JPH07311256A - Clutter suppressing circuit - Google Patents

Abstract

PURPOSE:To suitably detect a target signal having a low level even when the signal exists before or after a signal having a high level and spreading in a distance direction. CONSTITUTION:A threshold a value Tf is calculated by a low-pass filter 18 having higher response than that of a cell-averaging circuit 10. Since the value Tf reflects the variation in a high frequency as compared with a threshold value To obtained by the circuit 10, it becomes a smaller value than the value Tc before or after the target signal having a high level and spreading in a distance direction. In this case, a threshold value T calculator 22 obtains a weighted average of the values Tc and Tf and supplies it as the threshold value T to a subtracter 14.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a clutter suppressing circuit for suppressing clutter signals contained in radar signals.

[0002]

2. Description of the Related Art Received signals from radar devices (radar signals)
Usually includes a reflection signal from a target (target signal) and a reflection signal from another object (clutter signal). For example, in the case of a radar device for ships, all that is required is a target signal that is a reflection from another ship, but in reality, a sea surface reflection signal (sea clutter), rain / snow reflection signal (weather clutter), etc. Included in the signal.
If these clutter signals are kept, problems such as usability will occur such that it becomes difficult to capture the target image when the display based on the radar signal is performed. Therefore, various clutter suppression methods have been conventionally developed.

As one of the methods, a method called cell averaging LOG / CFAR (Constant False Alarm Rate) is known. This method focuses on the fact that the clutter signal in the logarithmically amplified radar signal generally has a certain variance with respect to the average value of the radar signal. That is, since the clutter signal has a constant variance as shown in FIG. 2A, subtracting the average value of the radar signal from the radar signal shown in FIG. As shown, the clutter signal is suppressed to a constant level.

FIG. 3 shows the configuration of a clutter suppression circuit using this method. The clutter suppression circuit shown in this figure includes a cell averaging (threshold T calculation) circuit 10,
The delay circuit 12, the subtraction circuit 14, and the amplification circuit 16 are included.

The cell averaging circuit 10 inputs the logarithmically amplified radar signal as an input signal X, and a time series of the input signal X {X _k : k = ... -n, -n + 1, ...- 1,
Based on 0, +1, ... N-1, n, ...}, the calculation based on the following equation (2) is performed to obtain the average value T _k of the radar signal level at time k. That is, the average value T _k is obtained by a calculation similar to the moving average, and the average value T _k is supplied to the subtraction circuit 14 as a threshold value T (hereinafter, referred to as T _k ) for time k. However, N is the number of data of the input signal X to be averaged.

[0006]

[Equation 2] In addition to the threshold value T, the subtraction circuit 14 includes a delay circuit 12
The input signal X delayed by is also input. The delay circuit 12 is a circuit for compensating the processing delay by the cell averaging circuit 10. Due to the delay in the delay circuit 12, the threshold value T input to the subtraction circuit 12 and the input signal X have the same value at the time k ( T _k , X _k ). The subtraction circuit 14 calculates X _k −T _k and supplies the result to the amplification circuit 16 as an output signal Y at time k (hereinafter referred to as Y _k ). The amplifier circuit 16 outputs the output signal Y after amplification.

[0007]

However, when clutter suppression is attempted by such a method, there is a low level target signal before or after a high level target signal having a spread in the distance direction (time series direction). In such a case, the target signal having a low level is suppressed like clutter and the detection probability thereof is lowered. That is, as is clear from the above equation (2), when the input signal to be averaged includes a high-level target signal, this target signal increases the average value (threshold value). If the threshold value of a low-level target signal at the time becomes higher due to the high-level target signal having a spread in the distance direction, the target signal that should appear in the output signal will be damaged as a result of the subtraction at that time. .

The present invention has been made to solve such a problem, and by operating a threshold value, a target signal having a high level and a spread in the distance direction is provided before or after the target signal. It is an object of the present invention to make it possible to detect a target signal with a suitable probability even when there is a low-level target signal.

[0009]

In order to achieve such an object, the clutter suppression circuit of the present invention has a radar signal X _{k-i to} X over a predetermined range k-i to k + i before and after time k.
a cell averaging means for obtaining a threshold value T _c at time k by averaging the level of _{k + i,} the clutter signal from the radar signal X _k by subtracting the threshold T _c from the radar signal X _k at time k Means to remove,
A means for obtaining a threshold T _f by low-pass filtering the radar signal X _k with a higher response than the cell averaging means, and a threshold T if the threshold T _c is smaller than the threshold T _f. and means for removing the clutter signals from the radar signal X _k by subtracting the determined threshold value T from the radar signal X _k at time k using equation (3) instead of _c, characterized in that it comprises a.

[0010]

## EQU3 ## T = αT _c + (1-α) T _f , where 0 ≦ α <1 (3)

[0011]

In the present invention, the cell averaging means averages the levels of the radar signals X _{k-i to} X _{k + i} over the predetermined range k-i to k + i before and after the time k, and the threshold value at the time k is thereby obtained. T _c is obtained. On the other hand, in the present invention, the threshold value T _f is obtained by low-pass filtering the radar signal X _k with a higher response than the cell averaging means. Normally, the threshold value T _c obtained by the cell averaging means is the time k.
Of the radar signal X _k , thereby removing the clutter signal from the radar signal X _k . On the other hand, when the threshold value T _c becomes smaller than the threshold value T _f , the radar signal X _k whose threshold value T is time k instead of the threshold value T _c.
, Which removes the clutter signal from the radar signal X _k .

That is, the threshold value T obtained with higher response than the threshold value T _c from the cell averaging means.
_When a target signal having a high level and a spread in the distance direction is present, _f becomes a value smaller than the threshold value T _c before and after the target signal, and the threshold value T _f and the threshold value T _c.
The threshold value T obtained by the weighted addition of is also smaller than the threshold value T _c . The threshold value T also reflects the levels of target signals existing before and after. Therefore, even if there is a low-level target signal before or after the high-level target signal having a spread in the distance direction, the presence of the high-level target signal and the threshold value T considering the level thereof Since the subtraction related to the clutter removal is performed, the target signal having the low level is detected with a suitable probability.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings. The same components as those of the conventional example shown in FIG. 3 are designated by the same reference numerals and the description thereof will be omitted.

FIG. 1 shows a clutter suppressing circuit showing an embodiment of the present invention. The circuit shown in this figure is
The circuit of the conventional example shown in FIG. 3 has a low-pass filter (T
_f calculation circuit) 18, level comparison circuit 20, and threshold value T
This is a configuration in which a calculation circuit 22 is added.

The low-pass filter 18 calculates the threshold value T _f by filtering the input signal X with a low pass band with a higher response than the cell averaging circuit 10. That is, the calculation in the cell averaging circuit 10 is a calculation similar to the moving average calculation as shown in the equation (2) and has a certain cutoff frequency. The low-pass filter 18 has a cutoff frequency higher than the cutoff frequency of the cell averaging circuit 10, and is a filter having good followability with respect to a change in the input signal X.
Therefore, the threshold value T _f obtained by the low-pass filter 18 is a value that reflects the high frequency fluctuation of the input signal X, as compared with the threshold value T _c obtained by the cell averaging circuit 10. For example, when a clutter signal having a high input level and a spread in the distance direction (time series direction) is input as the input signal X, the high level of the target signal is calculated by the equation (2) before and after the input signal X. Since it is reflected in the threshold value at time k (expressed as a threshold value T _c to distinguish it from that in the conventional example), T _f becomes smaller than the threshold value T _c .

The level comparison circuit 20 includes thresholds T _c and T
_f is compared, and the result is notified to the threshold T calculation circuit 22. When the threshold T _c is equal to or less than the threshold T _f , the threshold T calculation circuit 22 sets the threshold given to the subtraction circuit 14 to the threshold T _c obtained by the cell averaging circuit 10. , If the threshold value T _c is larger than the threshold value T _f , the threshold value T obtained by the equation (3) is supplied to the subtraction circuit 14 as a threshold value for subtraction. The subtraction circuit 14 receives the input signal X supplied via the delay circuit 12.
The clutter signal is suppressed by subtracting the threshold value T _c or T from.

Therefore, according to this embodiment, the threshold value T is corrected in accordance with the level of the input signal X, so that the level is high before or after the target signal having a high level and a spread in the distance direction. Even when a low target signal is present, it is possible to detect a target signal having a low level with a suitable probability. Further, since the threshold value T reflects the level of the target signal having the high level, the threshold value T does not unnecessarily decrease.

[0018]

As described above, according to the present invention,
The threshold value T _f is obtained by low-pass filtering the radar signal X _k with higher response than the cell averaging means, and the threshold value T _c obtained by the cell averaging means is smaller than the threshold value T _f . When it becomes smaller, the threshold value T of the equation (3) is used for clutter removal instead of the threshold value T _c , so that the level of the target signal before or after the target signal having a high level and spread in the distance direction is set. Even if there is a low target signal, this low level target signal can be detected with a suitable probability.

[Brief description of 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 diagram showing the contents of cell averaging LOG / CFAR, in which (a) shows the radar signal before subtraction,
(B) is a figure which shows the signal after a subtraction.

FIG. 3 is a block diagram showing a configuration of a clutter suppression circuit according to a conventional example.

[Explanation of symbols]

 10 cell averaging circuit 12 delay circuit 14 subtraction circuit 16 amplification circuit 18 low-pass filter 20 level comparison circuit 22 threshold value T calculation circuit

Claims (1)

[Claims] 1. A cell averaging means for obtaining a threshold value T _c at time k by averaging the levels of radar signals X _{k −i to} X _{k + i} over a predetermined range k−i to k + i before and after time k. , A means for removing the clutter signal from the radar signal X _k by subtracting the threshold value T _c from the radar signal X _k at the time k, in the clutter suppression circuit having a higher response than the cell averaging means. means for determining a threshold value T _f by low-pass瀘波the _k, the threshold the threshold T _c is determined by the equation instead of the threshold value T _c (1) If less than the threshold value T _f Means for removing the clutter signal from the radar signal X _k by subtracting T from the radar signal X _k at time k, and T = αT _c + (1-α) T _f , where 0 ≦ α <1 (1) A clutter suppression circuit comprising: