CN107976674A - A PRF selection method and system for dual-beam SAR based on optimal azimuth ambiguity - Google Patents

Abstract

The invention discloses a PRF (pulse repetition frequency) selection method and a system of a double-beam SAR (synthetic aperture radar) based on optimal azimuth ambiguity, which can obtain a proper azimuth direction double-beam Synthetic Aperture Radar (SAR) azimuth ambiguity calculation method through theoretical derivation and simulation analysis; obtaining the optimal parameters of the system Pulse Repetition Frequency (PRF) by calculating the position ambiguity of the system under different PRF frequencies and according to the optimal position ambiguity selection principle; the feasibility and the correctness of the method are verified through simulation tests. The method can effectively obtain the optimal result of the PRF in the design of the azimuth dual-beam SAR system, is a key step for completing the design of the azimuth dual-beam SAR system, and can effectively obtain the PRF optimal parameter of the system by calculating the azimuth ambiguity of the azimuth dual-beam SAR system and based on the optimal azimuth ambiguity principle.

Description

A PRF selection method and system for dual-beam SAR based on optimal azimuth ambiguity

technical field

The invention relates to a PRF selection method and system for dual-beam SAR based on optimal azimuth ambiguity, and belongs to the technical field of microwave remote sensing imaging.

Background technique

The azimuth dual-beam synthetic aperture radar (SAR) can revisit the target multiple times, and has application modes such as multi-angle observation and moving target detection. It is one of the development directions of the future SAR system.

The main technical problems of the azimuth dual-beam SAR system are: in the system design of the azimuth dual-beam SAR, it is difficult to determine the optimal value of the system key parameter pulse repetition frequency (PRF), which requires multiple iterations for design, and the design time and The design cost is higher.

Contents of the invention

The technical problem solved by the present invention is: in view of this, to overcome the deficiencies of the prior art, to provide a PRF selection method and system based on optimal azimuth ambiguity dual-beam SAR, by calculating the azimuth ambiguity of the dual-beam SAR system, and then Based on the optimal orientation fuzzy principle, the optimal parameters of the system PRF can be obtained effectively.

The technical solution solved by the present invention is: a PRF selection method based on optimal azimuth ambiguity dual-beam SAR, the steps are as follows:

(1) According to the antenna pattern of the azimuth dual-beam SAR, the location where the azimuth ambiguity appears in the Doppler domain is obtained, and the azimuth ambiguity of the azimuth dual-beam SAR is obtained according to the location where the azimuth ambiguity appears in the Doppler domain;

(2) According to the azimuth ambiguity of the azimuth dual-beam SAR in step (1), and then according to the sampling theorem and spectrum folding characteristics, formulate the optimal PRF selection method to obtain the optimal PRF, so that one of the dual beams Azimuth ambiguity occurs where the Doppler domain avoids another beam.

Also include step (3) as follows:

(3) Through the azimuth dual-beam SAR simulation test, the correctness of the PRF selection method in step (2) is verified.

According to the position where the azimuth ambiguity appears in the Doppler domain, the azimuth ambiguity AASR of the azimuth dual-beam SAR is obtained, and the steps are as follows:

In the formula, B _a is the Doppler bandwidth, B _off is the Doppler center frequency, G is the antenna pattern, PRF is the pulse repetition frequency, f is the Doppler frequency, and i is a non-zero integer.

Step (2) According to the azimuth ambiguity of the azimuth dual-beam SAR in step (1), and then according to the sampling theorem and spectrum folding characteristics, formulate the optimal PRF selection method to obtain the optimal PRF, so that one of the dual beams The azimuth ambiguity appears in the Doppler domain to avoid another beam, the steps are as follows:

(2.1) According to the azimuth ambiguity of the azimuth dual-beam SAR, determine the center frequency difference f _c of the two beams;

(2.2) When the optimal pulse repetition frequency PRF is selected as 1/(n+0.5) of the front-to-back center frequency difference 2 f _c , n is a positive integer, and the PRF is obtained, which can make the center of the other beam farthest from the main beam , resulting in minimal aliasing, as follows:

f _c is the center frequency difference between front and back

Step (2) Establish the azimuth dual-beam spaceborne SAR, obtain the optimal PRF through simulation calculation, and compare it with the optimal PRF calculated in step (2). If the optimal PRF differs within a certain range, then the optimal PRF calculated by the optimal PRF selection method selected in step (2) is correct; otherwise, the optimal PRF selection method selected in step (2) is incorrect.

A certain range is within 10Hz.

The present invention is a PRF selection system based on optimal azimuth fuzzy dual-beam SAR, comprising: a determination module and a PRF selection module;

The determination module obtains the position where the azimuth ambiguity appears in the Doppler domain according to the antenna pattern of the azimuth dual-beam SAR, and obtains the azimuth ambiguity of the azimuth dual-beam SAR according to the position where the azimuth ambiguity appears in the Doppler domain, and sends it to the PRF for selection module;

The PRF selection module formulates the optimal PRF selection method according to the azimuth ambiguity from the receiving azimuth received from the determination module to the dual-beam SAR, and then according to the sampling theorem and spectrum folding characteristics, and obtains the optimal PRF, so that one of the two beams The azimuth ambiguity of the beam avoids another beam where it occurs in the Doppler domain.

It also includes a verification module, which verifies the correctness of the PRF selection method in the PRF selection module through the azimuth dual-beam SAR simulation test.

According to the location where the azimuth ambiguity appears in the Doppler domain in the determination module, the azimuth ambiguity AASR of the azimuth dual-beam SAR is obtained, the formula is as follows:

In the formula, B _a is the Doppler bandwidth, B _off is the Doppler center frequency, G is the antenna pattern, PRF is the pulse repetition frequency, f is the Doppler frequency, and i is a non-zero integer.

The PRF selection module formulates the optimal PRF selection method according to the azimuth ambiguity of the dual-beam SAR in the azimuth direction, and then according to the sampling theorem and spectrum folding characteristics, and obtains the optimal PRF, so that the azimuth ambiguity of one of the two beams is Le field appears to avoid another beam, as follows:

The PRF selection module determines the center frequency difference _fc of the two beams according to the azimuth ambiguity of the azimuth double-beam SAR; when the optimal pulse repetition frequency _PRF is selected as 1/(n+0.5 ), n is a positive integer, and the PRF is obtained, which can make the center of the other beam farthest from the main beam and bring the minimum aliasing, as shown in the following formula:

f _c is the center frequency difference between front and back.

The advantage of the present invention compared with prior art is:

(1) The present invention deduces the azimuth ambiguity calculation method of double-beam SAR in azimuth, comprehensively considers Doppler bandwidth, Doppler center frequency, antenna pattern, pulse repetition frequency, Doppler frequency, proposes optimal azimuth The ambiguity selection criterion makes the optimal orientation ambiguity selection more accurate and improves the precision.

(2) The present invention can obtain the suitable azimuth direction dual-beam synthetic aperture radar (SAR) azimuth ambiguity calculation method through theoretical derivation and simulation analysis; Based on the selection principle of orientation ambiguity, the optimal parameters of system PRF are obtained; the feasibility and correctness of this method are verified by simulation experiments.

(3) According to the optimal orientation ambiguity selection criterion, the present invention provides an optimal PRF selection method, and combines various factors to accurately determine the optimal PRF.

(4) The present invention provides an optimal PRF calculation method for azimuth dual-beam SAR. According to the sampling theorem and spectrum folding characteristics, an optimal PRF selection method is formulated, and the azimuth ambiguity of the azimuth dual-beam SAR can be considered. Calculate the optimal PRF.

(5) The present invention provides an optimal PRF test verification method for azimuth dual-beam SAR, which can accurately determine the correctness of the optimal PRF.

Description of drawings

Fig. 1 is a flowchart of the present invention;

Fig. 2 is the geometric relationship diagram of the dual-beam SAR working mode of the present invention;

Fig. 3 is a time series relationship diagram of the dual-beam SAR working mode of the present invention;

Fig. 4 is a schematic diagram of azimuth dual-beam SAR Doppler frequency domain of the present invention;

Figure 5 shows the AASR curves of the system under different PRFs.

Detailed ways

The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

The invention discloses a PRF selection method and system for dual-beam SAR based on optimal azimuth ambiguity, which can obtain a suitable azimuth dual-beam synthetic aperture radar (SAR) azimuth ambiguity calculation method through theoretical derivation and simulation analysis; through calculation According to the azimuth ambiguity of the system at different pulse repetition frequencies (PRF), the optimal parameters of the system PRF are obtained according to the selection principle of the optimal azimuth ambiguity; the feasibility and correctness of this method are verified by simulation experiments. This method can effectively obtain the optimal result of the key parameter PRF in the design of the azimuth dual-beam SAR system, and is a key step in completing the design of the azimuth dual-beam SAR system. The optimal orientation fuzzy principle can effectively obtain the optimal parameters of the system PRF.

The present invention realizes a kind of PRF selection method based on optimal azimuth ambiguity dual-beam SAR, as shown in Figure 1, the method comprises the following steps:

Step 101: Calculate the azimuth ambiguity of the dual-beam SAR in azimuth;

Specifically, accompanying drawing 2 shows the geometric relationship diagram of the azimuth dual-beam SAR antenna. The azimuth dual-beam SAR antenna generates two symmetrical beams before and after the antenna (usually the grating lobe is generated by the phased array antenna, and the first grating The lobe is adjusted symmetrically with the main lobe), so that the same target area will be irradiated twice successively. When the system is working, the main lobe and the first grating lobe are used to transmit and receive pulses at the same time, and the two signals are mixed in a receiving window when receiving, as shown in Figure 3, where Tx _n-1 , Tx _n , Tx _n+1 represents the n-1th, nth, and n+1th pulses transmitted, and t _r represents the receiving time. Since the Doppler frequencies of the forward and backward beams are positive and negative, the two signals can be separated by band-pass filtering in the Doppler frequency domain, and then imaged separately. Since there are two beams in this mode, the signal received by the other beam is equivalent to ambiguous clutter when one beam is imaging, so the calculation method of the azimuth ambiguity (AASR) of the system is different from that of traditional SAR, and it needs to be re-analyzed and calculated. Azimuth ambiguity is caused by the pulse sampling working system of SAR in azimuth. The azimuth Doppler spectrum of the SAR echo signal is weighted by the two-way pattern of the antenna, and the azimuth is sampled using the pulse repetition frequency (PRF). In this way, the Doppler signal higher than the pulse repetition frequency will be folded into the processing bandwidth of the central part of the azimuth spectrum after being sampled, thus causing aliasing with the main signal spectrum and forming azimuth ambiguity. The azimuth ambiguity is the ratio of the azimuth ambiguity energy to the useful signal energy.

The intersecting grid area in Figure 4 is the energy of the imaging signal. After sampling by the PRF, the signals received at the positions of other side lobes (relative to the forward beam) of the antenna will also be aliased into the imaging signal, forming an azimuth ambiguous signal. It can be seen from the figure that if the system PRF is low, the strong fuzzy signal received by the backward beam will also be mixed into the imaging signal, which greatly deteriorates the imaging performance. Through analysis, the calculation formula of dual-beam SAR azimuth ambiguity (AASR) can be obtained as follows:

In the formula, B _a is the Doppler bandwidth, B _off is the Doppler center frequency, G is the antenna pattern, PRF is the pulse repetition frequency, f is the Doppler frequency, and i is a non-zero integer.

Step 102: According to the principle of selecting the optimal orientation ambiguity, obtain the optimal parameters of the system PRF;

According to the aliasing characteristics of the spectrum, the two groups of front and rear spectrums are extended on the frequency axis according to the PRF cycle, but the overlapping of the adjacent forward and backward spectra is also determined by the PRF. If the forward spectrum is used as a reference , then when the value of PRF increases, it can be understood that the backward spectrum slides along the forward spectrum, and at the same time, the extension period of the two groups of spectrum increases. When the two groups of forward and backward spectrums overlap completely, the aliasing cannot be separated at all. At this time, the center frequency difference of the forward and backward directions is an integer multiple of PRF; When the distances to the center of the frequency spectrum are equal (that is, the forward frequency band and the backward frequency band are evenly arranged on the frequency axis), the front-to-back center frequency difference is an integer multiple of the PRF plus half of the PRF. Therefore, when the PRF is selected as approximately 1/(n+0.5) of the front-to-back center frequency difference 2 f _c (n is a positive integer), the center of the grating lobe can be the farthest from the center of the main lobe, bringing the smallest aliasing , as shown in the following formula.

Step 103: verifying the feasibility and correctness of the method through a simulation test;

The above theory is verified by a spaceborne SAR system simulation. Construct reasonable spaceborne SAR system parameters, simulate the system PRF at the optimal azimuth ambiguity through the calculation method given in step 1, and then calculate the optimal PRF of the system through the calculation criteria given in step 2, and compare the two calculations As a result, if the optimal PRF calculated by the simulation calculation in step 1 is within a certain range from the optimal PRF calculated in step 2, it is determined that the optimal PRF calculated by the optimal PRF selection method selected in step 2 is correct; otherwise, The optimal PRF selection method selected in step 2 is incorrect. A certain range is within 10Hz.

A design example is used below to illustrate that the method of the present invention can obtain the optimal PRF of the azimuth dual-beam SAR. A spaceborne azimuth dual-beam SAR system is designed for simulation to verify the above theory. The parameters of the spaceborne azimuth dual-beam SAR system are shown in the table below:

Table 1 Simulation parameters of spaceborne SAR system

parameter value Average altitude of satellite orbit 517km Relative speed of star and earth 7462m/s Center frequency 9.6GHz speed of light 3.0×10 ⁸ m/s Azimuth Beamwidth 0.39° Double beam angle 3.94° PRF range 4000Hz～12000Hz forward center frequency 18655Hz

According to the optimal PRF selection criterion in step 2, the optimal PRF is selected as about 1/(n+0.5) of the difference between the front and rear center frequencies, and the optimal PRF that can be selected is 10660 Hz. The simulation analysis is carried out through the calculation method of step 1, and the PRF is traversed from 4000Hz to 12000Hz, and the AASR of the system under different PRFs is obtained, as shown in Figure 5. The abscissa in Figure 5 is the system PRF, and the ordinate is the azimuth ambiguity AASR. It can be seen that the system AASR has periodic deviations and folds. With the change of PRF, the first grating lobe and the main lobe are relatively translated, and the value of AASR fluctuates high and low. The maximum value of AASR is around 0dB, and it remains stable. It reflects the consistency of the backward and forward two-way antenna patterns, that is, when the main lobe and the grating lobe completely overlap, the useful signal energy is approximately equal to the fuzzy energy, resulting in AASR is 0dB. The minimum value decreases as the PRF increases, because the Doppler spectrum in the backward and forward regions can be separated by the Doppler bandpass filter, and as the PRF increases, the defolded The blur energy is relatively reduced, so the minimum value of AASR will gradually decrease with the oscillation. When the system obtains the optimal AASR position, the PRF is around 10650Hz, which is consistent with the theoretical calculation in step 2, and the simulation proves the correctness of the theoretical derivation.

A PRF selection system based on optimal azimuth ambiguity dual-beam SAR of the present invention, comprising: a determination module, a PRF selection module;

The determination module obtains the position where the azimuth ambiguity appears in the Doppler domain according to the antenna pattern of the azimuth dual-beam SAR, and obtains the azimuth ambiguity of the azimuth dual-beam SAR according to the position where the azimuth ambiguity appears in the Doppler domain, and sends it to the PRF for selection module;

The PRF selection module formulates the optimal PRF selection method according to the azimuth ambiguity from the receiving azimuth received from the determination module to the dual-beam SAR, and then according to the sampling theorem and spectrum folding characteristics, and obtains the optimal PRF, so that one of the two beams The azimuth ambiguity of the beam avoids another beam where it occurs in the Doppler domain.

It also includes a verification module, which verifies the correctness of the PRF selection method in the PRF selection module through the azimuth dual-beam SAR simulation test.

According to the location where the azimuth ambiguity appears in the Doppler domain in the determination module, the azimuth ambiguity AASR of the azimuth dual-beam SAR is obtained, the formula is as follows:

In the formula, B _a is the Doppler bandwidth, B _off is the Doppler center frequency, G is the antenna pattern, PRF is the pulse repetition frequency, f is the Doppler frequency, and i is a non-zero integer.

The PRF selection module formulates the optimal PRF selection method according to the azimuth ambiguity of the dual-beam SAR in the azimuth direction, and then according to the sampling theorem and spectrum folding characteristics, and obtains the optimal PRF, so that the azimuth ambiguity of one of the two beams is Le field appears to avoid another beam, as follows:

The PRF selection module determines the center frequency difference _fc of the two beams according to the azimuth ambiguity of the azimuth double-beam SAR; when the optimal pulse repetition frequency _PRF is selected as 1/(n+0.5 ), n is a positive integer, and the PRF is obtained, which can make the center of the other beam farthest from the main beam and bring the minimum aliasing, as shown in the following formula:

f _c is the center frequency difference between front and back.

The verification module verifies the correctness of the PRF selection method in step (2) through the azimuth dual-beam SAR simulation test, as follows:

Establish the azimuth dual-beam spaceborne SAR, calculate the optimal PRF through simulation, and compare it with the optimal PRF calculated in step (2), if the optimal PRF calculated by simulation is different from the optimal PRF calculated in step (2) Within a certain range, the optimal PRF calculated by the optimal PRF selection method selected in step (2) is correct; otherwise, the optimal PRF selection method selected in step (2) is incorrect.

The present invention can obtain a suitable azimuth dual-beam synthetic aperture radar (SAR) azimuth ambiguity calculation method through theoretical derivation and simulation analysis; by calculating the azimuth ambiguity of the system under different pulse repetition frequencies (PRF), according to the optimal azimuth ambiguity The selection principle is used to obtain the optimal parameters of the system PRF; the feasibility and correctness of this method are verified by simulation experiments. Moreover, the present invention derives the azimuth ambiguity calculation method of dual-beam SAR in azimuth, comprehensively considers Doppler bandwidth, Doppler center frequency, antenna pattern, pulse repetition frequency, and Doppler frequency, and proposes the optimal azimuth ambiguity The selection criterion makes the selection of the optimal orientation ambiguity more accurate and improves the precision.

The above are only preferred embodiments of the present invention and applied technical principles. Those skilled in the art will appreciate that the present invention is not limited to the preferred embodiments described herein, and various obvious changes, readjustments and substitutions can be made by those skilled in the art without departing from the protection scope of the present invention. Therefore, although the present invention has been described in detail through the above embodiments, the present invention is not limited to the above embodiments, and can also include more other equivalent embodiments without departing from the concept of the present invention, and the present invention The scope is determined by the scope of the appended claims.

Claims (10)

1. a kind of PRF selection method based on optimal azimuth ambiguity double-beam SAR, it is characterized in that the steps are as follows: (1) According to the antenna pattern of the azimuth dual-beam SAR, the location where the azimuth ambiguity appears in the Doppler domain is obtained, and the azimuth ambiguity of the azimuth dual-beam SAR is obtained according to the location where the azimuth ambiguity appears in the Doppler domain; (2) According to the azimuth ambiguity of the azimuth dual-beam SAR in step (1), and then according to the sampling theorem and spectrum folding characteristics, formulate the optimal PRF selection method to obtain the optimal PRF, so that one of the dual beams Azimuth ambiguity occurs where the Doppler domain avoids another beam. 2. a kind of PRF selection method based on optimal azimuth ambiguity dual-beam SAR according to claim 1, is characterized in that: also comprise step (3) as follows: (3) Through the azimuth dual-beam SAR simulation test, the correctness of the PRF selection method in step (2) is verified. 3. a kind of PRF selection method based on optimal azimuth ambiguity dual-beam SAR according to claim 1, is characterized in that: according to azimuth ambiguity occurs position in Doppler domain, obtain the azimuth ambiguity of azimuth dual-beam SAR AASR, the formula is as follows: <mrow><mi>A</mi><mi>A</mi><mi>S</mi><mi>R</mi><mo>=</mo><mfrac><mrow><munder><mo>&amp;Sigma;</mo><mrow><mi>i</mi><mo>&amp;NotEqual;</mo><mn>0</mn></mrow></munder><msubsup><mo>&amp;Integral;</mo><mrow><mo>-</mo><msub><mi>B</mi><mi>a</mi></msub><mo>/</mo><mn>2</mn><mo>+</mo><msub><mi>B</mi><mrow><mi>o</mi><mi>f</mi><mi>f</mi></mrow></msub></mrow><mrow><msub><mi>B</mi><mi>a</mi></msub><mo>/</mo><mn>2</mn><mo>+</mo><msub><mi>B</mi><mrow><mi>o</mi><mi>f</mi><mi>f</mi></mrow></msub></mrow></msubsup><mi>G</mi><mrow><mo>(</mo><mi>f</mi><mo>+</mo><mi>i</mi><mo>&amp;CenterDot;</mo><mi>P</mi><mi>R</mi><mi>F</mi><mo>)</mo></mrow><mi>d</mi><mi>f</mi></mrow><mrow><msubsup><mo>&amp;Integral;</mo><mrow><mo>-</mo><msub><mi>B</mi><mi>a</mi></msub><mo>/</mo><mn>2</mn><mo>+</mo><msub><mi>B</mi><mrow><mi>o</mi><mi>f</mi><mi>f</mi></mrow></msub></mrow><mrow><msub><mi>B</mi><mi>a</mi></msub><mo>/</mo><mn>2</mn><mo>+</mo><msub><mi>B</mi><mrow><mi>o</mi><mi>f</mi><mi>f</mi></mrow></msub></mrow></msubsup><mi>G</mi><mrow><mo>(</mo><mi>f</mi><mo>)</mo></mrow><mi>d</mi><mi>f</mi></mrow></mfrac></mrow> In the formula, B _a is the Doppler bandwidth, B _off is the Doppler center frequency, G is the antenna pattern, PRF is the pulse repetition frequency, f is the Doppler frequency, and i is a non-zero integer. 4. a kind of PRF selection method based on the dual-beam SAR of optimum azimuth ambiguity according to claim 1, is characterized in that: step (2) according to the azimuth of step (1) to the azimuth ambiguity of dual-beam SAR, then According to the sampling theorem and spectrum folding characteristics, formulate the optimal PRF selection method, obtain the optimal PRF, and make the azimuth ambiguity of one beam in the dual beam appear in the Doppler domain to avoid the other beam, the steps are as follows: (2.1) According to the azimuth ambiguity of the azimuth dual-beam SAR, determine the center frequency difference f _c of the two beams; (2.2) When the optimal pulse repetition frequency PRF is selected as 1/(n+0.5) of the front-to-back center frequency difference 2 f _c , n is a positive integer, and the PRF is obtained, which can make the center of the other beam farthest from the main beam , resulting in minimal aliasing, as follows: <mrow><mi>P</mi><mi>R</mi><mi>F</mi><mo>=</mo><mfrac><mrow><mn>2</mn><mo>&amp;CenterDot;</mo><msub><mi>f</mi><mi>c</mi></msub></mrow><mrow><mi>n</mi><mo>+</mo><mn>0.5</mn></mrow></mfrac><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>2</mn><mo>)</mo></mrow></mrow> f _c is the center frequency difference between front and back. 5. a kind of PRF selection method based on optimal azimuth ambiguity double-beam SAR according to claim 2, is characterized in that: step (3) passes the azimuth direction double-beam SAR simulation experiment, the PRF selection of verification step (2) The correctness of the method, the steps are as follows: Establish the azimuth dual-beam spaceborne SAR, calculate the optimal PRF through simulation, and compare it with the optimal PRF calculated in step (2), if the optimal PRF calculated by simulation is different from the optimal PRF calculated in step (2) Within a certain range, the optimal PRF calculated by the optimal PRF selection method selected in step (2) is correct; otherwise, the optimal PRF selection method selected in step (2) is incorrect. 6. A PRF selection method for dual-beam SAR based on optimal azimuth ambiguity according to claim 5, characterized in that: a certain range is within 10 Hz. 7. A PRF selection system based on optimal azimuth ambiguity dual-beam SAR, characterized in that it comprises: a determination module, a PRF selection module; The determination module obtains the position where the azimuth ambiguity appears in the Doppler domain according to the antenna pattern of the azimuth dual-beam SAR, and obtains the azimuth ambiguity of the azimuth dual-beam SAR according to the position where the azimuth ambiguity appears in the Doppler domain, and sends it to the PRF for selection module; The PRF selection module formulates the optimal PRF selection method according to the azimuth ambiguity from the receiving azimuth received from the determination module to the dual-beam SAR, and then according to the sampling theorem and spectrum folding characteristics, and obtains the optimal PRF, so that one of the two beams The azimuth ambiguity of the beam avoids another beam where it occurs in the Doppler domain. 8. a kind of PRF selection system based on optimal azimuth fuzzy double-beam SAR according to claim 7, is characterized in that: also comprise verification module, verification module passes through azimuth to double-beam SAR simulation test, verifies in the PRF selection module The correctness of the PRF selection method. 9. a kind of PRF selection system based on optimal azimuth ambiguity dual-beam SAR according to claim 7, is characterized in that: in the determination module, according to azimuth ambiguity occurs in the Doppler domain, obtains the azimuth to the dual-beam SAR Azimuth ambiguity AASR, the formula is as follows: <mrow><mi>A</mi><mi>A</mi><mi>S</mi><mi>R</mi><mo>=</mo><mfrac><mrow><munder><mo>&amp;Sigma;</mo><mrow><mi>i</mi><mo>&amp;NotEqual;</mo><mn>0</mn></mrow></munder><msubsup><mo>&amp;Integral;</mo><mrow><mo>-</mo><msub><mi>B</mi><mi>a</mi></msub><mo>/</mo><mn>2</mn><mo>+</mo><msub><mi>B</mi><mrow><mi>o</mi><mi>f</mi><mi>f</mi></mrow></msub></mrow><mrow><msub><mi>B</mi><mi>a</mi></msub><mo>/</mo><mn>2</mn><mo>+</mo><msub><mi>B</mi><mrow><mi>o</mi><mi>f</mi><mi>f</mi></mrow></msub></mrow></msubsup><mi>G</mi><mrow><mo>(</mo><mi>f</mi><mo>+</mo><mi>i</mi><mo>&amp;CenterDot;</mo><mi>P</mi><mi>R</mi><mi>F</mi><mo>)</mo></mrow><mi>d</mi><mi>f</mi></mrow><mrow><msubsup><mo>&amp;Integral;</mo><mrow><mo>-</mo><msub><mi>B</mi><mi>a</mi></msub><mo>/</mo><mn>2</mn><mo>+</mo><msub><mi>B</mi><mrow><mi>o</mi><mi>f</mi><mi>f</mi></mrow></msub></mrow><mrow><msub><mi>B</mi><mi>a</mi></msub><mo>/</mo><mn>2</mn><mo>+</mo><msub><mi>B</mi><mrow><mi>o</mi><mi>f</mi><mi>f</mi></mrow></msub></mrow></msubsup><mi>G</mi><mrow><mo>(</mo><mi>f</mi><mo>)</mo></mrow><mi>d</mi><mi>f</mi></mrow></mfrac></mrow> In the formula, B _a is the Doppler bandwidth, B _off is the Doppler center frequency, G is the antenna pattern, PRF is the pulse repetition frequency, f is the Doppler frequency, and i is a non-zero integer. 10. A kind of PRF selection system based on optimal azimuth ambiguity dual-beam SAR according to claim 7, characterized in that: the PRF selection module is based on the azimuth ambiguity of the dual-beam SAR, and then according to the sampling theorem and spectrum folding characteristics, formulate the optimal PRF selection method, obtain the optimal PRF, and make the azimuth ambiguity of one of the two beams avoid the position of the other beam in the Doppler domain, as follows: The PRF selection module determines the center frequency difference _fc of the two beams according to the azimuth ambiguity of the azimuth double-beam SAR; when the optimal pulse repetition frequency _PRF is selected as 1/(n+0.5 ), n is a positive integer, and the PRF is obtained, which can make the center of the other beam farthest from the main beam and bring the minimum aliasing, as shown in the following formula: <mrow><mi>P</mi><mi>R</mi><mi>F</mi><mo>=</mo><mfrac><mrow><mn>2</mn><mo>&amp;CenterDot;</mo><msub><mi>f</mi><mi>c</mi></msub></mrow><mrow><mi>n</mi><mo>+</mo><mn>0.5</mn></mrow></mfrac><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>2</mn><mo>)</mo></mrow></mrow> f _c is the center frequency difference between front and back.