A pseudo-random frequency-modulation ISAR image feature change method

Abstract

The application provides a pseudo-random frequency modulation ISAR image feature changing method, which comprises the following steps: step one, analyzing an ISAR signal model to obtain a frequency modulation; step two, adopting bandwidth and time width pseudo-random change to obtain a pseudo-random change frequency modulation, and generating a new LFM signal; and step three, mixing a control signal and a target signal, and changing a radar image feature after imaging processing. The application can make signal synthesis parameter acquisition relatively simple and fast, and only needs to acquire the bandwidth and pulse width of an LFM system ISAR radar signal. The frequency modulation of the control signal shows pseudo-random characteristics, so that the control and change of the image feature also show pseudo-random change characteristics. The synthesized control signal and the ISAR target signal are relatively close in time width and bandwidth, the difference is controlled within 5%, and the image feature control and change process is relatively concealed.

Description

[0001] The application is a method for changing the features of an inverse synthetic aperture radar (ISAR) image with pseudo-randomly changed frequency, and relates to the field of linear frequency modulation (LFM) system ISAR radar signal processing, in particular to a method for regulating and changing the features of an ISAR radar image with pseudo-randomly changed frequency.

[0002] As a high-resolution imaging wideband radar, ISAR is widely used in many fields. Based on advanced digital waveform synthesis technology, the influence of frequency change on imaging is studied for an LFM system ISAR, and the method of pseudo-randomly changed frequency is used, which is expected to quickly and conveniently realize the convenient regulation and covert change of ISAR image features.

[0003] The technical problem to be solved by the application is to realize the regulation and change of ISAR image features by using the signal forming technology of pseudo-randomly changed frequency.

[0004] The technical scheme adopted by the application is as follows:

[0005] Firstly, the ISAR signal model is analyzed to obtain the frequency change rate;

[0006] The LFM radar signal transmitted by the ISAR Wherein represents the fast time, t m represents the slow time; the signal bandwidth B and the time width T are obtained P , so that the frequency change rate is

[0007] Secondly, the bandwidth and time width are pseudo-randomly changed to obtain the pseudo-randomly changed frequency change rate, and based on a new LFM signal is generated;

[0008] By pseudo-randomly changing the bandwidth B and time width T P of the ISAR radar LFM signal, a new LFM signal bandwidth and time width are obtained, and the frequency change rate of the new LFM signal presents a pseudo-randomly changed characteristic; based on the frequency change rate a new LFM regulation signal is generated

[0009] Thirdly, the regulation signal and the target signal are mixed, and the radar image features are changed after imaging processing; ​​​

[0010] Will With the target echo signal After mixing, conventional range-Doppler imaging processing is performed.

[0011] The advantages of this invention are:

[0012] First, obtaining signal synthesis parameters is relatively simple and quick, requiring only the bandwidth and pulse width of the LFM-based ISAR radar signal.

[0013] Second, control signals The frequency modulation exhibits pseudo-random characteristics, which makes the modulation and alteration of image features also exhibit pseudo-random variation characteristics.

[0014] Third, the synthesized regulatory signals It is quite similar to the ISAR target signal in terms of time and bandwidth, with the difference controlled within 5%, and the process of image feature manipulation and alteration is relatively covert. [Attached Image Description]

[0015] Figure 1 This is a block diagram illustrating the implementation of a pseudo-random frequency modulated ISAR image feature alteration method.

[0016] Figure 2(a) is an image of the target scattering points.

[0017] Figure 2(b) is an ISAR image of the target.

[0018] Figure 3(a) shows the addition of pseudo-random frequency modulation within the range of 1%. and The target ISAR image after the modulation signal.

[0019] Figure 3(b) shows the addition of pseudo-random frequency modulation within the range of 5%. and The target ISAR image after the modulation signal.

Detailed Implementation Methods

[0020] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings:

[0021] This invention proposes a method for altering pseudo-random frequency-modulated ISAR image features, comprising the following steps:

[0022] The first step is to obtain the bandwidth, time width, and modulation frequency of the ISAR radar LFM signal.

[0023] Let the LFM radar signal transmitted by ISAR be...

[0024]

[0025] in, is a rectangular window function, j is a complex unit, represents the total time, the carrier frequency is f0. The signal bandwidth B and the time width T are obtained P , so the frequency modulation is

[0026] Second, in a certain range, the bandwidth and time width are pseudo-randomly changed to obtain a pseudo-randomly changed frequency modulation and based on a new LFM control signal is generated.

[0027] The bandwidth is limited and The bandwidth and time width are pseudo-randomly changed in a range of 5% of the corresponding parameters of the ISAR transmitted LFM signal,

[0028] that is and randsample represents a pseudo-random value, and a pseudo-randomly changed frequency modulation is obtained with signal waveform parameters to generate an LFM control signal

[0029]

[0030] where Δt represents the delay of the control signal.

[0031] Third, the control signal and the target signal are mixed, and the radar image features are changed after imaging processing.

[0032] The target is assumed to be a rotating platform target model, the rotating speed is ω (rad / s), and the center point distance from the radar is R0. The ISAR target echo signal can be represented as

[0033]

[0034] where i ∈ [1, I] represents a scattering point on the target, the coordinates in the target coordinate system are (x i ,y i ), and the total number is I; t i represents the time delay of the i-th scattering point. After distance-Doppler imaging processing, the result is

[0035]

[0036] where r represents the range, f represents the azimuth, T M represents the total time length in the slow time domain, sinc represents the sinc function, represents the signal wavelength, R Δim = R0+y i +x i ωt m-R ref R ref This is the reference range used in the de-firing frequency modulation processing technique commonly used in ISAR range imaging. (Due to the modulation signal...) Medium frequency It is pseudo-random and therefore cannot be focused after range-Doppler imaging, which will have the effect of changing the image features of the target imaging result H(r,f).

[0037] like Figure 1 The diagram shown is a block diagram of the pseudo-random frequency modulated ISAR image feature alteration method. Assume the LFM signal carrier frequency f0 = 10 GHz, B = 500 MHz, and the pulse length T... P =100µs, range sampling points 512, azimuth imaging pulses 256, radar repetition rate 1600Hz; the target scattering point image is shown in Figure 2(a), which is a civilian Yak-42 aircraft model, and its range-Doppler imaging results are shown in Figure 2(b). The bandwidth B is obtained by pseudo-randomly varying it within a range of 1%. Time width T P The time width is obtained by pseudo-random variation within a 1% range. Thus, a new modulation frequency is obtained. The range-Doppler imaging results after generating the LFM modulated signal and mixing it with the target signal are shown in Figure 3(a); the bandwidth B is obtained by pseudo-randomly varying it within a 5% range. Time width T P The time width is also controlled within a pseudo-random variation range of 5%. Thus, another set of modulation frequencies can be obtained. The range-Doppler imaging results after generating the LFM modulated signal and performing it together with the target signal are shown in Figure 3(b). By comparing the two sets of experimental results in Figure 3 with those in Figure 2(b), it can be seen that the target ISAR image features are significantly altered by the pseudo-random frequency modulated signal.

Claims

1. A method for altering pseudo-random frequency modulated ISAR image features, characterized in that: Includes the following steps: Step 1: Analyze the ISAR signal model to obtain its modulation frequency; LFM radar signal transmitted by ISAR in t indicates fast time. m Indicates slow time; obtains signal bandwidth B and time width T. P Thus, the frequency is tuned to Step two involves using pseudo-random variations in bandwidth and time width to obtain a pseudo-random frequency modulation (FCM), based on... Generate a new LFM signal; By analyzing the bandwidth B and time width T of the ISAR radar LFM signal P Perform pseudo-random variation processing to obtain a new LFM signal bandwidth. Time width Frequency modulation It exhibits pseudo-random variation characteristics; based on frequency modulation. Generate new LFM control signals Step 3: Mix the control signal and the target signal, perform imaging processing, and then change the radar image characteristics; Will With the target echo signal After mixing, conventional range-Doppler imaging processing is performed.

2. The pseudo-random frequency modulated ISAR image feature alteration method according to claim 1, characterized in that: In step one, let the LFM radar signal transmitted by ISAR be: in, Let j be a rectangular window function, where j is the complex unit. This indicates the full time period, with a carrier frequency of f0.

3. The pseudo-random frequency modulated ISAR image feature alteration method according to claim 1, characterized in that: In step two, the bandwidth is limited. and The parameters of the LFM signal transmitted by ISAR vary pseudo-randomly within 5% of the range. Right now randsample represents pseudo-random value selection, yielding the pseudo-random frequency modulation. by Generate LFM control signal based on signal waveform parameters. Where Δt represents the delay of the control signal.

4. The pseudo-random frequency modulated ISAR image feature alteration method according to claim 1, characterized in that: In step three, the target is assumed to be a turntable target model with a rotational speed of ω (rad / s) and a center point distance of R0 from the radar. Its ISAR target echo signal is then expressed as: Where i∈[1,I] represents the scattering point on the target, and its coordinates in the target coordinate system are (x... i ,y i ), the total number is I; t i This represents the time delay at the i-th scattering point.

5. The pseudo-random frequency modulated ISAR image feature alteration method according to claim 4, characterized in that: The result after distance-Doppler imaging processing is: Where r represents the range direction, f represents the azimuth direction, and T M Let sinc represent the total duration in the slow time domain, and sinc represent the symplectic function. R represents the signal wavelength. Δim =R0+y i +x i ωt m -R ref R ref This is the reference distance used in the demodulation technique commonly used in ISAR range imaging.

6. The pseudo-random frequency modulated ISAR image feature alteration method according to claim 5, characterized in that: Due to the control signal Medium frequency It is pseudo-random and therefore cannot be focused after range-Doppler imaging, which will have the effect of changing the image features of the target imaging result H(r,f).

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