JPS593710B2 - Land clutter removal device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for removing clutter, particularly land clutter, in a moving object display radar for detecting and displaying moving objects.

Land clutter is caused by reflections from stationary objects on the ground.

Since land clutter provides great protection when displaying moving objects, various means have been devised to eliminate it.

The most common is to convert the radar's video output to IPRP.
(Pulse Repetition Period)
51g which performs subtraction with the non-delayed signal with a delay of
1e-pulse canceller (for example, F
, E., Nathanson tRadar De
sign principles McGraw
-Hill tl 969 t Chapter -8)
, or an extension of this using multiple pulses m
ulti-pulsecarheailer
It is a clutter reduction method such as

However, as is well known, although land clutter is a reflection from a stationary object, the spectrum in the Doppler frequency range does not become a pure DC component due to the rotational movement of the antenna.

Therefore, when clutter is suppressed, signal components of moving objects having Doppler frequencies that fall within the clutter spectral region are also suppressed.

Therefore, even if the filter is optimally designed, there is a limit to signal detection in the clutter frequency range.

MIT's Muehe was considered to improve this point.
The step-scan antenna proposed by et al. (References: C, E, Muehe > Jr., and
L-Cartledge t“A HighPer
Formance Low Co5t + Air T
raffic Control Radar F
ebruaryl 973 * AD-759179)
It is.

In a step-scan type antenna, the antenna beam scans at a constant azimuth angle for a certain period of time, so that the clutter component in the received video output has a form similar to a DC component.

Therefore, by using a filter that suppresses the DC component, it is possible to ideally remove the rack.

However, this method has drawbacks such as a complicated antenna structure and a limited frequency band that can be applied due to the structure.

One way to remove clutter components is to memorize all the information on the display device, that is, all the signal information equivalent to one rotation of the antenna, and then use the stored information when scanning the next rotation. By taking the azimuthal information that the new skin provides and subtracting it from the information obtained by the new skin, a fairly ideal rack removal is possible.

However, this method requires a huge amount of storage, so it is difficult to put it into practical use unless storage devices become cheaper.

A possible method is to double the antenna system and obtain diversity in the azimuth direction.

Now, by shifting the two antennas A and B by θ radians in azimuth, they are rotated around the same rotation axis at the same angular velocity ω8.
Rotate with .

If the shape of the two antennas and the radar carrier used are the same, the delay time τ due to the azimuth θ can be calculated from these two antennas, excluding the effect of the difference in antenna height above the ground.
It is possible to obtain two similar racks and batons with a time difference of =θ/ω8.

However, the pulse emission time of antenna B, which follows later, is
It is assumed that the pulse emission time of the preceding antenna A is delayed by τ.

Of the two radar reception video outputs corresponding to these two antennas, if the temporally advanced signal is delayed by τ and the other video output is reduced, the clutter component can be ideally canceled out. becomes possible.

A conventional clutter removal method is used in which a single antenna is used to attract the reflected bumps caused by the preceding pulse from those caused by the subsequent pulses, but the pulse repetition period (PRP)
), the land clutter in the reflected batten was not the same and it was difficult to completely eliminate the land clutter.

According to the present invention, reflected waveforms scanned in the same azimuthal direction at different times by two antennas are used.
Although there is the problem of having to duplicate the antenna system,
Ideal eradication of land clutter becomes possible.

An embodiment of a radar transmitter according to the present invention according to the above basic principle is shown in FIG.

In the figure, it is assumed that two antennas A and B are shifted by an azimuth angle θ radian and are rotating around the same rotation axis C at the same angular velocity ω8.

The transmitter of this radar device includes a radar carrier generation circuit 1,
The radar carrier circuit 1 is composed of a pulse generator 2 and a modulation circuit 3, and the output of the radar carrier circuit 1 is modulated into a burst shape by the modulation circuit 3 by the output of the pulse generator 2.

A part of the output of the modulation circuit 3 is supplied to antenna A via a directional coupler 4 and a feeder 6, and the other part is supplied to antenna B via another directional coupler 5 and a feeder 7. has been done.

On the other hand, the radar reflected waves received by these antennas At B travel in reverse through the feeders 6 and 1 and reach the two directional couplers 4 and 5 mentioned above, and then are sent to one of the receivers of this radar. demodulation circuits 8 and 9, respectively, which constitute the section.

In the figure, τ is selected to be an integer multiple of the pulse repetition period in order to simplify the structure as much as possible, but if this is not the case, the output of the modulation circuit 3 is first passed through a delay circuit with a delay τ (not shown). After that, it is necessary to add it to the directional coupler 5.

Various configurations can be considered as the demodulation circuit (for example, the above-mentioned F and E.

However, for the sake of simplicity, single-channel synchronous detection is used here.

Of course, in the case of orthogonal detection that obtains two channels, in-phase and orthogonal, the method of the present invention may be applied to each of the in-phase and orthogonal components.

Furthermore, in the figure, the demodulation circuits 8 and 9 are depicted as directly receiving the carrier wave of the carrier wave circuit 1, but this is based on the principle and various modifications may be used in practice.
Since the demodulation technique itself is not directly related to the gist of the present invention, further explanation will be omitted.

Now, video outputs A' and B' of demodulation circuits 8 and 9
Assuming that antenna A is in front of antenna B by θ radians in the rotational direction, video output A' is temporally ahead of video output B' by 0708 seconds.

Therefore, if the waveform transmission characteristics including the antenna system until obtaining the two video outputs A' and B' can be made almost the same, then after the video output A' is delayed by τ by the delay circuit 10, the video output B' If it is reduced, it is possible to almost completely cancel out land and clutter, which are reflected waveforms from stationary objects.

Reference numeral 11 in FIG. 1 is a subtraction circuit for performing this subtraction.

On the other hand, the reflected waveform from a moving object has a Doppler shift that corresponds to the moving speed of the moving object in the radial direction, so
The video output is a sinusoidal oscillation with a Doppler frequency fd corresponding to the Doppler shift.

The phase difference of this sinusoidal vibration in video outputs A' and B' is 2πfdτ radians, so 2πfdτ = 2πk (
(k is an integer), this Doppler frequency signal is not canceled by the attenuation circuit 11.

On the other hand, since the reflected wave from the moving object is also canceled at the frequency of fd/τ, the amplitude transfer characteristic for the reflected wave from the moving object as seen from the output of the subtraction circuit 11 is based on the above-mentioned configuration. As far as it is concerned, the zero point is located at an integral multiple of 1/τ in the Doppler frequency domain, as shown by the solid line in FIG.

This zero point can be saved to some extent by the well-known stagger operation.

The amplitude transfer characteristic in this case is as shown by the dotted line in FIG.

In order to perform the stagger operation, it is necessary to make the repetition period of the sending pulse variable, and the pulse generator 2
It is necessary to add a pulse repetition period variable function to the method, but since the technology itself is well known, further explanation will be omitted.

Now, the output of the subtraction circuit 11 shown in FIG. 1 is then further subjected to some signal processing necessary for display, and then applied to the display device.

→If the signal indicates , the output of the subtraction circuit 11 is applied to, for example, a squaring circuit 12 and converted into power information, and then applied to a variable threshold circuit 13.

The variable threshold circuit is necessary because the received reflected power of the radar becomes weaker as the distance increases, so it is necessary to change the optimal threshold for signal detection.

The output of the variable threshold circuit 13 is sent to a display device and displayed.

As described above, the basic principle of the device of this invention is to rotate two antennas at the same angular velocity around the same rotation axis while shifting the two antennas by a certain angle in the azimuth direction. The purpose of this method is to remove land clutter components by delaying the waveform that is ahead in time and reducing it compared to the waveform that is behind the time.

Since two antennas are used, in addition to reflected signals, there is a concern about direct coupling between the antennas due to beam wraparound, but this problem can be ignored if the times of the pulse bursts sent from the two antennas are matched. can.

In order to implement this, it goes without saying that the output of the modulation circuit 3 may be branched and fed to two antennas, as shown in the embodiment of FIG.

When the times of the pulse bursts sent out from the two antennas are made to match, if τ=θ/ω8 is an integral multiple of PRP, reflected waves from stationary objects can be ideally canceled.

Conversely, if τ deviates from an integral multiple of PRP, this cancellation operation will be incomplete.

Next, another embodiment based on the basic principle of this invention will be described.

FIG. 3 should replace the delay circuit 10 and subtraction circuit 11 in FIG.

As described above, in the configuration shown in FIG. 1, many zero points occur on the amplitude transfer characteristic, as shown by the solid line in FIG. 2.

As mentioned above, this zero point can be removed to some extent by the stagger operation, but if the configuration shown in FIG. 3 is used, this zero point can be actively removed.

Reference numeral 30 in FIG. 3 is a delay circuit, 31 is a low-pass characteristic device having a low-pass characteristic for a sample sequence having a period of PRP, and 32 is a subtraction circuit corresponding to 11 in FIG.

Since the purpose of the present invention is to remove land clutter, signal processing for removing clutter may be performed on a spectral region where clutter exists.

Figure 3 is based on this idea, and shows the demodulation circuit 3 in Figure 1.
The video output A' is delayed by τ' seconds in the delay circuit 30 and then applied to the low-pass F wave generator 31.

The output of this low-pass P wave generator is further applied to a subtraction circuit 31, and subtraction is performed between it and the video output B' of the demodulation circuit 9 in FIG.

Since the low-pass P wave generator 31 must be designed so as not to cause waveform distortion to the clutter component, the amplitude characteristic in the clutter spectrum region is 1, the phase characteristic is a linear characteristic proportional to the frequency, and the clutter component has a linear characteristic proportional to the frequency. It is necessary to have amplitude characteristics that are sufficiently attenuated outside the spectral region.

Various configurations are possible for such an F-wave device, but
-fll is shown in Figure 4a.

The figure shows what is called a transversal filter, −
The delay time per section is equal to IPRP and consists of a tapped delay line 40, a weighting circuit 41 that weights each tap output, and an adder circuit 42 that adds the outputs of the weighting circuits.

It goes without saying that a linear phase characteristic can be obtained by making the weighted tapped delay line of the weighting circuit symmetrical about the center.

If the delay time obtained from the linear phase characteristic of the low-pass F-wave device 31 is τ'', then the delay time τ of the delay circuit 10 in FIG. 1, the delay time τ' of the delay circuit 31 in FIG. 3, and In contrast, the relationship τ=τ′+τ” is required.

An example of the amplitude characteristics of this low-frequency door transducer is shown in Figure 4.

Now, when the circuit configuration of FIG. 3 is used as a part of the configuration of FIG. 1, the amplitude transfer characteristic in the Doppler frequency domain of the output of the subtraction circuit 32 is as shown in FIG. 4C.

The zero point on the transfer characteristic seen in FIG. 2 disappears.

Note that Figure 4C shows the transfer characteristic for a signal with a Doppler shift from a moving object, and for a reflected signal from a stationary object that does not have a Doppler shift, the transfer characteristic becomes zero, and the subtraction in Figure 3 It will be clear from the foregoing description that nothing appears at the output of circuit 32.

In addition, if the zero point 1/τ closest to the zero frequency in the amplitude characteristic of the solid line in Figure 2 falls into the clutter spectrum region and falls into the passband of the mud wave characteristic in Figure 4, this zero point is It cannot be removed.

Therefore, in order to obtain the characteristics shown in FIG. 4C, it is desirable to select τ such that 1/τ is outside the clutter spectral region.

Finally, another embodiment based on the basic principle of this invention will be described with reference to FIG.

The circuit configuration of FIG. 5 should be replaced with circuits 10 to 12 of FIG. 1, and partially includes the circuit configuration of FIG. 3.

That is, in FIG. 5, two video inputs A' and B', a delay circuit 50, a low-pass P-wave unit 51, and a subtraction circuit 52 are respectively A' and B' and a delay circuit in FIG. 30, low-pass P-wave device 31 and subtraction circuit 3
2, but the remaining parts are new additions.

A feature of the circuit configuration shown in FIG. 5 is that the high-frequency signal component cut off by the low-pass P-wave unit 31 of the circuit configuration shown in FIG. 3 is effectively used.

A part of the output of the delay circuit 50 shown in FIG. can be easily obtained by subtracting the output of the low-pass p-wave converter 51 from the output of .

The outputs of the subtracting circuit 52 and the second subtracting circuit 54 described above are squared by two squaring circuits 55 and 56, respectively.
If the two are added together in the adder circuit 57, the power of the high-frequency signal component increases by up to 4 times, whereas the noise component only increases by at most 2 times, assuming that there is no correlation with each other. The signal-to-noise ratio can be expected to be improved by up to twice (=2 dB) compared to the circuit configuration.

In the above explanation, the discussion was based on the assumption that the same power is supplied to the two antenna systems, but the power on the side of antenna A used for erasing can be made smaller than that on the side of antenna B, as long as it does not interfere with the display. It is also possible to take

In this case, gain adjustment is required to match both levels upon erasure, but the basic idea of the invention does not change even in this case.

Further, in the above description, it is assumed that the same carrier wave is used for the two antennas, but it is also possible to use carrier waves whose frequencies are slightly shifted to operate the system with frequency diversity.

If such a method is used, the zero points of the amplitude transfer characteristic shown by the solid line in FIG. 2 can be removed to some extent without using a stagger operation.

In summary, by using the device of the present invention, land clutter, which is a signal reflected from a stationary object, can be almost completely eliminated by duplicating part of the antenna system and the circuit configuration of the transmitting/receiving section, and the land clutter, which is a reflected signal from a stationary object, can be almost completely eliminated. It is possible to provide a system that has sufficient transmission characteristics for reflected signals from.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment based on the basic principle of the present invention. In the figure, 1 is a carrier wave generation circuit, 2 is a pulse generator, 3 is a modulation circuit, 4 and 5 are directional couplings, A and B are two antenna systems, 8 and 9 are demodulation circuits, 10 is a delay circuit, 11 1 is a subtraction circuit, and 12 is a squaring circuit. Figure 2 is a diagram showing the characteristics of the device configuration in Figure 1;
The figure is a diagram for showing another embodiment based on the basic principle of the invention. The circuit configuration shown in FIG. 3 is used in place of parts 10 and 11 in FIG. 1, and 30 is a delay circuit, 31 is a low-pass F-wave device, and 32 is a subtraction circuit. 4a and 4b are the low-pass P-wave filters 3 of FIG. 3, respectively.
1 and an example of its transfer characteristic. Further, FIG. 4C is a diagram for explaining the characteristics of other embodiments including FIG. 3 obtained using such a low-pass P-wave device. FIG. 5 is a diagram for explaining another embodiment of the present invention, and is used in place of 10 to 12 in FIG. In the figure, 50 and 53 are delay circuits, 51 is a reduced pass filter, 52 and 54 are subtraction circuits, 55 and 56 are square circuits, and 57 is an addition circuit.

Claims (1)

[Scope of Claims] 1. Two antennas having the same shape and the same rotational speed installed on the same rotational axis at a certain azimuth angle, means for supplying radar transmission signals to these antennas, and these antennas. means for demodulating a video signal from a radar reflected signal received by the receiver; and means for delaying a temporally advanced video signal among the demodulated video signals by the time required for the antenna to rotate through the fixed azimuth angle. 1. A land clutter removal device comprising: subtracting means for outputting the difference between the temporally delayed video signal and the delayed video signal. 2. Two antennas with the same shape and rotational speed installed on the same rotational axis with a fixed azimuth angle difference, means for supplying radar transmission signals to these antennas, and radar reflections received by these antenna systems. Means for demodulating the video signal from the signal; means for delaying the temporally advanced video signal of the demodulated video signal by a predetermined first delay time; a low-pass filter that passes the band frequency component and has a second delay time obtained from the phase characteristic, and outputs the difference between the temporally delayed video signal and the output video signal of the low-pass filter. subtracting means, wherein the sum of the first and second delay times is set equal to the time required for the antenna to rotate through the constant azimuth angle. 3. Two antennas with the same shape and rotation speed installed on the same rotational axis with a fixed azimuth angle difference, means for supplying radar transmission signals to these antennas, and radar reflections received by these antennas. means for demodulating the video signal from the signal; first delay means for delaying the temporally advanced video signal of the demodulated video signal by a predetermined first delay time; , a low pass filter that passes a predetermined low frequency component and has a second delay time obtained from the phase characteristic; and a second delay time that delays the output of the first delay means by the second delay time. delay means; first subtraction means for outputting a difference between the temporally delayed video signal and the low-pass filter output video signal; means for outputting a difference between the output video signal and the output video signal; and means for outputting a signal corresponding to the sum of the outputs of the first and second subtraction means, the first and second delays A land clutter removal device characterized in that the sum of times is set equal to the time required for the antenna to rotate through the constant azimuth angle.