Pulse Doppler radar target distance-Doppler characteristic modulation method based on phase modulation surface

Abstract

The invention provides a pulse Doppler radar target distance-Doppler feature modulation method based on a phase modulation surface. The pulse Doppler radar target distance-Doppler feature modulation method comprises the steps of 1, presetting a coded modulation waveform; 2, modulating the position characteristics of the false target; 3, speed characteristic modulation of the false target; and step 4, deception of the CFAR detector is carried out. The PSS-based PD radar target distance-Doppler feature modulation method is innovatively provided, and the application of the PSS in the aspect of radar target multi-dimensional feature joint modulation is expanded. According to the invention, the cost is lower, the system complexity is lower, and the electromagnetic concealment is stronger. By applying different coding waveforms to the PSS feature modulation reflector, flexible and diversified feature modulation styles can be generated, the limitation of the traditional PD radar passive feature modulation method on functions of limited effect, insufficient flexibility and the like is overcome, and the advantage of reusability of an active feature modulation technology is also taken into account.

Description

Technical Field

[0001] A method for modulating the range-Doppler characteristics of a pulsed Doppler radar target based on a phase modulation surface according to the present invention belongs to the field of metasurface electromagnetic regulation. Specifically, it relates to the field of modulating the position and motion characteristics of a pulsed Doppler radar target. Further, it is to actively control the phase of the radar echo signal through a phase modulation surface, so that after the controlled echo signal passes through the radar signal processing system, the distribution of the radar target in the range-Doppler characteristic space will deviate significantly from the actual situation, forming a false electromagnetic situation, thereby causing the radar side to be unable to correctly perceive the actual target situation in the detected environment.

Background Art

[0002] Pulse-Doppler (PD) radar originated from Moving Target Indicator (MTI) radar. It obtains target position information through echo delay and calculates the motion characteristics of the target using the Doppler frequency shift of the received echo. PD radar is equipped with complex clutter suppression filters and has been widely used due to its excellent performance in detecting weak targets in strong clutter environments (such as ground reflections, weather phenomena such as sea waves or clouds and rain). On the other hand, characteristic modulation techniques that can change the detection results of PD radar for target position and speed characteristics have been widely studied by domestic and foreign research teams. However, there are many limitations in the existing characteristic modulation means, so exploring effective PD radar target characteristic modulation techniques has great theoretical value and practical prospects.

[0003] Measured by whether it actively emits electromagnetic waves, PD radar target feature modulation technology can be divided into two types: active and passive. Active feature modulation technology affects the radar system by emitting electromagnetic waves, while passive feature modulation technology relies on materials or objects that can reflect or absorb the energy of radar echoes. Currently, advanced active feature modulation systems usually integrate high-power transmitting devices to produce jamming or deceptive modulation effects. In particular, the intermittent sampling and forwarding technology based on Digital Radio Frequency Memory (DRFM) can generate flexible and rich modulation signals. However, with the continuous progress of radar detection and anti-countermeasure technologies, traditional active feature modulation technology is facing more and more challenges, including high cost, operational complexity, and easy detectability. In contrast, passive feature modulation technology inherently offers the advantages of rapid deployment, low cost, and high mobility. However, traditional passive methods such as chaff and corner reflectors can hardly effectively act on PD radar because the signals are easily filtered out by MTI filters due to their relatively slow movement. In recent years, passive electromagnetic modulation technology based on electromagnetic modulation materials has made it possible to flexibly control electromagnetic characteristics such as the amplitude, phase, frequency, and polarization of electromagnetic waves. A large number of studies have shown that it can effectively act on radar signals, thereby having a controllable impact on the results of radar target feature extraction.

[0004] The Phase-Switched Screen (PSS) is a switch-type phase electromagnetic modulation material, which was jointly proposed by Professor B. Chambers and Professor A. Tennant from the University of Sheffield in the UK in 2004 as a new type of radar absorbing material, mainly used to reduce the radar cross-section of the protected target. Different from traditional absorbing materials, the Phase-Switched Screen does not actually absorb the energy of electromagnetic waves, but by applying phase modulation to the incident electromagnetic waves, making the spectrum of its reflected signal fall outside the entire receiver bandwidth or have a controllable distribution within the receiver bandwidth. As a passive "thin and light" material, the Phase-Switched Screen has the ability to actively modulate the reflected signal and has great potential in the field of radar target feature modulation, and is attracting more and more attention from researchers.

[0005] In 2015, the research team led by Professor Feng Dejun of the National University of Defense Technology first recognized the application potential of PSS in the field of radar target feature modulation, and gradually studied the radar echo signal modulation characteristics of PSS, the time-domain coding control method of PSS, the matching filter characteristics of the linear frequency-modulated (LFM) signal reflected by PSS, the image feature modulation method of the imaging radar based on PSS, the radar target angle modulation method based on PSS, etc., and achieved rich research results. However, at present, the research on radar target feature modulation technology based on PSS mainly focuses on ranging and imaging radars, and the related research on PD radars is still blank. In addition, the flexible allocation of radar echo energy in the target feature space has always been a challenging problem in the field of radar target feature modulation technology based on electromagnetic control materials. Therefore, the present invention utilizes the phase manipulation characteristics of PSS for electromagnetic waves to simultaneously control the two main feature information of the position and speed of the radar target, thereby generating rich and variable target feature modulation results for PD radars, making it impossible for PD radars to accurately perceive the target situation in the detected environment.

Summary of the Invention

[0006] Aiming at the limitations of the existing active and passive feature modulation means of PD radars, the present invention proposes a method for modulating the range-Doppler features of PD radar targets based on a phase modulation surface. The core mechanism of this method is to use PSS to perform periodic coded phase modulation on radar echoes, which essentially destroys the phase coherence between radar pulses and realizes flexible control of the target distribution in the range-Doppler (RD) domain. This method can achieve electromagnetic stealth protection for targets and support a series of multi-purpose feature modulation effects for PD radars, including range deception, Doppler (velocity) deception, and two-dimensional joint deception of range-Doppler. In addition, even under low signal-to-noise ratio (SNR) conditions, this method can generate several effective main false targets. To achieve the above process of modulating the features of PD radar targets, the present method is implemented by the following steps:

[0007] Step 1: Preset the coded modulation waveform

[0008] The present invention uses a periodic coded voltage as an external excitation to enable PSS to regularly control the phase of the incident electromagnetic wave and reflect it, thereby generating flexible and diverse target feature modulation effects on PD radars, such as electromagnetic stealth, range feature deception, velocity feature deception, two-dimensional feature joint deception of range-velocity, etc. First, it is necessary to use an intelligence reconnaissance or electronic reconnaissance system to obtain the basic parameters of the target radar emission signal, including: carrier frequency f c , signal wavelength λ, pulse width T p , frequency modulation slope K, signal bandwidth B, pulse repetition frequency f rThe false radar target range-Doppler spectrum Y′(t,f) generated by modulating with the method of the present invention can be expressed as where: t is the time variable (there is a one-to-one mapping relationship with the target range R: c is the speed of light), f is the frequency variable (there is a one-to-one mapping relationship with the target velocity v: ), Y0(t,f) is the RD spectrum of the true target, ( denotes rounding down) represents the number of modulation harmonic components falling within the receiver passband, n is the number of the harmonic component, k n represents the energy intensity of the nth-order harmonic of the PSS coded modulation waveform spectrum (characterized by formula (4) below, and its value is controlled by the duty cycle β of the PSS coded waveform), f s is the modulation frequency corresponding to the PSS periodic coded waveform, mod(·) is the modulo operation. Therefore, the target feature modulation effect generated by the present invention is completely determined by both the PSS coded waveform and the basic parameters of the radar waveform: the distribution of false targets in the range dimension is jointly determined by the modulation frequency f s and the frequency modulation slope K of the radar signal; the distribution in the Doppler dimension is jointly determined by the modulation frequency f s and the pulse repetition frequency f r of the radar signal; the energy of the false target generated by the nth-order harmonic component is determined by the duty cycle β of the coded modulation waveform. Therefore, after obtaining the above basic parameters of the radar waveform, the corresponding PSS coded waveform can be determined according to the preset feature modulation effect. Multiple coded waveforms corresponding to different modulation styles can be written into the electrically controlled network with storage function in the PSS feature modulation reflector in advance, or a communication module can be embedded in the electrically controlled network to write the newly determined coded waveform in real time by using the host computer.

[0009] According to the modulated radar target RD spectrum Y′(t,f), both the range offset and velocity offset of the false target generated by the harmonic component are related to the modulation frequency f s , and the modulation results of the false target in terms of range and velocity features have strong coupling. However, due to the significant difference in the sensitivity to the frequency shift amount between the two, the range and velocity features of the false target can be controlled separately by finely setting the value of the modulation frequency f s .

[0010] Determine the modulation frequency f sAfter determining the duty cycle β, a corresponding high-low level coded control voltage can be applied to the PSS characteristic modulation reflector, causing the additional phase of the incident electromagnetic wave to continuously switch between 0 and 180°, which is equivalent to the reflection coefficient continuously switching between +1 and -1. This effect can be equivalently considered as superimposing the multiplicative modulation of p(t) on the incident radar signal s(t), and the reflected wave r(t) = s(t) × p(t). Since the multiplication in the time domain corresponds to the convolution operation in the frequency domain, the spectrum of the reflected wave can be expressed as where S(f) is the spectrum of the radar signal s(t), and P(f) is the spectrum of the coded modulation signal p(t). Since This indicates that the spectrum of the reflected signal has been harmonically extended due to the modulation effect of the PSS, that is where the frequency step of each harmonic component is equal to the modulation frequency f of the PSS phase control s .

[0011] Step 2: Location feature modulation of false targets

[0012] According to the frequency mismatch characteristic in the pulse compression process of the LFM signal, the reflected wave with a changed center frequency will have a range shift after pulse compression. That is, the echo with a center frequency shift of Δf will be shifted in range by The above phase-modulated echo r(t) consists of an infinite number of harmonic components centered at the center frequency f of the original signal s(t) c First, it enters the pre-stage band-pass filter of the radar receiver (the bandwidth is equal to the bandwidth B of the radar transmitted signal) and becomes r′(t). After band-pass filtering, 2N + 1 harmonic components are retained, that is r′(t) enters the matched filter of the PD radar. After pulse compression, it will change the range dimension characteristics of the target. Each harmonic can generate a false target with corresponding false location characteristics. The peak of the false target generated by the nth harmonic is located at where R0 is the location of the real target.

[0013] Step 3: Velocity feature modulation of false targets

[0014] Moving Target Detection (MTD) processing is a means for the PD radar to obtain target velocity feature information. Specifically, this process is achieved by extracting the Doppler frequency shift of the comb spectrum lines. From the perspective of the inter-pulse spectrum characteristics of the LFM pulse train signal, the frequency shift of nf caused by the harmonic generation through the PSS periodic phase coding modulation sIt is equivalent to generating 2N Doppler false targets with different false Doppler shifts. Based on the above principles, when the modulated echo is processed by MTD, the 2N velocity false targets with false Doppler (the targets generated by the 0th harmonic component have the same velocity as the real target) will be extracted. The false Doppler increment generated by the nth harmonic is equivalent to the frequency shift amount nf s , and the corresponding false target velocity is where v0 is the real target velocity. In practical applications, MTD processing has a limited velocity measurement frequency band range [0, +f r . The spectral energy outside the effective velocity measurement frequency band will fold back into this range. Therefore, the false target velocity generated by the nth harmonic can be rewritten as

[0015] Step 4: Deception of the CFAR detector

[0016] In summary, under the periodic phase control of the PSS feature modulation reflector, the phase-modulated echo, after being processed by the PD radar signal in Step 2 and Step 3, will generate electromagnetic false targets with false distance and false velocity characteristics in the RD domain, and the RD spectrum of the false targets is Y′(t, f). Finally, the energy of the false targets generated by the method proposed in the present invention can successfully deceive the constant false alarm (CFAR) detector in the RD domain of the target radar under a relatively large noise tolerance level, that is, effectively achieve the false situation deception effect in the distance-Doppler target feature space of the method proposed in the present invention.

[0017] The beneficial effects of the present invention are as follows:

[0018] First, an innovative PD radar target distance-Doppler feature modulation method based on PSS is proposed, expanding the application of PSS in the joint modulation of multi-dimensional features of radar targets.

[0019] Second, compared with the widely used active feature modulation method of PD radar, the new method proposed in the present invention has lower cost, lower system complexity, and stronger electromagnetic concealment.

[0020] Third, by applying different coding waveforms to the PSS feature modulation reflector, flexible and diverse feature modulation patterns can be generated, overcoming the limitations of traditional passive feature modulation methods of PD radar in terms of limited effect and insufficient flexibility, and also taking into account the reusable advantage of active feature modulation technology.

Description of the Drawings

[0021] Figure 1 is the flow chart of the PD radar target distance-Doppler feature modulation method.

[0022] Figure 2 is the PSS switch-type structure and its phase modulation principle.

[0023] Figure 3 These are the design details and layout of the PSS feature modulation reflector.

[0024] Figure 4(a) shows the 1-bit periodic coding modulation time-domain waveform of the PSS.

[0025] Figure 4(b) shows the spectrum of the 1-bit periodic coding modulation time-domain waveform of the PSS.

[0026] Figure 5 These are the spectrum comparisons of the LFM pulse train before and after periodic phase modulation by the PSS feature modulation reflector.

[0027] Figure 6 These are the typical LFM-PD radar signal processing procedures.

[0028] Figure 7(a) shows the position of the real target in the RD domain without modulation.

[0029] Figure 7(b) shows the range profile of the real target in the RD domain without modulation.

[0030] Figure 7(c) shows the velocity profile of the real target in the RD domain without modulation.

[0031] Figure 8(a) shows the target feature modulation in the RD domain when the modulation frequency f s = 10.1f r and the duty cycle β = 0.3.

[0032] Figure 8(b) shows the range profile in the RD domain when the modulation frequency f s = 10.1f r and the duty cycle β = 0.3.

[0033] Figure 8(c) shows the velocity profile in the RD domain when the modulation frequency f s = 10.1f r and the duty cycle β = 0.3.

[0034] Figure 8(d) shows the target feature modulation in the RD domain when the modulation frequency f s = 10.2f r and the duty cycle β = 0.3.

[0035] Figure 8(e) shows the range profile in the RD domain when the modulation frequency f s = 10.2f r and the duty cycle β = 0.3.

[0036] Figure 8(f) shows the velocity profile in the RD domain when the modulation frequency f s = 10.2f r and the duty cycle β = 0.3.

[0037] Figure 8(g) shows the target feature modulation in the RD domain when the modulation frequency f s = 20.1f r and the duty cycle β = 0.3.

[0038] Figure 8(h) shows the range profile in the RD domain when the modulation frequency f s = 20.1f r and the duty cycle β = 0.3.

[0039] Figure 8(i) shows the velocity profile in the RD domain when the modulation frequency f s = 20.1f r and the duty cycle β = 0.3.

[0040] Figure 9(a) shows the target feature modulation in the RD domain when the modulation frequency f s = 10.1f r and the duty cycle β = 0.5.

[0041] Figure 9(b) shows the range profile in the RD domain when the modulation frequency f s = 10.1f r and the duty cycle β = 0.5.

[0042] Figure 9(c) shows the velocity profile in the RD domain when the modulation frequency f s = 10.1f r and the duty cycle β = 0.5.

[0043] Figure 9(d) shows the target feature modulation in the RD domain when the modulation frequency f s = 10.1f r and the duty cycle β = 0.1.

[0044] Figure 9(e) shows the range profile in the RD domain when the modulation frequency f s = 10.1f r and the duty cycle β = 0.1.

[0045] Figure 9(f) shows the velocity profile in the RD domain when the modulation frequency f s = 10.1f r and the duty cycle β = 0.1.

[0046] Figure 10(a) shows the target feature modulation in the RD domain when the modulation frequency f s = 10f r and the duty cycle β = 0.5.

[0047] Figure 10(b) shows the target feature modulation in the RD domain when the modulation frequency f s = 0.1f r and the duty cycle β = 0.5.

[0048] Figure 10(c) shows the target feature modulation in the RD domain when the modulation frequency f s = 1.1B and the duty cycle β = 0.5.

[0049] Figure 11(a) shows the 2D CA-CFAR detection results in the RD domain when the signal-to-noise ratio SNR = 0 dB.

[0050] Figure 11(b) shows the 2D CA-CFAR detection results in the RD domain when the signal-to-noise ratio SNR = -15 dB.

[0051] Figure 11(c) shows the 2D CA-CFAR detection results in the RD domain when the signal-to-noise ratio SNR = -30 dB.

[0052] Figure 11(d) shows the 2D OS-CFAR detection results in the RD domain when the signal-to-noise ratio SNR = 0 dB.

[0053] Figure 11(e) shows the 2D OS-CFAR detection results in the RD domain when the signal-to-noise ratio SNR = -15 dB.

[0054] Figure 11(f) shows the 2D OS-CFAR detection results in the RD domain when the signal-to-noise ratio SNR = -30 dB.

Detailed implementation manners

[0055] To better understand the method of the present invention, the technical solution of the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0056] The applicable object of the present invention is a pulsed Doppler radar using a chirp signal. Taking an X-band radar system as an example, the radar transmission signal is a chirp pulse signal with a center frequency of 10.3 GHz, a bandwidth of 20 MHz, a pulse width of 50 μs, a pulse repetition frequency of 10 kHz, and the radar coherently accumulates 64 pulses at a time. Assume that the real target is located at a position 4 km away from the radar and moves at a radial velocity of 30 m / s relative to the radar. The PSS feature modulation reflector is assembled on the real target and moves along with it.

[0057] The present invention involves the following specific steps, and the specific process is as Figure 1 shown.

[0058] Step 1: Preset the coded modulation waveform

[0059] In the actual application process, the joint assistance of the electronic reconnaissance system and the intelligence reconnaissance system is required to provide data support for the subsequent coded waveform design. Using the electronic reconnaissance system and intelligence reconnaissance, the basic parameters of the radar transmission signal and the target are obtained, the carrier frequency f c = 10.3 GHz, the pulse width T p= 50 μs, signal bandwidth B = 20 MHz, pulse repetition frequency f r = 10 kHz. By calculation, the signal wavelength λ = c / f c = 2.92 cm, frequency modulation slope K = B / T p = 4×10 11 Hz / s, c is the speed of light, c = 3×10 8 m / s.

[0060] The PSS switch-type structure and its phase modulation principle are as Figure 2 shown. According to the basic parameters of the radar waveform obtained, the thickness of the dielectric layer is designed to be d = λ / 4 = 0.73 cm. The switch impedance layer is usually composed of unit structures integrated with active elements such as PIN diodes. By changing the bias voltage, the effective impedance of this layer changes, resulting in different electromagnetic responses. When the PIN diode is in the "ON" state, the impedance layer completely reflects the incident wave, and the reflected wave can be expressed as cos(2πf c t). In the "OFF" state, the impedance layer allows the incident wave to pass through the dielectric layer and reflect off the metal backplane. This increases the phase shift by π by increasing the path length of the wave by λ / 2. At this time, the reflected wave can be expressed as cos(2πf c t + π). The two electromagnetic response states of the switch impedance layer are pre-coded in binary. The external electronic control network provides external excitation mainly based on FPGA, and periodically applies the coded voltage to the PSS. This time-varying excitation causes the PIN diode to alternate between the "ON" and "OFF" states, enabling the impedance layer to periodically switch between the reflection and transmission states. The following specifically gives the preset examples of the coded modulation waveforms under several typical modulation styles:

[0061] ① Electromagnetic stealth: Design the modulation frequency f s of the PSS > B, so that (only the 0th-order component is retained), and set the duty cycle β of the coded waveform to 0.5, so that k0 = 0. Then the modulated target RD spectrum can be written as Y1′(t, f) = 0, and the electromagnetic stealth effect is achieved at this time;

[0062] ② Range feature deception: Design the modulation frequency f s of the PSS to be an integer multiple of the radar pulse repetition frequency f r , so that mod(nf s , f r ) = 0 holds for different n. Then the modulated target RD spectrum can be written as At this time, the range feature deception effect is achieved;

[0063] ③ Velocity feature deception: Design the modulation frequency f s of the PSS not to be the radar pulse repetition frequency f ran integer multiple of, and ensure that the modulation frequency f s is as small as possible, so that is less than the radar range resolution (simplified to: Nf s T p < 1), then the modulated target RD spectrum can be approximately written as At this time, the speed characteristic deception effect is achieved;

[0064] ④ Range-velocity characteristic two-dimensional deception: Design the modulation frequency f of the PSS s not an integer multiple of the radar pulse repetition frequency f r , and at least ensure that the magnitude of the modulation frequency f s is such that is greater than the radar range resolution (simplified to: Nf s T p > 1), then the modulated target RD spectrum can be written as At this time, the range-velocity characteristic two-dimensional deception effect is achieved. Finally, design the duty cycle β of the PSS coded modulation waveform to control the magnitude of k n , that is, the energy intensity of each false target. Figure 3 Shows the design details and layout of the PSS characteristic modulation reflector.

[0065] From the perspective of amplitude, since cos(2πf c t + π) = -cos(2πf c t + π), then the periodic phase reversal of the electromagnetic wave corresponds to the switching of the reflection coefficient of the PSS between 1 and -1. This process can be described by a 1-bit coded modulation waveform p(t), which can be given by the following formula:

[0066]

[0067] where t is the time variable, when then otherwise g represents the g-th period of the coded modulation waveform p(t) in the time domain, T s is the switching period of the PSS state, and its reciprocal is the modulation frequency f s , τ represents the duration of the reflection state within a modulation period, δ(·) is the impulse function, represents the convolution operation.

[0068] Assume that the phase coding sequence of the PSS is periodic and infinite, and p(t) and its spectrum P(f) can be expressed using Fourier series expansion as:

[0069]

[0070] where f is the frequency variable, and k n (n is an integer) represents the energy intensity of the nth harmonic, and the expression is:

[0071]

[0072] Figure 4(a) shows the time-domain waveform of the 1-bit coded modulation waveform p(t). Among them, represents the duty cycle of the periodic coded waveform. Figure 4(b) shows the spectrum P(f) of the coded modulation waveform p(t), which consists of a series of discrete spectral components, including the zero-order component and multiple groups of symmetric sideband harmonics, and the envelope is in the shape of a sinc function. Among them, the zero-order component represents the DC component in p(t). When the spectral range of the incident electromagnetic wave s(t) falls within the operating bandwidth of the PSS, the echo signal r(t) and its spectrum R(f) can be expressed as:

[0073] r(t) = s(t) × p(t) (5)

[0074]

[0075] where S(f) represents the spectrum of s(t). According to Equation (3) and Equation (6), after the periodic phase modulation by the PSS feature modulation reflector, the radar signal undergoes significant changes in the frequency domain. The spectrum of the original signal is extended simultaneously in the positive and negative directions along the frequency axis, and the step size is determined by the modulation frequency f s to form harmonic components symmetrically distributed around the center frequency, as Figure 5 shown. The energy of each harmonic component is controlled by the attenuation factor k n . As shown in Equation (4), k n is only determined by the duty cycle β of the PSS coded waveform. This means that the energy of all harmonic components can be controlled by adjusting the value of β.

[0076] As previously described, the pulse compression and MTD processing of PD radar are very sensitive to the changes in the echo signal spectrum. By inversely inferring the key parameters of the PSS coded waveform: the modulation frequency f s and the duty cycle β from the preset target feature modulation effect, the spectral structure of the reflected wave can be precisely controlled, thereby generating the corresponding preset feature modulation effect in the PD radar signal processing result.

[0077] Step 2: Location feature modulation of false targets

[0078] From the intrapulse-interpulse perspective, the two-dimensional time-domain expression of the LFM pulse waveform used by PD radar can be given by the following formula:

[0079]

[0080] where \(j\) is the imaginary unit, \(t\) m represents the slow-time variable, and let \(T\) r be the reciprocal of the pulse repetition frequency \(f\) r , \(R_0\) be the true target range, \(v_0\) be the true target velocity, and \(m\) represent the \(m\)-th pulse transmitted by the PD radar. Then \(t\) m can be expressed as:

[0081]

[0082] The radar transmits an LFM pulse train to the PSS feature modulation reflector. After phase-coding modulation, it returns along the original path to the PD radar receiver for signal processing. The typical LFM-PD radar signal processing flow is as Figure 6 shown. A group of pulse trains will successively go through the processes of pulse compression, MTI filtering, MTD processing, and CFAR detection, and finally the target detection result is displayed in the RD domain. Since the PD radar processes signals in the fast-slow two-dimensional time domain, in order to maintain the same signal form, slow-time sampling is introduced into the coded modulation waveform \(p(t)\), and its two-dimensional time-domain expression can be written as:

[0083]

[0084] After passing through the band-pass filter at the front end of the radar receiver, the modulated echo is convolved in the matched filter, and the pulse compression result \(y'(t,t\) m ) is given by:

[0085]

[0086] where represents the number of harmonic components falling within the half-passband of the receiver. In order to achieve an effective feature modulation effect, it is necessary to make \(f\) s <B. After pulse compression, the modulated echo will generate equally spaced symmetrically distributed range false targets around the true target, and the false target peaks are located at:

[0087]

[0088] where \(R_0\) is the true target position and \(K\) is the frequency modulation slope of the LFM signal transmitted by the radar.

[0089] Step 3: Velocity feature modulation of false targets

[0090] Compared with the echo without modulation, due to the periodic phase modulation, \(2N\) new harmonic components are introduced, and each harmonic carries an additional linear phase \(\exp[j\pi nf\) s (t + t\) m - 2mT\) r)]. By performing MTD processing on the pulse compression result, that is, performing slow-time DFT operation on y′(t,t m ), the false Doppler frequency shift amount of the false target generated by the nth harmonic in the RD domain can be deduced as:

[0091] f n =f d +nf s (12)

[0092] where f d is the Doppler frequency shift amount of the real target. Considering the velocity ambiguity problem in actual MTD processing, the above formula can be further written as:

[0093] f n =f d +mod(nf s ,f r ) (13)

[0094] The corresponding velocity measurement result still satisfies the linear mapping relationship between Doppler frequency shift and velocity Therefore, the velocity value of the false target generated by the nth harmonic can be expressed as:

[0095]

[0096] Formulas (11) and (14) reflect the two-dimensional feature coupling relationship of false targets in range-Doppler in the RD feature space. The method proposed in the present invention can flexibly generate false targets with range-Doppler coupling characteristics by precisely controlling the energy and frequency domain distribution of harmonics. Utilizing the above coupling effect, the method can achieve flexible and diverse target feature modulation effects, including electromagnetic stealth protection of targets, range deception, Doppler (velocity) deception, and range-Doppler two-dimensional joint deception, etc.

[0097] Figures 7(a)-(c) show the MTD results of real targets in the RD domain without modulation (temporarily omitting MTI and CFAR detection processing) and their range / velocity profiles, and the results conform to the set range of 4 km and velocity of 30 m / s. Figures 8(a)-(i) show the target feature modulation situations in the RD domain at different modulation frequencies f s (with a fixed duty cycle β = 0.3). Corresponding to Figures 8(a)-(c), (d)-(f), and (g)-(i), f s are 10.1f r , 10.2f r and 20.1f r respectively. When f sWhen set to the above three values, the distance intervals between the first-order false targets generated by periodic phase modulation (the false target closest to the target position, also known as the main false target) and the distance of the real target are 37.875 m, 38.25 m, and 75.375 m respectively, while the corresponding speed intervals are 14.49 m / s, 28.98 m / s, and 14.49 m / s. The simulation results show that when the modulation frequency f s takes the above values, the PSS feature modulation reflector produces a combined deception effect on the range-Doppler two-dimensional features of the PD radar, and the false target distance interval increases with the increase of f s . The false target speed interval is determined by the fractional part of f s / f r . In addition, the false target energy spreads in the range-Doppler domain centered on the real target according to the sinc function distribution.

[0098] Figs. 9(a)-(f) show the target feature modulation in the RD domain under different duty cycles β (fixed modulation frequency f s = 10.1f r ). The β corresponding to Figs. 9(a)-(c) and (d)-(f) are 0.5 and 0.1 respectively. Combining the results shown in Figs. 9(a)-(c), Figs. 8(a)-(c) and Figs. 9(d)-(f), when β is set to the above three values, the distance interval between the main false target and the real target is 37.875 m, and the corresponding speed interval is also 14.49 m / s. The main difference lies in the energy strength of the target. Based on the simulation results, in the RD domain, the energy of the real target decreases with the increase of the duty cycle β. When β = 0.5, the zero-order harmonic component corresponding to the real target position and the false target energy generated by the even-order harmonic components are both zero, thus achieving the effect of "hiding the real and showing the false". However, when β is small, a large zero-order attenuation factor k0 causes the energy of the real target to be greater than the energy of the false target, thus there may be a risk of exposure. In most cases, it is crucial to ensure that the coded modulation waveform p(t) has a large duty cycle. The simulation results clearly show the control effect of the duty cycle β on the energy intensity of each false target.

[0099] In addition, Figs. 10(a)-(c) show the unique modulation of the target features in the RD domain when f s takes special values (fixed duty cycle β = 0.5). The f s corresponding to Figs. 10(a)-(c) are 10f r , 0.1f r , and 1.1B respectively. As mentioned above, the simulation results show that when f s is an integer multiple of f r , the false targets are only distributed along the range dimension, presenting a pure range deception modulation effect. When f sis f r is a fractional multiple of and satisfies f s T P <<1, the false targets can be considered to be distributed only along the velocity dimension, presenting a modulation effect of pure velocity deception. In the extreme case, as shown in Fig. 10(c), when the modulation frequency satisfies f s >B, since all harmonic components generated by the echo through the PSS periodic phase modulation will fall outside the passband of the radar receiver's bandpass filter, and at this time β = 0.5, the energy of the zero-order harmonic component becomes zero, so that the PD radar cannot detect any effective targets, presenting an electromagnetic stealth effect of the targets.

[0100] Step 4: Deception of the CFAR detector

[0101] Although the periodic phase modulation of the echo by the PSS feature modulator will generate a large number of PD radar false targets, only the part that successfully passes the CFAR detector is considered valid. To evaluate the effectiveness of the method proposed in the present invention, the key parameters of p(t) are set as f s =10.1f r , β = 0.5, and the false alarm probability P fa =10 -6 . As shown in Figs. 11(a)-(f), for the two detection algorithms of 2D CA-CFAR and 2D OS-CFAR, under three signal-to-noise ratio conditions of -30dB, -15dB, and 0dB, simulation experiments were carried out on the effectiveness of the PSS feature modulator with periodic coding. The results show that the signal-to-noise ratio directly affects the number of effective false targets. At higher SNR values, the strong anti-noise performance of the LFM-PD radar in the RD domain enables high-order and low-energy false targets with significant attenuation to also pass through the CFAR detector, resulting in a large number of effective false targets being distributed in a large area of the RD domain. As the SNR decreases, more and more high-order and low-energy false targets are submerged in the noise background, resulting in a decrease in the number of effective false targets. As shown in Figs. 11(c) and 11(f), when the signal-to-noise ratio drops to -30dB, only the first-order false targets with a larger attenuation weight k n can pass through the constant false alarm detector to form effective false feature targets.

[0102] It can be easily seen from the above experimental results that the position, velocity, number, and energy intensity of the PD radar false targets generated based on the PSS feature modulator can be controlled by the coding waveform parameters f s and β. By presetting appropriate coding waveforms, flexible target feature modulation effects can be achieved on the PD radar to form diverse false electromagnetic postures, which is consistent with the theoretical analysis and also proves the effectiveness of the method proposed in this patent.

Claims

1. A method for modulating the range-Doppler characteristics of a pulsed Doppler radar target based on a phase modulation surface, characterized in that It includes the following steps: Step 1: Preset the coded modulation waveform Using the periodic coded voltage as an external excitation, the PSS regularly controls the phase of the incident electromagnetic wave and reflects it, thereby producing a flexible and diverse target feature modulation effect on the PD radar; Step 2: Location feature modulation of false targets According to the frequency mismatch characteristics in the pulse compression process of the LFM signal, the reflected wave with a changed center frequency will show a distance offset after pulse compression. That is, the echo with a center frequency shift of Δf will be offset in distance by The phase-modulated echo r(t) consists of an infinite number of harmonic components centered at the center frequency f c of the original signal s(t); Step 3: Velocity feature modulation of false targets Moving target detection (MTD) processing is a means for a PD radar to obtain target velocity characteristic information, which is achieved by extracting the Doppler frequency shift of the comb spectrum lines; the frequency shift amount nf brought about by generating harmonics through PSS periodic phase encoding modulation s is equivalent to generating 2N Doppler false targets with different false Doppler frequency shifts; Step 4: Deception of the CFAR detector Under the periodic phase control of the PSS feature modulation reflector, the phase-modulated echo, after being processed by the PD radar signal, will generate an electromagnetic false target with false distance and false velocity features in the RD domain, and the RD spectrum of the false target is Y′(t,f).

2. The pulse Doppler radar target range-Doppler feature modulation method based on a phase modulation surface according to claim 1, characterized in that: In step one, the basic parameters of the target radar emission signal are obtained using an intelligence reconnaissance or electronic reconnaissance system, including: carrier frequency f c , signal wavelength λ, pulse width T p , frequency modulation slope K, signal bandwidth B, pulse repetition frequency f r ; The false radar target range-Doppler spectrum Y′(t,f) generated by modulation is expressed as: where: t is a time variable, and there is a one-to-one mapping relationship with the target distance R: c is the speed of light, f is a frequency variable, and there is a one-to-one mapping relationship with the target velocity v: Y0(t,f) is the RD spectrum of the real target, denotes rounding down, represents the number of modulation harmonic components falling within the receiver passband, n is the number of the harmonic component, k n represents the nth-order harmonic energy intensity of the PSS coded modulation waveform spectrum, f s is the modulation frequency corresponding to the PSS periodic coding waveform, and mod(·) is the modulo operation.

3. The method for modulating the range-Doppler characteristics of a pulsed Doppler radar target based on a phase modulation surface according to claim 1 or 2, characterized in that: In step one, the distribution of false targets in the range dimension is jointly determined by the modulation frequency f s and the frequency modulation slope K of the radar signal; the distribution in the Doppler dimension is jointly determined by the modulation frequency f s , the pulse repetition frequency f r of the radar signal; the energy of false targets generated by the nth harmonic component is determined by the duty cycle β of the coded modulation waveform; therefore, after obtaining the basic parameters of the radar waveform, the corresponding PSS coded waveform is determined according to the preset characteristic modulation effect; Multiple coded waveforms corresponding to different modulation styles are pre-written into the electrically controlled network with a storage function in the PSS feature modulation reflector, or a communication module is embedded in the electrically controlled network, and the newly determined coded waveform is written in real time using the host computer.

4. The method for modulating the range-Doppler characteristics of a pulsed Doppler radar target based on a phase modulation surface according to claim 3, wherein: In step one, according to the modulated radar target RD spectrum Y′(t,f), both the range offset and the velocity offset of the false target generated by the harmonic components are related to the modulation frequency f s and the modulation results of the false target in terms of range and velocity characteristics have strong coupling. By setting the value of the modulation frequency f s , the range and velocity characteristics of the false target can be controlled separately.

5. The method for modulating the range-Doppler characteristics of a pulsed Doppler radar target based on a phase modulation surface according to claim 4, wherein: In Step 1, determine the modulation frequency f of the PSS coded modulation signal p(t) s and the duty cycle β, then apply the corresponding high-low level coded control voltage to the PSS characteristic modulation reflector, so that the additional phase of the incident electromagnetic wave is continuously switched between 0 and 180°, which is equivalent to the reflection coefficient being continuously switched between +1 and -1; multiply the incident radar signal s(t) by p(t), and the reflected wave r(t) = s(t) × p(t). Since the time-domain multiplication corresponds to the frequency-domain convolution operation, the spectrum of the reflected wave is expressed as where S(f) is the spectrum of the radar signal s(t), and P(f) is the spectrum of the coded modulation signal p(t). Since it shows that the spectrum of the reflected signal is harmonically extended by the modulation effect of the PSS, that is where the frequency step of each harmonic component is equal to the modulation frequency f of the PSS phase control s .

6. The pulse Doppler radar target range-Doppler feature modulation method based on a phase modulation surface according to claim 1, wherein: In step two, the pre-stage band-pass filter of the radar receiver becomes r′(t). After band-pass filtering, 2N + 1 harmonic components are retained, that is r′(t) enters the matched filter of the PD radar. After pulse compression, the range dimension characteristics of the target will be changed. Each harmonic can generate a false target with corresponding false position characteristics. The peak of the false target generated by the nth harmonic is located at where R0 is the real target position and K is the frequency modulation slope of the LFM signal transmitted by the radar.

7. The method for modulating the range-Doppler characteristics of a pulsed Doppler radar target based on a phase modulation surface according to claim 1 or 6, characterized in that: In Step 2, considering from the intra-pulse and inter-pulse perspectives, the two-dimensional time-domain expression of the LFM pulse waveform used by the PD radar is given by the following formula: where j is the imaginary unit, and t m represents the slow time variable. Let T r be the reciprocal of the pulse repetition frequency f r , R0 be the true target range, v0 be the true target velocity, and m represent the m-th pulse transmitted by the PD radar. Then t m is expressed as:

8. The method for modulating the range-Doppler characteristics of a pulsed Doppler radar target based on a phase modulation surface according to claim 7, characterized in that: In Step 2, the radar transmits an LFM pulse train to the PSS feature modulation reflector. After phase coding modulation, it returns to the PD radar receiver along the original path for signal processing. In order to maintain the same signal form, slow-time sampling is introduced into the coded modulation waveform p(t), and its two-dimensional time-domain expression is written as: After passing through the band-pass filter at the front end of the radar receiver, the modulated echo is convolved in the matched filter, and the pulse compression result y′(t, t m ) is given by: Among them, represents the number of harmonic components falling within the half passband of the receiver. To achieve an effective feature modulation effect, it is necessary to make f s < B; after pulse compression, the modulated echo will generate equally spaced and symmetrically distributed range false targets around the real target.

9. The method for modulating the range-Doppler characteristics of a pulsed Doppler radar target based on a phase modulation surface according to claim 1, characterized in that: In step three, when the modulated echo is processed by MTD, 2N velocity false targets with false Doppler will be extracted. The false Doppler increment generated by the nth harmonic is equal to the frequency shift amount nf s , and the corresponding false target velocity is where v0 is the real target velocity; in practical applications, MTD processing has a limited velocity measurement frequency band range [0, +f r . The spectral line energy outside the effective velocity measurement frequency band will fold back into this range. Therefore, the false target velocity generated by the nth harmonic is rewritten as 10. The method for modulating the range-Doppler characteristics of a pulsed Doppler radar target based on a phase modulation surface according to claim 9, characterized in that: In step 3, compared with the echo without modulation, 2N new harmonic components are introduced due to the periodic phase modulation, and each harmonic carries an additional linear phase exp[jπnf s (t + t m -2mT r )] that varies with slow time; by performing MTD processing on the pulse compression result, that is, performing a slow-time DFT operation on y′(t, t m ), the false Doppler frequency shift of the false target generated by the nth harmonic in the RD domain is obtained as follows: f n = f d + nf s where f d is the Doppler frequency shift of the real target; considering the velocity measurement ambiguity problem in actual MTD processing, the above formula is further written as: f n = f d + mod(nf s , f r ).

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