A fast scanning measurement method for large scale target near field RCS test

Abstract

The application discloses a fast scanning measurement method for large-size target near-field RCS (Radar Cross Section) testing. Under the condition of actual test site limitation, the application proposes a scheme of array composition, scanning mode and test method of a near-field bistatic measurement transmitting-receiving antenna, overcomes the condition limitation of the actual test site, optimizes RCS near-field measurement scanning, and establishes a near-field measurement high-efficiency data multiplexing method.

Description

Technical Field

[0001] This invention belongs to the technical field of electromagnetic measurement, specifically relating to a rapid scanning measurement method for near-field RCS testing of large-size targets. Background Technology

[0002] Radar Cross Section (RCS) measures the ability of radar-detected objects such as aircraft, vehicles, and ships to scatter electromagnetic waves. Therefore, RCS measurement and evaluation are essential when designing stealth targets of significant military value. However, measuring the RCS of a target aircraft is difficult because RCS calculation requires plane wave incidentness and that the receiver meets far-field (FF) conditions. Depending on the architecture of the RCS measurement system, the actual measurement of target RCS characteristics can be categorized into three types: far-field measurement, compact field measurement, and near-field measurement.

[0003] Near-field measurements obtain the target's near-field (NF) information and, with the assistance of a near-field-far field transformation (NFFFT) algorithm, yield the far-field RCS test results. Common post-processing algorithms are divided into imaging approximation algorithms and full-wave algorithms. Imaging approximation algorithms use near-field monostation measurement information to extrapolate the target's far-field monostation information. However, when dealing with complex targets involving multiple scattering and other complex coupling effects, approximation methods may not provide accurate calculation results. Full-wave methods require bistational measurements of the near field and use plane-wave synthesis (PWS) to approximate the field distribution within a certain region to the state of plane wave incidence. When high accuracy of the measurement results is required, bistational near-field measurements are typically performed, and a relevant full-wave method is used for near-field-far field transformation.

[0004] Near-field bistatic measurement technology can measure the far-field RCS at a relatively close distance to the target, thereby reducing the size of the microwave anechoic chamber and significantly lowering construction costs. This advantage comes at the cost of a dramatic increase in testing time. When performing near-field RCS measurement of a target in a specific incident direction, it is necessary to synthesize multiple incident waves from different incident directions within a spatial range around that incident direction into a single plane wave. For each incident wave, it is necessary to measure the bistatic near-field within a spatial range around it. Therefore, the overall testing complexity is O(N). 4(N represents the number of test points). When the target is large, existing microwave anechoic chambers cannot meet the requirements, and the near-field scanning test time is unacceptable, becoming a bottleneck for RCS near-field bistatic measurement. Therefore, this invention aims to optimize RCS near-field measurement scanning and establish an efficient data multiplexing method for near-field measurement. Considering the limitations of actual test site conditions, it proposes a scheme for the array configuration, scanning method, and testing method of near-field bistatic measurement transceiver antennas, providing theoretical support for the integration of a rapid near-field scanning measurement system for target RCS. Summary of the Invention

[0005] In view of this, the present invention provides a rapid scanning measurement method for near-field RCS testing of large-size targets, which can overcome the limitations of actual test site conditions, optimize near-field RCS measurement scanning, and establish an efficient near-field measurement data reuse method.

[0006] The technical solution for implementing the present invention is as follows:

[0007] A rapid scanning measurement method for near-field RCS testing of large-size targets includes the following steps:

[0008] Step 1: Construct multiple test points for transmitting antennas arranged in a single column and multiple test points for receiving antennas arranged in a two-dimensional curved surface. Set up a single transmitting antenna and multiple receiving antennas, which can move on their respective test points.

[0009] Step 2: Measure the background clutter, then measure the near-field value of the calibration body. Move the transmitting antenna to other test points, and at the same time change the distribution range of the corresponding receiving antennas. Repeat the measurement of the near-field value of the calibration body until all test points of the transmitting antennas have been tested.

[0010] Step 3: Measure the near-field value of the target object to obtain the canceled near-field value. Move the transmitting antenna to other test points and change the distribution range of the corresponding receiving antennas. Repeat the measurement of the near-field value of the target object until all test points of the transmitting antennas have been tested.

[0011] Step 4: Rotate the target's turntable to change the target's direction, and repeatedly measure the target's near-field value to obtain the canceled near-field value until the target completes one full rotation;

[0012] Step 5: Use the plane wave synthesis algorithm to solve for the transmission coefficient of each transmitting antenna;

[0013] Step 6: Use the near-field extrapolation algorithm to solve for the size of each plane spectrum;

[0014] Step 7: Combine the emission coefficient and the plane wave spectrum solution to calculate the far-field RCS.

[0015] Furthermore, step two specifically includes:

[0016] Step 2.1: Perform background clutter cancellation measurement, measuring the background field value E at each receiving antenna point. d ;

[0017] Step 2.2: Near-field measurement of the calibration body. A centrally symmetrical metal sphere is used for calibration. The electric field value E of the receiving antenna corresponding to a transmitting antenna point is measured. caliT Simultaneously, the original near-field value E of the calibration body under the radiation conditions of the receiving antenna is calculated. caliD ;

[0018] Step 2.3: Move the transmitting antenna and simultaneously change the distribution range of the corresponding receiving antenna, repeating the calibration measurement step 2.2 until all test points of all transmitting antennas have been tested.

[0019] Furthermore, step three specifically includes:

[0020] Step 3.1: Near-field measurement of the target object, measuring the electric field value E of the receiving antenna corresponding to a transmitting antenna point. tarT ;

[0021] Step 3.2: Perform near-field value conversion of the target body according to formula (1) to obtain the near-field value E after background clutter cancellation. tarD E tarT E caliT and E caliD All values ​​are electric field values ​​obtained under the same conditions of radiation from the transmitting antenna and reception from the receiving antenna.

[0022]

[0023] Step 3.3: Move the transmitting antenna and simultaneously change the distribution range of the corresponding receiving antennas. Repeat steps 3.1 and 3.2 until all test points of all transmitting antennas have been tested.

[0024] Furthermore, in step four, the target's single rotation angle Satisfies formula (2);

[0025]

[0026] Where λ is the free space wavelength corresponding to the test frequency, and R0 represents the maximum size of the target.

[0027] Furthermore, in step five, the transmitting antennas required for plane wave synthesis are not a single row during testing, but rather a transmitting antenna array formed by a row of transmitting antennas after the target has been relatively rotated. The positional distribution of this array and the total number of transmitting antennas are determined by the antenna distribution in a row of transmitting antenna positions required for plane waves at different elevation angles and target rotation angles that need to be considered for a single horizontal angle target.

[0028] Furthermore, in step six, the range of plane wave elevation angles to be calculated needs to be determined based on the height of the transmitting antenna. The number of receiving antennas required varies depending on the position of the transmitting antenna.

[0029] Beneficial effects:

[0030] 1. The transmitting antenna of the present invention moves on a fixed track in the column direction, while the target rotates at a fixed angle on the turntable, thereby generating data multiplexing of the transmitting antenna in the horizontal direction, which can reduce the number of antennas actually used and save measurement costs.

[0031] 2. The transmitting antenna of the present invention will simultaneously participate in the synthesis of multiple plane wave incident directions at the point of column movement, thereby generating vertical transmitting antenna data multiplexing, which can further improve the efficiency of fast scanning measurement at a single angle, reduce the number of near-field measurements, and thus reduce the overall test time.

[0032] 3. The present invention uses a method of moving and scanning multiple receiving antennas in a group, and moving the test points simultaneously. This method can obtain as much near-field test information as possible in a single measurement, balancing the efficiency of antenna usage and the total test time.

[0033] 4. This invention constructs a transmitting antenna array as algorithm input based on the angle and scanning range of the multiplexed transmitting antenna; and constructs corresponding near-field measurement regions and receiving antenna arrays as algorithm input based on transmitting antennas multiplexed at different elevation angles. The simulation of extrapolated far-field RCS can serve as an evaluation scheme for rapid scanning measurement technology, providing theoretical support for the integration of a near-field rapid scanning measurement system for target RCS. Attached Figure Description

[0034] Figure 1 This is a flowchart illustrating the execution of a rapid scanning measurement method for near-field RCS testing of large-size targets according to the present invention.

[0035] Figure 2 This is a schematic diagram of transmitting antenna multiplexing at different horizontal plane wave angles.

[0036] Figure 3 This is a schematic diagram of transmitting antenna multiplexing at different elevation plane wave angles.

[0037] Figure 4This is a schematic diagram illustrating the multiplexing of receiving antennas required for different transmitting antennas.

[0038] Figure 5 The results are the near-field measurement calculations of the single-station RCS in the example at an elevation angle of θ = 75°.

[0039] Figure 6 The results are the near-field measurement calculations of the single-station RCS in the example at an elevation angle θ = 90°.

[0040] Figure 7 The results are the near-field measurement calculations of the single-station RCS in the example at an elevation angle of θ = 105°. Detailed Implementation

[0041] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0042] This invention provides a rapid scanning measurement method for near-field RCS testing of large-size targets. Considering the limitations of actual testing sites, it proposes a near-field rapid scanning measurement system implementation scheme, addressing the following technical problems:

[0043] (1) The structure of the transmitting antenna and the scanning method of the transmitting antenna;

[0044] (2) Array configuration and scanning method of dual-station / multi-station receiving antennas;

[0045] (3) Extrapolation far-field RCS simulation strategy based on near-field multiplexing data.

[0046] like Figure 1 As shown, the specific solution of the present invention is as follows:

[0047] Step 1: Based on the actual test site conditions, establish the movement range of a series of transmitting antennas Tx. A single Tx can move back and forth between multiple test points within this range, and its angle range satisfies the projection angle from the measured elevation angle to the test static field.

[0048] Step 2: Based on the actual test site conditions, construct the moving range of the planar distributed receiving antenna Rx array. Multiple Rx can move up, down, left, and right at multiple test points within this range, or relative to a single Tx, one or more Rx can perform relative movement within a certain angle range.

[0049] Step 3: Perform background clutter cancellation measurement, measuring the background field value E at each Rx point. d .

[0050] Step 4: Near-field measurement of the calibration body. A centrally symmetrical metal sphere is used for calibration. The electric field value E of Rx corresponding to a transmitting antenna point Tx is measured. caliT Simultaneously, the original near-field value E of the calibration body under Rx radiation conditions was calculated.caliD .

[0051] Step 5: Vertically move the transmitting antenna Tx, while simultaneously changing the corresponding receiving antenna distribution range, and repeat step 4 of the calibration body measurement. Continue until all test points for all transmitting antennas have been tested.

[0052] Step 6: Near-field measurement of the target body, measuring the electric field value E of Rx corresponding to a Tx point. tarT .

[0053] Step 7: Convert the near-field value of the target body to obtain the canceled near-field value. Substitute it into the background clutter cancellation formula (1), where E tarT E caliT and E caliD All values ​​are electric field values ​​obtained under the same Tx radiation and Rx reception conditions.

[0054]

[0055] Step 8: Vertically move the transmitting antenna Tx, while simultaneously changing the corresponding receiving antenna distribution range. Repeat steps 6 and 7 until all test points for all transmitting antennas have been tested.

[0056] Step 9: Rotate the target's turntable to change the target's direction, and then repeat steps 6, 7, and 8. The rotation angle for each rotation satisfies formula (2) until the target completes one full rotation.

[0057]

[0058] Where λ is the free space wavelength corresponding to the test frequency, and R0 represents the maximum size of the target.

[0059] Step 10: Use the plane wave synthesis algorithm to solve for the transmission coefficient of each Tx. The Tx required for plane wave synthesis is not a single column during testing, but rather a transmitting antenna array formed by a column of Tx after the target has been relatively rotated. The positional distribution of this array and the total number of Tx antennas are determined by the target rotation angle that needs to be considered for a single horizontal angle target and the antenna distribution in a column of Tx points required for plane waves at different elevation angles. Referring to the illustration, firstly, the target rotation angle that needs to be considered for a single horizontal angle target is determined by the horizontal projection range (e.g., ...). Figure 2 As shown), the antenna distribution required for plane waves at different elevation angles in a series of Tx points is then determined by the projection range of the elevation angle (e.g., Figure 3 As shown in the figure, the final location distribution of the Tx array and the total number of Tx antennas corresponding to plane wave synthesis are determined.

[0060] The field distribution of a plane wave is formed by the sum of the electromagnetic fields radiated by the antenna, and its expression is shown in formula (3):

[0061]

[0062] Where M is the total number of transmitting antennas required for single-angle plane wave synthesis, and E syn For the electromagnetic field of the synthesized plane wave, E m For each Tx, the radiated electromagnetic field, a m The emission coefficient for each Tx.

[0063] In order to solve the composition problem in numerical computation, it is necessary to establish a test function and test the boundary conditions at the boundary of the test region, as shown in (4).

[0064]

[0065] Where, β p (r) is the test function defined on the surface of the plane wave synthesis region. RWG basis functions are generally used, and there are a total of P discrete basis functions on the surface of the plane wave synthesis region. The electromagnetic field expression for the unit radiation of the antenna is shown in (5) and (6).

[0066]

[0067]

[0068] in, Let k be the unit vector of the plane wave expansion, k be the beam, and r be the field point. O Let D be the coordinates of the center of the box where the field point is located, and D be the transfer direction vector. The plane wave expansion representing the equivalent current at the Tx antenna is expressed as shown in (7).

[0069]

[0070] Where r' is the source point, r S J represents the center coordinates of the box containing the source point. m This represents the equivalent surface current at the source point.

[0071] T L (k,x) represents the transfer function of the plane wave, expressed as (8).

[0072]

[0073] in, For the second type of Hankel function, P l Let L be the Legendre function. The upper limit of the summation L takes the value of (9).

[0074]

[0075] Where d is the maximum size of the planar spectral focusing box, and d0 is the accuracy parameter, which is generally between 3 and 4.

[0076] Formula (4) can be used to construct an M×P dimensional linear matrix C for solving. However, since the matrix is ​​not invertible, an approximate solution method is required to minimize the root mean square error of the problem. The expression is shown in (10).

[0077]

[0078] Where C matrix is ​​the matrix spanned by the left side of formula (4), and x is a m Zhang Cheng's column vector, b is Zhang Cheng's column vector calculated from the right side of formula (4). Formula (10) can be equivalent to solving equation (11).

[0079] C H Cx = C H b (11)

[0080] Where C H Let C be the conjugate transpose of matrix C. This equation is solved using the GMRES iterator.

[0081] Step 10: Use the near-field extrapolation algorithm to solve for the magnitude of each plane wave spectrum. Here, the range of plane wave elevation angles to be calculated needs to be determined based on the height of the transmitting antenna. The number of receiving antennas required for transmitting antennas at different locations varies. In practice, one or more Rx groups are usually moved bidirectionally relative to Tx to measure the near-field values ​​at all points.

[0082] The field distribution at Rx can be calculated from the plane wave spectrum, as shown in expression (12).

[0083]

[0084] Where K is the effective planar spectrum number of the target, q is the label of Rx, and a total of Q Rx points are used to receive the electric field. At this time, the field value test results of the Rx points are all scalars, so there is no need to use the test function to calculate the inner product for scalarization. The formula for calculating the near field value from the spherical spectrum is shown in (13).

[0085]

[0086] Where D is the distance from the center of the box to each Rx. This is the corresponding distance direction unit vector. For the plane wave expansion at the target location, the expression is consistent with (7). For the near-field extrapolation method, if the plane wave spectrum is used as the unknown, there is no need to use formula (7) for further calculation; only the transfer matrix T needs to be calculated. L and use b kθ , Refers to the plane wave spectrum θ and Unknown coefficients in the components. Formula (12) will form a K×Q dimensional linear matrix, and its solution method is similar to that of formula (11), so it will not be repeated here.

[0087] Step 11: RCS synthesis. Multiply the emission coefficient corresponding to each Tx antenna by the far-field electric field corresponding to the target direction, and superimpose the results of all Tx antennas to obtain the far-field electric field in that direction. Finally, calculate the far-field RCS. The calculation formula is shown in (14).

[0088]

[0089] The simulation results are as follows:

[0090] Example: Model of the US B-2 stealth bomber

[0091] This invention provides a simulation process for a complete near-field RCS test of the US B-2 stealth bomber. The model of the US B-2 stealth bomber is as follows: Figure 2 As shown, the B-2 bomber has a wingspan of 30m and a height of 4.5m. The static field region is slightly larger than the aircraft's wingspan, forming a cylinder with a radius of 32m and a height of 4.5m. The aircraft can rotate within this region for near-field measurements. The calculation frequency is 150MHz, and θ = 90° is calculated. The single-station scattering over the angular range is calculated with an electrical dimension of 32 electrical wavelengths. Assume an anechoic chamber configuration of 100m × 100m × 40m, a turntable with a height of up to 20m, and continuous azimuth rotation of 360 degrees. Based on the anechoic chamber dimensions and equipment setup, the bistatic near-field measurement points required for near-field extrapolation are distributed on a sphere with a radius of 70m.

[0092] A schematic diagram of transmitting antenna multiplexing at different horizontal plane wave angles is shown below. Figure 2 As shown, according to the Nyquist sampling theorem, the height spacing of the transmitting antennas is 2.18m. According to the projection theorem, the range of the transmitting antenna array in the theta direction is ±23.4°. Therefore, a single column has 27 transmitting antennas. Due to multiplexing calculations, the horizontal spacing is equivalent to a 1.22m horizontal spacing (rotating 1° for each measurement). The total directional range is ±35°. A schematic diagram of transmit antenna multiplexing at different elevation plane wave angles is shown below. Figure 3 As shown, the required Tx positions for plane waves at different elevation angles overlap, therefore, the multiplexing of plane waves at different elevation angles using Tx can be considered. In practice, the same Tx is usually moved up and down relative to the target attitude to radiate antennas at different angles. According to the projection theorem, approximately 18 antennas from the transmit antenna array are used for each angle. Ultimately, the number of transmit antennas participating in plane wave synthesis is 1159 at 90° incidence, and around 1000 at other angles. A schematic diagram of the multiplexing of receive antennas required for different transmit antennas is shown below. Figure 4As shown, according to the projection theorem, a transmitting antenna needs to participate in the calculation of a plane wave within ±35° of the phi direction. The receiving antennas are spaced 2.18m apart. Considering that a single transmitting antenna needs to participate in the composite calculation in multiple directions, the number of receiving antennas required varies depending on the location of the transmitting antenna, ranging from 1200 to 2400. In practice, one or more Rx groups are typically moved bidirectionally relative to Tx to measure the near-field values ​​at all points.

[0093] The calculation results are as follows Figure 5 , Figure 6 , Figure 7 As shown, the results correspond to single-station measurements at different elevation angles. It can be seen that the near-field extrapolation results are consistent with the scattering calculations using the MLFMA method. The root mean square error (RMSE) is 1.44 dB for the 75° principal polarization, 1.44 dB for the 90° principal polarization, and 1.49 dB for the 105° principal polarization. Finally, the comparison of the number of transmit and receive antennas required with and without multiplexing technology is shown in the table below. It can be seen that considering multiplexing significantly reduces the absolute coordinates of the actual measurement points, which greatly reduces the near-field scanning test time. Simultaneously, fewer transmit and receive antennas can be used for mobile measurements, saving on test hardware costs.

[0094]

[0095] In summary, the above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A rapid scanning measurement method for near-field RCS testing of large-size targets, characterized in that, Includes the following steps: Step 1: Construct multiple test points for transmitting antennas arranged in a single column and multiple test points for receiving antennas arranged in a two-dimensional curved surface. Set up a single transmitting antenna and multiple receiving antennas, which can move on their respective test points. Step 2: Measure the background clutter, then measure the near-field value of the calibration body. Move the transmitting antenna to other test points and change the distribution range of the corresponding receiving antennas. Repeat the measurement of the near-field value of the calibration body until all test points of the transmitting antennas have been tested. Step 3: Measure the near-field value of the target object to obtain the canceled near-field value. Move the transmitting antenna to other test points and change the distribution range of the corresponding receiving antennas. Repeat the measurement of the near-field value of the target object until all test points of the transmitting antennas have been tested. Step 4: Rotate the target's turntable to change the target's direction, and repeatedly measure the target's near-field value to obtain the canceled near-field value until the target completes one full rotation; Step 5: Use the plane wave synthesis algorithm to solve for the transmission coefficient of each transmitting antenna; In step five, the transmitting antennas required for plane wave synthesis are not the same row as during testing, but rather an array of transmitting antennas formed by a row of transmitting antennas after the target has been relatively rotated. The positional distribution of this array and the total number of transmitting antennas are determined by the target rotation angle and the antenna distribution required by the plane waves at different elevation angles in a row of transmitting antennas for a single horizontal angle target. The transmitting antennas move on a fixed track in the column direction, while the target rotates at a fixed angle on the turntable, thereby generating data multiplexing of the transmitting antennas in the horizontal direction. The transmitting antennas at the points where they move in the column direction will simultaneously participate in the synthesis of multiple plane wave incident directions, thereby generating data multiplexing of the transmitting antennas in the vertical direction. Step 6: Use the near-field extrapolation algorithm to solve for the size of each plane spectrum; In step six, the range of plane wave elevation angles to be calculated needs to be determined based on the height of the transmitting antenna. The number of receiving antennas required varies depending on the position of the transmitting antenna. Step 7: Combine the emission coefficient and the plane wave spectrum solution to calculate the far-field RCS.

2. The rapid scanning measurement method for near-field RCS testing of large-size targets as described in claim 1, characterized in that, Step two specifically includes: Step 2.1: Perform background clutter cancellation measurement, measuring the background field value at each receiving antenna location. ; Step 2.2: Near-field measurement of the calibration body. A centrally symmetrical metal sphere is used for calibration. The electric field value of the receiving antenna corresponding to a transmitting antenna point is measured. Simultaneously, the original near-field value of the calibration body under the radiation conditions of the receiving antenna is calculated. ; Step 2.3: Move the transmitting antenna and simultaneously change the distribution range of the corresponding receiving antenna, repeating the calibration measurement step 2.2 until all test points of all transmitting antennas have been tested.

3. The rapid scanning measurement method for near-field RCS testing of large-size targets as described in claim 2, characterized in that, Step three specifically includes: Step 3.1: Near-field measurement of the target object, measuring the electric field value of the receiving antenna corresponding to a transmitting antenna point. ; Step 3.2: Perform near-field value conversion of the target body according to formula (1) to obtain the near-field value after background clutter cancellation. ,in , and All values ​​are electric field values ​​obtained under the same conditions of radiation from the transmitting antenna and reception from the receiving antenna. （1） Step 3.3: Move the transmitting antenna and simultaneously change the distribution range of the corresponding receiving antennas. Repeat steps 3.1 and 3.2 until all test points of all transmitting antennas have been tested.

4. A rapid scanning measurement method for near-field RCS testing of large-size targets as described in any one of claims 1-3, characterized in that, In step four, the target's single rotation angle Satisfies formula (2); （2） in The free-space wavelength corresponding to the test frequency. R 0 indicates the maximum size of the target.

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