CN115877381A - Bistatic radar collaborative imaging method based on complementary random waveform - Google Patents
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Abstract
The invention belongs to the technical field of radar imaging, and relates to a bistatic radar collaborative imaging method based on complementary random waveforms, which is characterized by comprising the following steps: step 1: establishing a double-radar cooperative detection echo model, wherein both the radar 1 and the radar 2 adopt random frequency hopping waveforms; step 2: distance difference compensation is carried out, phase modulation items of phase differences of echo signals received by the radar 1 and the radar 2 are eliminated, and distance difference compensation is achieved; and 3, step 3: echo motion compensation is carried out, and components related to the velocity components in the echoes are eliminated, so that the echo motion compensation is realized; and 4, step 4: and performing frequency band fusion imaging, rearranging random frequency hopping of different frequency bands, performing zero filling processing on echo data on the defective frequency points, and performing coherent fusion imaging on echo signals of the two radars through FFT (fast Fourier transform) by setting tight constraint conditions. The imaging method of the invention designs a new frequency hopping waveform, realizes high-efficiency motion compensation, and adopts a coherent processing algorithm to realize frequency band fusion.
Description
Technical Field
The invention belongs to the technical field of ISAR imaging, and particularly relates to a bistatic radar collaborative imaging method based on complementary random waveforms.
Background
The ISAR (Inverse Synthetic Aperture Radar) Inverse Synthetic Aperture Radar analyzes the range delay and Doppler resolution of echo signals, and obtains the scattering intensity of each part of a target, namely the image of the target. At any point on a plane perpendicular to the direction of the radar line of sight, the distance to the radar is equal, called the equidistant plane. At any point on a plane parallel to the rotation axis and the radar sight line direction, the Doppler speeds are equal, the planes are called Doppler planes, and the scattering intensity of the target can be distributed along with the position by performing distance resolution and Doppler resolution on the target, so that the image of the target is obtained. The ISAR radar has all-weather, all-time and long-distance detection capability, is an important sensor for imaging detection of a missile-borne platform target, and plays a significant role in an accurate guidance task. However, with the advance of the target stealth technology and the rapid development of the interference countermeasure technology, for example, the reduction of the scattering Cross Section area of the target RCS (Radar Cross Section) Radar requires the Radar to adopt a large antenna aperture and a large bandwidth signal system, which brings a huge challenge to single Radar detection. Compared with single radar detection, the multi-radar cooperative detection can obtain more observation times, and can obtain the characteristics of different dimensions such as a target frequency domain, a polarization domain, an airspace and the like, so that the method is an important development direction for detecting the stealth target in a complex electromagnetic environment.
According to the existing multi-radar cooperative detection technology, according to different detection angles and radar working frequency bands, multi-radar cooperative detection modes can be divided into space cooperation and frequency band cooperation, and therefore space diversity benefits and frequency diversity benefits are obtained. The target detection and identification are realized by jointly processing a plurality of detection visual angle information, so that the probability of misjudgment of single visual angle detection is reduced. The latter carries out coherent processing on the frequencies of different radars to obtain higher range resolution and pulse pressure gain. For application scenarios of the missile-borne radar, multiple missile-borne radars fly along with the flight, the detection visual angle difference is small, and the target detection is usually performed in a frequency band fusion mode. Because different radars move at high speed and echo data are distributed on a plurality of discontinuous frequency bands, phase errors caused by platform movement need to be eliminated, system errors among different radar echoes are compensated, effective fusion of data of different frequency bands is realized on the basis, and the processing process relates to key technologies such as motion compensation, coherent registration and data fusion. In terms of Motion compensation, the document (see Liao Z K, hu J M, lu D W, et al. Motion analysis and compensation method for random stepped frequency using the pseudo random code [ J ]. IEEE access, 2018, 6 (1): 57643-57654) performs velocity estimation using cross-correlation of adjacent distance directions, and the compensation accuracy is determined by the bandwidth and cannot be applied to a radar waveform sensitive to Motion such as random frequency hopping. The target speed is determined by searching for the extreme cost function (S) (Li X, li G S, et al. Automatic of ISAR images based on the amount of reduction [ J ]. IEEE trans. On Aerospace and Electronic systems, 1999, 35 (4): 1240-1252). The algorithm performance is sensitive to the step size and range of the search. The literature (see Liao Z K, lu D W, hu J M, et al. Wave form design for random stepped frequency radio to estimate object velocity [ J ]. Electronics drivers, 2018, 54 (14): 894-896) proposes a velocity estimation method based on complementary code modulation, and realizes high-precision estimation of velocity by transmitting adjacent pulse trains with complementary relation, and has the disadvantages that two pulse trains are needed to complete velocity estimation, and radar data rate is reduced. In the aspect of coherent registration, the literature (see TIAN Biao, CHEN Zengping, and XU Shiyou. Sparse sub-base imaging based on parameter estimation of geometric phase of diffusion model [ J ]. IET radio frequency & Navigation, 2014, 8 (4): 318-326) divides the phase to be compensated between the sub-bands into two parts of linear phase and fixed phase, and utilizes an all-pole model to solve, and the pole order is difficult to determine when the defect is. In the literature (see TIAN Jihua, SUN Jinping, WANG Guohua, et al, multiband radio Signal coherent fusion Processing with IAA and apFFT [ J ]. IEEE Signal Processing Letters, 2013, 20 (5): 463-466), the linear phase term is solved by cross-correlation, and the fixed phase term is solved by Fast Fourier Transform (FFT), thereby avoiding the pole order problem. In terms of band data fusion, the literature (see BAI Xueru, ZHOU Feng, WANG Qi, et al. Space sub-band imaging of space targets in high-speed motion [ J ]. IEEE Transactions on Geoscience and remove Sensing, 2013, 51 (7): 4144-4154 and HU longjiang, XU Shiyou, WU Wenzhen, et al. Space sub-band ISAR imaging band on autoregressive mode and smoothened 84670, IEEE Sensors Journal, 2018, 18 (22): 9315-9323) images after filling in the defect band into a wideband signal, however, band estimation is based on existing band information and does not add additional information volume. In the literature (see ZHOU Feng and BAI Xue, high-resolution sparse sub-band imaging based on Bayesian echo with temporal principles [ J ]. IEEE Transactions on Geoscience and Remote Sensing, 2018, 56 (8): 4568-4580), after the target is subjected to azimuth dimension imaging, distance fusion imaging is performed by using a Bayesian learning algorithm, so that errors introduced by a band filling algorithm are avoided, however, the azimuth dimension imaging needs accurate target motion compensation, and the compensation precision of sub-band data needs further analysis.
The main defects of the existing missile-borne ISAR radar system detection are as follows: 1) Performing data level fusion after target trace information obtained by each radar is extracted respectively, wherein gains brought by radar target echo signal cooperative processing are not considered; 2) The band fusion algorithm is mainly carried out based on two ideas, one is to fill up a defective band by adopting a band extrapolation method and then carry out imaging, and the method is realized based on the existing band information without increasing extra information amount. The other idea is to extract scattering center parameters by adopting a modern spectrum, sparse representation or Bayesian method to carry out super-resolution range imaging, and because the original information of radar data is lost in the parameter extraction process, two-dimensional imaging is difficult to carry out.
Disclosure of Invention
The invention aims to provide a bistatic radar collaborative imaging method based on complementary random waveforms, which is used for designing a new frequency hopping waveform aiming at a bistatic radar collaborative detection scene, realizing efficient motion compensation, designing a coherent processing algorithm of bistatic echo signals, realizing frequency band fusion and improving the echo signal-to-noise ratio and the range resolution capability of a radar.
The specific technical scheme of the invention is a bistatic radar collaborative imaging method based on complementary random waveforms, which is characterized by comprising the following steps:
step 1: establishing a double-radar cooperative detection echo model, wherein both the radar 1 and the radar 2 adopt random frequency hopping waveforms, and then the emission signal models of the two radars are respectively expressed as the following formulas (I) and (II):
wherein the coherent processing bursts each compriseNSub-pulses having the same frequency hopping pattern and a pulse repetition period ofT r The pulse width isTWith random modulation frequency distribution over a given bandwidthBInner, the minimum hop step size is Δf =B / NLet the frequency hopping coefficients of radar 1 and radar 2 bec 1 (n) Andc 2 (n)，f 1 andf 2 the carrier frequencies of radar 1 and radar 2 respectively,
when the radar 1 transmits and receives the radar 1, the firstnThe sub-pulse echoes after mixing can be represented by the following formula (III):
wherein the target comprisesKA scattering center, initial time, secondkThe distance of each scattering center from the radar 1 isr 1k The target radial motion speed isvOf 1 atnThe distance of the scattering center from the radar 1 at the sub-pulse transmission time can be expressed asr 1k (n)=r 1k +vnT r ，σ k Is shown askThe intensity of the individual scattering centers is such that,
when the radar 2 transmits radar 2 to receivenThe sub-pulse echoes after mixing can be expressed as the following formula (V):
wherein, deltar=r 2k -r 1k Of 1 atkThe distance of each scattering center from the radar 2 isr 2k ，
The radar 2 emits an echo received by the radar 1s 21 (n) Echo received by radar 1 transmitting radar 2s 12 (n) Expressed as the following formulas (VI) and (VII), respectively:
step 2: distance difference compensation is performed, a phase modulation term of a phase difference of echo signals received by the radar 1 and the radar 2 is eliminated, distance difference compensation is achieved, and echoes after distance difference compensation can be expressed as the following formulas (IX) and (X) respectively:
and step 3: performing echo motion compensation, eliminating the component related to the velocity component in the echo to realize the echo motion compensation, wherein the echo after the echo motion compensation can be respectively expressed as the following formulas (XVI) and (XVII),
wherein the content of the first and second substances, f 0 =(f 1 +f 2 )/2 ，c(n)=c 1 (n)+(f 1 -f 2 )/2，c 2 (n)=-c 1 (n)；
and 4, step 4: and performing frequency band fusion imaging, rearranging random frequency hopping of different frequency bands, performing zero filling processing on echo data on the defective frequency points, and performing coherent fusion imaging on echo signals of the two radars through FFT (fast Fourier transform) by setting tight constraint conditions.
Furthermore, the echo motion compensation in step 3 is to set the frequency hopping coefficient of the pulse transmitted by the radar 1c 1 (n) The frequency hopping coefficient of the pulse transmitted by the radar 2 is equal in size and opposite in direction, whereinf 0 =（f 1 +f 2 ）/2 ，c(n)=c 1 (n)+(f 1 -f 2 )/2，c 2 (n)=-c 1 (n) Substituting formula (IX) and formula (X) may give:
the echo components are multiplied together, resulting in the following:
the target velocity of the sequence peak position after FFT is as follows (XIV):
whereink 0 After obtaining the target velocity for the peak position, eliminating the component related to the velocity component in the echo, and performing echo motion compensation on the formulas (XI) and (XII)。
Furthermore, the specific method for performing band fusion imaging in step 4 is to arrange the echo sequences of the formulas (XVI) and (XVII) from small to large according to the minimum frequency hopping step size, and the obtained echo sequence is represented as:
wherein the content of the first and second substances,L=M+2Nfor the length of the echo sequence after zero-filling,s all (n) Front ofNEach component iss 21 (n) Results of rearrangementNEach component iss 11 (n) The rearrangement result is effectively observed, and the defect frequency band is set to be [ 2 ]f 1 ，f 2 ]Defective bandwidthf 1 -f 2 For the step size Δ of frequency hoppingfIs integer multiple of (1), the corresponding zero-padding frequency point number isM=(f 1 -f 2 )/Δf-1, tos all (n) The high-resolution range profile obtained by performing FFT can be expressed as:
the double-base radar cooperative imaging method based on the complementary random waveform has the beneficial effects that 1) the double-base radar cooperative imaging method based on the complementary random waveform adopts the complementary random frequency hopping waveform to detect the target, and the phase item introduced by the platform distance difference can be effectively eliminated. 2) The fused range profile provides richer target detail information, and the estimated speed precision can meet the focusing requirement of high-resolution range imaging; 3) The resolution is improved by adopting a fusion imaging mode, the distribution area of the target scattering center is larger, the provided target detail information is richer, the ISAR imaging result under ideal speed compensation is given, and the focusing performance of the image is measured through the contrast. Practical use results show that the method can effectively realize the double-base motion compensation and the fusion detection of the target.
Drawings
FIG. 1 is a flow chart of a bistatic radar collaborative imaging method based on complementary random waveforms according to the present invention;
FIG. 2 is a schematic diagram of bistatic radar cooperative detection based on a complementary random waveform bistatic radar cooperative imaging method according to the present invention;
FIG. 3 is a schematic diagram of random frequency hopping complementary modulation of a bistatic radar based on a complementary random waveform cooperative imaging method of the bistatic radar of the present invention;
FIG. 4 is a schematic diagram of random frequency-hopping echo rearrangement zero-filling in the complementary random waveform-based bistatic radar collaborative imaging method according to the present invention;
FIG. 5 is a schematic diagram of an established scattering model of a target point;
FIG. 6 is a schematic diagram of a bistatic cooperative detection geometric scene using the bistatic radar cooperative imaging method based on complementary random waveforms of the present invention;
FIG. 7 (a) is a phase estimation result diagram introduced by distance difference compared with the fused range imaging effect using the method of the present invention;
FIG. 7 (b) is a graph of the result of target velocity estimation for fusion range imaging effect comparison using the method of the present invention (v tLOS =3m/s）；
FIG. 7 (c) is a diagram of a distance high-resolution imaging result of the radar 1 compared by the fusion distance imaging effect of the method of the present invention;
FIG. 7 (d) is a diagram of a radar 2 range high resolution imaging result comparing the fusion range imaging effect using the method of the present invention;
FIG. 7 (e) is a graph of the actual fused probe distance high resolution imaging result compared with the fused distance imaging effect using the method of the present invention;
FIG. 7 (f) is a graph of ideal fused detection range high resolution imaging results comparing the fused range imaging effects using the method of the present invention;
FIG. 8 (a) is a diagram of radar 1-ISAR high resolution imaging results comparing radar imaging results using the method of the present invention;
FIG. 8 (b) is a diagram of radar 2-ISAR high resolution imaging results comparing radar imaging results using the method of the present invention;
FIG. 8 (c) is a graph of actual fused ISAR imaging results compared with radar imaging results using the method of the present invention;
FIG. 8 (d) is an ideal fusion ISAR imaging result chart comparing radar imaging results using the method of the present invention.
Detailed Description
The technical scheme of the invention is further described in the following with the accompanying drawings of the specification.
As shown in fig. 1, the flow chart of the cooperative imaging method of bistatic radar based on complementary random waveform of the present invention is shown. The double-base radar cooperative detection principle is shown in figure 2, and the separated echo set is recorded ass 11 (t)，s 12 (t)，s 21 (t)，s 22 (t) }. WhereinS pq Representation radarp(p=1, 2) transmitting pulse, radarq(q=1, 2) echo representation in reception mode.
Firstly, two radar echo signals meeting the tight constraint condition of complementary characteristics are designed, then different band-pass filters are set according to the difference of carrier frequencies, and a separated target echo sequence is obtained. After echo components of 4 paths are obtained, modulation phase terms introduced by distance delay differences under different paths are calculated and used for compensating additional phases introduced by distance differences under different paths, distance difference correction of different paths is completed, and 4-path echo signals with the same distance delay are obtained. On the basis, a bibase random frequency hopping signal waveform pair with complementary characteristics is provided, two radars simultaneously transmit random frequency hopping signals with equal frequency hopping coefficients and opposite directions, 4-path echo signals with the same distance and time delay are obtained, and effective estimation of target speed can be achieved through simple multiplication processing and FFT conversion. And compensating the target echo based on the estimated speed to obtain random frequency hopping echo sequences of different frequency bands. And then, realizing high-resolution range imaging of random frequency hopping echoes of different frequency bands by adopting a rearrangement zero-filling mode. And finally, combining an envelope alignment and initial phase correction method in the ISAR imaging process to finish high-resolution two-dimensional imaging of the double-base cooperative detection.
The method comprises the following concrete implementation steps:
step 1: establishing a double-radar collaborative detection echo model:
the radar 1 and the radar 2 both adopt random frequency hopping waveforms, and coherent processing pulse trains both compriseNSub-pulses having the same frequency hopping pattern and a pulse repetition period ofT r Pulse width ofT. Random modulation frequency distribution in given bandwidthBInner, minimum hop step size is Δf=B / NThe frequency hopping coefficient being a range
[0，N-1]Is one length ofNThe frequency hopping coefficient usually traverses the interval and is not repeated in order to ensure that the frequency band is fully utilized. Let the frequency hopping coefficients of radar 1 and radar 2 bec 1 (n) Andc 2 (n) Then, the emission signal models of the two radars are respectively expressed as follows:
wherein the content of the first and second substances,f 1 andf 2 respectively, the carrier frequencies of radar 1 and radar 2.
Taking dual radar cooperative detection as an example, in order to separate out signal components of different radars, carrier frequencies of 2 radarsf 1 Andf 2 the difference of (c) needs to be larger than the radar signal modulation bandwidth and then realized by a corresponding band-pass filter. Set object comprisesKA scattering center, initial time, secondkThe distance of each scattering center from the radar 1 isr 1k . The radial motion speed of the target isvFirst, ofnThe distance of the scattering center from the radar 1 at the sub-pulse transmission time can be expressed asr 1k (n)=r 1k +vnT r . When the radar 1 transmits radar 1 to receive, the firstnThe sub-pulse echoes after mixing can be represented as:
whereinKIs the number of scattering centers contained in the target,σ k denotes the firstkThe intensity of each scattering center. For a cooperative detection scene of a plurality of missile-borne radars, the difference of radar detection visual angles is small, and the intensity and the speed of any scattering center on a target are basically consistent with those of two radars. Setting an initial time, the firstkThe distance of each scattering center from the radar 2 isr 2k When the radar 2 transmits radar 2 for reception, the secondnThe sub-pulse echoes after mixing can be represented as:
let a deltar=r 2k -r 1k The electromagnetic wave transmission time corresponding to the difference value of the two radar distances is represented, and the formula can be rewritten as follows:
obtaining, in a similar manner, the echo received by the radar 1 emitted by the radar 2s 21 (n) Echo received by radar 1 transmitting radar 2s 12 (n) Respectively expressed as:
step 2: distance difference compensation:
comparing the formula (III) with the formula (VII), it can be known that the echo signals received by the same radar have phase modulation terms introduced by different distance time delays, and the two echoes are subjected to phase modulationThe phase difference is processed, and the phase modulation item is eliminated, so that the distance difference compensation is realized.s 12 (n) Can be composed ofs 11 (n) Represents:
whereinφ(n)=2πΔr(f 1 +c 1 (n)Δf)/c，φ(n) Can be calculated from the echo signal received by the radar and used fors 12 (n) The distance difference of (a) is compensated for,s 22 (n) Ands 21 (n) Similarly, the distance-compensated echoes can be respectively represented as:
it can be seen that, after the radar distance difference is compensated, the echoes emitted by different radars have the same change rule, so that the subsequent echo-based radar has the same change rules 11 (n) Ands 21 (n) And performing cooperative detection.
And 3, step 3: echo motion compensation:
it can be seen from the echo model that there is a coupling term between the frequency change and the distance change of the random frequency hopping waveform, which leads to a high-order phase term introduced into the echo, and the high-resolution range image obtained by direct FFT has a serious defocus, so that the target motion needs to be compensated. As can be seen by comparing the formula (IX) with the formula (X),s 11 (n) Ands 21 (n) The method has the advantages that the same distance time delay and different signal frequencies are achieved, and if the random frequency hopping modes of the two radars are reasonably designed, the influence of frequency modulation can be eliminated, so that the target motion parameters can be quickly estimated.
Two partsThe random frequency hopping modulation law of radar is shown in figure 3, whereinf 0 =(f 1 +f 2 )/2 ，c(n)=c 1 (n)+(f 1 -f 2 )/2，c 2 (n)=-c 1 (n) It can be seen from the figure that the random frequency hopping signals transmitted by the two radars have complementary frequency change rules, i.e. the frequency hopping coefficient of the pulse transmitted by the radar 1c 1 (n) The frequency hopping coefficient is equal to the frequency hopping coefficient of the pulse transmitted by the radar 2 and opposite to the frequency hopping coefficient. Can be obtained by substituting formula (IX) and formula (X):
the echo components are multiplied together, resulting in the following:
in the formula (I), the compound is shown in the specification,
is not contained innThe variables involved, therefore, are constant values.
Containing the variables thereinc(n)，s’(n) Pulse number of followernAnd (4) changing. Thus, it is possible to provides(n) From a single frequency signal componentS 0 And a variation components’(n) And (4) forming. After FFT, the energy of the single-frequency signal components is coherently accumulated to form a peak values’(n) Still in the defocused state, the sequence peak position target speed after FFT transformation is as follows:
whereink 0 The peak position. After the target velocity is obtained, the component related to the velocity component in the echo is eliminated, and the echo motion compensation can be performed on the formulas (XI) and (XII). The compensated echo can be expressed as:
it can be seen that formula (XVI) and formula (XVII) describe the detection of random frequency hopping signals of the target in different frequency bands. And subsequently carrying out high-resolution imaging on the coherent fusion of the random frequency hopping echoes of different frequency bands.
And 4, step 4: and (3) band fusion imaging:
and rearranging the random frequency hopping of different frequency bands, carrying out zero filling processing on the echo data on the defective frequency points, and enabling the echo signals of the two radars to carry out coherent fusion imaging through FFT (fast Fourier transform) by setting tight constraint conditions. The tight constraint is that the defective bandwidth is integral multiple of the frequency hopping step length. The re-zero-filling process is shown in fig. 4, the echo sequences of the formulas (XVI) and (XVII) are arranged from small to large according to the minimum frequency hopping step, and the obtained echo sequence is represented as:
whereinL=M+2NIn order to be the length of the echo sequence after zero padding,s all (n) Front ofNComponent ofs 21 (n) Results of rearrangement afterNComponent ofs 11 (n) The rearrangement result is effective observation, and the defect frequency band is [ 2 ]f 1 ，f 2 ]Setting tight constraint for ensuring coherence of two radar frequency hopping signalsConditions are as follows: defective bandwidthf 1 -f 2 For step size Δ of frequency hoppingfIs integer multiple of (1), the corresponding zero-padding frequency point number isM=(f 1 -f 2 )/Δf-1。
As can be seen,s all (n) The defective band with the zero padding position located in the middle can be obtaineds all (n) Has a range resolution and a continuous bandwidth of 2BThe echo sequences of (a) are consistent in performance. Specific derivation procedure H The following were used:
is provided withs Δ (n) Is band 2f 1 -B，f 2 +B]Sampling step size of deltafIs determined by the effective observation sequence of (a),s H (n) Is band 2f 1 -B，f 1 +B]Sampling step size of deltafIn whichn=0,1,...,L-1. Then the
s H (n)=h 1 (n)s Δ (n)
s all (n)=h 2 (n)s Δ (n)
Wherein
，
Is a window function. The corresponding FFT results are:
s H (n) Ands all (n) The FFT imaging results of (a) are:
wherein, the first and the second end of the pipe are connected with each other,
which represents a convolution of the circumference of a circle,S Δ (k) Representing full frequency band effective observation sequencess Δ (n) FFT imaging result of (1). The FFT conversion result of the rectangular window function is a sinc envelope with the same shapeH 1 (k) AndH 2 (k). Therefore, after cyclic convolution, the spread modulation performance of the distance image peak value is consistent.
For is tos all (n) The high-resolution range profile obtained by performing FFT can be expressed as:
therefore, a high-resolution range imaging result under the double-base cooperative detection can be obtained, and a two-dimensional high-resolution range image of the target can be obtained by further combining the envelope alignment and the initial phase correction algorithm.
In one embodiment of the invention, the simulated target ship point scattering model is composed of 367 scattering points as shown in figure 5, and the shipl=120m，b=30m. The radar 1 transmits a signal at a carrier frequency off 1 =15GHz, the carrier frequency of the radar 2 transmission signal isf 2 =15.255GHz, and the signal bandwidths are allB=128MHz，ΔfAnd =1MHz. Modulation law of radar 1 signalc 1 (n) In the interval of [0 ],N-1]is one length ofNIs used to generate the random integer sequence. Radar 2 signal modulation lawc 2 (n)=-c 1 (n). The geometric scene of the double-base cooperative detection is shown in figure 6, the ship is positioned at the origin of a coordinate system OXYZ, and the double-detection platform is arranged at the heightH=1.73km flying in the forward direction along the Y axis and having the same flying speedv r The distance between the radar 1 and the center of the ship is 10km at the initial moment, the distance between the radar 2 and the center of the ship is 10.017km, the target is in a maneuvering state, and the projection component of the moving speed on the sight line of the radar isv tLOS And =3m/s. Azimuth angle theta and pitch angle beta of radar sight line in target coordinate system at initial momentθ =80 degrees and β =10 degrees, respectively.
The complementary random frequency hopping waveform of the method of the invention is adopted to detect the target, and the target echo sequence separated by the band-pass filter is expressed as as 11 (t)，s 12 (t)，s 21 (t)，s 22 (t) }. The phase introduced by calculating the radar distance difference is shown in fig. 7 (a), and the theoretical calculation result of the phase is also shown in the figure. Therefore, the method can effectively eliminate the phase term introduced by the platform distance difference. FIG. 7 (b) showss(n) The target velocity estimated from the peak position is 3.099m/s as a result of the FFT in (2). Fig. 7 (c) and fig. 7 (d) respectively show respective range imaging results of two radars, and due to the limited bandwidth, scattering centers are distributed in a small number of range units, and thus less detailed target information can be obtained. The high-resolution range image fused by the method is shown in figure 7 (e), and the high-resolution range image with ideal target speed compensation is provided.
Radar two-dimensional imaging results obtained by respectively carrying out envelope alignment and initial Phase correction by using an envelope cross correlation method and a PGA (Phase Gradient auto focus) method are shown in figures 8 (a) -8 (d), wherein the figures 8 (a) and 8 (b) respectively show independent imaging results of a radar 1 and a radar 2, due to the limitation of distance resolution, targets are distributed in a distance direction of about 15 distance units, scattering center distribution is fuzzy, and target profiles are unclear. The result obtained by Using the fusion imaging mode is shown in fig. 8 (c), because of the improvement of the resolution, the distribution area of the target scattering center is larger, and occupies about 50 distance units, the provided target detail information is richer, and simultaneously the ISAR imaging result under ideal speed compensation is given, and the focusing Performance of the image is measured by the Contrast (see m. Marrorella, b. Haywood, f. Berizzi, and e. Dalle mean. Performance Analysis of an ISAR Contrast-Based automatically focusing Algorithm used Real Data [ J. IEEE ar reference. Pp.30-35, 2003), wherein the actually fused ISAR image Contrast is 16.38, and the ISAR image Contrast under ideal compensation is 17.26. Experimental results show that the method can effectively realize the double-base motion compensation and the fusion detection of the target.
Although the present invention has been described in terms of the preferred embodiment, it is not intended that the invention be limited to the embodiment. Any equivalent changes or modifications made without departing from the spirit and scope of the present invention are also within the protection scope of the present invention. The scope of protection of the invention should therefore be determined with reference to the claims of the present application.
Claims (3)
1. A bistatic radar collaborative imaging method based on complementary random waveforms is characterized by comprising the following steps:
step 1: establishing a double-radar cooperative detection echo model, wherein random frequency hopping waveforms are adopted by the radar 1 and the radar 2, and then the emission signal models of the two radars are respectively expressed as the following formulas (I) and (II):
wherein each coherent processing burst comprisesNSub-pulses having the same frequency hopping pattern and a pulse repetition period ofT r Pulse = pulse width ofTWith randomly modulated frequencies distributed over a given bandwidthBInner, minimum hop step size is Δf =B / NLet the frequency hopping coefficients of radar 1 and radar 2 bec 1 (n) Andc 2 (n)，f 1 andf 2 respectively the carrier frequencies of radar 1 and radar 2,
when the radar 1 transmits radar 1 to receive, the firstnThe sub-pulse echoes after mixing can be expressed as follows（III）：
Wherein the target comprisesKA scattering center, initial time, firstkThe distance of each scattering center from the radar 1 isr 1k The target radial motion speed isvFirst, ofnThe distance between the scattering center and the radar 1 at the sub-pulse transmission time can be expressed asr 1k (n)=r 1k +vnT r ，σ k Denotes the firstkThe intensity of the individual scattering centers is such that,
when the radar 2 transmits radar 2 to receivenThe sub-pulse echoes after mixing can be expressed as the following formula (V):
wherein, deltar=r 2k -r 1k Of 1 atkThe distance of each scattering center from the radar 2 isr 2k ，
The radar 2 emits an echo received by the radar 1s 21 (n) Echo received by radar 1 transmitting radar 2s 12 (n) Expressed as the following formulas (VI) and (VII), respectively:
step 2: distance difference compensation is performed, a phase modulation term of a phase difference of echo signals received by the radar 1 and the radar 2 is eliminated, distance difference compensation is achieved, and echoes after distance difference compensation can be expressed as the following formulas (IX) and (X) respectively:
and 3, step 3: performing echo motion compensation, eliminating the component related to the velocity component in the echo to realize the echo motion compensation, wherein the echo after the echo motion compensation can be respectively expressed as the following formulas (XVI) and (XVII),
wherein the content of the first and second substances, f 0 =(f 1 +f 2 )/2 ，c(n)=c 1 (n)+(f 1 -f 2 )/2，c 2 (n)=-c 1 (n)；
and 4, step 4: and performing frequency band fusion imaging, rearranging random frequency hopping of different frequency bands, performing zero filling processing on echo data on the defective frequency points, and performing coherent fusion imaging on echo signals of the two radars through FFT (fast Fourier transform) by setting tight constraint conditions.
2. The cooperative imaging method for bistatic radar based on complementary random waveforms of claim 1, wherein the echo motion compensation in step 3 is to set the frequency hopping coefficient of the transmitted pulse of radar 1c 1 (n) The frequency hopping coefficient of the pulse transmitted by the radar 2 is equal in size and opposite in direction, whereinf 0 =（f 1 +f 2 ）/2 ，c(n)=c 1 (n)+(f 1 -f 2 )/2，c 2 (n)=-c 1 (n) Substituting formula (IX) and formula (X) may give:
the echo components are multiplied together, resulting in the following:
the target speed of the sequence peak position after FFT is as follows (XIV):
whereink 0 After the target velocity is obtained for the peak position, the component related to the velocity component in the echo is eliminated, and the echo motion compensation is performed on the equations (XI) and (XII).
3. The cooperative imaging method for bistatic radar based on complementary random waveforms according to claim 1, wherein the specific method for performing band fusion imaging in step 4 is to rank the echo sequences of formulas (XVI) and (XVII) from small to large according to a minimum frequency hopping step, and the obtained echo sequence is represented as:
wherein the content of the first and second substances,L=M+2Nin order to be the length of the echo sequence after zero padding,s all (n) Front of (2)NComponent ofs 21 (n) Results of rearrangementNComponent ofs 11 (n) The rearrangement result is effective observation, and the defect frequency band is [ 2 ]f 1 ，f 2 ]Defective bandwidthf 1 -f 2 For the step size Δ of frequency hoppingfIs integer multiple of (1), the corresponding zero-padding frequency point number isM=(f 1 -f 2 )/Δf-1, tos all (n) The high-resolution range profile obtained by performing FFT can be expressed as:
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