JPH057665B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a marine radar receiver for detecting a desired object (target), and in particular detects components caused by sea surface reflection (sea surface clutter) from an echo video signal. CFAR used for the purpose of removing
(Constant False Alarm Rate) circuit improvement.

(Prior art) The echo video signal, which is a received signal of a marine radar, includes not only reflected signals from target objects but also unwanted signals reflected from the sea surface, rain, snow, etc. (clutter). There is. Since these clutters impede detection of targets and increase the probability of false alarms, it is necessary for marine radars to take some means to suppress these clutters.

For this reason, in general, STC (Sensitivity Time Control) circuits for the purpose of removing sea surface clutter,
FTC (Fast Time) for the purpose of removing rain and snow clutter
Devices such as constant (constant) circuits are used to manually adjust the degree of clutter suppression. Additionally, a CFAR circuit has been proposed as a device that automatically stabilizes the false alarm probability due to clutter. In other words, since the clutter component contained in the echo video signal generally has Rayleigh distribution characteristics,
The CFAR circuit utilizes the fact that when the echo video signal due to Rayleigh distributed clutter is logarithmically compressed, the variance around the average value becomes constant.The basic circuit is shown in FIG. In FIG. 5, the echo video signal a input to the video input terminal 1 enters a tapped delay line 3 having a predetermined delay time. The outputs of each tap (1 to N) of the delay line 3 are added by an adder 4, and this output is sent to a depider 5.
is reduced to 1/N and becomes the average value (statistical signal c) of the echo video signal a in the time width determined by the delay line 3. On the other hand, the echo video signal b taken out from the center tap of the delay line 3 enters a subtracter or divider 6 and is subtracted or divided by the average value. This output signal f enters the threshold circuit 7, and the threshold signal g input from the terminal 8
The sea surface clutter component is suppressed to the receiver noise level and output from the video output terminal 2. If the echo video signal is actually logarithmically compressed, such as by changing the IF amplifier of the receiver to a logarithmic amplifier, a logarithmic CFAR circuit is used, and a subtracter is used. Linear CFAR if uncompressed
It becomes a circuit, and 6 uses a divider.

As an example, the operation in the case of logarithmic CFAR will be explained using the waveform diagram in FIG. As shown in the figure, the echo video signal a input from the input terminal 1 includes small target echoes (b) and land echoes (c), as well as sea surface clutter (b) due to sea surface reflection. The echo video signal b output from the center tap of the delay line 3 has a signal waveform delayed by a certain period of time from the input echo video signal a. Then, each echo video signal output from the terminals 1 to N of the delay line 3 is added by an adder 4, and a divider 5
is averaged and becomes the statistical signal c. Therefore,
The output signal f of the subtractor or divider 6 to which the statistics signal c is input has a signal waveform obtained by subtracting the component of the statistics signal c from the echo video signal b. Then,
The level of sea surface crucifera (a) becomes almost uniform.

By the way, since a considerable amount of sea surface clutter (a) remains in the output signal f shown in FIG. 6, it is insufficient as it is for the purpose of removing sea surface clutter. Therefore, the threshold signal g is added to the threshold input terminal 8, and the threshold signal g is subtracted by the threshold value (d) necessary to remove sea surface clutter by the threshold circuit 7, and the target echo (b) is obtained. will be detected.

In addition, in the CFAR circuit explained above, the average value is obtained using the delay line 3, but the average value can be simply obtained using an integrating circuit or low-pass filter using a resistor, capacitor coil, etc., which is cheaper than the delay line. Sometimes. In the case of a logarithmic CFAR circuit, an antilogarithmic converter is inserted before or after the threshold circuit 7 to release logarithmic compression, but this is not shown.

(Problems to be Solved by the Invention) However, the above-described CFAR circuit has the following two problems.

In general, marine radars use not only targets, but also
Suppressing land-based echoes is extremely dangerous to navigation. Therefore, as a means for removing clutter, it is necessary to use a method that does not suppress land or targets as much as possible. However, as shown in FIG. 6, the statistic signal c (average value) for subtraction increases both for the target object (b) component and also for the land (c) component. Therefore, the output signal f of the subtraction circuit 6
As shown in the figure, the target object (b) is suppressed and the land
(C) is also greatly suppressed, and if the threshold circuit 7 removes the sea surface clutter, there is a problem that the land echo will completely disappear.

In addition, the threshold circuit 7 also suppresses the sensitivity of a relatively long-distance area without sea surface clutter, which is used for subtraction with a fixed threshold value, as shown in FIG. 6(d). This requires the person operating the radar to adjust the threshold value (d) or use the CFAR circuit depending on whether the radar operator is detecting a target in the clutter or a target in an area without clutter. on/
It was very inconvenient to have the user select OFF.

The present invention has been made in order to solve the above-mentioned problems, and it automatically and effectively eliminates unnecessary suppression of target echoes such as ships and land echoes, and without reducing long-distance sensitivity. The purpose is to provide a CFAR circuit that removes sea surface clutter.

(Means for Solving the Problems) The present invention provides a statistics detection circuit that detects statistics within a predetermined time of an input echo video signal, and a statistics signal output from the statistics detection circuit from the echo video signal. A waveform correction circuit that removes a statistical signal component included in an echo video signal by subtracting the echo video signal or dividing the echo video signal by the statistical signal; In a CFAR circuit having a threshold circuit for removing signal components of The apparatus is equipped with a minimum value maintaining circuit that maintains and outputs the minimum value of the statistical signal that changes over time during the period until the next transmission trigger signal is input.

In a CFAR circuit according to another invention, in addition to the above-mentioned means, the initial value of the output signal of the lowest value maintaining circuit is maintained, and this value is attenuated with a time constant longer than the predetermined time in the statistics detection circuit. It is equipped with a Peter detector which sends out a threshold signal to a threshold circuit.

(Operation) It is well known that the average value (statistical signal) of sea level clutter generally decreases in inverse proportion to the cube of the distance. However, the average value (statistical signal) of the actual echo video signal also includes target echoes and echo components from land, so if these echo components are included, they will become smaller as the distance increases. In many cases, the size increases without increasing the size. Therefore, by adopting the above-mentioned configuration, only the lowest value is maintained from the statistics signal of the echo video signal obtained from time to time, and the echo video signal is subtracted or divided by this lowest value in the waveform correction circuit. Even if the statistic signal increases due to land or ground, the minimum value does not increase, and these echoes are not unnecessarily suppressed.

Note that the minimum value maintaining circuit is set up as its initial value by a transmission trigger signal or the like at a maximum value in the vicinity where the statistic signal due to the sea surface clutter becomes maximum immediately after transmission.

Furthermore, the basic function of the CFAR circuit is to keep the false alarm probability constant, and in order to suppress clutter to the receiver noise level, it is necessary to subtract it at an appropriate threshold. If this threshold value is fixed, the long-distance sensitivity will decrease as described above, so in the present invention, a peak detector that attenuates with a time constant longer than the time to obtain the average value (statistical signal) is used to detect the Since the initial minimum value is held and used as a threshold signal, the optimal threshold can be automatically set according to the level of sea surface clutter.

(Example) An example of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing a CFAR circuit according to an embodiment, and the same parts as in the conventional CFAR circuit of FIG. 5 are given the same reference numerals.

Echo video signal a input from input terminal 1
is input to a statistics detection circuit 12 consisting of a delay line 3, an adder 4, and a divider 5. The echo video signal a input to the statistics detection circuit 12 enters a tapped delay line 3 having a predetermined delay time. Each tap of day line 3 (1
~N) are added by an adder 4, and this output is converted to 1/N by a divider 5 and sent to the delay line 3.
The statistical signal (average value) c of the echo video signal a in the time width determined by . This statistical signal c
enters the lowest value maintenance circuit 9, and is set up to an initial value by the transmission trigger signal e inputted from the terminal 10. After setup, the output of the lowest value maintaining circuit 9 maintains only the lowest value of the statistics signal c that changes every moment. On the other hand, the echo video signal b taken out from the center tap of the delay line 3 enters a subtracter or divider 6 as a waveform correction circuit, and is subtracted or divided at the output of the minimum value maintaining circuit 9. That is, the statistical signal component corrected by the lowest value maintaining circuit 9 is removed from the echo video signal b. This output signal f is the threshold value circuit 7
It is subtracted by the threshold signal g input from terminal 8 to suppress sea surface clutter to the receiver noise level, and is output from video output terminal 2. If the echo video signal is actually logarithmically compressed, such as by changing the IF amplifier of the receiver to a logarithmic amplifier, it will become a logarithmic CFAR circuit, and a subtracter will be used as the waveform correction circuit 6, and a linear IF amplifier will be used. Linear CFAR if not logarithmically compressed
A divider is used as the waveform correction circuit 6.

As an example, the operation in the case of a logarithmic CFAR circuit is shown in the second section.
This will be explained using the waveform diagram in the figure. The echo video signal a, the center tap output signal b, and the statistic signal c (average value) output from the divider 5 have the same waveforms as those shown in FIG. 6 described above.

Further, in the output signal d of the minimum value maintenance circuit 9, an increase in the statistical signal c (average value) due to the target object echo and the land echo shown by the dotted line is cassetted. In response to the input of the transmission trigger signal f, the minimum value maintaining circuit 9 sets up an initial value at the shaded portion of the output signal d. The output signal f of the subtracter 6 is
The signal waveform is obtained by subtracting the output signal d of the lowest value maintenance circuit 9 from the echo video signal b, and the target echo (b)
and land echoes (c) are not unnecessarily suppressed. Next, in order to suppress the sea level clutter to the receiver noise level, the threshold signal g is applied to the threshold input terminal 8, and the threshold signal g is subtracted by the threshold value shown in FIG. , a target echo (b) and a land echo (c) are detected in the output signal h.

With the CFAR circuit configured in this way, the target echo (b) and land echo included in the statistical signal (average value) c output from the statistical detecting circuit 12
The increase caused by (c) is removed by the minimum value maintaining circuit 9. The output signal d of this minimum value maintaining circuit 9 is input to a subtractor or divider 6 as a waveform correction circuit. Therefore, the target echo (b) and the land echo in the output signal f of this waveform correction circuit are
Since (c) is not unnecessarily suppressed, only the sea surface clutter (a) component is removed from the output signal h of the video signal output terminal 2, and target echoes (b) and land echoes (c) clearly appear. .

FIG. 3 shows another embodiment, in which a peak detector 11 which attenuates with a longer time constant depending on the time width for obtaining the statistical signal c is added to the embodiment of FIG. The output signal d is held by this peak detector 11, and this output is input to the threshold circuit 7 as the threshold signal i. Note that the actual threshold value is adjusted so that the output of the peak detector 11 has sea level clutter almost equal to the receiver noise level. FIG. 4 shows the operation of this circuit, in which the output of the peak detector 11 (threshold signal i) has a peak detection output (ho) corresponding to the output signal d of the minimum value maintenance circuit 9 shown by the dotted line. is obtained. When sea surface clutter is small and the initial value of the minimum value maintenance circuit 9 is small, the peak detection output will also be small as in (f) and (g). As shown in the figure, the output signal f of the subtracter 6 is suppressed by a threshold circuit 7, and the output signal f is
is attenuated over time, indicating that long-range sensitivity is not unnecessarily suppressed. Note that the output signal f has a sloping threshold value (E) and a sea level clutter (A) that is at an almost constant level, so strictly speaking it cannot be said to be in an optimal state, but the divider 5 shown in FIG. The slope of the sea surface clutter level can be brought closer to the threshold slope by making the value of 1/N slightly larger than 1/N and making this output slightly smaller than the average value of the echo video signal.

Further, in the above embodiment, the delay line 3 is used in the statistics detection circuit 12 to detect the statistics signal (average value).
Needless to say, it can also be applied to the case where a statistical signal (average value) is obtained by simply using an integrating circuit or a low-pass filter. In addition, the present invention can be implemented with numerous modifications without departing from the gist thereof.

(Effects of the Invention) As detailed above, according to the present invention, target echoes such as ships and land echoes are not unnecessarily suppressed.
Furthermore, sea surface clutter can be automatically and effectively removed without reducing long-range sensitivity.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a CFAR circuit according to one embodiment of the present invention, FIG. 2 is an operating waveform diagram of the same circuit, FIG. 3 is a block diagram showing a CFAR circuit of another embodiment, and FIG. FIG. 5 is a block diagram showing a conventional CFAR circuit, and FIG. 6 is an operating waveform diagram of the same circuit. 1...Input terminal, 2...Output terminal, 3...Delay line, 4...Adder, 5...Divider, 6
...Subtractor or divider, 7...Threshold circuit,
8... Threshold input terminal, 9... Minimum value maintenance circuit, 10... Transmission trigger input terminal, 11... Peak detector, 12... Statistics detection circuit.

Claims (1)

[Claims] 1. A statistics detection circuit 12 that detects statistics within a predetermined time of an input echo video signal a.
and the statistics included in the echo video signal by subtracting the statistics signal e output from the statistics detection circuit from the echo video signal or dividing the echo video signal by the statistics signal. In a CFAR (Constant False Alarm Probability) circuit that has a waveform correction circuit 6 that removes a quantity signal component and a threshold circuit 7 that removes a signal component below a threshold value in the output signal f of this waveform correction circuit, It is inserted between the statistics detection circuit and the waveform correction circuit, and sets up the initial value of the statistics signal using the transmission trigger signal e, and changes the value as time elapses during the period until the next transmission trigger signal is input. The present invention is characterized by comprising a minimum value maintaining circuit 9 that maintains and outputs the minimum value in the statistical signal that changes as the time increases.
CFAR circuit. 2. Maintaining the initial value of the output signal d of the minimum value maintenance circuit, attenuating this value with a time constant longer than the predetermined time in the statistics detection circuit, and sending it to the threshold circuit as a threshold signal i. Claim 1, further comprising a peak detector 11.
CFAR circuit.