CN109557542B - Bistatic forward-looking SAR imaging method in diving mode - Google Patents
Bistatic forward-looking SAR imaging method in diving mode Download PDFInfo
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Abstract
The invention discloses a bistatic forward-looking SAR imaging method in a dive mode; an imaging model of reconnaissance aircraft and guided missile cooperation, vertical flight path and guided missile uniform velocity dive attack is provided, a geometric model of a uniform velocity dive bistatic forward-looking SAR is established, a corresponding slant-range process model is provided, and an imaging algorithm based on receiver fixed equivalence is provided aiming at the slant-range process characteristics. The invention enables the double-bullet matching model to adopt a bullet to attack a bullet for irradiation imaging, and the irradiation bullet is far away from an imaging area.
Description
Technical Field
The invention relates to a bistatic forward-looking SAR imaging method in a diving mode, which mainly researches diving imaging guidance of a reconnaissance machine and a guided missile in a matching mode, and in order to ensure safety, the reconnaissance machine irradiates outside a defense area, and the guided missile flies to a target area to acquire an echo signal and image. The scout plane is perpendicular to the motion track of the missile, and the missile dives to attack the target area, and a corresponding imaging algorithm is designed.
Background
The key for realizing intelligent striking and accurate striking is to improve the detection, identification and resolution capability of the final guide section of the accurately guided weapon. The terminal guidance radar of the active anti-ship missile mainly obtains the angle and distance information of a target through monopulse angle measurement or one-dimensional distance image, has low azimuth resolution, is not beneficial to the selection and tracking of a guide head on the target and the striking of a critical part, and is difficult to meet the increasing target identification and accurate striking requirements of an accurate guided weapon. Therefore, point target guidance, one-dimensional range image guidance and two-dimensional imaging guidance and even three-dimensional imaging guidance are inevitable trends in the development of radar seeker.
With the continuous development of Synthetic Aperture Radar (SAR) signal processing technology and processing devices and the trend of miniaturization and real-time sensor, the SAR and guidance are combined, so that the Radar seeker has imaging capability and becomes a new direction for the application of SAR imaging technology. Missile-borne SAR guidance can provide a high-resolution two-dimensional image of a target area, and has great advantages compared with one-dimensional imaging guidance.
The SAR sensor is applied to the end guidance of the anti-ship missile, and the following capabilities of the anti-ship missile can be improved: (1) target recognition capability. By adopting SAR imaging guidance, the target can be identified by using an image, the target condition can be accurately judged, and then the target with the maximum attack value is selected, so that the attack efficiency is improved; (2) and (4) anti-interference capability. The SAR imaging sensor is a typical microwave sensor, has strong weather interference resistance and is suitable for combat application under severe natural conditions; (3) and (4) damage judgment capability. The missile is used as a disposable weapon and can be used for hitting targets and simultaneously realizing information reconnaissance tasks.
Compared with the traditional flat flying bistatic SAR, the missile height changes along with time when the missile-borne SAR performs the dive motion, the bistatic operation track is more complex, and the application of the conventional bistatic forward-looking SAR imaging method is limited. In the conventional missile-borne SAR nose-down section research, the research on nose-down imaging of a single-base missile-borne SAR is mostly carried out, and the research is only limited to a missile-borne SAR nose-down section side-view and squint imaging method due to the system limitation of the single-base SAR. In order to obtain higher azimuth resolution, the receiving and transmitting platform of the bistatic forward-looking SAR cannot be close to a target area at the same time, so that the bistatic matching model needs to adopt a bullet to attack a bullet to irradiate and image, and the irradiated bullet needs to be far away from an imaging area.
Disclosure of Invention
1. Objects of the invention
The invention provides a bistatic forward-looking SAR imaging method in a diving mode in order to solve the imaging problem of a missile-borne SAR.
2. The technical scheme adopted by the invention
The invention provides a bistatic forward-looking SAR imaging method in a uniform-speed dive mode, which comprises the following steps:
1) carrying out range Fourier transform on the echo signal to a range frequency domain-azimuth time domain;
2) performing distance compensation, and equivalently fixing the receiver;
3) carrying out azimuth Fourier transform on the signal to a two-dimensional frequency domain and carrying out high-order coupling term compensation;
4) performing inverse range-to-Fourier transform on the signals to a range-Doppler domain, performing coordinate mapping, and converting data into a transmitter echo form;
5) multiplying the signal by a reference function, and performing Chirp Scaling operation;
6) performing range Fourier transform on the signal to a two-dimensional frequency domain, and performing range compression and migration correction;
7) performing range-to-Fourier inverse transformation on the signal to a range-Doppler domain;
8) performing azimuth compression on the signals;
9) and performing azimuth Fourier inverse transformation on the signals to obtain an imaging result.
3. The invention has the beneficial effects that:
(1) the invention establishes a constant-speed diving bistatic forward-looking SAR imaging method, mainly aims at diving imaging guidance of a reconnaissance plane and a guided missile, and in order to ensure safety, the reconnaissance plane irradiates outside a defense area, the guided missile flies to a target area, and echo signals are obtained and imaged.
(2) The invention establishes a geometric model of a constant-speed dive bistatic foresight SAR, provides a corresponding slope distance process model, and provides an imaging method based on receiver fixed equivalence aiming at the characteristics of the slope distance process;
(3) the invention enables a bistatic forward-looking SAR receiving and transmitting platform to be incapable of being close to a target area at the same time, so that a bistatic matching model adopts a bullet to attack a bullet for irradiation imaging, and the irradiation bullet is far away from an imaging area.
Drawings
FIG. 1 shows a geometric model of a constant-speed dive bistatic forward-looking SAR in the present invention.
In the figure, T, R represents a transmitter and a receiver, respectively, and P is a scattering point in the imaging region. O is the origin of coordinates, the negative direction of the transmitter flight direction is the x-axis, the projection of the receiver flight direction is the y-axis, and the z-axis refers to the sky. The transmitter operates in squint mode at a velocity v T Oblique angle of view theta T The synthetic aperture center time slant distance is R T0 . While the receiver flies towards the target area, this time setting
Is the angle of incidence, velocity is v R The synthetic aperture center time slant distance is R R0 。
FIG. 2 is a flow chart of a constant-speed dive bistatic forward-looking SAR imaging method based on receiver fixation equivalence.
Detailed Description
For the geometric model of the constant-speed dive bistatic forward-looking SAR, the slope distance course of the system at the time t can be represented as follows:
as shown in the formula (1), when the missile descends in a constant-speed diving mode, the missile advances towards a target, the receiver moves linearly relative to the target in a constant-speed mode, and a linear walking item and a fixed distance item are brought in a bistatic slant range process. During imaging processing, distance compensation is firstly carried out on a linear term in a distance frequency domain-azimuth time domain, the linear term is converted into an echo signal in a transmitter squint and receiver fixed mode, and then a mature monoradical SAR imaging method is adopted for processing.
Neglecting the non-backscattering coefficient of a ground target point, and the expression of the echo signal received by the missile during uniform-speed diving is
Wherein exp is a natural index, w r (τ) is a distance window function, w a (t) is an azimuthal window function, k r Distance-modulated frequency, c the speed of light, and λ the signal wavelength.
Using POSP to perform range-to-Fourier transform on the echo signal to range frequency domain-azimuth time domain to obtain
Wherein f is r Is the distance frequency, f c Is the signal center frequency. Then a distance compensation is carried out, the compensation function being
After compensation, equation (3) becomes
In the formula:
it can be seen that the distance compensation translates the ramp history of the dive bistatic forward-looking SAR to the sum of the ramp history of the transmitter and the fixed receiver in squint.
Using POSP to carry out azimuth Fourier transform on the formula (5) to obtain
S r (f r ,f a )＝∫S r (f r ,t)exp(-j2πf a t)dt (6)
Wherein f is a Is the azimuth frequency. Let the phase of the above integrated function be phi 2 Reuse of POSP to order
Solving the stationary phase point t, the method can obtain
Solved to obtain a stationary phase point
Substitution into phi 2 The two-dimensional frequency spectrum of the echo signal after the azimuth Fourier transform can be obtained as
In the formula: and the fourth exponential term is a distance direction and azimuth direction coupling term, and when an imaging method is deduced, a root sign needs to be expanded, and distance direction and azimuth direction decoupling processing is carried out.
When the frequency spectrum is spread, a Chebyshev polynomial is adopted
About f r Spread out to three orders.
Therefore, the temperature of the molten steel is controlled,
in the formula: k is i Where i is 0,1, …, n is the coefficient of Chebyshev polynomial, and the calculation formula is
Wherein, f k ' is a normalized Chebyshev node.
Substituting formula (10) for formula (9) to obtain
According to the above formula, the flow process of the CS algorithm is adopted herein.
Firstly, compensating high-order coupling terms in a two-dimensional frequency domain, and taking a reference distance R T_ref Compensation function of (a)
H hoc (f r ,f a ；R T_ref )＝exp(j8πR T_ref K 3 (f a )f r 3 ) (13)
After the compensation of the higher-order coupling term, the signal is subjected to range Doppler conversion to a range Doppler domain to obtain
In the formula:
mapping (15) the coordinates, i.e.
To obtain
In the formula (16), the new distance delay is cR T0 (K 1 (f a )-3K 3 (f a ) C) to be consistent with the conventional CS method, let the bending factor be C s (f a ) Then, then
C s (f a )＝c(K 1 (f a )-3K 3 (f a ))-1 (17)
Thereby, the formula (16) becomes
Taking the reference distance as R T_ref Setting the Chirp Scaling phase factor to
Multiplying formula (19) by formula (18) to obtain
It can be seen that the range migration amount at different distances is changed into R through the scale transformation of the range migration curve T_ ref C s (f a ) The bending amount can be corrected uniformly in the distance frequency domain directly. FFT is carried out on the equation (20) in the distance direction to obtain
The formula (21) is multiplied by the following compensation function, so that the distance compression and the distance migration correction can be simultaneously completed, and k is ignored m Taking the space-variant of the distance as the reference distance R T_ref To
Then, the formula (21) is transformed to a distance time domain and multiplied by the following compensation function, thereby completing azimuth compression and phase correction
Therefore, the specific steps of the imaging process are:
1) carrying out range Fourier transform on the echo signal to a range frequency domain-azimuth time domain;
2) performing distance compensation, and equivalently fixing the receiver;
3) and performing azimuth Fourier transform on the signal to a two-dimensional frequency domain and performing high-order coupling term compensation.
4) And performing range-to-Fourier inverse transformation on the signals to a range-Doppler domain, performing coordinate mapping, and converting the data into a transmitter echo form.
5) And multiplying the signal by a reference function to perform Chirp Scaling operation.
6) Performing range Fourier transform on the signal to a two-dimensional frequency domain, and performing range compression and migration correction;
7) performing range-to-Fourier inverse transformation on the signal to a range-Doppler domain;
8) performing azimuth compression on the signals;
9) and performing azimuth Fourier inverse transformation on the signals to obtain an imaging result.
Claims (2)
1. A bistatic forward-looking SAR imaging method in a uniform-speed dive mode is characterized by comprising the following steps:
1) carrying out range Fourier transform on the echo signal to a range frequency domain-azimuth time domain;
2) performing distance compensation, equivalently fixing the receiver, solving to obtain a stationary phase point, and obtaining an echo signal two-dimensional frequency spectrum after azimuth Fourier transform;
3) carrying out azimuth Fourier transform on the signal to a two-dimensional frequency domain and carrying out high-order coupling term compensation;
4) carrying out range-to-Fourier inverse transformation on the signals to a range-Doppler domain, carrying out coordinate mapping, and converting data into a transmitter echo form;
5) multiplying the signal by a reference function, and performing Chirpscaling operation;
6) performing range Fourier transform on the signal to a two-dimensional frequency domain, and performing range compression and migration correction;
7) performing range-to-Fourier inverse transformation on the signal to a range-Doppler domain;
8) performing azimuth compression on the signals;
9) performing azimuth Fourier inverse transformation on the signals to obtain an imaging result;
the method specifically comprises the following steps: during imaging processing, mapping the total echo delay time of a coordinate axis to the time taken by a transmitting signal to a target, and adopting the flow processing of a CS algorithm:
firstly, compensating a high-order coupling term in a two-dimensional frequency domain, and taking a reference distance R T_ref Compensation function of
H hoc (f r ,f a ；R T_ref )＝exp(j8πR T_ref K 3 (f a )f r 3 ) (13)
After the compensation of the higher-order coupling term, the signal is subjected to range Doppler conversion to a range Doppler domain to obtain
In the formula:
mapping (15) the coordinates, i.e.
To obtain
In the formula (16), the new distance delay is cR T0 (K 1 (f a )-3K 3 (f a ) C) to be consistent with the conventional CS method, let the bending factor be C s (f a ) Then, then
C s (f a )＝c(K 1 (f a )-3K 3 (f a ))-1 (17)
Thereby, the formula (16) becomes
Taking the reference distance as R T_ref Setting the Chirp Scaling phase factor to
Multiplying formula (19) by formula (18) to obtain
It can be seen that the range migration amount at different distances is changed into R through the scale transformation of the range migration curve T_ref C s (f a ) The bending amount can be corrected uniformly in a distance frequency domain directly; performing distance direction FFT on the formula (20) to obtain
Multiplying the formula (21) by the following compensation function to simultaneously complete range compression and range migration correction, and neglecting k m Taking the space-variant with distance at a reference distance R T_ref Is arranged at
Then, the formula (21) is transformed to a distance time domain and multiplied by the following compensation function, thereby completing azimuth compression and phase correction
2. The constant velocity dive mode bistatic forward-looking SAR imaging method according to claim 1, comprising the following steps:
the ramp history of the system at time t can be expressed as:
wherein R is T0 For transmitter zero-time skew, R R0 For receiver zero-time skew, v T Is transmitter speed, v R For receiver speed, θ T The expression of the echo signal received by the missile during uniform-speed diving for the oblique angle of the transmitter is
Wherein exp is a natural index, w r (τ) is a distance window function, w a (t) is an azimuthal window function, k r Distance direction modulation frequency, c is light speed, and lambda is signal wavelength;
using POSP to perform range-to-Fourier transform on the echo signal to range frequency domain-azimuth time domain to obtain
Wherein f is r Is the distance frequency, f c Is the signal center frequency; then a distance compensation is carried out, the compensation function being
After compensation, equation (3) becomes
In the formula:
the distance compensation enables the slope distance process of the dive bistatic forward-looking SAR to be converted into the sum of the slope distance process of a transmitter with strabismus and the slope distance process of a fixed receiver;
using POSP to carry out azimuth Fourier transform on the formula (5) to obtain
S r (f r ,f a )＝∫S r (f r ,t)exp(-j2πf a t)dt (6)
Wherein f is a For azimuthal frequency, let the phase of the above integrated function be Φ 2 Reuse of POSP, order
Solving for stationary phase point t * Obtained by
Solved to obtain a stationary phase point
Substitution into phi 2 The two-dimensional frequency spectrum of the echo signal after the direction Fourier transform can be obtained as
In the formula: the fourth exponential term is a distance direction and azimuth direction coupling term, and when an imaging method is deduced, a root sign needs to be unfolded, and distance direction and azimuth direction decoupling processing is carried out;
when the frequency spectrum is expanded, a Chebyshev polynomial is adopted
About f r Spreading to three orders;
therefore, the first and second electrodes are formed on the substrate,
wherein: k i Where i is 0,1, …, n is the coefficient of Chebyshev polynomial, and the calculation formula is
Wherein, f' k In order to be a normalized chebyshev node,
substituting formula (10) for formula (9) to obtain
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