CN113359131A - SAR low-interception radio frequency stealth system and design method thereof - Google Patents
SAR low-interception radio frequency stealth system and design method thereof Download PDFInfo
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Abstract
The invention relates to an SAR low interception radio frequency stealth system and a design method thereof, wherein the method comprises the following steps of S1: obtaining the equivalent backscattering coefficient NE sigma of noise0An equation for the standard SAR range; s2: obtaining an equation of the SAR low interception action distance according to the equation of the SAR action distance; s3: according to an equation of the SAR low interception action distance, one or more of related parameters of the SAR system are designed and changed so as to improve the low interception action distance of the SAR system; s4: an MIMO system SAR system based on orthogonal signals is adopted to reduce the equivalent radiation power of a radar transmitting end; s5: the method comprises the steps of carrying out agility processing on parameters of radar radiation signals, modulating by noise-like modulation, designing to obtain agility signal parameters with low interception waveforms, and designing to obtain a multi-parameter signal sorting SAR imaging algorithm with the low interception waveforms according to the agility signal parameters with the low interception waveforms. The method of the invention overcomes the defects of the prior art that most of the radio frequency stealth is low in interceptionThe study was directed to the deficiencies of both low-intercept waveform designs.
Description
Technical Field
The invention belongs to the technical field of radars, and particularly relates to an SAR low-interception radio frequency stealth system and a design method thereof.
Background
In modern war, the widespread use of electronic countermeasure equipment severely interferes with or disrupts the proper operation of the radar. The party with strong electronic countermeasure capability always has great advantages, and the battlefield initiative can be easier to master, so that the radio frequency stealth capability of the radar is imperative to be enhanced.
The stealth performance of the aircraft platform is improved, on one hand, the Radar cross-sectional area (RCS) of an airborne SAR (Synthetic Aperture Radar) is required to be efficiently reduced, and on the other hand, the requirements are provided for the low-interception radio frequency stealth design of the airborne SAR. If the SAR system does not carry out targeted low-interception radio frequency stealth design, the SAR system can be easily intercepted by electronic reconnaissance equipment of an enemy when the radar is started to work, so that the position of the flight platform is exposed, and the stealth characteristic of the flight platform is meaningless.
The meaning of low interception radio frequency stealth is to adopt multiple technical measures, destroy the detection and the receipt of the electronic countermeasure equipment of the other party, make the other party not detect the radar signal, or make the other party distinguish the radiation threat clearly, including: when the radar works, the electronic countermeasure equipment is difficult to find; when the radar works, the electronic countermeasure equipment is difficult to sort and identify radar radiation signals.
However, most of the existing low-interception radio frequency stealth researches are designed around low-interception waveform design, and the low-interception radio frequency stealth design of a radar system is rarely researched from a system level.
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides an SAR low-interception radio frequency stealth system and a design method thereof. The technical problem to be solved by the invention is realized by the following technical scheme:
the invention provides a design method of an SAR low-interception radio frequency stealth system, which comprises the following steps:
s1: obtaining the equivalent backscattering coefficient NE sigma of noise0Equation for standard SAR range:
wherein R is SAR action distance, PtFor radar peak power transmission, ratio is radar duty cycle, GtFor radar antenna transmission gain, GrFor radar antenna reception gain, prFor the distance resolution, the formula is rhor＝kr·C/2BiWhere C is the speed of light, BiFor the signal bandwidth, krWeighted broadening for pulse pressure, λ is radar operating wavelength, K is Boltzmann constant, T is noise temperature, NFIs the noise coefficient of the radar system, LSThe loss of the radar system is shown, v is the flying speed of the platform, and theta is the ground wiping angle;
s2: obtaining an equation of the SAR low interception action distance according to the equation of the SAR action distance:
wherein R isLPIAlpha is an interception factor, alpha is the interception distance of the SAR, the interception distance of the electronic countermeasure equipment to radar radiation signals is equal to the SAR action distance, k is a radar antenna emission pattern factor, P isrminFor the detection sensitivity of the electronic countermeasure equipment, LjFor electronic countermeasure equipment detecting receiver system loss, GjGain of the receiving antenna of the electronic countermeasure device, BjDigitizing the channel bandwidth for the electronic countermeasure device;
s3: according to the equation of the SAR low interception action distance, one or more of related parameters of the SAR system are designed and changed to improve the SAR low interception action distance, wherein the related parameters comprise radar working duty ratio, radar antenna receiving gain, radar system loss, radar system noise coefficient, platform flight speed, ground wiping angle and antenna emission beam side lobe;
s4: an MIMO system SAR system based on orthogonal signals is adopted to reduce the equivalent radiation power of a radar transmitting end;
s5: the method comprises the steps of carrying out agility processing on parameters of radar radiation signals, modulating the parameters by noise-like signals, designing to obtain agility signal parameters with low interception waveforms, and designing to obtain a multi-parameter signal sorting SAR imaging algorithm with the low interception waveforms according to the agility signal parameters with the low interception waveforms.
In an embodiment of the present invention, the S2 includes:
an equation of the interception distance of the electronic countermeasure equipment to the radar radiation signal is obtained,
wherein R isjFor interception of radar radiation signals by electronic countermeasure equipment, PtWhen detecting and receiving main lobe of beam transmitted by radar antenna, k is 1, and when detecting and receiving side lobe of beam transmitted by radar antenna, k is less than 1, BjDigitizing the channel bandwidth for the electronic countermeasure device; l isjSystem losses for the electronic countermeasure equipment;
according to the expression of the interception factor alpha, the electronic countermeasure equipment intercepts the radar radiation signal by a distance RjAnd SAR working distance R to obtain SAR low interception working distance RLPIThe equation of (a), wherein,
in an embodiment of the present invention, in the S3, designing to change relevant parameters of the SAR system includes:
double-base cooperative imaging is adopted, the receiving and transmitting isolation degree between a transmitting station and a receiving station is improved through the distance between double bases, double-base continuous wave work is realized, the radar work duty ratio is improved, and the low interception action distance of an SAR system is improved;
broadening azimuth-direction transmission beams of the radar antenna, dividing the radar receiving antenna into a plurality of sub-arrays, simultaneously forming narrow beams corresponding to a plurality of full-aperture antennas by a digital beam forming method, respectively filtering Doppler bandwidths in a wide beam range corresponding to the sub-arrays, and splicing the Doppler bandwidths into Doppler bandwidths required by imaging, so that the receiving gain of the radar antenna is improved, and the low interception action distance of the SAR system is increased;
when the area of the radar antenna is constant, the working frequency is increased, and when the receiving gain of the radar antenna is constant, the working frequency is reduced, so that the low interception action distance of the SAR system is increased;
the loss and the noise coefficient of the radar system are reduced, so that the low interception action distance of the SAR system is increased;
the flying speed of the platform is reduced, so that the low wiping angle is increased, and the low interception action distance of the SAR system is increased;
and the side lobe of the transmitted beam of the radar antenna is reduced, so that the low interception action distance of the SAR system is increased.
In one embodiment of the present invention, in the S4,
the method comprises the following steps of correcting an equation of SAR action distance of an SAR system adopting an MIMO system based on orthogonal signals and an equation of interception distance of electronic countermeasure equipment to radar radiation signals as follows:
wherein G ist' Transmission gain, G, for a single transmission sub-arraySBenefits the transmitting directional diagram obtained after the directional diagram of the transmitting and receiving antenna is synthesized;
obtaining a low interception action distance calculation formula of the MIMO system SAR system based on the orthogonal signals according to the corrected SAR action distance equation and the equation of the interception distance of the electronic countermeasure equipment to the radar radiation signals:
in an embodiment of the present invention, in S5, the parameters of the radar-radiated signal include PRI, PW, and PA.
In an embodiment of the present invention, in S5, the agile processing of the parameter of the radar radiation signal includes:
respectively generating random number sequences of PRI, PW and PA which are in accordance with standard normal distribution by a polar coordinate method;
and respectively carrying out linear transformation of standard normal distribution on the random number sequence to obtain the digital sequences of PRI, PW and PA which obey Gaussian distribution, and obtaining the agile PRI, the agile PW and the agile PA.
In one embodiment of the invention, in the S5, the multi-parameter signal sorting SAR imaging algorithm with low interception waveform comprises amplitude correction, azimuth resampling and CZT-based range migration correction, wherein,
the amplitude correction is used for correcting the amplitude of each echo pulse according to the change of the pulse width and the amplitude of the radiation signal, and the azimuth modulation caused by the change of the pulse width and the amplitude of the radiation signal is avoided;
the azimuth resampling is used for correcting azimuth non-uniform sampling caused by pulse repetition period change;
the CZT-based distance migration correction is used for correcting distance migration under high-resolution imaging aiming at a radiation signal of which the radiation modulation form is not linear frequency modulation.
The invention also provides an SAR low interception radio frequency stealth system which is designed by adopting an MIMO system SAR system based on orthogonal signals according to the design method in any embodiment.
Compared with the prior art, the invention has the beneficial effects that:
1. the SAR low interception radio frequency stealth system design method of the invention provides the design of the SAR low interception radio frequency stealth system from two levels, the first level is that the SAR adopts a larger receiving antenna, a double-base continuous wave operation and an orthogonal signal-based MIMO system, so that the electronic countermeasure equipment is difficult to detect the radiation signal of the SAR system, the second level is that the electronic countermeasure equipment is difficult to classify and identify the radiation signal of the SAR system by jointly agilely changing a plurality of characteristic parameters of the radiation signal of the SAR system, and the defect that most researches of the existing low interception radio frequency stealth system are designed around low interception waveforms is overcome.
2. The SAR low-interception radio frequency stealth system enhances the radio frequency stealth capability of the radar.
The foregoing description is only an overview of the technical solutions of the present invention, and in order to make the technical means of the present invention more clearly understood, the present invention may be implemented in accordance with the content of the description, and in order to make the above and other objects, features, and advantages of the present invention more clearly understood, the following preferred embodiments are described in detail with reference to the accompanying drawings.
Drawings
Fig. 1 is a flowchart of a method for designing an SAR low-interception radio frequency stealth system according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a bistatic continuous wave SAR operation provided by an embodiment of the present invention;
FIG. 3 is a graph of the SAR system range versus the intercept distance of the electronic countermeasure equipment provided by the embodiment of the present invention;
fig. 4 is a schematic diagram of an operation of a MIMO radar according to an embodiment of the present invention;
fig. 5 is a graph of the operating distance of a 4-transmission 4-reception MIMO-SAR system and the interception distance of an electronic countermeasure device according to an embodiment of the present invention;
FIG. 6 is a basic flow chart of signal sorting algorithm development and multi-parameter signal sorting according to an embodiment of the present invention;
FIG. 7 is a flow chart of a multi-parameter signal sorting SAR imaging algorithm with low interception waveforms provided by an embodiment of the present invention;
FIG. 8 is a graph of the variation of the radiation signal with agile pulse repetition period, pulse width, amplitude and modulation format provided by the embodiment of the invention;
fig. 9 is a graph of a simulation result of SAR imaging of a multi-parameter inter-pulse agility signal according to an embodiment of the present invention.
Detailed Description
In order to further explain the technical means and effects of the present invention adopted to achieve the predetermined invention purpose, the following will explain in detail a SAR low interception radio frequency stealth system and its design method according to the present invention with reference to the accompanying drawings and the detailed embodiments.
The foregoing and other technical matters, features and effects of the present invention will be apparent from the following detailed description of the embodiments, which is to be read in connection with the accompanying drawings. The technical means and effects of the present invention adopted to achieve the predetermined purpose can be more deeply and specifically understood through the description of the specific embodiments, however, the attached drawings are provided for reference and description only and are not used for limiting the technical scheme of the present invention.
Example one
Referring to fig. 1, fig. 1 is a flowchart of a method for designing an SAR low-interception radio frequency stealth system according to an embodiment of the present invention. As shown in the figure, the method for designing the SAR low-interception radio frequency stealth system of the present embodiment includes:
s1: obtaining the equivalent backscattering coefficient NE sigma of noise0Equation for standard SAR range:
wherein R is SAR action distance, PtFor radar peak power transmission, ratio is radar duty cycle, GtFor radar antenna transmission gain, GrFor radar antenna reception gain, prFor the distance resolution, the formula is rhor＝kr·C/2BiWhere C is the speed of light, BiFor the signal bandwidth, krWeighted broadening for pulse pressure, λ is radar operating wavelength, K is Boltzmann constant, T is noise temperature, NFIs the noise coefficient of the radar system, LSThe loss of the radar system is shown, v is the flying speed of the platform, and theta is the ground wiping angle;
s2: obtaining an equation of the SAR low interception action distance according to the equation of the SAR action distance:
wherein R isLPIAlpha is an interception factor, alpha is the interception distance of the SAR, the interception distance of the electronic countermeasure equipment to radar radiation signals is equal to the SAR action distance, k is a radar antenna emission pattern factor, P isrminFor the detection sensitivity of the electronic countermeasure equipment, LjFor electronic countermeasure equipment detecting receiver system loss, GjGain of the receiving antenna of the electronic countermeasure device, BjDigitizing the channel bandwidth for the electronic countermeasure device;
specifically, S2 includes:
an equation of the interception distance of the electronic countermeasure equipment to the radar radiation signal is obtained,
wherein R isjFor interception of radar radiation signals by electronic countermeasure equipment, PtWhen detecting and receiving main lobe of beam transmitted by radar antenna, k is 1, and when detecting and receiving side lobe of beam transmitted by radar antenna, k is less than 1, BjDigitizing the channel bandwidth for the electronic countermeasure device; l isjIs to combat the system losses of the device electronically.
In this embodiment, the electronic countermeasure device is a channelized high sensitivity receiver.
According to the expression of the interception factor alpha, the electronic countermeasure equipment intercepts the radar radiation signal by a distance RjAnd SAR working distance R to obtain SAR low interception working distance RLPIThe equation of (c).
WhereinThe interception factor alpha is the interception distance R of the electronic countermeasure equipment to radar radiation signalsjThe ratio of the SAR range to the SAR range R,
it should be noted that the signal bandwidth B of the high resolution SAR isiDigital channel bandwidth B of electronic countermeasure equipment far greater than high sensitivityjThus, f B is obtained according to the formula (4)i，
S3: according to an equation of the SAR low interception action distance, one or more of related parameters of the SAR system are designed and changed to improve the low interception action distance of the SAR system, wherein the related parameters comprise a radar working duty ratio, a radar antenna receiving gain, a radar system loss, a radar system noise coefficient, a platform flight speed, a ground clearance angle and an antenna emission beam side lobe;
it should be noted that when the relevant parameters of the electronic countermeasure equipment are determined, the upper limit of the low interception functioning distance of the SAR system can be calculated, and according to the expression (formula (2)) of the low interception functioning distance of the SAR, the relevant parameters of the SAR system can be changed through design, so that the design method capable of effectively improving the low interception functioning distance of the SAR system is provided.
Specifically, in S3, designing to change relevant parameters of the SAR system includes:
(a) the working duty ratio is improved;
it should be noted that the duty cycle of the conventional single-base SAR is limited by factors such as a blind area and an imaging width, and the space for increasing the working duty cycle is limited. In the embodiment, double-base cooperative imaging is adopted, the receiving and transmitting isolation degree between the transmitting station and the receiving station is improved through the distance between the double bases, double-base continuous wave work is realized, the radar work duty ratio is improved, and the low interception action distance of the SAR system is improved. Referring to fig. 2, fig. 2 is a schematic diagram of operation of a bistatic SAR according to an embodiment of the present invention, where (a) is a geometric model of a co-route bistatic SAR imaging, and (b) is a waveform of operation of the bistatic SAR.
(b) Improving the receiving gain of the antenna;
it should be noted that, in a general case, the SAR system utilizes the transmission gain and the reception gain of the whole antenna to realize high-resolution imaging on the ground. However, most of the existing SAR systems are multi-function radars, and in order to improve the range and detection accuracy of a lower resolution imaging mode or a GMTI (ground moving target indication) mode, a large azimuth aperture antenna is often adopted. However, in the strip mode, the azimuth resolution is 1/2 of the azimuth aperture of the antenna, so the SAR system usually adopts a mode of widening the azimuth transmitting directional diagram and the receiving directional diagram of the radar antenna to realize higher-resolution strip mode imaging.
In this embodiment, the azimuth transmission beam of the radar antenna is widened, the radar receiving antenna is divided into a plurality of sub-arrays, narrow beams corresponding to a plurality of full-aperture antennas are simultaneously formed by a digital beam forming method, the doppler bandwidths in the wide beam ranges corresponding to the sub-arrays are respectively filtered, and then the doppler bandwidths required by imaging are spliced, so that the receiving gain of the radar antenna is improved, and the low interception action distance of the SAR system is increased.
(c) Changing the working frequency of the radar antenna;
receiving gain of radar antenna
Therefore, when the area of the radar antenna is constant, the working frequency is increased, and when the receiving gain of the radar antenna is constant, the working frequency is reduced, so that the low interception action distance of the SAR system is increased.
(d) The loss and the noise coefficient of the radar system are reduced, so that the low interception action distance of the SAR system is increased;
(e) the flying speed of the platform is reduced, so that the low wiping angle is increased, and the low interception action distance of the SAR system is increased;
(f) and the side lobe of the transmitted beam of the radar antenna is reduced, so that the low interception action distance of the SAR system is increased.
As can be seen from the formula (2), when the duty ratio of the SAR system, the gain of the receiving antenna and the radar antenna workWhen the frequency, the radar system loss and the noise coefficient are determined, the SAR system determines the low interception action distance of the electronic countermeasure equipment with specific parameters under a specific flight platform, and the low interception action distance is irrelevant to the gain and the peak transmission power of the transmitting antenna. The EIRP (equivalent radiation power, equal to the product of peak emission power and emission antenna gain) corresponding to the SAR low interception action distance can be calculated and recorded as EIRPLPI. When EIRP of SAR system is larger than EIRPLPIWhen the SAR working distance R is more than PLPIHowever, at the same time, alpha is larger than 1, and at this time, the interception distance of the electronic countermeasure equipment to the radar radiation signal is larger than the action distance of the SAR, so that the radio frequency stealth capability of the radar cannot be realized.
Taking the SAR system parameters in table 1 and the electronic countermeasure equipment system parameters in table 2 as examples, the acting distance of the SAR system and the interception distance of the electronic countermeasure equipment are calculated, as shown in fig. 3, and fig. 3 is a graph of the acting distance of the SAR system and the interception distance of the electronic countermeasure equipment provided by the embodiment of the present invention.
TABLE 1 SAR System parameters
TABLE 2 electronic countermeasure Equipment System parameters
As can be seen from fig. 3, when the SAR system operating distance is less than 25.8km, the interception distance of the electronic countermeasure equipment is less than the radar operating distance; when the SAR system operating distance is more than 25.8km, the interception distance of the electronic countermeasure equipment is more than the radar operating distance, so that the low interception distance of the SAR system is less than 25.8 km. Substituting the parameters shown in the tables 1 and 2 into the formula (2) to calculate the low interception action distance R of the SARLPI<25.8km, thereby verifyingCorrectness of equation (2).
In addition, fig. 3 also reflects that the main lobe low interception operating distance of the conventional SAR system is far shorter than the farthest operating distance of the conventional SAR system under normal conditions, so that a further requirement is provided for improving the low interception operating distance of the SAR system to break through the operating distance calculated by the formula (2) and achieve the farthest operating distance of the SAR system.
Further, the design method as described in step S4 may be adopted to increase the farthest acting distance of the SAR system itself.
S4: an MIMO system SAR system based on orthogonal signals is adopted to reduce the equivalent radiation power of a radar transmitting end;
the maximum action distance of the SAR system is required to be reached, the equivalent radiation power of the radar is dispersed by the electronic countermeasure equipment, and the dispersed radiation power is synthesized at the radar receiving end, so that the long-distance low-interception imaging of the SAR system is realized.
An orthogonal signal-based Multiple Input Multiple Output (MIMO) system Synthetic Aperture Radar (SAR) system synthesizes a transmitting directional diagram of a radar at a receiving end, so that equivalent radiation power of the radar transmitting end is effectively reduced, radio frequency stealth is realized, and please refer to fig. 4 in combination, wherein fig. 4 is a working schematic diagram of the MIMO radar provided by the embodiment of the invention.
In S4, the equation of the SAR range of the SAR system using the MIMO system based on orthogonal signals and the equation of the intercept distance of the radar radiation signal by the electronic countermeasure equipment are modified as follows:
wherein G ist' Transmission gain, G, for a single transmission sub-arraySBenefits the transmitting directional diagram obtained after the directional diagram of the transmitting and receiving antenna is synthesized;
obtaining a low interception action distance calculation formula of the MIMO system SAR system based on the orthogonal signals according to the corrected SAR action distance equation and the equation of the interception distance of the electronic countermeasure equipment to the radar radiation signals:
as can be known from the formula (7), the low interception action distance of the MIMO system SAR based on the orthogonal signal is G of the low interception action distance of the conventional SAR systemSAnd (4) doubling. In general, the emission pattern benefits from GSThe number of the transmitting subarrays is equal to N of the SAR system with the MIMO system. Referring to fig. 5 in combination, fig. 5 is a graph of an acting distance and an intercepting distance of an electronic countermeasure equipment of a 4-transmission 4-reception MIMO-SAR system according to an embodiment of the present invention, and it can be seen from the graph that the farthest acting distance of the MIMO-SAR system itself is significantly increased compared to a conventional SAR system.
Further, considering that the radio frequency stealth capability of the radar is enhanced by designing the radar radiation signal of the SAR system, the design method of the radar radiation signal as described in step S5 may be adopted.
S5: the method comprises the steps of carrying out agility processing on parameters of radar radiation signals, modulating by noise-like modulation, designing to obtain agility signal parameters with low interception waveforms, and designing to obtain a multi-parameter signal sorting SAR imaging algorithm with the low interception waveforms according to the agility signal parameters with the low interception waveforms.
The key to whether the electronic countermeasure equipment can effectively interfere with the radar is whether the electronic countermeasure equipment can separate single radar radiation signals from randomly staggered signal streams under a high-density signal environment and identify radar radiation signals needing interference from the single radar radiation signals, namely signal sorting.
Referring to fig. 6 in combination, fig. 6 is a basic flow chart of signal sorting algorithm development and multi-parameter signal sorting according to an embodiment of the present invention, in which (a) is a basic flow chart of signal sorting algorithm development, and (b) is a basic flow chart of multi-parameter signal sorting. The signal sorting algorithm is gradually developed from single-parameter sorting into multi-parameter related sorting, and the multi-parameter signal sorting process is divided into three stages, wherein the first stage is pre-sorting by using characteristic parameters of CF (carrier frequency), DOA (direction of arrival), PW (pulse width) and PA (pulse amplitude). After the pulse signals are subjected to pretreatment, the pulse signals are diluted, then are sorted by a second-stage PRI (pulse repetition period) sorter, and after the PRI sorting is completed, the third-stage sorting is carried out to analyze the modulation form of the signals and carry out correlation sorting on the characteristics of the signals.
The electronic countermeasure equipment mainly analyzes the modulation form of the radar radiation signal according to parameters such as pulse repetition period, carrier frequency, pulse width, pulse amplitude and the like of the radar radiation signal, and realizes signal sorting.
In the embodiment, a plurality of parameters (PRI, PW and PA) of the radar radiation signal are subjected to agile processing, and a noise-like modulation form is adopted, so that the signal sorting difficulty of the electronic countermeasure equipment is increased, and the radio frequency stealth capability of the radar is enhanced.
Specifically, in S5, the agile processing is performed on the parameter of the radar radiation signal, including:
respectively generating random number sequences of PRI, PW and PA which are in accordance with standard normal distribution by a polar coordinate method;
and respectively carrying out linear transformation of standard normal distribution on the random number sequence to obtain the digital sequences of the PRI, the PW and the PA which obey Gaussian distribution, and obtaining the agile PRI, the agile PW and the agile PA.
Taking PRI as an example, the specific steps of the agile processing are described as follows:
the key to generating an agile PRI is to generate a sequence of random numbers representing the PRI. At present, a widely used method is to generate a random number sequence on a computer by a mathematical method. The polar coordinate method is the most common method for generating random number sequences which obey standard normal distribution, and the calculation steps are as follows:
(1) generating two independent U (0,1) random numbers U distributed in the same way1And U2；
(2) Let Vi＝2Ui-1(i ═ 1,2), and calculating S ═ V1 2+V2 2；
(3) If S > 1, returning to the step (1), otherwise, calculating
X1＝V1Y，X2＝V2Y。
A random number sequence following a standard normal distribution can be obtained by the above method. For random variables that obey a gaussian distribution, their linear functions still obey a gaussian distribution. Thus, the gaussian distribution of the variables can be obtained by a linear transformation of the standard normal distribution. For example, according to the polar method, a random number sequence X following a standard normal distribution may be first generated. Then, it is possible to obtain a obedient mean value of μ and a variance of σ2Y ═ σ X + μ of the gaussian distribution of (a). It can be assumed that PRI is a random number sequence following a Gaussian distribution with an average value of 1/FS，FSTo sample the frequency, let σ be an adjustable constant, and then get a PRI that obeys any gaussian distribution by changing σ, get a agile PRI.
The generation method of the agile PW and agile PA is similar to the generation method of the agile PRI, and is not described herein again.
Further, in S5, the noise-like modulation signal is generated as follows: since PRI and PW are agile, a limited set of noise-like modulated signals may be generated, stored in a code table from which noise-like modulated signals may be randomly selected for transmission as the radar operates.
Further, since the conventional SAR imaging algorithm is designed based on a constant radiation signal, in this embodiment, agile processing is performed on parameters of the radar radiation signal, noise-like modulation is performed, and agile signal parameters with a low intercepted waveform are designed, the SAR imaging algorithm also needs to be adjusted correspondingly based on the conventional SAR imaging algorithm, and a multi-parameter signal sorting SAR imaging algorithm with a low intercepted waveform is designed.
Referring to fig. 7, fig. 7 is a flowchart of a multi-parameter signal sorting SAR imaging algorithm with a low interception waveform according to an embodiment of the present invention, and as shown in the figure, the multi-parameter signal sorting SAR imaging algorithm with a low interception waveform according to the embodiment includes amplitude correction, azimuth resampling, and CZT-based distance migration correction, where the amplitude correction is used to correct the amplitude of each echo pulse according to the variation of the pulse width and the amplitude of a radiation signal, so as to avoid azimuth modulation caused by the variation of the pulse width and the amplitude of the radiation signal; azimuth resampling for correcting azimuth non-uniform sampling caused by pulse repetition period variation; and the CZT-based distance migration correction is used for correcting the distance migration under high-resolution imaging aiming at the radiation signal of which the radiation modulation form is not linear frequency modulation.
It should be noted that amplitude correction, azimuth resampling, and CZT-based range migration correction are all existing imaging algorithms, and detailed descriptions are omitted.
The SAR low interception radio frequency stealth system design method of the embodiment provides the design of the SAR low interception radio frequency stealth system from two levels, wherein the first level is that an electronic countermeasure device is difficult to detect a radiation signal of the SAR system by adopting a large receiving antenna, a double-base continuous wave operation and an orthogonal signal-based MIMO system, and the second level is that the electronic countermeasure device is difficult to classify and identify the radiation signal of the SAR system by jointly agilely changing a plurality of characteristic parameters of the radiation signal of the SAR system, thereby overcoming the defect that most researches of the existing low interception radio frequency stealth system are designed around low interception waveforms.
Further, the embodiment also provides an SAR low-interception radio frequency stealth system, which is designed according to the design method of the embodiment by adopting an MIMO system SAR system based on orthogonal signals. The SAR low-interception radio frequency stealth system of the embodiment enhances the radio frequency stealth capability of the radar.
Example two
The present embodiment further illustrates the effect of the first embodiment by combining with a simulation experiment.
1. Measured data acquisition system parameters
The parameters used for the simulation are as follows: the bandwidth is 150MHz, the wavelength is 0.0313m, the radar speed is 100m/s, the reference range is 8000m, and the sampling is 1024 in both range and azimuth.
2. Measured data imaging processing content and result
By using the design method of embodiment one, the PRF (pulse repetition frequency), PW, and PA can be kept agile, where PRF is the inverse of PRI, and simulation can be performed instead of PRI. Referring to fig. 8, fig. 8 is a graph of variation of radiation signals with an agile pulse repetition period, pulse width, amplitude and modulation format according to an embodiment of the present invention, where (a) is a pulse repetition period variation curve, (b) is a pulse width variation curve, (c) is a pulse amplitude variation curve, and (d) is a pulse modulation format agile. In (a) - (c) of the drawings, the PRF of each azimuth sampling point randomly increases or decreases centered at 650Hz, the PW of each azimuth sampling point randomly increases or decreases centered at 20 μ s, and the PA randomly increases or decreases centered at 3dB in each azimuth sampling point, and (d) illustrates a noise-like modulation signal.
3. Analysis of imaging results
Referring to fig. 9, fig. 9 is a graph of a simulation result of SAR imaging of a multi-parameter inter-pulse agility signal according to an embodiment of the present invention, where (a) is an echo data image, (b) is a pulse compressed image, (c) is a CZT-based range migration corrected image, and (d) is an azimuth pulse compressed image. The echo data shown in the graph (a) in fig. 9 is generated by transmitting the noise-like modulation signal in the graph (d) in fig. 8, and thereafter, the imaging results processed by the multi-parameter signal-sorting SAR imaging algorithm having a low intercept waveform as shown in the graphs (b) to (d) in fig. 9 are obtained. As can be seen from the figure, the target points are well focused in the imaging results shown in (d), which verifies the effectiveness of the low-intercept waveform design.
It should be noted that, in this document, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that an article or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of additional like elements in the article or device comprising the element.
The foregoing is a more detailed description of the invention in connection with specific preferred embodiments and it is not intended that the invention be limited to these specific details. For those skilled in the art to which the invention pertains, several simple deductions or substitutions can be made without departing from the spirit of the invention, and all shall be considered as belonging to the protection scope of the invention.
Claims (8)
1. A method for designing an SAR low-interception radio frequency stealth system is characterized by comprising the following steps:
s1: obtaining the equivalent backscattering coefficient NE sigma of noise0Equation for standard SAR range:
wherein R is SAR action distance, PtFor radar peak power transmission, ratio is radar duty cycle, GtFor radar antenna transmission gain, GrFor radar antenna reception gain, prFor the distance resolution, the formula is rhor＝kr·C/2BiWhere C is the speed of light, BiFor the signal bandwidth, krWeighted broadening for pulse pressure, λ is radar operating wavelength, K is Boltzmann constant, T is noise temperature, NFIs the noise coefficient of the radar system, LSThe loss of the radar system is shown, v is the flying speed of the platform, and theta is the ground wiping angle;
s2: obtaining an equation of the SAR low interception action distance according to the equation of the SAR action distance:
wherein R isLPIFor the low interception range of the SAR,alpha is an interception factor, alpha is 1, the interception distance of the electronic countermeasure equipment to radar radiation signals is equal to the SAR action distance, k is a radar antenna emission pattern factor, and P isrminFor the detection sensitivity of the electronic countermeasure equipment, LjFor electronic countermeasure equipment detecting receiver system loss, GjGain of the receiving antenna of the electronic countermeasure device, BjDigitizing the channel bandwidth for the electronic countermeasure device;
s3: according to the equation of the SAR low interception action distance, one or more of related parameters of the SAR system are designed and changed to improve the SAR low interception action distance, wherein the related parameters comprise radar working duty ratio, radar antenna receiving gain, radar system loss, radar system noise coefficient, platform flight speed, ground wiping angle and antenna emission beam side lobe;
s4: an MIMO system SAR system based on orthogonal signals is adopted to reduce the equivalent radiation power of a radar transmitting end;
s5: the method comprises the steps of carrying out agility processing on parameters of radar radiation signals, modulating the parameters by noise-like signals, designing to obtain agility signal parameters with low interception waveforms, and designing to obtain a multi-parameter signal sorting SAR imaging algorithm with the low interception waveforms according to the agility signal parameters with the low interception waveforms.
2. The SAR low interception radio frequency cloaking system design method according to claim 1, wherein said S2 comprises:
an equation of the interception distance of the electronic countermeasure equipment to the radar radiation signal is obtained,
wherein R isjFor interception of radar radiation signals by electronic countermeasure equipment, PtWhen detecting and receiving main lobe of beam transmitted by radar antenna, k is 1, and when detecting and receiving side lobe of beam transmitted by radar antenna, k is less than 1, BjDigitizing the channel bandwidth for the electronic countermeasure device; l isjSystem losses for the electronic countermeasure equipment;
according to the expression of the interception factor alpha, the electronic countermeasure equipment intercepts the radar radiation signal by a distance RjAnd SAR working distance R to obtain SAR low interception working distance RLPIThe equation of (a), wherein,
3. the method for designing the SAR low interception radio frequency cloaking system according to claim 2, wherein in the step S3, designing and changing relevant parameters of the SAR system comprises:
double-base cooperative imaging is adopted, the receiving and transmitting isolation degree between a transmitting station and a receiving station is improved through the distance between double bases, double-base continuous wave work is realized, the radar work duty ratio is improved, and the low interception action distance of an SAR system is improved;
broadening azimuth-direction transmission beams of the radar antenna, dividing the radar receiving antenna into a plurality of sub-arrays, simultaneously forming narrow beams corresponding to a plurality of full-aperture antennas by a digital beam forming method, respectively filtering Doppler bandwidths in a wide beam range corresponding to the sub-arrays, and splicing the Doppler bandwidths into Doppler bandwidths required by imaging, so that the receiving gain of the radar antenna is improved, and the low interception action distance of the SAR system is increased;
when the area of the radar antenna is constant, the working frequency is increased, and when the receiving gain of the radar antenna is constant, the working frequency is reduced, so that the low interception action distance of the SAR system is increased;
the loss and the noise coefficient of the radar system are reduced, so that the low interception action distance of the SAR system is increased;
the flying speed of the platform is reduced, so that the low wiping angle is increased, and the low interception action distance of the SAR system is increased;
and the side lobe of the transmitted beam of the radar antenna is reduced, so that the low interception action distance of the SAR system is increased.
4. The SAR low interception radio frequency cloaking system design method according to claim 3, characterized in that in said S4,
the method comprises the following steps of correcting an equation of SAR action distance of an SAR system adopting an MIMO system based on orthogonal signals and an equation of interception distance of electronic countermeasure equipment to radar radiation signals as follows:
wherein G ist' Transmission gain, G, for a single transmission sub-arraySBenefits the transmitting directional diagram obtained after the directional diagram of the transmitting and receiving antenna is synthesized;
obtaining a low interception action distance calculation formula of the MIMO system SAR system based on the orthogonal signals according to the corrected SAR action distance equation and the equation of the interception distance of the electronic countermeasure equipment to the radar radiation signals:
5. the SAR low interception radio frequency cloaking system design method according to claim 4, characterized in that in S5, the parameters of the radar radiation signal include PRI, PW and PA.
6. The SAR low interception radio frequency cloaking system design method according to claim 5, wherein in the S5, the agile processing of the parameters of the radar radiation signal includes:
respectively generating random number sequences of PRI, PW and PA which are in accordance with standard normal distribution by a polar coordinate method;
and respectively carrying out linear transformation of standard normal distribution on the random number sequence to obtain the digital sequences of PRI, PW and PA which obey Gaussian distribution, and obtaining the agile PRI, the agile PW and the agile PA.
7. The SAR low interception radio frequency cloaking system design method according to claim 5 characterized in that in S5, multi-parameter signal sorting SAR imaging algorithm with low interception waveform, including amplitude correction, azimuth resampling and CZT based range migration correction,
the amplitude correction is used for correcting the amplitude of each echo pulse according to the change of the pulse width and the amplitude of the radiation signal, and the azimuth modulation caused by the change of the pulse width and the amplitude of the radiation signal is avoided;
the azimuth resampling is used for correcting azimuth non-uniform sampling caused by pulse repetition period change;
the CZT-based distance migration correction is used for correcting distance migration under high-resolution imaging aiming at a radiation signal of which the radiation modulation form is not linear frequency modulation.
8. An SAR low-interception radio frequency stealth system is characterized in that the SAR low-interception radio frequency stealth system adopts an MIMO system SAR system based on orthogonal signals and is designed according to the design method of any one of the claims 1 to 7.
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