JP3725669B2 - Clutter suppression method and circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a clutter suppression method and circuit for suppressing clutter included in a radar signal.
[0002]
[Prior art and its problems]
The radar signal includes a reflected signal (clutter) from another object in addition to a reflected signal (target signal) from a target to be detected. Examples of marine radars include reflection from the sea surface, ie, sea clutter, and reflection from rain clouds, ie, weather clutter. When an image is displayed based on the radar signal, an image showing these clutters is an obstacle to visually recognizing the target image. Therefore, various clutter suppression methods have been developed so far.
[0003]
FIG. 2 shows an example of a conventional clutter suppression circuit. The circuit shown in this figure has a scan correlation circuit 1 including an arithmetic circuit 10, a memory circuit 11, and a coordinate recognition circuit 12. The memory circuit 11 has a capacity capable of storing radar signals for at least one scan (period in which the radar antenna beam rotates once). The coordinate recognition circuit 12 accesses the memory circuit 11 based on a signal indicating the direction of the radar antenna, the transmission timing, and the like, and updates the content based on the output signal Y _n of the arithmetic circuit 10. The arithmetic circuit 10 performs the following weighted addition based on the radar signal, that is, the input signal X _n related to the current scan obtained by reception and the output signal Y _n-1 at the time of the previous scan stored in the memory circuit 11. Number 1]
Y _n = α · X _n + (1−α) · Y _n−1 (X _n ≧ T) (1)
Y _n = (1−α) · Y _n−1 (X _n <T) (2)
Where α: weight, 0 <α ≦ 1
The output signal Y _n is generated by performing T: threshold value n: scan number. As can be read from Equation (1), the processing performed by the arithmetic circuit 10 using the memory circuit 11 is the output signal Y _n-1 at the previous scan among the signals included in the input signal X _n at the current scan. Is a process of enhancing a signal having a relatively strong correlation with respect to the image, that is, a scan correlation process. In general, a strong correlation appears between scans for a target signal among signals included in a radar signal, whereas a relatively weak correlation appears for a signal whose amplitude level is not constant, such as clutter. The scan correlation process can stabilize the target signal and suppress clutter.
[0004]
Further, the threshold value T for determining whether to use the equation (1) or the equation (2) is set so that the weight α is set to zero for the input signal X _n having a relatively low level and is excluded from the object of the weighted addition. It plays the role of improving the clutter suppression effect. That is, even when the input signal X _n is below the threshold value T, the expression (1) is used when the input signal X _n is below the threshold value T as compared with the case where the expression (1) is used (FIG. 3). Is used (FIG. 4), the level of the clutter signal in the output signal Y _n decreases. That is, the ratio of target signal to clutter (S / C ratio) increases. 3 and 4 are diagrams assuming that the level of the target signal is a constant value, and the level normalized by the level of the target signal is used as the vertical axis. Further, α = 0.5 and T = 0.8.
[0005]
However, dynamic level fluctuations often appear in the clutter signal. For example, the level of sea clutter dynamically changes depending on the difference in wave height due to the difference in direction such as downwind or upwind. The above-described conventional method has a problem that S / C that can detect a target signal with a small level difference from the clutter sufficiently accurately when dynamic fluctuations appear in the clutter cannot be realized. It should be noted that the clutter level can be suppressed to some extent by adjusting STC (Sensitivity Time Control), gain, and the like, but it is not possible to cope with the fluctuations only by suppressing the clutter level.
[0006]
SUMMARY OF THE INVENTION
One object of the present invention is to enable accurate detection of a target signal even when dynamic fluctuations appear in the clutter. In the clutter suppression method and circuit according to the present invention, this object is achieved by introducing adaptive processing to set a threshold value used in scan correlation processing.
[0007]
That is, in the clutter suppression method according to the present invention, first, a radar signal obtained by sequential reception in the current scan is input as a processing target radar signal. Next, the threshold value is adaptively set so that the higher the level of the processing target radar signal is, the higher the level is, and the larger the variation in the level of the processing target radar signal is. Further, the input radar signal to be processed is weighted and added to the processing radar signal on the memory circuit. At this time, if the level of the processing target radar signal is below the threshold value, the weighting addition is performed by setting the weight multiplied by the processing target radar signal to zero. Then, the signal obtained as a result of the weighted addition is output as a post-processing radar signal and stored in a memory circuit to be used as a processing radar signal in the next scan. Further, the clutter suppression circuit according to the present invention compares the level of the radar signal to be processed and its threshold value, weighted addition of the radar signal to be processed and the radar signal for processing, and writes the result to the memory circuit, etc. In addition to the scan correlation circuit that performs the above processing, a threshold setting circuit that performs processing related to adaptive setting of the threshold is provided.
[0008]
In this way, by setting the threshold value in accordance with the level of the radar signal to be processed and its variation, for example, the sea clutter whose level dynamically changes and the target signal having a relatively low level are processed. The target signal can be detected sufficiently stably even when it is included in the radar signal. At this time, STC adjustment or the like is not necessary. Also, regarding the level of the processing target radar signal and the variation in level, regarding the processing target radar signal, the cell average and the average deviation in the moving cell sandwiching the current time are obtained, and the threshold is set based on the obtained cell average and the average deviation. By setting each time, it can be reflected in the setting of the threshold value. For example, a cell averaging circuit for calculating cell average, an average deviation calculating circuit for calculating average deviation, and an adding circuit for adding these cell averages and average deviation are provided, and the output of the adding circuit is used as a threshold value. Good. In that case, it is preferable to provide a delay circuit for delaying the input of the cell average to the adder circuit to compensate for the operation delay until the average deviation is obtained after the cell average is obtained. In addition, a delay circuit that delays the input of the processing target radar signal to the scan correlation circuit is provided, and calculation from when the processing target radar signal is obtained by reception until the threshold corresponding to the processing target radar signal is set. It is preferable to compensate for the delay.
[0009]
Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. In the following description, reference numerals common to those in the prior art shown in FIGS. 2 to 4 are used, and a part of the description is omitted. However, this is for simplification of the description, and the present invention is combined with other types of prior art. be able to. In addition, the sea clutter is taken as an example in explaining the problems of the prior art, but the present invention can be applied to a radar other than a radar that is affected by sea surface reflection such as a marine or harbor radar.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a configuration of a clutter suppression circuit according to an embodiment of the present invention. The circuit shown in this figure includes a threshold value setting circuit 2 in addition to the scan correlation circuit 1. The threshold value setting circuit 2 includes a cell averaging circuit 21 that calculates the cell average of the input signal _Xn and an average deviation calculation circuit 23 that calculates an average deviation. The cell average here is a period (distance) of a predetermined length across the current time, that is, the average value of the radar signal level in the moving cell (usually, the moving average value obtained without taking the current radar signal level into the calculation basis). When the discretized time is represented by an integer k, the following equation
A _{n, k} = {ΣX _{n, ki} + ΣX _{n, k + i} } / N (3)
However, Σ: Total from i = 1 to i = N / 2 N: Size of moving cell (number of samples), 2 or more natural number X _{n, k} : X _{n at} time k
Given in. The cell average gives an average level of the radar signal at N points (samples) before and after the current point k. The average deviation is an index indicating how much the radar signal level is within the mobile cell with respect to the cell average, that is, an index indicating the level fluctuation range of the radar signal. Number 3]
D _{n, k} = Σ | X _{n, k + i} −A _{n, k + i} | / N (4)
However, Σ: given by the sum from i = −N / 2 to i = N / 2.
[0011]
The threshold setting circuit 2 has an adding circuit 24 for setting a threshold T _{n, k} for time k based on the obtained cell average An _{n, k} and the obtained average deviation D _{n, k.} are doing. For example, the adder circuit 24 has the following formula:
T _{n, k} = A _{n, k} + D _{n, k} (5)
The threshold value T _{n, k} is set in the arithmetic circuit 10. However, for example D _n, A _n on which multiplied by some factor _k, may be added to the _k, D _n, A _n when _k is smaller than the predetermined _{value, k} is directly threshold T _{n, k} You may make a cut off. The threshold setting circuit 2 includes delay circuits 20 and 22. The delay circuit 20 compensates for an operation delay from when the input signal X _{n, k} related to time k is given to when the threshold value T _{n, k} is set, so that the input signal X _{n, This} circuit delays the supply of _k . The delay circuit 22 to compensate for the operation delay of the cell average A _n, from _k is determined to an average deviation D _{n, k} is obtained, the cell average A _{n, k} supplied to the summing circuit 24 This is a circuit that delays. The delay times of the delay circuits 20 and 22 may be determined by design or testing.
[0012]
In the arithmetic circuit 10, processing is performed on the same principle as the above-described equations (1) and (2). However, in this embodiment, the threshold value T _{n, k} is set for each time according to the cell average An _{n, k} and the average deviation D _{n, k} . Therefore, the output signal in the present embodiment is different from the above-described conventional technique.
Y _{n, k} = α · X _{n, k} + (1−α) · Y _{n−1, k} (X _{n, k} ≧ T _{n, k} ) (6)
Yn _{, k} = (1- [alpha]). _{Yn-1, k} ( _{Xn, k} < _{Tn, k} ) (7)
The value given by.
[0013]
As described above, in this embodiment, the threshold value T _{n, k} is set at each time according to the cell average An _{n, k} and the average deviation D _{n, k} , so that the level changes randomly. Such clutter can also be reflected in the estimated scan correlation process, and the effect of suppressing clutter and improving the S / C ratio can be obtained. In addition, a sensitivity adjustment operation such as STC is unnecessary. Furthermore, while taking out the low frequency during the clutter fluctuation by the cell averaging, the amplitude or degree of the clutter fluctuation is taken out by the average deviation, and these are reflected in the threshold setting. Although the fluctuation is reflected in the threshold setting, the threshold itself does not show unstable fluctuation.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a clutter suppression circuit according to an embodiment of the present invention.
FIG. 2 is a block diagram showing a configuration of a clutter suppression circuit according to a conventional technique.
FIG. 3 is a diagram showing the effect of scan correlation processing that does not involve processing switching by a threshold value.
FIG. 4 is a diagram showing the effect of scan correlation processing with processing switching by a threshold value.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Scan correlation circuit, 2 Threshold setting circuit, 10 Arithmetic circuit, 11 Memory circuit, 12 Coordinate recognition circuit, 20, 22 Delay circuit, 21 Cell averaging circuit, 23 Average deviation calculation circuit, 24 Adder circuit

Claims (5)

The radar signal obtained by sequential reception in the current scan is input as a processing target radar signal, the input processing target radar signal is weighted and added to the processing radar signal on the memory circuit, and the resulting signal is converted into a post-processing radar signal. In the clutter suppression method for outputting as a signal and storing it on the memory circuit to be used as a processing radar signal in the next scan,
When the level of the processing target radar signal is below the threshold, the weight to be multiplied by this processing target radar signal is set to zero and the above weighted addition is performed. Further, the higher the level of the processing target radar signal, the higher the level. A clutter suppression method, wherein the threshold value is adaptively set so as to increase and to increase as the level variation of the processing target radar signal increases. The clutter suppression method according to claim 1, wherein a cell average and an average deviation in a moving cell sandwiching a current time are obtained for a radar signal to be processed, and the threshold value is set for each time based on the obtained cell average and the average deviation. The clutter suppression method characterized by setting to. A radar signal obtained by sequential reception in the current scan is input as a processing target radar signal, and becomes zero when the level of the input processing target radar signal is below a threshold value. As described above, while switching the weight related to the processing target radar signal, the processing target radar signal is weighted and added to the processing radar signal on the memory circuit, and the resulting signal is output as a post-processing radar signal and the next scan A scan correlation circuit stored on the memory circuit for use as a processing radar signal at
A threshold setting circuit for adaptively setting the threshold value such that the higher the level of the radar signal to be processed, the higher the level of the radar signal to be processed, and the greater the variation in the level of the radar signal to be processed;
A clutter suppression circuit comprising: 4. The clutter suppression circuit according to claim 3, wherein the threshold setting circuit has a cell averaging circuit for obtaining a cell average in a moving cell sandwiching the current time with respect to the processing target radar signal, and an average in the moving cell with respect to the processing target radar signal. A clutter suppression circuit comprising: an average deviation calculation circuit for obtaining a deviation; and an addition circuit for setting the threshold value at each time by adding the obtained cell average and the average deviation . 5. The clutter suppression circuit according to claim 4, wherein the threshold value setting circuit delays the input of the cell average to the adder circuit so as to compensate for the operation delay from when the cell average is obtained until the average deviation is obtained. The first delay circuit and the radar to be processed to the scan correlation circuit in order to compensate for a calculation delay from when the radar signal to be processed is obtained by reception until a threshold corresponding to the radar signal to be processed is set. A clutter suppression circuit comprising: a second delay circuit that delays input of a signal.

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