CN109541556B - Method for identifying frequency shift interference of linear frequency modulation signal - Google Patents
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Abstract
The invention discloses a method for identifying frequency shift interference of linear frequency modulation signals, which comprises the following steps: preprocessing signals received by a radar, and dividing the signals into three paths before matching filtering; respectively passing the three signals through a complete matched filter and a designed half-bandwidth filter, and extracting the output voltage peak value of each half-bandwidth matched filter; setting a threshold; and determining whether the target is a frequency shift interference false target or not according to the ratio of the matched output peaks of the target under different half-bandwidth matched filters and the set gate. The invention can meet the real-time identification of the radar to multiple targets; the application range is wide, and various interference system false targets can be identified; the defects of an anti-interference method for tracking the front edge of the radar are made up, and the detection and identification of the interference of the active leading false target can be realized; the method has the advantages of few implementation steps, small calculation amount, capability of meeting the real-time identification of the radar on multiple targets, no additional requirement on a hardware system and easiness in engineering implementation.
Description
Technical Field
The invention belongs to the field of radar anti-interference, and particularly relates to a method for identifying frequency shift interference of linear frequency modulation signals.
Background
Chirp signals (also called chirp signals) are widely used in various pulse compression radars today because they can obtain a long range and a high range resolution. The existing coupling characteristics of distance and Doppler frequency shift enable the system radar to be easily subjected to frequency shift interference, namely when targets such as airplanes and missiles find stable detection and tracking signals of enemy radars, long-distance vivid false targets are usually generated before and after the targets are identified as linear frequency modulation signals by using a frequency shift interference technology, so that the radar is difficult to identify the real targets, and if interference signals with different frequency shift quantities are repeatedly forwarded by an interference party, the generated leading and lagging false target groups greatly consume radar resources. The significant survival challenge faced by the chirp radar makes the search for a real-time effective frequency shift interference false target identification method have important military application value.
The existing radar identification method for the frequency shift interference false target mainly comprises two methods: frequency shift measurement, fractional Fourier transform (FRFT). The frequency shift measurement method calculates the center frequency of an echo signal by using a digital receiver processing technology and calculates the frequency shift amount of an interference signal according to the center frequency so as to judge whether the target is a frequency shift interference false target. The method has the disadvantages that the time width and the bandwidth of the frequency shift interference signal are the same as those of the real target echo, and the interference machine can completely change the time width and the bandwidth of the forwarded interference signal by using the DRFM technology, control the center frequency of the interference signal, ensure that the center frequency is the same as that of the target echo signal, and enable the frequency shift measurement method to be invalid. The fractional Fourier transform method is used for identifying a frequency shift interference false target transmitted by an interference machine by combining the fractional Fourier transform with a matching result in a radar receiver and utilizing target speed information obtained by the fractional Fourier transform estimation and distance information obtained by pulse compression. In practical application, the method needs a large amount of optimal FRFT rotation angle searching work, and the timeliness of false target identification is influenced. In summary, the existing frequency shift interference false target identification methods all have respective disadvantages and shortcomings, and no effective frequency shift interference false target identification method is applied to the linear frequency modulation radar.
Patent 201510598123.4 discloses a method for identifying frequency shift interference by half-bandwidth matched filtering, which also adopts a method combining half-bandwidth matching and perfect matching, comparing the output peak ratio of radar receiving signals passing through the half-bandwidth matched filter and the perfect matched filter, under ideal conditions, the ratio of the output peak value to target echo will ensure strict 1/2, and the ratio of the output peak value to frequency shift interference will be less than 1/2, thus realizing identification of frequency shift interference distance false target. Compared with the method, the method utilizes the difference of the frequency shift interference signal and the real echo in frequency to a greater extent, and the ratio for identifying the true and false targets obtained by the method under the same condition is about twice that of the method, so that the false targets are easier to identify.
Disclosure of Invention
The invention aims to provide a method for identifying frequency shift interference of a linear frequency modulation signal, which solves the problems that the frequency shift interference pulse pressure of a linear frequency modulation broadband radar is beneficial to large, the fidelity of a generated false target is high, and the false target is difficult to identify quickly and effectively.
The technical solution for realizing the purpose of the invention is as follows: a method for identifying frequency shift interference of a linear frequency modulation signal comprises the following steps:
the method comprises the following steps: preprocessing signals received by a radar, and dividing the signals into three paths before matching filtering:
firstly, preprocessing signals received by a radar receiver by filtering, low-noise amplification, down-conversion and band-pass filtering, then dividing the received radar signals into three paths, wherein the first path is used for normal target detection, and the second path and the third path respectively extract peak values and compare the peak values to identify whether the target is a false target;
step two: respectively passing the three signals through a complete matched filter, a first half-bandwidth matched filter and a second half-bandwidth matched filter, and extracting output voltage peak values of the half-bandwidth matched filters:
the first half-bandwidth matched filter is close to the low-frequency side of the perfect matched filter in the frequency domain, the second half-bandwidth matched filter is close to the high-frequency side of the perfect matched filter in the frequency domain, and the widths of the first half-bandwidth matched filter and the second half-bandwidth matched filter are half of the widths of the perfect matched filters; setting linear frequency modulation signal x (T), radar transmitting frequency modulation signal bandwidth as B, pulse width as T, its complete matching filter response h 0 (t) first half-band matched filter impulse response h L (t) second half-bandwidth matched filter impulse response h R (t) are respectively:
wherein ". sup" denotes conjugation, t 0 Outputting peak value moment for matched filtering, wherein k is a normalization coefficient, and t is time;
passing the linear frequency-modulated signal x (t) through a three-way matched filter h 0 (t)、h L (t)、h R (t) the frequency domain expression of the output is:
wherein X (f) is the Fourier transform of the chirp signal x (t), H 0 (f) For a perfectly matched filter response h 0 (t) Fourier transform, H L (f) Matching the impulse response h of the filter for the first half-band L (t) Fourier transform, H R (f) For the second half-bandwidth matched filter impulse response h R (t) Fourier transform, Y 0 (f) For the chirp signal x (t) through a perfect-matched filter h 0 Fourier transform of the post output, Y L (f) Passing the chirp signal x (t) through a first half-band matched filter pulse h L Fourier transform of the post output, Y R (f) Passing the second half-bandwidth matched filter pulse h for the chirp signal x (t) R (t) Fourier transform of the post output.
And (3) carrying out Fourier inverse transformation on the result of the formula II, and calculating a signal matching output peak value:
wherein, | · | is absolute value calculation, j is an imaginary unit, f represents frequency, and t represents time;
setting the matching output peak values as y 0max 、y Lmax 、y Rmax Here y 0max ＝max(|y 0 (t)|)y Lmax ＝max(|y L (t)|)，y Rmax ＝max(|y R (t) |), max (·) is calculated by taking the maximum value;
step three: setting a threshold:
taking into account the Doppler shift f due to the target velocity d ＝2v max /λ＝2v max f 0 C mismatch, determining threshold:
wherein v is max The maximum speed of a measurable target of the radar, lambda is the wavelength of a radar transmission signal, f 0 Carrier frequency for radar transmitted signal, c speed of light, f d Doppler shift brought to target velocity, n h Is an upper threshold, n l Is a lower threshold;
step four: when the first path of matched filtering output is subjected to radar target detection processing to detect a target, determining whether the target is a frequency shift interference false target according to the ratio of matched output peak values of the target under different half-frequency-width matched filters and a set threshold:
compared with the prior art, the invention has the remarkable advantages that:
(1) the method skillfully utilizes the frequency characteristics of the start and the end of the frequency shift interference signal, and the identification process of the false target is not influenced by the center frequency of the frequency shift interference signal. The method effectively overcomes the defect that the identification of frequency shift interference is invalid due to the fact that an interference machine controls the center frequency of an interference signal in a frequency shift measurement method.
(2) The application range is wide, and various interference system false targets can be identified. The invention can identify the distance multiple false targets generated by applying linear frequency modulation signal distance-Doppler coupling characteristics, such as intermittent sampling forwarding interference, frequency shift interference and the like.
(3) The defects of an anti-interference method for tracking the front edge of the radar are made up, and the detection and identification of the interference of the active leading false target can be realized;
(4) the radar can be used for identifying the false target in real time. The invention has the advantages of less implementation steps, small calculation amount and easy engineering implementation. The target can be judged to be true or false by using one signal echo theoretically, space-time resources of the radar are saved, the efficiency is high, the speed is high, and the real-time identification of the radar to multiple targets can be met.
Drawings
Fig. 1 is a general flowchart of a false target identification method according to the present invention.
Fig. 2 is a block diagram of a system for identifying a frequency-shift interference decoy according to the present invention.
Fig. 3 is a schematic diagram of the relationship between the matched filters and the radar received signal spectrum according to the present invention.
Fig. 4(a) is a schematic diagram of the amplitude of the interference signal and the echo signal after passing through the first path of perfect matching filter in the simulation experiment performed by the present invention.
Fig. 4(b) is a schematic diagram of the result of commercial false target identification obtained by comparing the amplitudes of the interference signal and the echo signal after passing through the second left half-bandwidth matched filter with the amplitudes of the interference signal and the echo signal after passing through the third right half-bandwidth matched filter in the simulation experiment performed by using the invention.
Detailed Description
The present invention is described in further detail below with reference to the attached drawing figures.
In order to enable the radar to effectively identify the false target generated by the frequency shift interference in time, support is provided for subsequent anti-interference means, and the false target identification is also an important key point (false target identification) in radar frequency shift interference resistance. The invention utilizes the difference of the frequency characteristics of a target echo signal, a frequency shift interference signal and a matched filter, adopts a method of combining half-frequency width matching and complete matching, compares the ratio of the output peak values of the signals received by a radar after the radar receives and detects a target and the output peak values of the signals passing through two half-frequency width matched filters, ensures the strict 1:1 of the ratio to the target echo under an ideal condition, and is more than or less than 1 of the ratio to the frequency shift interference, thereby realizing the rapid identification of a frequency shift interference false target according to the difference.
With reference to fig. 1 and 2, the simulation experiment is implemented by using a Matlab simulation platform based on a general-purpose computer, and the simulation parameters are set as follows: the pulse width T of radar emission signal is 50us, the bandwidth B is 10MHz, and the carrier frequency f 0 3000MHz, interference frequency shift amount f dj -1Mhz, -3Mhz, the false target lead distance generated at this time is 750m, 2250 m.
The invention relates to a method for realizing frequency shift interference identification by half-bandwidth matched filtering, which comprises the following specific steps of:
the method comprises the following steps: preprocessing signals received by a radar, and dividing the signals into three paths before matching filtering:
firstly, signals received by a radar receiver are subjected to pre-processing of filtering, low-noise amplification, down-conversion and band-pass filtering, then the received radar signals are divided into three paths, the first path is used for normal target detection, and the second path and the third path respectively extract peak values and compare the peak values to identify whether the targets are false targets.
Here a perfect match filter H 0 (f) And left and right half-bandwidth matched filters H L (f)、H R (f) The spectral relationship of (a) is shown in fig. 3. FIG. 3 is a spectrum relationship between each matched filter and the radar receiving signal according to the present invention, from which it can be seen that when the true target echo signal X (f) passes through H L (f)、H R (f) The ratio of the post-output signal peak value is kept to be strict 1:1 (when the frequency shift caused by noise and target speed is not considered, the frequency shift is considered in practical application), and the frequency shift interference signal X J (f) Through H L (f)、H R (f) The ratio of the peak values of the post-output signals is larger or smaller than 1:1, which is the basis for identifying the frequency shift interference decoys.
Step two: respectively passing the three signals through a complete matched filter, a first half-bandwidth matched filter and a second half-bandwidth matched filter, and extracting output voltage peak values of the half-bandwidth matched filters:
the first half-bandwidth matched filter is close to the low-frequency side of the perfect matched filter in the frequency domain, the second half-bandwidth matched filter is close to the high-frequency side of the perfect matched filter in the frequency domain, and the widths of the first half-bandwidth matched filter and the second half-bandwidth matched filter are half of the widths of the perfect matched filters; setting linear frequency modulation signal x (T), radar transmission frequency modulation signal bandwidth as B and pulse width as T, then its complete matching filter response h 0 (t), first half-band matched filter impulse response h L (t), second half-bandwidth matched filter impulse response h R (t) are each h 0 (t)、h L (t)、h R (t)，h 0 (t)、h L (t)、h R (t) the expression is given in equation one:
wherein "+" denotes conjugation, t 0 And k is a normalization coefficient for the time of the peak value output by matched filtering.
Passing the linear frequency-modulated signal x (t) through a three-way matched filter h 0 (t)、h L (t)、h R (t) outputs are each Y 0 (f)、Y L (f)、Y R (f)，Y 0 (f)、Y L (f)、Y R (f) The calculation method is shown in formula II:
wherein X (f) is the Fourier transform of the chirp signal x (t), H 0 (f) For a perfectly matched filter response h 0 (t) Fourier transform, H L (f) For a first half-band matched filter impulse response h L (t) Fourier transform, H R (f) For the second half-bandwidth matched filter impulse response h R (t) Fourier transform, Y 0 (f) For the chirp signal x (t) through a perfect match filter h 0 Fourier transform of the post output, Y L (f) Passing the chirp signal x (t) through a first half-band matched filter pulse h L Fourier transform of the post output, Y R (f) Passing the second half-bandwidth matched filter pulse h for the chirp signal x (t) R (t) Fourier transform of the post output.
For Y 0 (f)、Y L (f)、Y R (f) The result of (3) is subjected to Fourier inverse transformation, and a signal matching output peak value y is calculated 0 (t)|、|y L (t)|、|y R (t)|，|y 0 (t)|、|y L (t)|、|y R The expression of (t) | is shown in formula three:
wherein, | is absolute value operation, j is an imaginary unit, f represents frequency, and t represents time.
Setting the matching output peak values as y 0max 、y Lmax 、y Rmax Where y is 0max ＝max(|y 0 (t)|)y Lmax ＝max(|y L (t)|)，y Rmax ＝max(|y R (t) |), max (·) is the maximum value operation;
step three: and setting a threshold.
Taking into account the Doppler shift f due to the target velocity d ＝2v max /λ＝2v max f 0 C mismatch, determining threshold n l 、n h ，n l 、n h The expression is shown in formula four:
wherein v is max The maximum speed of a measurable target of the radar, lambda is the wavelength of a radar transmission signal, f 0 Carrier frequency for radar transmission, c speed of light, B bandwidth for radar transmission of frequency-modulated signal, f d Doppler shift brought to target velocity, n h Is an upper threshold, n l Is the lower threshold.
Maximum radial velocity v of radar measurement target max 3400m/s (10 times the speed of sound), the amount of doppler shift by the target velocity is f d The threshold may be slightly relaxed considering noise etc. at 68kHz (it can be seen that the mismatch due to the target speed is very limited), where the threshold is set to n cl ＝98.0％、n cr ＝102.0％。
Step four: when the first path of matched filtering output is subjected to radar target detection processing to detect a target, whether the target is a frequency shift interference false target is determined according to the ratio of matched output peak values of the target under different half-frequency-width matched filters and a set threshold, and the judgment method is shown in a formula five.
Will be provided with
And n l 、n h By contrast, if H 0 If yes, the target is judged to be a true target, otherwise, the target is a false target. In order to clearly illustrate the false target identification effect of the invention, the radar receiving signal is arranged to pass through H on the premise of not influencing the simulation correctness 0 (f) All of which are 1, see fig. 4(a), and the radar reception signal passes through H L (f) And H 0 (f) The corresponding target output peak values of (a) are compared. Target echo can be seen from FIG. 4(b) through H L (f) And H 0 (f) The ratio of the post-amplitude is about 1:1 and the ratio of the frequency-shift interference signal is no longer 1:1, about 40% and 80%, respectively, far below the set threshold n l And the false target can be judged, and the identification of the frequency shift interference false target is realized.
Claims (1)
1. A method for identifying frequency shift interference of a linear frequency modulation signal is characterized by comprising the following steps:
the method comprises the following steps: preprocessing signals received by a radar, and dividing the signals into three paths before matching filtering:
firstly, preprocessing a signal received by a radar receiver by filtering, low-noise amplification, down-conversion and band-pass filtering, then dividing the received radar signal into three paths, wherein the first path is used for normal target detection, and the second path and the third path respectively extract peak values and compare the peak values to identify whether the target is a false target;
step two: respectively passing the three signals through a complete matched filter, a first half-bandwidth matched filter and a second half-bandwidth matched filter, and extracting output voltage peak values of the half-bandwidth matched filters:
the first half-bandwidth matched filter is close to the low-frequency side of the perfect matched filter in the frequency domain, the second half-bandwidth matched filter is close to the high-frequency side of the perfect matched filter in the frequency domain, and the widths of the first half-bandwidth matched filter and the second half-bandwidth matched filter are half of the widths of the perfect matched filters; setting linear frequency modulation signal x (T), radar transmitting frequency modulation signal bandwidth as B, pulse width as T, its complete matching filter response h 0 (t), first half-band matched filteringWave filter impulse response h L (t) second half-bandwidth matched filter impulse response h R (t) are respectively:
wherein ". sup" denotes conjugation, t 0 Outputting peak value time for matched filtering, wherein k is a normalization coefficient, and t is time;
passing the linear frequency-modulated signal x (t) through a three-way matched filter h 0 (t)、h L (t)、h R (t) the frequency domain expression of the output is:
wherein X (f) is the Fourier transform of the chirp signal x (t), H 0 (f) For a perfectly matched filter response h 0 (t) Fourier transform, H L (f) For a first half-band matched filter impulse response h L (t) Fourier transform, H R (f) Matching the filter impulse response h for the second half bandwidth R (t) Fourier transform, Y 0 (f) For the chirp signal x (t) through a perfect match filter h 0 Fourier transform of the post output, Y L (f) Passing the chirp signal x (t) through a first half-band matched filter pulse h L Fourier transform of the post output, Y R (f) Pulsing h through a second half-bandwidth matched filter for a chirp signal x (t) R (t) post output fourier transform;
and (3) carrying out Fourier inverse transformation on the result of the formula II, and calculating a signal matching output peak value:
wherein, | · | is absolute value calculation, j is an imaginary unit, f represents frequency, and t represents time;
setting the matching output peak values as y 0max 、y Lmax 、y Rmax Here y 0max ＝max(|y 0 (t)|)，y Lmax ＝max(|y L (t)|)，y Rmax ＝max(|y R (t) |), max (·) is the maximum value operation;
step three: setting a threshold:
taking into account the Doppler shift f due to the target velocity d ＝2v max /λ＝2v max f 0 C caused mismatch, determine threshold:
wherein v is max The maximum speed of a measurable target of the radar, lambda is the wavelength of a radar transmission signal, f 0 Carrier frequency for radar transmitted signal, c speed of light, f d Doppler shift brought about by target velocity, n h Is an upper threshold, n l Is a lower threshold;
step four: when the first path of matched filtering output is processed by radar target detection to detect a target, determining whether the target is a frequency shift interference false target according to the ratio of matched output peaks of the target under different half-bandwidth matched filters and in combination with a set threshold:
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