CN104614715A - Measurement calibration and polarimetric calibration device for target bistatic radar cross section and measurement calibration method thereof - Google Patents
Measurement calibration and polarimetric calibration device for target bistatic radar cross section and measurement calibration method thereof Download PDFInfo
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Abstract
The invention discloses a measurement calibration and polarimetric calibration device (BPARC) for a target bistatic radar cross section (RCS) and a measurement calibration method thereof. The device is a double-antenna active polarization radar calibration (PARC) device with a biaxial rotation control mechanism, can simultaneously solve the problems of target bistatic RCS calibration and target bistatic polarization scattering matrix measurement calibration under the bistatic scattering measurement condition. As the bistatic RCS measurement calibration device, the BPARC can overcome the defects of the existing technology and device and ensures the stability of an RCS calibration value under different bistatic angles. As the polarimetric calibration device for bistatic polarization scattering matrix measurement, the BPARC retains all advantages of the existing single-station polarimetric calibration PARC device and also can be used for bistatic polarimetric calibration. The functions of bistatic RCS calibration and bistatic polarimetric calibration of the traditional PARC are increased.
Description
Technical Field
The invention relates to the technical field of Radar Cross Section (RCS) measurement and processing of targets, in particular to a device (BPARC for short) for measuring, calibrating and polarizing a target double-station Radar Cross Section (RCS) and a measuring and calibrating method thereof.
Background
A schematic diagram of the geometry of a two-station Radar Cross Section (RCS) measurement is shown in fig. 1. The two-station radar equation can be expressed as:
in the formula, PrIs the receiver input power (W); ptIs a transmitter power (W); gr,GtGain (dimensionless) for the receive antenna and the transmit antenna, respectively; l istTotal loss for the transmit channel (dimensionless); l isrTotal loss for the receive channel (dimensionless); rt,RrThe distances (m) from the target to the transmitting antenna and the receiving antenna respectively;is a target two-station scattering cross section (m)2) (ii) a λ is the radar operating wavelength (m).
According to the relative calibration principle (see the article [1] Huangpekang Master eds., [ Radar target characteristic Signal ], chapter 6, astronavigation Press, 1993.), a dual-station RCS measurement calibration equation can be derived from a dual-station radar equation as follows:
in the formulaIn order for the target dual-station RCS,a dual station RCS as a calibration volume; prCAnd PrTRespectively measuring the echo power received by the radar when the target body and the target are measured; sTAnd SCRespectively measuring a target and a radar complex echo signal when a calibration body is measured in single RCS measurement sampling; kbThe calibration constants corresponding to the two-station measurement geometry are:
in the formula RtTAnd RtCRespectively representing the target distance and the calibration body distance of the transmitting channel; rrTAnd RrCRespectively representing the target distance and the calibration body distance of the receiving channel; l istTAnd LtCRespectively representing the total loss of the transmitting channel when the target is measured and the target body is measured; l isrTAnd LrCThe total loss of the receiving channel is shown for the target and target under test, respectively. K if the target distance and the calibration body distance are determined in the test and the two-station geometric relationship remains unchangedbThe value is also a definite constant.
When a double-station RCS is selected to measure a calibration body, the scattering characteristic of the calibration body can be stabilized in a relatively small level range along with the change of the measured double-station angle, so that the high enough double-station calibration precision is ensured.
However, some conventional single-station RCS measures a calibration object such as a metal sphere, a metal cylinder, a dihedral and trihedral reflector, a metal plate, etc., and the scattering characteristics thereof exhibit an oscillation characteristic as the double station angle increases, and are not suitable for use as a calibration object when the double station angle is large. For example, the two-station scattering of a metal ball calibration volume is calculated by the Mie exact solution, and the normalized RCS versus two-station angle is shown in fig. 2 when the ka value is 20. It is clear that for small two-station angle measurements, metal spheres are still suitable for two-station RCS calibration, but as the two-station angle exceeds 90 °, the oscillation behavior becomes more and more severe, which will affect the calibration accuracy to a large extent.
Therefore, how to find or design a proper two-station scattering calibration body is a technical problem of two-station RCS measurement calibration.
The prior art analysis related to the present invention is as follows:
prior Art-1 use of a conventional calibration volume having a simple shape
This is currently the most commonly used technique in dual station RCS measurement calibration. Bradley et al (see [2] C.J.Bradley, P.J.Collins, et al, "An introduction of two stationary calibration objects," IEEE Trans.on Geosistence and remove Sensing, Vol.43, No.10, Oct.2005: 2177-.
The defects of the prior art-1: bradley et al have just studied the calibration characteristics of some traditional calibration bodies and their suitability in the calibration of two-station measurement, and have not solved the key problem that the two-station scattering of the calibration body appears to fluctuate with the oscillation of the two-station angle and cause large calibration error as the two-station angle increases.
Prior art-2 designing special double-station calibration body
Monzon proposed a two-station calibration body design (see document [4] C. Monzon, "A cross-polarized biostatic calibration device for RCS measurements," IEEE trans. on Antennas and amplification, Vol.51, No.4, April 2003: 833-. Numerical calculations show that such a calibration body has good two-station scattering properties and cross-polarization scattering properties.
The defects of the prior art-2: the main technical drawbacks that have led to such a calibration body design that have not been really put to practical engineering include three aspects: (1) because the metal conducting wire needs to be wound on the dielectric cylinder body strictly according to a certain inclination angle, the solid processing and manufacturing are difficult; (2) the accurate calculation of the theoretical scattering value of the calibration device has certain difficulty; (3) how the machining error affects the calibration precision is difficult to analyze.
Prior art-3: deriving scaling functions for two-station measurements from two single-station measurements using two transmitters and receivers
Alexander and Currie et al (see [5] N.T. Alexander, N.C. Currie, M.T. Tuley, "Calibration of stationary RCS measurements," Proc.of Antenna Measurement technique Association 1995 Symposium, Columbus, OH, Nov.1995: 166. quadrature. 171, and [6] N.C. Current, N.T. Alexander, M.T. Tuley, "Unique Calibration for stationary radio Measurement measurements," Proc.IEEE 1996 National reference, Ann Arbor, Mich, May: 142. 147. was measured in a dual-station system (TSC) which solves the problem of dual-station Measurement of the United states air force vertical Scattering test field (TSC. RAR. TM.), and was measured by a dual-station Calibration system (TSC. TM. S), which was measured by a dual-station Calibration system (for dual-station Measurement), as shown by the dual-Calibration station Calibration analysis and Calibration system (1996) and was measured by a dual-station (RCS).
The main advantages of the prior art-3: the method can adopt the traditional single-station calibration body, such as a metal ball, a metal cylinder, a corner reflector, a flat plate and the like as the double-station calibration body, can finish the double-station RCS measurement and calibration only by twice single-station and twice double-station measurements, has the calibration body uncertainty easily meeting the engineering application requirements, and can not be influenced by the large dispersion fluctuation of the calibration double-station under the condition of large double-station angles, because the RCS theoretical value of the calibration body only needs to adopt the single-station RCS theoretical value.
The main defects of the prior art-3 are as follows: (1) while the conventional dual-station RCS measurement only needs one transmitter and one receiver and is placed at a required position (as shown in FIG. 1), two RCS measurement radars (including the transmitter and the receiver) with basically consistent performance must be used by adopting the calibration technology, so that the system cost is multiplied; (2) because the calibration function can be derived only by two times of single-station measurement and two times of double-station measurement data of the calibration body, factors influencing the calibration error are increased, and compared with a calibration method adopting a single transmitter and a single receiver, the RCS calibration uncertainty is increased.
Prior art-4: active polarization radar scaling device (PARC)
An Active polarization calibrator (PARC) is mostly used for polarization channel calibration and RCS calibration in polarization scatterometry.
Suppose that the receiving channel transmission matrix R, the transmitting channel transmission matrix T and the background clutter I of the radar result in the measured polarization scattering matrix of the targetSmThere is a deviation from the target true polarization scattering matrix S. Measured value SmAnd the true value S of the target PSM satisfy the following relation (see document [7 ]]Xiaozhihe, Chao Zengming, Jiangxing, Wang Cheng, radar target polarization scattering holding measurement technology [ J]Systematic engineering and electronics, 1996, (3):13-32.)
Sm＝R·S·T+I (4)
The aim of polarization calibration is to recover the true PSM of the target from the measured data as undistorted as possible, and solve the equation into
S＝R-1·(Sm-I)·T-1 (5)
It can be seen that the transmit-receive channel transmission matrix R, T and the background noise I of the system must be simultaneously obtained to achieve the calibration of any target.
The common practice for polarization calibration is: controlling the background clutter of the test environment to be low enough to ignore the influence on the measurement, and approximately I is 0, or directly measuring a background clutter matrix I and performing background vector subtraction processing, on the basis, using a known target of the theoretical PSM as a polarization calibration body, combining measured PSM data, and establishing a quantitative relation between the theoretical PSM and the measured value after background cancellation by an equation (4), wherein the quantitative relation comprises:
M＝Sm-I＝R·S·T (6)
to solve for the calibration parameters R, T of the radar measurement system, there are the following simplified equations
S＝R-1·M·T-1 (7)
The most basic PARC is an active transponder with a fiber delay line, and its simple structure is shown in FIG. 5 (see document [8] K. Sarabandi, F. T. Ulaby, "Performance characteristics of polar active radars and a new single antenna design," IEEE Transactions on Antennas and Propagation, 1992, 40(10): 1147-. The working process is as follows: the receiving antenna receives radar signals from the space, and clutter outside a radar working frequency band is filtered by a band-pass filter after the signals are amplified by an amplifier; the measuring distance is equivalently changed by adjusting the delay size of the delay line, so that background clutter on a fixed distance can be removed to enable I to be 0 approximately; and finally, the data is forwarded out through a forwarding antenna so as to be received and processed by the radar.
Compared with a passive calibration body, the radar scattering cross section of the PARC is not limited by physical size, and the size of the PARC can be changed by adjusting an attenuator, and the theoretical calculation value is as follows:
in the formula, GTAnd GRGain, G, for PARC transmitting and receiving antennas, respectivelyLoopThe total gain of the whole loop except the antenna in fig. 5. The RCS size can also be generally measured by a relative scaling method.
The transceiving antenna of PARC generally adopts a horn antenna, and the antenna has a single linear polarization mode. As shown in fig. 6, the angle between the linear polarization state of the antenna and the horizontal X axis (horizontal right) is referred to as the polarization angle of the antenna. If the polarization angle of the PARC receiving antenna is thetarThe polarization angle of the repeater antenna being thetatThen the theoretical PSM of PARC is:
in the formula, ht＝[cosθt sinθt]TAnd hr＝[cosθr sinθr]TThe superscript T represents the transpose operation of the matrix or vector for the Jones vectors for PARC transmit and receive antennas, respectively.
As can be seen from equation (9), if the dual antenna PARC design is adopted, the polarization angle θ of the receiving antenna is changedrAnd the polarization angle theta of the repeater antennatObtaining theoretical polarization scattering matrix S of various polarization angle combinations of PARCP1，SP2，…，SPnCombined with corresponding polarization scattering matrix measurements Mp1，Mp2，…，MpnAnd a plurality of equation sets can be written according to the formula (7), and then the transmission matrixes R and T of the receiving and transmitting channels can be solved, so that the acquisition of polarization calibration parameters is completed. This is the basic starting point of the present invention.
In practical applications, the accuracy of the obtained polarization calibration parameters depends to a large extent on whether the theoretical PSM of the calibration body selected for the polarization calibration measurement is accurate and whether the equation for solving the polarization calibration parameters is robust.
Single antenna PARC:
document [8] proposes a structure of a single antenna PARC, and a schematic block diagram thereof is shown in fig. 5. Inside the antenna aperture plane there is a pair of mutually orthogonally placed feeds for receiving and retransmitting signals, respectively, so that the polarizations of the received and retransmitted signals are always mutually orthogonal, as shown in fig. 7. By rotating the antenna to different angular positions around the radar line of sight, the transmit-receive polarization state of the antenna changes accordingly, so that different polarization scattering matrixes are obtained.
The basic steps for using this PARC polarization calibration are [8 ]:
(1) calculating theoretical value S of polarization scattering matrix of single antenna PARC under two posturesP1、SP2；
(2) Measurement value Mm for measuring single antenna PARC in two attitudesp1、Μp2In the measurement, the received echo is delayed to a distance away from the PARC through the processing of a delay line, so that the influence of the background clutter I can be eliminated;
(3) mixing the above materialsP1、ΜP2、SP1And SP2And (3) establishing an equation set through the formula (7), and solving transmitting and receiving channel transmission matrixes R and T of the single-station radar system for transmitting and receiving the common antenna.
Although the single antenna PARC proposed in document [8] has a simple structure and can perform polarization calibration well, the following disadvantages still exist:
(1) the receiving polarization mode and the transmitting polarization mode of the PARC antenna are always orthogonal, the polarization of the transmitting antenna and the receiving antenna can not be combined at will, the form of a theoretical polarization scattering matrix is greatly reduced, and many polarization scattering matrixes in special forms can not be obtained through the single antenna PARC, such as an identity matrix, so that the application range of the single antenna PARC is limited;
(2) because the scheme measures the PARC of the rotation angles of some antennas to obtain the measured data, when a small rotation angle error exists in the measurement, the calibration precision is influenced;
(3) the polarization calibration parameter extraction algorithm using such PARC is less robust: once the system measurement value at a certain time in the calibration process has an abnormal value or a large error, the precision of the extracted calibration parameter is greatly reduced;
(4) because a pair of orthogonal polarization feed sources work simultaneously, a polarization filtering device cannot be adopted to improve the polarization isolation of the antenna, and the polarization calibration precision is influenced to a great extent.
Dual antenna PARC:
the document [9] (M.He, Y.Z.Li, S.P.Xiao, et al., "Scheme of dynamic polarization," Electronics Letters, 2012, 48(4): 237: "238"), proposes a PARC system based on digital RF memory, and its overall design block diagram is shown in FIG. 5. The system carries out discretization sampling on received signals, stores the discretized signals in a digital radio frequency memory, operates the discretized signals in the memory in all relevant processing of the signals, and converts the signals into analog signals for forwarding through a D/A converter after the processing. The receiving antenna and the forwarding antenna of the PARC are controlled to rotate at different angular speeds through the rotary table, the PARC transmitting and receiving antenna always rotates in the radar measurement process, and an active polarization calibration method based on a frequency domain is provided based on the PARC structure.
The basic steps of polarization calibration using such PARC are:
(1) PARC transmitting-receiving antenna respectively using angular velocity omegarAnd ωtRotating, measuring the measured value (M)P；
(2) Polarize scattering matrix S by PARC theoryPAnd the measured value μmPRewritten as a 4x1 vectorAnd and rewriting the error model described by the formula (3) into <math>
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</math> Wherein <math>
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</math> （Kroneker product) which fully describes the radar system calibration parameters;
(3) will be provided withTaking Fourier transform on both sides simultaneously to obtainAnd the solution is only established when the rotation angular velocities of the PARC receiving antenna and the forwarding antenna are not equal;
document [9] discloses a frequency dynamic polarization calibration method based on a dual-antenna digital PARC structure, which can theoretically achieve the calibration of a radar system, but has the following disadvantages:
(1) the technical design is very complex. The digital PARC has very high requirements on the sampling rate of A/D and D/A, the sequential logic of a digital radio frequency memory circuit is complex, the operations of signal writing and reading, delay processing and the like all need clocks for control, and in a circuit running at high speed, competition and hazard easily occur, so that the whole system can not work stably and normally;
(2) the calibration parameter solving process and algorithm are complex;
(3) the cost is high, the reliability and the stability need to be verified, and no real product and practical application report is found at present.
The defects of the prior art-4: the two PARC devices adopting the single antenna or double antenna design belong to a single-station polarization calibration PARC device, and cannot be used for double-station measurement RCS calibration and double-station measurement polarization calibration, so that the problems of double-station measurement RCS calibration and polarization calibration are not solved.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: the invention provides a double-antenna PARC device with a double-shaft rotation control mechanism and a measurement calibration method thereof, which can simultaneously solve the problems of target double-station RCS calibration and target double-station polarization scattering matrix measurement polarization calibration under the condition of double-station scattering measurement. As a double-station RCS measurement calibration device, the device can overcome all the defects of the prior art-1-4, and ensure the stability of RCS calibration values under different double-station angles; as a polarization calibration device for dual-station polarization scattering matrix measurement, the polarization calibration device not only retains all the advantages of the existing single-station polarization calibration PARC device, but also can be used for dual-station polarization calibration, and the dual-station RCS calibration and dual-station polarization calibration functions of the traditional PARC device are added.
The technical scheme adopted by the invention is as follows: a radar target two-station scatterometry RCS calibration and polarization calibration device comprises a receiving antenna, a transmitting antenna, two azimuth-sight double-axis rotation units, an azimuth rotation driving and controlling device, a pitching rotation driving and controlling device, a radio frequency combination and a power supply combination, wherein:
the receiving antenna is used for receiving radiation signals of the transmitting antenna of the double-station measuring radar, and the radiation signals are fed to the radio frequency combination by the radio frequency cable;
the radio frequency combination comprises the following steps: the system comprises an amplifier, a filter, a delay line and an attenuator which are connected in sequence, wherein the amplifier, the filter, the delay line and the attenuator are used for amplifying, filtering and delaying a double-station measuring radar radiation signal received by a receiving antenna to obtain an output signal, and the output signal is fed to a transmitting antenna through a radio frequency cable after the level of the output signal is adjusted by the attenuator;
the transmitting antenna is used for completing the radiation of the radio frequency signal to the double-station measuring radar receiving antenna;
the two azimuth-sight line biaxial rotation units are as follows: one for placing the receiving antenna and the other for placing the transmitting antenna,
the azimuth rotation driving and controlling device comprises: the two-axis rotation unit is used for controlling the rotation of the azimuth-sight two-axis rotation unit around the azimuth direction;
the sight line rotation driving and controlling device comprises: the two-axis rotation unit is used for controlling the rotation of the azimuth-sight two-axis rotation unit around a sight line axis;
the power supply combination is as follows: power supply for the device.
Furthermore, the receiving antenna and the transmitting antenna are respectively composed of a horn antenna, meanwhile, in order to reduce cross polarization coupling errors of the antennas as much as possible and improve polarization isolation ratio, a micro-strip polarization filter device is additionally arranged at the opening surface of each antenna, each horn antenna is arranged on an azimuth-sight double-shaft rotating unit with angle codes, each antenna can independently rotate around the sight line of the radar and rotate around the azimuth under the control of an azimuth rotation driving and controlling device and a sight rotation driving and controlling device, and the azimuth-sight double-shaft rotating unit with the angle codes can simultaneously give out accurate position information of the sight line rotation angle and the azimuth rotation angle of the antennas.
Further, the azimuth-line-of-sight biaxial rotation unit: the device mainly comprises a sight line rotating stepping motor, a sight line angle encoder, an azimuth turntable, an azimuth angle encoder and a matching installation interface between the sight line rotating stepping motor and an antenna; wherein the sight line refers to a connecting line between a double-station measuring radar transmitter and a receiving antenna or between a double-station measuring radar receiver and a transmitting antenna; the 'azimuth' refers to the rotation speed and the rotation angle position of each antenna around the sight line of the radar antenna in the xOy plane when the measurement radar is erected in the xOy plane, and can be accurately controlled in real time through the sight line rotation and driving controller.
Furthermore, the azimuth rotation driving and controlling device completes rotation of the receiving antenna and the transmitting antenna in the azimuth direction by controlling the azimuth turntable, gives azimuth position information of each antenna through the azimuth encoder, and can be remotely controlled by the double-station scattering measurement system controller through the remote control interface.
Further, the sight line rotation driving and controlling device: the rotation of the receiving antenna and the transmitting antenna around the sight line axis is completed by controlling the sight line rotating motor, the sight line angle position information of each antenna is given through a sight line angle encoder, and the sight line rotating drive and controller can be remotely controlled by the double-station scattering measurement system controller through a remote control interface.
The method for the double-station measurement and calibration processing corresponding to the device for measuring, calibrating and polarizing the target double-station radar scattering cross section comprises the following specific steps:
step-1: a BPARC device calibration comprising:
adjusting the line-of-sight rotation mechanisms of the BPARC receiving antenna and the forwarding antenna to make the initial polarization angles of the two antennas consistent, and controlling the rotation speeds of the two rotation mechanisms to keep the same rotation speed w of the receiving and transmitting antennasrAt a constant speed, wherein wr＝wtThe unit rad/s is used for ensuring that the polarization of a transmitting and receiving antenna of the BPARC is completely consistent all the time in the whole measuring process, therefore, a stepping motor can be adopted as a rotating mechanism, the radar is used for measuring a group of data when the antenna stops when rotating to an angle, then the antenna is controlled to rotate to the next angular position, and the control and the measurement are repeated in such a way, so that the polarization of the transmitting and receiving antenna can be ensured to be changed synchronously;
the angle encoder accurately records the angle gamma of the antenna, and the number of the BPARC turns can be calculated by N-gamma/360 deg. In the measurement, the BPARC double antenna can be controlled to carry out the measurement of a whole circle, so that the selection of the initial polarization angle is ensured to have no influence on the whole calibration process;
step-2: installation of a BPARC device comprising:
the BPARC polarization calibration device provided by the invention is arranged on a calibration bracket, and the time delay parameter of the BPARC polarization calibration device is adjusted according to the requirement of a measuring radar system;
aiming at the current given double-station angle measured by the double stations, the azimuth turntable is controlled, the BPARC receiving antenna is aligned to the transmitting antenna of the measuring radar, the BPARC forwarding antenna is aligned to the receiving antenna of the measuring radar, and the BPARC installing step needs to be repeated every time the measuring double-station angle is changed;
step-3: polarization calibration measurements and data logging
Maintaining the position of an azimuth turntable well adjusted according to a given measurement double-station angle to be fixed, controlling a BPARC sight line rotating mechanism to enable two BPARC antennae to rotate at a constant speed and a low speed, measuring a radar transmitting signal and receiving an echo signal radiated by a BPARC forwarding antenna on the assumption that the two BPARC antennae rotate for N circles in total, recording echo signals of all polarization channels at different positions in the BPARC antenna line rotation process of the measurement radar sight line, and obtaining all measurement data;
repeating the BPARC measurement and data recording steps each time the measurement dual-station angle is changed;
step-4: polarization calibration parameter extraction
Solving through the measurement data in the step-3 to obtain all polarization calibration parameters of the measurement radar system;
when the measurement double-station angle is changed every time, the BPARC measurement data under the double-station angle is required to be used for solving the calibration parameters of the measurement radar system;
step-5: target dual-station polarization measurement
Installing a target to be tested, and recording target echoes of all polarization channels by a double-station measuring radar;
under the same two-station angle, the same measuring radar can be adopted to carry out two-station measurement on a plurality of same or different targets;
step-6: polarization calibration process
According to the polarization calibration parameters obtained in the step-4 and the target measurement value measured in the step-5, the polarization calibration of the measured target can be completed by applying the formula (6-1), and the real polarization scattering matrix value of the target is obtained;
in the formula Measured values under 4 polarization combinations of the target to be calibrated; stA true polarization scattering matrix of a target to be calibrated; rHHAnd RVVGain factors, T, for HH-polarized and VV-polarized receiving channels of the measurement system, respectivelyHHAnd TVVFor measuring the gain factors of the HH polarized and VV polarized transmit channels of the system,for measuring the cross polarization factor of the system, the 8 parameters are polarization calibration parameters of the measurement system which are required to be solved by measuring the BPARC through the steps-1 to-5 and processing the measurement data;
under the same two-station angle, when the same measuring radar is adopted to carry out two-station measurement on a plurality of same or different targets, the same set of calibration parameters can be used for carrying out polarization calibration processing;
if only the two-station RCS measurement and calibration are carried out, the 6 steps can be further simplified, the polarization calibration parameters do not need to be extracted, only the two-station calibration equation given by the formula (6-2) is needed to carry out the measurement and the RCS calibration calculation,
in the formulaIn order for the target dual-station RCS,theoretical two-station RCS for BPARC; prCAnd PrTRespectively measuring the echo power received by the radar when the target body and the target are measured; sTAnd SCRespectively measuring a target and a radar complex echo signal when a calibration body is measured in single RCS measurement sampling; kbA calibration constant corresponding to the geometric relationship of the two-station measurement;
theoretical two-station RCS value for BPARCThe calculation formula is as follows;
in the formula, GTAnd GRGains, G, for the BPARC transmit and receive antennas, respectivelyLoopThe total gain of the whole loop except the antenna in the BPARC; the theoretical RCS value of BPARC can also be measured by relative calibration measurements using another calibration standard with known RCS values;
scaling constant K corresponding to geometric relation of two-station measurementbThe calculation of (2) is then performed by equation (6-4) according to the two-station measurement geometry:
in the formula RtTAnd RtCRespectively representing the target distance and the calibration body distance of the transmitting channel; rrTAnd RrCRespectively representing the target distance and the calibration body distance of the receiving channel; l istTAnd LtCRespectively representing the total loss of the transmitting channel when the target is measured and the target body is measured; l isrTAnd LrCRespectively representing the total loss of a receiving channel when the target is measured and the target body is measured; k if the target distance and the calibration body distance are determined in the test and the two-station geometric relationship remains unchangedbThe value is a definite constant.
The technical scheme of the invention has the following beneficial effects:
1) the rotatable dual-antenna active calibration device BPARC provided by the invention reserves all the advantages of the existing PARC device, simultaneously solves the problems of dual-station RCS measurement calibration and polarization calibration, and has a series of important advantages which are not possessed by the traditional calibration body and PARC device.
2) Compared with the conventional simple calibration body (prior art-1 and) in the document [2, 3, 4 ]: the transmitting and receiving antennas of the BPARC are controllable in pointing direction and can be accurately aligned with the transmitting and receiving antennas of the double-station measuring radar, so that the influence of measuring the size of the double-station angle is avoided, and the problems that the RCS of the traditional simple calibration body has large fluctuation along with the increase of the double-station angle and the accurate calibration of the RCS measurement of the large double-station angle is difficult to complete are solved.
3) Compared with the technique (prior art-3) in the document [5, 6 ]: when the BPARC is used as a double-station measurement calibration and calibration device, the double-station measurement radar does not need two sets of transmitting and receiving devices, and can complete double-station measurement, calibration and polarization calibration only by one transmitter and one receiver.
4) Compared to the single antenna PARC (prior art-4) in document [8 ]: (1) advantages of the BPARC of the present invention include the ability to perform dual station measurement RCS calibration and polarization calibration: (2) a polarization filter can be additionally arranged, so that the polarization isolation degree of the PARC antenna is greatly improved, and the measurement and calibration accuracy is favorably provided; (3) the receiving and transmitting polarization combination forms are various, and more diversified polarization scattering characteristic signals can be provided, so that the selection of polarization calibration measurement and processing schemes can be diversified, and the realizability in different engineering applications is ensured.
5) Compared with the digital PARC (Prior Art-4) in the document [9 ]: (1) the BPARC of the invention can be used for completing the calibration and polarization calibration of the double-station measurement RCS; (2) A/D and D/A processing is not needed to be carried out on the radio frequency signals, and the received and forwarded signals are ensured not to be distorted; (3) the structure is simple, the performance is stable, the research and development cost is low, and the engineering is easy to realize; (4) the receiving and transmitting polarization combination forms are various, and more diversified polarization scattering characteristic signals can be provided, so that the selection of polarization calibration measurement and processing schemes can be diversified, and the realizability in different engineering applications is ensured.
Drawings
FIG. 1 is a schematic diagram of a dual-station RCS measurement geometry;
FIG. 2 is a graph of normalized RCS versus dual standing angle for a metal sphere;
FIG. 3 is a design two station scaling apparatus for Monzon;
FIG. 4 is a schematic diagram of dual station RCS measurement calibration using two transmitters and receivers;
FIG. 5 is a block diagram of a PARC architecture;
fig. 6 is a front view of a dual antenna PARC antenna;
fig. 7 is a front view of a single antenna PARC antenna;
FIG. 8 is a block diagram of a dual antenna digital PARC architecture;
FIG. 9 is a schematic diagram of the overall structure of a dual-station scatterometry RCS calibration and polarization calibration apparatus;
FIG. 10 is a schematic diagram of the geometric relationship between the dual-station scatterometry RCS calibration and polarization calibration device and the dual-station measurement radar in a specific measurement.
Detailed Description
The invention is further described with reference to the following figures and specific examples.
Fig. 9 shows a schematic diagram of an overall structure of a device for calibrating a target dual-station radar scattering cross section measurement and polarization calibration according to the present invention.
In fig. 9, the calibration and polarization calibration apparatus for dual-station scatterometry RCS is composed of a receiving antenna, a transmitting antenna, two azimuth-line-of-sight dual-axis rotation units, an azimuth rotation driving and controlling unit, a pitch rotation driving and controlling unit, a radio frequency combination, a power supply combination, a related matching installation interface, a remote control interface, and other functional modules. Wherein:
receive and transmit antennas: the receiving antenna is used for receiving radiation signals of the two-station measuring radar transmitting antenna, the radio frequency cable feeds the radio frequency combination, the radio frequency cable feeds the transmitting antenna to complete radiation of the radio frequency signals to the two-station measuring radar receiving antenna after the output signal level is adjusted through amplification, filtering, time delay and an attenuator, and the radiation is shown in fig. 10. The receiving and transmitting Antennas are each composed of a horn antenna, and at the same time, in order to reduce the cross polarization coupling error of the antenna and increase the polarization isolation ratio as much as possible, a microstrip polarization filter device is added at each antenna aperture (see document [10] m. kuloglu, C-C Chen, "ultra wideband polarization filter (UWB-EMPF) application to controlled antenna for passive cross polarization-polarization reduction, IEEE Antennas and amplification Antennas, 2013, 55(2): 280). Each horn antenna is arranged on an azimuth-sight double-shaft rotating mechanism with angle codes, and the controller controls each antenna to independently rotate around the radar sight and around the azimuth, and can give accurate position information of the sight angle and the azimuth angle of the antenna.
Azimuth-line-of-sight biaxial rotation unit: the line-of-sight rotary stepping motor is mainly composed of a line-of-sight rotary stepping motor, a line-of-sight angle encoder, an azimuth rotary table, an azimuth angle encoder, a matched mounting interface between the line-of-sight rotary stepping motor and an antenna, and the like. Wherein "line of sight" refers to the connection between the two-station measuring radar transmitter and the receiving antenna of the device of the present invention, or between the two-station measuring radar receiver and the transmitting antenna of the device of the present invention in fig. 10; "azimuth" refers to the angle of rotation in the plane of xOy, shown as angle θ in fig. 10, when the measurement radar is mounted in the plane of xOy in fig. 10. Through the sight line rotation and driving controller, the rotation speed and the rotation angle position of each antenna around the sight line of the radar antenna can be accurately controlled in real time. Through the azimuth rotation and driving controller, the rotation speed and the rotation angle position of each antenna around the sight line of the azimuth plane radar antenna can be accurately controlled in real time. In operation, the receive and transmit antennas of the dual-station scatterometry RCS calibration and polarization calibration apparatus are rotated in azimuth to align the transmitter and receiver antennas of the dual-station survey radar, respectively, and a typical polarization combination elevation view when the line-of-sight axis is rotated to a different position is shown in fig. 6.
Azimuth rotation drive and controller: the rotation of the receiving and transmitting antennas of the device in the azimuth direction is completed by controlling the azimuth turntable, and azimuth position information of each antenna is given by the azimuth encoder. The azimuth rotary drive and controller may be remotely controlled by the dual-station scatterometry system controller via a remote control interface.
Visual line rotation driving and controlling device: the receiving and transmitting antennas of the device of the invention rotate around the visual axis by controlling the visual line rotating motor, and the visual line angle position information of each antenna is given by the visual line angle encoder. The line-of-sight rotary drive and controller may be remotely controlled by the dual-station scatterometry system controller via a remote control interface.
Radio frequency combination: and the signal is fed to the transmitting antenna to finish the signal radiation to the antenna of the double-station measuring radar receiver after the amplification, the filtering and the time delay of the double-station measuring radar transmitting signal received by the receiving antenna are finished, and the level of the output signal is adjusted by the attenuator. The operation principle of the amplifier, filter, attenuator, delay line, power supply, etc. in fig. 10 is the same as that of the conventional PARC and will not be discussed here.
Power supply combination: and completing the power supply of all units and components of the device.
Hereinafter, we refer to the device designed by the present invention as a dual-station active polarization calibrator (BPARC device), which is referred to as BPARC device for short, to distinguish from the conventional PARC device that can only be used for single-station measurement RCS calibration and polarization calibration.
By adopting the device BPARC designed by the invention, when the measuring double-station angle changes, the rotation of the azimuth turntable of the BPARC can be synchronously controlled, so that the receiving antenna of the BPARC is always kept consistent with the sight line of the transmitting antenna of the measuring radar, and the forwarding antenna of the BPARC is always kept consistent with the sight line of the transmitting antenna of the measuring radar. Therefore, no matter how large the double-station angle is measured, the theoretical RCS value and the polarization characteristic of the BPARC do not change along with the double-station angle, thereby ensuring that the BPARC device can be used for completing single-station and double-station RCS measurement and polarization calibration in various application occasions.
The transmitting and receiving antenna of the BPARC device can work in various polarization combinations, and polarization scattering matrixes of various common passive calibration bodies are realized through BPARC simulation, so that the application range of the BPARC can be greatly expanded. In addition, the polarization filter is additionally arranged on the mouth surface of the receiving antenna and the transmitting antenna, so that the cross polarization isolation degree can be greatly improved, and the negative influence of cross polarization coupling existing in the PARC of a single antenna on polarization calibration is solved.
Another important advantage of the present invention over known PARC devices is that due to the rotatable dual antenna design, different combinations of attitude of the receiving and transmitting antennas can be combined to form different polarization combinations, thereby allowing different polarization calibration measurement schemes and calibration algorithms to be designed, as discussed below.
The calibration principle and scheme of the two-station polarization measurement are introduced as follows:
since the quantitative polarization measurement and calibration process generally requires the completion of calibration on 4 polarization channels, it is ensured that the finally obtained target polarization scattering matrix measurement is accurate, which means that the measurement calibration of the target RCS under 4 polarization combinations is completed at the same time. Therefore, we do not discuss the two-station RCS measurement calibration problem separately below, but focus on the principle and scheme of polarization measurement calibration.
The polarization calibration model of equation (3) is rewritten in matrix form as:
equation (8) can be decomposed as follows:
in the formula, <math>
<mrow>
<msubsup>
<mi>ϵ</mi>
<mi>R</mi>
<mi>H</mi>
</msubsup>
<mo>=</mo>
<mfrac>
<msub>
<mi>R</mi>
<mi>HV</mi>
</msub>
<msub>
<mi>R</mi>
<mi>HH</mi>
</msub>
</mfrac>
<mo>,</mo>
<msubsup>
<mi>ϵ</mi>
<mi>R</mi>
<mi>V</mi>
</msubsup>
<mo>=</mo>
<mfrac>
<msub>
<mi>R</mi>
<mi>VH</mi>
</msub>
<msub>
<mi>R</mi>
<mi>VV</mi>
</msub>
</mfrac>
<mo>,</mo>
<msubsup>
<mi>ϵ</mi>
<mi>T</mi>
<mi>H</mi>
</msubsup>
<mo>=</mo>
<mfrac>
<msub>
<mi>T</mi>
<mi>VH</mi>
</msub>
<msub>
<mi>T</mi>
<mi>HH</mi>
</msub>
</mfrac>
<mo>,</mo>
<msubsup>
<mi>ϵ</mi>
<mi>T</mi>
<mi>V</mi>
</msubsup>
<mo>=</mo>
<mfrac>
<msub>
<mi>T</mi>
<mi>HV</mi>
</msub>
<msub>
<mi>T</mi>
<mi>VV</mi>
</msub>
</mfrac>
</mrow>
</math> to measure the cross-polarization factor of the system.
According to equation (9), in polarization calibration, the parameters are determined by a series of measurements and calculationsRHH、RVV、THHAnd TVVThe polarization calibration can be completed, and the method for obtaining the polarization calibration parameters by the BPARC proposed by the present invention can be varied. The following describes 3 typical schemes, each of which is discussed for a given measurement dual-station angle, and when the measurement dual-station angle is changed, because the two azimuth turntables of the BPARC are controlled to align its receiving antenna and forwarding antenna with the transmitting antenna and receiving antenna of the dual-station measurement radar, respectively, the polarization measurement and calibration principles and procedures under different dual-station angles are identical.
Scheme-1:
in the BPARC transmitting/receiving antenna, one antenna is kept fixed (i.e. working in a fixed polarization state), and the other antenna can rotate 0-360 degrees (i.e. the polarization state can be changed within a range of 0-360 degrees). Examples are as follows:
first, the transponder antenna is kept unchanged at 45 ° linear polarization, i.e. θtThe receiving antenna rotates within the range of 0-360 degrees when the angle is 45 degrees. From equation (6), the PSM of PARC at this time is:
wherein,this coefficient can be obtained by scaling PARC according to other scales before polarization calibration, and can also be calculated by equation (5), all considered as known quantities herein. It can be seen that in this case, the components of the polarization scattering matrix of the PARC are the polarization angle θ with the receiving antennarThe change is in sine and cosine law.
Take thetar90 ° and θrWhen the value is 0 °, the theoretical PSMs in the corresponding postures are expressed by the formula (8):
the following expressions (11a) and (11b) are developed by substituting the following expressions (8):
is provided with
Secondly, the receiving antenna remains unchanged at a linear polarization of 45 °, i.e. θrAnd when the angle is 45 degrees, the forwarding antenna rotates within the range of 0-360 degrees.
Take thetat90 ° and θtWhen the angle is 0 °, the theoretical PSMs in the corresponding postures are:
the following expressions (14a) and (14b) are developed by substituting the following expressions (8):
is provided with
Will be provided withAndby substituting formula (9), we can obtain:
all calibration parameters of the system are calculated, and the calibration work of any target to be calibrated can be completed. Assume that the target measurement value to be calibrated is MtTrue polarization scattering matrix S of target to be calibratedtAs can be seen from equations (10) and (11), the following polarization calibration equation is available:
scheme-2:
the polarization mode of the forwarding antenna and the polarization mode of the receiving antenna are kept the same, and the two antennas rotate synchronously within the range of 0-360 degrees. At this time, the polarization scattering matrix of PARC is:
therefore, the PARC can be regarded as a comprehensive calibration body with the scattering property of the pseudo-dihedral corner reflector and the scattering property of the metal ball, so that the active polarization calibration can be realized by imitating the traditional passive polarization calibration method adopting the dihedral corner reflector and the metal ball. The specific process is as follows.
From the matrix multiplication, equation (9) can be written as:
the PARC polarized scattering matrix of formula (19) is respectively brought into formulas (20a) to (20d), i.e. PARC at different rotation angles is measured, and the obtained measured values are:
taking the HH polarization channel as an example, the measurement values obtained from measuring the rotated PARCPerforming Fourier series expansion, including:
constant terms and 2-order fourier coefficients are extracted from equation (22), and are compared with equation (21a), which is easy to obtain:
the unknown quantity AHH＝RHH·THHConsidered as an unknown quantity, the equations (23a), (23b) and (23c) can be formed by 3 equationsIs solved to obtain 3 unknowns AHH，Similarly, the remaining unknowns A can be solved by performing Fourier series expansion and equation system solution on the HV, VH and VV polarization channelsHV＝RHH·TVV，AVH＝RVV·THH，AVV＝RVV·TVV，
Therefore, all system parameters in the polarization measurement error model are obtained by solving, and polarization calibration can be carried out on the target to be calibrated.
Assume that the target measurement to be calibrated is Knowing its true polarization scattering matrix with the target to be calibrated Satisfy the requirement of
By working up formula (24), it is possible to obtain:
in the formulaAccording to the obtained system parameters and the measured value of the target to be calibrated, solving of the true polarization scattering matrix of the target to be calibrated can be completed through the formulas (25a) to (25 d).
Scheme-3:
the polarization modes of the forwarding antenna and the receiving antenna are mutually orthogonal and synchronously rotate. I.e. always satisfying the relation:at this time, the polarization scattering matrix of PARC is:
in this case, the polarization calibration operation is equivalent to the single antenna PARC described in document [2], as shown in fig. 4.
Parameters in an error model of the system are 8 at most, and theoretically, data under any three groups of different attitude combinations can be used for polarization calibration. For example, for ease of calculation, we choose θt,1＝0°、θt,2＝45°、θt,3Angle theta equal to 90 DEGt,1、θt,2、θt,3When the two are respectively brought into the formula (26), the PSM corresponding to each posture is
Will be provided withRespectively replace the formula (9) to expand, comprising:
the first element (HH component) of the three matrices is processed as follows:
by substituting formula (30) for formula (29), we can obtain:
equation (31) is a one-dimensional quadratic equation whose solution is:
the selection of `, + - `, in formula (32) follows such thatPrinciple of (1).
And solving other parameters of the system. Can be directly obtained from the formula (30)Andcan be solved by the following two ways: (1) by solving for similaritiesThe solution equation is solved, so that the solving process is troublesome; (2) the special relation among the parameters is utilized to solve.
The second method is used for solving the problem.
In the formula (28)Andthe combination (28) can determine
In the formula (28)Andandcan determine
While
All calibration parameters of the system are obtained, and the calibration work of any target to be calibrated can be realized by the formula (18).
In addition to the three examples listed above, there may be other combinations of PARC transmit and receive antennas, so that more forms of polarization scattering matrices are available for polarization calibration, which are not listed here.
The implementation process and application of the invention are described as follows:
to further illustrate how the BPARC proposed by the present invention can be applied in polarization calibration, the implementation process will be described by taking the above scheme-2 as an example. The measurement and calibration procedures are completely similar when other schemes are adopted.
The measurement and polarization calibration processing steps are as follows:
step-1: BPARC device tuning
Adjusting the line-of-sight rotation mechanisms of the BPARC receiving antenna and the forwarding antenna to make the initial polarization angles of the two antennas consistent, and controlling the rotation speeds of the two rotation mechanisms to keep the same rotation speed w of the receiving and transmitting antennasr(wr＝wtUnit rad/s) to ensure that the transmit-receive antenna polarization of the BPARC is always completely consistent throughout the measurement. Therefore, the rotating mechanism can adopt a stepping motor, the radar can be used for measuring a group of data when the antenna stops when rotating to an angle, then the antenna is controlled to rotate to the next angle position, and the control and the measurement are repeated, so that the polarization of the transmitting and receiving antenna can be ensured to be synchronously changed.
The angle encoder accurately records the angle gamma of the antenna, and the number of the BPARC turns can be calculated by N-gamma/360 deg. In the measurement, the BPARC dual antenna can be controlled to perform the measurement of a whole circle, so that the selection of the initial polarization angle is ensured to have no influence on the whole calibration process.
Step-2: installation of BPARC devices
The BPARC polarization calibration device provided by the invention is arranged on a calibration bracket, and the delay parameters of the calibration bracket are adjusted according to the requirements of a radar system.
For the current double-station angle measured by the double stations, the azimuth turntable is controlled, so that the BPARC receiving antenna is aligned with the transmitting antenna of the measuring radar, and the BPARC forwarding antenna is aligned with the receiving antenna of the measuring radar, as shown in fig. 10. This step is repeated each time the measurement double stand angle is changed.
Step-3: polarization calibration measurements and data logging
And keeping the position of the well adjusted azimuth turntable fixed, and controlling a sight line rotating mechanism of the BPARC to enable the two antennas of the BPARC to rotate at a constant speed and a low speed, wherein N turns are supposed to be passed. Measuring radar emission signals and receiving echo signals radiated by a BPARC forwarding antenna, recording echo signals of all polarization channels at different positions of the BPARC antenna in the process of rotating around the line of sight of the measuring radar, and recording all obtained PSM measurement data as
Step-4: polarization calibration parameter extraction
Will be provided withThe polarization components are expanded by Fourier series, respectively
Wv represents all polarization states, constant terms and 2-order term coefficients are extracted, corresponding term coefficients are made to be equal by combining formula (21), and all polarization calibration parameters of the radar system can be obtained by the process according to the scheme-2;
step-5: target dual-station polarization measurement
Installing the target to be measured, and recording target echoes of all polarization channels by a double-station measuring radar, and assuming that the PSM measured value is Mt；
Step-6: polarization calibration process
According to the polarization calibration parameters obtained in the step-4 and the target PSM measured value measured in the step-5, the polarization calibration of the measured target can be completed by applying the formula (18), and the real PSM value S of the target is obtainedt。
Note that if only two-station RCS measurement and calibration are needed, the above 6 steps can be further simplified, and no polarization calibration parameter needs to be extracted, only the measurement and RCS calibration calculation is performed according to the two-station calibration equation given by the formula (2), wherein the theoretical RCS value of BPARCThe scaling constant K is still calculated from equation (8)bThe calculation of (2) is then performed by equation (3) based on the two-station measurement geometry. And are not described in detail.
In addition, other alternatives of the present invention are set forth below:
(1) the horn antenna used in PARC of the present invention can be replaced by other types of linearly polarized antennas;
(2) the polarization combination of the transmitting and receiving antenna in the invention has infinite variety, different polarization calibration measurement schemes and polarization calibration parameter extraction algorithms can be designed according to different polarization combinations, and the invention is not limited to the 3 schemes which have been exemplified.
The art related to the present invention is not described in detail.
Claims (6)
1. The utility model provides a can be used to target two-station radar cross section measurement calibration and polarization calibrating device which characterized in that: the device comprises a receiving antenna, a transmitting antenna, two azimuth-sight double-axis rotating units, an azimuth rotating drive and controller, a pitching rotating drive and controller, a radio frequency combination and a power supply combination, wherein:
the receiving antenna is used for receiving radiation signals of the transmitting antenna of the double-station measuring radar, and the radiation signals are fed to the radio frequency combination by the radio frequency cable;
the radio frequency combination comprises the following steps: the system comprises an amplifier, a filter, a delay line and an attenuator which are connected in sequence, wherein the amplifier, the filter, the delay line and the attenuator are used for amplifying, filtering and delaying a double-station measuring radar radiation signal received by a receiving antenna to obtain an output signal, and the output signal is fed to a transmitting antenna through a radio frequency cable after the level of the output signal is adjusted by the attenuator;
the transmitting antenna is used for completing the radiation of the radio frequency signal to the double-station measuring radar receiving antenna;
the two azimuth-sight line biaxial rotation units are as follows: one of them is used for placing the receiving antenna, and the other is used for placing the transmitting antenna;
the azimuth rotation driving and controlling device comprises: the two-axis rotation unit is used for controlling the rotation of the azimuth-sight two-axis rotation unit around the azimuth direction;
the sight line rotation driving and controlling device comprises: the two-axis rotation unit is used for controlling the rotation of the azimuth-sight two-axis rotation unit around a sight line axis;
the power supply combination is as follows: power supply for the device.
2. The calibration and polarization calibration device for the target two-station radar cross section measurement according to claim 1, wherein: the receiving antenna and the transmitting antenna are respectively composed of a horn antenna, meanwhile, in order to reduce cross polarization coupling errors of the antennas and improve polarization isolation ratio as much as possible, a micro-strip polarization filter device is additionally arranged at the opening surface of each antenna, each horn antenna is arranged on an azimuth-sight double-shaft rotating unit with angle codes, each antenna can independently rotate around a radar sight line and rotate around the azimuth under the control of an azimuth rotation driving and controlling device and a sight line rotation driving and controlling device, and the azimuth-sight double-shaft rotating unit with the angle codes can simultaneously give out accurate position information of sight line rotation angles and azimuth rotation angles of the antennas.
3. The calibration and polarization calibration device for the target two-station radar cross section measurement according to claim 1, wherein: the azimuth-sight line biaxial rotation unit comprises: the device mainly comprises a sight line rotating stepping motor, a sight line angle encoder, an azimuth turntable, an azimuth angle encoder and a matching installation interface between the sight line rotating stepping motor and an antenna; wherein the sight line refers to a connecting line between a double-station measuring radar transmitter and a receiving antenna or between a double-station measuring radar receiver and a transmitting antenna; the 'azimuth' refers to the rotation speed and the rotation angle position of each antenna around the sight line of the radar antenna in the xOy plane when the measurement radar is erected in the xOy plane, and can be accurately controlled in real time through the sight line rotation and driving controller.
4. The calibration and polarization calibration device for the target two-station radar cross section measurement according to claim 3, wherein: the azimuth rotation driving and controlling device completes rotation of the receiving antenna and the transmitting antenna in the azimuth direction by controlling the azimuth turntable, gives azimuth position information of each antenna through the azimuth encoder, and can be remotely controlled by the double-station scattering measurement system controller through the remote control interface.
5. The calibration and polarization calibration device for the target two-station radar cross section measurement according to claim 3, wherein: the sight line rotation driving and controlling device comprises: the rotation of the receiving antenna and the transmitting antenna around the sight line axis is completed by controlling the sight line rotating motor, the sight line angle position information of each antenna is given through a sight line angle encoder, and the sight line rotating drive and controller can be remotely controlled by the double-station scattering measurement system controller through a remote control interface.
6. A calibration method for measurement of a scattering cross section of a target double-station radar, according to any one of claims 1 to 5, the calibration and polarization calibration device for measurement of the scattering cross section of the target double-station radar is characterized in that: the method and steps of the two-station measurement and calibration process are as follows:
step-1: a BPARC device calibration comprising:
adjusting the line-of-sight rotation mechanisms of the BPARC receiving antenna and the forwarding antenna to make the initial polarization angles of the two antennas consistent, and controlling the rotation speeds of the two rotation mechanisms to keep the same rotation speed w of the receiving and transmitting antennasrAt a constant speed, wherein wr＝wtThe unit rad/s is used for ensuring that the polarization of a transmitting and receiving antenna of the BPARC is completely consistent all the time in the whole measuring process, therefore, a stepping motor can be adopted as a rotating mechanism, the radar is used for measuring a group of data when the antenna stops when rotating to an angle, then the antenna is controlled to rotate to the next angular position, and the control and the measurement are repeated in such a way, so that the polarization of the transmitting and receiving antenna can be ensured to be changed synchronously;
the angle encoder accurately records the angle gamma of the antenna, and the number of the BPARC turns can be calculated by N-gamma/360 deg. In the measurement, the BPARC double antenna can be controlled to carry out the measurement of a whole circle, so that the selection of the initial polarization angle is ensured to have no influence on the whole calibration process;
step-2: installation of a BPARC device comprising:
the BPARC polarization calibration device provided by the invention is arranged on a calibration bracket, and the time delay parameter of the BPARC polarization calibration device is adjusted according to the requirement of a measuring radar system;
aiming at the current given double-station angle measured by the double stations, the azimuth turntable is controlled, the BPARC receiving antenna is aligned to the transmitting antenna of the measuring radar, the BPARC forwarding antenna is aligned to the receiving antenna of the measuring radar, and the BPARC installing step needs to be repeated every time the measuring double-station angle is changed;
step-3: polarization calibration measurements and data logging:
maintaining the position of an azimuth turntable well adjusted according to a given measurement double-station angle to be fixed, controlling a BPARC sight line rotating mechanism to enable two BPARC antennae to rotate at a constant speed and a low speed, measuring a radar transmitting signal and receiving an echo signal radiated by a BPARC forwarding antenna on the assumption that the two BPARC antennae rotate for N circles in total, recording echo signals of all polarization channels at different positions in the BPARC antenna line rotation process of the measurement radar sight line, and obtaining all measurement data;
repeating the BPARC measurement and data recording steps each time the measurement dual-station angle is changed;
step-4: polarization calibration parameter extraction:
solving through the measurement data in the step-3 to obtain all polarization calibration parameters of the measurement radar system;
when the measurement double-station angle is changed every time, the BPARC measurement data under the double-station angle is required to be used for solving the calibration parameters of the measurement radar system;
step-5: target two-station polarization measurement:
installing a target to be tested, and recording target echoes of all polarization channels by a double-station measuring radar;
under the same two-station angle, the same measuring radar can be adopted to carry out two-station measurement on a plurality of same or different targets;
step-6: polarization calibration process
According to the polarization calibration parameters obtained in the step-4 and the target measurement value measured in the step-5, the polarization calibration of the measured target can be completed by applying the formula (6-1), and the real polarization scattering matrix value of the target is obtained;
in the formula Measured values under 4 polarization combinations of the target to be calibrated; stA true polarization scattering matrix of a target to be calibrated; rHHAnd RVVGain factors, T, for HH-polarized and VV-polarized receiving channels of the measurement system, respectivelyHHAnd TVVFor measuring the gain factors of the HH polarized and VV polarized transmit channels of the system,for measuring the cross polarization factor of the system, the 8 parameters are polarization calibration parameters of the measurement system which are required to be solved by measuring the BPARC through the steps-1 to-5 and processing the measurement data;
under the same two-station angle, when the same measuring radar is adopted to carry out two-station measurement on a plurality of same or different targets, the same set of calibration parameters can be used for carrying out polarization calibration processing;
if only the two-station RCS measurement and calibration are carried out, the 6 steps can be further simplified, the polarization calibration parameters do not need to be extracted, only the two-station calibration equation given by the formula (6-2) is needed to carry out the measurement and the RCS calibration calculation,
in the formulaIn order for the target dual-station RCS,theoretical two-station RCS for BPARC; prCAnd PrTRespectively measuring the echo power received by the radar when the target body and the target are measured; sTAnd SCRespectively measuring a target and a radar complex echo signal when a calibration body is measured in single RCS measurement sampling; kbA calibration constant corresponding to the geometric relationship of the two-station measurement;
theoretical two-station RCS value for BPARCThe calculation formula is as follows;
in the formula, GTAnd GRGains, G, for the BPARC transmit and receive antennas, respectivelyLoopThe total gain of the whole loop except the antenna in the BPARC; the theoretical RCS value of BPARC can also be measured by relative calibration measurements using another calibration standard with known RCS values;
scaling constant K corresponding to geometric relation of two-station measurementbThe calculation of (2) is then performed by equation (6-4) according to the two-station measurement geometry:
in the formula RtTAnd RtCRespectively representing the target distance and the calibration body distance of the transmitting channel; rrTAnd RrCRespectively representing the target distance and the calibration body distance of the receiving channel; l istTAnd LtCRespectively representing the total loss of the transmitting channel when the target is measured and the target body is measured; l isrTAnd LrCRespectively representing the total loss of a receiving channel when the target is measured and the target body is measured; k if the target distance and the calibration body distance are determined in the test and the two-station geometric relationship remains unchangedbThe value is a definite constant.
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