A method for coherent processing of echo signals based on space-based multi-source fusion of external radiation

Abstract

The present invention provides a method for coherent processing of echo signals based on space-based external radiation multi-source fusion. The main problem faced by space-based external radiation source radars is that the landing power of navigation satellites and other satellites is relatively weak. In addition to extending the accumulation time, using rich navigation satellite signals for coherent accumulation of multi-satellite signals is another important way to improve the gain. The basic idea of the present invention is to use the target reflection signal of one navigation satellite as a reference to perform coherent processing on the target reflection signal of another navigation satellite, compensate for the phase difference caused by the different sources of the two satellites, so as to achieve coherent accumulation of the echoes of signals from different satellites.

Description

Technical Field

[0001] The present invention belongs to the field of radar signal processing, and relates to a method for coherent processing of echo signals. Background Art

[0002] The main problem based on the space-based passive radar is that the downlink power of navigation satellites is relatively weak. In addition to extending the accumulation time, using the rich navigation satellite signals for coherent accumulation of multi-satellite signals is another important way to improve the gain.

[0003] Currently, the application of the target reflection signals of multiple navigation satellites is still limited to non-coherent accumulation. This is because although navigation satellites are strict time synchronization systems, the signals are not coherent with each other, and it is impossible to directly perform coherent accumulation on the target echo signals of multiple navigation satellites. The gain of non-coherent accumulation is about times. Taking the fusion of three satellites as an example, the maximum gain loss of non-coherent accumulation reaches 2.39 dB. Summary of the Invention

[0004] The technical problem to be solved by the present invention is: The present invention proposes a method for coherent processing of echo signals based on space-based external radiation multi-source fusion. Through the coherent processing method proposed by the present invention, the fixed phase shift and linear phase shift parameters between the echo signals of multiple navigation satellite signals that are not coherent with each other can be estimated, so as to realize the coherent processing of the echoes of multiple navigation satellites, achieve multi-source coherent fusion, improve the processing gain, and expand the radar detection range.

[0005] The technical solution of the present invention is: A method for coherent processing of echo signals based on space-based external radiation multi-source fusion, taking the target reflection signal of one navigation satellite as a reference, performing coherent processing on the target reflection signal of another navigation satellite, and compensating for the phase difference caused by different sources of the two satellites, so as to realize the coherent accumulation of the echoes of different satellite signals.

[0006] A method for coherent processing of echo signals based on space-based external radiation multi-source fusion includes the following steps:

[0007] (1) Use the direct wave receiving antenna to receive all navigation satellite signals, complete the calculation of navigation information, determine the position of each navigation satellite, and extract the PRN code information of the direct wave signal; use the echo receiving antenna to receive the navigation satellite signals reflected by the target, that is, the echo signals;

[0008] (2) Establish a spherical coordinate system with the detection radar coordinate as the origin, and convert the positions of multiple navigation satellites to the spherical coordinate system;

[0009] (3) Select a navigation satellite from the above navigation satellites as the reference satellite. According to the distance difference between the reference satellite and the other navigation satellites to the origin of the spherical coordinate system, compensate for the different target distances caused by the different positions of the navigation satellites.

[0010] (4) For each navigation satellite, perform correlation processing on the echo signal processed in step (3) using its PRN code to obtain the correlation result for each navigation satellite signal, and then obtain the correlation function of the correlation results between the reference satellite and each of the other navigation satellites.

[0011] (5) For each correlation function, estimate the incoherent parameters existing between the target scattering signals of the corresponding two navigation satellites, determine the estimated value of the linear phase shift and the fixed phase shift, and then combine them into the phase difference of the corresponding two navigation satellites.

[0012] (6) Based on the above phase difference, compensate for the phase difference caused by the different sources of the two navigation satellites, so as to realize the coherent accumulation of the echo signals of different navigation satellite signals.

[0013] Preferably, in step (3), the different target distances caused by the different positions of the navigation satellites are compensated in the following manner:

[0014] Let the distance difference Δγ between the reference satellite and navigation satellite m to the origin of the spherical coordinate system m , then the echo sequence of navigation satellite m is uniformly shifted backward or forward by round(Δγ m / FS) points. When shifting forward, the front-end signal removes the shifted-out part and zeros are filled at the back-end; when shifting backward, the back-end signal removes the shifted-out part and zeros are filled at the front-end; where round() is the ceiling function and FS is the distance unit length corresponding to the radar sampling rate.

[0015] Preferably, the estimated value of the linear phase shift is determined in the following manner:

[0016] Search for the position where the peak of the correlation function is located. Let its peak position be N, then the estimated value of the linear phase shift N′ is the number of points in the circular convolution during the determination of the correlation function.

[0017] Preferably, the value of N′ satisfies the following relationship: N′·prf is equal to twice the bandwidth of the direct wave signal, where prf is the signal repetition frequency of the navigation signal.

[0018] Preferably, the fixed phase shift is determined in the following manner:

[0019] Define the function:

[0020]

[0021] Search for the minimum value of H1(Δα1) at a preset step size in the interval [0, 2π]. Assume that when Δα1 = α1', H1(Δα1) has the minimum value, then fix the phase shift α1 = α1'.

[0022] Among them, α2 is the estimated value of the linear phase shift, N is the position where the peak of the correlation function is located, k is the signal frequency factor, and the specific calculation method is k = 2*π*f c , f c is the direct wave frequency of the navigation satellite, and m1(p) and m2(p) are the Fourier transform results of the echo signal R G (n), respectively, and p is the sampling point.

[0023] Preferably, in order to obtain sufficiently high calculation accuracy, the search step size is greater than or equal to

[0024] Preferably, the phase difference formula between two navigation satellites is as follows:

[0025] Among them, α2 is the estimated value of the linear phase shift, α1 is the fixed phase shift, k is the signal frequency factor, and the specific calculation method is k = 2*π*f c , f c is the direct wave frequency of the navigation satellite.

[0026] The advantages of the present invention compared with the prior art are as follows: The echo signal coherence processing method based on space-based external radiation multi-source fusion proposed by the present invention does not rely on the data communication link between multiple sources to transmit clock differences, but estimates the incoherent parameters of the target scattering signals of two / multiple transmit sources through the echo signals, and realizes the coherence processing of multi-source echoes by obtaining the linear phase shift and the fixed phase shift. The present invention coherently processes the navigation satellite signals that could not be multi-source fused originally, and improves the processing gain of the external radiation source radar. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is the multi-source fusion architecture diagram of the present invention;

[0028] Figure 2 is the amplitude before coherent accumulation in the embodiment of the present invention;

[0029] Figure 3 is the amplitude value after multi-source fusion after coherence in the embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0030] The main problem faced by space-based external radiation source radars is that the landing power of navigation and other satellites is relatively weak. In addition to extending the accumulation time, coherent accumulation of multi-satellite signals using the abundant navigation satellite signals is another important way to improve the gain. The basic idea of the present invention is to use the target reflection signal of one navigation satellite as a reference to perform coherence processing on the target reflection signal of another navigation satellite, compensate for the phase difference caused by the different sources of the two satellites, and thus achieve coherent accumulation of the echoes of different satellite signals.

[0031] A method for coherent processing of echo signals based on space-based external radiation multi-source fusion, as Figure 1 shown, the steps are as follows:

[0032] (1) Use the direct wave receiving antenna to receive the navigation satellite signal, and use the parallel code phase search method to complete the calculation of the navigation information. The steps are to multiply the input digital intermediate frequency signal orthogonally with the local carrier and then perform Fourier transform (FFT or DFT), then multiply it conjugately with the Fourier transform of the code, then perform inverse Fourier transform (IFFT), and finally take the absolute value squared. Under a certain threshold, the visible navigation satellite signal is captured. After analyzing the characteristics of the direct wave signal, the carrier and PRN code information of the direct wave signal are extracted for use in target echo calculation. Use the echo receiving antenna to receive the navigation satellite signal reflected by the target, that is, the echo signal; in the present invention, the bandwidths of the direct wave signals of all navigation satellites are the same.

[0033] (2) Establish a spherical coordinate system with the coordinates of the detection radar as the origin, and convert the positions of multiple navigation satellites from the rectangular coordinate system to the above-mentioned spherical coordinate system; among them, the navigation satellite coordinate system is the geocentric rectangular coordinate system, with the geocenter as the coordinate origin, its Z-axis pointing to the geodetic north pole, the X-axis pointing to an intersection of the reference meridian plane and the earth's equator, and the X, Y, and Z axes form a right-handed rectangular coordinate system. The conversion target coordinate system takes the receiving radar as the origin, the antenna beam center pointing as the spherical coordinate reference plane, θ represents the azimuth angle, β is the elevation angle, and it is negative when rotating clockwise from the antenna aperture exit direction and positive when rotating counterclockwise, and γ is the distance from the target to the radar.

[0034] The coordinate conversion process is as follows: Let the position of the receiving detection radar be (x1, y1, z1), and the position of any navigation satellite be (x2, y2, z2), satisfying the following transformation relationship:

[0035]

[0036]

[0037]

[0038] where R is the average radius of the earth.

[0039] (3) The converted navigation satellite positions in step 2 can be used to compensate for the differences in target distances caused by different navigation satellite positions.

[0040] Let the distance difference measured by the direct waves of two navigation satellites 1 and 2 be Δγ. Then, taking navigation satellite 1 as the reference, the echo sequence of navigation satellite 2 is uniformly shifted backward or forward by round(Δγ / FS) points. When shifting forward, the front-end signal removes the shifted-out part and zeros are filled at the back end; when shifting backward, the back-end signal removes the shifted-out part and zeros are filled at the front end. FS is the distance unit length corresponding to the radar sampling rate, and the specific calculation method is:

[0041] FS = c / fs

[0042] where c is the speed of light and fs is the radar sampling frequency, with the unit of Hz.

[0043] The remaining navigation satellites are compensated and processed in the same way with navigation satellite 1 as the reference.

[0044] (4) The compensated echo signals in step 3 are correlated using the ranging codes obtained from the direct waves to obtain the correlation results of multiple groups of navigation satellite signals, which are R1(n) and R2(n) …… R G (n), and they are uniformly calculated according to the following formula:

[0045]

[0046] In the formula, G(t) is the C / A code sequence as a function of time; the subscript G represents the navigation satellite number respectively, t is the time, τ is the correlation delay, and T CA is the length of the navigation signal pseudo-random code.

[0047] Assuming that navigation satellite 1 is the reference satellite, the correlation functions of R1(n) and the correlation results of the remaining navigation satellites are obtained respectively. Taking navigation satellite 2 as an example:

[0048] The correlation function X(n) of R1(n) and R2(n) is obtained according to the following formula:

[0049]

[0050] In the formula represents the N'-point circular convolution, and * represents taking the conjugate. Among them, in order to make the accuracy of the obtained linear phase shift α2 high enough, N' should be large enough. We take N' such that N'Δf is equal to twice the bandwidth of the direct wave signal; Δf is the step size for each signal search, which is related to the detection radar system and is a fixed value. In this example, the value is 1 kHz.

[0051] (5) Search for the position of the peak of X(n). The position of the peak is N, and the estimated value of the linear phase shift α2 is:

[0052]

[0053] (6) Define a function:

[0054]

[0055] Search for the minimum value of H1(Δα1) at a step size of 0.05 in the interval [0, 2π]. Assume that when Δα1 = α1', H1(Δα1) has the minimum value, then fix the phase shift α1 = α1'.

[0056] (7) After obtaining the fixed phase shift α1 and the linear phase shift α2, let: the phase difference between the two navigation satellites

[0057]

[0058] In this way, the y2(k) signal coherent with y1(k) is obtained.

[0059] To verify the effectiveness of the echo signal coherence processing method based on space-based external radiation multi-source fusion proposed by the present invention, the accumulation signal-to-noise ratios before and after using the method proposed by the present invention are simulated and compared as Figure 2 、 3 shown. It can be seen from the figure that before the coherence processing, the amplitudes of the target's individual accumulations are 2.02×10^7 and 1.585×10^7 respectively. After the coherence processing using the method proposed by the present invention, the amplitude of the signal after the two signals are coherently fused reaches 3.486×10^7, and the highest sidelobe is only 1.23×10^7.

[0060] The content not described in detail in the specification of the present invention belongs to the well-known technology of those skilled in the art.

Claims

1. A method for coherent processing of echo signals based on space-based external radiation multi-source fusion, characterized in that It includes the following steps: (1) Use the direct wave receiving antenna to receive all navigation satellite signals, complete the calculation of navigation information, determine the position of each navigation satellite, and extract the PRN code information of the direct wave signal; Use the echo receiving antenna to receive the navigation satellite signals reflected by the target, i.e., the echo signals; (2) Establish a spherical coordinate system with the detection radar coordinates as the origin, and convert the positions of multiple navigation satellites to the spherical coordinate system; (3) Select one navigation satellite from the above navigation satellites as the reference satellite, and compensate for the different target distances caused by the different positions of the navigation satellites according to the distance differences between the reference satellite and the other navigation satellites to the origin of the spherical coordinate system; (4) For each navigation satellite, perform correlation processing on the echo signal processed in step (3) using its PRN code to obtain the correlation results for each navigation satellite signal, and further obtain the correlation function of the correlation results between the reference satellite and each of the other navigation satellites; (5) For each correlation function, estimate the incoherent parameters existing between the target scattering signals of the corresponding two navigation satellites, determine the estimated value of the linear phase shift and the fixed phase shift, and then combine them into the phase difference of the corresponding two navigation satellites; (6) According to the above phase difference, compensate for the phase difference caused by the different sources of the two navigation satellites, so as to realize the coherent accumulation of the echoes of different navigation satellite signals; The estimated value of the linear phase shift is determined by the following method: Search for the peak position of the correlation function. Let the peak position be N. Then the estimated value of the linear phase shift N' is the number of points in the circular convolution during the determination of the correlation function; The fixed phase shift is determined by the following method: Define the function: Search for the minimum value of H1(Δα1) at preset step lengths in the interval [0,2π]. Suppose that when Δα1 = α1', H1(Δα1) has the minimum value, then the fixed phase shift α1 = α1'; Among them, α2 is the estimated value of the linear phase shift, N is the position where the peak of the correlation function is located, k is the signal frequency factor, and the specific calculation method is k = 2*π*f c , f c is the direct wave frequency of the navigation satellite, m1(p) and m2(p) are the Fourier transform results of the echo signal R G (n), respectively, and p is the sampling point.

2. The method according to claim 1, characterized in that: In step (3), the different target distances caused by the different positions of the navigation satellites are compensated by the following method: Let the distance difference Δγ between the reference star and the navigation satellite m from the origin of the spherical coordinate system m , then the echo sequence of the navigation satellite m is uniformly shifted backward or forward by round(Δγ m / FS) points. When shifting forward, the front-end signal removes the shifted-out part and zeros are filled at the back-end; when shifting backward, the back-end signal removes the shifted-out part and zeros are filled at the front-end. Among them, round() is the ceiling function, and FS is the distance unit length corresponding to the radar sampling rate.

3. The method according to claim 1, characterized in that: The value of N′ satisfies the following relationship: N′·prf is equal to twice the bandwidth of the direct wave signal, where prf is the signal repetition frequency of the navigation signal.

4. The method according to claim 1, characterized in that: To obtain a sufficiently high computational accuracy, the search step size is greater than or equal to 5. The method according to claim 1, characterized in that: The phase difference formula for two navigation satellites is as follows: Among them, α2 is the estimated value of the linear phase shift, α1 is the fixed phase shift, and k is the signal frequency factor. The specific calculation method is k = 2*π*f c , f c is the direct wave frequency of the navigation satellite.

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