A Joint Design Method of Pulse Repetition Frequency for Satellite-missile Bistatic SAR

Abstract

The present invention discloses a satellite-missile bistatic SAR pulse repetition frequency joint design method. In view of the fact that the design of the satellite-missile bistatic SAR pulse repetition frequency has become a bottleneck problem restricting the application of the satellite-missile bistatic SAR, an innovative satellite-missile bistatic SAR pulse repetition frequency joint design method is proposed, that is, the pulse repetition frequency coprime technology of the transmitting end is used to realize the deambiguation of the receiving end echo, avoiding the conventional method of using multiple channels or multiple missile-borne SAR receiving ends for joint deambiguation, and greatly reducing the complexity of the system hardware implementation through signal processing.

Description

Technical Field

[0001] The invention relates to a satellite-missile bistatic SAR pulse repetition frequency joint design method, which is a part of radar system design and belongs to the radar technical field. Background Art

[0002] The Star-Bomb Dual-Base SAR technology is derived from the Basic Strengthening Project. The dual-base SAR technology is based on the technology of transmitting and receiving separately. Compared with the single-base SAR, it has the advantages of small size, high weight, and strong anti-interference ability. Specifically, there are the following aspects:

[0003] 1. Obtain target imaging results and ground object information different from monostatic SAR

[0004] Due to the reasons of working frequency band, imaging mechanism and geometric configuration, the microwave images obtained by the monostatic SAR system will have corner effect (i.e. corner reflector effect), mirror effect, shadow effect, close-range compression, high wall collapse and other phenomena, resulting in radiation distortion and geometric distortion. In bistatic SAR, since the transmitter and receiver are placed on different platforms, there are differences in the spatial position of the transmitter and the receiver, and these image features will have some changes (such as corner effect, etc.), and these image features that are different from monostatic SAR (such as scattering characteristics and light and dark relationship, etc.) can provide new information about the ground objects, making bistatic SAR and monostatic SAR complementary in application. Their combination can improve the image dimension of ground observation and obtain richer ground object information.

[0005] 2. Good concealment, anti-interference and anti-interception performance

[0006] Bistatic SAR can achieve this by "transmitting far away and receiving near". The transmitting station works in an area far away from the target area, which reduces the possibility of being hit. The receiving station works in a passive receiving state, which is relatively concealed, which is conducive to its own protection and strong battlefield survivability. Secondly, interference with the transmitting station will not affect the work of the receiving station. Therefore, this system has good anti-detection and anti-interference capabilities. Bistatic radar working in the "transmitting far away and receiving near" mode can effectively reduce the transmitting power of the transmitter and improve its own anti-interception performance. Therefore, for bistatic radar, it has strong anti-interference and high concealment.

[0007] 3. Low cost and flexible configuration

[0008] The dual-base SAR receiving platform does not require active transmitting components such as transmitters, high-power amplifiers, and signal sources, which greatly saves volume, weight, and power consumption. Therefore, the ground imaging system can be lightweight, miniaturized, and low-power, and can be loaded on small drones, making it possible to use smaller drones as a carrier platform. In addition, the dual-base SAR can be expanded to multiple bases, using the free flight of different platforms to obtain radar imaging images of the target area, greatly improving the platform flexibility.

[0009] 4. Possess forward-looking imaging capabilities that conventional radars do not have

[0010] In general, the working mode of imaging radar is oblique or positive side view, that is, the vertical direction of the radar movement direction and the transmission beam movement direction form an angle greater than or equal to 0 degrees. This working mode leads to certain limitations and blind spots in the illumination of the radar beam, such as the detection of the area directly in front of the radar. However, under actual application conditions, objects directly in front of the radar movement are more strategically important than objects on the side. However, when a single base illuminates the area directly in front of the radar, the direction of the Doppler resolution is parallel to the direction of the slant range resolution, resulting in the loss of the ability to resolve cells directly below it, making it impossible to image the area directly in front of the radar. Therefore, under the special transceiver platform model of the dual-base, the direction of the Doppler resolution and the direction of the slant range resolution form a certain angle, which can achieve imaging of the target directly in front of the radar.

[0011] However, for satellite-missile dual-base SAR, the core part of its system design - pulse repetition frequency (PRF) design often has contradictions. In order to meet the imaging needs of satellite-borne SAR itself, the pulse repetition frequency is generally taken to be 3000Hz to 6000Hz. At this time, if the PRF is further increased, the imaging mapping band will become smaller. For the missile-borne SAR receiver, the platform flight speed usually reaches more than Mach 3, the antenna diameter is generally about 0.1m, and the pulse repetition frequency is generally 10000Hz to 15000Hz. Obviously, there is a conflict in the selection of pulse repetition frequency between satellite-borne SAR and missile-borne SAR.

[0012] At this time, as the missile-borne SAR receiver, there is serious ambiguity in the azimuth. The conventional solution to this problem is to use multiple receiving channels or multiple missile-borne SAR receivers to jointly resolve the ambiguity. However, the price paid is huge. Multiple channels not only increase the size and weight of a single missile-borne receiver, but also place high demands on the consistency of multiple channels. If multiple missile-borne SAR receivers are used, the complexity and cost of the system will also be greatly increased, and there will also be great uncertainty in the path planning and coordination of multiple SAR receivers. In summary, the pulse repetition rate of the satellite-missile bistatic SAR has become a bottleneck problem restricting the use of the satellite-missile bistatic SAR. Summary of the invention

[0013] The purpose of the present invention is to overcome the shortcomings of the prior art, and to propose an innovative method for the joint design of pulse repetition frequency of star-missile dual-base SAR to address the difficulties and bottleneck problems in the design of pulse repetition frequency of star-missile dual-base SAR. That is, the mutual prime technology of pulse repetition frequency at the transmitting end is used to solve the problem of deambiguation of echo at the receiving end, and the complexity of system implementation is reduced by signal processing.

[0014] The technical solution adopted by the present invention is:

[0015] A method for joint design of pulse repetition frequency of satellite-missile bistatic SAR, the steps are as follows:

[0016] (1) According to the design constraints of the pulse repetition frequency PRF, the pulse repetition frequency PRF0 of the missile-borne SAR receiver is designed to meet the following two requirements: (a) the pulse repetition time PRT exceeds the duration of the echo signal; (b) the azimuth direction meets the requirements of the sampling theorem;

[0017] (2) The satellite SAR payload alternately transmits two sets of pulse signals with mutually prime pulse repetition frequencies;

[0018] (3) The two echo sequences corresponding to the two groups of pulse signals in step (2) are subjected to separate imaging processing to obtain two SAR images that are severely blurred due to undersampling;

[0019] (4) Calculate the minimum azimuth spacing between blurred images in the basic coprime SAR imaging image;

[0020] (5) When the minimum azimuth distance between the blurred images is greater than the size of the ship target, the coherence coefficient of the two images is calculated, and then the targets appearing at the same position in the two images are judged to be the same target based on the coherence coefficient.

[0021] (6) If they are the same target, then the target is determined to be the real target and the defuzzification is completed.

[0022] Furthermore, the design constraints of the pulse repetition frequency PRF are specifically:

[0023]

[0024] Among them, η1 and η2 are margins, t w is the duration of the range echo, B a is the azimuth bandwidth;

[0025] B in different modes a The calculation formulas are different, namely

[0026]

[0027] V s is the platform speed, D a is the antenna azimuth size, N is the number of receiving subarrays, D ae is the equivalent antenna azimuth dimension.

[0028] Furthermore, the SAR satellite payload alternately transmits two sets of pulse signals with pulse repetition frequencies of PRF1 = PRF0 / N1 and PRF2 = PRF0 / N2, ​​where N1 and N2 are mutually prime positive integers, and N2 = N1 + 1. The pulse repetition frequencies PRF1 and PRF2 meet the pulse repetition frequency selection requirements for on-board imaging.

[0029] Furthermore, after separate imaging processing, the two echo sequences corresponding to the two groups of pulse signals with pulse repetition frequencies PRF1 and PRF2 will obtain two SAR images s1(x,r) and s2(x,r) that are severely blurred due to undersampling, where x and r are the azimuth and distance coordinates of the pixels, respectively.

[0030] Furthermore, the step (4) calculates the minimum azimuth spacing between blurred images in the basic mutually prime SAR imaging image, specifically:

[0031] In images s1(x,r) and s2(x,r), the blurred image of the target appears at different azimuths and distances, and only the real image of the target appears at the same position in the two SAR images;

[0032] For image s1(x,r), the target position after imaging is

[0033]

[0034] For image s2(x,r), the target position after imaging is

[0035]

[0036] Among them, i1 and i2 are integers, λ is the wavelength of the radar signal, r0 is the range of the radar to the target, v sis the satellite speed relative to the Earth, are the azimuth and distance coordinates of image s1(x,r), are the azimuth and distance coordinates of image s2(x,r) respectively; only when i1 / i2=N1 / N2, the image of the target appears at the same position in the two images; the minimum azimuth spacing between the blurred images in images s1(x,r) and s2(x,r) in basic coprime SAR imaging is obtained as

[0037]

[0038] Among them, λ is the wavelength of the radar transmission signal, r0 is the range of the radar to the target, v s is the satellite speed relative to the earth, PRF0 is the pulse repetition frequency of the missile-borne SAR receiver, and N1 and N2 are mutually prime positive integers.

[0039] Furthermore, the calculation of the coherence coefficient is shown as follows:

[0040]

[0041] Where c(x,r) is the correlation coefficient function, s1(x,r) and s2(x,r) represent the SAR image functions with pulse repetition frequencies PRF1 and PRF2, respectively, and x and r are the azimuth and range coordinates of the pixel, respectively; is the conjugate complex image of image s2(x,r).

[0042] Furthermore, if the coherence coefficient is 1, the target is determined to be the desired real target, and the defuzzification is completed.

[0043] Furthermore, the process of generating the final SAR image from two blurred mutually prime SAR images can be described as follows:

[0044]

[0045] Among them, c(x,r) is the correlation coefficient function, s1(x,r) and s2(x,r) represent the SAR image functions with pulse repetition frequencies PRF1 and PRF2 respectively, x and r are the azimuth and distance coordinates of the pixel respectively, and s(x,r) is the final generated SAR image function.

[0046] Compared with the reference documents, the present invention has the following beneficial effects:

[0047] Aiming at the problem that the pulse repetition frequency design of satellite-missile bistatic SAR has become a bottleneck problem restricting the application of satellite-missile bistatic SAR, the present invention innovatively proposes a joint design method of the pulse repetition frequency of satellite-missile bistatic SAR, namely, utilizing the pulse repetition frequency coprime technology of the transmitting end to realize the deambiguation of the echo at the receiving end, avoiding the conventional method of utilizing multiple channels or multiple missile-borne SAR receiving ends for joint deambiguation, and greatly reducing the complexity of the system hardware implementation through signal processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 The following is a schematic diagram of the basic mutually prime SAR transmission pulse sequence (taking N1=4 and N2=5 as an example);

[0049] Figure 2 The following are the basic coprime SAR simulation ship imaging results, where (a) is the unambiguous standard SAR image, (b) is the blurred image when the ship size exceeds the upper limit; (c) is the coprime SAR imaging result; (d) is the standard SAR imaging result. DETAILED DESCRIPTION

[0050] For satellite-borne radars, the pulse repetition frequency (PRF) generally ranges from 3000 to 6000 Hz. For missile-borne receivers, the antenna size is relatively small, typically about 0.1 m, and the flight speed is relatively fast, reaching more than 1.5 km / s. At this time, the PRF design of the missile-borne receiver should be above 15000 Hz. Obviously, there is a contradiction between the RPF of the transmitter and the missile-borne receiver. If the PRF design of the satellite-borne transmitter is adopted, the receiver will have serious azimuth ambiguity. Therefore, we propose to use the design technology of basic PRF coprime coding to solve the above problems.

[0051] The basic principle of this method is as follows:

[0052] The basic PRF coprime coded SAR system (hereinafter referred to as the basic coprime SAR) alternately transmits two sets of pulse signals with pulse repetition frequencies PRF1 = PRF0 / N1 and PRF2 = PRF0 / N2 (such as Figure 1 As shown, taking the transmission and reception of positive frequency modulated pulses as an example), where N1 and N2 are mutually prime positive integers (such as 3 and 4, 3 and 5), without loss of generality, it is assumed here that N2 = N1 + 1. Wherein, PRF0 is the pulse repetition frequency that satisfies the unambiguous imaging of the missile-borne receiving end.

[0053] The design constraints of PRF0 are

[0054]

[0055] Among them, η1 and η2 are margins, generally 0.1 to 0.2, t w is the duration of the range echo, B a is the azimuth bandwidth. a The calculation formulas are different, namely

[0056]

[0057] V s is the platform speed, D a is the antenna azimuth size, N is the number of receiving subarrays, D ae is the equivalent antenna azimuth dimension.

[0058] For a conventional single-channel SAR system, at the time t corresponding to all positive integers n, n =n / PRF0, the radar is sending and receiving pulse signals. For the basic coprime SAR system, the radar will only send and receive signals at times that are integer multiples of N1 and N2 (i.e. n=n1N1 or n=n2N2, n1 and n2 are positive integers). After the two echo sequences corresponding to the transmission frequencies PRF1 and PRF2 are processed separately, two SAR images s1(x,r) and s2(x,r) with severe blurring due to undersampling will be obtained, where x and r are the azimuth and range coordinates of the pixel, respectively. Although these two SAR images are severely blurred, their resolution is the same as that of the fully sampled SAR image with a repetition frequency of PRF0. Since the undersampling multiples are coprime, the blurred images of the target appear at different azimuths and distances in the images s1(x,r) and s2(x,r), and only the true image of the target appears at the same position in the two SAR images. For image s1(x,r), the target position after imaging is

[0059]

[0060] For image s2(x,r), the target position after imaging is

[0061]

[0062] Among them, i1 and i2 are integers, λ is the wavelength of the radar signal, r0 is the range of the radar to the target, v s is the satellite speed relative to the Earth, are the azimuth and distance coordinates of image s1(x,r), are the azimuth and distance coordinates of image s2(x,r), respectively, and PRF0 is the pulse repetition frequency of the missile-borne SAR receiver. From the above formula, it can be seen that the image of the target appears in the same position in the two images only when i1 / i2=N1 / N2. Since N1 and N2 are relatively prime, this situation only holds when i1=iN1 and i2=iN2, that is,

[0063]

[0064] Where i is an integer, λ is the wavelength of the radar signal, r0 is the range of the radar to the target, v s is the speed of the satellite relative to the earth, and PRF0 is the pulse repetition frequency of the missile-borne SAR receiver.

[0065] The basic mutual prime SAR imaging mode can significantly reduce the amount of data collected by echoes and obtain high-resolution SAR images without blurring in the sea area. The amount of data collected by basic mutual prime SAR imaging is 2.3 times that of conventional fully oversampled SAR.

[0066]

[0067] The azimuth distance between the blurred images in images s1(x,r) and s2(x,r) is

[0068]

[0069] Where i1 and i2 are integers, λ is the wavelength of the radar signal, r0 is the range from the radar to the target, v s is the speed of the satellite relative to the earth, and PRF0 is the pulse repetition frequency of the missile-borne SAR receiver.

[0070] Therefore, the minimum azimuth spacing between the blurred images in images s1(x,r) and s2(x,r) in basic coprime SAR imaging is

[0071]

[0072] This minimum azimuth spacing also limits the maximum target size in the sea scene. When the ship target size is larger than this value, due to mutual aliasing, it is impossible to obtain an unambiguous final SAR image by judging the coherence coefficient, such as Figure 2 shown.

[0073] At this time, the blurred images in images s1(x,r) and s2(x,r) are the same as those in conventional fully oversampled SAR images. Due to the suppression of the antenna pattern, the energy of the blurred images is more than 20dB lower than the real image of the target. Therefore, the coherence coefficient of images s1(x,r) and s2(x,r) is obtained, and then the targets appearing at the same position in the two images are judged to be the same target according to the coherence coefficient, so as to obtain the final SAR image without blur.

[0074] The present invention proposes a method for jointly designing pulse repetition frequency of a satellite-missile bistatic SAR, and the specific steps are as follows:

[0075] 1) According to the design constraints of PRF (Pulse repetition frequency)

[0076]

[0077] The pulse repetition frequency of the missile-borne SAR receiver is designed to meet the following two requirements: (a) the PRT (Pulse repetition time) exceeds the duration of the echo signal; (b) the azimuth direction meets the requirements of the sampling theorem. The designed pulse repetition frequency is set to PRF0.

[0078] 2) The SAR satellite payload alternately transmits two sets of pulse signals with pulse repetition frequencies of PRF1 = PRF0 / N1 and PRF2 = PRF0 / N2 (e.g. Figure 1 As shown, taking the transmission and reception of positive frequency modulated pulses as an example), where N1 and N2 are mutually prime positive integers (such as 3 and 4, 3 and 5), without loss of generality, it is assumed here that N2 = N1 + 1. Among them, the pulse repetition frequencies of PRF1 and PRF2 meet the pulse repetition frequency selection requirements for on-board imaging.

[0079] 3) After the two echo sequences corresponding to the transmission frequencies PRF1 and PRF2 are processed separately, two SAR images s1(x,r) and s2(x,r) with serious blurring caused by undersampling will be obtained, where x and r are the azimuth and distance coordinates of the pixels, respectively.

[0080] 4) Since the undersampling multiples are mutually prime, the blurred images of the target appear at different positions and distances in the images s1(x,r) and s2(x,r), and only the real image of the target appears at the same position in the two SAR images. For image s1(x,r), the position of the target after imaging is

[0081]

[0082] For image s2(x,r), the target position after imaging is

[0083]

[0084] Among them, i1 and i2 are integers, λ is the wavelength of the radar signal, r0 is the range of the radar to the target, v s is the satellite speed relative to the Earth, are the azimuth and distance coordinates of image s1(x,r), are the azimuth and distance coordinates of image s2(x,r) respectively. It can be seen from the above formula that the target image appears at the same position in the two images only when i1 / i2=N1 / N2. The minimum azimuth distance between the blurred images in images s1(x,r) and s2(x,r) in basic coprime SAR imaging is obtained as

[0085]

[0086] Among them, λ is the wavelength of the radar transmission signal, r0 is the range of the radar to the target, v s is the satellite speed relative to the earth, PRF0 is the pulse repetition frequency of the missile-borne SAR receiver, and N1 and N2 are mutually prime positive integers.

[0087] 5) When the minimum azimuth spacing between blurred images is greater than the size of the ship target, the coherence coefficient of images s1(x,r) and s2(x,r) is calculated, and then the targets appearing at the same position in the two images are judged to be the same target based on the coherence coefficient. The calculation of the coherence coefficient is shown in the following formula:

[0088]

[0089] Where c(x,r) is the correlation coefficient function, s1(x,r) and s2(x,r) represent the SAR image functions with pulse repetition frequencies PRF1 and PRF2, respectively, and x and r are the azimuth and range coordinates of the pixel, respectively. is the conjugate complex image of image s2(x,r).

[0090] (6) If the coherence coefficient is 1, the target is judged to be the real target and the deambiguation is completed. At this time, the process of generating the final SAR image from two blurred mutually prime SAR images can be described as:

[0091]

[0092] Among them, c(x,r) is the correlation coefficient function, s1(x,r) and s2(x,r) represent the SAR image functions with pulse repetition frequencies PRF1 and PRF2 respectively, x and r are the azimuth and distance coordinates of the pixel respectively, and s(x,r) is the final generated SAR image function.

Claims

1. A method for joint design of pulse repetition frequency of satellite-missile bistatic SAR, characterized in that Here are the steps: (1) According to the design constraints of the pulse repetition frequency PRF, the pulse repetition frequency PRF0 of the missile-borne SAR receiver is designed to meet the following two requirements: (a) the pulse repetition time PRT exceeds the duration of the echo signal; (b) the azimuth direction meets the requirements of the sampling theorem; (2) The satellite SAR payload alternately transmits two sets of pulse signals with mutually prime pulse repetition frequencies; (3) The two echo sequences corresponding to the two groups of pulse signals in step (2) are subjected to separate imaging processing to obtain two SAR images that are severely blurred due to undersampling; (4) Calculate the minimum azimuth spacing between blurred images in the basic coprime SAR imaging image; (5) When the minimum azimuth distance between the blurred images is greater than the size of the ship target, the coherence coefficient of the two images is calculated, and then the targets appearing at the same position in the two images are judged to be the same target based on the coherence coefficient. (6) If they are the same target, then the target is determined to be the real target and the defuzzification is completed; The design constraints of the pulse repetition frequency PRF are specifically: Among them, η1 and η2 are margins, t w is the duration of the range echo, B a is the azimuth bandwidth; B in different modes a The calculation formulas are different, namely V s is the platform speed, D a is the antenna azimuth size, N is the number of receiving subarrays, D ae is the equivalent antenna azimuth dimension.

2. The method for joint design of pulse repetition frequency of satellite-missile bistatic SAR according to claim 1, characterized in that: The SAR satellite payload alternately transmits two sets of pulse signals with pulse repetition frequencies of PRF1=PRF0 / N1 and PRF2=PRF0 / N2, ​​where N1 and N2 are mutually prime positive integers, and N2=N1+1; the pulse repetition frequencies PRF1 and PRF2 meet the pulse repetition frequency selection requirements for on-board imaging.

3. The method for joint design of pulse repetition frequency of satellite-missile bistatic SAR according to claim 2, characterized in that: After separate imaging processing, the two echo sequences corresponding to the two groups of pulse signals with pulse repetition frequencies PRF1 and PRF2 will obtain two SAR images s1(x,r) and s2(x,r) that are severely blurred due to undersampling, where x and r are the azimuth and distance coordinates of the pixels, respectively.

4. The method for joint design of pulse repetition frequency of satellite-missile bistatic SAR according to claim 3, characterized in that: The step (4) calculates the minimum azimuth spacing between blurred images in the basic mutually prime SAR imaging image, specifically: In images s1(x,r) and s2(x,r), the blurred image of the target appears at different azimuths and distances, and only the real image of the target appears at the same position in the two SAR images; For image s1(x,r), the target position after imaging is For image s2(x,r), the target position after imaging is Among them, i1 and i2 are integers, λ is the wavelength of the radar signal, r0 is the range of the radar to the target, v s is the satellite speed relative to the Earth, are the azimuth and distance coordinates of image s1(x,r), are the azimuth and distance coordinates of image s2(x,r) respectively; only when i1 / i2=N1 / N2, the image of the target appears at the same position in the two images; the minimum azimuth spacing between the blurred images in images s1(x,r) and s2(x,r) in basic coprime SAR imaging is obtained as Among them, v s is the speed of the satellite relative to the Earth.

5. The method for joint design of pulse repetition frequency of satellite-missile bistatic SAR according to claim 4, characterized in that: The calculation of the coherence coefficient is shown below: Among them, c(x,r) is the correlation coefficient function, is the conjugate complex image of image s2(x,r).

6. The method for joint design of pulse repetition frequency of satellite-missile bistatic SAR according to claim 5, characterized in that: If the coherence coefficient is 1, the target is judged to be the desired real target, and the defuzzification is completed.

7. The method for joint design of pulse repetition frequency of satellite-missile bistatic SAR according to claim 6, characterized in that: The process of generating the final SAR image from two blurred mutually prime SAR images can be described as: Among them, s(x,r) is the final generated SAR image function.

Images