JP7520601B2 - Radar device and radar signal processing method - Google Patents

Description

This embodiment relates to a radar device and a radar signal processing method.

Inverse synthetic aperture radar (ISAR) is known as a radar device that outputs the absolute position of a target shape. This ISAR radar device uses the movement and attitude change of the target to increase the resolution, but it is pointed out that the absolute position of the target shape cannot be output because the size of the Doppler axis of the target shape changes due to the rotation of the target, and problems such as the inability to correctly observe the direction of the target axis cannot be observed.

SAR method (ISAR), Yoshida, 'Revised Radar Technology', Institute of Electronics, Information and Communication Engineers, pp. 280-283 (1996) Monopulse, Yoshida, 'Revised Radar Technology', Institute of Electronics, Information and Communication Engineers, pp. 260-264 (1996) SAR method (range compression), Ouchi, 'Fundamentals of Synthetic Aperture Radar for Remote Sensing', Tokyo Denki University Press, pp. 131-149 (2003) Pulse Compression, Yoshida, 'Revised Radar Technology', Institute of Electronics, Information and Communication Engineers, pp. 278-280 (1996) Compressed Sensing, Toyoki Hoshikawa, 'Performance Comparison of Compressed Sensing Algorithms for DOA Estimation of Multi-band Signals', 2018 15TH WORKSHOP ON POSITIONING NAVIGATION AND COMMUNICATIONS (2018) Generalized Hough Transform, Ohashi, 'Arbitrary Figure Detection Algorithm Using Generalized Hough Transform, Graphics and CAD', 37-5, pp.33-39 (1989)

As described above, with conventional ISAR radar devices, the size of the Doppler axis of the target shape changes due to target rotation, so it was not possible to output the absolute position of the target shape, and there was a problem in that, for example, the direction of the target axis could not be observed correctly.

The objective of this embodiment is to provide a radar device and a radar signal processing method that can output the absolute position of the target shape regardless of the target rotation, thereby enabling the direction of the target axis to be correctly observed.

In order to solve the above problems, the radar device according to this embodiment is a radar device that uses signals that are pulse modulated or continuous wave modulated and transmitted by multiple subarrays consisting of one or more antenna elements arranged one-dimensionally or two-dimensionally, performs FFT (DBF) processing on the angle axis of the received signals of the multiple subarrays, sets the two-dimensional absolute position (X, Y) or three-dimensional absolute position (X, Y, Z) of the target reflection point, converts the polar coordinates of the range axis and angle axis into orthogonal coordinates of the X-axis and Y-axis (and Z-axis, in the case of three dimensions) using the results of the FFT (DBF) processing, extracts the target range of the X-axis and Y-axis (and Z-axis, in the case of three dimensions) that exceeds a predetermined amplitude threshold using the amplitude sum value projected in amplitude onto the X-Y plane (and Y-Z plane, in the case of three dimensions), and calculates the angular axis CS (compressed amplitude) of the received signals of the multiple subarrays. At least one of the following CS processing is performed: compressed sensing or range axis CS processing, points that fall within the target range are extracted from the results of the CS processing, and the two-dimensional absolute position (X, Y) or three-dimensional absolute position (X, Y, Z) of the extracted points is output.

In other words, using the signals transmitted and received by antenna elements (sub-arrays) arranged in two dimensions (one dimension), FFT processing is performed on the angle axes of the AZ and EL axes (AZ axis or EL axis), and further FFT processing is performed on the range frequency axis to output the three-dimensional (two-dimensional) absolute position (X, Y, Z) of the target reflection point, the polar coordinates of the range axis, AZ axis and EL axis are converted to orthogonal coordinates of the X, Y and Z axes, and the range of the X-axis, Y-axis and Z axis that exceeds a predetermined amplitude threshold is extracted using the amplitude sum value projected in amplitude onto the X-Y plane and Y-Z plane, CS processing is performed on the AZ and EL axes (AZ axis or EL axis), and from the data that has been CS processed on the range frequency axis, only points within the extraction range of the aforementioned FFT results are extracted, and the three-dimensional (two-dimensional) absolute position (X, Y, Z) of the target reflection point is output. In this way, angle axis CS processing is performed using the signals from the antenna elements (subarray), and CS processing is also performed on the range axis if necessary.Furthermore, by limiting the target range using output using FFT processing instead of CS processing, false detections can be suppressed and the absolute value of the target shape can be output.

FIG. 1 is a block diagram showing the configuration of a receiving system of a radar device according to a first embodiment. FIG. 2 is a block diagram showing an example of an antenna sub-array receiving system in the first embodiment. FIG. 3 is a waveform diagram showing received signals of each sub-array in the first embodiment. FIG. 4 is a flowchart showing the flow of a process for extracting a target range from a subarray signal input in the first embodiment. FIG. 5 is a diagram showing how the sub-array DBF is formed from the reception outputs of the sub-arrays in the first embodiment. FIG. 6 is a diagram showing how a target range is extracted from the sub-array DBF output based on an amplitude threshold in the first embodiment. FIG. 7 is a diagram showing how signals on the subarray-range frequency axis are acquired by signals with a selected Doppler bank in the first embodiment. FIG. 8 is a diagram showing how an angle-range axis signal is acquired from a subarray-range frequency axis signal by angle axis CS processing and range axis CS processing in the first embodiment. FIG. 9 is a flowchart showing the flow of processing for extracting a target range from a subarray signal input through angle axis CS processing and range frequency axis CS processing in the first embodiment. FIG. 10 is a diagram defining the position of the target range with respect to the flight direction and moving speed of the onboard aircraft in the first embodiment. FIG. 11 is a diagram showing the target range extraction process in the first embodiment. FIG. 12 is a diagram defining the position of a target relative to the flight path of the onboard aircraft in the first embodiment. FIG. 13 is a block diagram showing the configuration of a receiving system of a radar device according to the second embodiment. FIG. 14 is a flowchart showing the target range extraction process according to the second embodiment. FIG. 15 is a diagram for explaining a process of converting an ISAR image to the center of an imaging range in the second embodiment. FIG. 16 is a diagram showing how a target axis range defined by a target axis and a target width centered on the target axis is extracted in the second embodiment.

The following describes the embodiment with reference to the drawings.

First Embodiment
1 is a block diagram showing the configuration of a receiving system of a radar device according to the first embodiment. The transmitting system is omitted for simplicity. The transmitting system controls the beam direction of the antenna so that the real aperture beam of the onboard radar is always irradiated on the target, and transmits pulses at PRI (Pulse Repetition Interval) intervals.

In FIG. 1, the antenna 11 is composed of N (N>1) subarrays 111-11N, each of which has one or more antenna elements arranged therein. In the reception system, the reception output of each of the subarrays 111-11N is frequency converted by frequency converters 121-12N, then converted to a digital signal by AD converters 131-13N, and the Doppler axis signal is obtained by performing FFT processing on each slow-time axis by slow-time FFTs 141-14N, and target detection processing is performed by the signal processing unit described below.

The signal processing section includes a DBF (Digital Beam Forming) synthesizer 15, a first target range extractor 16, a signal extractor 17, a range frequency converter 18, a CS (compress sensing) processor 19, and a second target range extractor 20.

The DBF combiner 15 performs FFT processing between subarrays on the Doppler axis signals obtained by the Slow-time FFTs 141 to 14N to form multiple beams in the angle space and obtain the Doppler angle axis signals.

The first target range extractor 16 measures the angle, range, and Doppler of the amplitude components exceeding a predetermined amplitude threshold from the Doppler angle axis signal obtained by the beam formed by the DBF combiner 15, and extracts a Doppler bank indicating the target range.

The signal extractor 17 inputs the Doppler axis signals obtained by the N slow-time FFTs 141 to 14N, and extracts the fast-time axis signals for each subarray for each Doppler bank in the target range extracted by the first target extractor 16.

The range frequency converter 18 inputs the fast-time axis signal for each subarray extracted by the signal extractor 17, performs FFT processing to convert it to the range frequency axis, and obtains a range frequency axis signal.

The CS processor 19 inputs the range frequency axis signal, performs angle axis CS processing, and then performs range frequency axis CS processing for each angle.

The second target range extractor 20 converts the target range on the angle-range frequency axis extracted by the first target range extractor 16 into the X-Y axis, and extracts targets within the target range converted into the X-Y axis from the CS processing results of the angle-range frequency axis.

The processing operations in the above configuration will be explained with reference to Figures 2 to 12.

Figure 2 is a block diagram showing an example of the receiving system of the subarrays 111 to 11N of the antenna 11 shown in Figure 1, and Figure 3 is a waveform diagram showing the received signals of each subarray shown in Figure 1. The example shown in Figure 2 is an array in which antenna elements (subarrays) of the AZ (or EL) axis are arranged one-dimensionally. In this embodiment, the arrival direction of the AZ (or EL) axis is calculated by compressed sensing (hereinafter, CS) processing of the AZ (or EL) axis. If angle measurement is required for the other EL (or AZ) axis, as shown in Figure 2, ΔEL (or ΔAZ) is output by the difference beam divided into two upper and lower (or left and right) apertures, and in the AZ (or EL) axis, an analog synthesized signal is used to combine with the Σ beam, which is the sum of the entire aperture, and phase monopulse angle measurement (see Non-Patent Document 1) is performed using the angle measurement error and angle measurement table.

The antenna 11 is oriented so that the real aperture beam of the onboard radar is always irradiated on the target, and acquires data for each pulse (slow-time) transmitted at the PRI (Pulse Repetition Interval) interval, in range cell units (fast-time) within the PRI. That is, the RF signals #1 to #N received by the subarrays (including antenna elements, omitted below) 111 to 11N are frequency-converted to the IF band by frequency converters 121 to 12N, respectively, and further converted to digital signals by AD converters 131 to 13N. These received signals #1 to #N are signals on the fast-time-slow-time axis, as shown in Figure 3.

The process of extracting the target range from the digitized subarray reception signals #1 to #N is explained below.

Figure 4 is a flowchart showing the process of extracting a target range from a subarray signal input, Figure 5 is a waveform diagram showing how a subarray DBF is formed from the received output of the subarray, and Figure 6 is a diagram showing how a target range is extracted from the subarray DBF output based on an amplitude threshold.

According to the process shown in Figure 4, when the subarray reception signals #1 to #N shown in Figure 5(a) are input in the reception system with the above configuration (step S11), the slow-time FFT 141 to 14N perform slow-time FFT processing for each PRI#1 to PRI#L, and the subarray reception signals #1 to #N on the fast-time axis are obtained in the banks #1 to #L corresponding to PRI#1 to PRI#L shown in Figure 5(b), and the DBF (Digital Beam Forming) combiner 15 performs DBF synthesis between the subarrays (step S12), projects the amplitude waveforms on the X and Y axes (step S13), and the first target range extractor 16 extracts the target range (X-Y axis) of the range-angle axis shown in Figure 5(c) (step S14). DBF synthesis is FFT processing between the subarrays, and forms multiple beams in the angle space. From this Doppler angle axis signal, the range of Doppler and angle that exceeds a specified amplitude threshold can be extracted as the target range (X-Y axis), as shown in Figure 6.

Here, Figure 7 shows how a signal on the subarray-range frequency axis is obtained using a signal that has been selected from a Doppler bank, Figure 8 shows how a signal on the angle-range frequency axis is obtained from the signal on the subarray-range frequency axis by CS processing on the angle axis and CS processing on the range frequency axis, Figure 9 is a flowchart showing the process flow for extracting the target range from the subarray signal input through angle axis CS processing and range frequency axis CS processing, Figure 10 is a diagram that defines the position of the target range relative to the flight direction and movement speed of the aircraft to be loaded, Figure 11 is a diagram showing the target range extraction process, and Figure 12 is a diagram that defines the position of the target relative to the flight path of the aircraft to be loaded.

In this embodiment, the subarray reception signals #1 to #N shown in FIG. 7(a) are input, and slow-time FFT processing is performed for each PRI#1 to PRI#L in slow-time FFTs 141 to 14N. Then, as shown in FIG. 7(b), subarray reception signals #1 to #N on the fast-time axis are obtained in banks #1 to #L corresponding to PRI#1 to PRI#L, respectively. This makes it possible to extract signals on the fast-time axis for each subarray for each Doppler bank in the extracted target range. Then, by performing FFT processing on the signals on the fast-time axis and converting them to the range frequency axis, it is possible to extract signals on the subarray-range frequency axis shown in FIG. 7(c).

The above process will now be described in detail.
In the case of modulation within the pulse, such as a signal that has been pulse compressed (see Non-Patent Document 3), the reference signal, which is a modulated signal, is subjected to FFT processing and multiplied by the signal on the frequency axis to obtain the signal Sigr(f) on the range frequency axis (see Non-Patent Document 4).

first,

The phase of each subarray received signal #1 to #N from the phase center of the entire antenna aperture is given by the following equation:

Next, consider compressed sensing processing (see Non-Patent Document 5). Using the same method as in Figure 5, a signal on the subarray-range frequency axis shown in Figure 7(c) is obtained from a signal with a selected Doppler bank. If this signal is taken as the input signal Y and the wave source as Xang, it can be expressed by the following equation.

次に、（４）式を用いて、Xがスパースであることを用いると、次式を最小化するXangを算出することができる（非特許文献５参照）。

Next, by using equation (4) and utilizing the fact that X is sparse, it is possible to calculate Xang that minimizes the following equation (see Non-Patent Document 5).

Next, consider performing CS processing on the range frequency axis. Figure 8 shows how the input signal changes. After performing angle axis CS processing, perform range axis CS processing for each angle. If the input signal for each angle axis is Yr and the wave source is Xr, it can be expressed by the following equation.

The element in the nth column of Ar is a vector that corresponds to the distance when the wave source xn is present.

ここで、（９）式を用いて、Xがスパースであることを用いると、次式を最小化するXrを算出することができる（非特許文献５参照）。

Here, by using equation (9) and utilizing the fact that X is sparse, it is possible to calculate Xr that minimizes the following equation (see Non-Patent Document 5).

Xrに対応するｍが算出できれば、次式により距離を算出することができる。

If m corresponding to Xr can be calculated, the distance can be calculated using the following formula.

The overall processing flow of the above angle axis and range frequency axis CS processing is shown in Figure 9. Figure 9 shows the processing flow from subarray signal input through angle axis CS processing and range frequency axis CS processing to extract the target range, but the order of the angle axis and range frequency axis CS processing may be reversed.

In FIG. 9, first, a subarray signal is input, and then a fast-time axis signal is input (step S21), and fast-time axis FFT processing is performed (step S22), a fast-time axis reference signal is set (step S23), the fast-time axis reference signal is subjected to FFT processing (step S24), the fast-time axis signal is multiplied by the FFT-processed fast-time axis reference signal (frequency axis multiplication) (step S25), and CS processing of the angle axis is performed (step S26). Here, it is determined whether CS processing has been completed for all range frequency cells (step S27), and if not, the range frequency cells are changed (step S28), and the process returns to step S26 to perform angle axis CS processing. If CS processing for all range frequency cells has been completed in step S27, an observation matrix for the range frequency axis is set (step S28), and CS processing for the range frequency axis is performed (step S29). Here, it is determined whether CS processing has been completed for all angle cells (step S30), and if not, the angle cells are changed (step S31), and the process returns to step S29 to perform range frequency axis CS processing. If CS processing has been completed for all angle cells in step S30, the process ends.

The distance R and angle θ can be calculated from the above, so to convert them into three-dimensional coordinates, refer to Figure 10 and convert them using the following formula.

Next, in order to suppress the effects of false positives due to CS processing, the target range of the range-angle axis extracted in Figure 6 is converted to the X-Y axis as shown in Figure 11(a) and the amplitude range exceeding a specified amplitude threshold is extracted as the target range, and signals within the target range are extracted as shown in Figure 11(b), with the rest treated as noise to suppress false positives. In addition, the target axis is extracted from the distribution of targets within the target range.

As a result, when a target is present at a position at an AZ angle direction and distance R relative to the flight path (Y-axis direction) of the aircraft on the X-Y axis as shown in Figure 12(a), a high-resolution image of the target is obtained on the X-Y axis as shown in Figure 12(b).

Second Embodiment
In the first embodiment, a method for increasing the resolution of the angle axis-range frequency axis (or the XY axis, which is the converted coordinate system) using CS processing has been described. In the present embodiment, a method for extracting a target axis using a high-resolution image will be described. There are methods for extracting the target axis that use the generalized Hough transform (see Non-Patent Document 5), but here, a method using template matching will be described.

Figure 13 is a block diagram showing the configuration of the receiving system of the radar device according to the second embodiment, Figure 14 is a flowchart showing the target range extraction process, Figure 15 is a diagram for explaining the process of converting the ISAR image to the center of the imaging range, and Figure 16 is a diagram showing how the target axis range defined by the target axis and the target width centered on the target axis is extracted.

As shown in FIG. 13, the receiving system of the radar device according to this embodiment is characterized in that a signal processor 21 for extracting the target axis is added after the second target range extractor 20 of the first embodiment. In FIG. 13, the same parts as those in FIG. 1 are denoted by the same reference numerals, and duplicated explanations will be omitted here.

In this embodiment, as shown in FIG. 14, first, the ISAR image obtained by the second target range extractor 20 is converted to the image center of the range-angle axis (step S41). To convert to the image center, for the target image of the X-Y axis shown in FIG. 15(a), the amplitude is projected onto the X-axis and Y-axis as shown in FIG. 15(b), an amplitude threshold is set for the result, the range of the X-axis and Y-axis exceeding this is extracted as the target range, and the center (Xtc, Ytc) of the target range is calculated. By converting the center of this target range to the imaging center (Xc, Yc), the target image is moved as shown in FIG. 15(c). This makes it possible to reduce the number of templates for the target center axis.

Next, to extract the target axis, a template is prepared with the center (Xc, Yc) of the target range shown in FIG. 16 as the center, with a range of a certain width Wtn (n=1 to Nw) and a certain angle θtn (n=1 to Nt) set to 1 (reference) and the rest set to 0. This template is multiplied by the amplitude of the image centered on the image, and added (step S42), the result of the addition is saved (step S42), and the template with the maximum result of the addition is extracted (steps S44 to S46). The template is used to extract the target axis range (θt and Wt) (step S47), the brightness of the target axis range is enhanced (step S48), and the series of processes is completed. This allows the target axis range defined by the target axis θt and the target width Wt centered on the target axis to be extracted.

As described above, the radar device according to the above embodiment can output the absolute position of the target shape even when the target rotates, and can correctly observe, for example, the direction of the target axis.

The present invention is not limited to the above-described embodiment as it is, and in the implementation stage, the components can be modified and embodied without departing from the gist of the invention. In addition, various inventions can be formed by appropriately combining multiple components disclosed in the above-described embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, components from different embodiments may be appropriately combined.

11...antenna, 121-12N...frequency converter, 131-13N...AD converter, 141-14N...slow-time FFT, 15...DBF (Digital Beam Forming) combiner, 16...first target range extractor, 17...signal extractor, 18...range frequency converter, 19...CS (compress sensing) processor, 20...second target range extractor.

Claims (5)

A radar device using a signal that is pulse modulated or continuous wave modulated by a plurality of subarrays each having one or more antenna elements arranged one-dimensionally or two-dimensionally,
an FFT processing means for performing FFT (DBF) processing on the angle axis of the reception signals of the plurality of subarrays;
a target range extraction means for setting a two-dimensional absolute position (X, Y) or a three-dimensional absolute position (X, Y, Z) of a target reflection point, converting the polar coordinates of the range axis and the angle axis into orthogonal coordinates of the X axis and the Y axis (and the Z axis, in the case of three dimensions) using the processing result of said FFT processing means, and extracting a target range of the X axis and the Y axis (and the Z axis, in the case of three dimensions) that exceeds a predetermined amplitude threshold using an amplitude sum value projected onto the X-Y plane (and the Y-Z plane, in the case of three dimensions);
CS processing means for performing at least one of angular axis CS (compressed sensing) and range axis CS processing on the received signals of the plurality of subarrays;
a position output means for extracting points that fall within the target range from the processing result of the CS processing means, and outputting the two-dimensional absolute position (X, Y) or three-dimensional absolute position (X, Y, Z) of the extracted points. The radar device according to claim 1 further comprises a target axis extraction means for extracting a target axis using the two-dimensional absolute position (X, Y) or three-dimensional absolute position (X, Y, Z) of the target reflection point. The radar device according to claim 2, wherein the target axis extraction means generates a target image by collecting points obtained by the position output means, and moves the target image to the image center of the range-angle axis. The radar device according to claim 2, wherein the target axis extraction means prepares a template with a range of a predetermined width and angle set to 0 with the center of the target range as the reference, multiplies and adds this template with the amplitude of the image centered on the image, stores the sum, extracts the template with the maximum sum, and extracts the target axis range using the template. A radar signal processing method for detecting a target using a signal transmitted and received by a signal processing device of a radar device using a plurality of subarrays each having one or a plurality of antenna elements arranged one-dimensionally or two-dimensionally, the signal processing device comprising:
The signal processing device,
performing an FFT (DBF) process on the angular axis of the received signals of the plurality of subarrays;
a step of setting a two-dimensional absolute position (X, Y) or a three-dimensional absolute position (X, Y, Z) of the target reflection point, converting the polar coordinates of the range axis and the angle axis into the orthogonal coordinates of the X axis and the Y axis (and the Z axis, in the case of three dimensions) using the processing result of the FFT (DBF) processing, and extracting the target range of the X axis and the Y axis (and the Z axis, in the case of three dimensions) that exceeds a predetermined amplitude threshold using the amplitude sum value projected onto the X-Y plane (and the Y-Z plane, in the case of three dimensions) ;
performing at least one of angle axis CS (compressed sensing) and range axis CS processing on the received signals of the plurality of subarrays;
extracting points that fall within the target range from the processing result of the CS processing, and outputting the two-dimensional absolute position (X, Y) or the three-dimensional absolute position (X, Y, Z) of the extracted points;
A radar signal processing method comprising :

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