A SAR illumination region inversion method

Abstract

A SAR irradiation area inversion method, which draws a zebra graph by using received spaceborne SAR radar signals; determines a SAR radar incidence angle range by using the zebra graph and a signal repetition frequency parameter; determines a pulse amplitude through the incidence angle range, and determines a real incidence angle by using a fitting processing; and inverts a spaceborne SAR radar irradiation area by using the determined incidence angle and spaceborne SAR radar orbit information. The method is suitable for reconnaissance and countermeasure of spaceborne synthetic aperture radar (SAR) in the field of electronic countermeasure, and the inverted spaceborne SAR radar irradiation area has high reliability and good accuracy, so that accurate radar working state information can be provided for spaceborne synthetic aperture radar (SAR) information countermeasure, and the irradiation area inverted by the method can be used for protection of important areas in wartime.

Description

Technical Field

[0001] This invention relates to a method for inverting the illumination area of ​​a spaceborne synthetic aperture radar (SAR), and particularly to a method for detecting and countering spaceborne synthetic aperture radar (SAR) signals, belonging to the field of spaceborne synthetic aperture radar (SAR) signal processing. Background Technology

[0002] Spaceborne synthetic aperture radar (SAR) is a high-resolution reconnaissance imaging radar that can acquire ground images in real time, around the clock, and has excellent visibility of vegetation and the ground. It has a wide range of military applications and also poses a significant threat to the military of various countries.

[0003] In spaceborne synthetic aperture radar (SAR), due to the high altitude of the satellite orbit, at any given time, not only are signal echoes from multiple observed areas propagating in the air, but also multiple nadir echo signals. Since the power of the nadir echo is much greater than that of echoes from other directions, the position of the echo window must avoid interference from non-human-induced signals such as transmitted signals and nadir signals. To prevent the echoes from being blocked by transmitted pulses across the entire mapping area by spaceborne SAR, the echoes must be received within a reception time several transmitted pulses later. Therefore, the pulse repetition frequency should be reasonably selected so that the nadir echoes are constrained between the transmitted pulse and the useful echo signal acquisition window, and the useful echo signal acquisition window for the observation area should be located within the transmission pulse repetition period.

[0004] A zebra plot is a set of curves plotted based on the relationship between the incident angle and the pulse repetition frequency. This graph resembles a rhombus, hence its common name, zebra plot, or rhombus plot. In radar design, the zebra plot is typically used to select the pulse repetition frequency. Furthermore, in the process of retrieving the illumination area of ​​a spaceborne synthetic aperture radar (SAR), the possible incident angle of the SAR can be inferred from the received SAR radar signal pulse repetition frequency and the zebra plot.

[0005] To effectively counter spaceborne synthetic aperture radar (SAR) systems, it is essential to acquire their effective signals in a timely manner, accurately determine their illumination area, operating mode, and illumination range, and provide accurate radar operational status intelligence for countering spaceborne SAR systems.

[0006] Because reconnaissance satellites orbit the Earth, their overpass times can be calculated. However, this overpass time limitation restricts observation of specific targets. Furthermore, when a spaceborne synthetic aperture radar (SAR) operates in a specific orbit and its operating mode is determined, its zebra pattern is also fixed. Therefore, the illumination area of ​​the spaceborne SAR at different time periods can be calculated. For the location of the area to be protected, based on the front-end inversion, jamming equipment can be activated when the radar illuminates the protected area to maintain its integrity. Summary of the Invention

[0007] The purpose of this invention is to provide a method for inverting SAR illumination areas. This method can draw a radar zebra diagram based on the parameters of the received spaceborne synthetic aperture radar (SAR) signal, identify possible incident angles, and compare them with the pulse amplitude of the received SAR signal to obtain the incident angle of the spaceborne SAR, thereby inverting the SAR illumination area.

[0008] The technical solution of this invention is:

[0009] A method for inverting SAR illumination areas, characterized by comprising the following steps:

[0010] Step 1: Receive the spaceborne synthetic aperture radar signal and extract the pulse repetition frequency, pulse width, and pulse amplitude.

[0011] Step 2: Draw a radar zebra diagram using the altitude information and obtained pulse width from the spaceborne synthetic aperture radar; the altitude information of the spaceborne synthetic aperture radar is information that can be known in advance.

[0012] Step 3: Determine the possible radar incident angle range θ based on the radar zebra pattern drawn in Step 2 and the pulse repetition frequency obtained in Step 1. i , i = 1…n;

[0013] Step 4: For each θ in Step 3 i The pulse amplitude A of the corresponding signal i ;

[0014] Step 5: Transfer each A i The pulse amplitude obtained in step 1 is fitted, and the incident angle θ of the spaceborne synthetic aperture radar is determined based on the fitting result.

[0015] Step 6: Use the determined incident angle θ and the orbital position of the spaceborne synthetic aperture radar to invert the illumination area of ​​the spaceborne synthetic aperture radar.

[0016] The steps for drawing the zebra map in step 2 are as follows:

[0017] Step 21: Calculate the leading edge echo time and trailing edge echo time of the interference band of the spaceborne synthetic aperture radar signal;

[0018] Step 22: Calculate the leading edge echo time and trailing edge echo time of the satellite nadir signal interference of the spaceborne synthetic aperture radar.

[0019] Step 23: Based on the leading edge echo time and trailing edge echo time obtained in Step 21 and Step 22, determine the incident angles corresponding to the leading edge echo time and the trailing edge echo time, respectively.

[0020] Step 24: Draw the zebra map.

[0021] In step 4, according to θ i Determine the pulse amplitude A of the corresponding signal. i The method can be obtained from the following formula.

[0022]

[0023] Among them, P t R is the transmit power of the spaceborne synthetic aperture radar; G(0) is the distance from the satellite antenna to the radar receiver; N is the number of synthetic aperture radar antenna elements; d is the spacing between synthetic aperture radar antenna elements; λ is the wavelength of the synthetic aperture radar signal; A e This represents the effective area of ​​the radar receiver antenna.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] (1) This invention draws a radar zebra diagram based on the received spaceborne SAR radar parameters, and combines the zebra diagram with the pulse repetition frequency of the received spaceborne SAR radar signal to determine the possible range of incident angles, thus narrowing the possible range of incident angles and reducing the workload of inversion. Since radar zebra diagrams are mainly used to select radar parameters in radar system design, the inversion method uses reverse thinking, draws a zebra diagram based on the received radar parameters, and then inverts the radar incident angle based on the zebra diagram. Combining the zebra diagram with the inversion of the radar illumination area makes it possible to invert the illumination area of ​​spaceborne radar.

[0026] (2) This invention utilizes the signal pulse amplitude measured by a radar receiver to simulate the pulse amplitude at various possible signal incident angles. When the simulated pulse amplitude matches the actual measured pulse amplitude, the incident angle is determined, thereby retrieving the illumination area of ​​the spaceborne SAR radar. Through experiments and intelligence data verification, the inversion method demonstrates high reliability and small error in the inverted area.

[0027] (3) The method for inverting the illumination area of ​​spaceborne SAR in this invention is mainly used in the field of electronic warfare for reconnaissance and countermeasures against spaceborne synthetic aperture radar (SAR). The inverted illumination area of ​​spaceborne SAR provides accurate radar operating status intelligence for information countermeasures against spaceborne synthetic aperture radar (SAR) and can be used for the protection of key areas during wartime. Attached Figure Description

[0028] Figure 1 This is a flowchart of the present invention;

[0029] Figure 2 Create a flowchart for the radar zebra diagram;

[0030] Figure 3 Example image of a zebra;

[0031] Figure 4 This is a pulse amplitude fitting plot;

[0032] Figure 5 This is a diagram showing the spatial relationship between the spaceborne SAR radar and the illuminated area. Detailed Implementation

[0033] The specific real-time method of the present invention will be further described below with reference to the accompanying drawings.

[0034] The method for inverting the illumination area of ​​a spaceborne synthetic aperture radar (SAR) provided by this invention first extracts the pulse repetition frequency (PRF), pulse width, and pulse amplitude of the signal received from the radar receiver; then, it uses the extracted pulse width and the known radar altitude to draw a zebra diagram, and determines the possible incident angle range θ1, θ2, ..., θ3 based on the signal PRF and the drawn zebra diagram. n Simulation was used to obtain the corresponding radar incident angles θ. i When (i = 1…n), the pulse amplitude A received by the receiver i Finally, regarding A i By comparing the pulse amplitude A with that of the real signal, the incident angle θ of the radar can be determined when the two pulse amplitudes coincide. Then, the incident angle θ and the radar's orbital position can be used to invert the SAR illumination area.

[0035] Overall process

[0036] The specific process is as follows: Figure 1 As shown in the flowchart, the specific processing steps are as follows:

[0037] (1) Receive the spaceborne SAR signal using a radar receiver and obtain the pulse width, pulse repetition frequency and pulse amplitude parameters in the SAR signal by measurement.

[0038] (2) Drawing the radar zebra pattern: Drawing the radar zebra pattern requires pulse width parameters and altitude information of the spaceborne SAR. The specific method for drawing the radar zebra pattern is described in the next section. The altitude mentioned refers to the known altitude of the satellite.

[0039] (3) Determine the possible radar incident angle range θ1, θ2, ..., θ by combining the radar zebra diagram and the signal pulse repetition frequency. n .

[0040] (4) For each possible incident angle θ i Simulations were performed using (i = 1…n) to obtain the radar speed at θ. i At the incident angle, the signal pulse amplitude A received by the radar receiver is... i .

[0041] Based on the detection equation of the spaceborne SAR radar antenna, A can be derived. i With θ i The relationship between them is:

[0042]

[0043] Where: P t R is the transmit power of the spaceborne SAR radar; R is the distance from the satellite antenna to the ground receiving equipment; G(0) is the maximum value of the main lobe gain; N is the number of antenna elements; d is the element spacing; λ is the operating wavelength; A e The effective area of ​​the receiving antenna.

[0044] (5) Using A i By fitting the amplitude of the received pulse to the actual received pulse amplitude A, an A that coincides with the actual received pulse amplitude A is obtained. i Thus, the radar incident angle θ = θ i .

[0045] (6) The illumination area of ​​the spaceborne synthetic aperture radar (SAR) is derived from the radar incident angle θ and the radar orbital position.

[0046] Zebra drawing

[0047] In practical applications, a planar diagram of the pulse repetition frequency and incident angle must first be drawn to ensure that the selection of the surveying zone and pulse repetition frequency avoids interference from the transmitted signal and the nadir signal. For example... Figure 2 The diagram shows the process of drawing the zebra plot module. The zebra plot is a set of curves drawn based on the relationship between the incident angle or the distance from the ground point to the satellite and the pulse repetition frequency.

[0048] The specific steps for drawing a zebra pattern are as follows:

[0049] First, calculate the echo times of the leading and trailing edges of the transmitted signal interference band. Then, calculate the echo times of the leading and trailing edges of the nadir signal interference band. Finally, draw the zebra diagram. The diamond-shaped area in the zebra diagram represents the selected area for the mapping zone.

[0050] Step 1: Calculate the echo times of the leading and trailing edges of the interference band of the transmitted signal.

[0051] Leading echo time T e1 for

[0052] T e1 =j·T pri -T p -T g (1)

[0053] Back edge echo time T e2 for:

[0054] T e2 =j·T pri +T p +T g (2)

[0055] Among them, T p T is the pulse width; pri T is the pulse repetition period; g To protect the time zone;

[0056] j∈{j min j min +1, ..., j max};j min =int(T e min ·f p min ), j max =int(T e max ·f p max );

[0057] And T e min and T e max These are the echo times corresponding to the nearest and farthest slant range kernels of the selected observation area, respectively; f p min The minimum and maximum pulse repetition frequencies are selected.

[0058] Step 2: Calculate the leading and trailing edge echo times of the signal interference at the nadir point.

[0059] The leading times are as follows:

[0060] The trailing edge times are as follows:

[0061] Where h is the satellite altitude; i∈{i min i min +1, ..., imax};i min =j min -int[(T nad +T p +2T g )·f p min ]; i max =j max -int[T nad ·f p max ].

[0062] The start time of the sub-satellite echo is:

[0063] Step 3: Calculate the corresponding incident angle based on the echo times given in Step 1 and Step 2.

[0064] Step 4: Draw a zebra map based on the data calculated in the previous steps.

[0065] like Figure 3 The image shows an example of a zebra diagram. The values ​​of the pulse repetition frequency and the incident angle should be selected within the diamond-shaped area. For example, when the pulse repetition frequency measured by the radar receiver is 3300Hz, draw a dotted vertical line with a frequency of 3300Hz in the colored diamond area of ​​the zebra diagram, including five segments. Find the corresponding incident angle ranges for each segment, which are {[32.5°~34.2°], [37.8°~39°], [42.2°~43°], [45.6°~46.4°], [48.8°~49.2°]}, which is the desired incident angle range.

[0066] like Figure 4 The image shown is a pulse amplitude fitting plot. The simulated actual received pulse amplitude A and the simulated incident angle θ are given. i The radar receiver receives the signal pulse amplitude A. i The fitting situation. When AA i When the mean and variance are less than the set values ​​ΔA and δA, determine A and A'. i Fitting.

[0067] like Figure 5 The diagram shows the spatial relationship between the spaceborne SAR radar and the illuminated area. Point A is the location of the spaceborne SAR radar, point B is the location of the illuminated area, point C is the nadir point, and L is the ground distance between points B and C. Re is the Earth's radius, θ is the angle of incidence, α is the geocentric angle, and β is the viewing angle. The positional information of points A and C can be obtained from the satellite ephemeris.

[0068] Based on the spatial positional relationships of the spaceborne SAR radar, the relationship between the incident angle θ, the geocentric angle α, and the viewing angle β is as follows:

[0069]

[0070] θ = α + β

[0071] Therefore, once the radar incident angle θ is obtained, the geocentric angle α can be obtained using the two formulas above.

[0072] Based on the spatial relationship, the distance between point B in the irradiated area and point C below the star is calculated as: L = Re·sinα

[0073] Since the location of the nadir point C is known, the location information of point B, the illumination area of ​​the spaceborne SAR satellite, can be obtained by combining the ground distance L between the illumination point and the nadir point, thus obtaining the illumination area of ​​the spaceborne SAR radar.

[0074] By using received real spaceborne SAR radar signals, the illumination area of ​​the spaceborne SAR radar is inverted. Through literature review and experimental verification, the inversion results are proven to be reliable. The error between the inverted illumination area and the real spaceborne SAR radar illumination area is small and can be controlled within 5km. This inversion method can provide accurate intelligence information for the reconnaissance and countermeasures of spaceborne synthetic aperture radar (SAR).

[0075] The parts of this invention not described in detail are common knowledge to those skilled in the art.

Claims

1. A method for inverting SAR illumination areas, characterized in that, Includes the following steps: Step 1: Receive the spaceborne synthetic aperture radar signal and extract the pulse repetition frequency, pulse width, and pulse amplitude. Step 2: Draw a radar zebra diagram using the altitude information and obtained pulse width from the spaceborne synthetic aperture radar; the altitude information of the spaceborne synthetic aperture radar is information that can be known in advance. Step 3: Determine the possible radar incident angle range θ based on the radar zebra pattern drawn in Step 2 and the pulse repetition frequency obtained in Step 1. i , i = 1…n; Step 4: Based on each θ from Step 3 i The pulse amplitude A of the corresponding signal is calculated. i ; Step 5: Transfer each A i The pulse amplitude obtained in step 1 is fitted, and the incident angle θ of the spaceborne synthetic aperture radar is determined based on the fitting result. Step 6: Use the determined incident angle θ and the orbital position of the spaceborne synthetic aperture radar to invert the illumination area of ​​the spaceborne synthetic aperture radar.

2. The SAR illumination area inversion method according to claim 1, characterized in that, The steps for drawing the zebra map in step 2 are as follows: Step 21: Calculate the leading edge echo time and trailing edge echo time of the interference band of the spaceborne synthetic aperture radar signal; Step 22: Calculate the leading edge echo time and trailing edge echo time of the satellite nadir signal interference of the spaceborne synthetic aperture radar. Step 23: Calculate the incident angles corresponding to the leading edge echo time and the trailing edge echo time based on the leading edge echo time and trailing edge echo time obtained in Step 21 and Step 22, respectively. Step 24: Draw the zebra map.

3. The SAR illumination area inversion method according to claim 1, characterized in that, In step 4, according to θ i Determine the pulse amplitude A of the corresponding signal. i The method can be obtained from the following formula. Among them, P t R is the transmit power of the spaceborne synthetic aperture radar; G(0) is the distance from the satellite antenna to the radar receiver; N is the maximum value of the main lobe gain of the synthetic aperture radar antenna; d is the number of elements in the synthetic aperture radar antenna array; λ is the wavelength of the synthetic aperture radar signal; A e This represents the effective area of ​​the radar receiver antenna.

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