JP4881209B2 - Target detection device - Google Patents

Description

  The present invention relates to a target detection apparatus that is applied to, for example, a radar apparatus and detects a target from a received signal, and more particularly to a technique for detecting a target on a time-frequency axis.

  Conventionally, it is applied to radar equipment that detects and tracks a target, for example, receives a reflected wave reflected by a target pulse signal, generates a received signal by converting the received reflected wave into an electrical signal, and receives it. 2. Description of the Related Art A target detection device that detects a target by performing signal processing on a signal is known. In such a target detection apparatus, an S / N ratio (signal to thermal noise power ratio) is improved by integrating reception signals obtained by receiving a plurality of reflected waves (hits).

  However, multiple hits should be sent to and received from a target that moves at high speed (hereinafter referred to as “high-speed target”) or a target that is faster than distance resolution (in other words, a target detection device with high range resolution). The signal obtained by (1) includes a deviation in the target range direction and has an upper limit on the number of hits that can be integrated. That is, integration loss due to range walk occurs. Therefore, there is a limit in improving the SN ratio by integration, and there is a problem that the target detection performance is inferior.

  In order to deal with such a problem, a short-time Fourier transform (hereinafter referred to as “STFT: Short Time Fourier Transform”) is used for a target that appears only for a short time, such as when a shift in the target range direction occurs. Target detection devices have been developed for detection.

  FIG. 9 is a block diagram showing the configuration of such a conventional target detection apparatus. The target detection apparatus includes an STFT unit 1, a constant false alarm rate (hereinafter referred to as “CFAR: Constant False Alarm Rate”) unit 4, a detection unit 5, and a distance calculation unit 6.

  This target detection apparatus operates as follows. That is, a reception signal obtained by receiving a reflected wave with an antenna (not shown) is sent to the STFT unit 1. The STFT unit 1 converts the received signal into a signal on the time-frequency axis by short-time Fourier transform. The short-time Fourier transform is described in Non-Patent Document 1, for example. A signal on the time-frequency axis obtained by the short-time Fourier transform in the STFT unit 1 is sent to the CFAR unit 4.

  The CFAR unit 4 calculates a threshold level from the signal level around the signal of interest in the signal on the time-frequency axis sent from the STFT unit 1, and compares the calculated threshold level with the signal of interest to calculate the target level. The signal with the false alarm probability being suppressed to a certain low level is generated. The CFAR process is described in detail, for example, in Non-Patent Document 2, but will be briefly described below.

  FIG. 10 is a block diagram showing a configuration of a linear CFAR unit that performs normalization by arithmetic mean as an example of the CFAR unit 4. The CFAR unit 4 includes a delay circuit 41, an adding unit 42, an averaging processing unit 43, and a dividing unit 44.

  The delay circuit 41 delays the input signal xi and then sends it to the adder 42 and the divider 44. The adding unit 42 adds N pieces of data sent from the delay circuit 41 during a certain period and sends the data to the averaging processing unit 43. The averaging processing unit 43 calculates an average value of the N pieces of data sent from the adding unit 42 and sends it to the dividing unit 44. The division unit 44 divides the data sent from the delay circuit 41 by the average value and outputs the result. The output of the division unit 44 is sent to the detection unit 5 as the output of the CFAR unit 4. The CFAR unit 4 may be realized by a logarithmic CFAR unit that performs normalization with a geometric mean.

The detection unit 5 detects the target based on the signal sent from the CFAR unit 4. A signal representing the target detected by the detection unit 5 is sent to the distance calculation unit 6. The distance calculation unit 6 calculates the distance to the target based on the signal representing the target sent from the detection unit 5. A signal representing the distance to the target calculated by the distance calculation unit 6 is output to the outside as the output of the target detection device.
Sugawara, “Wavelet Beginners Guide”, Tokyo Denki University Press, pp. 23-24 (1995) Sekine, "Radar signal processing technology", IEICE, pp. 96-106 (1991)

  However, the above-described conventional target detection device using short-time Fourier transform has a problem that when the high-speed target is a small target, the SN ratio is small and the target may not be detected.

  The present invention has been made in order to solve the above-described problems. Even if a high-speed target is a small target, a target detection device that can reliably detect the small target and improve target detection performance. It is to provide.

  In order to achieve the above-described problem, the invention according to claim 1 is a short-time Fourier transform unit that converts an input received signal into a signal on a time-frequency axis by a short-time Fourier transform, and a short-time Fourier transform unit. A local maximum value extracting unit for extracting the local maximum value of the signal on the time-frequency axis converted by the above, and a cell existing in a predetermined range in the time axis direction of the local maximum value extracted by the local maximum value extracting unit An integration unit that emphasizes the local maximum value by adding the signals of, and a threshold calculated based on a signal of a cell that exists in a predetermined range in the time axis direction and the frequency axis direction of the local maximum value emphasized in the integration unit, or A determination unit that determines whether or not a predetermined fixed threshold is exceeded, and a detection unit that detects a target based on a determination result by the determination unit are provided.

According to a second aspect of the invention, in the first aspect of the invention, the signal on the time-frequency axis emphasized by integrating the signal on the time-frequency axis converted by the short-time Fourier transform unit is obtained. An emphasis unit to be generated is provided, and the maximum value extraction unit extracts a maximum value of the signal on the time-frequency axis emphasized by the enhancement unit.

  According to a third aspect of the present invention, in the first aspect of the present invention, the cell existing in a predetermined range in the time axis direction of the maximum value extracted by the maximum value extraction unit with respect to the target detected by the detection unit Or centroid distance calculation using a signal exceeding a predetermined threshold among signals of cells existing in a predetermined range in the time axis direction of the maximum value extracted by the maximum value extraction unit A distance calculation unit that calculates the distance to the target is provided.

  According to the first aspect of the present invention, the signals of the cells existing in the predetermined range in the time axis direction of the maximum value of the signal obtained by the short-time Fourier transform become substantially the same frequency on the frequency axis and are on the time axis. Since the signals on the time axis are integrated and emphasized, integration loss due to range walk is reduced, and even if the high-speed target is a small target, the small target is reliably detected and target detection performance is improved. Can be made.

According to the second aspect of the present invention, the signal on the time-frequency axis emphasized by integrating the signal on the time-frequency axis obtained by the conversion in the short-time Fourier transform unit is generated and maximized. Since it is given to the value extraction unit, the target detection performance can be further improved as compared with the first aspect of the invention.

  According to the third aspect of the present invention, the signal of the cell existing in the predetermined range in the time axis direction of the maximum value extracted by the maximum value extraction unit or the time of the maximum value extracted by the maximum value extraction unit Since the gravity center distance calculation is performed using the signal exceeding the predetermined threshold among the signals of the cells existing in the predetermined range in the axial direction, the target distance accuracy can be increased, with higher accuracy than the range resolution, The distance to the target can be calculated.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following description, the same constituent elements as those of the conventional target detection apparatus described in the background art will be described with the same reference numerals as those of the conventional target detection apparatus.

  FIG. 1 is a block diagram illustrating the configuration of the target detection apparatus according to the first embodiment of the present invention. The target detection apparatus includes an STFT unit 1, a maximum value extraction unit 2, an integration unit 3, a CFAR unit 4, and a detection unit 5.

  The STFT unit 1 converts a reception signal (time domain signal) sent from the outside into a signal on the time-frequency axis by short-time Fourier transform. A signal on the time-frequency axis obtained by the short-time Fourier transform in the STFT unit 1 is sent to the maximum value extracting unit 2.

  The local maximum extracting unit 2 extracts P local maximum values from the maximum value to the Pth (P is a positive integer) from the signal on the time-frequency axis sent from the STFT unit 1. The P maximum values extracted by the maximum value extraction unit 2 are sent to the integration unit 3.

  For each of the P local maximum values on the time-frequency axis sent from the local maximum value extracting unit 2, the integrating unit 3 is M (M is a positive integer) existing in front of the local maximum time axis direction. Then, the emphasized local maximum value is generated by adding the signals of M cells existing in the rear to the local maximum value and performing integration. The emphasized maximum value generated by the integration unit 3 is sent to the CFAR unit 4.

  The CFAR unit 4 corresponds to the determination unit of the present invention. As described in detail in the background art section, the CFAR unit 4 calculates the threshold level from the signal level around the signal of interest in the signal on the time-frequency axis sent from the integration unit 3, The calculated threshold level and the signal of interest are compared to determine the presence or absence of a target, and a signal with a false alarm probability suppressed to a certain low level is generated. The signal generated by the CFAR unit 4 is sent to the detection unit 5.

  The detection unit 5 detects the target based on the signal sent from the CFAR unit 4. A signal representing the target detected by the detection unit 5 is output to the outside as a target signal detected by the target detection device.

  Next, the operation of the target detection apparatus according to Embodiment 1 of the present invention configured as described above will be described. The reception signal input to the STFT unit 1 is composed of range cell data as shown in FIG. In the case of a target that moves at a higher speed than the distance resolution, when attempting to integrate the data of each range cell, as shown in FIG. 2 (a), it gradually deviates from the range cell, so PRI (Pulse Repetition Interval; pulse repetition) As shown in FIG. 2B, the PRI data obtained for each (cycle) appears only for a short time.

  If the movement of the target based on this received signal is expressed on the PRI-range axis, the oblique data T shown in FIG. The inclination of this data T is determined by the target speed and PRI. The STFT unit 1 performs a short-time Fourier transform on such a received signal for each range cell. As a result, the input received signal is converted into a signal on the time-frequency axis as shown in FIGS. 3B to 3D and sent to the local maximum value extraction unit 2.

  If the received signal includes a target, a signal representing the target is detected across several range cells. In this case, assuming that the target speed is substantially constant, the values on the frequency (speed) axis are the same as shown in FIGS. 3B to 3D, and different positions on the time axis (PRI axis). A signal representing the target appears.

  When a plurality of targets are included in the received signal, a plurality of signals representing the targets appear. Therefore, the maximum value extraction unit 2 extracts the maximum value from the signal on the time-frequency axis sent from the STFT unit 1. To the integration unit 3.

  For each of the maximum values from the maximum value to the P-th maximum value, the integration unit 3 outputs a cell signal in a predetermined range in the time axis direction around the maximum value on the time-frequency axis, specifically, M ahead of the maximum value. The signal of the number of cells and the signal of the rearward M cells are added to the maximum value and integrated, and the signal of the maximum value is emphasized as shown in FIG. FIG. 4 shows the state of processing executed by the integration unit 3. The emphasized signal on the time-frequency axis calculated by the integration unit 3 is sent to the CFAR unit 4.

  As shown in FIG. 5, the CFAR unit 4 receives Nt × Nf cells (in the present invention) around the emphasized maximum value for the emphasized signal on the time-frequency axis sent from the integrating unit 3. (Corresponding to “cells existing in a predetermined range in the time axis direction and the frequency axis direction”), the CFAR process is executed, and when the threshold is exceeded by this CFAR process, the emphasized maximum value is sent to the detection unit 5. send.

  The detection unit 5 detects the emphasized maximum value sent from the CFAR unit 4 as a target. Then, by executing the above processing for P local maximum values, one or more targets are detected and output as target signals.

  In addition, when integrating the signal of a cell in a predetermined range in the time axis direction around the maximum value to the maximum value and integrating, the technique of integrating after detection is fundamental, but it is a coherent integration including the phase. In addition, the maximum value of the frequency bank can be used by using Fourier transform.

  Further, the CFAR unit 4 is used as the determination means of the present invention, and the threshold determined by the CFAR process is used to detect the target. However, when determining the threshold, instead of the CFAR process, Nt × Nf cell An average value of thermal noise, a fixed threshold, or the like can be used.

  Furthermore, the target detection apparatus according to the first embodiment can be modified so as to add an emphasis unit 8 between the STFT unit 1 and the local maximum extraction unit 2 as shown in FIG. Similarly to the integration unit 3 described above, the enhancement unit 8 generates a signal on the time-frequency axis that is enhanced by combining the signals on the time-frequency axis sent from the STFT unit 1 by integration, It is sent to the local maximum extraction unit 2. According to this configuration, the target detection performance can be further improved.

  The target detection apparatus according to the second embodiment of the present invention is configured to calculate the distance to the target with high accuracy with respect to the target signal detected by the target detection apparatus according to the first embodiment.

  FIG. 7 is a block diagram illustrating a configuration of the target detection apparatus according to the second embodiment of the present invention. This target detection device is configured by adding a centroid distance calculation unit 7 to the target detection device according to the first embodiment.

The center-of-gravity distance calculation unit 7 is a signal that appears in a plurality of ranges for each maximum value, that is, a signal of a cell in a predetermined range in the time axis direction around the maximum value on the time-frequency axis (M cells ahead of the maximum value). The distance is calculated by the following equation (1). This center-of-gravity distance calculation can increase the distance accuracy beyond the distance required for each range cell. The state of the center of gravity distance calculation is shown in FIG.

here,
Rm; distance of m cells (m = 1 to M)
Am; amplitude of m cell (m = 1 to M)
Note that the center-of-gravity distance calculation unit 7 does not use all 2M + 1 signals in the time axis direction around the maximum value, but uses only the signals that exceed a predetermined threshold out of 2M + 1 signals, for example. It can also be configured to perform a distance calculation. According to this configuration, the distance can be calculated while eliminating noise and the like, and thus a more accurate distance can be obtained.

  The target detection apparatus according to the present invention can be configured by combining the target detection apparatus according to the first or second embodiment with techniques such as MTI (Moving Target Indicator) and pulse compression.

  The target detection apparatus according to the present invention can be used for a radar apparatus or a reception apparatus that detects and / or tracks a target.

It is a block diagram which shows the structure of the target detection apparatus which concerns on Example 1 of this invention. It is a figure which shows the received signal input into the target detection apparatus which concerns on Example 1 of this invention. It is a figure for demonstrating the STFT process performed with the target detection apparatus which concerns on Example 1 of this invention. It is a figure for demonstrating the integration process performed with the target detection apparatus which concerns on Example 1 of this invention. It is a figure for demonstrating the local maximum extraction process performed with the target detection apparatus which concerns on Example 1 of this invention. It is a block diagram which shows the structure of the modification of the target detection apparatus which concerns on Example 1 of this invention. It is a block diagram which shows the structure of the target detection apparatus which concerns on Example 2 of this invention. It is a figure for demonstrating the gravity center distance calculation process performed with the target detection apparatus which concerns on Example 2 of this invention. It is a figure for demonstrating the structure of the conventional target detection apparatus. FIG. 10 is a block diagram illustrating a detailed configuration of a CFAR unit illustrated in FIG. 9.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 STFT part 2 Maximum value extraction part 3 Integration part 4 CFAR part 5 Detection part 7 Center of gravity distance calculation part 8 Enhancement part

Claims (3)

A short-time Fourier transform unit that transforms an input received signal into a signal on a time-frequency axis by a short-time Fourier transform;
A maximum value extraction unit that extracts the maximum value of the signal on the time-frequency axis transformed by the short-time Fourier transform unit;
An integration unit that emphasizes the maximum value by adding a signal of a cell existing in a predetermined range in the time axis direction of the maximum value to the maximum value extracted by the maximum value extraction unit;
Judgment means for judging whether a threshold calculated based on a signal of a cell existing in a predetermined range in the time axis direction and the frequency axis direction of the maximum value emphasized in the integration unit or a predetermined fixed threshold is exceeded. When,
A detection unit for detecting a target based on a determination result by the determination unit;
A target detection apparatus comprising: An enhancement unit that generates a signal on the time-frequency axis that is enhanced by integrating the signal on the time-frequency axis converted by the short-time Fourier transform unit;
The target detection apparatus according to claim 1, wherein the maximum value extraction unit extracts a maximum value of a signal on the time-frequency axis emphasized by the enhancement unit. With respect to the target detected by the detection unit, the signal of the cell existing in a predetermined range in the time axis direction of the maximum value extracted by the maximum value extraction unit, or the maximum value extracted by the maximum value extraction unit A distance calculation unit that calculates a distance to a target by performing a centroid distance calculation using a signal that exceeds a predetermined threshold among signals of cells existing in a predetermined range in the time axis direction of The target detection apparatus according to claim 1, wherein:

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