JP6687297B1 - Radar signal processing device and radar signal processing method - Google Patents

Abstract

By repeatedly radiating the transmission pulse from each of the plurality of apertures in the antenna unit (1) having a plurality of apertures in the azimuth direction at unequal time intervals, the transmission pulse reflected by the target as a reception pulse, A beam forming unit (3) that forms a plurality of beams by using the reception pulse received by each of the plurality of apertures and formed by the beam forming unit (3) An equal interval part (5) for equalizing the time intervals of a plurality of reception pulses included in each of a plurality of beams, and a plurality of receptions whose time intervals are equalized by the equal interval part (5). Each of the beams including the pulses is converted into a Doppler frequency domain signal, and a plurality of Doppler frequency domain signals are output, and a plurality of Doppler frequency domain output signals are output. Signal combiner (7) for combining signals in the Raler frequency domain and outputting a combined signal of a plurality of signals in the Doppler frequency domain, and converting the combined signal output from the signal combiner (7) into a signal in the time domain The radar signal processing device is configured to include the inverse conversion unit (8).

Description

  The present invention relates to a radar signal processing device and a radar signal processing method for forming a plurality of beams using reception pulses received by a plurality of apertures in an antenna section.

The following Non-Patent Document 1 discloses a Synthetic Aperture Radar (SAR) signal processing device mounted on a radar device.
The SAR signal processing device disclosed in Non-Patent Document 1 includes an antenna having a plurality of openings in the azimuth direction which is the traveling direction of the radar device.
The SAR signal processing device disclosed in Non-Patent Document 1 repeatedly emits a transmission pulse from each of a plurality of apertures at equal time intervals and receives a transmission pulse reflected by a target when the radar device is moving. Received as a pulse at each of the plurality of apertures.
The SAR signal processing device disclosed in Non-Patent Document 1 converts a received signal for a received pulse received at each of a plurality of apertures into a Doppler frequency domain signal.
Then, the SAR signal processing device disclosed in Non-Patent Document 1 performs a reconstruction process that cancels out ambiguity, which is the folding of the signal generated in the received signal.

G. Krieger, N. Gebert, and A. Moreira, “Unambiguous SAR Signal Reconstruction From Nonuniform Displaced Phase Center Sampling”, IEEE Trans. Geosci. Remote Sensing, 2004

  A method using an antenna having a plurality of openings in the azimuth direction, which is the traveling direction of the radar device, is known in order to realize both high resolution and wide area observation in the SAR image (Non-Patent Document 1). On the other hand, as a method that contributes to wide area observation, a stagger trigger method is known in which a transmission pulse is repeatedly emitted at unequal intervals and a transmission pulse reflected by a target is repeatedly received as a reception pulse. By using an antenna having a plurality of apertures and transmitting and receiving pulses by the staggered trigger method, it is expected to realize a wider area observation than the method disclosed in Non-Patent Document 1. However, since the SAR signal processing device disclosed in Non-Patent Document 1 is based on the premise that the transmission pulse is repeatedly emitted at equal time intervals, there is a problem that the staggered trigger observation cannot be applied.

  The present invention has been made to solve the above-mentioned problems, and makes it possible to apply staggered-triggered observations and repeatedly emits transmission pulses at equal time intervals in a SAR having a plurality of openings in the azimuth direction. It is an object of the present invention to obtain a radar signal processing device and a radar signal processing method that can contribute to wide-area observation of SAR images rather than the case.

In the radar signal processing device according to the present invention, the transmission pulse is repeatedly radiated from each of the plurality of openings in the antenna section having a plurality of openings in the azimuth direction at unequal time intervals, and the radiation is repeatedly radiated at the unequal time intervals. It is reflected to the transmission pulse the target that is, using the received pulses received in each of the reflected transmitted pulses a plurality of openings, and a beam forming unit for forming a plurality of beams, which is formed by the beam forming section After reducing the frequency of each Doppler frequency in the plurality of beams, and equalizing the time intervals of the plurality of received pulses included in each of the plurality of beams whose Doppler frequency is reduced by the frequency reducing unit , Each Doppler frequency in multiple beams containing multiple received pulses with equal time intervals Back to the Doppler frequency before being reduced by the number reducing unit, and equispaced unit for outputting a plurality of beams that returned the Doppler frequency, a plurality of output by equispaced portions beams each beam Doppler frequency domain A signal that converts a signal and outputs multiple Doppler frequency domain signals, and a signal that combines the multiple Doppler frequency domain signals output from the conversion section and outputs a composite signal of multiple Doppler frequency domain signals A combination unit and an inverse conversion unit that converts the combined signal output from the signal combination unit into a signal in the time domain are provided.

  According to the present invention, in the SAR having a plurality of apertures in the azimuth direction, it is possible to contribute to wide area observation of the SAR image, compared to the case where the transmission pulse is repeatedly emitted at equal time intervals.

1 is a configuration diagram showing a radar signal processing device according to a first embodiment. 3 is a hardware configuration diagram showing hardware of a signal processing unit 2 in the radar signal processing device according to the first embodiment. FIG. It is a hardware block diagram of a computer in case the signal processing part 2 of a radar signal processing apparatus is implement | achieved by software or firmware. 6 is a flowchart showing a radar signal processing method which is a processing procedure of the signal processing unit 2. FIG. 3 is an explanatory diagram showing an example of a plurality of openings included in an antenna unit 1 mounted on a moving body and a plurality of beams formed by a beam forming unit 3. FIG. 3 is an explanatory diagram showing an example of a transmission pulse repeatedly transmitted from the antenna unit 1 at unequal time intervals and a reception pulse repeatedly received by the antenna unit 1 at unequal time intervals. It is explanatory drawing which shows an example of the k-th beam with which the angle of the directivity direction formed by the beam forming part 3 is (theta) _k = 0. It is explanatory drawing which shows an example of the kth beam in which the angle of the directivity direction formed by the beam forming part 3 is θ _k ≠ 0. It is explanatory drawing which shows the beam formed by the beam forming part 3 implementing DBF of an elevation direction. It is explanatory drawing which shows the reduction process of the Doppler frequency of the beam by the frequency reduction part 4. FIG. 6 is an explanatory diagram showing an example of a plurality of reception pulses included in a beam after Doppler frequency reduction output from the frequency reduction unit 4. It is explanatory drawing which shows an example of the approximate curve calculated | required from the some received pulse contained in the beam after Doppler frequency reduction. FIG. 7 is an explanatory diagram showing an example of a plurality of received pulses whose time intervals are made equal by an equalizing section 5. When transmission pulses are repeatedly emitted from the plurality of apertures at unequal time intervals, each transmission pulse is reflected by one target, and a plurality of reception pulses are received by each of the plurality of apertures, either It is explanatory drawing which shows an example of the phase of the received signal containing the some received pulse received with one aperture. It is explanatory drawing which shows an example of the phase of the beam formed from the received pulse received by the beam forming part 3 by several apertures. It is explanatory drawing which shows an example of the phase of the beam in which the Doppler frequency was reduced by the frequency reduction part 4. FIG. 6 is an explanatory diagram showing an example of a signal obtained by transforming the synthesized signal output from the signal synthesis unit 7 into the time domain by the inverse transformation unit 8. It is explanatory drawing which shows the signal of the Doppler frequency domain in the beam shown in FIG. It is explanatory drawing which shows the signal of the Doppler frequency domain in the beam shown in FIG. 15 formed of the beam forming part 3. It is explanatory drawing which shows the signal of the Doppler frequency region in the beam which the Doppler frequency was reduced by the frequency reduction part 4 in FIG. 7 is an explanatory diagram showing a combined signal by the signal combining unit 7. FIG. It is explanatory drawing which shows the reduction process of the Doppler frequency of the beam by the frequency reduction part 4. It is explanatory drawing which shows the reduction process of the Doppler frequency of the beam by the frequency reduction part 4.

  Hereinafter, in order to describe the present invention in more detail, embodiments for carrying out the present invention will be described with reference to the accompanying drawings.

Embodiment 1.
FIG. 1 is a configuration diagram showing a radar signal processing device according to the first embodiment.
FIG. 2 is a hardware configuration diagram showing hardware of the signal processing unit 2 in the radar signal processing device according to the first embodiment.
In FIG. 1, the antenna unit 1 has a plurality of openings in each of the azimuth direction and the elevation direction.
The antenna unit 1 repeatedly radiates a transmission pulse from each of the plurality of openings at unequal time intervals, and repeatedly receives each of the transmission pulses reflected by the target as a reception pulse at each of the plurality of openings.
The antenna unit 1 includes an analog-digital converter (hereinafter referred to as “A / D converter”), and converts a reception signal for each reception pulse received at each of a plurality of apertures from an analog signal to a digital signal. Convert.
The antenna unit 1 outputs the digital received signal to the beam forming unit 3 of the signal processing unit 2.

The signal processing unit 2 includes a beam forming unit 3, a frequency reduction unit 4, an equal spacing unit 5, a conversion unit 6, a signal combination unit 7, an inverse conversion unit 8 and an image reproduction unit 9.
The signal processing unit 2 reproduces the SAR image by performing various kinds of signal processing on the digital received signal output from the antenna unit 1.

The beam forming unit 3 is realized by, for example, the beam forming circuit 11 shown in FIG.
The beam forming unit 3 forms a plurality of beams by using the reception signals of the respective reception pulses received by the plurality of apertures in the antenna unit 1.
The beam forming unit 3 outputs each of the formed beams to the frequency reducing unit 4.

The frequency reduction unit 4 is realized by, for example, the frequency reduction circuit 12 shown in FIG.
The frequency reduction unit 4 reduces each Doppler frequency in the plurality of beams formed by the beam forming unit 3.
The frequency reduction unit 4 outputs each of the plurality of beams after the Doppler frequency reduction to the equal spacing unit 5.

The equalizing section 5 is realized by, for example, the equalizing circuit 13 shown in FIG.
The equalization unit 5 equalizes the time intervals of the plurality of received pulses included in each of the plurality of beams whose Doppler frequencies have been reduced by the frequency reduction unit 4.
The equalizing section 5 restores the respective Doppler frequencies in a plurality of beams including a plurality of received pulses whose time intervals are equalized to the Doppler frequency before being reduced by the frequency reducing section 4 to set the Doppler frequency. Each of the returned multiple beams is output to the conversion unit 6.

The conversion unit 6 is realized by, for example, the conversion circuit 14 shown in FIG.
The conversion unit 6 converts each beam whose Doppler frequency has been returned by the equalization unit 5 into a Doppler frequency domain signal.
The conversion unit 6 outputs each of the signals in the Doppler frequency domain to the signal synthesis unit 7.

The signal composition unit 7 is realized by, for example, the signal composition circuit 15 shown in FIG.
The signal combiner 7 combines the signals in the Doppler frequency domain output from the converter 6, and outputs the combined signal of the signals in the Doppler frequency domain to the inverse transformer 8.

The inverse conversion unit 8 is realized by, for example, the inverse conversion circuit 16 shown in FIG.
The inverse transforming unit 8 transforms the combined signal output from the signal synthesizing unit 7 into a time domain signal, and outputs the time domain signal to the image reproducing unit 9.

The image reproduction unit 9 is realized by, for example, the image reproduction circuit 17 shown in FIG.
The image reproduction unit 9 reproduces the SAR image from the time domain signal output from the inverse conversion unit 8.

In FIG. 1, the beam forming unit 3, the frequency reduction unit 4, the equalization unit 5, the conversion unit 6, the signal synthesis unit 7, the inverse conversion unit 8 and the image reproduction, which are the components of the signal processing unit 2 in the radar signal processing device. It is assumed that each of the units 9 is realized by dedicated hardware as shown in FIG. That is, the signal processing unit 2 in the radar signal processing device is realized by the beam forming circuit 11, the frequency reduction circuit 12, the equalization circuit 13, the conversion circuit 14, the signal synthesis circuit 15, the inverse conversion circuit 16, and the image reproduction circuit 17. I am assuming something.
Here, each of the beam forming circuit 11, the frequency reduction circuit 12, the equal interval circuit 13, the conversion circuit 14, the signal synthesis circuit 15, the inverse conversion circuit 16, and the image reproduction circuit 17 is, for example, a single circuit, a composite circuit, A programmed processor, a parallel programmed processor, an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or a combination thereof is applicable.

The components of the signal processing unit 2 in the radar signal processing device are not limited to those realized by dedicated hardware, but the signal processing unit 2 is realized by software, firmware, or a combination of software and firmware. It may be one.
The software or firmware is stored in the memory of the computer as a program. The computer means hardware that executes a program, and corresponds to, for example, a CPU (Central Processing Unit), a central processing unit, a processing unit, an arithmetic unit, a microprocessor, a microcomputer, a processor, or a DSP (Digital Signal Processor). To do.

FIG. 3 is a hardware configuration diagram of a computer when the signal processing unit 2 of the radar signal processing device is realized by software, firmware, or the like.
When the signal processing unit 2 of the radar signal processing device is realized by software, firmware, or the like, the beam forming unit 3, the frequency reduction unit 4, the equal spacing unit 5, the conversion unit 6, the signal synthesis unit 7, the inverse conversion unit 8 A program for causing a computer to execute the processing procedure of the image reproducing unit 9 is stored in the memory 21. Then, the processor 22 of the computer executes the program stored in the memory 21.
FIG. 4 is a flowchart showing a radar signal processing method which is a processing procedure of the signal processing unit 2.

  2 shows an example in which each of the components of the signal processing unit 2 in the radar signal processing device is realized by dedicated hardware, and in FIG. 3, the signal processing unit 2 in the radar signal processing device is software or firmware. An example realized by the above is shown. However, this is merely an example, and some of the constituent elements in the signal processing unit 2 may be realized by dedicated hardware and the remaining constituent elements may be realized by software or firmware.

Next, the operation of the radar signal processing device shown in FIG. 1 will be described.
The radar signal processing device shown in FIG. 1 is mounted on a moving body such as an artificial satellite or an airplane, and moves as the moving body moves.
In the radar signal processing device shown in FIG. 1, it is assumed that the moving body is moving at a uniform speed at a velocity V _plt .

The antenna unit 1 has a plurality of openings in each of the azimuth direction and the elevation direction.
FIG. 5 is an explanatory diagram showing an example of a plurality of openings included in the antenna unit 1 mounted on a moving body and a plurality of beams formed by the beam forming unit 3.
The plurality of beams formed by the beam forming unit 3 are beams formed in a virtual space used for calculation in the signal processing unit 2, and are not beams formed in a real space.
The azimuth direction coincides with the moving direction x of the moving body. The elevation direction is the direction indicated by the arrow EL in FIGS. 5 and 9. The elevation direction is a direction orthogonal to the moving direction x.
In FIG. 5, the y-axis direction is the ground range direction, and the z-axis direction is a direction orthogonal to the moving direction x and the y-axis direction of the moving body.
In the example of FIG. 5, the antenna unit 1 has two openings in the azimuth direction and two openings in the elevation direction.
The beam forming unit 3 performs digital beam forming (DBF: Digital Beam Forming) by using digital reception signals of the reception pulses received by the plurality of apertures, respectively, so that K (K is 2 or more). A number of beams (which is an integer) can be formed. The beam forming unit 3 can form a beam having a narrower beam width as the number of openings of the antenna unit 1 increases. In the example of FIG. 5, two beams are formed by the beam forming unit 3. However, this is only an example, and three or more beams may be formed.

Since the radar signal processing device shown in FIG. 1 applies the staggered trigger observation, the antenna unit 1 repeatedly radiates a transmission pulse from each of the plurality of apertures toward the target at unequal time intervals.
After radiating each transmission pulse toward the target, the antenna unit 1 repeatedly receives each transmission pulse reflected by the target as a reception pulse at each of the plurality of apertures.

FIG. 6 is an explanatory diagram showing an example of a transmission pulse repeatedly transmitted from the antenna unit 1 at unequal time intervals and a reception pulse repeatedly received by the antenna unit 1 at unequal time intervals.
In FIG. 6, PRI (Pulse Repetition Interval) is a pulse repetition period. In the staggered trigger observation, the PRI changes as the azimuth time elapses.
In the radar signal processing device shown in FIG. 1, since the antenna unit 1 has a plurality of openings in the elevation direction, even if the time widths of the azimuth times at the time of receiving two received pulses are partially overlapped, The two received pulses can be separated. Therefore, the radar signal processing device shown in FIG. 1 can observe the received pulse even if the time width of the received pulse is wider than PRI. Since the process itself for separating the two received pulses is a known technique, detailed description thereof will be omitted.
It is known that in the staggered trigger type observation using an antenna having a plurality of openings in the elevation direction, it is possible to perform wide area observation while maintaining high resolution without lowering the pulse repetition frequency PRF (Pulse Repetition Frequency). There is.

The antenna unit 1 uses an internal A / D converter to convert a received signal for each received pulse received at each of the plurality of apertures from an analog signal to a digital signal.
The antenna unit 1 outputs the digital received signal to the beam forming unit 3 of the signal processing unit 2.

The beam forming unit 3 forms a plurality of beams, as shown in FIG. 5, by using the digital reception signals of the respective reception pulses received by the plurality of apertures in the antenna unit 1 (see FIG. 4). Step ST1).
The beam forming unit 3 outputs each of the formed beams to the frequency reducing unit 4.

The process of forming a plurality of beams by the beam forming unit 3 will be specifically described below.
The number of openings that the antenna unit 1 has in the azimuth direction is N (N is an integer of 2 or more), and the intervals between adjacent openings in the N openings are constant. Is.
Further, the number of openings that the antenna unit 1 has in the elevation direction is M (M is an integer of 2 or more), and the spacing between the openings adjacent to each other in the M openings. Is constant.
The beam forming unit 3 can form a plurality of beams by performing DBF, for example, by using digital reception signals of reception pulses received by (N × M) apertures.

式（１）において、τは、レンジ時刻、ηは、アジマス時刻、λは、受信パルスの波長、ｄは、アジマス方向に並んでいるＮ個の開口における、互いに隣り合う開口間のそれぞれの間隔である。The beam forming unit 3 can form K (K is an integer of 2 or more) beams by performing, for example, DBF in the azimuth direction using the digital received signal. The number of beams that can be formed by the beam forming unit 3 is not limited by the number of apertures included in the antenna unit 1. However, the beam forming unit 3 can form a beam having a narrow beam width as the number of apertures increases.
Of the K beams, the k-th (k = 1, ..., K) -th beam has a directivity of θ _k at an angle from the direction perpendicular to the elevation direction and the azimuth direction, and N It is assumed that the digital received signal of the received pulse received by the n-th (n = 1, ..., N) -th opening of the above is s _n (τ, η).
At this time, the kth beam s _k (τ, η) has the angle θ _{k in} the pointing direction of the kth beam and the received signal s _n (τ, η) as shown in the following equation (1). ) And.

In equation (1), τ is the range time, η is the azimuth time, λ is the wavelength of the received pulse, and d is the distance between the adjacent openings of the N openings arranged in the azimuth direction. Is.

FIG. 7 is an explanatory diagram showing an example of the kth beam formed by the beam forming unit 3 in which the angle of the directivity direction is θ _k = 0.
FIG. 8 is an explanatory diagram showing an example of the kth beam formed by the beam forming unit 3 in which the angle of the directivity direction is θ _k ≠ 0.
In FIGS. 7 and 8, two openings are arranged in the azimuth direction. For convenience of description, the left opening is referred to as a first opening and the right opening is referred to as a second opening. .
The beam forming unit 3 can form a beam in an arbitrary direction of the virtual space by performing DBF in the azimuth direction, as shown in FIGS. 7 and 8.
Among the beams formed in the virtual space by the beam forming unit 3, the beam shown in FIG. 7 is a beam that is oriented so that the angle of the orientation direction is θ _k = 0, and the beam shown in FIG. The beam is oriented so that the angle of direction is θ _k ≠ 0.
7 and 8 show a first beam containing a plurality of received pulses received by a first aperture and a second beam containing a plurality of received pulses received by a second aperture. There is.
The beam formed by the beam forming unit 3 has a narrower beam width than the first and second beams.

The beam forming unit 3 can form a beam in an arbitrary direction of the virtual space even in the ground range direction y by performing the DBF in the elevation direction using the digital received signal.
FIG. 9 is an explanatory diagram showing a beam formed by the beam forming unit 3 performing DBF in the elevation direction.
In FIG. 9, two openings are arranged side by side in the elevation direction. For convenience of description, in the figure, the lower left opening is referred to as a third opening, and the upper right opening is referred to as a fourth opening. .
FIG. 9 shows a third beam including a plurality of received pulses received by the third aperture and a fourth beam including a plurality of received pulses received by the fourth aperture.
The beam formed by the beam forming unit 3 is a beam directed in an arbitrary direction of the virtual space and has a narrower beam width than the third and fourth beams.

As described above, the beam forming unit 3 forms the plurality of beams by performing the DBF in the azimuth direction and the DBF in the elevation direction using the digital received signal.
However, in the following description, for simplification of description, it is assumed that the beam forming unit 3 performs only DBF in the azimuth direction to form K beams.
The beam forming unit 3 outputs each of the formed K beams _sk (τ, η) (k = 1, ..., K) to the frequency reducing unit 4.

In order for the image reproducing unit 9 in the subsequent stage to reproduce the SAR image, it is necessary to combine the plurality of beams formed by the beam forming unit 3.
As a method of combining the plurality of beams formed by the beam forming unit 3, a method of converting each beam into a signal in the Doppler frequency domain and combining the signals in the plurality of Doppler frequency domains can be used.
As a method of converting each beam into a signal in the Doppler frequency domain, a method of performing discrete Fourier transform of each beam in the azimuth direction can be used. However, the discrete Fourier transform in the azimuth direction cannot be easily performed when the time intervals of the plurality of received pulses included in the beam are unequal.

  The radar signal processing apparatus shown in FIG. 1 includes an equal interval unit 5 that equalizes the time intervals of a plurality of received pulses included in each of a plurality of beams. The equalization of the time intervals of the plurality of received pulses can be realized by performing pulse interpolation processing, for example. However, when the Doppler frequency of the beam is high, it is difficult to accurately interpolate a new pulse because the phase rotation of the beam is fast, and the equal interval unit 5 post-processes the pulse interpolation process. It may not be possible to carry out with the accuracy required for. When the Doppler frequency of the beam is low, the phase rotation of the beam is slow, so that a new pulse can be interpolated with high accuracy, and the equal interval unit 5 performs the pulse interpolation processing with the accuracy required for post-processing. It is likely to be possible.

Upon receiving the K beams _sk (τ, η) from the beam forming unit 3, the frequency reduction unit 4 reduces the respective Doppler frequencies in the K beams _sk (τ, η) (step of FIG. 4). ST2).
The frequency reduction unit 4 outputs each of the K beams s _{dc, k} (τ, η) after the Doppler frequency reduction to the equalization unit 5.
It is assumed that the radar signal processing device shown in FIG. 1 fixes the beam in an arbitrary direction during observation and performs strip map observation for observing the target. When performing strip map observation, the frequency reduction unit 4 operates the phase of the beam _sk (τ, η) in the azimuth time domain so that the center frequency of the Doppler frequency of the beam is observed at 0 Hz. The center frequency of the frequencies can be reduced.
FIG. 10 is an explanatory diagram showing a beam Doppler frequency reduction process by the frequency reduction unit 4.
In the example of FIG. 10, the frequency reduction unit 4 reduces the Doppler frequency f _k in the beam pointing direction that matches the center frequency of the Doppler frequency so that the center frequency of the Doppler frequency of the beam is observed at 0 Hz.

The processing of manipulating the phase of the beam s _k (τ, η) in the azimuth time domain is performed by the following formula so that the Doppler frequency f _k in the beam pointing direction that matches the center frequency of the beam Doppler frequency is observed at 0 Hz. It is expressed as (2).

式（３）において、ｓ’_ｅｓ，ｋ（τ，η）は、等間隔化部５によりドップラー周波数が、周波数低減部４により低減される前のドップラー周波数に戻されたビームである。
等間隔化部５は、ドップラー周波数を戻したＫ本のビームｓ’_ｅｓ，ｋ（τ，η）のそれぞれを変換部６に出力する。When the equal spacing unit 5 receives the K beams s _{dc, k} (τ, η) (k = 1, ..., K) after the Doppler frequency reduction from the frequency reduction unit 4, the equalization unit 5 reduces the Doppler frequency. The time intervals of the plurality of reception pulses included in each of the K beams s _{dc, k} (τ, η) are equalized (step ST3 in FIG. 4). Hereinafter, K beams including a plurality of received pulses with equal time intervals are expressed as “ _{ses, k} (τ, η)”.
Next, the equalization unit 5 reduces the respective Doppler frequencies of the K beams _{ses, k} (τ, η) by the frequency reduction unit 4 as shown in the following Expression (3). The processing for returning to the Doppler frequency of is performed (step ST4 in FIG. 4).

In Expression (3), s ′ _{es, k} (τ, η) is the beam whose Doppler frequency has been returned to the Doppler frequency before being reduced by the frequency reducing unit 4 by the equalization unit 5.
The equalization unit 5 outputs each of the K beams s ′ _{es, k} (τ, η) whose Doppler frequency has been returned to the conversion unit 6.

Hereinafter, the equalizing process of the time intervals by the equalizing unit 5 will be specifically described.
FIG. 11 is an explanatory diagram showing an example of a plurality of received pulses included in the beam after the Doppler frequency reduction output from the frequency reduction unit 4.
As shown in FIG. 11, the plurality of received pulses included in the beam s _{dc, k} (τ, η) after the Doppler frequency reduction output from the frequency reduction unit 4 have the azimuth times received at unequal time. It is an interval. In FIG. 11, a plurality of “” indicate reception pulses received by the antenna unit 1.

First, the equalization unit 5 uses, for example, an interpolation method to extract a plurality of received pulses included in the beam s _{dc, k} (τ, η) after the Doppler frequency reduction as shown in FIG. Find an approximate curve. The approximate curve is a curve that passes through a plurality of received pulses included in the beam s _{dc, k} (τ, η). The curved line passing through the plurality of received pulses includes a straight line passing through the plurality of received pulses.
FIG. 12 is an explanatory diagram showing an example of an approximate curve obtained from a plurality of received pulses included in the beam after the Doppler frequency reduction. In FIG. 12, a plurality of “” indicate received pulses received by the antenna unit 1, respectively.

Next, as shown in FIG. 13, the equalizing section 5 makes an approximation curve so that the azimuth times of the plurality of reception pulses included in the beam s _{dc, k} (τ, η) are evenly spaced. The new received pulse is interpolated to.
FIG. 13 is an explanatory diagram showing an example of a plurality of reception pulses whose time intervals are made equal by the equalizing section 5. In FIG. 13, a plurality of “*” s indicate received pulses whose time intervals are equalized by the equalization unit 5, respectively.
Since the process itself of interpolating a new reception pulse on the approximate curve so that the azimuth times of the plurality of reception pulses are evenly spaced is a known technique, detailed description thereof will be omitted.

When the conversion unit 6 receives the K beams s ′ _{es, k} (τ, η) whose Doppler frequencies have been returned from the equalization unit 5, for example, the respective beams s ′ _{es, k} (τ, η). 4) is subjected to discrete Fourier transform in the azimuth direction to convert each beam into a signal in the Doppler frequency domain (step ST5 in FIG. 4).
The conversion unit 6 outputs each of the K Doppler frequency domain signals to the signal synthesis unit 7.

  Upon receiving the K Doppler frequency domain signals from the transforming section 6, the signal synthesizing section 7 synthesizes the K Doppler frequency domain signals and inversely transforms the K Doppler frequency domain signals. (STEP ST6 of FIG. 4).

Upon receiving the combined signal from the signal combining unit 7, the inverse transform unit 8 transforms the combined signal into a signal in the time domain by, for example, performing an inverse discrete Fourier transform of the combined signal (step ST7 in FIG. 4).
The inverse conversion unit 8 outputs the time domain signal to the image reproduction unit 9.
When the image reproduction unit 9 receives the time domain signal from the inverse conversion unit 8, the image reproduction unit 9 reproduces the SAR image from the time domain signal (step ST8 in FIG. 4).
The processing itself for reproducing the SAR image from the signal in the time domain is a well-known technique, and thus detailed description thereof is omitted.

The waveform of the real part of the phase of the beam in the time domain including a plurality of received pulses and the intensity of the beam in the Doppler frequency region including a plurality of received pulses will be described below.
FIG. 14 shows the case where the transmission pulses are repeatedly radiated from the plurality of apertures at unequal time intervals, each transmission pulse is reflected by one target, and the plurality of reception pulses are received by each of the plurality of apertures. FIG. 4 is an explanatory diagram showing an example of a phase of a reception signal including a plurality of reception pulses received by any one aperture. In FIG. 14, a plurality of “x” indicate received pulses at sampling points.
In the figure, the plurality of reception pulses surrounded by ellipses are reception pulses having a higher Doppler frequency than the plurality of reception pulses not surrounded by ellipses.
When the PRF bandwidth at the time of observation is narrower than the Doppler bandwidth of the reception pulse received at each aperture, ambiguity that is signal folding occurs in the Doppler frequency region of the reception pulse.

FIG. 18 is an explanatory diagram showing signals in the Doppler frequency domain in the beam shown in FIG.
From FIG. 18, it can be seen that ambiguity occurs when the PRF bandwidth at the time of observation is narrower than the Doppler bandwidth of the received pulse received at each aperture.
The signal in the Doppler frequency domain shown in FIG. 18 is subjected to equalization processing of time intervals for a plurality of reception pulses included in the beam shown in FIG. 14 and discrete Fourier transform processing for the beam after the equalization processing. It is obtained by doing.

FIG. 15 is an explanatory diagram showing an example of the phases of the beams formed from the reception pulses received by the beam forming unit 3 through the plurality of apertures. In FIG. 15, a plurality of "x" s indicate received pulses at sampling points.
In the example of FIG. 15, the beam forming unit 3 forms three beams having different directing directions. In the figure, the plurality of reception pulses surrounded by ellipses are reception pulses having a higher Doppler frequency than the plurality of reception pulses not surrounded by ellipses. The plurality of received pulses not enclosed by an ellipse include the plurality of received pulses described in the central diagram of FIG. 15 and the plurality of received pulses enclosed by an ellipse in the leftmost diagram of FIG. Also, a plurality of reception pulses on the side with earlier azimuth time are included. Further, the plurality of reception pulses not enclosed by the ellipse also include the plurality of reception pulses on the side of which the azimuth time is later than the plurality of reception pulses enclosed by the ellipse in the rightmost diagram in FIG. .

FIG. 19 is an explanatory diagram showing signals in the Doppler frequency region in the beam shown in FIG. 15 formed by the beam forming unit 3.
In the example of FIG. 19, three Doppler frequency domain signals are shown. The signals in the three Doppler frequency regions correspond to the three beams shown in FIG. 15, respectively. Each Doppler frequency domain signal is subjected to time interval equalization processing for a plurality of reception pulses included in each beam shown in FIG. 15 and discrete Fourier transform processing for the beam after the equal interval processing. It is obtained by doing.
The Doppler bandwidth of the beam formed by the beam forming unit 3 has a value according to the number of apertures included in the antenna unit 1. If the Doppler bandwidth of the beam formed by the beam forming unit 3 is designed to be narrower than the bandwidth of the PRF at the time of observation, it occurs in the Doppler frequency region before the beam is formed by the beam forming unit 3. The ambiguity is eliminated by the beam forming unit 3. The beam forming unit 3 can form a beam in a Doppler frequency range wider than the PRF bandwidth at the time of observation.

FIG. 16 is an explanatory diagram showing an example of the phase of the beam whose Doppler frequency has been reduced by the frequency reduction unit 4. In FIG. 16, a plurality of “x” indicate received pulses at sampling points.
In the example of FIG. 16, each of the center frequencies of the Doppler frequencies of the three beams shown in FIG. 15 is shifted to 0 Hz by the frequency reduction unit 4, so that the Doppler frequency is reduced.
Since the frequency reduction unit 4 reduces the Doppler frequency, the phase rotation speed of the beam becomes slower than before the reduction of the Doppler frequency. Therefore, as compared with before the reduction of the Doppler frequency, the accuracy of equalization of the plurality of received pulses by the equalization unit 5 is improved.

FIG. 20 is an explanatory diagram showing signals in the Doppler frequency region in the beam shown in FIG. 16 in which the Doppler frequency has been reduced by the frequency reduction unit 4.
In the example of FIG. 20, three Doppler frequency domain signals are shown. The signals in the three Doppler frequency regions correspond to the three beams shown in FIG. 16, respectively. Each Doppler frequency domain signal is subjected to equalization processing of time intervals for a plurality of reception pulses included in each beam shown in FIG. 16 and discrete Fourier transform processing for the beam after the equalization processing. It is obtained by doing.
The center frequency f _k of the Doppler frequency of each beam is shifted to 0 Hz by the frequency reduction unit 4.

FIG. 17 is an explanatory diagram showing an example of a signal obtained by transforming the synthesized signal output from the signal synthesis unit 7 into the time domain by the inverse transformation unit 8.
In the example of FIG. 17, a plurality of received pulses included in the signal obtained by converting the combined signal shown in FIG. 21 into the time domain by the inverse transformation unit 8 are shown. In the figure, a plurality of “⋄” indicate received pulses at sampling points.
The time intervals of the plurality of reception pulses included in the signal in the time domain are equalized.

FIG. 21 is an explanatory diagram showing a combined signal by the signal combining unit 7.
The signal combining unit 7 reduces the Doppler frequency of the signal in the Doppler frequency region on the left side of the drawing in FIG. 20 and the Doppler frequency of the signal in the Doppler frequency region on the right side of the drawing in FIG. 20 by the frequency reducing unit 4. To the Doppler frequency before resetting (see FIG. 19).
In the example of FIG. 21, a signal in the Doppler frequency domain in which the signal in the Doppler frequency domain in which the Doppler frequency is returned and the signal in the central Doppler frequency domain in FIG. It is written.
In the Doppler frequency domain, the ambiguity is eliminated, and the Doppler bandwidth of the combined Doppler frequency domain signal becomes approximately the same as the Doppler bandwidth of the received pulse received at each aperture.
Further, the Doppler bandwidth of the synthesized signal in the Doppler frequency domain is wider than the Doppler bandwidth of the signal in the Doppler frequency domain shown in FIG. 18 because the ambiguity is eliminated.
Therefore, the SAR image reproduced by the image reproducing unit 9 is a high-resolution image with no aliasing.

  In the first embodiment, the transmission pulse reflected from the target is generated by repeatedly radiating the transmission pulse from each of the plurality of openings in the antenna unit 1 having the plurality of openings in the azimuth direction at unequal time intervals. When a pulse is received as a received pulse at each of the plurality of apertures, the received signal received at each of the plurality of apertures is used to provide a beam forming unit 3 that forms a plurality of beams. Configured the device. In addition, the radar signal processing device includes an equal interval unit 5 that equalizes the time intervals of a plurality of received pulses included in each of the plurality of beams formed by the beam forming unit 3, and an equal interval unit 5. And a conversion unit 6 that converts each beam including a plurality of received pulses whose time intervals are made equal to each other into a signal in the Doppler frequency domain and outputs the signals in the Doppler frequency domain, The radar signal processing device synthesizes a plurality of signals in the Doppler frequency domain output from the conversion unit 6, and outputs a synthesized signal of a plurality of signals in the Doppler frequency domain, and outputs from the signal synthesis unit 7. And an inverse transform unit 8 for transforming the synthesized signal thus generated into a signal in the time domain. Therefore, the radar signal processing device can contribute to wide area observation of the SAR image as compared with the case where the transmission pulse is repeatedly emitted at equal time intervals in the SAR having a plurality of openings in the azimuth direction.

  Further, the first embodiment has a plurality of openings in each of the azimuth direction and the elevation direction, and the transmission pulse is repeatedly emitted from each of the plurality of openings at unequal time intervals, and the transmission pulse reflected by the target is transmitted. The radar signal processing device is configured to include the antenna unit 1 that receives a pulse as a reception pulse at each of the plurality of apertures. Therefore, the radar signal processing device can further realize wide area observation of the SAR image as compared with the case where the radar signal processing device includes an antenna unit having a plurality of openings only in the azimuth direction.

Embodiment 2.
In the radar signal processing device of the first embodiment, the frequency reduction unit 4 reduces the center frequency of the Doppler frequency so that the center frequency of the Doppler frequency of the beam is observed at 0 Hz.
In the second embodiment, the radar signal processing device in which the frequency reducing unit 4 reduces the respective Doppler frequencies in the plurality of beams by eliminating the change in the center frequencies of the plurality of beams with the passage of the azimuth time is described. explain.
The configuration of the radar signal processing device according to the second embodiment is the same as that of the radar signal processing device according to the first embodiment, as shown in FIG.

In the observation of scanning the beam during observation such as spotlight observation or sliding spotlight observation, the Doppler frequency of the beam can be reduced by a method other than the method of shifting the Doppler frequency f _k in the beam pointing direction to 0 Hz. It is possible.
In the case of spotlight observation or sliding spotlight observation, the same area is irradiated with the transmission pulse by performing beam scanning that changes the radiation direction of the transmission pulse while moving. Therefore, the beam pointing direction at the time of beam formation may be changed as the azimuth time elapses. In this case, the center frequency of the beam formed into a beam changes as the azimuth time elapses, as shown in FIG.

式（４）において、ｆ_ｋは、ビーム指向方向のドップラー周波数のうち、アジマス時刻に伴った変化をしない定数成分、Ｒ_ｒｃ（η）は、アンテナ部１から放射されたビームの回転中心と、アンテナ部１が有している全ての開口との距離である。FIG. 22 is an explanatory diagram showing a beam Doppler frequency reduction process by the frequency reduction unit 4.
The center frequency of the beam before the Doppler frequency is reduced by the frequency reduction unit 4 changes as the azimuth time elapses.
The frequency reduction unit 4 reduces the Doppler frequency of the beam by eliminating the change in the center frequency of the beam with the passage of the azimuth time.
The Doppler frequency reduction processing by eliminating the change in the center frequency of the beam is expressed by the following equation (4).

In Expression (4), f _k is a constant component of the Doppler frequency in the beam pointing direction that does not change with azimuth time, R _rc (η) is the rotation center of the beam radiated from the antenna unit 1, It is the distance from all the openings of the antenna unit 1.

In the second embodiment, the frequency reduction unit 4 reduces the Doppler frequencies of the plurality of beams by eliminating the change in the center frequency of the plurality of beams with the passage of the azimuth time. However, this is merely an example, and the frequency reduction unit 4 determines the Doppler frequency f _k in the beam pointing direction so that the Doppler frequency f _k in the beam pointing direction is observed at 0 Hz as shown in FIG. Both the reduction processing and the processing for eliminating the change in the center frequency of the beam may be executed.
FIG. 23 is an explanatory diagram showing the processing for reducing the Doppler frequency of the beam by the frequency reducing unit 4.

  In the invention of the present application, it is possible to freely combine the respective embodiments, modify any of the constituent elements of each of the embodiments, or omit any of the constituent elements of each of the embodiments within the scope of the invention. .

  INDUSTRIAL APPLICABILITY The present invention is suitable for a radar signal processing device and a radar signal processing method for forming a plurality of beams by using received pulses received by a plurality of apertures in an antenna section.

  DESCRIPTION OF SYMBOLS 1 antenna section, 2 signal processing section, 3 beam forming section, 4 frequency reducing section, 5 equal spacing section, 6 converting section, 7 signal combining section, 8 inverse converting section, 9 image reproducing section, 11 beam forming circuit, 12 Frequency reduction circuit, 13 equal spacing circuit, 14 conversion circuit, 15 signal synthesis circuit, 16 inverse conversion circuit, 17 image reproduction circuit, 21 memory, 22 processor.

Claims (6)

Transmission pulses are repeatedly emitted at unequal time intervals from each of the plurality of apertures in the antenna section having a plurality of apertures in the azimuth direction, and the transmission pulses repeatedly emitted at the unequal time intervals are reflected to the target. the transmission pulse the reflected using the received pulses received by each of the plurality of openings, and a beam forming unit for forming a plurality of beams,
A frequency reduction unit that reduces each Doppler frequency in the plurality of beams formed by the beam forming unit,
After equalizing the time intervals of the plurality of reception pulses included in each of the plurality of beams whose Doppler frequency has been reduced by the frequency reducing unit, the plurality of reception pulses including the time intervals being equal intervals are included. Each Doppler frequency in the plurality of beams is returned to the Doppler frequency before being reduced by the frequency reducing unit, and an equal spacing unit that outputs a plurality of beams with the returned Doppler frequency ,
Converting each of the plurality of beams output by the equalizing section to a signal in the Doppler frequency domain, a conversion section that outputs a signal in the multiple Doppler frequency domain,
A signal synthesizing unit that synthesizes a plurality of Doppler frequency domain signals output from the converting unit and outputs a synthesized signal of the plurality of Doppler frequency domain signals,
An inverse transform unit that transforms the synthesized signal output from the signal synthesis unit into a signal in the time domain,
Signal processing device including the.   Having a plurality of apertures in each of the azimuth direction and the elevation direction, repeatedly radiating a transmission pulse from each of the plurality of apertures at unequal time intervals, and using the transmission pulse reflected by the target as a reception pulse. The radar signal processing device according to claim 1, further comprising: an antenna unit for receiving at each of the plurality of openings. The radar signal processing device according to claim 1 or 2, further comprising an image reproducing unit that reproduces an image from the signal in the time domain. Wherein the frequency reduction unit, said by shifting the respective center frequencies of the plurality of beams formed by the beam forming unit, wherein the claim 1, characterized in that to reduce the respective Doppler frequencies in the plurality of beams Item 4. The radar signal processing device according to any one of Items 3 . If the center frequencies of the plurality of beams formed by the beam forming unit change with the passage of the azimuth time, the frequency reduction unit eliminates the change of the center frequencies with the passage of the azimuth time. The radar signal processing device according to any one of claims 1 to 3 , wherein each of the plurality of beams reduces the Doppler frequency. The beam forming unit has a plurality of apertures in the azimuth direction, and the transmit pulses are repeatedly emitted from the plurality of apertures in the antenna unit at unequal time intervals, and the transmit pulse reflected by the target is the receive pulse. As, when received at each of the plurality of apertures, using the received pulse received at each of the plurality of apertures, forming a plurality of beams,
The frequency reduction unit reduces each Doppler frequency in the plurality of beams formed by the beam forming unit,
The equalization unit equalizes the time intervals of the plurality of reception pulses included in each of the plurality of beams whose Doppler frequency has been reduced by the frequency reduction unit , and then equalizes the time intervals. Each Doppler frequency in the plurality of beams containing the received pulse, returned to the Doppler frequency before being reduced by the frequency reduction unit, and outputs a plurality of beams that returned the Doppler frequency,
The conversion unit converts each of the plurality of beams output by the equalization unit to a signal in the Doppler frequency domain, and outputs a signal in the multiple Doppler frequency domain,
The signal combining unit combines the signals of the plurality of Doppler frequency regions output from the conversion unit, and outputs a combined signal of the signals of the plurality of Doppler frequency regions,
A radar signal processing method, wherein an inverse transforming unit transforms the combined signal output from the signal combining unit into a signal in the time domain.

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