Passive radar device and target detection method

Abstract

To provide a passive radar device that satisfies limitation of frequency allocation and can conduct a search over a wide range.SOLUTION: According to an embodiment, a passive radar device is provided with a reference wave antenna, a passive phased array antenna, and a target detection unit. The reference wave antenna receives a direct wave of a radio wave emitted from an artificial satellite on a stationary orbit and outputs a reference signal. The passive phased array antenna receives an indirect wave of the radio wave reflected on a target and outputs a received signal. The target detection unit detects the target based on the reference signal and the received signal.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] SUMMARY OF THE INVENTION Embodiments of the present invention relate to a passive radar device and a target detection method. [Background technology]

[0002] Radio waves are a finite resource, and various infrastructures operate by sharing a limited frequency band. In recent years, the frequency band available for radar systems has been limited due to factors such as the expansion of frequencies for cellular phone systems. With frequency allocations limited, passive radars are attracting attention. However, the wavelengths of the radio waves used in terrestrial digital television broadcasting and radio broadcasting are long, making it difficult to apply phased arrays using existing angle measurement technology, as the antenna size would be too large.

[0003] Therefore, there is considerable room for consideration of passive radars that use shorter wavelength radio waves. For example, if a passive radar device using BS (Broadcasting Satellite) broadcast waves or CS (Communication Satellite) communication waves can be realized, it may be possible to detect targets at least between the ground and geostationary orbit. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] "Revised Radar Technology," edited by Takashi Yoshida, Institute of Electronics, Information and Communication Engineers, October 1, 1996 (first edition) [Non-patent document 2] Malanowski,'Signal Processing For Passive Bistatic Radar'ARTEC- HOUSE,sec.2.4.1The Ambiguity Function of a Noise Signal(2019) Summary of the Invention [Problem to be solved by the invention]

[0005] Both broadcast and communication waves (hereafter referred to as broadcast waves) are transmitted as continuous waves rather than pulses, which presents unique challenges. For example, the amount of received data can be so large that the computational load required for signal processing becomes enormous, resulting in significant resource consumption. Furthermore, the detection accuracy requirements change depending on the position, speed, and distance of the target to be detected. Furthermore, the size of targets varies enormously, making it difficult to expect stable detection accuracy for all targets. Technology that can solve these challenges is desired.

[0006] Therefore, an object is to provide a passive radar device and a target detection method that can satisfy the restrictions of frequency allocation and search a wide area. [Means for solving the problem]

[0007] According to an embodiment, a passive radar device includes a reference wave antenna, a passive phased array antenna, and a target detection unit. The reference wave antenna receives direct radio waves emitted from a satellite in geostationary orbit and outputs a reference signal. The passive phased array antenna receives indirect radio waves reflected by a target and outputs a received signal. The target detection unit detects a target based on the reference signal and the received signal. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram showing an example of an operation mode of a passive radar device according to an embodiment. [Figure 2] FIG. 2 is a system diagram showing an example of a passive radar device 500 according to an embodiment. [Figure 3] FIG. 3 is a block diagram showing an example of a receiving system of the passive radar device 500 shown in FIG. [Figure 4] FIG. 4 is a diagram showing an example of a calculation system related to receiving beamforming. [Figure 5]FIG. 5 is a functional block diagram showing an example of a processing system of the signal processing unit 6 and the radar control unit 7. As shown in FIG. [Figure 6] FIG. 6 is a diagram showing an example of a processing system related to clutter suppression processing. [Figure 7] FIG. 7 is a diagram showing an example of a processing system related to the correlation processing 63 in FIG. [Figure 8] FIG. 8 is a diagram for explaining reduction of the amount of data in correlation processing. DETAILED DESCRIPTION OF THE INVENTION

[0009] In many areas, satellite broadcast waves are transmitted in the Ku band, which is between 12 and 18 gigahertz. The wavelengths in this band are short enough that existing phased array technology can be applied. However, for example, the bandwidth of one satellite broadcast channel in Japan is approximately 35 MHz, and if continuous waves are received and processed as is, the amount of distance data becomes enormous. While this has the advantage of increasing Doppler resolution, the huge amount of data consumes a large amount of computer resources, resulting in enormous calculation costs.

[0010] Furthermore, aircraft are generally required to search over long distances, have a relatively large RCS, and operate at high speeds. On the other hand, AI (Artificial Intelligence) drones and similar devices are required to search over short distances, have a relatively small RCS, and operate at low speeds. As such, the characteristics of each target vary widely. Furthermore, while a coarse range resolution is acceptable for long distances, there is a latent demand for finer range resolution for close distances to reduce observation errors. Radars that are required to search for targets over a wide area must satisfy conflicting conditions when selecting radar parameters such as range resolution, Doppler resolution, and integration time.

[0011] Next, an embodiment capable of solving the above-mentioned problem will be described. <Configuration> Figure 1 is a diagram showing an example of the operation mode of a passive radar device 500 according to an embodiment. In Figure 1, broadcast waves emitted via downlink from a broadcast satellite (BS) 100 in geostationary orbit are captured by a terrestrial passive radar device 500. The passive radar device 500 receives both a direct wave (reference signal) arriving directly from the broadcast satellite 100 and an indirect wave (received signal) arriving after the broadcast wave is reflected by a target, and calculates the correlation between the two signals to detect a target 400. If the positions of the passive radar device 500 and the broadcast satellite 100 are known, the position of the target 400 can be obtained using a bistatic positioning method.

[0012] The passive radar device 500 includes a reference wave antenna 1 and a search antenna 200. The reference wave antenna 1 is fixed with its aperture facing the broadcasting satellite 100, and can be, for example, a commercially available BS antenna. The search antenna 200 is fixed to a mount equipped with, for example, a signal processing system 300.

[0013] The signal processing system 300 is a computer having a processor and a memory, and detects a target based on the reference signal output from the reference wave antenna 1 and the received signal output from the search antenna 200 .

[0014] The search antenna 200 is, for example, a subarray phased array receiving antenna having multiple element antennas. In this embodiment, it is a so-called passive phased array DBF (Digital Beam Forming) antenna. This type of antenna can simultaneously form multiple receiving beams for coverage in three-dimensional space.

[0015] Fig. 2 is a system diagram showing an example of the passive radar device 500 shown in Fig. 1. In Fig. 2, BS broadcast waves arriving at a reference wave antenna 1 are input to a reference wave receiving unit 3. The reference wave receiving unit 3 suppresses unnecessary wave components from the antenna input and inputs a reference signal derived from the direct wave to a reception processing unit 5. The reception processing unit 5 performs quadrature digital conversion on the reference signal to generate a baseband digital signal (reference wave I / Q signal: reference data) and inputs it to a signal processing unit 6.

[0016] The search antenna 200 includes a plurality of subarray antennas 4 (#1 to #N). Each subarray antenna 4 has element antennas 2 (#1 to #K), which are, for example, dipole antennas. Indirect waves arriving from the target 400 are captured by each element antenna 2 of the subarray antenna 4, and a received signal is generated after undergoing processes such as unnecessary wave suppression and subarray synthesis.

[0017] The received signals (#1 to #N) from the subarray antennas 4 (#1 to #N) are input to the reception processing unit 5. The reception processing unit 5 performs quadrature digital conversion on each of the received signals (#1 to #N) to generate baseband digital signals (received I / Q signals: received data), and inputs these signals to the signal processing unit 6.

[0018] The signal processing unit 6 forms a receiving beam from the digital received data by DBF calculation, and after clutter suppression, obtains a correlation output with an improved S / N (signal-to-noise ratio) by taking the cross-correlation between the beamformed data and the reference wave I / Q data. Furthermore, the signal processing unit 6 detects targets from the correlation output using a 2D CFAR or the like, and outputs target detection information based on the bistatic positioning method to the radar control unit 7.

[0019] The radar control unit 7 converts the target detection information from bistatic coordinates to Cartesian coordinates based on the position coordinates of the broadcasting satellite 100 and the passive radar device 500. Furthermore, the radar control unit 7 performs target movement prediction processing using an extended Kalman filter or the like to establish a target track. The target track is output to the display unit 8 and displayed visually. Furthermore, the radar control unit 7 outputs control signals such as an integration time to the signal processing unit 6 based on the target movement prediction results.

[0020] Figure 3 is a block diagram showing an example of the receiving system of the passive radar device 500 shown in Figure 2. The subarray antenna 4 includes a low-noise amplifier 41, an image-rejection mixer 42, and a subarray combiner 43. The indirect waves that reach the element antennas 2 of the subarray antenna 4 are amplified by the low-noise amplifier 41, and then passed through the image-rejection mixer 42, where the carrier signal of approximately 12 GHz is down-converted to an intermediate frequency of, for example, approximately 1 GHz, to generate an IF signal. The IF signals from each subarray antenna 4 are combined in the subarray combiner 43 to generate a received signal.

[0021] The reference wave receiving unit 3 includes a low-noise amplifier 31 and an image rejection mixer 32, similar to those of the subarray antenna 4, and down-converts a carrier signal of approximately 12 GHz to an intermediate frequency of, for example, about 1 GHz to obtain an IF signal. Then, a delay line 33 applies a group delay equivalent to that of the subarray combiner 43 to the IF signal, generating a reference signal.

[0022] The reception processing unit 5 includes N+1 circuit systems 50 corresponding to the N subarray antennas 4 and one reference wave receiving unit 3. Each circuit system 50 includes an amplifier 51, a mixer 52, an analog filter 53, an analog-to-digital (A / D) converter 54, and an I / Q detector 55.

[0023] The IF signal of the reference signal is converted into digital form in the circuit system 50, and a baseband digital data sequence (I, Q signals: complex numbers) is generated and output as reference data. The IF signals of the received signals #1 to #N are converted into digital form in each circuit system 50, and a baseband digital data sequence (I, Q signals: complex numbers) is generated and output as received data #1 to #N. Here, since the bandwidth of one channel of a BS broadcast wave is approximately 35 MHz, the data rate of the reference data and the received data is set to a 35 MHz rate.

[0024] Next, the operation of the above configuration will be explained with reference to the circuit system and processing system of each functional block. <effect> Figure 4 shows an example of a calculation system related to receive beamforming. Here, receive beamforming will be explained along with its relationship with the subarray aperture arrangement. In Figure 4, subarray I and Q signals of the total aperture, number L of subarrays, are input. Here, the aperture is divided into four, aperture (1), aperture (2), aperture (3), and aperture (4), and receive beams are formed.

[0025] To form multiple beams, for example, J types of beam weights are prepared according to the number of beams corresponding to the search coverage area, and J sets of beam forming weights W (number of subarrays N) are provided to form J beams simultaneously. Then, by multiplying the input signal by the J types of beam weights, Σ beam 1,...,Σ beam J, ΔAZ beam 1,...,ΔAZ beam J, ΔEL beam 1,...,ΔEL beam J are formed.

[0026] Here, Σ is the sum beam, which is obtained by adding the entire aperture after weight multiplication. Also, Δ represents the difference beam. ΔAZ is the difference between the beams obtained by dividing the antenna aperture into two in the azimuth direction. Similarly, ΔEL is the difference between the beams obtained by dividing the antenna aperture into two in the elevation direction. Σ, ΔAZ, and ΔEL can each be calculated using equation (1).

[0027]

number

[0028] 5 is a functional block diagram showing an example of a processing system of the signal processing unit 6 and the radar control unit 7. The signal processing unit 6 performs beam forming processing 61 in the digital domain by DBF using received data #1 to #N (I and Q signals at a rate of 35 MHz) from each subarray antenna (#1 to #N). DBF allows angle, distance, and velocity measurement to be performed simultaneously.

[0029] Next, using the reference data (I, Q signals at a rate of 35 MHz), clutter suppression processing 62 is performed on the beamformed beam data sequence (I, Q signals at a rate of 35 MHz). A correlation process 63 is performed using the beam data sequence after clutter suppression processing and the data sequence of the reference signal. The correlation process result includes bistatic velocity information and bistatic distance information. Using this correlation process result, a target detection and estimation process 64 is performed, and the target is detected through two-dimensional CFAR processing of the velocity and distance. The target detection information (bistatic target distance, bistatic target velocity, target angle) is passed to the radar control unit 7.

[0030] In the radar control unit 7, bistatic target movement prediction processing 71 in bistatic coordinates is performed using a linear Kalman filter or the like based on target detection information. The predicted position is converted from the bistatic coordinate system to a Cartesian coordinate system to generate target information (target distance, target speed, target angle). This target information is passed to target position measurement processing 72. The target position measurement processing 72 performs target movement prediction processing 73 in a Cartesian coordinate system using, for example, an extended Kalman filter, thereby establishing the target's trajectory. Furthermore, the radar control unit 7 may perform TWS (Track While Scan) using position prediction by the Kalman filter.

[0031] FIG. 6 is a diagram showing an example of a processing system related to clutter suppression processing. FIG. 6 shows a processing system for one of J beams. The clutter suppression processing 62 is a process for removing direct wave signals and clutter signals from the beam data sequence generated by the beam forming processing 61. That is, clutter is suppressed by adaptive filter processing 621 using beam reception data (I / Q) of the Σ beam formed by the beam forming processing 61 and reference data (I / Q). In FIG. 6, the beam reception data (I / Q) and the reference data (I / Q) that has passed through the adaptive filter processing 621 are synthesized by synthesis processing 622, and a portion of the output is fed back to the adaptive filter processing 621. For example, an RLS algorithm or a lattice filter algorithm can be used as the adaptive filter. Furthermore, using a blocked lattice filter is expected to reduce computational costs.

[0032] Next, the correlation processing 63 according to the embodiment will be described in detail. An ambiguity function is used to perform target detection and estimation processing 64. The ambiguity function in the embodiment is calculated as a function of speed and distance. That is, the ambiguity function can be obtained by multiplying the delayed signal of the complex conjugate of the beam reception data and the reference signal data after clutter suppression and performing a fast Fourier transform (FFT).

[0033] In existing technology, correlation processing calculations are performed using one type of delay resolution (for example, 1 / 35 MHz ≒ 29 ns) and one type of data rate (for example, 35 MHz). In other words, both the delay resolution and the data rate are fixed. However, performing similar correlation processing calculations on continuous waves requires an extremely large amount of data, consuming a large amount of computer resources, making it unrealistic.

[0034] Therefore, in this embodiment, the delay resolution and data rate are varied during correlation processing calculations to prevent the amount of data required for calculations from increasing indefinitely. This will be explained in detail with reference to Figures 7 and 8.

[0035] Fig. 7 is a diagram showing an example of a processing system related to correlation processing in an embodiment. Fig. 7 shows a processing system for one of J beams. In Fig. 7, for example, in a search range from distance 0 to distance (delay) A, a delay resolution of 1 / 35 MHz is set at a rate of 35 MHz. This search range corresponds to the sequence Filter_A → Resample_A → FFT_A, and the number of data in this sequence is Data Count_A.

[0036] In addition, in the search range from distance A to B, for example, a delay resolution of 1 / 20 MHz is set at a 20 MHz rate. This search range corresponds to the sequence Filter_B → Resample_B → FFT_B, and the number of data in this sequence is Data Count_B.

[0037] Similarly, in the search range from distance B to C, for example, the delay resolution is set to 1 / 15 MHz at a 15 MHz rate. In the search range from distance C to D, for example, the delay resolution is set to 1 / 10 MHz at a 10 MHz rate. In the search range from distance D to E, for example, the delay resolution is set to 1 / 5 MHz at a 5 MHz rate. The number of data for each search range is Data Count_C, Data Count_D, and Data Count_E.

[0038] Each sequence corresponding to each distance receives the product of the beam reception data and the complex conjugate (I, Q) of the reference signal data, and the band is limited by Filter_A to Filter_E, which are set with bands corresponding to the distance (delay), and the output is resampled at different rates. The longer the search range, the coarser the resampling clock (resample filter). Then, by performing FFT processing on the resampled data for each distance, correlation processing data for each search range is obtained. In other words, by resampling the data sequence before correlation after band-limiting it according to the distance (delay), the data can be thinned out, making it possible to reduce the amount of calculation required for correlation processing.

[0039] This allows for a significant reduction in the overall data volume, and therefore in the computational costs. Furthermore, the combination of delay range, delay resolution, and bandwidth (data rate) can be freely varied, allowing for flexible operation in accordance with the coverage area and integration time of the target with a single device. Flexibility can be further enhanced by implementing the correlation processing 63 in Figure 7 in software as part of DBF processing.

[0040] Fig. 8 is a diagram for explaining the reduction of data volume in correlation processing. As shown in the two-dimensional array in Fig. 8, when the distance to the target (search range) is short, the distance resolution and Doppler resolution are reduced to achieve precise observation although the amount of data is large. On the other hand, as the distance to the target increases, the distance resolution and Doppler resolution are increased to prevent the amount of data from becoming unnecessarily large. This type of processing can reduce the amount of data.

[0041] To observe slow-moving targets such as drones, it is necessary to improve the Doppler resolution. In this embodiment, by taking advantage of the fact that BS broadcast waves are continuous waves, the Doppler resolution can be significantly improved compared to general pulse radars, which have a limited number of pulse hits. Furthermore, the amount of data can be reduced by resampling the multiplication of the beam reception data and the complex conjugate (I, Q) of the reference signal data.

[0042] On the other hand, when observing high-speed targets such as aircraft, the necessary distance and velocity resolution can be achieved by taking advantage of the fact that BS broadcast waves are continuous waves.

[0043] <Summary of composition and function> In the embodiment, a passive phased array DBF radar device using BS broadcast waves is disclosed. However, because BS broadcast waves are continuous waves, the amount of data in correlation processing becomes too large with existing technology. Therefore, in the embodiment, an increase in the amount of data can be prevented by creating a resampled data sequence whose bandwidth is variable according to the distance. This is described in more detail in (1) to (7).

[0044] (1) Equipped with a reference wave antenna and receiver that captures the direct wave (reference signal) of BS broadcast waves. (2) Equipped with a phased array DBF antenna and DBF receiver that captures indirect waves (search signals) from BS broadcast waves. (3) The transmission frequency of (1) and (2) is, for example, the Ku band in the case of BS broadcast waves. This allows the size of the phased array DBF antenna (2) to be kept within reasonable limits. (4) The direct wave (reference signal) and indirect wave (search signal) are frequency-converted and converted into digital data by an A / D converter. After beamforming using the data sequence of the indirect wave, clutter suppression is performed using the data sequence of the direct wave. (5) Next, the indirect wave data is given a delay corresponding to a predetermined distance range (for example, 30 km-20 km, 20 km-10 km, 10 km-1 km, 1 km-100 m, 100 m-0 m) to create a data string. (6) The indirect wave data after beamforming in (4) is complex-multiplied by the direct wave data with a delay that is variable depending on the search range in (5). The complex multiplication result is input to a digital filter processing system provided for each of several bandwidths (e.g., 35 MHz, 20 MHz, 15 MHz, 10 MHz, 5 MHz). (7) In the digital filter processing system of (6), the input complex multiplication result is band-limited, resampled, and then FFT-processed to obtain the Doppler cross-correlation corresponding to multiple distance ranges. (8) The data sequence in (7) is an ambiguity function consisting of two dimensions: range (delay) and Doppler. From this two-dimensional information, the target is detected using a bistatic positioning method, and the target's range information and velocity information (Doppler shift) are calculated.

[0045] <Effects> As described above, according to the embodiment, it is possible to reduce the amount of data required for signal processing and reduce calculation costs. Furthermore, by matching the range resolution and Doppler resolution to the search range, it is possible to obtain sufficient resolution (observation accuracy) for a wide range of targets, from high-speed targets at long distances to low-speed targets at short distances.

[0046] As described above, by using satellite broadcast waves as a passive radar device, it is possible to avoid consuming further radio wave bandwidth, which is a finite resource, and to avoid exceeding the specified frequency allocation restrictions. Furthermore, by limiting the bandwidth and performing resampling processing according to distance and speed, it is possible to reduce the computational cost of signal processing and obtain sufficient range and velocity resolution to match the characteristics of the target, from long-distance, high-speed targets to close, low-speed targets. Therefore, according to the embodiments, it is possible to provide a passive radar device and target detection method that can satisfy frequency allocation restrictions and search a wide area.

[0047] Although the embodiment has been described above, this embodiment is presented as an example and is not intended to limit the scope of the invention. For example, although the calculation of the target position using a bistatic positioning method has been described, the same discussion can be applied to a more general multistatic method using multiple radio wave sources and multiple receiving points.

[0048] Furthermore, this novel embodiment may be embodied in various other forms, and various omissions, substitutions, and modifications may be made without departing from the spirit of the invention. This embodiment and its modifications are included within the scope and spirit of the invention, and are also included in the claims and their equivalents. [Explanation of symbols]

[0049] 1...reference wave antenna, 2...element antenna, 3...reference wave receiving unit, 4...subarray antenna, 5...reception processing unit, 6...signal processing unit, 7...radar control unit, 8...display unit, 26...recorder, 31...low noise amplifier, 32...image suppression mixer, 33...delay line, 41...low noise amplifier, 42...image suppression mixer, 43...subarray combiner, 50...circuit system, 51...amplifier, 52...mixer, 53...analog filter, 54 ...Analog / digital converter, 55...I / Q detector, 61...beam forming processing, 62...clutter suppression processing, 63...correlation processing, 64...estimation processing, 71...bistatic target movement prediction processing, 72...target position measurement processing, 73...target movement prediction processing, 100...broadcast satellite, 200...search antenna, 300...signal processing system, 400...target, 500...passive radar device, 621...adaptive filter processing, 622...synthesis processing.

Claims

1. a reference wave antenna that receives direct waves of radio waves transmitted as continuous waves from an artificial satellite in geostationary orbit and outputs a reference signal; a passive phased array antenna that receives the indirect radio waves reflected by a target and outputs a received signal; a reception processing unit that generates baseband reference data from the reference signal and generates baseband reception data from the reception signal; a signal processing unit that calculates detection information of the target at a resolution according to a search range based on the reference data, the received data, and position information of the artificial satellite, The signal processing unit generating a data string by performing a correlation process on the reference data and the received data; A passive radar device in which the distance resolution is varied by changing the delay amount of the reference data involved in the correlation processing calculation using a delay means that varies the delay amount according to the search range.

2. The passive radar device according to claim 1 , wherein the signal processing unit calculates the position of the target by a multistatic positioning method based on the reference data, the received data, and the position information of the artificial satellite.

3. The passive radar device according to claim 1 , wherein the signal processing unit calculates the position of the target by a bistatic positioning method based on the reference data, the received data, and position information of the artificial satellite.

4. the signal processing unit generates a data string by performing a correlation process between the reference data and the received data; The passive radar device according to claim 1, wherein the frequency resolution is varied by limiting the band of the data string using a band filter whose band is variable according to the search range.

5. The passive radar device according to claim 1 , wherein the signal processing unit includes means for resampling the data sequence at a rate set in accordance with the search range.

6. The passive radar device according to claim 1 , wherein the signal processing unit performs beamforming in a digital domain based on the received data.

7. The passive radar device of claim 1, further comprising a control unit that performs TWS (Track While Scan) using position prediction by a Kalman filter based on the target detection information.

8. A step of receiving a direct wave of a radio wave transmitted as a continuous wave from an artificial satellite in a geostationary orbit by a reference wave antenna and outputting a reference signal; receiving the indirect radio wave reflected by a target using a passive phased array antenna and outputting a received signal; generating baseband reference data from the reference signal and generating baseband received data from the received signal by a computer; and a step of calculating, by a computer, detection information of the target at a resolution according to a search range based on the reference data, the received data, and position information of the artificial satellite, The step of calculating the target detection information includes: generating a data string by a computer through correlation processing of the reference data and the received data; a delay means for varying the delay amount according to the search range by a computer, thereby varying the delay amount of the reference data related to the correlation processing operation, thereby varying the distance resolution.

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