Embodiment 1 of the present invention, the RCS results of a square metal plate (500mm*100mm*4mm) at frequencies of 12GHz and 18GHz are obtained by theoretical values, traditional NFFFT calculation formula and NFFFRT calculation formula, respectively. Among them, (a) is the calculation result for f=12GHz and R=3m, and (b) is the calculation result for f=18GHz and R=3m.

[0048] The attached diagram is labeled as follows: 1 is the transmitting antenna, 2 is the receiving antenna, 3 is the target under test, 4 is the turntable, 5 is the antenna, and 6 is the metal plate under test. Detailed Implementation

[0049] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings.

[0050] A method for calculating near-field and far-field RCS based on distance compensation includes the following steps:

[0051] Step 1: Adjust the polarization state of the transmitting antenna according to the required electromagnetic wave incident polarization (vertical polarization or horizontal polarization). A schematic diagram of the testing setup is shown below. Figure 1 As shown, it includes a transmitting antenna 1, a receiving antenna 2, and a turntable 4; wherein, the transmitting antenna 1 and the receiving antenna 2 are placed horizontally side by side and facing the target 3 to be tested, and the center height of the antennas is consistent with the center height of the target 3 to be tested. The target 3 to be tested is placed directly above the turntable 4 and can rotate with the turntable.

[0052] Step 2: Extract the test echo signal received by the target in the near field. Specifically as follows:

[0053] (1) Adjust the polarization state of the receiving antenna to be consistent with that of the transmitting antenna, and place both the transmitting antenna and the receiving antenna directly opposite the rotation center of the turntable at a distance R0.

[0054] (2) Empty the turntable and place a sufficiently large metal plate between the turntable and the antenna, directly in front of the turntable. Control the turntable to rotate using the motor control module to perform measurements, measuring the distance between the transmitting and receiving antennas. parameter;

[0055] (3) Empty the turntable, remove the metal plate, and start rotating the turntable by controlling the motor control module to perform measurements, measuring the distance between the transmitting and receiving antennas. parameter;

[0056] (4) The target to be measured is placed on the turntable. The turntable is rotated by controlling the motor control module to perform the measurement, measuring the distance between the transmitting antenna and the receiving antenna. parameter;

[0057] (5) Calculate the echo signal received by the target in the near field. The specific formula is as follows:

[0058]

[0059] Step 3: Based on the distance R0 from the antenna position to the turntable rotation center and the included angle Test echo signal received by the target in the near field The NFFFT formula for traditional near-field and far-field transforms is calculated as follows:

[0060] (1) If the scattered field is equivalent to a secondary radiation field with each scattering center as the source, then the test echo signal received in the near field can be expressed as:

[0061]

[0062] r' is the vector pointing from any scattering point P on the target to the rotation center O of the turntable. The vector pointing from the midpoint of the antenna to the center of the target is denoted as . The measurement position is denoted in polar coordinates as . The inherent characteristics of the scattering center of the target under test;

[0063] (2) Rearranging equation (2) into the weighted integral of the Green's function in two-dimensional free space with respect to the objective function, we get:

[0064]

[0065] k is the free-space wavenumber of the electromagnetic wave, since the test distance satisfies R0 >> r' maxThe above formula can be simplified to:

[0066]

[0067] in, It characterizes the inherent scattering properties of the target, independent of the test conditions. max The maximum size of the target to be measured;

[0068] (3) The relationship between the echo signal of the target under the test condition of infinity and the inherent scattering characteristics of the target is as follows:

[0069]

[0070] (4) Combining the near-field test equations, we can obtain the following near-far-field transformation relationship:

[0071]

[0072] Where, N0=kr' max +10;

[0073] Step 4: Since the distance from the test surface to the target changes with the rotation angle when the target rotates during near-field testing, a distance compensation function is added to equation (6) to obtain the NFFFRT formula with the added distance compensation function, as follows:

[0074]

[0075] in

[0076] Figure 2 This is a schematic diagram illustrating the principle of distance change between the target and the antenna in this invention. In the diagram, 5 represents the antenna, 6 represents the metal plate under test, R0 is the distance from the antenna tip to the center of the object under test, and L is the maximum size of the object under test. Let be the angle at which the object under test rotates during the test. During this rotation, the distance from the antenna end to the nearest point on the object changes with the rotation angle. The changes are specifically as follows: Therefore, this invention uses this distance to compensate for the NFFFRT formula.

[0077] Example 1

[0078] In this embodiment, a square metal plate with a length of 500mm, a width of 100mm, and a thickness of 4mm and a metal cylinder with a diameter of 30mm and a length of 300mm were used as the test targets. Under vertical polarization incident light, a double-ridged waveguide horn antenna with an operating frequency of 1-18GHz was used as the transmitting antenna and the receiving antenna, respectively. Tests were conducted at frequencies of 12GHz and 18GHz, with a near-field test distance of 3m.

[0079] In this embodiment, the near-field echo signal test results of the square metal plate are substituted into formulas (1)-(7) for calculation, and the RCS calculation results of the square metal plate at f=12GHz and f=18GHz when the test distance R0=3m are obtained. Simultaneously, the RCS results calculated by the NFFFFT formula of this invention, the RCS results calculated by the traditional NFFFT formula, and the theoretical RCS values ​​are compared, and the comparison results are as follows: Figure 3 As shown.

[0080] As can be seen from the figure, the RCS value calculated by the traditional NFFFT formula has a large error compared with the theoretical value, exceeding 1 dBsm at some angles, and the error tends to increase as it deviates further from 0°. In contrast, the RCS value calculated by the NFFFFT formula proposed in this invention has a better fit with the theoretical value and good overlap across the entire angle range, indicating that the calculation method of this invention has excellent accuracy in the angle range of -180° to 180° and has strong applicability.

[0081] The above description is merely a specific embodiment of the present invention. Any feature disclosed in this specification may be replaced by other equivalent or similar features unless otherwise specified. All disclosed features, or steps in all methods or processes, may be combined in any way except for mutually exclusive features and / or steps.

Claims

1. A method for calculating near-field and far-field RCS transformation based on distance compensation, characterized in that, Includes the following steps: Step 1: Adjust the polarization state of the transmitting antenna according to the required electromagnetic wave incident polarization requirements; Step 2: Extract the test echo signal received by the target in the near field. Specifically as follows: Step 2.

1. Adjust the polarization of the receiving antenna to match that of the transmitting antenna. The transmitting and receiving antennas are set at a distance R0 from the center of rotation of the turntable. Step 2.

2. Do not place the target to be tested on the turntable. Place a metal plate directly in front of the turntable, rotate the turntable and perform measurements. Measure the distance between the transmitting antenna and the receiving antenna at this time. parameter; Step 2.

3. Without placing the target on the turntable, remove the metal plate, rotate the turntable, and perform the measurement. Measure the distance between the transmitting and receiving antennas at this point. parameter; Step 2.

4. Place the target to be tested on the turntable, rotate the turntable and perform measurements, measuring the distance between the transmitting antenna and the receiving antenna at this time. parameter; Step 2.

5. Calculate the echo signal received by the target in the near field. The specific formula is as follows: Step 3: Based on the distance R0 between the antenna and the rotation center of the turntable, and the rotation angle of the target under test... and the echo signal obtained in step 2 The NFFFT formula for traditional near-field and far-field transforms is obtained, and the specific formula is as follows: in, Let P be the vector pointing from any scattering point P on the target to the center of rotation O of the turntable. The coordinates of a certain measurement position of the target in polar coordinates are... k is the free-space wavenumber of electromagnetic waves, N0 = kr' max +10, r' max The maximum size of the target to be measured. Here, j is the Hankel function, and j is the imaginary unit. Step 4: Perform range compensation on the traditional near-field and far-field transform NFFFT formula obtained in Step 3, using the shortest distance from the antenna end to the target as the compensation principle, to obtain the final NFFFT formula, specifically: in, 2. The RCS near-field and far-field transformation calculation method as described in claim 1, characterized in that, The testing system in step 1 specifically includes a transmitting antenna, a receiving antenna, and a turntable. The transmitting and receiving antennas are placed horizontally side by side and facing the target under test. The center height of the antennas is consistent with the center height of the target under test. The target under test is placed directly above the turntable and can rotate with the turntable.

3. The RCS near-field and far-field transformation calculation method as described in claim 1, characterized in that, In step 1, the polarization state of the transmitting antenna is either vertical or horizontal.

4. The RCS near-field and far-field transformation calculation method as described in claim 1, characterized in that, In step 2, parameter, Parameters and The parameters are used after time-domain gating. calculate.

5. The RCS near-field and far-field transformation calculation method as described in claim 1, characterized in that, In step 2, the size of the metal plate must be larger than the size of the target to be tested, and it must be placed in a way that completely covers the target to be tested.

6. The RCS near-field and far-field transformation calculation method as described in claim 1, characterized in that, In step 2, the isolation between the receiving antenna and the transmitting antenna must be less than -40dB.

7. The RCS near-field and far-field transformation calculation method as described in claim 1, characterized in that, In step 2, the turntable rotates at an angle ranging from -180° to +180°.

8. The RCS near-field and far-field transformation calculation method as described in claim 1, characterized in that, The specific process of step 3 is as follows: (1) If the scattered field is equivalent to a secondary radiation field with each scattering center as the source, then the test echo signal received in the near field can be expressed as: Let P be the vector pointing from any scattering point P on the target to the center of rotation O of the turntable. The vector pointing from the midpoint of the antenna to the center of the target being measured. The inherent characteristics of the scattering center of the target under test; (2) Rearranging equation (4) into the weighted integral of the Green's function in two-dimensional free space with respect to the objective function, we get: Since the test distance satisfies R0 >> r' max The above formula can be simplified to: in, It characterizes the inherent scattering properties of the target, regardless of the test conditions; (3) The relationship between the target's echo signal in the far field and the target's inherent scattering characteristics is as follows: (4) Combining the near-field test relations (6) and (7), we can obtain the following near-field and far-field transformation relations: Where N0=kr' max +10.

9. The RCS near-field and far-field transformation calculation method as described in claim 1, characterized in that, In step 3, the distance R0 between the antenna position and the rotation center of the turntable should meet the near-field test conditions, i.e., R0 < 2D. 2 / λ, where D is the maximum size of the antenna or the target under test, and λ is the wavelength corresponding to the operating frequency.

Images