A shipborne ground wave radar ship target and ground clutter separation method

Abstract

The application discloses a shipborne ground wave radar ship target and ground clutter separation method, and relates to the field of shipborne ground wave radar target detection. The basic steps are as follows: obtaining radar frequency domain data and attitude data; performing target detection based on the frequency domain data and marking target points in the maximum frequency shift range of ground clutter; separating stationary targets (stationary ship targets and ground clutter) and moving ship targets by utilizing the difference in target Doppler shift characteristics; separating stationary ship target echoes and ground clutter by utilizing the difference in target echo amplitude variation characteristics; and accurately estimating target azimuth and positioning the target by utilizing the characteristics that different azimuth stationary targets produce different Doppler shifts. According to the characteristics that the change of platform speed influences target echo Doppler shift, the difference in frequency shift characteristics and the difference in echo amplitude variation characteristics of ship target echoes and ground clutter are utilized to realize the differentiation of stationary ship target echoes, moving ship target echoes and ground clutter, and the positioning of stationary targets is completed.

Description

TECHNICAL FIELD

[0001] The present application relates to a shipborne ground wave radar target detection and separation method, in particular to a shipborne ground wave radar ship target and ground clutter separation method. BACKGROUND

[0002] High frequency ground wave radar utilizes the characteristics of 3-30MHz high frequency band vertical polarization wave diffraction propagation along the sea surface, which can realize large-scale, over-the-horizon continuous detection of sea surface ship targets. High frequency ground wave radar is usually divided into shore-based ground wave radar and shipborne ground wave radar. In the echo spectrum of shore-based ground wave radar, according to the characteristics that the ship target echo produces Doppler shift due to the movement of the ship target itself, the moving ship target and the static target (static ship and ground clutter) can be well distinguished; but since the static ship target echo and the ground clutter are both concentrated at zero Doppler frequency, there is a problem that it is difficult to distinguish the static ship target and the ground clutter.

[0003] Under the shipborne ground wave radar system, the Doppler shift of the target echo is not only affected by the speed of the target itself, but also by the speed and direction of the shipborne platform. Therefore, the static ship target and the ground clutter originally located at zero Doppler frequency under the shore-based ground wave radar system will have Doppler shift, and then move away from zero Doppler frequency, making it possible to separate the static ship target and the ground clutter. However, in this case, the ground clutter and the static ship target echo may be mixed with the moving ship target echo, and the problem of difficult separation may occur.

[0004] At present, there is little research on the separation method of ground wave radar ship target and ground clutter in the detection of ground wave radar ship target. Only a small number of researchers have carried out research on the detection method of static ship target within the range of ground clutter under the shore-based ground wave radar system, such as Ji Yonggang et al. who proposed a method of distinguishing static ship target and ground clutter by using the echo signal characteristics of zero Doppler. So far, no researchers have carried out research on the separation method of ground wave radar target and ground clutter under the shipborne moving platform. The present application combines the characteristics of shipborne ground wave radar, takes advantage of the fact that the shipborne platform can change the speed of sailing, and fully utilizes the detection results of shipborne ground wave radar under different speeds, and proposes a separation method of ship target echo and ground clutter suitable for shipborne ground wave radar, which effectively solves the problem of mixing of ground clutter, static ship target echo and moving ship target echo. SUMMARY

[0005] (I) Technical problems to be solved

[0006] The purpose of the present application is to provide a method of distinguishing mixed static ship target, moving ship target and ground clutter by using radar echo frequency domain data and attitude data before and after the speed of the shipborne platform changes, and to improve the target detection performance.

[0007] (II) Technical Solution

[0008] The application comprises the following steps:

[0009] (1) Obtain two frames of shipborne ground wave radar frequency domain data at different speeds while the heading of the shipborne platform remains unchanged and the corresponding platform attitude data Z(t1), Z(t2).

[0010] The shipborne platform sails at a constant speed for a period of time, and the frequency domain data of the ground wave radar at time t1 is obtained (in the form of a distance-Doppler two-dimensional spectrum) and platform attitude data Z(t1), wherein r represents a distance unit, r∈[1,r max ], r max is the number of distance units, d represents a Doppler unit, d∈[1,d max ], d max is the number of Doppler units; the attitude data Z(t1) of the shipborne platform includes a speed Z(t1).v1, a heading angle Z(t1).θ1, a longitude Z(t1).lon1, and a latitude Z(t1).lat1.

[0011] The heading of the shipborne platform remains unchanged, and the shipborne platform sails at a constant speed after slowly decelerating for a period of time, and the frequency domain data of the radar at time t2 is obtained and attitude data Z(t2), including a speed Z(t2).v2, a heading angle Z(t2).θ2, a longitude Z(t2).lon2, and a latitude Z(t2).lat2.

[0012] (2) Calculate the maximum frequency shift ranges ΔF1 and ΔF2 of the ground clutter and , and perform target detection within ΔF1 and ΔF2, respectively.

[0013] Due to the influence of the motion of the shipborne platform, the echo of a stationary target in the radar frequency domain data produces a frequency shift on the Doppler dimension, and the frequency shift formula is:

[0014]

[0015] where f d is the Doppler frequency shift of the echo of a stationary target, v p is the speed of the shipborne platform, λ is the wavelength of the radar, and φ is the angle between the line connecting the land and the shipborne platform and the heading of the platform, φ∈[0,π].

[0016] The maximum frequency shift range ΔF1 of the ground clutter at time t1 is:

[0017]

[0018] at time t2 The maximum frequency shift range ΔF2 of ground clutter is:

[0019]

[0020] at time t1 Target detection is performed, in which target points outside the maximum frequency shift range of ground clutter are moving ship targets, and mixed phenomena of moving ship echoes, stationary ship echoes and ground clutter exist within the ground clutter frequency shift range. The target points within the maximum frequency shift range ΔF1 of ground clutter are Tar d (t1) = [Tar1(t1), Tar2(t1), ···, Tar i (t1), ···, Tar N (t1)], i = [1, 2, ···, N], and N represents the number of detected target points. CA-CFAR uses the method of summing and averaging the amplitudes of selected reference cells to calculate the detection threshold T D :

[0021] T D = kμ

[0022]

[0023] where T D is the CA-CFAR detection threshold, k represents the threshold factor, μ is the average of the detection background amplitude, L is the number of reference cells, and x i is the amplitude of the i-th reference cell. Similarly, target detection is performed on to obtain the target points Tar d (t2) = [Tar1(t2), Tar2(t2), ···, Tar j (t2), ···, Tar M (t2)], j = [1, 2, ···, M], and M represents the number of detected target points.

[0024] (3) Based on the Doppler frequency shift characteristics of the target, moving ship targets and stationary targets are distinguished.

[0025] Unlike stationary targets affected only by the ship-borne platform, the Doppler frequency shift of moving ship targets is also affected by the speed of the ship itself. In combination with the attached Figure 3 , the Doppler values and distance values of Tar d (t1) and Tar d (t2) are used to determine the stationary target points and moving ship target points in Tar d (t1). According to the cosine theorem, the target point Tar1(t1) in Tar d (t1) is at The theoretical distance r2' in the equation can be expressed as:

[0026]

[0027] Tar1(t1) in The theoretical angle φ2' between the center and the platform bow is:

[0028] φ2'=φ1+Δφ

[0029]

[0030]

[0031] Tar1(t1) in The theoretical Doppler value f2' is:

[0032]

[0033] If the actual Tar d (t2) There exists a target point Tar j (t2) satisfies:

[0034]

[0035] This indicates that Tar1(t1) is a stationary target. If no target point meets the requirements, then Tar1(t1) is a moving vessel target. Here, r1 is the distance value of Tar1(t1), f1 is the Doppler value of Tar1(t1), φ1 is the angle between Tar1(t1) and the platform's bow direction, Δφ is the change in the angle between Tar1(t1) and the platform's bow direction before and after the platform decelerates, and f2 is the distance between Tar1(t1) and the platform's bow direction. j The Doppler frequency shift value of (t2), r2 is Tar j The distance value of (t2).

[0036] (4) Further distinguish stationary ship targets and ground clutter based on the target echo amplitude characteristics.

[0037] Based on the stationary targets selected in step three, the shipborne platform maintains a constant speed of v2 for a period of time to acquire multiple batches of radar echo frequency domain data. By analyzing the multi-moment echo amplitude variation characteristics of the targets, the stationary targets are classified into stationary ship targets (RT). s and ground clutter tar g .

[0038]

[0039] Where, ΔA max A represents the maximum value of the amplitude change at multiple time points before and after each target. rms It is the root mean square of the maximum amplitude changes of all stationary targets at multiple time intervals.

[0040] (5) Based on the angle between the stationary target and the ship-borne platform, the longitude and latitude of the target are accurately positioned.

[0041] Based on the stationary target separated by the above steps, the longitude and latitude of the stationary target are positioned by using the formula of the angle between the target and the heading of the platform and the longitude and latitude formula.

[0042] Calculation of the angle between the line connecting the target and the ship-borne platform and the true north:

[0043] θ = φ + θ1

[0044]

[0045] Longitude and latitude calculation formula:

[0046]

[0047]

[0048] Where f is Tar i (t1) Doppler shift of the stationary target point; θ is the angle between the line connecting the target and the ship-borne platform and the true north; φ is the angle between Tar i (t1) and the heading of the ship-borne platform; R is the distance; lon t , lat t are the longitude and latitude of the stationary target respectively; ARC is the radius of the Earth's equator, about 6371Km.

[0049] (Three) beneficial effects

[0050] The present application provides a method for separating the mixed ground clutter of ship-borne ground wave radar, stationary ship target echo and moving ship target echo, which utilizes the advantage of changing speed of the ship-borne ground wave radar platform, uses real-time acquired radar frequency domain data and attitude data, first performs target detection based on platform attitude data and radar frequency domain data and determines the target point within the maximum frequency shift range of ground clutter; then separates the stationary target and the moving ship target by using the difference in target Doppler shift characteristics; subsequently separates the stationary ship target and the ground clutter by using the difference in long-time echo amplitude change characteristics of the target; finally accurately estimates the target azimuth and positions the target by using the characteristics that different azimuth stationary targets produce different Doppler shifts. BRIEF DESCRIPTION OF DRAWINGS

[0051] Figure 1 is the basic flowchart of the present application.

[0052] Figure 2 is a schematic diagram of ship-borne ground wave radar ground clutter.

[0053] Figure 3The figure is a schematic diagram of shipborne ground wave radar for detecting stationary ship targets.

[0054] Figure 4 The figure is a characteristic diagram of amplitude of stationary ship targets and ground clutter based on measured data. DETAILED DESCRIPTION

[0055] In order to make the purpose, content and advantages of the present application more clear, the specific embodiments of the present application are described in further detail below with reference to the accompanying drawings:

[0056] Reference Figure 1 The specific implementation steps of the present application are as follows:

[0057] (1) Obtain shipborne ground wave radar frequency domain data of two frames at different speeds of the shipborne platform with unchanged heading and corresponding platform attitude data Z(t1), Z(t2).

[0058] The shipborne platform sails at a constant speed for a period of time, and the shipborne ground wave radar echo frequency domain data of the shipborne platform and the attitude data Z(t1) of the shipborne platform are obtained.

[0059] Frequency domain data wherein r represents a distance unit, r∈[1, r max ], r max is the number of distance dimension units, d represents a Doppler unit, d∈[1, d max ], d max is the number of Doppler dimension units; the attitude data Z(t1) of the shipborne platform includes the speed Z(t1).v1, the heading angle Z(t1).θ1, the longitude Z(t1).lon1 and the latitude Z(t1).lat1.

[0060] The shipborne platform sails at a constant speed for a period of time after slowly reducing the speed while keeping the heading unchanged, and the radar frequency domain data and the attitude data Z(t2) are obtained, which include the speed Z(t2).v2, the heading angle Z(t2).θ2, the longitude Z(t2).lon2 and the latitude Z(t2).lat2.

[0061] (2) Calculate the ground clutter maximum frequency shift range ΔF1 and ΔF2 of the frequency domain data and and perform target detection within ΔF1 and ΔF2, respectively.

[0062] Due to the influence of the shipborne platform, the stationary targets (ground clutter and stationary ship targets) produce frequency shift on the Doppler dimension in the radar frequency domain data, and the frequency shift formula is as follows:

[0063]

[0064] Where f d For the Doppler frequency shift of the echo from a stationary target, v p Let λ be the speed of the shipborne platform, λ be the radar wavelength, and φ be the angle between the line connecting the land and the shipborne platform and the bow direction of the platform, where φ∈[0,π].

[0065] time t1 The maximum frequency shift range ΔF1 of the ground clutter is:

[0066]

[0067] time t2 The maximum frequency shift range ΔF2 of ground clutter is:

[0068]

[0069] At time t1 Target detection was performed. Targets outside the maximum frequency shift range of ground clutter were identified as moving vessels. Within the ground clutter frequency shift range, there was a mixture of echoes from moving vessels, stationary vessels, and ground clutter. Targets within the maximum frequency shift range ΔF1 of ground clutter were identified as Tar. d (t1)=[Tar1(t1),Tar2(t1),···,Tar i (t1),···,Tar N [t1)], i = [1, 2, ..., N], where N represents the number of detected target points. CA-CFAR calculates the detection threshold T by summing and averaging the amplitudes of the selected reference cells. D :

[0070] T D =kμ

[0071]

[0072] Where T D Here, k represents the CA-CFAR detection threshold, μ is the mean amplitude of the detected background, L is the number of reference cells, and x is the threshold factor. i Let be the amplitude of the i-th reference unit. Similarly, for... Target detection is performed to obtain the target point Tar within the maximum frequency shift range ΔF2 of ground clutter. d (t2)=[Tar1(t2),Tar2(t2),···,Tar j (t2),···,Tar M [(t2)], j = [1, 2, ..., M], where M represents the number of detected target points.

[0073] The results of CA-CFAR detection include moving ship targets, stationary ship targets and ground clutter targets presented as point targets. In order to obtain more complete ground clutter detection results, the present application selects the distance dimension protection unit as 1 and the reference unit as 2 on the range-Doppler spectrum; the Doppler dimension protection unit as 1 and the reference unit as 5.

[0074] (3) Distinguishing moving ship targets and stationary targets based on target Doppler shift characteristics.

[0075] Unlike stationary targets affected only by the ship-borne platform, the Doppler shift of moving ship targets is also affected by the speed of the ship itself. The Doppler shift of Tar d (t1) and Tar d (t2) is used to distinguish moving ship targets and stationary targets mixed in the ground clutter range of the ship-borne platform attitude data Z(t1) and Z(t2).

[0076] To determine whether the target point in Tar d (t1) is a stationary target, it is mainly determined whether the distance and Doppler shift of the point change accordingly with the change of the speed of the ship-borne platform. Tar i (t1) and v1, v2 are used to predict the distance r2' and Doppler shift f2' of Tar i (t1) in the frequency domain data at t2, and if there is a point Tar d (t2) in Tar j (t2) whose distance r2 and Doppler shift f2 are the same as the predicted point, it indicates that Tar i (t1) is a stationary target, and if there is no Tar j (t2) that meets the requirements, it indicates that Tar i (t1) is a moving target. The specific process is as follows:

[0077] In combination with the attached Figure 3 , according to the cosine theorem, the theoretical distance r2' of the target point Tar1(t1) in Tar d (t1) in can be expressed as:

[0078]

[0079] The theoretical angle φ2' between Tar1(t1) in Tar d (t1) and the heading of the platform in is:

[0080] φ2' = φ1 + Δφ

[0081]

[0082]

[0083] The theoretical Doppler value f2' of Tar 1 (t1) in

[0084]

[0085] If the actual Tar d 1 (t1) has a target point Tar j 2 that satisfies:

[0086]

[0087] It is explained that Tar1 (t1) is a stationary target, and if there is no target point that satisfies the requirement, it is explained that Tar1 (t1) is a moving ship target, where r1 is the distance value of Tar 1 (t1), f1 is the Doppler value of Tar 1 (t1), φ1 is the included angle between Tar 1 (t1) and the bow direction of the platform, Δφ is the change amount of the included angle between Tar 1 (t1) and the bow direction of the platform before and after the deceleration of the ship-borne platform, f2 is the Doppler shift value of Tar 1 (t1), and r2 is the distance value of Tar 1 (t1). j j

[0088] (4) Further distinguish stationary ship targets and ground clutter based on target echo amplitude characteristics.

[0089] Under the ship-borne ground wave radar system, due to the movement of the ship-borne platform, the echo amplitudes of ground objects, islands and stationary ship targets are not fixed, but change with time. However, unlike fixed targets such as ground objects or islands, stationary ship targets are also affected by moving ocean currents, which will produce swinging or even rotating phenomena, making their echo amplitudes change more.

[0090] Maintain the ship-borne platform to sail at a constant speed v2, based on the stationary targets screened in step three, obtain multiple batches of radar echo frequency domain data, and divide the stationary targets into stationary ship targets tar s 1 and ground clutter tar g 2 through the change characteristics of the echo amplitudes of the targets at multiple times.

[0091]

[0092] Where ΔA max is the maximum amplitude change amount of each target at multiple times, and A rms is the root mean square of the maximum amplitude change amount of all stationary targets at multiple times.

[0093] Since the stationary ship target is a point target and the ground clutter is a surface target, when analyzing the echo amplitude, for the point target, only the amplitude variation amount ΔA of multiple moments of the point target needs to be counted max ; and for the surface target, the surface target needs to be cut, and the maximum value ΔA of the amplitude variation amount of multiple moments of each point in the surface target is counted according to a preset region max , and ΔA max of each point is compared with A rms to determine whether it is a stationary ship target.

[0094] (5) Latitude and longitude positioning based on the angle between the stationary target and the ship-borne platform after accurate estimation.

[0095] Based on the stationary target separated according to the above steps, the latitude and longitude position of the stationary target is positioned by using the formula of the angle between the target and the heading direction of the platform and the latitude and longitude formula.

[0096] Calculation of the angle between the line connecting the target and the ship-borne platform and the north:

[0097] θ = φ + θ1

[0098]

[0099] Latitude and longitude calculation formula:

[0100]

[0101]

[0102] Where f is the Doppler frequency shift of the stationary target point in Tar i (t1) ; θ is the angle between the line connecting the target and the ship-borne platform and the north; φ is the angle between Tar i (t1) and the heading direction of the ship-borne platform; R is the distance; lon t and lat t are the longitude and latitude of the stationary target, respectively; ARC is the radius of the earth equator, about 6371Km.

[0103] The innovation of the present application lies in the following aspects:

[0104] The application takes advantage of the changeable speed of the shipborne ground wave radar platform, separates the static target and the moving ship target based on the platform attitude data and the radar frequency domain data under different speeds, and uses the difference of the target Doppler shift characteristics; for the static ship target and the ground clutter, the application separates the two according to the difference of the long-time echo amplitude characteristics of the targets; and the application uses the characteristics that different azimuth targets form different Dopplers to accurately estimate the target azimuth and position the target. The target azimuth estimation method proposed by the application is based on the target frequency shift and the platform attitude data, has few influencing factors, is high in accuracy, and the obtained estimation result can be used to verify the accuracy of other static target azimuth angle estimation methods.

Claims

1. A method for separating ship targets from ground clutter using a shipborne ground wave radar, comprising the following steps: (1) When the bow direction of the shipborne platform remains unchanged, acquire two frames of shipborne ground wave radar frequency domain data at different speeds. And the corresponding platform attitude data Z(t1), Z(t2); The shipborne platform travels at a constant speed for a period of time, acquiring distance-Doppler two-dimensional frequency domain data at time t1 using ground-wave radar. And platform attitude data Z(t1), where r represents the distance cell, r∈[1,r] max ], r max Let be the number of distance dimension units, and d represent the Doppler units, d∈[1,d]. max ], d max The number of Doppler elements; the attitude data Z(t1) of the shipborne platform, including speed Z(t1).v1, heading angle Z(t1).θ1, longitude Z(t1).lon1 and latitude Z(t1).lat1; Maintaining the shipborne platform's bow direction unchanged, the vessel slowly decelerates and then travels at a constant speed for a period of time to acquire range-Doppler frequency domain data at radar time t2. and attitude data Z(t2), including speed Z(t2).v2, heading angle Z(t2).θ2, longitude Z(t2).lon2 and latitude Z(t2).lat2; (2) Calculation and The maximum frequency shift ranges of ground clutter are ΔF1 and ΔF2, and target detection is performed within ΔF1 and ΔF2 respectively; Due to the motion of the shipborne platform, the echo of a stationary target experiences a frequency shift in the Doppler dimension of the radar frequency domain data. The formula for this frequency shift is: Where f d For the Doppler frequency shift of the echo from a stationary target, v p Let λ be the speed of the shipborne platform, λ be the radar wavelength, and φ be the angle between the line connecting the land and the shipborne platform and the bow direction of the platform, where φ∈[0,π]. time t1 The maximum frequency shift range ΔF1 of the ground clutter is: time t2 The maximum frequency shift range ΔF2 of ground clutter is: At time t1 Target detection is performed. Targets outside the maximum frequency shift range of ground clutter are all moving vessel targets. Within the ground clutter frequency shift range, there is a mixture of echoes from moving vessels, stationary vessels, and ground clutter. Targets within the maximum frequency shift range ΔF1 of ground clutter are defined as Tar(t1) = [Tar1(t1), Tar2(t1), ..., Tar...]. i (t1),···,Tar N [t1], i = [1, 2, ..., N], where N represents the number of detected target points. CA-CFAR calculates the detection threshold T by summing and averaging the amplitudes of the selected reference units. D : T D =kμ Where T D Here, k represents the CA-CFAR detection threshold, μ is the mean amplitude of the detected background, L is the number of reference cells, and x is the threshold factor. i For the magnitude of the i-th reference unit, similarly for... Target detection is performed to obtain the target points Tar(t2) = [Tar1(t2), Tar2(t2), ..., Tar2(t2)] within the maximum frequency shift range ΔF2 of ground clutter. j (t2),···,Tar M (t2)], j=[1,2,···,M], M represents the number of detected target points; (3) Distinguish between moving ship targets and stationary targets based on the target's Doppler frequency shift characteristics. Unlike stationary targets affected only by the ship's platform, the Doppler shift of a moving vessel target is also affected by its own speed. Using the Doppler values ​​and distance values ​​of Tar(t1) and Tar(t2), we can determine the stationary target point and the moving vessel target point in Tar(t1). According to the law of cosines, the target point Tar1(t1) in Tar(t1) is located at... The theoretical distance r2′ in the equation can be expressed as: in, At time t1, the distance between the target and the radar is r1, the angle between the target and the platform's direction of travel is φ1, v represents speed, and t represents time; exist The theoretical angle φ2' between the center and the platform bow is: φ2′=φ1+Δφ Tar1(t1) in The theoretical Doppler value f2' is: If there exists a target point Tar(t2) in the actual Tar(t2) j (t2) satisfies: This indicates that Tar1(t1) is a stationary target. If no target point meets the requirements, then Tar1(t1) is a moving vessel target. Here, r1 is the distance value of Tar1(t1), f1 is the Doppler value of Tar1(t1), φ1 is the angle between Tar1(t1) and the platform's bow direction, Δφ is the change in the angle between Tar1(t1) and the platform's bow direction before and after the platform decelerates, and f2 is the distance between Tar1(t1) and the platform's bow direction. j The Doppler value of (t2), r2 is Tar j The distance value of (t2); (4) Further distinguish stationary ship targets from ground clutter based on the target echo amplitude characteristics. Based on the stationary targets selected in step three, the shipborne platform maintains a constant speed of v2 for a period of time to acquire multiple batches of radar echo frequency domain data. By analyzing the multi-moment echo amplitude variation characteristics of the targets, the stationary targets are classified into stationary ship targets (RT). s and ground clutter tar g : Where, ΔA max A represents the maximum value of the amplitude change at multiple time points before and after each target. rms The root mean square of the maximum amplitude changes of all stationary targets at multiple time intervals; (5) Based on the accurate estimation of the angle between the stationary target and the shipborne platform, the latitude and longitude are determined. Based on the stationary target separated by the above steps, the latitude and longitude position of the stationary target is located using the formula of the angle between the target and the platform's heading and the formula of latitude and longitude. Calculation of the angle between the line connecting the target and the shipborne platform and true north: θ = φ + Z(t1).θ1 Latitude and longitude calculation formula: Where f is Tar i (t1) represents the Doppler frequency shift of the stationary target point; θ is the angle between the line connecting the target and the shipborne platform and true north; φ is the Tar... i (t1) is the angle between the ship and the bow of the platform; R is the distance; ARC is the radius of the Earth's equator, which is 6371 km.

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