JP2697627B2 - Correlation processing device and image reproduction processing device for synthetic aperture radar - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a correlation processing device for obtaining a correlation between a predetermined signal and reference data corresponding to the predetermined signal, and in particular, to reduce the amount of data by thinning out the data after the correlation processing. The present invention relates to a correlation processing device and an image reproduction processing device for a synthetic aperture radar using the same.

[0002]

2. Description of the Related Art Correlation processing is used in examining the strength of association between two signals. For example, in the field of radar, resolution is improved by correlating a transmitted signal with an echo signal reflected back from an object. There are various types of radars, and a typical example is a pulse radar. This means that pulsed radio waves are periodically transmitted from the antenna,
The distance to the object is detected based on the time required for the transmitted radio wave to reflect off the object and return. The resolution greatly depends on the pulse width to be transmitted, and the resolution can be increased by shortening the pulse width.

If the pulse width is shortened in order to increase the resolution, it is difficult to receive the returned radio wave due to the attenuation of the energy until the light is reflected back to the object. Therefore, when shortening the pulse width, it is necessary to increase the power of radio waves to be transmitted or to increase the size of the antenna in response to this, which has been a factor of increasing the size of the device and increasing the cost. Therefore, various radar apparatuses using a pulse compression technique for shortening a pulse width in a pseudo manner by using a correlation process have been proposed.

One of the pulse compression techniques is to reduce the pulse width by using an FM modulation pulse (frequency modulation pulse) as a transmission wave and correlating the returned echo signal with a predetermined reference wave. There is a technique to obtain the same effect as described above. In a radar device using this pulse compression technology,
During a period corresponding to one pulse width, an FM modulated pulse whose frequency gradually increases or decreases is transmitted.
Then, the frequency of the returned echo signal is checked, and based on the frequency, it is determined at which timing of the transmission wave the transmitted radio wave has returned as the echo signal. As a result, the same effect as that of reducing the pulse width of the transmission wave by the frequency resolution is obtained.
The frequency of the returned signal is examined by correlating the echo data with the reference data sequence corresponding to the transmitted FM modulated pulse.

In a synthetic aperture radar for detecting the state of the earth's surface by remote sensing from an artificial satellite or an aircraft, a two-dimensional area is detected. For example, two-dimensional detection is performed by moving a satellite equipped with a radar at a constant speed in a predetermined direction and periodically transmitting an FM modulation pulse in a direction perpendicular to this direction. The direction of movement of the satellite is called the azimuth direction, and the direction perpendicular thereto is called the range direction. In such a synthetic aperture radar, a predetermined correlation process is performed on the received echo signal in the range direction and the azimuth direction, and a visualized image is obtained. Further, it is common practice to improve the resolution in the range direction using the FM modulation pulse described above.

A synthetic aperture radar temporarily stores a received echo signal in a memory and performs various arithmetic processes over a sufficient time to obtain an image corresponding to the echo signal. Some are visualized as images in real time. The process of creating a visualized image is performed first in the range direction and then in the azimuth direction. The amount of computation required to create a visualized image based on the echo signal is very large. For this reason, in a device that requires a particularly high speed, such as a quickle device that forms an image in real time, a correlation processing circuit with a thinning function is used to thin out the correlation processing result in the range direction, and the amount of calculation in the azimuth direction is reduced. It is often done to reduce.

FIG. 5 schematically shows a conventional correlation processing circuit having a thinning function. A predetermined reference data sequence 102 according to the FM modulation pulse to be transmitted is input to the correlation circuit 101. The echo signal 103 from which the transmitted FM-modulated pulse is reflected by any object and returned is input to the correlation circuit 101. The correlation circuit 101 obtains a correlation between the reference data sequence 102 and the echo signal 103, and outputs the result as correlation data 104. The thinning circuit 105 thins out the correlation data 104 at a predetermined ratio to reduce the amount of data to be output. Output data 106 output from the thinning circuit 105 is input to a subsequent processing circuit (not shown), and is displayed as an image by performing a predetermined operation there.

As another apparatus for shortening the processing time until an image is obtained, an image reproduction processing apparatus for a synthetic aperture radar in which the processing content to be calculated is simplified and the processing speed is improved is disclosed in Japanese Patent Laid-Open No. 61-13371. Is disclosed. The image reproduction processing apparatus of the synthetic aperture radar performs a fast Fourier transform process omitting the bit inversion process on the image data compressed in the range direction, and performs a correlation operation between the resulting data and a predetermined reference function. It is carried out. Then, fast inverse Fourier transform without bit inversion processing is performed on the result of the correlation processing. The high-speed Fourier transform process is simply performed to replace data in the time domain with data in the frequency domain and to execute the correlation process as a simple multiplication at high speed. Therefore, since the results of the fast Fourier transform aligned in the frequency domain are not required, the processing is speeded up by omitting the bit inversion processing in the fast Fourier transform and the fast inverse Fourier transform.

Further, in an image reproducing apparatus of a synthetic aperture radar, generally, the resolution in the azimuth direction is improved by utilizing the Doppler effect. The distance to the object can be obtained by analyzing the received echo signal in the range direction. However, the position information in the azimuth direction obtained from the received echo signal has a certain width according to the width of the pulse to be transmitted and the moving speed of the synthetic aperture radar in the azimuth direction. Assuming that the object reflecting the transmitted wave is stationary, the distance to the object gradually decreases as the synthetic aperture radar moves in the azimuth direction, and changes to become longer again after approaching the closest. Therefore, utilizing this property, the position at which the object is closest to the object is obtained by calculation to improve the resolution in the azimuth direction.

[0010]

SUMMARY OF THE INVENTION As described above,
In a radar apparatus that thins out the correlation data after performing the correlation processing in the range direction and reduces the amount of calculation thereafter, information included in the received echo signal may be missing due to the thinning. For example, when the correlation between the reference data sequence and the received echo signal is obtained, the effect of the echo signal reflected back to a small object may appear only in one piece of correlation data. In such a case,
If the correlation data is simply decimated, there is a case where the correlation data having the effect is decimated.
When the information about the object is lost due to the thinning, the object disappears from the visualized image, so that there is a problem that an accurate image cannot be reproduced.

When the resolution in the azimuth direction is to be improved by utilizing the Doppler effect, it is necessary to accurately recognize a change in the distance from the object. However, if the data subjected to the correlation processing in the range direction is simply thinned out, the continuity of the change in the distance to the object is destroyed. Therefore, there is a problem that the resolution in the azimuth direction cannot be improved by using the Doppler effect.

On the other hand, in an image reproduction processing apparatus of a synthetic aperture radar disclosed in Japanese Patent Laid-Open No. 61-13371,
Since the speed of the arithmetic processing is increased by a method other than the thinning-out, no information is lost in the reproduced image. However, simplification of the content of operation alone has not been able to increase the processing speed so that a visualized image can be obtained in real time.

It is an object of the present invention to thin out data after correlation processing without losing information included in data obtained as a result of performing correlation processing, and to reduce the amount of data to be processed in subsequent calculations. It is an object of the present invention to provide a correlation processing device that can perform the above.

[0014]

According to the first aspect of the present invention, a high-speed conversion of a reference data sequence in a time domain, which is prepared in advance to obtain a correlation with a predetermined signal, into data in a frequency domain. A Fourier transform circuit, a multiplying circuit for multiplying the data converted by the fast Fourier transform circuit and a predetermined window function for removing a frequency component higher than a certain frequency contained therein, and the multiplying circuit A fast inverse Fourier transform circuit for converting data in the frequency domain obtained as a result of the multiplication by time into data in the time domain; and a correlation between the data converted by the fast inverse Fourier transform circuit and the predetermined signal. A correlation circuit that takes
The data obtained as a result of correlation by this correlation circuit is output from the correlation circuit in the amount of data after the data is thinned out.
The ratio of the amount of data to
Of the window function for the sampling frequency of the road
Equal to or greater than the certain frequency
The correlation processing device is provided with a thinning circuit for thinning at a predetermined ratio.

That is, according to the first aspect of the present invention, the high frequency component of the reference data sequence to be correlated with the signal is removed in advance, and the correlation between the reference data sequence from which the high frequency component has been removed and the signal is obtained. ing. As a result, even if a signal is such that its effect appears only in one correlation result in the correlation with the reference data sequence including a high frequency component, the effect appears in a plurality of correlation results. Therefore, even if such correlation results are decimated, the effect of the signal also appears on the decimated correlation data, so that the data amount after the correlation processing can be reduced without losing the information included in the signal. Can be reduced.

According to the first aspect of the present invention, the predetermined rate of thinning by the thinning circuit is equal to or greater than the ratio of the sampling frequency of the fast Fourier transform circuit to the fixed frequency to be removed by the window function. Is set. For example, when thinning out the data resulting from the correlation processing to a quarter, the window function is set so that a frequency component higher than a quarter of the sampling frequency of the fast Fourier transform circuit is removed. Thereby, when a correlation is obtained with the original reference data sequence, 1
Even for a signal whose effect appears only in one correlation data, the effect appears in four or more correlation data. Therefore, it is possible to surely leave the influence of the signal on the data that has been thinned to one fourth.

According to the second aspect of the present invention, a fast Fourier transform circuit for converting a reference data sequence in a predetermined time domain into data in a frequency domain according to a waveform of a transmission wave transmitted by the synthetic aperture radar, A multiplication circuit for multiplying the data converted by the fast Fourier transform circuit and a predetermined window function for removing a frequency component higher than a certain frequency included in the data, and a result obtained by multiplication by the multiplication circuit A high-speed inverse Fourier transform circuit that converts the obtained data in the frequency domain to data in the time domain, and the data converted by the fast inverse Fourier transform circuit and the transmission wave received by the synthetic aperture radar. A correlation circuit for correlating in the range direction with the echo signal, and a result obtained by correlating in the range direction by the correlation circuit. A thinning circuit for thinning out the data at a predetermined ratio, and an azimuth direction processing circuit for performing predetermined processing in the azimuth direction based on the data thinned out by the thinning circuit are provided in the image reproduction processing device of the synthetic aperture radar. Let me.

That is, according to the second aspect of the invention, in the correlation processing in the range direction of the synthetic aperture radar, the reference data sequence from which the high-frequency component has been removed is used. As a result, the influence of the information included in the echo signal also appears in the data after the correlation processing remaining after the thinning. In addition, since the processing in the azimuth direction is performed on the reduced data after thinning, the processing time in the azimuth direction can be reduced, and a visualized image can be obtained in a short time. In addition, since the information contained in the echo signal is not lost even when the data is thinned, the resolution in the azimuth direction can be improved by using, for example, the Doppler effect.

According to the third aspect of the present invention, the predetermined ratio for thinning out by the thinning-out circuit is set to be equal to or larger than the ratio of the fixed frequency to be removed by the window function to the sampling frequency of the fast Fourier transform circuit. ing. As a result, the influence of the information included in the echo signal can be reliably left in the thinned data.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to embodiments.

FIG. 1 shows an outline of a circuit configuration of a correlation processing apparatus according to an embodiment of the present invention. A reference data sequence 12 is input to the fast Fourier transform circuit 11 as reference data for obtaining a correlation. The reference data sequence 12 is predetermined according to a signal to be correlated, and is a group of digital signals represented by 16 bits. For example, when the pulse transmitted by the synthetic aperture radar is an FM modulation pulse, a reference data string obtained by converting the amplitude of a waveform corresponding to this pulse into a digital signal having a resolution of 16 bits at a predetermined sampling rate is input. . The fast Fourier transform circuit 11 converts the input reference data sequence 12 from data in the time domain to data in the frequency domain. The data converted by the fast Fourier transform circuit 11 is input to the multiplication circuit 13, where it is multiplied by a predetermined window function 14. The window function 14 is preset so as to remove a frequency component higher than a predetermined frequency included in the reference data string converted into the frequency domain. By converting to the frequency domain in this way, it is possible to remove high-frequency components of the reference data sequence by simply performing multiplication with the window function 12.

The output of the multiplication circuit 13 is input to the fast inverse Fourier transform circuit 15. Fast inverse Fourier transform circuit 1
Numeral 5 is for converting data in the frequency domain into data 16 in the time domain. These fast Fourier transform circuits 1
1. The multiplication circuit 13 and the fast inverse Fourier transform circuit 15 convert the reference data string 12 into a signal from which the high-frequency components have been removed. The converted reference data sequence is hereinafter referred to as a thinning-out reference data sequence 16. The thinning-out reference data sequence 16 is a set of 16-bit digital signals, like the original reference data sequence. The thinning-out reference data sequence 16 is input to the correlation circuit 18 as a reference data sequence for correlating with the signal 17, and is set in the correlation circuit as a reference function. If you do not want to change the reference data column,
Once the thinning-out reference data sequence 16 is set in the correlation circuit 18, it need not be changed thereafter.

A signal 17 to be correlated with the reference data sequence 12 is input to the correlation circuit 18. Signal 1
Reference numeral 7 denotes a digital signal having a bit width of 16 bits, which is sequentially input to the correlation circuit 18 at a predetermined cycle. The correlation circuit 18 performs a convolution operation between the set reference data sequence 16 for thinning and the signal 17 that is sequentially input, and outputs correlation data 19 obtained as a result one after another. The correlation data 19 output from the correlation circuit 18 is input to a thinning circuit 21 so that the data is thinned at a predetermined rate. The rate of thinning by the thinning circuit 21 depends on the window function 14 multiplied by the multiplying circuit 13. For example, when thinning out the data after the correlation processing to a quarter of the data amount, the window function 14 of the fast Fourier transform circuit 11 removes a frequency component higher than a quarter of the sampling frequency. The settings are made.

FIG. 2 shows an outline of the circuit configuration of the correlation circuit and the thinning circuit. Correlation circuit 18
Are the first to Nth registers 31 ₁ to 31 3 connected in series
1 _N, and first to _N-th reference registers 32 _{1 to} 32 _N for holding the reference data sequence 16. The signal 17 held in the registers 31 _{1 to} 31 _N and the reference data string 1
6 to the first to Nth multipliers 33 _{1 to} 33 _N
And an addition circuit 34 for calculating the sum of these outputs. In the correlation circuit 18 of the embodiment, N is 500, and the register 31, the reference register 32, and the multiplier 33
Are provided 500 each. Register 31
_A clock signal 35 is input to each of _{1 to} 31 _N and the adder circuit 34. Each time the clock signal 35 is inputted, the contents held in the registers 31 _{1 to} 31 _N are updated. As a result, the clock signal 35
The signal 17 is shifted one stage at a time from left to right in FIG. The signal 17 is sequentially inputted in synchronization with the clock signal 35. Therefore, the signals 17 from the latest input to the last 500 samples are held by the 500 registers 31.

The first to N-th reference data registers 32 ₁ to 32 ₁
32 _N holds thinning-out reference data for 500 samples. For example, one cycle of the FM modulation pulse is divided into 500 at equal intervals, and the values of the reference data sequence 16 for thinning obtained by sampling each of them are sequentially held. . First to Nth multipliers 33
_{1 to} 33 _N multiply the signal 17 held in the registers 31 _{1 to} 31 _N by the reference data held in the reference registers 32 _{1 to} 32 _N , respectively. Each time the signal 17 is input, the adder circuit 34 outputs the first to first N
And outputs the multiplier 33 ₁ ~ 33 _N correlation data 19 taking the sum of the results that have been multiplied by. As described above, the correlation circuit 18 outputs the signal 1 for the latest 500 samples.
Correlation data 19 obtained by correlating 7 with the reference data sequence
Are sequentially output in synchronization with a clock signal.

The thinning circuit 18 comprises a counter 36 for controlling the rate of thinning, and an output register 37 for holding the correlation data 19. The clock signal 35 is input to the counter 36. For example, when the correlation data 19 is thinned to one fourth, 4 is set as the count value of the counter 35. With this setting, the counter 36 outputs the count-over signal 38 every time the clock signal 35 is counted for four clocks. The output register 37 holds the correlation data 19 every time the count-over signal 38 is input. As a result, the data 39 output from the thinning circuit 21 is reduced to a quarter of the correlation data 19.

In the correlation processing apparatus shown in FIG. 1, the thinning-out reference data sequence 16 from which the high-frequency component of the reference data sequence 12 has been removed is generated by the fast Fourier transform circuit 11, the multiplication circuit 13 and the fast Fourier transform circuit 15. . This is used as reference data of the correlation circuit 18. Depending on the input signal 17, when a correlation is obtained with the original reference data sequence 12 including a high frequency component, the effect may appear only on one correlation data 19. Even with such a signal 17, the correlation between the signal 17 and the thinning-out reference data sequence 16 from which high-frequency components have been removed has an effect on a plurality of correlation data 19. As a result, even if the correlation data 19 is decimated, the influence of the signal appears on the decimated correlation data, so that the information contained in the signal is not completely lost due to the decimation. Thereby, the data amount of the correlation data 19 can be reduced without causing a loss of information. Taking a correlation with the reference data sequence from which the high-frequency component has been removed can provide the same effect as applying a blur to an image, for example, by taking a radar image as an example. Therefore, even for a small object, it is possible to obtain a correlation result in which the object spreads to a certain extent.

The rate at which thinning is performed by the thinning circuit 21 is closely related to the high cutoff frequency of the window function 14.
For example, when thinning is performed so as to reduce the correlation data to a quarter, it is necessary to make the influence of the signal 17 appear in four or more correlation data 19. For this reason, when thinning out the data amount so as to reduce it to a quarter, the window function 14 must be set so as to remove a frequency band equal to or more than a quarter of the sampling frequency of the fast Fourier transform circuit 11. Goodbye. That is, it is necessary to remove the high-frequency side to reduce the bandwidth to one-fourth or less.

Next, a case where the correlation processing apparatus is applied to a synthetic aperture radar will be described.

FIG. 3 shows an outline of an image reproduction processing apparatus of a synthetic aperture radar using the correlation processing apparatus shown in FIG. The synthetic aperture radar 41 transmits a pulse wave 42 from an antenna unit (not shown), and receives an echo pulse 43 reflected back from an object. The transmission wave generation circuit 44 is a part that generates the transmission wave 35 as a reference signal of the pulse wave 42 to be transmitted from the antenna unit and the reference data sequence 12 corresponding thereto. The transmission wave generation circuit 44 has a function to reduce the pulse width in a pseudo manner.
An FM modulation pulse is generated as the transmission wave 35. The transmission wave generation circuit 44 includes a ROM (Read Only Memory) (not shown), an address generation circuit, and a digital / analog conversion circuit. Data obtained by quantizing the amplitude of the FM modulation pulse to be output at a predetermined sampling interval is stored in the ROM in advance. These are sequentially read out by an address generation circuit, and the values are converted into analog signals by a digital / analog converter and output. Further, the data stored in the ROM is output as the reference data string 12 as it is.

Echo pulse 4 reflected back from the object
3 is received by the synthetic aperture radar 41 and input to the range direction processing circuit 48 as the echo signal 17. The range direction processing circuit 48 includes a transmission wave generation circuit 44.
Is input. The range direction processing circuit 48 has the same configuration as the correlation processing device shown in FIG. Here, similarly to the correlation processing apparatus shown in FIG.
It is assumed that the thinning rate and the window function are set so as to reduce the number by one. Thereafter, the range direction processing circuit 48
1 will be described based on FIG.
The output 49 of the range direction processing circuit 48 is input to the azimuth direction processing circuit 51. The display device 52 is configured to display a visible image corresponding to the echo signal 17 in real time based on data 53 obtained as a result of performing calculations in the range direction and the azimuth direction.

The range direction processing circuit 48 generates the thinning-out reference data sequence 16 based on the reference data sequence 12 input from the transmission wave generation circuit 44. This is performed by the fast Fourier transform circuit 11, the multiplying circuit 13, and the fast inverse Fourier transform circuit 15, similarly to the correlation processing apparatus shown in FIG. The thinning-out reference data sequence 16 from which the high-frequency component of the original reference data sequence 12 has been removed is set in the correlation circuit 18,
Thereafter, detection by the synthetic aperture radar 41 is started.

The pulse wave 43 transmitted from the synthetic aperture radar 41 and reflected back to some object is successively correlated with the thinning reference data 16 already set in the correlation circuit 18. Thus, two effects can be obtained simultaneously by correlating with the reference data 16 for thinning. One of them is to obtain the same effect as pseudo-shortening the pulse width by correlating with the reference data sequence. The other is to filter the high frequency components of the correlation data 19 according to the thinning rate. By correlating with the reference data sequence from which the high-frequency component has been removed in advance as described above, these two effects can be obtained at the same time, and there is no need to separately perform processing for removing the high-frequency component of the correlation data. Thereby, the processing time in the range direction can be reduced. Thinning circuit 2
1 thins out the correlation data 19 output from the correlation circuit 18 at a preset thinning rate. Accordingly, the data amount of the correlation data 49 output from the range direction processing circuit 48 is reduced by thinning.

The azimuth direction processing circuit 51 performs various operations on the correlation data 49 output from the range direction processing circuit 48. Since the azimuth direction processing circuit performs an operation on the data that has been thinned out by the range direction processing circuit 48, the amount of data to be processed is reduced, and the processing speed can be increased. In the azimuth direction processing circuit 51, for example, processing for improving resolution using the Doppler effect is performed.

FIG. 4 shows the circuit configuration of the azimuth direction processing circuit. The output 49 of the range direction processing circuit is input to the fast Fourier transform circuit 61, and the correlation data in the time domain is converted into data in the frequency domain. The output of the fast Fourier transform circuit 61 is input to the multiplying circuit 62. The multiplication circuit 62 is connected to a multiplication coefficient generation circuit 63 that sequentially generates multiplication coefficients. Multiplication circuit 6
Reference numeral 2 denotes a circuit for multiplying data input from the fast Fourier transform circuit 61 by a predetermined multiplication coefficient input from the multiplication coefficient generation circuit 65. Here, in order to perform processing in the azimuth direction using the Doppler effect,
A multiplication coefficient that forms a Doppler filter is generated. The result of the multiplication by the multiplication circuit 62 is input to the fast inverse Fourier transform circuit 64 and converted into correlation data 65 in the time domain. The power calculation circuit 66 is a circuit that calculates the squares of the values of the real part and the imaginary part of the correlation data 65, adds them, and takes the square root thereof.
That is, the power calculation circuit 66 calculates the absolute value of the phase information included in the correlation data 65 and converts it into image data representing the density of the image. The image data 67 is stored in a predetermined position of the image memory 71 in accordance with the address signal 69 output from the address generation circuit 68. The image memory 71 can store an image for one screen. The image stored in the image memory 71 is read by the display device 52 of FIG. 3 and displayed on a display such as a CRT (not shown). Thus, a visualized image is formed based on the echo signal 17 received by the synthetic aperture radar 41.

The correlation data 49 output from the range direction processing circuit 48 is thinned out and the data amount is reduced. However, since correlation is obtained between the reference data sequence 16 for thinning from which high-frequency components have been removed, the information of the object included in the echo signal 17 is not lost in the correlation data 49 after thinning. Remaining. Therefore, the continuity of the position information of the object is not broken. Therefore, it is possible to improve the resolution by utilizing the Doppler effect in the processing in the azimuth direction. Also, in the range direction processing circuit 48, since the thinning-out reference data 16 only needs to be created once at first, the echo signal 17
During the reception, the fast Fourier transform and the inverse fast Fourier transform, which require a large amount of computation, are not performed. Therefore, processing in the range direction can be performed at high speed. In contrast, FIG.
In the azimuth direction processing circuit 51 shown in (1), fast Fourier transform processing and fast inverse Fourier transform processing must be performed in parallel with the reception of the echo signal 17. Therefore, the azimuth direction processing circuit 51 in the range direction processing circuit 48
Reducing the amount of data to be processed has a significant effect on shortening the processing time for image reproduction.

In the embodiment described above, the correlation data is 4
Although the case of thinning out to reduce it by a factor of 1 was shown,
The rate of thinning is not limited to this. Also, the synthetic aperture radar has been described as an example, but it goes without saying that the apparatus can be applied to any apparatus that needs to reduce the amount of calculation performed on the data after the correlation processing.

[0038]

As described above, according to the first aspect of the present invention, the high-frequency component of the reference data sequence to be correlated is removed in advance, and the reference data sequence from which the high-frequency component has been removed and the predetermined signal are output. Have a correlation between This allows
Even if the signal contains information whose influence is apparent only in one correlation result in the correlation with the original reference data sequence, a plurality of signals can be obtained by correlating with the reference data sequence after removing high-frequency components. The effect will appear on the correlation result. Therefore, the correlation result after thinning out also
Since the influence of the information included in the original signal appears, the amount of data after the correlation processing can be reduced without any loss of information. Further, since special processing for removing the high-frequency component after the correlation processing with the reference data sequence is not required, the processing can be speeded up and the apparatus can be simplified.

Further, according to the first aspect of the present invention, the predetermined ratio for thinning out by the thinning-out circuit is set to be equal to or larger than the ratio of the fixed frequency to be removed by the window function to the sampling frequency of the fast Fourier transform circuit. Have been. For example, when thinning out the data resulting from the correlation processing to a quarter, a window function is set so that a frequency component higher than a quarter of the sampling frequency of the fast Fourier transform circuit is removed. I have. As a result, the influence of the information included in the signal can be reliably left in the data after the thinning.

According to the second aspect of the present invention, in the processing in the range direction of the echo signal received by the synthetic aperture radar, the correlation processing is performed using the reference data sequence from which the high frequency component has been removed. Then, the data after the correlation processing is thinned out at a predetermined ratio. As a result, the influence of the information included in the echo signal also appears in the data after the correlation processing remaining after the thinning. In addition, since the processing in the azimuth direction is performed on the reduced data after the thinning, a two-dimensional image can be visualized at high speed.
In addition, since the information contained in the echo signal is not lost even if the data is thinned, the resolution in the azimuth direction can be improved by using, for example, the Doppler effect.

According to the third aspect of the present invention, the predetermined ratio for thinning out by the thinning-out circuit is equal to or greater than the ratio of the sampling frequency of the fast Fourier transform circuit to the fixed frequency to be eliminated by the window function. It is set large. As a result, the influence of the information included in the echo signal can be reliably left in the thinned data.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an outline of a circuit configuration of a correlation processing apparatus according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a circuit configuration of a correlation circuit and a thinning circuit of the correlation processing apparatus shown in FIG.

FIG. 3 is a block diagram showing an outline of an image reproducing apparatus of a synthetic aperture radar according to one embodiment of the present invention.

FIG. 4 is a block diagram showing an outline of a circuit configuration of an azimuth direction processing circuit of the image reproducing apparatus of the synthetic aperture radar shown in FIG.

FIG. 5 is a block diagram showing an outline of a conventionally used correlation processing circuit with a thinning function.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Fast Fourier transform circuit 12 Reference data sequence 13 Multiplier circuit 14 Window function 15 Fast inverse Fourier transform circuit 16 Decimation reference data sequence 17 Input signal 18 Correlation circuit 19 Correlation data 21 Decimation circuit 41 Synthetic aperture radar 48 Range direction processing circuit 51 Azimuth Direction processing circuit

Continuation of the front page (56) References JP-A-4-223288 (JP, A) JP-A-60-140942 (JP, A) JP-A-60-130908 (JP, A) JP-A-61-260300 (JP, A) JP-A-63-26112 (JP, A) JP-A-63-281511 (JP, A) JP-A-1-10717 (JP, A) JP-A-2-98717 (JP, A) 5-273267 (JP, A)

Claims (3)

(57) [Claims] 1. A fast Fourier transform circuit for converting a reference data sequence in a time domain prepared in advance to obtain a correlation with a predetermined signal into data in a frequency domain, and a transform by the fast Fourier transform circuit A multiplication circuit that multiplies the data after the multiplication by a predetermined window function for removing a frequency component higher than a certain frequency included in the data, and a frequency domain obtained by multiplication by the multiplication circuit. A fast inverse Fourier transform circuit for converting data into data in the time domain; a correlation circuit for correlating the data converted by the fast inverse Fourier transform circuit with the predetermined signal; The data obtained as a result of the sampling is output from the correlation circuit in the amount of data after the data is thinned out.
The ratio of the amount of data to
Of the window function for the sampling frequency of the road
Equal to or greater than the certain frequency
A thinning circuit for thinning at a predetermined ratio. 2. A waveform of a transmission wave transmitted by a synthetic aperture radar.
Of the reference data sequence in a predetermined time domain according to the
Fast Fourier transform circuit to convert to data in wave number domain
And the data converted by this fast Fourier transform circuit.
Frequency components and frequency components higher than a certain frequency
Multiplier for multiplying by a predetermined window function for removing
And the frequency domain obtained as a result of multiplication by this multiplication circuit.
High-speed reverse converter that converts data in the time domain to data in the time domain.
And Rie conversion circuit, de converted by the inverse fast Fourier transform circuit
Data and the transmission received by the synthetic aperture radar
Correlate with the wave echo signal in the range direction
A correlation circuit and the correlation circuit correlates in the range direction.
Thinning out the data obtained as a result at a specified rate
Circuit and the data after being thinned out by this thinning circuit.
Azimuth direction processing that performs predetermined processing in the azimuth direction
Of synthetic aperture radar, characterized by comprising a sense circuit
Image reproduction processing device. 3. The data after thinning by the thinning circuit.
Of the data amount to the data amount output from the correlation circuit
Is the sampling frequency of the fast Fourier transform circuit.
And the ratio of the constant frequency to be removed by the window function
Claims that are higher or larger
Item 3. An image reproduction processing device for a synthetic aperture radar according to Item 2.