JPH1164486A - Method and circuit for suppressing clutter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect a target signal accurately even when a dynamic fluctuation appears in the clutter by introducing an adaptive processing into the setting of a threshold being employed at the scan correlation processing. SOLUTION: An adder circuit 24 adds the cell average and the mean deviation of input signals X to set a threshold value. An operating circuit 10 sets a zero specific gravity for the input signal X having a level lower than the threshold value and performs weighted addition of the input signal X and a radar signal on a memory circuit 11. Since the threshold is set adaptively at each point of time depending on on the level and the fluctuation of the input signal X, even such a clutter as exhibiting a random level fluctuation can be suppressed properly.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a clutter suppressing method and circuit for suppressing clutter included in a radar signal.

[0002]

2. Description of the Related Art A radar signal includes a reflection signal (clutter) from another object in addition to a reflection signal from a target to be detected (target signal). In the example of the marine radar, there are reflections from the sea surface, that is, sea clutter, and reflections from rain clouds or the like, that is, weather clutter. When an image is displayed based on the radar signal, the image showing these clutters hinders the visual recognition of the target image. For this reason, various clutter suppression techniques have been developed.

FIG. 2 shows an example of a conventional clutter suppression circuit. The circuit shown in FIG. 1 includes 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 at least one
It has a capacity to store radar signals for scanning (a period during which the beam of the radar antenna rotates once). Coordinate recognition circuit 12 accesses the memory circuit 11 based on a signal indicating the azimuth and transmission timing of the radar antenna or the like, and updated based the contents of the output signal Y _n of the arithmetic circuit 10. The arithmetic circuit 10 performs the following weighted addition based on the radar signal relating to the current scan obtained by reception, that is, the input signal _Xn and the output signal Yn _-1 at the time of the previous scan stored in the memory circuit 11.

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 T: threshold value n: scan number The output signal Y _n is generated. Equation (1)
The arithmetic circuit 10 is connected to the memory circuit 11 so that
Is a process of emphasizing a signal having a relatively strong correlation with the output signal Y _n-1 of the previous scan among the signals included in the input signal X _n of the current scan, that is, the scan correlation. Processing. Generally, among signals included in a radar signal, a target signal has a strong correlation between scans, whereas a signal whose amplitude level is not constant, such as clutter, has a relatively weak correlation. By the scan correlation processing, the target signal can be stabilized and clutter can be suppressed.

The threshold value T for determining whether to use the equation (1) or the equation (2) is set such that the weight α is set to zero for the input signal X _n having a relatively low level, and the threshold value T By removing it, it plays the role of improving the clutter suppression effect. That is, when the input signal X _n is as adapted to use the equation (1), even when they are below the threshold T compared with (FIG. 3), the input signal X _n is the threshold value T
When to use equation (2) when below the can (Fig. 4), the level of the clutter signal in the output signal Y _n is reduced. That is, the ratio of the target signal to the clutter (S / C ratio) increases. 3 and 4 are diagrams assuming that the level of the target signal is a constant value, and the vertical axis uses the level standardized by the level of the target signal. Also, α = 0.
5, T = 0.8.

[0005] However, a dynamic level fluctuation often appears in the clutter signal. For example, the level of the sea clutter dynamically changes due to a difference in wave height due to a difference in direction such as leeward or upwind. The above-described conventional method has a problem that when dynamic fluctuations appear in the clutter, it is not possible to realize an S / C that can sufficiently accurately detect a target signal having a small level difference from the clutter. STC (Sensitivity T)
You can control the clutter level to some extent by adjusting the (ime Control), gain, etc., but it is not possible to cope with the fluctuation only by suppressing the clutter level.
Please be careful.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to enable accurate detection of a target signal even when dynamic fluctuations appear in clutter. In the clutter suppression method and circuit according to the present invention, this object is achieved by introducing adaptive processing to the setting of the threshold used in scan correlation processing.

That is, in the clutter suppression method according to the present invention, first, a radar signal obtained by successive reception in the current scan is input as a radar signal to be processed. Next, the threshold value is adaptively set so as to increase as the level of the radar signal to be processed increases, and to increase as the level variation of the radar signal to be processed increases. 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 radar signal to be processed is lower than the threshold value, the weight for multiplying the radar signal to be processed is set to zero and weighted addition is performed. Then, the signal obtained as a result of the weighted addition is output as a processed radar signal and stored in a memory circuit for use as a processing radar signal in the next scan. Also, the clutter suppression circuit according to the present invention may be configured to compare the level of the radar signal to be processed with its threshold value, to perform weighted addition of the radar signal to be processed and the radar signal for processing, and to write the result to a memory circuit. 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.

As described above, when the threshold value is set, the setting is performed in accordance with the level of the radar signal to be processed and the variation thereof, so that, for example, a sea clutter whose level dynamically fluctuates and a target signal of a relatively low level can be used. Can be detected sufficiently stably even when is included in the radar signal to be processed. At that time, S
There is no need to adjust TC or the like. The level of the radar signal to be processed and the level variation are obtained by calculating the cell average and the average deviation in the moving cell sandwiching the current time with respect to the radar signal to be processed, and setting the threshold based on the obtained cell average and the average deviation. By setting for each time, it can be reflected in the setting of the threshold. For example, a cell averaging circuit for obtaining a cell average,
An average deviation calculating circuit for obtaining an average deviation, and an adding circuit for adding these cell averages and the average deviation may be provided, and the output of the adding circuit may be used as a threshold. In this 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 from when the cell average is obtained to when the average deviation is obtained. In addition, a delay circuit for delaying the input of the radar signal to be processed to the scan correlation circuit is provided, and an operation is performed from when the radar signal to be processed is obtained by reception until a threshold value corresponding to the radar signal to be processed is set. It is preferable to compensate for the delay.

Hereinafter, a preferred embodiment of the present invention will be described with reference to the drawings. In the following description, the same reference numerals as those in the related art shown in FIGS. 2 to 4 are used, and the description is partially omitted. However, this is for the sake of simplification of the description, and the present invention is also combined with other types of the related art. be able to. Although sea clutter is taken as an example when describing the problems of the prior art, the present invention can be applied to other radars such as radars for ships or harbors that are affected by sea surface reflection.

[0010]

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 has a cell averaging circuit 21 for obtaining a cell average of the input signal _Xn and an average deviation calculating circuit 23 for obtaining an average deviation. The cell average here is a period (distance) of a predetermined length sandwiching the present time, that is, the average value of the radar signal level in the moving cell (usually, the moving average value obtained without taking the radar signal level at the present time into the basis of calculation). And the discrete time is represented by an integer k, the following expression

A _{n, k} = X _{n, ki} + ΣX _{n, k + i} ｝ / N (3) where Σ: the sum from i = 1 to i = N / 2 N: size of moving cell ( Sample number), 2 or more natural numbers X _{n, k} : given by X _n at time k. The cell average gives an average level of the radar signal at N time points (samples) before and after the current time k. The average deviation is an index indicating how much the level of the radar signal has a deviation from the cell average in the moving cell, that is, an index indicating the level variation width of the radar signal.

D _{n, k} = Σ | X _{n, k + i} −A _{n, k + i} | / N (4) where Σ: the sum from i = −N / 2 to i = N / 2 Given by

The threshold value setting circuit 2 is an adding circuit for setting a threshold value T _{n, k} for time k based on the obtained cell average A _{n, k} and the obtained average deviation D _{n, k.} 2
Four. The addition circuit 24 is, for example, the following equation

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 _k, when D _{n, k} is smaller than a predetermined value A
_{The truncation} may be performed to set _{n, k as it is} to the threshold value T _{n, k} . The threshold value setting circuit 2 includes delay circuits 20 and 22
have. The delay circuit 20 compensates for an operation delay from when the input signal X _{n, k} relating to the time k is given to when the threshold value T _{n, k} is set, so that the input signal X _{n, This} is a circuit that delays the supply of _k . Further, the delay circuit 22 compensates for an operation delay from when the cell average A _{n, k} is obtained to when the average deviation D _{n, k} is obtained.
This is a circuit for delaying the supply of the cell average _{An, k} to the adding circuit 24. The delay time of the delay circuits 20 and 22 is
It may be determined by design or test.

The arithmetic circuit 10 performs processing according to the same principle as the above-described equations (1) and (2). However, in the present embodiment, the threshold value T _{n, k} is set for each time according to the cell average A _{n, k} and the average deviation D _{n, k} . Therefore, the output signal in the present embodiment is different from the above-described prior art.

Y _{n, k} = α · X _{n, k} + (1−α) · Y _{n−1, k} (X _{n, k} ≧ T _{n, k} ) (6) Y _{n, k} = (1 −α) · Y _{n−1, k} (X _{n, k} <T _{n, k} ) (7)

As described above, in the present embodiment, the threshold T
_{Since n, k} is set for each time according to the cell average A _{n, k} and the average deviation D _{n, k} , even for clutter whose level changes randomly, the fluctuation is estimated and the scan correlation is estimated. This can be reflected in the processing, and the effect of suppressing clutter and improving the S / C ratio can be obtained. Also,
No sensitivity adjustment operation such as STC is required. Further, while the low frequency during the clutter fluctuation is extracted by cell averaging, the amplitude or degree of the clutter fluctuation is extracted by the average deviation, and these are reflected in the threshold value 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 illustrating a configuration of a clutter suppression circuit according to an embodiment of the present invention.

FIG. 2 is a block diagram illustrating a configuration of a clutter suppression circuit according to one related art.

FIG. 3 is a diagram illustrating an effect of a scan correlation process without switching a process based on a threshold.

FIG. 4 is a diagram illustrating the effect of scan correlation processing involving switching of processing based on a threshold.

[Explanation of symbols]

1 scan correlation circuit, 2 threshold setting circuit, 10
Arithmetic circuit, 11 memory circuit, 12 coordinate recognition circuit, 2
0,22 delay circuit, 21 cell averaging circuit,
23 average deviation calculation circuit, 24 addition circuit.

Claims (5)

[Claims] 1. A radar signal obtained by successive reception in a current scan is input as a processing target radar signal, the input processing target radar signal is weighted and added to a processing radar signal on a memory circuit, and a signal obtained as a result is obtained. Is output as a processed radar signal and stored in the memory circuit to be used as a processing radar signal in the next scan, when the level of the radar signal to be processed is below a threshold value. Then, the weight for multiplying the radar signal to be processed is set to zero, and the above-mentioned weighted addition is performed. Further, the higher the level of the radar signal to be processed, the larger the level and the variation in the level of the radar signal to be processed. A clutter suppression method, wherein the threshold value is adaptively set so that the threshold value increases as the value increases. 2. The clutter suppression method according to claim 1, wherein a cell average and an average deviation in a moving cell sandwiching the current time are obtained for the radar signal to be processed, and the threshold value is determined based on the obtained cell average and the average deviation. A clutter suppression method characterized in that a value is set for each time. 3. A radar signal obtained by successive reception in a current scan is input as a radar signal to be processed, and is non-zero when the level of the input radar signal to be processed is less than a threshold value and is not likely to become zero. The weight of the radar signal to be processed is added to the radar signal for processing on the memory circuit while the weight of the radar signal to be processed is switched so that the predetermined value of the radar signal is obtained, and the resulting signal is output as the processed radar signal. And a scan correlation circuit stored on the memory circuit to be used as a radar signal for processing in the next scan, and the level of the radar signal to be processed increases as the level of the radar signal to be processed increases. Threshold setting circuit that adaptively sets the above threshold so that the larger the variation of , Clutter suppressing circuit comprising: a. 4. The clutter suppression circuit according to claim 3, wherein the threshold setting circuit calculates a cell average in a moving cell sandwiching the current time with respect to the radar signal to be processed, and a cell averaging circuit with respect to the radar signal to be processed. An average deviation calculation circuit for obtaining an average deviation in the moving cell; and an addition circuit for setting the threshold value at each time by adding the obtained cell average and the average deviation. Clutter suppression circuit. 5. The clutter suppression circuit according to claim 4, wherein said threshold value setting circuit compensates for an operation delay from when the cell average is obtained to when an average deviation is obtained, and said cell average is supplied to said adder circuit. And a scan correlation circuit for compensating for an operation delay from when a processing target radar signal is obtained by reception to when a threshold value corresponding to the processing target radar signal is set. And a second delay circuit for delaying the input of the radar signal to be processed to the clutter suppression circuit.