JP2020201133A - Synthetic Aperture Radar System, Synthetic Aperture Radar Method, and Synthetic Aperture Radar Program - Google Patents

Abstract

To provide a synthetic aperture radar system, with which an increase in data volume due to an increase in the number of antenna apertures needed for heightened performance is suppressed.SOLUTION: In a synthetic aperture radar system having a plurality of antenna apertures, a range Fourier transform unit 40 converts each of a plurality of receive signals 32 having been received by the plurality of antenna apertures into a frequency domain by range Fourier transformation, and outputs the result as a plurality of converted receive signals 61. A code modulation unit 41 multiplies each of a plurality of code-modulating codes 332 mutually having orthogonality to each of the plurality of converted receive signals 61 and outputs the result as a plurality of modulated receive signals 62. A multiplexing unit 43 multiplexes the plurality of modulated receive signals 62 and records the result as a multiplexed signal 34 in a data recording unit 150.SELECTED DRAWING: Figure 2

Description

The present invention relates to synthetic aperture radar systems, synthetic aperture radar methods, and synthetic aperture radar programs. In particular, the present invention relates to a synthetic aperture radar system, a synthetic aperture radar method, and a synthetic aperture radar program that form a plurality of transmission / reception beams by controlling the antenna aperture surface to realize high resolution and a wide observation width.

Synthetic aperture radar is called SAR (Synthetic Aperture Radar). In the following, the synthetic aperture radar may be referred to as SAR. SAR is mounted on a flying object such as an artificial satellite or an aircraft. SAR is a sensor that can observe the surface of the earth day and night through clouds or fog. The main parameters for SAR are azimuth resolution and range observation width. Due to the imaging principle of SAR, these parameters are in a reciprocal relationship. However, the multi-beam SAR method that divides the aperture of the antenna and processes the received data for each divided aperture makes it possible to improve the azimuth resolution and expand the range observation width at the same time.
The multi-beam SAR method that realizes high azimuth resolution and wide range observation width is a well-known technique, and is shown in Non-Patent Document 1 and Non-Patent Document 2 below.

In the SAR system adopting the multi-beam SAR method, it is possible to realize a wide area and high resolution. However, since it is necessary to record data for each antenna opening, the amount of data recorded as a SAR system increases. As the amount of data increases, it is necessary to increase the number or capacity of data recording units, which poses a problem in terms of system resources and cost. Therefore, various methods are being studied in order to reduce the amount of data in the multi-beam SAR method. Non-Patent Document 3 discloses a method for improving the efficiency of the multi-beam SAR method using on-board quantization.

N. Gebert and G. Krieger, "Azimuth Phase Adaptation on Transform for High-Resolution Wide-Swath SAR Imaging," in IEEE Geoscience and Remote Sensing. 6, no. 4, pp. 782-786, Oct. 2009. Qisong Wu, Mengdao Xing, Zheng Bao and Hongzhu Shi, "Wide swath, high range resolution imaging with MIMO-SAR," 2009 IET International, 1-6. M. Martone, et al. , "An Efficient Own Quantization Strategy for Multi-Channel SAR Systems", EUSAR2018, pp. 572-577, June 6, 2018. Kazuo Ouchi, Basics of Synthetic Aperture Radar for Remote Sensing (2nd Edition Revised Edition), Tokyo Denki University Press, 2009.

The efficiency improvement method of Non-Patent Document 3 is effective only when the quantization error is uniformly distributed on the Doppler band and the quantization error is relatively large. Since such a condition is not satisfied by the actual SAR observation data, it is difficult to use the efficiency improvement method of Non-Patent Document 3 in the actual SAR observation data.

An object of the present invention is to suppress an increase in the amount of data due to an increase in the numerical aperture of antennas required for high performance in a multi-beam SAR system.

The synthetic aperture radar system according to the present invention is
A range Fourier transform unit that performs range Fourier transform on each of a plurality of received signals received by a plurality of antenna openings and outputs them as a plurality of converted received signals.
A code modulation unit that multiplies each of the plurality of conversion reception signals by each of a plurality of code modulation codes having orthogonality to each other and outputs the plurality of modulation reception signals.
It is provided with a multiplexing unit that multiplexes the plurality of modulated reception signals and records them as a multiplexing signal in the data recording unit.

In the synthetic aperture radar system according to the present invention, the amount of data is reduced by multiplexing the received signals acquired from a plurality of antenna openings after the code modulation unit performs the code modulation process. Therefore, according to the synthetic aperture radar system according to the present invention, it is possible to suppress an increase in hardware resources even when the numerical aperture of antennas required for high performance is increased.

The block diagram which shows the synthetic aperture radar system which concerns on Embodiment 1. The block diagram of the signal processing part of the synthetic aperture radar system which concerns on Embodiment 1. FIG. The block diagram of the signal demodulation part of the synthetic aperture radar system which concerns on Embodiment 1. FIG. A hardware configuration example of a synthetic aperture radar system according to a modified example of the first embodiment. FIG. 6 is a conceptual diagram when the range Fourier transform process, the decharp process, the code modulation process, the multiplexing process, and the code demodulation process according to the first embodiment are performed. The flow chart which shows the operation of the synthetic aperture radar system which concerns on Embodiment 1. FIG. The block diagram which shows the synthetic aperture radar system which concerns on the modification of Embodiment 1. The block diagram which shows the signal processing part which concerns on the modification of Embodiment 1. A hardware configuration example showing a synthetic aperture radar system according to a modified example of the first embodiment.

Hereinafter, the present embodiment will be described with reference to the drawings. In each figure, the same or corresponding parts are designated by the same reference numerals. In the description of the embodiment, the description will be omitted or simplified as appropriate for the same or corresponding parts.

Embodiment 1.
*** Explanation of configuration ***
FIG. 1 is a configuration diagram showing a synthetic aperture radar system 100 according to the present embodiment. Hereinafter, the synthetic aperture radar system 100 may be referred to as a SAR system.
The synthetic aperture radar system 100 is a device that repeatedly irradiates an observation region with a pulse signal, receives a reflected wave reflected by the observation target 200, and reproduces a SAR image using the received signal.

FIG. 2 is a configuration diagram of a signal processing unit 140 of the synthetic aperture radar system 100 according to the present embodiment.
FIG. 3 is a configuration diagram of a signal demodulation unit 102 of the synthetic aperture radar system 100 according to the present embodiment.
The signal processing unit 140 multiplies each of the plurality of received signals 32 received from the plurality of antenna openings by each of the plurality of code modulation codes 332 having orthogonality to each other to multiplex the plurality of received signals 32. The reception signal 32 is a digital reception signal digitized by the reception unit 130. The multiplexed signal is the multiplexed signal 34.
The signal demodulation unit 102 multiplies the multiplexed signal 34 multiplexed by the signal processing unit 140 by the code demodulation code 221 to generate the image spectrum 36.
In this embodiment, it is assumed that the number of antenna openings is N. N is a natural number. That is, the transmitting / receiving antenna 120 has N antenna openings from the antenna opening 1 to the antenna opening N.

The excitation unit 107 generates a pulse signal 31 and outputs it to the transmission unit 108 and the reception unit 130 at regular time intervals. The pulse signal 31 generated by the excitation unit 107 is a chirp signal. A chirp signal is a signal whose frequency change rate changes with time. The rate of change in the frequency of the chirp signal is called the char plate.
The pulse width, bandwidth, time interval between the char plate and the pulse of the pulse signal 31 generated by the excitation unit 107 is determined by the control signal 30 output from the control unit 106.
The transmission unit 108 amplifies the signal level of the pulse signal 31 output from the excitation unit 107 to a desired signal level, and outputs the signal level to the transmission / reception switch 109. The transmission / reception switching device 109 includes N transmission / reception switching devices of the transmission / reception switching device n (n = 1, 2, ..., N).

The transmission / reception switch n (n = 1, 2, ..., N) switches between the transmission state and the reception state of the transmission / reception antenna. In the transmission state, the transmission / reception switch n (n = 1, 2, ..., N) outputs the signal from the transmission unit 108 to the antenna opening n (n = 1, 2, ..., N) of the transmission / reception antenna 120. In the receiving state, the transmission / reception switch n (n = 1, 2, ..., N) receives the signal of the reflected wave received by the antenna opening n (n = 1, 2, ..., N) of the transmission / reception antenna 120 of the receiving unit 130. Output to the receiving unit n (n = 1, 2, ..., N).

The transmitting / receiving antenna 120 includes antenna openings n (n = 1, 2, ..., N) divided in the azimuth direction. The transmission / reception antenna 120 irradiates the pulse signal output from the transmission / reception switch n (n = 1, 2, ..., N) toward the observation region, and a part of the pulse signal reflected by the observation target 200 is RF ( Radio Frequency) Received as a signal. The transmission / reception antenna 120 outputs an RF signal to the transmission / reception switch n (n = 1, 2, ..., N). In FIG. 1, one transmitting / receiving antenna 120 has a transmitting function and a receiving function. However, there may be a plurality of transmitting and receiving antennas, and the transmitting antenna and the receiving antenna may be separately separated.

The receiving unit n (n = 1, 2, ..., N) converts the RF signal output from the transmission / reception switching device n (n = 1, 2, ..., N) into a baseband signal. Then, the receiving unit n (n = 1, 2, ..., N) converts the baseband signal into a digital reception signal, that is, a reception signal 32 by the A / D (Analog to Digital) converter. The receiving unit n (n = 1, 2, ..., N) outputs the received signal 32 to the signal processing unit 140. Further, the receiving unit n (n = 1, 2, ..., N) converts the pulse signal 31 output from the excitation unit 107 into a digital transmission signal, that is, a transmission signal 33 by the A / D converter, and is a signal processing unit. Output to 140.

The signal processing unit 140 converts the received signal 32 and the transmitted signal 33 output by the receiving unit n (n = 1, 2, ..., N) into a frequency domain. Further, the signal processing unit 140 converts the transmission signal 33 into the reference signal 331. The signal processing unit 140 performs a code modulation process of multiplying the received signal 32 by the reference signal 331 and the code modulation code 332 in the frequency domain. Then, the signal processing unit 140 generates the multiplexing signal 34 by performing the multiplexing processing on the N received signals 32. The signal processing unit 140 outputs the multiplexed signal 34 to the data recording unit 150.

The data recording unit 150 records the multiplexed signal 34 output by the signal processing unit 140, and outputs the multiplexed signal 34 to the data transfer unit 101 at a desired time.
The data transfer unit 101 maps the multiplexed signal 34 to the radio frequency signal 35 and outputs it to the signal demodulation unit 102.

The signal demodulation unit 102 demodulates the multiplexing signal 34 from the radio frequency signal 35 output by the data transfer unit 101 to obtain the demodulated multiplexing signal 63. The signal demodulation unit 102 performs a code demodulation process of multiplying the demodulated multiplexed signal 34, that is, the demodulation multiplexing signal 63 by the code demodulation code 221. The signal demodulation unit 102 generates an image spectrum 36 corresponding to N received signals received at each antenna opening. Then, the signal demodulation unit 102 outputs the image spectrum 36 to the image reproduction unit 103.

The image reproduction unit 103 performs a process of reproducing the SAR image 37 using the image spectrum 36 output by the signal demodulation unit 102.

The display 104 includes a processor such as a GPU (Graphic Processing Unit). The display 104 displays the SAR image 37 reproduced by the image reproduction unit 103 on the display.

The control unit 106 is a circuit that controls the timing at which the transmission / reception antenna 120 switches transmission / reception and the timing at which the excitation unit 107 generates the pulse signal 31.

<Detailed description of signal processing unit 140>
The range Fourier transform unit n (n = 1, 2, ..., N) of the signal processing unit 140 shown in FIG. 2 has the received signal 32 output from the receiving unit n (n = 1, 2, ..., N) and The transmission signal 33 is converted into a frequency domain.
The recording device 42 of the signal processing unit 140 is a reference signal having a complex conjugate relationship of the transmission signal 33 in the frequency space from the transmission signal 33 output from the range Fourier transform unit n (n = 1, 2, ..., N). Generate and retain 331. The recording device 42, coded modulation code ^{_{C n m (n = 1,2,}} ..., N) having orthogonality to each other are recorded. Ie, n number of code modulation code C _n ^m are each having orthogonality. Recording device 42, coded modulation code ^{_{C n m (n = 1,2,}} ..., N) and outputs the code modulation unit 41.

The code modulation multiplier n (n = 1, 2, ..., N) of the code modulation unit 41 is a reference signal for each received signal 32 output by the range Fourier change unit n (n = 1, 2, ..., N). By multiplying 331 and the code modulation code C _n ^m (n = 1, 2, ..., N), it is converted into a digital received signal having orthogonality with each other. That is, the n digital received signals obtained by multiplying the _n received signals by the code modulation code C ^nm (n = 1, 2, ..., N) have orthogonality to each other. Specifically, the first digital reception signal and the second digital reception signal have orthogonality. Here, having orthogonality means having two functions, that is, mathematical properties in which signals can be synthesized and separated.

In FIG. 2, the multiplication of the code modulation code C _n ^m (n = 1, 2, ..., N) is performed in parallel. However, the received signal 32 output by the range Fourier change unit n (n = 1, 2, ..., N) is multiplied in order by the code modulation code C _n ^m (n = 1, 2, ..., N). May be good. At this time, only one code modulation multiplier may be used. The recording device 42 holds the received signal 32 output from the range Fourier transform unit n (n = 1, 2, ..., N), and holds the reference signal 331 and the code modulation code C _n ^m (n = 1, 2, ..., N). , N) and the received signal 32 is output to the code modulation multiplier.

The recording apparatus 42 in FIG. 2 code modulation code ^{_{C n m (n = 1,2,}} ..., N) shows an example for holding. However, the code modulation code may be calculated each time the code modulation multiplier is used to multiply the received signal 32 by the code modulation code.
In FIG. 2, the reference signal 331 held in the recording device 42 is calculated from the transmission signal 33 output by the range Fourier transform unit n (n = 1, 2, ..., N) for each observation. However, the reference signal 331 corresponding to all the pulse signals generated by the excitation unit 107 may be stored, and the reference signal 331 corresponding to the generated pulse signal may be output to the code modulation multiplier. At this time, the excitation unit 107 of FIG. 1 does not transmit the pulse signal 31 to the reception unit 130.
Further, although it is assumed here that the reference signal 331 is multiplied by the received signal 32, the present invention is not limited to this, and the received signal 32 may not be multiplied by the reference signal 331. At this time, the excitation unit 107 of FIG. 1 does not transmit the pulse signal 31 to the reception unit 130, and the reference signal 331 is not recorded in the recording device 42 of the signal processing unit 140.

Specifically, the multiplexing unit 43 of the signal processing unit 140 is equipped with a well-known multiplexer circuit, and the N received signals 32 output by the code modulation unit 41 are combined into one multiplexing signal 34 in the frequency space. Synthesize.

The demodulator 201 of the signal demodulation unit 102 of FIG. 3 demodulates the multiplexing signal 34 from the radio frequency signal 35 output from the data transfer unit 101, and serves as the demodulation multiplexing signal 63 as the code demodulator n of the code demodulation unit 202 ( Output to n = 1, 2, ..., N).
The recording device 203 of the signal demodulation unit 102 records the code demodulation code 221 corresponding to the radio frequency signal 35, and outputs the code demodulation code 221 to the code demodulation multiplier n (n = 1, 2, ..., N) of the code demodulation unit 202. To do.

The signal demodulation multiplier n (n = 1, 2, ..., N) multiplies the multiplexed signal 34 by the code demodulation code n (n = 1, 2, ..., N) in the frequency space to generate the image spectrum 36. To do.
In FIG. 3, the multiplication of the code demodulation code is performed in parallel, but the code demodulation code C _n ^d (n = 1, 2, ..., N) is sequentially assigned to the multiplexed signal 34 output by the demodulator 201. May be multiplied by. At this time, only one code demodulation multiplier is required, and the recording device 203 holds the multiplexed signal 34 output from the demodulator 201, and the code demodulation code C _n ^d (n = 1, 2, ..., N). At the same time, the multiplexing signal 34 is output to the code modulation multiplier.

In FIG. 3, the recording device 203 records the code demodulation code C _n ^d (n = 1, 2, ..., N) and outputs it to the code demodulation multiplier. However, the recording apparatus 203 is code modulation code ^{_{C n m (n = 1,2,}} ..., N) holds, may calculate the code demodulator code when multiplying the code demodulator code multiplexed signal.

FIG. 4 is a diagram showing a hardware configuration example of a computer used in the synthetic aperture radar system 100 according to the present embodiment.
As a specific example, the synthetic aperture radar system 100 includes an electronic circuit 909, a memory 921, and a communication device 950.

Each of the range Fourier transform unit 40, the code modulation unit 41, the multiplexing unit 43, and the recording device 42, which are the components of the signal processing unit 140, is realized by the electronic circuit 909, which is dedicated hardware. Specifically, the electronic circuit 909 is a range Fourier transform circuit, a code modulation circuit, a multiplexing circuit, and a recording device circuit. Further, each of the demodulator 201, the code demodulation unit 202, and the recording device 203, which are the components of the signal demodulation unit 102, is realized by the electronic circuit 909, which is dedicated hardware. Specifically, the electronic circuit 909 is a demodulation circuit, a code demodulation circuit, and a recording device circuit.

Here, the range Fourier conversion circuit, the code modulation circuit, the multiplexing circuit, the recording device circuit, the demodulation circuit, and the code modulation circuit are specifically described as a single circuit, a composite circuit, a programmed processor, an ASIC, an FPGA, and the like. Or a combination of these is applicable. ASIC is an abbreviation for Application Specific Integrated Circuit. FPGA is an abbreviation for Field-Programmable Gate Array.

The components of the signal processing unit 140 and the signal demodulation unit 102 are not limited to those realized by dedicated hardware. The components of the signal processing unit 140 and the signal demodulation unit 102 may be realized by software, firmware, or a combination of software and firmware. Software or firmware is stored as a program in computer memory. A computer is hardware that executes a program, and specifically includes hardware such as a CPU, a central processing unit, an arithmetic unit, and a microcomputer. CPU is an abbreviation for Central Processing Unit.

Specifically, the computer memory 921 corresponds to a storage device such as a non-volatile or volatile semiconductor memory such as RAM, ROM, flash memory, EEPROM, a magnetic disk, or a compact disk. RAM is an abbreviation for Random Access Memory. ROM is an abbreviation for Read Only Memory. EEPROM is an abbreviation for Electrically Erasable Programmable Read Only Memory.

The computer communication device 950 has a receiver and a transmitter. The communication device 950 is wirelessly connected to a communication network such as a LAN, the Internet, or a telephone line. The communication device 950 is specifically a communication chip or a NIC. LAN is an abbreviation for Local Area Network. NIC is an abbreviation for Network Interface Card.

FIG. 5 is a conceptual diagram when the range Fourier transform process, the decharp process, the code modulation process, the multiplexing process, and the code demodulation process according to the present embodiment are performed.
With reference to FIG. 5, the principle of data amount reduction by range Fourier transform processing, decharp processing, code modulation processing, multiplexing processing, and code demodulation processing will be described. In the example of FIG. 5, decharp processing is performed by multiplying the received signal by the reference signal. However, the received signal may be subjected to code modulation processing without performing decharp processing. At this time, the code demodulation code C _n ^d (n = 1, 2, ..., N) used in the code demodulation process is the complex conjugate of the transmission signal to the code modulation code C _n ^m (n = 1, 2, ..., N). Is the result of multiplying by the complex conjugate.

When the time is τ and the char plate is K, the transmitted wave _Et (τ) transmitted by the synthetic aperture radar system 100 is expressed as in Equation 1.

The received wave _Er (τ) received by the synthetic aperture radar system can be represented by the same waveform as the transmitted wave _Et (τ), and one of the received signals obtained from the nth antenna aperture is Equation 2. Represented.

この送信波の複素共役Ｅ_ｔ ^＊（τ）が上述の参照信号３３１となる。参照信号３３１は受信波と逆のチャープレートを示す。送信波の複素共役Ｅ_ｔ ^＊（τ）と受信波のフーリエ変換の積を取ることで、受信波のチャープレートは打ち消される。このチャープレートの打消しはデチャープ処理と呼ばれる。デチャープ後の受信信号Ｓ_{ｄｅｃｈｉｒｐ}（ｆ）は数４のように求められる。ここでＦはフーリエ変換を示し、ｆは周波数空間上であることを示す。

デチャープされた受信信号に対して、あるチャープレートＫ_ｎを持つ信号Ｃ_ｎ ^ｍ（ｆ）を周波数空間上で乗算することで、数５のように受信信号のチャープレートをチャープレートＫ_ｎに変更することができる。

ここで互いに異なるチャープレートを持ち、高い直交性を示す信号を符号変調コードと呼ぶ。互いに高い直交性を持つ２つの符号変調コードの一例を数６に示す。ここで、τ_０は受信信号のパルス幅である。また、Ｋ_１，Ｋ_２は同じ符号を持つチャープレートとする。そして、Ｆ^−１は逆フーリエ変換である。

ここで示した符号変調コードは一例であり、符号変調コードＣ_ｎ ^ｍ（ｎ＝１，２，…，Ｎ）の形式は互いに直交性を持っている形式であればよい。具体的には、線形、非線形、あるいは多項式でもよい。 Since the transmitted wave and the received wave are represented by the same waveform, the complex conjugate _St ^* (τ) of the transmitted wave is the complex conjugate of the received wave. The complex conjugate _Et ^* (τ) of the transmitted wave is represented by Equation 3.

The complex conjugate _Et ^* (τ) of this transmitted wave becomes the above-mentioned reference signal 331. The reference signal 331 indicates a char plate opposite to the received wave. The char plate of the received wave is canceled by taking the product of the complex conjugate _Et ^* (τ) of the transmitted wave and the Fourier transform of the received wave. This cancellation of the char plate is called decharp processing. The received signal S _dechirp (f) after decharping is obtained as shown in _Equation 4. Here, F indicates the Fourier transform, and f indicates that it is on the frequency space.

Against Dechapu received signal, changes the signal C _{n m} ^(f) having a certain chirp K _n is multiplied by a frequency space, the chirp rate of the received signal as in equation 5 to the chirp K _n can do.

Here, a signal having different char plates and showing high orthogonality is called a code modulation code. Equation 6 shows an example of two code modulation codes having high orthogonality with each other. Here, τ ₀ is the pulse width of the received signal. Further, K ₁ and K ₂ are char plates having the same code. And F ^-1 is an inverse Fourier transform.

The code modulation code shown here is an example, and the form of the code modulation code C _n ^m (n = 1, 2, ..., N) may be any form having orthogonality to each other. Specifically, it may be linear, non-linear, or polynomial.

As shown in FIG. 5, the received signals output from the antenna openings n (n = 1, 2, ..., N) are decharped, and each received signal is multiplied by a code modulation code in the frequency space. N received signals with different char plates are generated. The multiplication process of this code modulation code is referred to as a code modulation process.

ここで、Ｓ_{ｍｏｄ＿ｎ}（ｆ）とＳ_{ｍｕｌｔｉ}（ｆ）のデータ量は同一であるため、多重化処理後の受信信号Ｓ_{ｍｕｌｔｉ}（ｆ）は多重化処理前のＮ個の受信信号に対して１／Ｎのデータ量となる。 The N received signals after the code modulation processing have orthogonality with each other and can be combined. The process of synthesizing the received signal is called the multiplexing process.
When the received signal n (n = 1, 2, ..., N) after the code modulation processing is expressed as S _{mod_n} (f), the received signal S _multi (f) after the multiplexing processing is expressed as _Equation 7. ..

Here, since the amount of data of S _{mod_n} (f) and S _multi (f) is the same, the received signal S _multi (f) after the multiplexing process is 1 for the N received signals before the multiplexing process. The amount of data is / N.

Code division multiple access is one of the multiple access technologies in the wireless communication field. The code division multidimensional method is a method in which communication is performed by assigning a different spectral diffusion code to each signal using one frequency. However, the code division multidimensional method is different from this method because the transmission signal is spread over a wider band than the frequency band of the modulated signal by using the spreading code.

In order to reproduce the SAR image, it is necessary to generate an image spectrum obtained from the N received signals before the multiplexing process from the received signal S _multi (f) after the multiplexing process. A matched filter is used to generate this image spectrum. In matched filtering, signals that are highly correlated with each other are multiplied in frequency space. This makes it possible to extract a desired signal even if an unnecessary signal component is synthesized in the signal. An example of a matched filter process used in a synthetic aperture radar is disclosed in Non-Patent Document 4.

In the code demodulation process, the received signal S _multi (f) after the multiplexing process is multiplied by the code demodulation signal C _n ^d (f) having a complex conjugate relationship with the code modulation code C _n ^m (f) in the frequency domain. To do. By the matched filtering process by this multiplication, only the signal of the same char plate K _n as the code modulation code C _n ^m (f) shows high correlation, and the image spectrum is extracted. The reason why the image spectrum is extracted is that the char plates contained in the synthesized signal are highly orthogonal, and the matched filter processing can obtain a high correlation only with the signal of the desired char plate.

よって、抽出された画像スペクトラムは符号変調処理、多重化処理、および符号復調処理を実施しなかった場合と同様の信号であり、画像再生処理によって処理が可能である。
以上が符号変調処理、多重化処理、および符号復調処理によるデータ量削減の原理である。 The code demodulation code C _n ^d (f) (n =) which has a complex conjugate relationship with the code modulation code C _n ^m (f) (n = 1, 2, ..., N) for the received signal after the multiplexing process. Multiplying 1, 2, ..., N) is called code demodulation processing. In the code demodulation processing, it is possible to extract the image spectrum of a signal having N different char plates in the received signal after the multiplexing processing. The image spectrum extracted at this time is the same as the product of the received signal of Equation 2 and the Fourier transform of the complex conjugate of the received signal. The image spectrum _Gn (f) is expressed as in Equation 8.

Therefore, the extracted image spectrum is a signal similar to the case where the code modulation processing, the multiplexing processing, and the code demodulation processing are not performed, and can be processed by the image reproduction processing.
The above is the principle of data reduction by code modulation processing, multiplexing processing, and code demodulation processing.

*** Explanation of operation ***
FIG. 6 is a diagram showing the operation of the synthetic aperture radar processing according to the present embodiment.

In step S101, the control unit 106 outputs the pulse signal parameters stored in the control unit 106 to the excitation unit 107 by the control signal 30. The pulse signal parameter is specifically one of a combination of parameters such as pulse width, bandwidth, char plate, or time interval between pulses.

In step S102, the excitation unit 107 receives the control signal 30 and repeatedly generates the pulse signal 31 indicated by the control signal 30. Then, the excitation unit 107 outputs the pulse signal 31 to the transmission unit 108, which is an amplification unit. The excitation unit 107 outputs a part or all of the generated all-pulse signals to the receiving unit 130. Here, the timing of outputting the pulse signal 31 to the receiving unit 130 is assumed to be before the receiving unit 130 outputs the pulse signal 31 to the transmitting unit 108, but the timing is not limited to this. The excitation unit 107 may simultaneously transmit a pulse signal to the transmission unit 108 and the reception unit 130, or may transmit the pulse signal 31 to the reception unit 130 after outputting all the pulse signals to the transmission unit 108.

In step S103, when the transmission unit 108 receives the pulse signal 31 from the excitation unit 107, the transmission unit 108 amplifies the signal level of the pulse signal 31 to a desired signal level, and transmits and receives the amplified pulse signal n (n = 1). , 2, ..., N).
When the transmission / reception switching device n (n = 1, 2, ..., N) receives the amplified pulse signal from the transmission unit 108, the transmission / reception switching device n (n = 1, 2, ..., N) transmits the pulse signal to the antenna opening n (n = 1, 2, ..., N) of the transmission / reception antenna 120. Output to N).

In step S104, the transmission / reception antenna 120 irradiates the pulse signal received from the transmission / reception switch n (n = 1, 2, ..., N) toward the observation region. Specifically, the irradiated pulse signal is scattered on the ground surface, and a part of the scattered wave returns to the transmission / reception antenna 120 as a reflected wave of the pulse signal. The antenna opening n (n = 1, 2, ..., N) of the transmission / reception antenna 120 receives the reflected wave as an RF signal and outputs it to the transmission / reception switch n (n = 1, 2, ..., N).

In step S105, when the transmission / reception switch n (n = 1, 2, ..., N) receives the RF signal from the transmission / reception antenna 120, the transmission / reception switch n (n = 1, 2, ..., N) transmits the RF signal to the receiving unit n (n = 1, 2, ..., N). Output.

In step S106, when the receiving unit n (n = 1, 2, ..., N) receives the RF signal from the transmission / reception switching device n (n = 1, 2, ..., N), the frequency of the RF signal is changed. Convert to baseband signal. The receiving unit n (n = 1, 2, ..., N) converts the baseband signal from an analog signal to a digital signal at a predetermined sampling rate to obtain the receiving signal 32. Then, the receiving unit n (n = 1, 2, ..., N) outputs the received signal 32 to the signal processing unit 140. Further, the receiving unit n (n = 1, 2, ..., N) converts the pulse signal 31 received from the exciting unit 107 into a digital signal and uses it as the transmitting signal 33. Then, the receiving unit n (n = 1, 2, ..., N) outputs the transmission signal 33 to the signal processing unit 140.

In step S107, the range Fourier transform unit 40 of the signal processing unit 140 performs range Fourier transform on each of the plurality of received signals 32 received by the plurality of antenna openings n (n = 1, 2, ..., N). It is converted into a frequency domain and output as a plurality of converted reception signals 61.
The code modulation unit 41 of the signal processing unit 140 multiplies each of the plurality of conversion reception signals 61 by each of the plurality of code modulation codes 332 having orthogonality to each other, and outputs the plurality of modulation reception signals 62. The code modulation unit 41 multiplies each of the plurality of conversion reception signals 61 by the reference signal 331 for performing the decharp process for canceling the char plate, and multiplies each of the plurality of code modulation codes 332, and is different from each other. A plurality of modulated reception signals 62 having a char plate are output. The reference signal 331 is a signal having a complex conjugate relationship with each of the plurality of transmission signals 33 corresponding to the plurality of received signals 32.
The multiplexing unit 43 multiplexes the plurality of modulated reception signals 62 and records them as the multiplexing signals 34 in the data recording unit 150.

The range Fourier transform unit 40 receives the reception signal n (n = 1, 2, ..., N) output by the reception unit n (n = 1, 2, ..., N) and the transmission signal n (n = 1, 2, ..., N). , N) is converted into the frequency domain. Among them, the transmission signal n (n = 1, 2, ..., N) is recorded in the recording device 42. The recording device 42 converts the transmission signal n (n = 1, 2, ..., N) into the reference signal n (n = 1, 2, ..., N). The code modulation multiplier n (n = 1, 2, ..., N) of the code modulation unit 41 is a reference signal recorded in the recording device 42 in the received signal n (n = 1, 2, ..., N) in the frequency domain. Multiply n (n = 1, 2, ..., N) and the code modulation code C _n ^m (n = 1, 2, ..., N) in the frequency space. By this multiplication, the received signals n (n = 1, 2, ..., N) after the code modulation processing are combined, and the multiplexed signal 34 is generated. The multiplexing unit 43 of the signal processing unit 140 outputs the multiplexing signal 34 to the data recording unit 150.

Here, the processing content of the signal processing unit 140 will be specifically described.
The signal processing unit 140 Fourier transforms the received signal 32 and the transmitted signal 33 output by the receiving unit n (n = 1, 2, ..., N) in the range direction by the range Fourier transform unit 40, and the received signal after the Fourier transform. 32 is output to the code modulation unit 41.
In this embodiment, the transmission signal 33 and the code modulation code of the reference signal 331 and N is the complex conjugate relationship ^{_{C n m (n = 1,2,}} .., N) when holding the the recording device 42 To do. Therefore, the range Fourier transform unit 40 Fourier transforms the transmission signal 33 received from the reception unit 130 in the range direction and outputs it to the recording device 42.
The recording device 42 generates and holds the reference signal 331 which has a complex conjugate relationship of the digital transmission signal after the range Fourier transform.

Recording device 42, the reference signal and the code-modulated code ^{_{C n m (n = 1,2,}} .., N) and outputs the code modulation unit 41. Specifically, the code modulation unit 41 multiplies the signal output from the reception unit 1 by using the code modulation multiplier 1 in the order of the reference signal and the code modulation code C ₁ ^m . Further, the code modulation unit 41 multiplies the signal output from the reception unit 2 in the order of the reference signal and the code modulation code C ₂ ^m by using the code modulation multiplier 2. Then, the code modulation unit 41 multiplies the signal output from the reception signal from the reception unit N in the order of the reference signal and the code modulation code C _N ^m by using the code modulation multiplier N. The code modulation unit 41 outputs the received signal n (n = 1, 2, ..., N) after the code modulation process to the multiplexing unit 43.
The multiplexing unit 43 multiplexes the received signal n (n = 1, 2, ..., N) after the code modulation processing, that is, the modulated reception signal 62, and records it in the data recording unit 150 as the multiplexing signal 34. Specifically, the multiplexing unit 43 synthesizes the received signal n (n = 1, 2, ..., N) after the code modulation processing received from the code modulation unit 41 in the frequency space, and generates a multiplexing signal. 34 is output to the data recording unit 150.

In step S108, when the data recording unit 150 receives the multiplexing signal 34, the data recording unit 150 records the multiplexing signal 34 and outputs a part or all of the multiplexing signal 34 to the data transfer unit 101 at a desired time.

In step S109, the data transfer unit 101 receives the multiplexing signal 34, maps the multiplexing signal 34 to a radio frequency signal, and irradiates the space toward the signal demodulation unit 102.

In step S110, the demodulator 201 of the signal demodulation unit 102 demodulates the multiplexing signal 34 and outputs it as a demodulation multiplexing signal 63.
Further, the code demodulation unit 202 of the signal demodulation unit 102 is obtained by multiplying the demodulation multiplexing signal 63 by each of the plurality of code demodulation codes 221 each having a complex conjugate relationship with each of the plurality of code modulation codes 332. The image spectrum 36 including the plurality of spectra is output. Signal demodulator 102 is specifically configured to receive the multiplexed signal 34, with respect to the multiplexed signal 34, code demodulator code recorded in the recording device ^{_{203 C n d (n = 1,2}} , ..., N) is multiplied, and the image spectrum n (n = 1, 2, ..., N) is output to the image reproduction unit 103.

Here, the processing content of the signal demodulation unit 102 will be specifically described.
The demodulator 201 of the signal demodulation unit 102 receives the radio frequency signal to which the multiplexed signal 34 is mapped from the data transfer unit 101, and demodulates the multiplexed signal 34. Then, the demodulator 201 outputs the demodulated multiplexed signal 34 to the code demodulation unit 202.
The recording device 203 code demodulates the code demodulation code C _n ^d (n = 1, 2, ..., N) which has a complex conjugate relationship with the code modulation code C _n ^m (n = 1, 2, ..., N). Output to unit 202. Specifically, the code demodulation unit 202 multiplies the multiplexed signal 34 by the code demodulation code C ₁ ^d on the frequency space using the code demodulation multiplier 1. Further, the code demodulation unit 202 multiplies the multiplexed signal 34 by the code demodulation code C ₂ ^d on the frequency space using the code demodulation multiplier 2. Further, the code demodulation unit 202 multiplies the multiplexed signal 34 by the code demodulation code C _N ^d on the frequency space using the code demodulation multiplier N. The code demodulation multiplier n (n = 1, 2, ..., N) of the code demodulation unit 202 generates an image spectrum n (n = 1, 2, ..., N), and the image spectrum n (n = 1, 2) , ..., N) is output to the image reproduction unit 103.

In step S111, the image reproduction unit 103 reproduces an image using the image spectrum 36. Specifically, the image reproduction unit 103 receives an image spectrum n (n = 1, 2, ..., N) from the signal demodulation unit 102 and performs image reproduction processing such as range compression or azimuth compression to obtain a SAR image. Play 37.
The display 104 displays the SAR image 37 reproduced by the image reproduction unit 103 on the display.

*** Other configurations ***
<Modification example 1>
FIG. 7 is a configuration diagram showing a synthetic aperture radar system 100 according to a modified example of the present embodiment. Further, FIG. 8 is a configuration diagram showing a signal processing unit 140 according to a modified example of the present embodiment.
In step S102 of the present embodiment, it is assumed that the excitation unit 107 transmits the pulse signal 31 to the reception unit 130, and the signal processing unit 140 generates the reference signal 331. However, as described above, it is not necessary to generate the reference signal 331. When the reference signal 331 is not generated, the configuration diagram of the synthetic aperture radar system 100 is as shown in FIG. The excitation unit 107 does not transmit the pulse signal 31 to the reception unit 130. Then, as shown in FIG. 7, when the reference signal is not generated, the transmission signal is not transmitted to the recording device 42 and the reference signal is not generated.

<Modification 2>
In the present embodiment, the range Fourier transform unit 40, the code modulation unit 41, the multiplexing unit 43, the recording device 42, which are the components of the signal processing unit 140, and the demodulator 201, which is the component of the signal demodulation unit 102, The code demodulation unit 202 and the recording device 203 are realized by a dedicated electronic circuit 909. However, these functional elements may be implemented in software.

FIG. 9 is a hardware configuration example showing the synthetic aperture radar system 100 according to the modified example of the present embodiment.
The synthetic aperture radar system 100 is a computer. The synthetic aperture radar system 100 includes a processor 910 and other hardware such as a memory 921, an auxiliary storage device 922, an input interface 930, an output interface 940, and a communication device 950. The processor 910 is connected to other hardware via a signal line and controls these other hardware.

The synthetic aperture radar system 100 includes the above-mentioned code modulation unit 41, multiplexing unit 43, recording device 42, demodulator 201, code demodulation unit 202, and recording device 203 as functional elements. These functional elements are referred to as each functional element of the synthetic aperture radar system 100.

Each functional element of the synthetic aperture radar system 100 is realized by software. The recording devices 42 and 203 may be provided in the memory 921.

Processor 910 is a device that executes a synthetic aperture radar program. The synthetic aperture radar program is a program that realizes the functions of each functional element of the synthetic aperture radar system 100.
The processor 910 is an IC (Integrated Circuit) that performs arithmetic processing. Specific examples of the processor 910 are a CPU, a DSP (Digital Signal Processor), and a GPU (Graphics Processing Unit).

The auxiliary storage device 922 is a storage device that stores data. A specific example of the auxiliary storage device 922 is an HDD. Further, the auxiliary storage device 922 may be a portable storage medium such as an SD (registered trademark) memory card, CF, NAND flash, flexible disc, optical disk, compact disc, Blu-ray (registered trademark) disc, or DVD. HDD is an abbreviation for Hard Disk Drive. SD® is an abbreviation for Secure Digital. CF is an abbreviation for CompactFlash®. DVD is an abbreviation for Digital Versaille Disc.

The input interface 930 is a port connected to an input device such as a mouse, keyboard, or touch panel. Specifically, the input interface 930 is a USB (Universal Serial Bus) terminal. The input interface 930 may be a port connected to a LAN (Local Area Network).
The output interface 940 is a port to which a cable of an output device such as a display is connected. Specifically, the output interface 940 is a USB terminal or an HDMI (registered trademark) (High Definition Interface Interface) terminal. Specifically, the display is an LCD (Liquid Crystal Display).

The memory 921 and the communication device 950 are the same as those described with reference to FIG.

The synthetic aperture radar program is loaded into processor 910 and executed by processor 910. In the memory 921, not only the synthetic aperture radar program but also the OS (Operating System) is stored. Processor 910 executes the synthetic aperture radar program while executing the OS. The synthetic aperture radar program and the OS may be stored in the auxiliary storage device 922. The synthetic aperture radar program and OS stored in the auxiliary storage device 922 are loaded into the memory 921 and executed by the processor 910. A part or all of the synthetic aperture radar program may be incorporated in the OS.

The synthetic aperture radar system 100 may include a plurality of processors that replace the processor 910. These plurality of processors share the execution of the synthetic aperture radar program. Each processor, like the processor 910, is a device that executes a synthetic aperture radar program.

Data, information, signal values and variable values used, processed or output by the synthetic aperture radar program are stored in memory 921, auxiliary storage 922, or a register or cache memory in processor 910.

The "part" or "device" of each functional element of the synthetic aperture radar system 100 may be read as "processing", "procedure" or "process". Further, "processing" of code modulation processing, multiplexing processing, demodulation processing, code demodulation processing, and recording processing may be read as "program", "program product", or "computer-readable recording medium on which the program is recorded". ..

The synthetic aperture radar program causes a computer to execute each process, each procedure or each process in which the "part" of each of the above parts is read as "process", "procedure" or "process". In addition, the synthetic aperture radar method corresponds to each procedure in which the "part" of each of the above parts is read as "procedure".
Synthetic aperture radar programs may be provided stored on a computer-readable recording medium. The synthetic aperture radar program may also be provided as a program product.

Each of the processor and the electronic circuit is also called a processing circuit. That is, the functions of each functional element of the synthetic aperture radar system 100 are realized by the processing circuit.

*** Explanation of the effect of this embodiment ***
A synthetic aperture radar system that employs a multi-beam SAR system can achieve a wider area and higher resolution. However, since it is necessary to record data for each reception aperture, the amount of data to be recorded as a synthetic aperture radar system increases. As the amount of data increases, it is necessary to increase the capacity of the data recording unit and the data transfer speed, which increases system resources and costs. In order to prevent the increase of system resources and costs while maintaining the observation performance, it is necessary to consider a method of reducing the amount of data.
In the synthetic aperture radar system according to the present embodiment, the amount of data is reduced by multiplexing the received signals of the reflected waves acquired from the plurality of antenna openings after the code modulation processing by the signal processing unit. Further, the signal demodulation unit performs code demodulation processing on the multiplexed received signal to generate an image spectrum corresponding to the numerical aperture of the antenna. Therefore, according to the synthetic aperture radar system according to the present embodiment, it is possible to suppress an increase in hardware resources while ensuring azimuth resolution and observation width.

In the synthetic aperture radar system according to the present embodiment, the signal processing unit converts the received signal received by the N antenna openings of the transmitting and receiving antennas into a digital received signal in the frequency space, and the multiplexing process is performed after the code modulation process. The data capacity is reduced by implementing. Then, the signal demodulation unit generates an image spectrum corresponding to N received signals by code demodulation processing, so that a high-performance SAR image can be reproduced while suppressing an increase in data capacity and hardware.
This embodiment is suitable for a synthetic aperture radar system equipped with a plurality of antenna apertures and reproducing a synthetic aperture radar image using a multi-beam SAR method.

In the above-described first embodiment, each functional element of the synthetic aperture radar system has been described as an independent functional block. However, the configuration of the synthetic aperture radar system does not have to be the configuration as in the above-described embodiment. The functional block of the synthetic aperture radar system may have any configuration as long as it can realize the functions described in the above-described embodiment. Further, the synthetic aperture radar system may be a system including a plurality of devices instead of one device.
Further, in the first embodiment, a plurality of parts may be combined and carried out. Alternatively, one part of this embodiment may be implemented. In addition, this embodiment may be implemented in any combination as a whole or partially.
That is, in the first embodiment, it is possible to freely combine the respective embodiments, modify any component of each embodiment, or omit any component in each embodiment.

It should be noted that the above-described embodiments are essentially preferred examples and are not intended to limit the scope of the present invention, the scope of application of the present invention, and the scope of use of the present invention. The above-described embodiment can be variously modified as needed.

30 control signal, 31 pulse signal, 32 received signal, 33 transmitted signal, 34 multiplexed signal, 35 radio frequency signal, 36 image spectrum, 37 SAR image, 40 range Fourier converter, 41 code modulator, 42 recorder, 43 Multiplexing unit, 61 conversion reception signal, 62 modulation reception signal, 63 demodulation multiplexing signal, 100 composite aperture radar system, 101 data transfer unit, 102 signal demodulation unit, 103 image reproduction unit, 104 display, 106 control unit, 107 Excitation unit, 108 transmitter, 109 transmit / receive switch, 120 transmit / receive antenna, 130 receiver, 140 signal processing unit, 150 data recording unit, 200 observation target, 201 demodulator, 202 code demodulation unit, 203 recording device, 221 code demodulation Code, 331 reference signal, 332 code modulation code, 909 electronic circuit, 910 processor, 921 memory, 922 auxiliary storage, 930 input interface, 940 output interface, 950 communication device.

Claims (6)

A range Fourier transform unit that converts each of a plurality of received signals received by a plurality of antenna openings into a frequency domain by range Fourier transform and outputs them as a plurality of converted received signals.
A code modulation unit that multiplies each of the plurality of conversion reception signals by each of a plurality of code modulation codes having orthogonality to each other and outputs the plurality of modulation reception signals.
A synthetic aperture radar system including a multiplexing unit that multiplexes the plurality of modulated reception signals and records them as a multiplexing signal in a data recording unit. The synthetic aperture radar system
A demodulator that demodulates the multiplexed signal and outputs it as a demodulated multiplexed signal,
A code demodulation unit that outputs an image spectrum consisting of a plurality of spectra obtained by multiplying the demodulation-multiplexed signal by each of a plurality of code demodulation codes having a complex conjugate relationship with each of the plurality of code modulation codes. When,
The synthetic aperture radar system according to claim 1, further comprising an image reproduction unit that reproduces an image using the image spectrum. The code modulation unit
Each of the plurality of converted received signals is multiplied by a reference signal for performing a decharping process for canceling the char plate, and each of the plurality of code modulation codes is multiplied, and the plurality of modulated receptions having different char plates are received. The synthetic aperture radar system according to claim 1 or 2, which outputs a signal. The code modulation unit
The synthetic aperture radar system according to claim 3, wherein the reference signal having a complex conjugate relationship with each of the plurality of transmission signals corresponding to the plurality of reception signals is multiplied by each of the plurality of conversion reception signals. In the synthetic aperture radar method of the synthetic aperture radar system,
The range Fourier transform unit converts each of the plurality of received signals received by the plurality of antenna openings into a frequency domain by performing range Fourier transform, and outputs them as a plurality of converted received signals.
The code modulation unit multiplies each of the plurality of conversion reception signals by each of the plurality of code modulation codes having orthogonality to each other, and outputs the plurality of modulation reception signals.
A synthetic aperture radar method in which a multiplexing unit multiplexes the plurality of modulated reception signals and records them as multiplexed signals in a data recording unit. In the synthetic aperture radar program of the synthetic aperture radar system
Range Fourier transform processing that converts each of a plurality of received signals received by a plurality of antenna openings into a frequency domain by range Fourier transform and outputs them as a plurality of converted received signals.
A code modulation process in which each of the plurality of converted received signals is multiplied by each of a plurality of code modulation codes having orthogonality to each other and output as a plurality of modulated received signals.
A synthetic aperture radar program that causes a computer to perform a multiplexing process that multiplexes the plurality of modulated reception signals and records them as multiplexed signals in a data recording unit.

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