Multi-mode synthetic aperture radar wave position selection method with variable pulse width

Abstract

The invention relates to a multi-mode synthetic aperture radar wave position selection method with variable pulse width. The method comprises the following steps: initializing design parameters of a spaceborne radar system; determining a pulse repetition frequency constraint range of each imaging mode; calculating a time sequence constraint relation between a radar echo receiving window and sub-satellite point echo interference and transmitted pulse interference, decoupling pulse width, and drawing a zebra map; according to the zebra diagram, gradually selecting wave positions from a low angle to a high angle, and determining a lower visual angle central angle and a pulse repetition frequency of each wave position; when the wave position meeting the time sequence constraint relation cannot be selected from a certain lower view angle, the duty ratio in the design parameters is adjusted, and the time sequence constraint relation and the pulse width are regenerated; and outputting the selected wave position result. By optimizing the pulse parameters, the problem that the pulse width of a traditional SAR system is limited in multiple modes is solved, and the flexibility and imaging performance of the system are remarkably improved.

Description

Technical Field

[0001] The invention relates to the technical field of synthetic aperture radar (SAR), and in particular to a wave position selection method of a multi-mode synthetic aperture radar with a variable pulse width. Background Art

[0002] Synthetic aperture radar (SAR) is a high-resolution microwave remote sensing imaging radar. It has been widely used in environmental monitoring, resource exploration, mapping, and scientific research due to its all-day, all-weather working ability and high-resolution imaging of surface features. The SAR system uses synthetic aperture technology to increase the equivalent aperture by using the relative motion between the radar antenna and the target, thereby achieving high-resolution imaging.

[0003] In SAR systems, beam position selection is one of the key links to achieve efficient imaging. The beam position, that is, the direction of the radar beam, determines the radar's observation area and imaging coverage. Reasonable beam position design can ensure the effective reception of radar signals, avoid signal interference, and maximize the radar's observation efficiency. However, existing technologies have many challenges and limitations in beam position selection.

[0004] First, the SAR system will be affected by a variety of interference factors during operation, the most notable of which are subsatellite point echo interference and transmission pulse conflict. Subsatellite point echo may drown out other useful signals, resulting in reduced imaging quality. Transmission pulse conflict will affect the reception of echo signals. The existence of these two types of interference requires that the wave position selection be optimized under the premise of meeting the interference-free condition. In order to avoid this interference, the timing relationship of the pulses needs to be precisely controlled.

[0005] In addition, the performance of the SAR system is also affected by azimuth ambiguity and range ambiguity. The existence of these two ambiguities will reduce the imaging quality, so it is necessary to calculate and optimize them during the wave position selection process to ensure that the ambiguity is within an acceptable range.

[0006] In the prior art, although there are some methods that attempt to solve the above problems, these methods can often only optimize for a single working mode, and the pulse width needs to be fixed at the beginning of the design, which limits the adaptability of the system to multiple working modes. In addition, the azimuth bandwidth under different imaging modes varies greatly, further exacerbating the complexity of system design.

[0007] In summary, the existing technology has many deficiencies in SAR wave position selection, and lacks a SAR wave position selection method that can flexibly adjust the pulse width and PRF and meet the requirements of multiple working modes. Summary of the invention

[0008] In view of the above analysis, the present invention aims to disclose a multi-mode synthetic aperture radar wave position selection method with variable pulse width; solves the problem of limited pulse width of traditional SAR system in multi-mode, and significantly improves the flexibility and imaging performance of the system.

[0009] The present invention discloses a variable pulse width multi-mode synthetic aperture radar wave position selection method, comprising:

[0010] Step S1, initializing the design parameters of the spaceborne radar system including different imaging modes and duty cycles;

[0011] Step S2, determining the constraint range of the pulse repetition frequency of each imaging mode according to the design parameters;

[0012] Step S3, calculating the timing constraint relationship between the radar echo receiving window and the sub-satellite point echo interference and the transmission pulse interference, decoupling the pulse width, and drawing a zebra diagram; the horizontal axis range of the zebra diagram is the pulse repetition frequency constraint range; the vertical axis range is the radar viewing angle range;

[0013] Step S4: According to the zebra diagram, wave positions are gradually selected from low angles to high angles, and the lower viewing angle center angle and pulse repetition frequency of each wave position are determined;

[0014] When a wave position that satisfies the timing constraint relationship cannot be selected from a certain lower viewing angle, return to step S1 to adjust the duty cycle in the design parameters to regenerate the timing constraint relationship and pulse width;

[0015] Step S5, output the selected wave position results including the viewing angle, pulse width and pulse repetition frequency under the wave position.

[0016] The present invention can achieve one of the following beneficial effects:

[0017] The variable pulse width multi-mode synthetic aperture radar wave position selection method disclosed in the present invention improves the imaging quality and anti-interference ability of the SAR system in various working modes by optimizing the relationship between pulse width and pulse repetition frequency (PRF), and significantly improves the flexibility and imaging performance of the system. It is suitable for the design and application of high-resolution imaging radar systems in the fields of aerospace and aviation. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The drawings are only for the purpose of illustrating specific embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0019] Figure 1 It is a flow chart of a method for selecting a wave position of a multi-mode synthetic aperture radar with a variable pulse width in an embodiment of the present invention;

[0020] Figure 2 is a signal timing relationship diagram in an embodiment of the present invention;

[0021] Figure 3 A zebra diagram drawn in an embodiment of the present invention;

[0022] Figure 4 Schematic diagram of the distribution of wave positions in a zebra diagram in an embodiment of the present invention. DETAILED DESCRIPTION

[0023] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used to illustrate the principles of the present invention together with the embodiments of the present invention.

[0024] Embodiment 1

[0025] An embodiment of the present invention discloses a method for selecting a wave position of a multi-mode synthetic aperture radar with a variable pulse width, such as Figure 1 As shown, including:

[0026] Step S1, initializing the design parameters of the spaceborne radar system including different imaging modes and duty cycles;

[0027] Step S2, determining the constraint range of the pulse repetition frequency of each imaging mode according to the design parameters;

[0028] Step S3, calculating the timing constraint relationship between the radar echo receiving window and the sub-satellite point echo interference and the transmission pulse interference, decoupling the pulse width, and drawing a zebra diagram; the horizontal axis range of the zebra diagram is the pulse repetition frequency constraint range; the vertical axis range is the radar viewing angle range;

[0029] Step S4: According to the zebra diagram, wave positions are gradually selected from low angles to high angles, and the lower viewing angle center angle and pulse repetition frequency of each wave position are determined;

[0030] When a wave position that satisfies the timing constraint relationship cannot be selected from a certain lower viewing angle, return to step S1 to adjust the duty cycle in the design parameters to regenerate the timing constraint relationship and pulse width;

[0031] Step S5, output the selected wave position results including the viewing angle, pulse width and pulse repetition frequency under the wave position.

[0032] Specifically, in step S1, the initialized satellite radar system design parameters include: antenna length D, satellite orbit height H, average radius of the earth R e , center frequency f c , duty cycle η of the transmitted signal, echo receiving window protection time T g , Width of the surveying band W g 、Minimum downward viewing angle θmin 、Maximum downward viewing angle θ max ; Wherein, the duty cycle η is a variable duty cycle, which is adjusted according to the timing constraint relationship; the imaging modes include: strip imaging mode, beam imaging mode, and sliding focusing imaging mode.

[0033] Echo receiving window protection time T g =T g =μT p , μ∈[0,1], μ is the proportion of protection time in pulse width.

[0034] The solution of this embodiment can perform wave position selection for the strip imaging mode, the beam imaging mode, and the sliding focus imaging mode; and the wave position selection can be adjusted by adjusting the duty cycle η and the proportion μ of the protection time in the pulse width.

[0035] Specifically, in step S2, it includes:

[0036] Step S201, calculate the minimum value of the pulse repetition frequency range; in different imaging modes, determine the minimum value PRF of the pulse repetition frequency that is dynamically adjusted to avoid azimuth ambiguity according to the respective azimuth bandwidths; min ;

[0037] Where, for strip imaging mode, the instantaneous Doppler bandwidth B a The antenna bandwidth B 3 Completely determined, that is, B a =B 3 .

[0038] The beam pointing does not change during the imaging observation, so the azimuth frequency history only shifts in time, and no new Doppler bandwidth B is introduced due to the change of the Doppler center. d Considering the oversampling factor of 1.3, the total azimuth bandwidth of the stripe pattern is:

[0039] B sm =B a +B d =B 3

[0040] Therefore, the stripe mode PRF minimum expression is:

[0041] PRF min =1.3*B 3 .

[0042] The antenna bandwidth is Vs is the satellite speed, λ is the radar signal wavelength, and α is the antenna azimuth beamwidth;

[0043] Satellite speed V s, the radar signal wavelength λ and the antenna azimuth beam width α are calculated according to the design parameters of the spaceborne radar system initialized in step S1;

[0044] The expression of radar signal wavelength λ is:

[0045]

[0046] The expression of antenna azimuth beamwidth α is:

[0047]

[0048] According to Kepler's laws and the law of universal gravitation, the satellite velocity V is calculated s The expression is:

[0049]

[0050] Among them, G is the gravitational constant, which is about 6.674×10^-11m 3 / (kg·s 2 ), M is the mass of the Earth, about 5.965×10^24kg, R e The average radius of the Earth is approximately 6371 km.

[0051] For the spotlight imaging mode, the beam will stare around a fixed rotating point on the surface, introducing a new instantaneous Doppler bandwidth, which is related to the rotation angle Δθ. The instantaneous Doppler bandwidth B a The expression side is:

[0052]

[0053] The Doppler center also changes as the beam angle changes during gaze, resulting in a new bandwidth B d , and the beam azimuth width L s Related, the expression is:

[0054] Therefore, the total azimuthal bandwidth of the beamforming mode is:

[0055]

[0056] Therefore, the expression of the minimum PRF in the beamforming mode is:

[0057]

[0058] Among them, for the sliding focus imaging mode, its characteristics are between strip and bunching, and the beam will stare around a virtual rotating point underground to observe, so as to take into account both high resolution and large azimuth width. At this time, the azimuth full-scene Doppler bandwidth will consist of a 3dB bandwidth and a slowly increasing scanning bandwidth, and its expression is:

[0059]

[0060] r is the distance between the underground virtual rotation point and the ground surface. Therefore, the minimum value expression of the PRF in the sliding mode is:

[0061]

[0062] Step S202, calculate the maximum value of the pulse repetition frequency range; determine the maximum value of the pulse repetition frequency PRF to avoid interference with the transmitted pulse according to the radar mapping band width and the lower viewing angle limit max ;

[0063] Specifically, they include:

[0064] 1) Divide the radar's lower viewing angle range into a lower viewing angle sequence according to a set step value;

[0065] Preferably, the viewing angle step value is designed to be 0.1 degree, and the sequence is θ∈[θ min :0.1:θ max ];

[0066] 2) Calculate the corresponding incident angle for each downward viewing angle in the downward viewing angle sequence:

[0067]

[0068] Among them, θ n is the nth lower view in the lower view sequence, β n is the corresponding incident angle;

[0069] 3) Calculate the time spread T of the signal within the mapping bandwidth w :

[0070]

[0071] T p is the pulse width; W g is the width of the ground-to-survey swath;

[0072] In this embodiment, the ground range mapping band width requirement is converted into the slant range width, and the time spread T of the signal within the mapping bandwidth is calculated. w , and the delay of the pulse width is also taken into account.

[0073] 4) Obtain the maximum value of the pulse repetition frequency corresponding to each incident angle according to the constraint conditions satisfied by the time spread;

[0074] In order to avoid interference from transmitted pulses, it is required that all echoes in the surveying band can reach the receiver within the same pulse repetition period, and the time dispersion T w The constraints that need to be met are:

[0075] T w ≤PRT-T p

[0076] Among them, PRT is the pulse repetition period, which is the inverse of the pulse repetition frequency; therefore

[0077] In the entire downward viewing angle range, the incident angle increases with the increase of the downward viewing angle, and the design width W of the surveying band is g is a fixed value, so the time spread of the survey band is T w It is positively correlated with the incident angle.

[0078] In this embodiment, the duty cycle η is introduced to replace T p , then the inequality becomes:

[0079]

[0080] The maximum value of the pulse repetition frequency corresponding to each incident angle determined by the above formula is

[0081] 5) taking the minimum value among the maximum values ​​of the pulse repetition frequency corresponding to each incident angle as the maximum value of the pulse repetition frequency range;

[0082] PRF max (n) and the incident angle β n It decreases with the increase of the incident angle, showing a negative correlation. Considering that an excessively large PRF will cause a surge in the data rate, in this embodiment, all PRF max The minimum value of (n) is used as a constraint, that is,

[0083] PRF max =min[PRF max (1),…,PRF max (n),…,PRF max (N)];

[0084] Wherein, N is the length of the lower viewing angle sequence.

[0085] Step S203: divide the PRF range into PRF sequences according to the set PRF interval.

[0086] Preferably, the step value of PRF is 1, and the sequence is prf∈[PRF min :1:PRF max ].

[0087] Specifically, step S3 includes:

[0088] Step S301, determining the signal timing relationship between the signal receiving window and the sub-satellite point echo and the transmitting pulse;

[0089] In such Figure 2 In the signal timing diagram shown, it is assumed that the sub-satellite echo of the first pulse arrives at the receiving end after the kth transmitted pulse, and the echo width is 2T p The useful echo signal of the observation band begins to arrive at the receiving end after the i-th transmission pulse, where T f and T n are the two-way delays from SAR to the far and near ends of the observation band, T nad It is the round-trip delay at the sub-satellite point.

[0090] Step S302: determine the first timing constraint condition of the signal receiving window and the sub-satellite point echo signal timing in each mode; the first timing constraint condition is to add a receiving window protection time T before and after the receiving window. g After that, the sub-satellite point echo of any pulse arrives before or after the receiving window to avoid the sub-satellite point echo interfering with the reception of the mapping band signal;

[0091] Specifically, the process of determining the first timing constraint condition includes:

[0092] 1) Add receiving window protection time T before and after the receiving window g After that, the first timing constraint condition for the sub-satellite point echo of any pulse to arrive before or after the receiving window is expressed as:

[0093]

[0094] Among them, Δ 1 The time interval from transmitting the pulse to receiving the sub-satellite point echo within this transmitting cycle; Δ 2 It is the time interval from transmitting pulse to receiving radar echo within this transmitting cycle;

[0095] Δ 1 +2T p ≤Δ 2 -T g The condition for the sub-satellite point echo to arrive before the receiving window;

[0096] Δ 2 +T w +T g ≤Δ 1 It is the condition that the sub-satellite point echo arrives after the receiving window.

[0097] 2) Δ 1 and Δ 2 With variable T f , T n and T nadSubstituting, the first timing constraint is obtained as

[0098]

[0099] 3) In the whole range of incident angle, T f and T n Unified as the delay T, the first timing constraint in step 2) is modulo the PRT, and the integer PRT is eliminated to obtain the expression of the first timing constraint:

[0100]

[0101] Since any incident angle can be either near or far, T f and T n Unified as delay T.

[0102] Step S303, determining the second timing constraint condition of the signal receiving window and the transmitting pulse in different modes; the second timing constraint condition is that all echoes in the observation band can reach the receiver within the same pulse repetition period, and the transmitting pulse does not fall into the echo receiving window;

[0103] Spaceborne SAR generally uses a shared antenna for transmission and reception. In order to avoid interference from transmitted pulses, it is required that all echoes in the observation band can reach the receiver within the same pulse repetition period.

[0104] Specifically,

[0105] 1) Considering the protection pulse, the second timing constraint condition that the transmitted pulse does not fall into the echo receiving window is:

[0106]

[0107] 2) T f and T n Unified as T, the above formula is modulated by PRT, and i PRTs are eliminated at the same time. The second timing constraint is:

[0108] T p +T g ≤mod(T,PRT)≤PRT-T p -T g

[0109] Step S304: Using the duty cycle η in the design parameters, the PRF and pulse width T in the first and second timing constraints are calculated. p Decoupling; Under the first and second timing constraints, the pulse width and protection time of each wave position are adjusted individually by adjusting the duty cycle η and the proportion of protection time in the pulse width μ;

[0110] Using the duty cycle η in the design parameters, PRF and pulse width T P The decoupling expression is:

[0111]

[0112] Echo receiving window protection time T g Expressed as: T g =μT p , μ∈[0,1], μ is the proportion of protection time in pulse width.

[0113] Step S305, drawing a zebra diagram according to the first and second timing constraints;

[0114] In the zebra diagram, the horizontal axis is PRF and the vertical axis is the viewing angle under the radar.

[0115] The viewing angle step value is designed to be 0.1 degree, and the sequence is θ∈[θ min :0.1:θ max ], the step value of PRF is 1, and the sequence is prf∈[PRF min :1:PRF max ].

[0116] Specifically, step S4 includes:

[0117] Step S401: Based on the width Wg and track height H constraints, min Start designing the wave position and determine the downward viewing angle θ of the center of each wave position i , the overlap ratio requirement must be met.

[0118] Step S402, calculate and extend half of the surveying bandwidth from the central wave position to the near point and the far point respectively to meet the width requirement;

[0119] Then the near and far viewing angles of the corresponding wave positions are calculated, and the PRF that meets the constraints is selected on the zebra diagram, and the selected wave positions are numbered bi,i=1,2,3….

[0120] Step S403: When a wave position that meets the requirements cannot be selected from a certain lower viewing angle, return to step S1, optimize the duty cycle η, and regenerate the echo signal timing constraint condition.

[0121] Preferably, while optimizing the duty cycle, the proportion μ of the protection time in the pulse width may also be optimized together to generate an echo signal timing constraint condition that better meets the requirements.

[0122] Step S404, switch to the next central viewing angle, repeat step S402, and finally form a design wave position covering all design requirement viewing angles.

[0123] Specifically, in step S5, the wave position selection results that meet the design conditions are output, including the lower viewing angle, pulse width, and pulse repetition frequency of each wave position.

[0124] Embodiment 2

[0125] An embodiment of the present invention discloses a specific implementation case of a variable pulse width multi-mode synthetic aperture radar wave position selection method;

[0126] During the implementation process,

[0127] Step S1, initialized satellite radar system design parameters; as shown in Table 1:

[0128] parameter Numeric Antenna length D 4.2m Satellite orbit height H 505km <![CDATA[The average radius of the Earth, R e > 6371km <![CDATA[Center frequency f c > 13.5GHz The duty cycle of the transmitted signal η 14％ <![CDATA[Receiving window protection time T g > <![CDATA[0.2*T p ]]> <![CDATA[Surveying and mapping strip width W g > 20km <![CDATA[Minimum downward viewing angle θ min > 10.82° <![CDATA[Maximum lower viewing angle θ max > 48.69° Imaging Mode Strip

[0129] Step S2, determining the constraint range of the pulse repetition frequency of each imaging mode according to the design parameters;

[0130] Specifically, they include:

[0131] 1) Calculate the antenna 3dB beamwidth;

[0132] The wavelength calculated based on the above design parameters is:

[0133]

[0134] The antenna azimuth beamwidth obtained is:

[0135]

[0136] The satellite speed obtained is:

[0137]

[0138] Then, the antenna 3dB beamwidth calculated based on the wavelength, antenna azimuth beamwidth and satellite speed is:

[0139]

[0140] 2) Calculate the stripe mode PRF minimum value:

[0141] The strip pattern azimuth bandwidth is a fixed value, consistent with the 3dB beamwidth; that is,

[0142] Strip mode azimuth bandwidth B sm =B 3 =3619Hz.

[0143] Considering the 1.3x oversampling factor, the actual stripe mode PRF minimum is:

[0144] PRF min=1.3*3619=4705Hz

[0145] 3) Calculate the maximum value of the stripe mode PRF:

[0146] The designed viewing angle step value is 0.1 degree, and the viewing angle sequence is θ∈[θ min :0.1:θ max ], N is the length of the next viewing sequence; θ min =10.82°,θ max =48.69°

[0147] The minimum angle of incidence is:

[0148]

[0149] The maximum incident angle is:

[0150]

[0151] The maximum value of PRF is determined by the duty cycle η, the design width of the ground survey band W g and the incident angle β, substituting the above parameters, the PRF of the minimum and maximum viewing angles max They are 6475Hz and 25955Hz respectively.

[0152] 4) Set the PRF step value to 1, and the PRF sequence within the range is [4705:1:6475].

[0153] Step S3, calculating the timing constraint relationship between the radar echo receiving window and the sub-satellite point echo interference and the transmission pulse interference, decoupling the pulse width, and drawing a zebra diagram; the horizontal axis range of the zebra diagram is the pulse repetition frequency constraint range; the vertical axis range is the radar viewing angle range;

[0154] The first timing constraint condition of the signal receiving window and the sub-satellite point echo signal timing in the strip mode is:

[0155]

[0156] The second timing constraint between the signal receiving window and the transmitting pulse in strip mode is:

[0157] T p +T g ≤mod(T,PRT)≤PRT-T p -T g

[0158] Draw a zebra map based on the viewing angle range and the above constraints, such as Figure 3 The horizontal axis is PRF and the vertical axis is the bottom viewing angle.

[0159] Step S4: According to the zebra diagram, wave positions are gradually selected from low angles to high angles, and the lower viewing angle center angle and pulse repetition frequency of each wave position are determined;

[0160] Determine the viewing angles at each wave position. According to step S402, a set of viewing angles is designed as shown in Table 2.

[0161] Table 2 Viewing angle under wave position

[0162]

[0163]

[0164] When a wave position that satisfies the timing constraint relationship cannot be selected from a certain lower viewing angle, return to step S1, adjust the duty cycle in the design parameters, and regenerate the timing constraint relationship and pulse width;

[0165] Complete the wave position selection, obtain the lower viewing angle and pulse repetition frequency of each wave position, and a set of sample wave positions are distributed in the zebra diagram as follows Figure 4 The wave position design results are shown in Table 3.

[0166] Table 3 Strip mode wave position design results

[0167]

[0168]

[0169] Step S5, output the wave position results selected in Table 3 including the viewing angle, pulse width and pulse repetition frequency under the wave position.

[0170] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for selecting wave position of a multi-mode synthetic aperture radar with variable pulse width, characterized in that: include: Step S1, initializing the design parameters of the spaceborne radar system including different imaging modes and duty cycles; Step S2, determining the constraint range of the pulse repetition frequency of each imaging mode according to the design parameters; Step S3, calculating the timing constraint relationship between the radar echo receiving window and the sub-satellite point echo interference and the transmission pulse interference, decoupling the pulse width, and drawing a zebra diagram; Step S4: According to the zebra diagram, wave positions are gradually selected from low angles to high angles, and the lower viewing angle center angle and pulse repetition frequency of each wave position are determined; When a wave position that satisfies the timing constraint relationship cannot be selected from a certain lower viewing angle, return to step S1 to adjust the duty cycle in the design parameters to regenerate the timing constraint relationship and pulse width; Step S5, output the selected wave position results including the viewing angle, pulse width and pulse repetition frequency under the wave position.

2. The variable pulse width multi-mode synthetic aperture radar wave position selection method according to claim 1 is characterized in that: In step S1, the initialized satellite radar system design parameters include: antenna length D, satellite orbit height H, average radius of the earth R e , center frequency f c , duty cycle η of the transmitted signal, echo receiving window protection time T g , Width of the surveying band W g 、Minimum downward viewing angle θ min 、Maximum downward viewing angle θ max ; The duty cycle η is a variable duty cycle, which is adjusted according to the timing constraint relationship; the imaging modes include: strip imaging mode, beam imaging mode, and sliding focusing imaging mode.

3. The variable pulse width multi-mode synthetic aperture radar wave position selection method according to claim 1 is characterized in that: In step S2, it includes: Step S201, calculate the minimum value of the pulse repetition frequency range; in different imaging modes, determine the minimum value PRF of the pulse repetition frequency that is dynamically adjusted to avoid azimuth ambiguity according to the respective azimuth bandwidths; min ; Step S202, calculate the maximum value of the pulse repetition frequency range; determine the maximum value of the pulse repetition frequency PRF to avoid interference with the transmitted pulse according to the radar mapping band width and the lower viewing angle limit max ; Step S203: divide the PRF range into PRF sequences according to the set PRF interval.

4. The variable pulse width multi-mode synthetic aperture radar wave position selection method according to claim 3 is characterized in that: In step S201, For strip imaging mode, the PRF minimum expression is: PRF min =1.3*B3; Wherein, B3 is the antenna bandwidth; For the spotlight imaging mode, the PRF minimum expression is: Among them, V s is the satellite speed, λ is the radar signal wavelength, Δθ is the beam rotation angle; For the sliding focus imaging mode, the PRF minimum expression is: Among them, r is the distance between the underground virtual rotation point and the ground target, R is the slant distance between the satellite and the ground target, and L a is the synthetic aperture length.

5. The variable pulse width multi-mode synthetic aperture radar wave position selection method according to claim 3 is characterized in that: Step S202 includes: 1) Divide the radar's lower viewing angle range into a lower viewing angle sequence according to a set step value; 2) Calculate the corresponding incident angle for each downward viewing angle in the downward viewing angle sequence: Among them, θ n is the nth lower view in the lower view sequence, β n is the corresponding incident angle; 3) Calculate the time spread T of the signal within the mapping bandwidth w : T p is the pulse width; W g is the width of the ground-to-survey swath; 4) Obtain the maximum value of the pulse repetition frequency corresponding to each incident angle according to the constraint conditions satisfied by the time spread; The maximum value of pulse repetition frequency corresponding to each incident angle 5) The minimum value of the pulse repetition frequency maximum values ​​corresponding to each incident angle is taken as the maximum value of the pulse repetition frequency range.

6. The variable pulse width multi-mode synthetic aperture radar wave position selection method according to claim 3 is characterized in that: In step S3, Step S301, determining the signal timing relationship between the signal receiving window and the sub-satellite point echo and the transmitting pulse; Step S302: determine the first timing constraint condition between the signal receiving window and the sub-satellite point echo signal in each mode; the first timing constraint condition is to add a receiving window protection time T before and after the receiving window. g After that, the sub-satellite point echo of any pulse arrives before or after the receiving window to avoid the sub-satellite point echo interfering with the reception of the mapping band signal; Step S303, determining the second timing constraint conditions of the signal receiving window and the transmitting pulse in different modes; The second timing constraint is that all echoes in the observation band can reach the receiver within the same pulse repetition period, and the transmitted pulse does not fall into the echo receiving window; Step S304: Using the duty cycle η in the design parameters, the PRF and pulse width T in the first and second timing constraints are calculated. p Decoupling; Under the first and second timing constraints, the pulse width and protection time of each wave position are adjusted individually by adjusting the duty cycle η and the proportion of protection time in the pulse width μ; Step S305, drawing a zebra diagram according to the first and second timing constraints; the horizontal axis range of the zebra diagram is the pulse repetition frequency constraint range; the vertical axis range is the radar viewing angle range.

7. The variable pulse width multi-mode synthetic aperture radar wave position selection method according to claim 3 is characterized in that: In step S3, the first timing constraint is expressed as: Where T is the round-trip delay from SAR to one end of the observation band; T nad is the round-trip delay at the sub-satellite point; PRT is the pulse repetition time; T g is the echo receiving window protection time; T p is the pulse width.

8. The variable pulse width multi-mode synthetic aperture radar wave position selection method according to claim 3 is characterized in that: In step S3, the second timing constraint condition is expressed as: T p +T g ≤mod(T,PRT)≤PRT-T p -T g 。 9. The variable pulse width multi-mode synthetic aperture radar wave position selection method according to claim 3 is characterized in that: Step S4 includes: Step S401: Based on the width W g , track height H constraint, from the minimum downward viewing angle θ min Start designing the wave position and determine the downward viewing angle θ of the center of each wave position i , the overlap rate requirement must be met. Step S402, calculate and extend half of the surveying bandwidth from the central wave position to the near point and the far point respectively to meet the width requirement; Then the near and far viewing angles of the corresponding wave positions are calculated, and the PRF that meets the constraints is selected on the zebra diagram, and the selected wave positions are numbered bi,i=1,2,3…. Step S403: When a wave position that meets the requirements cannot be selected from a certain lower viewing angle, return to step S1, optimize the duty cycle η, and regenerate the echo signal timing constraint condition. Step S404, switch to the next central viewing angle, repeat step S402, and finally form a design wave position covering all design requirement viewing angles.

10. The variable pulse width multi-mode synthetic aperture radar wave position selection method according to any one of claims 2 to 9, characterized in that: Echo receiving window protection time T g =T g =μT p , μ∈[0,1], μ is the proportion of protection time in pulse width.

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