1. A constant false alarm detection method for an external radiation source radar self-adaptive time-sharing clutter map is characterized by comprising the following steps:

step 1, performing clutter interval division based on a radar range-Doppler spectrum, and dividing a clutter unit from an airspace according to a distance unit and a Doppler unit;

step 2, establishing a two-dimensional clutter matrix according to clutter interval division, wherein each clutter unit is a matrix element and is a unit to be detected;

step 3, initializing a clutter matrix;

step 4, firstly deducing whether the clutter map has a constant false alarm characteristic, and then carrying out constant false alarm detection based on the clutter map, wherein the detection is to process and compare the sampling value of the radar at each unit with the value stored in the clutter map, if the sampling value is higher than the processed detection threshold value, the target is determined to be present, otherwise, the target is determined not to be present;

and step 5, establishing a forgetting factor for updating the clutter map according to time, and updating and iterating the clutter map according to a corresponding rule of the current scanning echo sampling value to form a latest clutter map, so that the clutter map can carry out self-adaptive updating and iterating along with time, and the false alarm performance superior to that of an airspace CFAR can be exerted under the detection condition that the airspace environment is more complex.

2. The adaptive time-sharing clutter map constant false alarm detection method of the external radiation source radar according to claim 1, characterized in that: the specific implementation of step 1 is as follows,

if the maximum detection distance radius of the radar is R_mThe clutter map is divided into a plurality of clutter units in two dimensions of distance and azimuth, and the distance size and the azimuth size of each clutter unit are respectively delta R_cAnd Δ a, each clutter map unit is divided into M × N resolution units, the range resolution and the azimuth resolution of the resolution units are Δ R and Δ θ, respectively, then there are:

the data in a certain clutter unit q is obtained by two-dimensional averaging of all resolution units in the current unit:

x in the above formula_m,nThe clutter unit storage value is updated together with the historical clutter storage unit value.

3. The adaptive time-sharing clutter map constant false alarm detection method of the external radiation source radar according to claim 1, characterized in that: the specific implementation manner of the step 2 is as follows;

taking each clutter unit divided by the clutter map as a unit d to be detected_iEach unit to be detected has a buffer space-ordered array G special for buffering its historical sampling data_iThe buffer space has a length of N and the data therein are maintained in an ordered arrangement from small to large, G_iThe structure of (a) is represented by the following formula;

G_i＝[g₁ ... g_(N+1)/2-1 g_(N+1)/2 g_(N+1)/2+1 ... g_N]

to facilitate subsequent calculation and extraction of ordered array G_iIs estimated median value x_midThe length N needs to be odd and is an ordered array G_iG of_(N+1)/2To estimate the median x_midDefining half-length K as (N-1)/2, then ordered array G_iCan be expressed as

G_i＝[g₁ ... g_K g_K+1 g_K+2 ... g_2K+1]

In the above formula, g_K+1Is an ordered array G_iIs estimated median value x_mid；

The ordered arrays corresponding to all units to be detected in the radar echo jointly form an ordered matrix M, namely a clutter matrix.

4. The adaptive time-sharing clutter map constant false alarm detection method of the external radiation source radar according to claim 3, characterized in that: the specific implementation manner of performing the clutter matrix initialization in step 3 is as follows,

firstly, a specific CFAR algorithm is used for calculating a unit d to be detected_iIs detected by the detection threshold T_i；

Secondly, willAssign value to ordered array G_iAll buffer units in (1), i.e. g₁-g_NWhere α is the CFAR threshold factor.

5. The adaptive time-sharing clutter map constant false alarm detection method of the external radiation source radar according to claim 1, characterized in that: the specific implementation of step 4 to derive whether the clutter map has constant false alarm characteristics is as follows,

it is known that the square-law detected sample value q follows an exponential distribution, and the probability density function PDF thereof is as follows:

in the above formula, H₀Finger assumes no target, H₁The method is characterized in that a target is assumed, mu is clutter power, s is a target signal-to-clutter ratio, and a moment-mother function MGF of q is defined as:

in the above formula λ is an arbitrary value within the expected existence interval defined by the moment mother function, and since wq also follows an exponential distribution with a mean value of w μ, according to M_wq(λ)＝(1-wμλ)^-1Substituting it into the above formula and developing it to obtain:

P_n＝wq_n+w(1-w)q_n-1+w(1-w)²q_n-2+…

then there is

Due to P_n-1Become random variables to obtain

According to the definition formula of the moment mother function, the above formula is actually a moment mother function of the random variable P, namely

PD＝M_P(λ)|_{λ＝-T/[μ(1+s)]}

The detection probability can be obtained by substituting lambda

Setting the target signal-to-clutter ratio s to 0 to obtain a false alarm probability formula when the target is judged to be present under the condition of no target as follows:

it can be seen from the above formula that the false alarm probability of clutter map detection is only related to the iteration number l, the threshold factor α and the forgetting factor w, and is independent of clutter power, so that the clutter map detection has a constant false alarm characteristic.

6. The adaptive time-sharing clutter map constant false alarm detection method of the external radiation source radar according to claim 1, characterized in that: the specific implementation manner of updating and iterating the clutter map in the step 5 is as follows,

let q_nIs the echo range Doppler spectrum after nth radar scan and square law detection, w is a forgetting factor, P_nRepresenting clutter background estimation value obtained after nth scanning of clutter map unit, threshold factor alpha is coefficient enabling clutter map detection to have constant false alarm performance, T is detection threshold, and T is alpha.P_n-1It can be seen that the echo range Doppler spectrum of the nth scan is formed after the nth-1 scanComparing the clutter maps to make a decision; the clutter map after the n-1 scanning is obtained by performing iterative updating according to the following formula:

P_n＝(1-w)P_n-1+wq_n

as shown in the above formula, w is a forgetting factor, P_nThe method is updated through a first-order recursive filter, and the above formula is developed to know that the iterative process is actually exponential weighted average, and the formula is as follows:

l in the above formula is the total scanning times of the radar, namely the total iteration times of clutter map updating, and L refers to the current scanning times;

when the number of iterations is L, the threshold factor α can be calculated by the following false alarm rate formula with the threshold factor:

where l is the current iteration number.

7. The adaptive time-sharing clutter map constant false alarm detection method of the external radiation source radar according to claim 1, characterized in that: the principle of updating the forgetting factor in the step 5 is that after the clutter map is constructed, if the number of targets in the new frame of the echo RD spectrum is large, the clutter map updating mechanism with the small forgetting factor can enable the existing clutter map to be influenced by the new frame of the RD spectrum data as little as possible, and further avoid the detection performance deterioration caused by the large fluctuation of the clutter map detection threshold, so that the forgetting factor is adaptively adjusted according to time to improve the time domain performance of the radar clutter map detection of the external radiation source.

Background

As shown in fig. 1, which is a schematic diagram of a dual-base extraterrestrial radiation source radar system, a non-cooperative radiation source such as a television tower or a broadcasting station is used as a transmitting station to transmit electromagnetic wave signals into the air, and the signals reach a receiving station via target reflection in the process of propagation. After receiving the signal, the reference channel capable of receiving the direct wave of the radiation source and the target (monitoring) channel capable of receiving the target reflection echo signal at the receiving end perform subsequent processing by using the direct wave signal as a reference and the target reflection echo signal, so that target parameter information such as distance, speed, angle and the like can be obtained, and the detection and tracking of the double-base terrestrial external radiation source radar system on the target can be realized.

S in FIG. 1_dAs a reference channel antenna, S_rIs the target (monitoring) channel antenna. After a target with a certain distance and speed relative to the transmitting-receiving double station acts on electromagnetic waves emitted by a radiation source, a target echo signal received by a monitoring channel has a certain time delay and Doppler frequency shift relative to a reference channel signal, and the two parameters can be obtained through subsequent processing. The distance difference between the transmitting and receiving double stations and the target and the distance between the transmitting and receiving double stations can be obtained through time delay, and the distance difference is called double-base distance. The obtained double-base distance is not the actual distance between the receiving station and the target, and the actual distance and the actual speed of the target also need to be obtained indirectly through the spatial geometrical position relation of the transmitting and receiving stations, which is different from the traditional single-station active radar.

The external radiation source radar does not emit energy, but utilizes a signal emitted by an external radiation source as a detection signal to extract target parameter information from an echo signal reflected by a received target. Compared with the traditional single-station active radar, the radar mainly has the following advantages:

(1) has the property of anti-stealth

At present, a stealth target in an electromagnetic environment generally only greatly reduces a Radar scattering cross section (RCS) of backscattering, and forward and lateral scattering is still strong. The bistatic external radiation source radar can detect the stealth target by receiving forward and lateral scattered echo signals of the target, and most external radiation sources work in very high frequency, ultrahigh frequency and other low frequency bands with extremely poor action of stealth wave-absorbing materials, so that the bistatic external radiation source radar has the capability of anti-stealth.

(2) Has low altitude detection capability

The multi-purpose high tower of external radiation source such as broadcast signal and TV station signal erects and sends the wave beam downwards, just can cover low latitude scope and its transmission signal frequency generally lower, and the wavelength is longer for the signal can pass low latitude barrier through the diffraction, and these two conditions make external radiation source radar have the ability of low latitude detection.

(3) Has stealth and anti-interference capability

The external radiation source radar which does not emit electromagnetic signals is difficult to detect by a reconnaissance system, and the problem that the active radar is easily interfered by signals of the external radiation source can be avoided due to the external radiation source, and the survival capability and the anti-impact capability can be improved.

Besides, the method also has the advantages of environmental protection, no electromagnetic pollution, low cost and the like. It also has some problems: the low-gain omnidirectional radiation mode is adopted for pursuing wide area coverage, so that the power of echo signals received by the system is lower; in addition, the energy of a target echo signal received by the external radiation source radar system is often lower than that of a direct wave, the lower signal-to-noise ratio directly affects the detection capability of the system on a target, and particularly when the environment is complex, the detection performance of the traditional airspace single-frame CFAR method is also deteriorated, such as detection rate loss and false alarm rate increase. Therefore, in order to solve these problems, it is necessary to search a more suitable target detection method for the radar of the external radiation source aiming at the characteristics of the radar of the external radiation source.

Therefore, the invention provides a self-adaptive time-sharing clutter map constant false alarm detection method suitable for a complex clutter environment based on the characteristic of separate transmitting and receiving of an external radiation source radar.

Disclosure of Invention

The invention provides a clutter map constant false alarm detection method based on self-adaptive time sharing, aiming at the problem of detecting an external radiation source radar target in a complex clutter environment.

The technical scheme adopted by the invention is as follows: a clutter map constant false alarm target detection method suitable for an external radiation source radar comprises the following steps:

step 1, performing clutter interval division based on a radar range-Doppler spectrum, and dividing a clutter unit from an airspace according to a distance unit and a Doppler unit;

step 2, establishing a two-dimensional clutter matrix according to clutter interval division, wherein each clutter unit is a matrix element and is a unit to be detected;

step 3, initializing a clutter matrix;

step 4, firstly deducing whether the clutter map has a constant false alarm characteristic, and then carrying out constant false alarm detection based on the clutter map, wherein the detection is to process and compare the sampling value of the radar at each unit with the value stored in the clutter map, if the sampling value is higher than the processed detection threshold value, the target is determined to be present, otherwise, the target is determined not to be present;

and step 5, establishing a forgetting factor for updating the clutter map according to time, and updating and iterating the clutter map according to a corresponding rule of the current scanning echo sampling value to form a latest clutter map, so that the clutter map can carry out self-adaptive updating and iterating along with time, and the false alarm performance superior to that of an airspace CFAR can be exerted under the detection condition that the airspace environment is more complex.

Further, the specific implementation manner of step 1 is as follows,

if the maximum detection distance radius of the radar is R_mThe clutter map is divided into a plurality of clutter units in two dimensions of distance and azimuth, and the distance size and the azimuth size of each clutter unit are respectively delta R_cAnd Δ a, each clutter map unit is divided into M × N resolution units, the range resolution and the azimuth resolution of the resolution units are Δ R and Δ θ, respectively, then there are:

the data in a certain clutter unit q is obtained by two-dimensional averaging of all resolution units in the current unit:

x in the above formula_m,nThe clutter unit storage value is updated together with the historical clutter storage unit value.

Further, the specific implementation manner of step 2 is as follows;

taking each clutter unit divided by the clutter map as a unit d to be detected_iEach unit to be detected has a buffer space-ordered array G special for buffering its historical sampling data_iThe buffer space has a length of N and the data therein are maintained in an ordered arrangement from small to large, G_iThe structure of (a) is represented by the following formula;

G_i＝[g₁ ... g_(N+1)/2-1 g_(N+1)/2 g_(N+1)/2+1 ... g_N]

to facilitate subsequent calculation and extraction of ordered array G_iIs estimated median value x_midThe length N needs to be odd and is an ordered array G_iG of_(N+1)/2To estimate the median x_midDefining half-length K as (N-1)/2, then ordered array G_iCan be expressed as

G_i＝[g₁ ... g_K g_K+1 g_K+2 ... g_2K+1]

In the above formula, g_K+1Is an ordered array G_iIs estimated median value x_mid；

The ordered arrays corresponding to all units to be detected in the radar echo jointly form an ordered matrix M, namely a clutter matrix.

Further, the specific implementation manner of performing the clutter matrix initialization in step 3 is as follows,

firstly, a specific CFAR algorithm is used for calculating a unit d to be detected_iIs detected by the detection threshold T_i；

Secondly, willAssign value to ordered array G_iAll buffer units in (1), i.e. g₁-g_NWhere α is the CFAR threshold factor.

Further, the specific implementation of step 4 to derive whether the clutter map has the constant false alarm characteristic is as follows,

it is known that the square-law detected sample value q follows an exponential distribution, and the probability density function PDF thereof is as follows:

in the above formula, H₀Finger assumes no target, H₁The method is characterized in that a target is assumed, mu is clutter power, s is a target signal-to-clutter ratio, and a moment-mother function MGF of q is defined as:

in the above formula λ is an arbitrary value within the expected existence interval defined by the moment mother function, and since wq also follows an exponential distribution with a mean value of w μ, according to M_wq(λ)＝(1-wμλ)^-1Substituting it into the above formula and developing it to obtain:

P_n＝wq_n+w(1-w)q_n-1+w(1-w)²q_n-2+…

then there is

Due to P_n-1Become random variables to obtain

According to the definition formula of the moment mother function, the above formula is actually a moment mother function of the random variable P, namely

PD＝M_P(λ)|_{λ＝-T/[μ(1+s)]}

The detection probability can be obtained by substituting lambda

Setting the target signal-to-clutter ratio s to 0 to obtain a false alarm probability formula when the target is judged to be present under the condition of no target as follows:

it can be seen from the above formula that the false alarm probability of clutter map detection is only related to the iteration number l, the threshold factor α and the forgetting factor w, and is independent of clutter power, so that the clutter map detection has a constant false alarm characteristic.

Further, the specific implementation manner of the update iteration of the clutter map in step 5 is as follows,

let q_nIs the echo range Doppler spectrum after nth radar scan and square law detection, w is a forgetting factor, P_nRepresenting clutter background estimation value obtained after nth scanning of clutter map unit, threshold factor alpha is coefficient enabling clutter map detection to have constant false alarm performance, T is detection threshold, and T is alpha.P_n-1Comparing the echo range-Doppler spectrum of the nth scanning with a clutter map formed after the nth-1 scanning to judge; the clutter map after the n-1 scanning is obtained by performing iterative updating according to the following formula:

P_n＝(1-w)P_n-1+wq_n

as shown in the above formula, w is a forgetting factor, P_nThe method is updated through a first-order recursive filter, and the above formula is developed to know that the iterative process is actually exponential weighted average, and the formula is as follows:

l in the above formula is the total scanning times of the radar, namely the total iteration times of clutter map updating, and L refers to the current scanning times;

when the number of iterations is L, the threshold factor α can be calculated by the following false alarm rate formula with the threshold factor:

where l is the current iteration number.

Further, the principle of updating the forgetting factor in the step 5 is that after the clutter map is constructed, if the number of targets in the new frame of the echo RD spectrum is large, the clutter map updating mechanism with the small forgetting factor can enable the existing clutter map to be influenced by the new frame of the RD spectrum data as little as possible, so that the detection performance deterioration caused by the large fluctuation of the clutter map detection threshold is avoided, and therefore the forgetting factor is adaptively adjusted according to time to improve the time domain performance of the clutter map detection of the external radiation source radar.

Compared with the prior art, the external radiation source radar target detection method provided by the invention has high stability, has outstanding advantages in a complex ground object environment, has positive effect on improving the detection performance of a radar system, and has great significance for the practical application of the external radiation source radar.

Drawings

FIG. 1: is a schematic diagram of a dual-base terrestrial external radiation source radar system;

FIG. 2: is a schematic diagram of clutter map distance-azimuth division;

FIG. 3: is the actual measurement range Doppler spectrum of the external radiation source radar;

FIG. 4: the method is a schematic block diagram of a self-adaptive time-sharing clutter map detection method;

FIG. 5: is a clutter map self-adaptive updating schematic diagram;

FIG. 6: detecting an effect graph of a uniform single-target environment clutter graph;

FIG. 7: clutter map detection effect maps under the uniform multi-target environment;

FIG. 8: a clutter map detection effect map under a clutter edge environment;

FIG. 9 average detection probability in a homogeneous environment;

FIG. 10: detecting results of all methods in a uniform single-target environment;

FIG. 11: a uniform single target environment detection threshold;

FIG. 12: detecting results of all methods in a uniform multi-target environment;

FIG. 13: a uniform multi-target environment detection threshold;

FIG. 14: clutter edge iteration false alarm rate performance;

FIG. 15: and detecting a threshold in the clutter edge environment.

Detailed Description

In order to facilitate the understanding and implementation of the present invention for those of ordinary skill in the art, the present invention is further described in detail with reference to the accompanying drawings and examples, it is to be understood that the embodiments described herein are merely illustrative and explanatory of the present invention and are not restrictive thereof.

The invention provides a target detection method suitable for an external radiation source radar, which specifically comprises the following steps:

step 1: and performing clutter interval division based on the radar range-Doppler spectrum, and dividing a clutter unit from an airspace according to the distance unit and the Doppler unit.

As shown in FIG. 2(a), if the maximum detection range radius of the radar is R_mThe clutter map is divided into a plurality of clutter units in two dimensions of distance and azimuth, and the distance size and the azimuth size of each clutter unit are respectively delta R_cAnd Δ a.

In addition, as shown in FIG. 2(b), each clutter unit can be further divided into M × N resolution units, and the range resolution and the azimuth resolution of the resolution units are Δ R and Δ θ, respectively, and then

The data in a certain clutter unit q is obtained by two-dimensional averaging of all resolution units in the current unit

X in the above formula_m,nThe representation is a resolution cell in the clutter cell. And each clutter map is updated, the average value of each clutter unit is calculated according to each resolution unit value in the clutter unit to serve as the current clutter unit value, and the clutter unit stored value is updated together with the historical clutter unit value.

In the bistatic-ground-external-radiation-source radar system, since the measurement of the target is presented by the non-actual distance and velocity such as the bistatic distance and the bistatic velocity, the clutter map cannot be directly established in the space, but is established by using the Range-Doppler (RD) spectrum containing the bistatic distance and the bistatic Doppler shift information of the target, for example, fig. 3 is the RD spectrum of some measured data of the bistatic-ground-external-radiation-source radar.

In fig. 3, the amplitudes are distinguished by colors, and the horizontal and vertical coordinates are the doppler cell and the bibase range cell, respectively. Because the radar is repeatedly scanned in the detection range, the RD spectrum, which is the echo data obtained after each scanning, is iteratively updated into the clutter map according to a certain rule, and the stored value in the clutter unit is iterated to be converged after a plurality of times of scanning through the interframe accumulation process carried out according to the scanning period, so that the background estimation data with the better current unit is obtained, and therefore, the influence of the strong change of the clutter on the spatial domain on the detection performance of the clutter map is not large, and the target detection is convenient.

Step 2: and establishing a two-dimensional clutter matrix according to the clutter interval division, wherein each clutter unit is a matrix element.

Partitioning clutter mapsEach clutter unit is used as a unit d to be detected_iEach unit to be detected has a buffer space-ordered array G special for buffering its historical sampling data_iThe buffer space has a length of N and the data therein are maintained in an ordered arrangement from small to large, G_iThe structure of (A) is shown in the following formula.

G_i＝[g₁ ... g_(N+1)/2-1 g_(N+1)/2 g_(N+1)/2+1 ... g_N]

To facilitate subsequent calculation and extraction of ordered array G_iIs estimated median value x_midThe length N needs to be odd and is an ordered array G_iG of_(N+1)/2To estimate the median x_mid. Defining half-length K as (N-1)/2, then ordered array G_iCan be expressed as

G_i＝[g₁ ... g_K g_K+1 g_K+2 ... g_2K+1]

In the above formula, g_K+1Is an ordered array G_iIs estimated median value x_mid。

And the ordered arrays corresponding to all units to be detected in the radar echo jointly form an ordered matrix M.

And step 3: after the clutter matrix is established, initialization is required. Initialization is to assign initial values to each ordered array in the algorithm, and obtain better initial performance of iteration through a specific initialization strategy.

The initialization operation flow comprises the following steps:

firstly, a specific CFAR algorithm is used for calculating a unit d to be detected_iIs detected by the detection threshold T_i；

Secondly, willAssign value to ordered array G_iAll buffer units in (1), i.e. g₁-g_NWhere α is the CFAR threshold factor.

In the invention, the OS-CFAR method with better detection performance under the multi-target environment is selected as an initialization strategy, so that the detection performance basically the same as that of the OS-CFAR can be obtained at the initial stage of iteration of the algorithm, and the algorithm can further converge towards the optimal detector performance on the basis of the detection performance of the OS-CFAR after passing through the initial stage of iteration.

And 4, step 4: firstly deducing whether the clutter map has a constant false alarm characteristic, then carrying out constant false alarm detection based on the clutter map, wherein the CFAR detection of the radar clutter map of the external radiation source comprises two steps of detection and updating, wherein the detection is to process and compare radar range-Doppler spectrum data with a value stored in the clutter map, and if the amplitude of a range-Doppler spectrum unit is higher than a processed detection threshold value, judging that a target exists, otherwise, judging that no target exists.

The detection rate PD and the false alarm probability PFA of clutter map detection are deduced, and it is known that the sampling value q subjected to square-law detection follows exponential distribution, and the probability density function PDF is as follows

In the above formula, H₀Finger assumes no target, H₁The finger assumes that there is a target, mu is the clutter power, and s is the target signal-to-clutter ratio. The moment mother function MGF of q is defined as:

in the above formula, λ is an arbitrary value within the expected existence interval defined by the intalox function. And since wq also follows an exponential distribution with a mean value of w μ, according to M_wq(λ)＝(1-wμλ)^-1Substituting it into the above formula and developing it to obtain

P_n＝wq_n+w(1-w)q_n-1+w(1-w)²q_n-2+…

Then there is

Due to P_n-1Become random variablesTo obtain

According to the definition formula of the moment mother function, the above formula is actually a moment mother function of the random variable P, namely

PD＝M_P(λ)|_{λ＝-T/[μ(1+s)]}

The detection probability can be obtained by substituting lambda

Setting the target signal-to-clutter ratio s to 0 to obtain a false alarm probability formula when the target is judged to be present under the condition of no target as follows

It can be seen from the above formula that the false alarm probability of clutter map detection is only related to the iteration number l, the threshold factor α and the forgetting factor w, and is independent of clutter power, so that the clutter map detection has a constant false alarm characteristic.

Further, the specific implementation manner of step 5 is as follows,

the clutter map updating strategy directly influences the detection performance of the constant false alarm detection in different time periods, processing and caching the echo data of the multi-frame unit to be detected through algorithm iterative calculation, and calculating the detection threshold by using the historical data of the unit to be detected, wherein a schematic diagram of the algorithm is shown in fig. 5.

Q in FIG. 4_nIs the echo range Doppler spectrum after nth radar scan and square law detection, w is a forgetting factor, P_nRepresenting clutter background estimation value obtained after nth scanning of clutter map unit, threshold factor alpha is coefficient enabling clutter map detection to have constant false alarm performance, T is detection threshold, and T is alpha.P_n-1It can be seen that the echo range-Doppler spectrum of the nth scan is compared with the clutter map formed after the nth-1 scan for judgmentAnd (6) determining. The clutter map after the n-1 scanning is obtained by performing iterative updating according to the following formula:

P_n＝(1-w)P_n-1+wq_n

as shown in the above equation, w is a forgetting factor, which is a preset parameter that affects the iteration speed, the target interference resistance, and the feedback real-time performance of the algorithm. Because the double-base ground external radiation source radar mostly adopts a fixed station mode to detect, the construction of the converged clutter map can be completed before the real detection is carried out, so the forgetting factor can actually measure the clutter level in the radar layout area and fit the empirical function relationship of the clutter and the forgetting factor. P_nThe method is updated through a first-order recursive filter, and the iterative process is actually subjected to exponential weighted averaging by expanding the formula

L in the above formula is the total radar scanning times, namely the total iteration times of clutter map updating, and L refers to the current scanning times.

As shown in fig. 5, the function of the delay unit is to delay the received data by one frame time for calculating the detection threshold T and the clutter background estimation value P_nWherein q is_nThe amplitude value of the echo of the unit to be detected at the nth radar scanning period moment is obtained; the comparator is used for comparing the detection threshold with the echo amplitude of the unit to be detected, and if the detection threshold is larger than the threshold T, the target is detected.

When the number of iterations is L, the threshold factor α can be calculated by the following false alarm rate formula with the threshold factor:

where l is the current iteration number.

From fig. 5, it can be known that the strength of the memory effect of the clutter map is mainly dominated by the forgetting factor w, which directly determines the influence of new echo range doppler spectrum data on the clutter map during the update iteration of the clutter map.

The main principle of the forgetting factor w is that after the clutter map is constructed, if a new frame of echo RD spectrum has more targets, the clutter map updating mechanism with the smaller forgetting factor can enable the existing clutter map to be influenced by the new frame of RD spectrum data as little as possible, and further avoid the detection performance deterioration caused by the large fluctuation of the clutter map detection threshold, so that the forgetting factor is considered to be adaptively adjusted according to time to improve the time domain performance of the detection of the clutter map of the external radiation source radar.

And (4) simulating and detecting performance analysis, and respectively carrying out detection simulation and performance comparison analysis.

Firstly, in order to evaluate the performance of different methods, simulation analysis is respectively carried out on the detection performance under the conditions of single target, multiple targets and clutter edges.

Firstly, under a uniform single-target environment, a clutter map forgetting factor w performs simulation analysis on the influence of an external radiation source radar clutter map detection performance, and simulation conditions are as follows: square law detection is adopted in the Rayleigh clutter background, the background environment is a uniform single-target environment, the clutter background power is 20dB, the exponential distribution is obeyed, the distribution parameter is 1, and the false alarm probability PFA of the detector is set to be 10^-6The detection results when weak targets with signal-to-noise ratios of 15dB randomly appear at a certain position of a sample with a length of 200, a clutter map is constructed, and forgetting factors w adopted by iterative update of the clutter map are 1/8 and 1/128 are shown in fig. 6.

The results in the simulation effect graph shown in fig. 6 are all detected, but it can be observed that when the forgetting factor is different, the detection threshold presented by the clutter map detection algorithm is obviously different. When the forgetting factor is 1/8, the fluctuation range of the clutter map threshold is large, and the deviation from the optimal threshold is large overall, because the forgetting factor is large, if the fluctuation range of the clutter of the unit to be detected is large or a target appears, a large echo sampling value is iterated into the clutter map to raise the threshold greatly, so that the peak of the threshold in the map appears; the clutter map threshold with the forgetting factor of 1/128 is obviously better than the situation when the forgetting factor is 1/8, the peak is reduced, the fluctuation width is reduced, and the threshold value is closer to the optimal threshold overall; in addition, as can be seen by comparing with the detection threshold map of the spatial CFAR, the detection threshold fluctuation width of the clutter map is generally narrow, because the nearby units in the spatial domain do not influence each other due to the target.

Secondly, simulation under multiple targets with uniform background is carried out, other simulation conditions except the target number are unchanged, observation and analysis are carried out on the detection effect, and the result is shown in fig. 7.

It can be seen from fig. 7 that when the forgetting factor is 1/8, the weaker target of the adjacent two targets is not detected because the amplitude is lower than the threshold, and it can still be observed that although the width of the threshold peak is not large, the number of peaks is obviously greater than that in the case of the single target, because the clutter map with the larger forgetting factor has more sampling values iterated into the clutter map after the targets are greater, so that the threshold is raised and the degree is obviously greater than that in the case of the single target; and when the forgetting factor is 1/128, both targets are detected, and although the detection threshold deviates from the optimal threshold and fluctuates obviously compared with the single target, the overall fluctuation degree of the threshold is obviously smaller compared with the clutter map with the forgetting factor of 1/8.

Compared with the results of fig. 6, it can be found that the detection threshold fluctuation of the multi-target situation is larger, the number of peaks is more, the multi-target detection performance is deteriorated under the same iteration number, and the only difference is the target number, so that under the visible multi-target situation, the clutter map with a larger forgetting factor has a certain degree of detection performance loss during detection.

Thirdly, simulation is performed in a clutter edge environment, no target is added at this time, other simulation conditions except the background and the target are unchanged, and the observation and detection result is shown in fig. 8.

It can be observed from fig. 8 that the performance of the clutter map in the clutter edge environment is significantly superior to that of the airspace CFAR method, and even if a clutter peak which is very likely to generate a false alarm when the detection is performed by the airspace CFAR method appears in the high-power edge region of the clutter, the two clutter maps do not generate a false alarm, and the threshold at the clutter edge is also ideal. However, by comparing clutter maps with forgetting factors of 1/8 and 1/128, the threshold result of the former is poor, the threshold of the latter is observed to be almost fitted with the optimal threshold, and the clutter map false alarm performance with the smaller forgetting factor is more excellent.

From the clutter map detection simulation results of the above three background environments, we can draw the following conclusions: the clutter map can cope with the situation that the clutter background of an airspace is complex due to the independence of the clutter map unit in the airspace; when the targets exist or even a large number of targets exist, the threshold value of the clutter map unit is raised due to the memory effect; when no target is added in the clutter edge environment, the false alarm prevention capability is superior, so that the method can be well adapted to the weak target detection scene in the complex environment faced by the external radiation source radar.

Although the timing of the clutter map can help it cope with more complex environments, in the case of more targets, the timing memory may become a disadvantage. From the simulation result, the strength of the memory effect of the clutter map is mainly dominated by a forgetting factor w, which directly determines the influence of new echo data on the clutter map when the clutter map is updated and iterated.

The simulation result proves that the fluctuation of the detection threshold is small when the forgetting factor is small, obviously the fluctuation influences the detection performance, the clutter maps with the forgetting factors w of 1/4, 1/8, 1/16, 1/32 and 1/128 are used for detecting the sample by adjusting the signal-to-noise ratio of the target in the sample, and the detection probability results obtained after 1000 Monte-Carlo experiments are averaged.

In fig. 9, it can be seen that the detection probability of each clutter map increases with the increase of the target signal-to-clutter ratio, and when the target signal-to-clutter ratio is about 15dB, the relative difference between the methods is large. Under the condition that the signal-to-clutter ratio of the target is fixed, along with the reduction of the forgetting factor, the detection probability gradually rises, the detection rate of the clutter map detector with the forgetting factor of 1/128 is very close to the performance of the theoretical optimal detector, the superiority of the detection performance when the forgetting factor is small is fully embodied, and therefore theoretically better detection and false alarm performance are achieved after the clutter map detector is applied to an external radiation source radar constant false alarm detection link.

(II) Performance comparison experiment

Simulation analysis was performed mainly on the above-mentioned cell average CA-CFAR, maximum selection GO-CFAR, minimum selection SO-CFAR, order statistics OS-CFAR, change index VI-CFAR, and the improved clutter map CM-CFAR proposed by the present invention. And comparing the detection performance of each constant false alarm detection algorithm under three most representative clutter environments, namely a uniform single target, a uniform multi-target and a clutter edge environment. The experimental method not only compares the variation trend of the average detection rate of each method along with the increase of the target signal-to-noise ratio, but also visually compares the difference between the detection threshold of each method and the optimal detection threshold.

The general simulation conditions are set as follows: false alarm probability PFA is preset to 10^-6(ii) a The total length N of the reference samples of CA-CFAR, GO-CFAR, SO-CFAR, OS-CFAR and VI-CFAR is set to be 30, and the lengths of the front protection window and the rear protection window are respectively 3; the rate value of OS-CFAR is 3/4; VI-CFAR decision threshold K_VIAnd K_MRSet to 4.76 and 1.086, respectively; the forgetting factor w of the CM-CFAR is improved to 1/128.

(1) Uniform single target environment

The simulation background is used as a uniform single-target environment, and the simulation result is shown in FIG. 10.

As can be seen from FIG. 10, among the results of the three mean-value classes CFAR, the CA-CFAR is optimal, which is a spatial domain method better suitable for a uniform environment, the GO-CFAR is suboptimal, and the SO-CFAR is worst, because the SO-CFAR discards more valid values after being selected to be small, which results in detection loss. The detection effects of the OS-CFAR and the VI-CFAR are superior to those of the SO-CFAR and inferior to those of the other two. The detection probability PD of the improved CM-CFAR is closer to the optimal detector and is obviously better than the spatial CFAR.

In order to compare the differences of the algorithms more simply and intuitively, the detection thresholds are calculated by processing the clutter data by using each CFAR algorithm, and the detection thresholds and the echo data obtained by each CFAR algorithm are shown in fig. 11.

As can be seen from fig. 11, each algorithm can detect a target in a uniform single-target environment, but the CA-CFAR raises the detection thresholds on both sides due to the echo intensity of the unit where the target is located, and the GO-CFAR raises the thresholds more and deteriorates most seriously due to the selection of a larger average value in the reference windows on both sides; the thresholds of the SO-CFAR and the VI-CFAR are similar because the strategy of VI-CFAR selection is biased to be smaller. The threshold of the CM-CFAR is closest to the optimal detection threshold and the neighboring threshold values are hardly affected by the target.

(2) Uniform multi-target environment

The simulation background is used as a uniform multi-target environment, and the simulation result is shown in fig. 12.

As can be seen from FIG. 12, in the airspace CFAR, the GO-CFAR has the worst detection effect, and the analysis of the GO-CFAR in the foregoing shows that the threshold is raised too much due to more targets, so that the detection performance is deteriorated, and the CA-CFAR is the worst except for the GO-CFAR, and the reason is similar to that of GO; it can be seen that the detection performance of the OS-CFAR under the multi-target condition is superior in the spatial CFAR, but the detection performance of the CM-CFAR is better than that of the OS-CFAR, and the detection probability is closest to the optimal detector.

Similarly, each CFAR algorithm is used to process the clutter data to calculate a detection threshold, and the detection threshold and the echo data obtained by each CFAR algorithm are shown in fig. 13.

As can be seen from the observation of FIG. 13, the thresholds of CA-CFAR and GO-CFAR are indeed raised much, which leads to the missed detection of the adjacent weak targets, and the VI and SO results are still similar; although the CM-CFAR has better detection probability, the threshold fluctuation is larger, and the OS-CFAR threshold of the airspace is quite stable and is more suitable for multi-target scenes. In the simulation of the adjacent strong and weak targets, the CA and the GO generate a target mutual shielding effect, so that the detection is missed.

(3) Clutter edge environment

The clutter edge environment is adopted for simulation, and the simulation result is shown in fig. 14.

As can be seen from fig. 14, the SO-CFAR in the space-domain CFAR has a sudden change in false alarm rate at the clutter edge due to the principle of selecting a small mean value, and thus has poor performance; false alarms also appear in CA-CFAR and OS-CFAR, and CFAR in other airspaces has better performance. It can be observed that the performance of the CM-CFAR becomes stable and very close to the preset value.

Similarly, the clutter data is processed by each CFAR algorithm to calculate a detection threshold, and the detection threshold and the echo data obtained by each CFAR algorithm are shown in fig. 15.

As can be seen from fig. 15, the detection threshold obtained by CM-CFAR fluctuates above and below the optimal detection threshold, and is more suitable for the optimal detection threshold compared to other comparison reference algorithms. Other CFAR algorithms have the problem of lifting threshold in advance or in a delayed way in a clutter edge area, for example, CA-CFAR, GO-CFAR, OS-CFAR, VI-CFAR and the like have the problem of lifting detection threshold in a low-power area, and a target is possibly shielded; the detection threshold of the SO-CFAR is raised in a high-power area, which may cause the false alarm probability to increase.

In conclusion, for a radar system which adopts a fixed station mode to detect a weak target in a complex environment, such as a double-base ground external radiation source radar, a clutter map has ideal detection and false alarm performance, and can also easily cope with a detection scene of an airspace environment mutation by utilizing the stability of a time sequence.

Compared with the prior art, the adaptive time-sharing clutter map constant false alarm target detection method for the external radiation source radar has high stability, has outstanding advantages in a complex ground object environment, has a positive effect on improving the detection performance of a radar system, and is very significant for the practical application of the external radiation source radar.

The above description of the preferred embodiments is intended to be illustrative, and not to be construed as limiting the scope of the invention, which is defined by the appended claims, and all changes and modifications that fall within the metes and bounds of the claims, or equivalences of such metes and bounds are therefore intended to be embraced by the appended claims.