CN112731329A - Method and system for improving isolation degree of long-distance echo and short-distance clutter of harmonic radar - Google Patents

Method and system for improving isolation degree of long-distance echo and short-distance clutter of harmonic radar Download PDF

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CN112731329A
CN112731329A CN202011601933.8A CN202011601933A CN112731329A CN 112731329 A CN112731329 A CN 112731329A CN 202011601933 A CN202011601933 A CN 202011601933A CN 112731329 A CN112731329 A CN 112731329A
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harmonic
sequence
distance
clutter
frequency
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CN112731329B (en
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雷杰
罗朝鹏
姜和俊
孙绪仁
吴美武
唐云峰
张勇
瞿诗华
武思瑶
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Shanghai Institute of Microwave Technology CETC 50 Research Institute
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
    • G01S7/41—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00 using analysis of echo signal for target characterisation; Target signature; Target cross-section
    • G01S7/414—Discriminating targets with respect to background clutter

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Abstract

The invention provides a method and a system for improving the isolation degree of long-distance echoes and short-distance clutter of a harmonic radar, which comprises the steps of adopting frequency and time coding modulation, forming excitation signals according to a designed code element sequence, amplifying and transmitting; mixing and filtering the harmonic wave receiving signal by using a reference signal, inhibiting carrier frequency and digitizing to obtain a vector reflecting a target distance by the harmonic wave channel; restoring the phase sequence of the elements in the vector according to the frequency sequence corresponding to the designed code element sequence, and estimating the power spectrum; weighting according to the power spectrum, and adopting a larger weight value for the pre-polymerization points which exceed the clutter map more; obtaining target information of the harmonic channel through power weighted target cluster aggregation; and respectively processing each subharmonic to finally obtain target multi-harmonic information. The method and the system for improving the isolation degree of the long-distance echo and the short-distance clutter of the harmonic radar greatly improve the isolation degree of the long-distance target echo and the short-distance clutter of the harmonic radar.

Description

Method and system for improving isolation degree of long-distance echo and short-distance clutter of harmonic radar
Technical Field
The invention relates to the technical field of near-field nonlinear target harmonic radar detection, in particular to a method and a system for improving the isolation degree of long-distance echoes and short-distance clutter of a harmonic radar.
Background
The harmonic radar firstly transmits a specially designed waveform to irradiate a nonlinear target, and then detects the characteristics of reradiated harmonic signals of the nonlinear target after the nonlinear target is excited by the waveform, so that the targets with internal nonlinear characteristics, such as semiconductor nodes, metal nodes and the like, can be detected and positioned. Compared with the traditional linear radar, the harmonic radar has excellent linear clutter resistance. Can be widely applied to the fields of national defense, security inspection, public security, traffic, urban construction and the like.
The echo of a long-distance target received by the harmonic radar must have a certain degree of isolation from the short-distance clutter so as to separate the higher harmonic echo from the target at a longer distance from the short-distance clutter. Because the echo intensity of the target harmonic wave is rapidly attenuated along with the increase of the distance, taking a second harmonic wave radar as an example, under the condition of the same transmitting power, the signal-to-noise ratio of a receiving signal of the harmonic wave radar is in an inverse proportion to the sixth power of the distance, and therefore, the nonlinear target harmonic wave echo at a long distance is weak. While near the harmonic radar antenna, nonlinear objects exist, introducing clutter of the same frequency as the harmonic signal. In order to distinguish a target from a clutter, a harmonic radar has a very urgent need for improving the isolation between a long-distance target echo and a short-distance clutter in the application of a long-distance detection scene.
The time-sharing technology of transceiving is the most traditional method for improving the isolation degree of radar long-distance target echo and short-distance clutter, and the method respectively transmits and receives electromagnetic waves in different time windows. Because the electromagnetic wave signal is not transmitted in the receiving time window, the isolation between the long-distance target echo and the short-distance clutter is improved to a certain extent.
In the application of harmonic radar, the above prior art has the following disadvantages:
1. when the time-sharing technology is applied to harmonic radar, the nonlinear target detection within a range of tens of meters is difficult to realize due to the constraint of the transmitting-receiving conversion speed. The propagation speed of electromagnetic waves in the air is close to 30 ten thousand kilometers per hour, the 1 microsecond two-way delay is about 150 meters, and in order to realize the transceiving time-sharing work within the range of tens of meters, the conversion of transmitting and receiving needs to be realized in picosecond magnitude, which has great difficulty in engineering, thereby limiting the application of the technology;
2. when the transceiving time-sharing technology is applied to a harmonic radar, the receiver is in a non-receiving state during transmitting, so that strong target detection in a distance corresponding to a transmitting time window cannot be realized, a distance blind area is generated, and super-clutter detection on a strong target echo signal in a short-distance range cannot be realized.
In view of the above-mentioned related technologies, the inventor considers that the method for improving the isolation between the long-range target echo and the short-range clutter by using the traditional transmit-receive time-sharing technology has poor applicability in the field of harmonic radar, and therefore, a technical solution needs to be provided to improve the above technical problems.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide a method and a system for improving the isolation degree of long-distance echoes and short-distance clutter of a harmonic radar.
The method for improving the isolation degree of the long-distance echo and the short-distance clutter of the harmonic radar comprises the following steps:
step 1: adopting frequency and time coding modulation, selecting a group of code elements from a code element set to form an excitation signal according to a design sequence without repetition, and amplifying and transmitting the excitation signal;
step 2: the reference signal recovery and the transmission of the harmonic receiving channel are carried out simultaneously, and the excitation reference signal of the harmonic channel is recovered from the code element set;
and step 3: carrying out frequency mixing and low-pass filtering processing on the received harmonic signals by using the harmonic reference signals obtained in the step (2), and after carrier frequency suppression and digitization through frequency mixing and filtering, obtaining phase signals reflecting signal distances on the harmonic channels;
and 4, step 4: rearranging the reset phase signal sequence obtained in the step (3) according to the element values in the sequence number vector and carrying out power spectrum estimation to obtain a power spectrum sequence of the harmonic wave receiving channel;
and 5: obtaining a clutter map without a target in the environment learning stage for the power spectrum sequence obtained in the step 4, and obtaining a power spectrum sequence exceeding the clutter map in the target detection stage, wherein the sequence reflects the distance factor of a signal in the pre-polymerization stage;
step 6: performing power weighting target cluster aggregation on the elements in the power spectrum sequence exceeding the clutter map obtained in the step 5, wherein the power weighting is performed according to the element power values in the sequence;
and 7: and (4) sequentially processing the higher harmonic channel according to the steps 2 to 6 to obtain the target information of the multi-harmonic channel.
Preferably, the step 1 adopts frequency and time coding modulation, and the single emission pulse period is TrThe width of a single code element is tau, and the frequency of the code element set is fLT，fLT+(N-1)Δf]，fLTRepresenting the frequency of an initial code element set, wherein the minimum frequency difference between any two frequency points in the code element set is delta f, and the number of code elements is N; selecting a code element from the code element set according to a pre-designed sequence without repetition at intervals of tau within the time range from 0 to (N-1) tau, amplifying and transmitting, wherein the sequence number of the transmitted code element sequence is [ I ] in sequence0 I1 … IN-1]In which In∈[0，N-1]，n＝[0 1 … N-1]The frequency of the corresponding symbol sequence can be expressed as
Figure BDA0002869029280000037
Wherein
Figure BDA0002869029280000038
Figure BDA0002869029280000039
Preferably, the reference signal recovery and the transmission of the harmonic receiving channel in the step 2 are performed simultaneously according to the symbol sequence [ I0 I1 … IN-1]And sequentially recovering the transmission reference signals of the harmonic channels from the symbol set.
Preferably, the step 3 sequentially performs frequency mixing and low-pass filtering on the received harmonic signal by using the harmonic reference signal obtained in the step 2, a complex phase value is obtained within each symbol duration of the processed kth harmonic channel, and consecutive N symbols are arranged to form a complex phase vector; because the received target echo and clutter signals contain the two-way time delay 2R/c of the electromagnetic wave transmission process, wherein R represents the one-way slant distance of the electromagnetic wave back-and-forth receiving and transmitting link, and c represents the transmission speed of the electromagnetic wave, after the carrier frequency is suppressed by frequency mixing and filtering, the harmonic channel obtains the phase signal related to the echo distance, and on the kth harmonic channel, the complex phase signal sequence is recorded as
Figure BDA0002869029280000031
Preferably, the step 4 comprises:
step 4.1: construct column vector E ═ {1, Ejω,…,ejω(N-1)}TIn which ω is taken
Figure BDA0002869029280000032
Pi is a circumferential rate constant;
step 4.2: calculating N multiple phase signal sequences phik(n) autocorrelation matrix:
Figure BDA0002869029280000033
where r (i) is the autocorrelation matrix of the input phase sequence:
Figure BDA0002869029280000034
step 4.3: calculating N complex phase signal sequences phi of the kth channelkPower spectrum of (n):
Figure BDA0002869029280000035
preferably, the step 5 is to obtain the power spectrum sequence P of the harmonic channel obtained in the step 4k(m) estimating clutter maps C without targets in the environmental learning stage respectivelyk(M) comparing the clutter maps in the signal detection stage to obtain a power estimation vector of the difference between the signal exceeding the clutter maps and the clutter maps, and recording the power estimation vector as Mk(P, m), where P represents the value of an element in the sequence and m represents the power spectral vector P of the element in the sequencek(m) the number, which reflects the distance factor of the signal during the prepolymerization stage.
Preferably, said step 6 is on the vector MkPerforming power weighted target cluster aggregation on elements in (p, m), and performing power weighting Wk(p) is based primarily on the vector MkIn the (p, m) element power values, a larger weight is adopted for pre-polymerization points which exceed the clutter map more, and correspondingly, a smaller weight is adopted for pre-polymerization points which exceed the clutter map less; the vector to be aggregated after power weighting is recorded as
Figure BDA0002869029280000036
The target cluster aggregation is mainly carried out according to Euclidean distances among elements to be aggregated in the vector, and the distance vector of each point to be aggregated to other points is Dj＝{d1,...,dN-1In which d isi＝nr-nqAnd r ≠ q, j ∈ e[0,1,…,N-1](ii) a One cluster can be represented as Ic＝{Ii,...,Ii+VAt a distance that must satisfy di≤ε,...,dV≦ ε, the choice of ε is determined primarily by the resolution of the power spectrum, with the higher the resolution of the power spectrum, the smaller ε. And obtaining target information of a k-th harmonic receiving channel through power weighted target cluster aggregation.
Preferably, the step 7 sequentially processes K higher harmonic channels in total, which are K e [1,2, … K ], according to the steps 2 to 6, so as to obtain power weighted target cluster aggregation information of K groups of harmonic channels.
The invention also provides a system for improving the isolation degree of the long-distance echo and the short-distance clutter of the harmonic radar, which is characterized by comprising the following modules:
module M1: adopting frequency and time coding modulation, selecting code elements from a code element set according to a design sequence to form an excitation signal for amplification and transmission;
module M2: the reference signal recovery and the transmission of the harmonic receiving channel are carried out simultaneously, and the transmission reference signal of the harmonic channel is recovered from the code element set;
module M3: the harmonic reference signal obtained by the module M2 for the received harmonic signal is sequentially subjected to frequency mixing and low-pass filtering, and after carrier frequency suppression and digitization through frequency mixing and filtering, the harmonic channel obtains a phase signal reflecting the signal distance;
module M4: rearranging the reset phase signal sequence obtained by the module M4 according to element values in the sequence number vector, and estimating a power spectrum to obtain the power spectrum of each frequency point in the harmonic wave receiving channel;
module M5: for the harmonic power spectrum sequence obtained by the module M4, obtaining a clutter map without a target in the environment learning stage, and obtaining a signal exceeding the clutter map in the target detection stage, wherein the factor reflects the distance factor of the signal in the pre-polymerization stage;
module M6: performing power weighting target cluster aggregation on elements in the signal vector exceeding the clutter map obtained by the module M5, wherein the power weighting is performed according to the element power value in the vector;
module M7: and sequentially processing the higher harmonic channels by adopting modules 2 to 6 to obtain power weighted target cluster aggregation information of each harmonic channel.
Preferably, said module M1 uses frequency and time coding modulation, and the single emission pulse period is TrThe width of a single code element is tau, and the frequency of the code element set is fLT，fLT+(N-1)Δf]，fLTRepresenting the frequency of an initial code element set, wherein the minimum frequency difference between any two frequency points in the code element set is delta f, and the number of code elements is N; selecting a code element from the code element set according to a pre-designed sequence without repetition at intervals of tau within the time range from 0 to (N-1) tau, amplifying and transmitting, wherein the sequence number of the transmitted code element sequence is [ I ] in sequence0 I1 … IN-1]In which In∈[0，N-1]，n＝[0 1 … N-1]The frequency of the corresponding symbol sequence can be expressed as
Figure BDA0002869029280000041
Wherein
Figure BDA0002869029280000042
Compared with the prior art, the invention has the following beneficial effects:
1. the isolation between the long-distance target echo and the short-distance clutter of the harmonic radar is greatly improved;
2. the method has the super clutter detection function of the near-distance stronger target echo signal;
3. the power weighted target cluster aggregation technology is adopted to replace the traditional method for calculating the target position by hard decision after constant false alarm, so that the detection probability is improved while the constant false alarm is kept.
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Other features, objects and advantages of the invention will become more apparent upon reading of the detailed description of non-limiting embodiments with reference to the following drawings:
FIG. 1 is a block diagram of the system of the present invention;
FIG. 2 is a schematic diagram of an implementation method for improving the isolation between the long-distance target echo and the short-distance clutter of the harmonic radar based on frequency time coding according to the present invention.
Detailed Description
The present invention will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the invention, but are not intended to limit the invention in any way. It should be noted that it would be obvious to those skilled in the art that various changes and modifications can be made without departing from the spirit of the invention. All falling within the scope of the present invention.
Referring to fig. 1 and 2, the present invention mainly transmits a frequency-time coding waveform to improve the isolation between the long-distance target echo and the short-distance clutter of the harmonic radar, and meanwhile, the present invention has the function of detecting the super-clutter of the short-distance stronger target echo signal, and improves the detection probability while maintaining the constant false alarm through the power weighted target cluster aggregation technology. The core content of the invention is mainly the waveform capable of improving the isolation degree of the long-distance target echo and the short-distance clutter and the processing method thereof. The method mainly comprises a frequency-time coding waveform generation module (I), a receiving preprocessing module (II), a power spectrum estimation module (II), a clutter suppression module (II), a power weighted target cluster aggregation module (II) and the like.
The frequency-time coding waveform (I) is mainly designed for realizing high isolation of a long-distance target echo and a short-distance clutter in a harmonic radar, and mainly adopts the following steps:
step 1: in N is equal to [ 0N-1 ]]Randomly without repetition, on a uniformly distributed probability density over a set of [ I ]0 I1 … IN-1]In which In∈[0 N-1]，n＝[0 1 … N-1]。
Step 2: encoding a set of modulation symbols [ f ] in frequency and timeLT fLT+(N-1)Δf]Wherein, a symbol is selected for amplification and transmission according to the sequence generated in step 1, and the frequency of the corresponding symbol sequence can be expressed as
Figure BDA0002869029280000051
Wherein
Figure BDA0002869029280000052
The minimum frequency difference between any two frequency points in the code element set is delta f, the number of the code elements is N, and the single emission pulse period is TrThe single symbol width is τ.
The receiving preprocessing mainly completes the functions of harmonic reference signal generation, frequency mixing and filtering, received signal recovery, power spectrum estimation, signal search and aggregation and the like, and mainly adopts the following steps:
step 1: k sets of local harmonic reference signals are generated from the local transmit signal. The signal and the sequence of transmitted symbols [ I ]0 I1… IN-1]In association, the reference signal for the kth harmonic channel may be written as
Figure BDA0002869029280000061
Figure BDA0002869029280000062
Wherein the argument t of rect (t/τ) is [ - τ/2 [ - τ/2 [ ]]The value in the range is 1, the value in the other ranges is 0, and K belongs to [1,2, … K ]]。
Step 2: mixing and filtering the received harmonic signals by taking local harmonics as reference, and digitizing to convert the received higher harmonic signals into corresponding multichannel baseband digital signal sequences;
and step 3: in K receiving channels, in turn
Figure BDA0002869029280000063
The N multiple phase signal sequences are based on the sequence number vector [ I0 I1… IN-1]The values of the middle elements are rearranged in an ascending or descending order in sequence, and the result is recorded as phik(n)；
And 4, step 4: signal vector phi perceived for K harmonic receive channelsk(n) sequentially performing high resolution power spectrum estimation. Obtaining the signal power P of each harmonic channelk(n)＝{P1,P2,…,PN}。
Wherein, the power spectrum estimation in step 4 comprises the following steps:
step 4.1: construct a column vector E of 1,ejω,…,ejω(N-1)}Tin which ω is taken
Figure BDA0002869029280000064
And pi is a circumferential rate constant.
Step 4.2: calculating N multiple phase signal sequences phik(n) autocorrelation matrix:
Figure BDA0002869029280000065
where r (i) is the autocorrelation matrix of the input phase sequence:
Figure BDA0002869029280000066
step 4.3: calculating N complex phase signal sequences phi of the kth channelkPower spectrum of (n):
Figure BDA0002869029280000067
clutter suppression (II) is mainly performed by the harmonic power spectrum sequence Pk(m) performing an analysis to suppress clutter components therein and to implement power weighting. Specifically, the method comprises the following steps:
step 1: estimating a vector P for a sequence of harmonic power spectra during an environmental learning phasek(m) analyzing to obtain clutter map C without targetk(m)；
Step 2: and comparing the clutter maps in the target detection stage to obtain a power estimation vector of the difference value between the signal exceeding the clutter maps and the clutter maps, and recording the power estimation vector as Mk(P, m), where P represents the value of an element in the sequence and m represents the power spectral vector P of the element in the sequencek(m) the number, which reflects the distance factor of the signal during the prepolymerization stage;
the power weighted target cluster aggregation (II) mainly completes the power weighted target cluster aggregation from the perceived high-resolution power spectrum of the baseband signal of the harmonic receiving channel, and specifically comprises the following steps:
step 1: for vector MkPerforming power weighted target cluster aggregation on elements in (p, m), and performing power weighting Wk(p) is based primarily on the vector MkAnd (p, m) using a larger weight for the pre-polymerization points which are more beyond the clutter map, and correspondingly using a smaller weight for the pre-polymerization points which are less beyond the clutter map. The vector to be aggregated after power weighting is recorded as
Figure BDA0002869029280000071
Step 2: the target cluster aggregation is mainly carried out according to Euclidean distances among elements to be aggregated in the vector, and the distance vector of each point to be aggregated to other points is Dj＝{d1,...,dN-1In which d isi＝nr-nqAnd r ≠ q, j ∈ [0,1, …, N-1]. One cluster can be represented as Ic＝{Ii,...,Ii+VAt a distance that must satisfy di≤ε,...,dVEpsilon is selected mainly according to the resolution ratio of a power spectrum, the higher the resolution ratio of the power spectrum is, the smaller epsilon is, target information of a k-th harmonic receiving channel is obtained through power weighted target cluster aggregation, and after the target cluster aggregation, a received signal becomes a target aggregation cluster.
The invention also provides a system for improving the isolation degree of the long-distance echo and the short-distance clutter of the harmonic radar, which comprises the following modules:
module M1: selecting a symbol from the symbol set according to the sequence without repetition for amplification and transmission by adopting frequency and time coding modulation;
module M2: the reference signal recovery and the transmission of the harmonic receiving channel are carried out simultaneously, and the transmission reference signal of the harmonic channel is recovered from the code element set;
module M3: the harmonic reference signal obtained by the module M2 for the received harmonic signal is sequentially subjected to frequency mixing and low-pass filtering, and after carrier frequency suppression and digitization through frequency mixing and filtering, the harmonic channel obtains a phase signal sequence related to the signal distance;
module M4: rearranging the reset phase signal sequence obtained by the module M3 according to the element values in the sequence number vector of the reset phase signal sequence and estimating the power spectrum to obtain the power spectrum of each frequency point in the harmonic wave receiving channel;
module M5: for the power spectrum sequence of the harmonic channel obtained by the module M4, obtaining a clutter map without a target in the environment learning stage, and obtaining a signal exceeding the clutter map in the signal detection stage, wherein the factor reflects the distance factor of the signal in the pre-polymerization stage;
module M6: performing power weighting target cluster aggregation on elements in the vector, wherein the power weighting is performed according to the element power value in the vector;
module M7: and sequentially processing the higher harmonic channel control modules 2 to 6 to obtain multi-harmonic target information.
The module M1 adopts frequency and time coding modulation, and the single emission pulse period is set as TrThe width of a single code element is tau, and the frequency of the code element set is fLT，fLT+(N-1)Δf]，fLTRepresenting the frequency of an initial code element set, wherein the minimum frequency difference between any two frequency points in the code element set is delta f, and the number of code elements is N; selecting a code element from the code element set according to a pre-designed sequence without repetition at intervals of tau within the time range from 0 to (N-1) tau, amplifying and transmitting, wherein the sequence number of the transmitted code element sequence is [ I ] in sequence0 I1 … IN-1]In which In∈[0，N-1]，n＝[0 1 … N-1]The frequency of the corresponding symbol sequence can be expressed as
Figure BDA0002869029280000081
Wherein
Figure BDA0002869029280000082
Figure BDA0002869029280000083
The method for improving the isolation degree of the long-distance target echo and the short-distance clutter of the harmonic radar greatly improves the isolation degree of the long-distance target echo and the short-distance clutter of the harmonic radar; the method has the super clutter detection function of the near-distance stronger target echo signal; the power weighted target cluster aggregation technology is adopted to replace the traditional method for calculating the target position by hard decision after constant false alarm, so that the detection probability is improved while the constant false alarm is kept.
Those skilled in the art will appreciate that, in addition to implementing the system and its various devices, modules, units provided by the present invention as pure computer readable program code, the system and its various devices, modules, units provided by the present invention can be fully implemented by logically programming method steps in the form of logic gates, switches, application specific integrated circuits, programmable logic controllers, embedded microcontrollers and the like. Therefore, the system and various devices, modules and units thereof provided by the invention can be regarded as a hardware component, and the devices, modules and units included in the system for realizing various functions can also be regarded as structures in the hardware component; means, modules, units for performing the various functions may also be regarded as structures within both software modules and hardware components for performing the method.
The foregoing description of specific embodiments of the present invention has been presented. It is to be understood that the present invention is not limited to the specific embodiments described above, and that various changes or modifications may be made by one skilled in the art within the scope of the appended claims without departing from the spirit of the invention. The embodiments and features of the embodiments of the present application may be combined with each other arbitrarily without conflict.

Claims (10)

1. A method for improving the isolation degree of long-distance echoes and short-distance clutter of a harmonic radar is characterized by comprising the following steps:
step 1: adopting frequency and time coding modulation, selecting a group of code elements from a code element set to form an excitation signal according to a design sequence without repetition, and amplifying and transmitting the excitation signal;
step 2: the reference signal recovery and the transmission of the harmonic receiving channel are carried out simultaneously, and the excitation reference signal of the harmonic channel is recovered from the code element set;
and step 3: carrying out frequency mixing and low-pass filtering processing on the received harmonic signals by using the harmonic reference signals obtained in the step (2), and after carrier frequency suppression and digitization through frequency mixing and filtering, obtaining phase signals reflecting signal distances on the harmonic channels;
and 4, step 4: rearranging the reset phase signal sequence obtained in the step (3) according to the element values in the sequence number vector and carrying out power spectrum estimation to obtain a power spectrum sequence of the harmonic wave receiving channel;
and 5: obtaining a clutter map without a target in the environment learning stage for the power spectrum sequence obtained in the step 4, and obtaining a power spectrum sequence exceeding the clutter map in the target detection stage, wherein the sequence reflects the distance factor of a signal in the pre-polymerization stage;
step 6: performing power weighting target cluster aggregation on the elements in the power spectrum sequence exceeding the clutter map obtained in the step 5, wherein the power weighting is performed according to the element power values in the sequence;
and 7: and (4) sequentially processing the higher harmonic channel according to the steps 2 to 6 to obtain the target information of the multi-harmonic channel.
2. The method according to claim 1, wherein the step 1 employs frequency and time coded modulation, and the single transmission pulse period is TrThe width of a single code element is tau, and the frequency of the code element set is fLT，fLT+(N-1)Δf]，fLrRepresenting the frequency of an initial code element set, wherein the minimum frequency difference between any two frequency points in the code element set is delta f, and the number of code elements is N; selecting a code element from the code element set according to a pre-designed sequence without repetition at intervals of tau within the time range from 0 to (N-1) tau, amplifying and transmitting, wherein the sequence number of the transmitted code element sequence is [ I ] in sequence0 I1 … IN-1]In which In∈[0，N-1]，n＝[0 1 … N-1]The frequency of the corresponding symbol sequence can be expressed as
Figure FDA0002869029270000011
Wherein
Figure FDA0002869029270000012
3. The method of claim 1, wherein the step 2 of recovering the reference signal of the harmonic receiving channel is performed simultaneously with the step of transmitting the reference signal according to the symbol sequence [ I [ ]0 I1 … IN-1]And sequentially recovering the transmission reference signals of the harmonic channels from the symbol set.
4. The method according to claim 1, wherein the step 3 sequentially performs frequency mixing and low-pass filtering on the received harmonic signal by using the harmonic reference signal obtained in the step 2, and obtains a complex phase value in each symbol duration of the kth harmonic channel after the processing, and the consecutive N symbols are arranged into a complex phase vector; because the received target echo and clutter signals contain the two-way time delay 2R/c of the electromagnetic wave transmission process, wherein R represents the one-way slant distance of the electromagnetic wave back-and-forth receiving and transmitting link, and c represents the transmission speed of the electromagnetic wave, after the carrier frequency is suppressed by frequency mixing and filtering, the harmonic channel obtains the phase signal related to the echo distance, and on the kth harmonic channel, the complex phase signal sequence is recorded as
Figure FDA0002869029270000026
5. The method for improving the isolation between the far-range echo and the near-range clutter of the harmonic radar according to claim 1, wherein the step 4 comprises:
step 4.1: construct column vector E ═ {1, Ejω，…，ejω(N-1)}TIn which ω is taken
Figure FDA0002869029270000021
Pi is a circumferenceA rate constant;
step 4.2: calculating N multiple phase signal sequences phik(n) autocorrelation matrix:
Figure FDA0002869029270000022
where r (i) is the autocorrelation matrix of the input phase sequence:
Figure FDA0002869029270000023
step 4.3: calculating N complex phase signal sequences phi of the kth channelkPower spectrum of (n):
Figure FDA0002869029270000024
6. the method for improving the isolation between the far-range echo and the near-range clutter of the harmonic radar according to claim 1, wherein the step 5 is performed on the power spectrum sequence P of the harmonic channel obtained in the step 4k(m) estimating clutter maps C without targets in the environmental learning stage respectivelyk(M) comparing the clutter maps in the signal detection stage to obtain a power estimation vector of the difference between the signal exceeding the clutter maps and the clutter maps, and recording the power estimation vector as Mk(P, m), where P represents the value of an element in the sequence and m represents the power spectral vector P of the element in the sequencek(m) the number, which reflects the distance factor of the signal during the prepolymerization stage.
7. The method of claim 1, wherein the step 6 is performed on a vector MkPerforming power weighted target cluster aggregation on elements in (p, M), and performing power weighting Mk(p) is based primarily on the vector MkElement power values in (p, m) for out of clutter mapsMore pre-polymerization points adopt larger weight values, and correspondingly, the pre-polymerization points which exceed the clutter map and are less adopt smaller weight values; the vector to be aggregated after power weighting is recorded as
Figure FDA0002869029270000025
The target cluster aggregation is mainly carried out according to Euclidean distances among elements to be aggregated in the vector, and the distance vector of each point to be aggregated to other points is Dj＝{d1，...，dN-1In which d isi＝nr-nqAnd r ≠ q, j ∈ [0,1, …, N-1](ii) a One cluster can be represented as Ic＝{Ii，...，Ii+VAt a distance that must satisfy di≤ε，...，dV≦ ε, the choice of ε is determined primarily by the resolution of the power spectrum, with the higher the resolution of the power spectrum, the smaller ε. And obtaining target information of a k-th harmonic receiving channel through power weighted target cluster aggregation.
8. The method for improving the isolation degree of the far-distance echo and the near-distance clutter of the harmonic radar according to claim 1, wherein the step 7 sequentially processes K higher harmonic channels in total of K e [1,2, … K ] according to the steps 2 to 6 to obtain power weighted target cluster aggregation information of the K groups of harmonic channels.
9. The utility model provides a system for improve distant echo of harmonic radar and closely clutter isolation degree which characterized in that includes following module:
module M1: adopting frequency and time coding modulation, selecting code elements from a code element set according to a design sequence to form an excitation signal for amplification and transmission;
module M2: the reference signal recovery and the transmission of the harmonic receiving channel are carried out simultaneously, and the transmission reference signal of the harmonic channel is recovered from the code element set;
module M3: the harmonic reference signal obtained by the module M2 for the received harmonic signal is sequentially subjected to frequency mixing and low-pass filtering, and after carrier frequency suppression and digitization through frequency mixing and filtering, the harmonic channel obtains a phase signal reflecting the signal distance;
module M4: rearranging the reset phase signal sequence obtained by the module M4 according to element values in the sequence number vector, and estimating a power spectrum to obtain the power spectrum of each frequency point in the harmonic wave receiving channel;
module M5: for the harmonic power spectrum sequence obtained by the module M4, obtaining a clutter map without a target in the environment learning stage, and obtaining a signal exceeding the clutter map in the target detection stage, wherein the factor reflects the distance factor of the signal in the pre-polymerization stage;
module M6: performing power weighting target cluster aggregation on elements in the signal vector exceeding the clutter map obtained by the module M5, wherein the power weighting is performed according to the element power value in the vector;
module M7: and sequentially processing the higher harmonic channels by adopting modules 2 to 6 to obtain power weighted target cluster aggregation information of each harmonic channel.
10. The system according to claim 9, wherein the module M1 employs frequency and time coded modulation, and the single transmission pulse period is TrThe width of a single code element is tau, and the frequency of the code element set is fLT，fLT+(N-1)Δf]，fLrRepresenting the frequency of an initial code element set, wherein the minimum frequency difference between any two frequency points in the code element set is delta f, and the number of code elements is N; selecting a code element from the code element set according to a pre-designed sequence without repetition at intervals of tau within the time range from 0 to (N-1) tau, amplifying and transmitting, wherein the sequence number of the transmitted code element sequence is [ I ] in sequence0 I1 … IN-1]In which In∈[0，N-1]，n＝[0 1 … N-1]The frequency of the corresponding symbol sequence can be expressed as
Figure FDA0002869029270000041
Wherein
Figure FDA0002869029270000042
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