A method for selecting frequency modulation bandwidth of linear frequency modulation pulse signal for ultra-low altitude detection

Abstract

The present application discloses a method for selecting a frequency modulation bandwidth of a linear frequency modulation pulse signal for ultra-low altitude detection, comprising the following steps: setting a constraint condition for the frequency modulation bandwidth of an ultra-low altitude target; and obtaining a range of the frequency modulation bandwidth according to the constraint condition. The present application determines the range of the frequency modulation bandwidth by setting a constraint condition for the frequency modulation bandwidth. Within this frequency modulation bandwidth range, the target signal can be correctly detected, and the target signal is not split. The target signal and the mirror interference signal are separated in the one-dimensional range image and the range-Doppler image, and the angular error of the target signal is small.

Description

Technical Field

[0001] The invention belongs to the technical field of transmitting waveforms for detecting ultra-low altitude targets by radar, and in particular relates to a method for selecting a frequency modulation bandwidth of a linear frequency modulation pulse signal applied to ultra-low altitude detection. Background Art

[0002] There are two main challenges faced by radar when looking downward to detect ultra-low altitude targets. First, ultra-low altitude targets have slow speeds, low altitudes, and small radar scattering cross-sections, making it difficult for radars to find and accurately detect target signals. Second, when radars look downward to detect ultra-low altitude targets, they are subject to two types of passive interference: environmental clutter interference under the irradiation of radar waves and multipath echo interference generated by target environment coupling. Environmental clutter interference reduces the radar's probability of detecting targets, making it impossible to stably detect and track targets, while multipath echo interference is usually close to the target signal in distance and speed. The combination of the two increases the error of radar angle measurement, causing the radar beam to deviate from the direction of the target signal, reducing the target signal power, and causing the radar to lose the target signal.

[0003] In order to improve the performance of radar downward detection of ultra-low altitude targets, the radar transmission waveform can use linear frequency modulation pulse signal, which has the following advantages: increasing pulse width can increase the transmission power, thereby increasing the detection distance; linear frequency modulation pulse waveform modulation can improve the distance resolution and the ranging accuracy; if pulse Doppler processing is used for multiple echo pulse signals, it is beneficial to improve the detection probability of the target; increasing the frequency modulation bandwidth can reduce the clutter power, increase the ratio of target signal to clutter (signal-to-clutter ratio), and the target detection probability will also increase; increasing the frequency modulation bandwidth can also separate the target and multipath signals on the one-dimensional echo range image or the two-dimensional range-Doppler map, which provides the possibility of filtering out multipath interference. However, when the frequency modulation bandwidth increases to a certain extent, it is possible to cause the target signal to split on the one-dimensional echo range image or the two-dimensional range-Doppler map, which will increase the radar angle measurement error and cause the radar beam to deviate from the target signal direction.

[0004] It can be seen that when the radar uses linear frequency modulation pulse signals to detect ultra-low altitude targets, the choice of frequency modulation bandwidth directly affects the detection accuracy and tracking accuracy, and the size of the frequency modulation bandwidth is affected by three aspects: radar parameters, target parameters and environmental parameters.

[0005] In order to ensure a higher detection probability and a smaller angular error, the influence of radar parameters, target parameters and environmental parameters on radar detection should be comprehensively considered, and the frequency modulation bandwidth should be reasonably selected according to certain rules. Summary of the invention

[0006] In order to ensure that the radar has a higher detection probability and a smaller angular error when using a linear frequency modulated pulse signal to detect ultra-low altitude targets, the present application proposes a method for selecting a frequency modulation bandwidth of a linear frequency modulated pulse signal applied to ultra-low altitude detection.

[0007] To achieve the above objectives, this application provides the following solutions:

[0008] A method for selecting a frequency modulation bandwidth of a linear frequency modulation pulse signal applied to ultra-low altitude detection comprises the following steps:

[0009] Set constraints on the frequency modulation bandwidth of ultra-low altitude targets;

[0010] According to the constraint condition, the range of the frequency modulation bandwidth is obtained.

[0011] Preferably, the constraint conditions include: sampling conditions, detection conditions, target and image interference separation conditions, target non-splitting conditions and coherent accumulation conditions.

[0012] Preferably, the sampling condition is: the upper limit of the frequency modulation bandwidth is less than or equal to the sampling frequency.

[0013] Preferably, the detection condition is: the lower limit of the frequency modulation bandwidth is greater than or equal to the minimum frequency modulation bandwidth, the minimum frequency modulation bandwidth is the frequency modulation bandwidth when the signal-to-noise ratio is greater than or equal to the detection threshold; the signal-to-noise ratio is the ratio of the echo pulse signal to the clutter interference around the target.

[0014] Preferably, the method for obtaining the signal-to-noise ratio includes: receiving a plurality of radar echo pulse signals; mixing and low-pass filtering the radar echo pulse signals to obtain a baseband signal; pulse compressing the baseband signal to obtain a matched filter signal; rearranging the matched filter signal to obtain a two-dimensional pulse compression signal; performing a digital Fourier transform on each column of the two-dimensional pulse compression signal to obtain a range-Doppler map of the echo pulse signal; finding the maximum value of the echo pulse signal in the range-Doppler map, taking the average value of the clutter around the maximum value of the echo pulse signal, and the ratio of the maximum value of the echo pulse signal to the average value of the clutter is the signal-to-noise ratio.

[0015] Preferably, the target and image interference separation condition is: the lower limit of the frequency modulation bandwidth is greater than or equal to c / (2Hsinβ), where c represents the speed of light, H represents the height of the target geometric center from the environmental surface, and β refers to the elevation angle of the radar target line.

[0016] Preferably, the target non-splitting condition is: the upper limit of the frequency modulation bandwidth is less than or equal to c / (2L), where c represents the speed of light and L represents the projection length of the target on the radar target line.

[0017] Preferably, the coherent accumulation condition is: the upper limit of the frequency modulation bandwidth is less than or equal to c / (T c V r ), where c is the speed of light, T c represents the total coherent accumulation time, V r represents the radial velocity of the radar and the target, T c It is obtained by multiplying the pulse repetition period and the number of pulses.

[0018] Preferably, the range of the frequency modulation bandwidth is:

[0019] The lower bound of the FM bandwidth is B min for:

[0020] B min =max{c / (2Hsinβ),B1}

[0021] The upper limit of the FM bandwidth is B max for:

[0022] B max =min{c / (2L),c / (T c V r ), f s}

[0023] The frequency modulation bandwidth range is [B min , B max ].

[0024] The beneficial effects of this application are:

[0025] The present application discloses a method for selecting a frequency modulation bandwidth of a linear frequency modulation pulse signal applied to ultra-low altitude detection. By setting constraints on the frequency modulation bandwidth, the range of the frequency modulation bandwidth is determined. Within this frequency modulation bandwidth range, the target signal can be correctly detected and the target signal is not split. The target signal and the mirror interference signal are separated in a one-dimensional range image and a range-Doppler image, and the angular error of the target signal is small. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solution of the present application, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0027] Figure 1 This is a schematic diagram of the constraint conditions that the frequency modulation bandwidth needs to meet in the first embodiment of the present application;

[0028] Figure 2This is a schematic diagram of the relationship between the frequency modulation bandwidth of the linear frequency modulation pulse signal and the sampling frequency spectrum in the first embodiment of the present application;

[0029] Figure 3 This is a schematic diagram for explaining the signal-to-noise ratio in Example 1 of the present application;

[0030] Figure 4 Schematic diagram of a curve showing the variation of the signal-to-noise ratio with the frequency modulation bandwidth in the first embodiment of the present application;

[0031] Figure 5 This is a schematic diagram of the mutual position relationship between the target and the image interference in the first embodiment of the present application;

[0032] Figure 6 This is a schematic diagram of target splitting in a one-dimensional range image under different frequency modulation bandwidths in Example 1 of the present application;

[0033] Figure 7 This is a schematic diagram of the relative position between the radar and the target environment in the second embodiment of the present application;

[0034] Figure 8 A curve diagram showing the variation of the signal-to-noise ratio with the frequency modulation bandwidth in the second embodiment of the present application;

[0035] Fig. 9 This is a schematic diagram of the spectrum of the baseband echo signal when the frequency modulation bandwidth is 18 MHz in the second embodiment of the present application;

[0036] Fig.10 This is a schematic diagram of the separation effect of the target signal and the image interference signal under the one-dimensional range image when the frequency modulation bandwidth is 18MHz in the second embodiment of the present application;

[0037] Fig.11 This is a range-Doppler schematic diagram of the echo signal after pulse Doppler processing in Example 2 of the present application;

[0038] Fig.12 This is a schematic diagram of the relationship between the target signal and the mirror interference signal in the echo signal range-Doppler diagram in the second embodiment of the present application. DETAILED DESCRIPTION

[0039] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0040] In order to make the above-mentioned objects, features and advantages of the present application more obvious and easy to understand, the present application is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0041] Embodiment 1: A method for selecting a frequency modulation bandwidth of a linear frequency modulation pulse signal applied to ultra-low altitude detection, comprising the following steps:

[0042] Step 1: Calculate the range of FM bandwidth under each of the five constraints, see Figure 1 .

[0043] 1) The FM bandwidth must meet the sampling conditions, namely:

[0044] B≤f s (1)

[0045] Among them, f s is the sampling frequency in MHz. This condition is the basis of radar signal processing and a restriction on the upper limit of the frequency modulation bandwidth.

[0046] 2) The signal-to-noise ratio SCR must meet the detection conditions, namely:

[0047] B≥B1(SCR≥detection threshold) (2)

[0048] Among them, the received radar echo pulse signals are mixed and low-pass filtered to become baseband signals, and then pulse compressed to obtain matched filter signals. Then the one-dimensional pulse compression signal is rearranged into a two-dimensional data matrix, see Figure 2 , so that each column corresponds to the pulse compression echo data with the same range gate sampling time interval of one pulse repetition period, and finally each column is subjected to digital Fourier transform to obtain the range-Doppler map of the echo signal. Find the maximum value of the target signal in the range-Doppler map, take the average value of the clutter around the target signal, and the ratio of the two is the signal-to-clutter ratio (SCR), see Figure 3 When SCR is greater than the detection threshold, usually 15dB, the minimum FM bandwidth is B1, see Figure 4 .

[0049] This condition is a restriction on the lower limit of the FM bandwidth. The ratio of the target signal to the clutter interference around the target, that is, the signal-to-clutter ratio (SCR), must be greater than a certain threshold so that the target can be detected.

[0050] 3) The frequency modulation bandwidth meets the target and image interference separation conditions, that is:

[0051] B ≥ c / (2Hsinβ) (3)

[0052] Where c represents the speed of light, H represents the height of the target geometric center from the environment surface, and β refers to the elevation angle of the radar target line. When the frequency modulation bandwidth meets this condition, the target signal and the image interference signal will be separated in the one-dimensional echo range image or the two-dimensional range-Doppler image. Figure 5 .

[0053] This condition is a restriction on the lower limit of the frequency modulation bandwidth. The separation of target and image interference can eliminate the influence of multipath interference and thus reduce the angular error of target detection.

[0054] 4) The FM bandwidth meets the condition that the target signal is not split:

[0055] B≤c / (2L) (4)

[0056] Where c represents the speed of light, L represents the projection length of the target on the radar target line. When the frequency modulation bandwidth meets this condition, the target signal will not be split in the one-dimensional echo range image or the two-dimensional range-Doppler image. Figure 6 .

[0057] This condition is a restriction on the upper limit of the FM bandwidth. Non-splitting of the target ensures that the target signal is not reduced and makes the angular error of target detection as small as possible.

[0058] 5) Coherent accumulation condition, that is, the radial moving distance between the radar and the target does not exceed the radar range resolution unit:

[0059] B≤c / (T c V r ) (5)

[0060] Where c is the speed of light, T c represents the total coherent accumulation time, V r Represents the radial velocity of the radar and the target. c It is obtained by multiplying the pulse repetition period and the number of pulses. When the frequency modulation bandwidth meets this condition, the target pulses after pulse compression in the rearranged two-dimensional data matrix are still in the same distance unit, ensuring the effectiveness of coherent accumulation.

[0061] This condition is a restriction on the upper limit of the frequency modulation bandwidth. Linear frequency modulation pulse signals usually require multiple pulses to be accumulated to increase the probability of target detection. However, the upper limit of the number of accumulated pulses is limited by the coherent accumulation condition.

[0062] Step 2: Determine the range of the FM bandwidth according to the FM bandwidth constraints determined above. The lower bound of the FM bandwidth is min for:

[0063] B min =max{c / (2Hsinβ),B1} (6)

[0064] The upper limit of the FM bandwidth is B max for:

[0065] B max=min{c / (2L),c / (T c V r ), f s} (7)

[0066] In summary, the range of FM bandwidth is [B min , B max ].

[0067] Within this frequency modulation bandwidth, the target signal can be correctly detected without splitting, the target signal and the image interference signal are separated in the one-dimensional range image and the range-Doppler image, and the angular error of the target signal is small.

[0068] Embodiment 2: To verify the effectiveness and feasibility of the present invention, a calculation example is used to illustrate.

[0069] The radar is an active system, that is, the transmitting and receiving antennas are both inside the radar, the antenna sidelobe level is -20dB, the antenna beam points to the center of the target, both the incident and receiving are VV polarization, the operating frequency is in the Ku band, the intermediate frequency is 45MHz, and the sampling frequency is f s The pulse width is 2μs, the pulse repetition frequency is 100kHz, the pulse accumulation number is 256, and the relative dielectric constant of the environment is ε r =6–j4, ambient RMS height h x 1×10 -3 m, the target is a metal cylinder with a diameter of 0.5m and a length of 6m, and its axis is along the x-axis. The radar position vector is S p (3000m, 0m, 1500m), velocity vector is V s (-200m / s, 0, 0), the target position vector is T p (0, 0, 50m), velocity vector V t is (200m / s, 0, 0), the target radar elevation angle is θ, and the relative position relationship between the radar and the target environment can be found in Figure 7 .

[0070] By using the method of the present invention, the five conditions that the frequency modulation bandwidth needs to meet are: 1) The sampling condition determines that the upper limit of the frequency modulation bandwidth is f s ; 2) Detection conditions, set the signal-to-noise ratio detection condition that the target signal needs to meet to 15dB, corresponding to Figure 8 Point P in the figure shows that the FM bandwidth should be greater than B1, i.e. 8 MHz; 3) The target and image interference separation condition, at this time, according to the radar and target position, β is calculated to be 25.8°, and the FM bandwidth should be greater than 7 MHz; 4) The target is not split, the radial length L is calculated to be 5.39 m, and the FM bandwidth should be less than 27.7 MHz; 5) The coherent accumulation condition, T c2.56ms, V r The frequency modulation bandwidth should be less than 325MHz. min 8MHz, B max is 27.7MHz, so the FM bandwidth range is [8MHz, 27.7MHz]. Figures 9 to 12 The frequency spectrum of the echo signal is given when the frequency modulation bandwidth is 18MHz. It can be seen from the figure that the frequency modulation bandwidth is 18MHz. Under this frequency modulation bandwidth, the target and image interference can be separated, and the absolute value of the target angle error calculated by the radar is less than 0.1° within the frequency modulation bandwidth range [8MHz, 27.7MHz].

[0071] The embodiments described above are only descriptions of the preferred methods of the present application, and are not intended to limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements made to the technical solutions of the present application by ordinary technicians in this field should fall within the scope of protection determined by the claims of the present application.

Claims

1. A method for selecting a frequency modulation bandwidth of a linear frequency modulation pulse signal for ultra-low altitude detection, characterized in that: The following steps are involved: Set constraints on the frequency modulation bandwidth of ultra-low altitude targets; According to the constraint condition, obtaining the range of the frequency modulation bandwidth; The constraint conditions include: sampling conditions, detection conditions, target and image interference separation conditions, target non-splitting conditions and coherent accumulation conditions; The detection condition is: the lower limit of the frequency modulation bandwidth is greater than or equal to the minimum frequency modulation bandwidth, the minimum frequency modulation bandwidth is the frequency modulation bandwidth when the signal-to-noise ratio is greater than or equal to the detection threshold; the signal-to-noise ratio is the ratio of the echo pulse signal to the clutter interference around the target; The method for obtaining the signal-to-noise ratio comprises: receiving a plurality of radar echo pulse signals; mixing and low-pass filtering the radar echo pulse signals to obtain a baseband signal; pulse compressing the baseband signal to obtain a matched filter signal; rearranging the matched filter signal to obtain a two-dimensional pulse compression signal; performing a digital Fourier transform on each column of the two-dimensional pulse compression signal to obtain a range-Doppler diagram of the echo pulse signal; finding the maximum value of the echo pulse signal in the range-Doppler diagram, taking the average value of the clutter around the maximum value of the echo pulse signal, and the ratio of the maximum value of the echo pulse signal to the average value of the clutter is the signal-to-noise ratio.

2. The method for selecting the frequency modulation bandwidth of a linear frequency modulation pulse signal for ultra-low altitude detection according to claim 1, characterized in that: The sampling condition is: the upper limit of the frequency modulation bandwidth is less than or equal to the sampling frequency.

3. The method for selecting the frequency modulation bandwidth of a linear frequency modulation pulse signal for ultra-low altitude detection according to claim 1, characterized in that: The target and image interference separation condition is: the lower limit of the frequency modulation bandwidth is greater than or equal to c / (2Hsinβ), where c represents the speed of light, H represents the height of the target geometric center from the environment surface, and β refers to the elevation angle of the radar target line.

4. The method for selecting the frequency modulation bandwidth of a linear frequency modulation pulse signal for ultra-low altitude detection according to claim 1, characterized in that: The target non-splitting condition is that the upper limit of the frequency modulation bandwidth is less than or equal to c / (2L), where c represents the speed of light and L represents the projection length of the target on the radar target line.

5. The method for selecting the frequency modulation bandwidth of a linear frequency modulation pulse signal for ultra-low altitude detection according to claim 1, characterized in that: The coherent accumulation condition is: the upper limit of the frequency modulation bandwidth is less than or equal to c / (T c V r ), where c is the speed of light, T c represents the total coherent accumulation time, V r represents the radial velocity of the radar and the target, T c It is obtained by multiplying the pulse repetition period and the number of pulses.

6. The method for selecting the frequency modulation bandwidth of a linear frequency modulation pulse signal for ultra-low altitude detection according to claim 1, characterized in that: The range of the frequency modulation bandwidth is: The lower bound of the FM bandwidth is B min for: B min =max{c / (2Hsinβ),B1} Among them, B1 is the minimum frequency modulation bandwidth, β refers to the elevation angle of the radar target line, H is the height of the target geometric center from the environment surface, and c is the speed of light; The upper limit of the FM bandwidth is B max for: B max =min{c / (2L),c / (T c V r ),f s } Where L represents the projection length of the target on the radar target line, T c represents the total coherent accumulation time, V r represents the radial velocity of the radar and the target, f s Indicates the sampling frequency; The frequency modulation bandwidth range is [B min , B max ].

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