Air-to-air ISAR imaging and target information combined PRF design method

Abstract

The invention belongs to the technical field of ISAR imaging, and particularly relates to an air-to-air ISAR imaging and target information combined PRF design method, which comprises the following steps: step 1, in an air search tracking state, stably tracking a target to obtain a target radial speed VTar, a target distance RTar and a radar beam azimuth angle theta az and a pitch angle theta el; step 2, according to the target distance RTar, calculating a PRF value range by taking avoidance of transmitted pulses and altitude line echoes as a principle; step 3, calculating a main clutter center fdc and a main clutter width Ba according to the wave beam azimuth angle theta az and the pitch angle theta el; step 4, according to the target radial speed VTar, setting the requirement that the PRF needs to meet by taking the principle that the target echo signal is located outside the main clutter in the azimuth frequency domain; and step 5, searching in the PRF value range, and selecting the PRF value meeting the requirement.

Description

Technical Field

[0001] The present application belongs to the field of ISAR imaging technology, and specifically relates to a PRF design method for air-to-air ISAR imaging combined with target information. Background Art

[0002] Inverse synthetic aperture radar (ISAR) obtains high-resolution images of non-cooperative targets by processing the Doppler effect formed by the relative motion between the radar and the target. It is widely used in the detection and identification of targets such as aircraft, ships, and space.

[0003] At present, ISAR imaging of aerial targets is mainly achieved by ground-based radars. Due to the influence of the earth's curvature, the imaging distance is greatly limited. Compared with ground-based radars, airborne radars can detect targets earlier and can track and monitor aerial targets in a larger range. Combined with air-to-air ISAR imaging technology, high-resolution images of distant aerial targets can be obtained, thereby realizing functions such as long-range target recognition and classification, which is of great significance for early warning detection, target reconnaissance and air control.

[0004] However, due to the high flying speed, strong maneuverability and small size of aerial targets, it is very difficult for airborne radar to perform ISAR imaging on them. Therefore, at present, airborne radar is mainly used for searching and tracking aerial targets, treating the target as a "point" and obtaining information such as the distance, angle, and speed of the target from the received echo and the direction of its own antenna beam. With the increasing demand for aerial target identification, it is urgent to adopt ISAR imaging technology to obtain high-resolution images of aerial targets, provide information such as the shape and size of the target, and provide support for high-precision identification of aerial targets.

[0005] Different from the upward beam-looking mode of ground-based radar for ISAR imaging of aerial targets, when the aerial target is flying at a lower altitude than the radar carrier, the radar beam will work in the downward beam-looking mode. At this time, the echo of the ground scene will also be received by the radar together with the aerial target echo, forming the main clutter interference. In serious cases, it will drown out the aerial target echo, seriously affecting air-to-air ISAR imaging. In addition, since airborne radars usually use a pulse system, the transmission and reception share the same antenna, and there are also transmission pulses, altitude line echoes and target echoes in the air at the same time.

[0006] Limiting the radar pulse repetition frequency (PRF) can prevent the transmitted pulse and altitude line echo from falling within the echo receiving window, and can separate the echo signal of the aerial target from the main clutter, providing convenience for air-to-air ISAR imaging. For this reason, this application is proposed. Summary of the invention

[0007] The purpose of this application is to provide a PRF design method for air-to-air ISAR imaging combined with target information. In view of the problem that the air target echo is easily submerged by the main clutter during air-to-air ISAR imaging by airborne radar, a PRF value suitable for air-to-air ISAR imaging is optimized, while avoiding the emission pulse and altitude line echo, the air target echo is placed outside the main clutter in the azimuth frequency domain, so that the target echo signal can be effectively separated for imaging processing, thereby realizing high-resolution ISAR imaging of airborne targets by airborne radar.

[0008] The technical solution of this application is:

[0009] A PRF design method for air-to-air ISAR imaging combined with target information, comprising:

[0010] Step 1: In the air search and tracking state, stably track the target and obtain the target radial velocity V Tar 、Target distance R Tar and the radar beam azimuth θ az and the pitch angle θ el ;

[0011] Step 2: According to the target distance R Tar , based on the principle of avoiding the transmission pulse and height line echo, calculate the PRF value range;

[0012] Step 3: According to the beam azimuth angle θ az , pitch angle θ el , calculate the main clutter center f dc and the main clutter width B a ;

[0013] Step 4: According to the target radial velocity V Tar The principle is to make the target echo signal outside the main clutter in the azimuth frequency domain, and set the requirements that the PRF needs to meet;

[0014] Step 5: Search within the PRF value range and select a PRF value that meets the requirements.

[0015] According to at least one embodiment of the present application, in the above-mentioned PRF design method for air-to-air ISAR imaging combined with target information,

[0016] In step 2, according to the target distance R Tar , based on the principle of avoiding the transmit pulse and height line echo, the PRF value range is calculated as follows:

[0017]

[0018] in,

[0019] f p is the transmit pulse repetition frequency;

[0020] H is the flight altitude of the carrier aircraft;

[0021] c is the propagation speed of electromagnetic waves,

[0022] T p is the transmit pulse width;

[0023] N r The number of sampling points of the echo signal in the range direction;

[0024] F s is the sampling frequency;

[0025] T g To protect the time width;

[0026] m is the serial number of the transmitted pulse.

[0027] According to at least one embodiment of the present application, in the above-mentioned PRF design method for air-to-air ISAR imaging combined with target information, in step 3, according to the beam azimuth angle θ az , pitch angle θ el , calculate the main clutter center f dc , specifically:

[0028]

[0029] in,

[0030] V a is the carrier speed;

[0031] λ is the radar operating wavelength.

[0032] According to at least one embodiment of the present application, in the above-mentioned PRF design method for air-to-air ISAR imaging combined with target information, in step 3, according to the beam azimuth angle θ az , pitch angle θ el , calculate the main clutter width B a , specifically:

[0033]

[0034] θ bw is the 3dB beamwidth of the transmitting antenna.

[0035] According to at least one embodiment of the present application, in the above-mentioned PRF design method for air-to-air ISAR imaging combined with target information, in step 4, according to the target radial velocity V Tar The principle is to make the target echo signal outside the main clutter in the azimuth frequency domain, and set the requirements that the PRF needs to meet. When the clear area is located on both sides of the main clutter, the requirements that the PRF needs to meet are set as follows: or Where k is a constant protection factor.

[0036] According to at least one embodiment of the present application, in the above-mentioned PRF design method for air-to-air ISAR imaging combined with target information, in step 4, according to the target radial velocity V Tar In principle, the target echo signal is outside the main clutter in the azimuth frequency domain, and the requirements that the PRF needs to meet are set. When the clear area is located in the middle of the main clutter, the requirements that the PRF needs to meet are set as follows:

[0037] According to at least one embodiment of the present application, in the above-mentioned PRF design method for air-to-air ISAR imaging combined with target information, in step 5, a search is performed within the PRF value range to select a PRF value that meets the requirements, and when the clear area is located on both sides of the main clutter, a digital filter is used Among them, f a Indicates the azimuth frequency.

[0038] According to at least one embodiment of the present application, in the above-mentioned PRF design method for air-to-air ISAR imaging combined with target information, in step 5, a search is performed within the PRF value range to select a PRF value that meets the requirements, and when the clear area is located in the middle of the main clutter, a digital filter is used

[0039] This application has at least the following beneficial technical effects:

[0040] A PRF design method for air-to-air ISAR imaging combined with target information is provided. Compared with the method of designing the PRF value in advance in conventional airborne radar ISAR imaging, the influence of different target motion characteristics on the target echo and main clutter distribution is fully considered. The PRF is designed and optimized online according to the target information to avoid the emission pulse and height line echo, and the target echo is placed in a clear area outside the main clutter frequency band, so that the target signal can be effectively separated for imaging processing, and a high-resolution ISAR image of the aerial target can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 Schematic diagram of the PRF design method for air-to-air ISAR imaging combined with target information provided in this application;

[0042] Figure 2 Schematic diagram of the working geometry of airborne radar air-to-air ISAR imaging provided for this application;

[0043] Figure 3 A schematic diagram of the airborne radar air-to-air ISAR imaging echo signal timing provided for this application;

[0044] Figure 4Schematic diagram of the clear area provided for this application located on both sides of the main clutter;

[0045] Figure 5 Schematic diagram of the clear area provided for this application being located in the middle of the main clutter;

[0046] Figure 6 A schematic diagram of the PRF design results at different azimuth angles under a set of typical parameters provided in this application;

[0047] Figure 7 A schematic diagram of echo data simulation when the PRF design provided in this application is inappropriate;

[0048] Figure 8 A schematic diagram of a PRF design method for air-to-air ISAR imaging combined with target information provided in this application, designing PRF, and performing echo data simulation.

[0049] In order to better illustrate the present embodiment, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of the actual product. In addition, the drawings are only used for illustrative purposes and should not be construed as limitations on the present application. DETAILED DESCRIPTION

[0050] In order to make the technical solution and advantages of the present application clearer, the technical solution of the present application will be described in further detail in detail and in detail with reference to the accompanying drawings. It can be understood that the specific embodiments described here are only partial embodiments of the present application, which are only used to explain the present application, not to limit the present application. It should be noted that, for the convenience of description, only the parts related to the present application are shown in the accompanying drawings, and other related parts can refer to the general design.

[0051] In addition, unless otherwise defined, the technical terms or scientific terms used in the description of this application shall have the usual meanings understood by those of ordinary skill in the art to which this application belongs. The words used in the description of this application to indicate orientation are only used to indicate relative directions or positional relationships. When the absolute position of the described object changes, its relative positional relationship may also change accordingly. The word "include" used in the description of this application indicates that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, but does not exclude other elements or objects.

[0052] In addition, it should be noted that, unless otherwise clearly specified and limited, the words "installation", "connection" and similar terms used in the description of this application should be understood in a broad sense. For example, the connection can be a fixed connection or a detachable connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium. Technical personnel in the field can understand its specific meaning in this application according to the specific circumstances.

[0053] A PRF design method for air-to-air ISAR imaging combined with target information, such as Figure 1 shown.

[0054] Step 1: In the air search and tracking state, stably track the target and obtain the target radial velocity V Tar 、Target distance R Tar and the radar beam azimuth θ az and the pitch angle θ el .

[0055] Airborne radar air-to-air ISAR imaging working geometry, such as Figure 2 As shown in the figure, before performing ISAR imaging of aerial targets, the radar works in the air search and tracking state, finds the target and performs stable tracking, and the target radial velocity V can be obtained. Tar 、Target distance R Tar and the radar beam azimuth θ az and the pitch angle θ el And other information, and output the target track number.

[0056] After the operator selects the aerial target to be imaged and the imaging resolution, the radar is controlled to enter the air-to-air ISAR working mode. After receiving the control command, the radar designs the PRF based on the target information.

[0057] Step 2: According to the target distance R Tar , based on the principle of avoiding the transmitted pulse and height line echo, calculate the PRF value range.

[0058] The time series of airborne radar air-to-air ISAR imaging echo signal, such as Figure 3 As shown in the figure, due to the limited flying altitude of the radar carrier, the sampleable time window between the transmit pulse and the altitude line echo is short, which often cannot meet the target echo sampling length requirement. Therefore, when designing the PRF, the target echo signal needs to be placed within the sampleable time window between the altitude line echo and the next transmit pulse. Therefore, the restrictions of the transmit pulse and altitude line echo on the PRF are:

[0059]

[0060] in,

[0061] f p is the transmit pulse repetition frequency;

[0062] H is the flight altitude of the carrier aircraft;

[0063] c is the propagation speed of electromagnetic waves,

[0064] T p is the transmit pulse width;

[0065] Nr The number of sampling points of the echo signal in the range direction;

[0066] F s is the sampling frequency;

[0067] T g To protect the time width;

[0068] m is the serial number of the transmitted pulse, which is 1, 2, 3, etc.

[0069] Then the PRF value range is obtained as follows:

[0070] When the PRF lower limit calculation result is higher than the upper limit calculation result, it indicates that there is no suitable PRF to be selected.

[0071] Step 3: According to the beam azimuth angle θ az , pitch angle θ el , calculate the main clutter center f dc and the main clutter width B a .

[0072]

[0073] in,

[0074] V a is the carrier speed;

[0075] λ is the radar operating wavelength.

[0076] Assume that the 3dB beamwidth of the transmitting antenna is θ bw , then the beam width between zero points is 2θ bw The clutter width corresponding to the beam width is generally defined as the main clutter width B a , which can be expressed as:

[0077]

[0078] Step 4: According to the target radial velocity V Tar The PRF requirement is set based on the principle that the target echo signal is outside the main clutter in the azimuth frequency domain.

[0079] In order to prevent the target echo signal from falling into the main clutter, it is necessary to optimize the PRF design according to the target radial velocity so that the target echo signal is in a clear area outside the main clutter in the azimuth frequency domain. There are two situations according to the distribution of the main clutter and the clear area.

[0080] When the clear area is located on both sides of the main clutter, such as Figure 4 As shown, the main clutter interval can be expressed as:

[0081]

[0082] in,

[0083] Frac(·) is the baseband partial function taking the azimuth frequency;

[0084] In order to make the target echo signal avoid the main clutter, the PRF needs to meet the following requirements:

[0085] or

[0086] in,

[0087] k is a constant protection factor, which can be between 1.2 and 1.5.

[0088] When the clear area is located in the middle of the main clutter, Figure 5 As shown, the main clutter interval can be expressed as:

[0089]

[0090] In order to make the target echo signal avoid the main clutter, the PRF needs to meet the following requirements:

[0091]

[0092] Step 5: Search within the PRF value range and select a PRF value that meets the requirements.

[0093] Searching within the PRF value range can avoid the transmitted pulse and altitude line clutter. According to the requirements that PRF needs to meet, the PRF value is selected so that the target echo signal is in a clear area outside the main clutter. Therefore, a digital filter can be used in the azimuth frequency domain to filter out the main clutter and only retain the target echo signal. In this way, the influence of the main clutter can be avoided during air-to-air ISAR imaging processing.

[0094] When the clear area is located on both sides of the main clutter, the digital filter L(f a ) can be expressed as:

[0095]

[0096] in,

[0097] f a is the azimuth frequency.

[0098] When the clear area is located in the middle of the main clutter, the digital filter L(f a ) can be expressed as:

[0099]

[0100] According to the PRF redundancy, azimuth spectrum truncation can be used to implement azimuth downsampling during filtering, thereby reducing the amount of data for ISAR imaging processing.

[0101] The PRF design results at different azimuth angles under a set of typical parameters are as follows: Figure 6 As shown, the white area is the optional PRF value. Select a PRF value from the black area and the white area respectively to simulate the echo data. Select a PRF value from the black area to simulate the echo data. The simulation results are shown in Figure 7 As shown in the figure, select the PRF value from the white area to simulate the echo data. The simulation results are shown in Figure 8 As shown, it can be seen that when the PRF design is not appropriate, the target echo signal will be submerged by the main clutter, and it is difficult to extract the target signal for ISAR imaging processing. By adopting the PRF value designed in this application, the target echo will be outside the main clutter frequency band, so that the target signal can be effectively extracted for ISAR imaging processing, thereby realizing ISAR imaging of the target by the airborne radar.

[0102] So far, the technical solution of the present application has been described in conjunction with the preferred embodiments shown in the accompanying drawings. Those skilled in the art should understand that the scope of protection of the present application is obviously not limited to these specific embodiments. Without departing from the principles of the present application, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will fall within the scope of protection of the present application.

Claims

1. A PRF design method for air-to-air ISAR imaging combined with target information, characterized in that: include: Step 1: In the air search and tracking state, stably track the target and obtain the target radial velocity V Tar 、Target distance R Tar and the radar beam azimuth θ az and the pitch angle θ el ; Step 2: According to the target distance R Tar , based on the principle of avoiding the transmission pulse and height line echo, calculate the PRF value range; Step 3: According to the beam azimuth angle θ az , pitch angle θ el , calculate the main clutter center f dc and the main clutter width B a ; Step 4: According to the target radial velocity V Tar The principle is to make the target echo signal outside the main clutter in the azimuth frequency domain, and set the requirements that the PRF needs to meet; Step 5: Search within the PRF value range and select a PRF value that meets the requirements.

2. The PRF design method for air-to-air ISAR imaging combined with target information according to claim 1, characterized in that: In step 2, according to the target distance R Tar , based on the principle of avoiding the transmit pulse and height line echo, the PRF value range is calculated as follows: in, f p is the transmit pulse repetition frequency; H is the flight altitude of the carrier aircraft; c is the propagation speed of electromagnetic waves, T p is the transmit pulse width; N r The number of sampling points of the echo signal in the range direction; F s is the sampling frequency; T g To protect the time width; m is the serial number of the transmitted pulse.

3. The PRF design method for air-to-air ISAR imaging combined with target information according to claim 2 is characterized in that: In step 3, according to the beam azimuth angle θ az , pitch angle θ el , calculate the main clutter center f dc , specifically: in, V a is the carrier speed; λ is the radar operating wavelength.

4. The PRF design method for air-to-air ISAR imaging combined with target information according to claim 3 is characterized in that: In step 3, according to the beam azimuth angle θ az , pitch angle θ el , calculate the main clutter width B a , specifically: θ bw is the 3dB beamwidth of the transmitting antenna.

5. The PRF design method for air-to-air ISAR imaging combined with target information according to claim 4 is characterized in that: In step 4, according to the target radial velocity V Tar The principle is to make the target echo signal outside the main clutter in the azimuth frequency domain, and set the requirements that the PRF needs to meet. When the clear area is located on both sides of the main clutter, the requirements that the PRF needs to meet are set as follows: or Where k is a constant protection factor.

6. The PRF design method for air-to-air ISAR imaging combined with target information according to claim 5, characterized in that: In step 4, according to the target radial velocity V Tar In principle, the target echo signal is outside the main clutter in the azimuth frequency domain, and the requirements that the PRF needs to meet are set. When the clear area is located in the middle of the main clutter, the requirements that the PRF needs to meet are set as follows:

7. The PRF design method for air-to-air ISAR imaging combined with target information according to claim 6, characterized in that: In step 5, search within the PRF value range and select the PRF value that meets the requirements. When the clear area is located on both sides of the main clutter, use the digital filter Among them, f a Indicates the azimuth frequency.

8. The PRF design method for air-to-air ISAR imaging combined with target information according to claim 7, characterized in that: In step 5, search within the PRF value range and select the PRF value that meets the requirements. When the clear area is located in the middle of the main clutter, use the digital filter

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