JP7257885B2 - Synthetic Aperture Radar System, Synthetic Aperture Radar Method, and Synthetic Aperture Radar Program - Google Patents

Description

The present invention relates to a synthetic aperture radar system, a synthetic aperture radar method, and a synthetic aperture radar program. 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 and reception beams by antenna aperture control and achieve high resolution and a wide observation width.

Synthetic aperture radar is called SAR (Synthetic Aperture Radar). Synthetic aperture radar may be referred to as SAR below for description. A SAR is mounted on a flying object such as an artificial satellite or an aircraft. SAR is a sensor capable of penetrating clouds or fog and observing the surface of the earth day or night. The key parameters for SAR are azimuth resolution and range swath. These parameters are in a conflicting relationship from the imaging principle of SAR. However, the multi-beam SAR method, in which the aperture of the antenna is divided and the reception data is processed for each divided aperture, makes it possible to improve both the azimuth resolution and the expansion of the range observation width.
A multi-beam SAR system that achieves high azimuth resolution and a wide range observation width is a well-known technology, and is disclosed in Non-Patent Document 1 and Non-Patent Document 2 below.

An SAR system employing the multi-beam SAR method can achieve widening and high resolution. However, since it is necessary to record data for each antenna aperture, the amount of data recorded in the SAR system increases. As the amount of data increases, it is necessary to increase the number or capacity of data recording units, which poses problems in terms of system resources and costs. Therefore, various methods are being studied to reduce the amount of data in the multi-beam SAR system. Non-Patent Document 3 discloses a method for improving the efficiency of a multi-beam SAR system using on-board quantization.

N. Gebert andG. Krieger, "Azimuth Phase Center Adaptation on Transmit for High-Resolution Wide-Swath SAR Imaging," in IEEE Geoscience and Remote Sensing Letters, vol. 6, no. 4, pp. 782-786, Oct. 2009. Qsong Wu, Mengdao Xing, Zheng Bao and Hongzhu Shi, “Wide swath, high range resolution imaging with MIMO-SAR,” 2009 IET International Radar Conference, Guilin, p. 1-6. M. Martone, et al. , "An Efficient Onboard Quantization Strategy for Multi-Channel SAR Systems", EUSAR2018, pp. 572-577, June 6, 2018. Kazuo Ouchi, Fundamentals of Synthetic Aperture Radar for Remote Sensing (Second 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 over the Doppler band and the quantization error is relatively large. Since the actual SAR observation data does not satisfy such a condition, it is difficult to use the efficiency improvement method of Non-Patent Document 3 with the actual SAR observation data.

SUMMARY OF THE INVENTION An object of the present invention is to suppress an increase in the amount of data associated with an increase in the antenna numerical aperture required for high performance in a multi-beam SAR system.

A synthetic aperture radar system according to the present invention includes:
a Range Fourier transform unit that Range Fourier transforms each of a plurality of received signals received by a plurality of antenna apertures and outputs a plurality of transformed received signals;
a code modulation unit that multiplies each of the plurality of transformed received signals by each of a plurality of code modulation codes having orthogonality to each other and outputs a plurality of modulated received signals;
and a multiplexing unit that multiplexes the plurality of modulated received signals and records them in a data recording unit as a multiplexed signal.

In the synthetic aperture radar system according to the present invention, the received signals acquired from a plurality of antenna apertures are multiplexed after the code modulation processing is performed by the code modulation section, thereby reducing the amount of data. Therefore, according to the synthetic aperture radar system of the present invention, it is possible to suppress an increase in hardware resources even when the antenna numerical aperture required for high performance is increased.

1 is a configuration diagram showing a synthetic aperture radar system according to Embodiment 1; FIG. 3 is a configuration diagram of a signal processing unit of the synthetic aperture radar system according to Embodiment 1; FIG. 4 is a configuration diagram of a signal demodulator of the synthetic aperture radar system according to Embodiment 1. FIG. FIG. 10 is a hardware configuration example of a synthetic aperture radar system according to a modification of the first embodiment; FIG. FIG. 4 is a conceptual diagram when range Fourier transform processing, dechirping processing, code modulation processing, multiplexing processing, and code demodulation processing according to Embodiment 1 are performed. 4 is a flowchart showing the operation of the synthetic aperture radar system according to Embodiment 1; FIG. FIG. 2 is a configuration diagram showing a synthetic aperture radar system according to a modification of Embodiment 1; FIG. 4 is a configuration diagram showing a signal processing unit according to a modification of Embodiment 1; FIG. 4 is a hardware configuration example showing a synthetic aperture radar system according to a modification of the first embodiment;

The present embodiment will be described below with reference to the drawings. In each figure, the same reference numerals are given to the same or corresponding parts. In the description of the embodiments, the description of the same or corresponding parts will be omitted or simplified as appropriate.

Embodiment 1.
*** Configuration description ***
FIG. 1 is a configuration diagram showing a synthetic aperture radar system 100 according to this embodiment. Below, 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 a pulse signal onto an observation area, receives a reflected wave reflected by an observation target 200, and reproduces an SAR image using the received signal.

FIG. 2 is a configuration diagram of signal processing section 140 of synthetic aperture radar system 100 according to the present embodiment.
FIG. 3 is a configuration diagram of signal demodulator 102 of synthetic aperture radar system 100 according to the present embodiment.
The signal processing unit 140 multiplies each of a plurality of received signals 32 received from a plurality of antenna apertures by each of a plurality of mutually orthogonal code modulation codes 332 to multiplex the plurality of received signals 32 . The received signal 32 is a digital received signal digitized by the receiver 130 . The multiplexed signal is multiplexed signal 34 .
The signal demodulator 102 multiplies the multiplexed signal 34 multiplexed by the signal processor 140 by the code demodulation code 221 to generate the image spectrum 36 .
Assume that the number of antenna apertures is N in this embodiment. N is a natural number. That is, the transmitting/receiving antenna 120 has N antenna apertures from antenna aperture 1 to antenna aperture N. FIG.

The excitation section 107 generates a pulse signal 31 and outputs it to the transmission section 108 and the reception section 130 at regular time intervals. The pulse signal 31 generated by the excitation section 107 is a chirp signal. A chirp signal is a signal whose rate of change in frequency changes with time. The rate of change of the frequency of the chirp signal is called the chirp rate.
The pulse width, bandwidth, chirp rate and time interval between pulses of the pulse signal 31 generated by the excitation section 107 are determined by the control signal 30 output from the control section 106 .
Transmitting section 108 amplifies the signal level of pulse signal 31 output from excitation section 107 to a desired signal level, and outputs it to duplexer 109 . The duplexer 109 comprises N duplexers n (n=1, 2, . . . , N).

A duplexer n (n=1, 2, . . . , N) switches the transmitting and receiving antennas between the transmitting state and the receiving state. In the transmission state, the duplexer n (n=1, 2, . . . , N) outputs the signal from the transmitter 108 to the antenna aperture n (n=1, 2, . In the receiving state, the duplexer n (n=1, 2, . Output to receiving unit n (n=1, 2, . . . , N).

The transmitting/receiving antenna 120 has antenna apertures n (n=1, 2, . . . , N) divided in the azimuth direction. The transmitting/receiving antenna 120 irradiates the observation area with a pulse signal output from the transmission/reception switch n (n=1, 2, . Radio Frequency) signal. Transmitting/receiving antenna 120 outputs RF signals to duplexers 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 and receiving antennas may be separate.

The receiver n (n=1, 2, . . . , N) converts the RF signal output from the duplexer 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 received signal, that is, a received signal 32 by an A/D (Analog to Digital) converter. The receiver n (n=1, 2, . . . , N) outputs the received signal 32 to the signal processor 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 an A/D converter, and the signal processing unit output to 140.

The signal processing unit 140 transforms the reception signal 32 and the transmission signal 33 output by the reception unit n (n=1, 2, . . . , N) into the frequency domain. Also, the signal processing unit 140 converts the transmission signal 33 into a reference signal 331 . The signal processing unit 140 performs code modulation processing in which the received signal 32 is multiplied by the reference signal 331 and the code modulation code 332 in the frequency domain. Then, the signal processing unit 140 generates a multiplexed signal 34 by multiplexing the N received signals 32 . The signal processing section 140 outputs the multiplexed signal 34 to the data recording section 150 .

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

The signal demodulation unit 102 demodulates the multiplexed signal 34 from the radio frequency signal 35 output from the data transfer unit 101 to generate a demodulated multiplexed signal 63 . The signal demodulation unit 102 performs code demodulation processing for multiplying the demodulated multiplexed signal 34 , that is, the demodulated multiplexed signal 63 by the code demodulation code 221 . A signal demodulator 102 generates an image spectrum 36 corresponding to the N received signals received at each antenna aperture. The signal demodulator 102 then outputs the image spectrum 36 to the image reproducer 103 .

The image reproduction unit 103 performs processing for reproducing the SAR image 37 using the image spectrum 36 output by the signal demodulation unit 102 .

The display device 104 includes a processor such as a GPU (Graphic Processing Unit). The display device 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 between transmission and reception and the timing at which the excitation unit 107 generates the pulse signal 31 .

<Detailed Description of Signal Processing Unit 140>
Range Fourier transform unit n (n=1, 2, . . . , N) of signal processing unit 140 shown in FIG. Transmit signal 33 is transformed into the frequency domain.
The recording device 42 of the signal processing unit 140 converts the transmission signal 33 output from the range Fourier transform unit n (n=1, 2, . 331 is generated and held. Also, the recording device 42 records code-modulation codes C _nm (n=1, 2, . . . , N) that are orthogonal ^to each other. That is, the n code-modulation codes C _nm are orthogonal to ^each other. The recording device 42 outputs the code modulation code C _nm (n=1, 2, . . . , N) to the code modulation section ⁴¹ .

The code modulation multiplier n (n=1, 2, . . . , N) of the code modulation unit 41 generates a reference signal for each received signal 32 output by the range Fourier change unit n (n=1, 2, . 331 and ^the code modulation code C _nm (n=1, 2, . That is, n digital ^received signals obtained by multiplying n received signals by code modulation codes C _nm (n=1, 2, . . . , N) have orthogonality to each other. Specifically, the first digital received signal and the second digital received signal have orthogonality. Here, having orthogonality means having a mathematical property that two functions, ie, signals, can be combined and separated.

In FIG. 2, multiplication of code-modulation codes C _nm (n=1, 2, . . . , N) is performed ⁱⁿ parallel. However, by sequentially multiplying the received signal ³² output by the range Fourier transform unit n (n=1, 2, . . . , N) by the code modulation code C _nm (n=1, 2, . . . , N), good too. At this time, only one code modulation multiplier is required. A recording device 42 holds the received signal 32 output from the range Fourier transform unit n ( _n =1, 2, ^. , N) to the code modulation multiplier.

Also, FIG. 2 shows an example ⁱⁿ which the recording device 42 holds code-modulation codes C _nm (n=1, 2, . . . , N). However, the code-modulation code may be calculated each time the code-modulation multiplier multiplies 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 pulse signals generated by the excitation section 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 in FIG. 1 does not transmit the pulse signal 31 to the reception unit 130 .
Further, although it is assumed here that the received signal 32 is multiplied by the reference signal 331 , 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 in 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 .

The multiplexing unit 43 of the signal processing unit 140 specifically includes a well-known multiplexer circuit, and converts the N received signals 32 output from the code modulation unit 41 into one multiplexed signal 34 in the frequency space. Synthesize.

The demodulator 201 of the signal demodulator 102 in FIG. 3 demodulates the multiplexed signal 34 from the radio frequency signal 35 output from the data transfer unit 101, and outputs the demodulated multiplexed signal 63 to the code modulator n ( 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 it to the code demodulation multiplier n (n=1, 2, . . . , N) of the code demodulation unit 202. do.

A signal demodulation multiplier n (n=1, 2, . . . , N) multiplies the multiplexed signal 34 by a code demodulation code n (n=1, 2, . do.
In FIG. 3, code-demodulation code multiplication is performed in parallel, and code-demodulation code _C ^nd (n=1, 2, . can 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 records the code-demodulation code _C ^nd (n=1, 2, . . . , N). together with the multiplexed signal 34 to the code modulation multiplier.

In FIG ^{. 3, the recording device 203 records the code-demodulation code C nd} ₍ n=1, 2, . . . , N) and outputs it to the code-demodulation multiplier. However, the recording device 203 may hold the code modulation code C _nm (n=1, 2, . . . , N) and calculate the code demodulation ^code when multiplying the multiplexed signal by the code demodulation code.

FIG. 4 is a diagram showing a hardware configuration example of a computer used in the synthetic aperture radar system 100 according to this embodiment.
Synthetic aperture radar system 100 illustratively comprises electronic circuitry 909 , memory 921 , and 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 components of the signal processing unit 140, is implemented by an electronic circuit 909 that is dedicated hardware. The electronic circuit 909 is specifically a range Fourier transform circuit, a code modulation circuit, a multiplexing circuit and a recording device circuit. The demodulator 201, the code demodulator 202, and the recording device 203, which are components of the signal demodulator 102, are each realized by an electronic circuit 909, which is dedicated hardware. The electronic circuit 909 is specifically a demodulator circuit, a code demodulator circuit, and a recorder circuit.

Here, the range Fourier transform circuit, the code modulation circuit, the multiplexing circuit, the recording device circuit, the demodulation circuit, and the code modulation circuit are specifically single circuits, composite circuits, programmed processors, ASICs, FPGAs, or a combination of these. ASIC is an abbreviation for Application Specific Integrated Circuit. FPGA is an abbreviation for Field-Programmable Gate Array.

The constituent elements of signal processing section 140 and signal demodulation section 102 are not limited to those realized by dedicated hardware. The components of signal processing section 140 and signal demodulation section 102 may be realized by software, firmware, or a combination of software and firmware. Software or firmware is stored as a program in a computer's 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.

The memory 921 of the computer specifically corresponds to a storage device such as non-volatile or volatile semiconductor memory such as RAM, ROM, flash memory, EEPROM, magnetic disk, or 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.

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 NIC. LAN is an abbreviation for Local Area Network. NIC is an abbreviation for Network Interface Card.

FIG. 5 is a conceptual diagram when range Fourier transform processing, dechirping processing, code modulation processing, multiplexing processing, and code demodulation processing according to this embodiment are performed.
The principle of data amount reduction by range Fourier transform processing, dechirping processing, code modulation processing, multiplexing processing, and code demodulation processing will be described with reference to FIG. In the example of FIG. 5, the dechirping process of multiplying the received signal by the reference signal is performed. However, code modulation processing may be performed on the received signal without performing dechirping processing. At this time ^, the code demodulation code C _nd ( _n =1, 2 ^, . is the complex conjugate of the result of multiplying by .

When the time is τ and the chirp rate is K, the transmission wave E _t (τ) transmitted by the synthetic aperture radar system 100 is expressed as Equation 1.

The received wave E _r (τ) received by the synthetic aperture radar system can be represented by the same waveform as the transmitted wave E _t (τ). 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 S _t ^* (τ) of the transmitted wave is the complex conjugate of the received wave. The complex conjugate E _t ^* (τ) of the transmitted wave is expressed by Equation (3).

The complex conjugate E _t ^* (τ) of this transmission wave becomes the reference signal 331 described above. A reference signal 331 indicates a chirp rate opposite to that of the received wave. The chirp rate of the received wave is canceled by taking the product of the complex conjugate E _t ^* (τ) of the transmitted wave and the Fourier transform of the received wave. This chirp rate cancellation is called dechirping. The received signal S _dechirp (f) after dechirping is obtained by Equation (4). Here, F indicates Fourier transform, and f indicates frequency space.

By multiplying the dechirped received signal by a signal C _nm (f) having a certain chirp rate ^K _n in the frequency space, the chir rate of the received signal is changed to the chir rate K _n as shown in Equation 5. can do.

Here, signals having different chirp rates and exhibiting high orthogonality are called code modulation codes. Formula 6 shows an example of two code-modulation codes having high orthogonality to each other. where τ ₀ is the pulse width of the received signal. Also, K ₁ and K ₂ are chir plates having the same sign. And F ⁻¹ is the inverse Fourier transform.

The code-modulation code shown here is an example, and the code-modulation code C _n ^m (n=1, 2, . Specifically, it may be linear, nonlinear, or polynomial.

As shown in FIG. 5, dechirping is performed on received signals output from antenna apertures n (n=1, 2, . . . , N), and each received signal is multiplied by a code modulation code in frequency space, N received signals with different chirp rates are generated. This code modulation code multiplication processing is referred to as code modulation processing.

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 code modulation processing are mutually orthogonal and can be combined. The process of combining received signals is called multiplexing.
Representing the received signal n (n=1, 2, . . . , N) after code modulation processing as S _{mod —} n (f), the received signal S _multi (f) after multiplexing processing is expressed as Equation 7. .

Here, since the data amounts of S _{mod — n} (f) and S _multi (f) are the same, the received signal S _multi (f) after multiplexing is 1 for N received signals before multiplexing. /N data amount.

One of the multiple access techniques in the wireless communication field is the code division multidimensional system. The code division multidimensional system is a system in which communication is performed by assigning a different spread spectrum code to each signal using one frequency. However, in the code division multidimensional system, the transmission signal is spread over a wider band than the frequency band of the modulated signal using a spreading code, which is different from the present system.

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

In the code demodulation process, the received signal S _multi (f) after multiplexing is _multiplied by the code demodulated signal C ^nd (f) having a complex conjugate relationship with the code modulation code C _nm (f) ⁱⁿ the frequency domain. do. By this multiplication-based matched filtering process, only the signal with the same chirp rate 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 chirp rate contained in the synthesized signal is highly orthogonal, and high correlation can be obtained only for the desired chir rate signal by the matched filtering process.

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。 Code demodulation code _{C nd} (f) ^{( n} = 1, 2, . . . , N) is called a code demodulation process. The code-demodulation process allows extraction of the image spectrum of signals with N different chirp rates in the received signal after multiplexing. The image spectrum extracted at this time is the same as the product of the received signal of Expression 2 and the Fourier transform of the complex conjugate of the received signal. The image spectrum G _n (f) is expressed as in Equation (8).

Therefore, the extracted image spectrum is the same signal as when code modulation processing, multiplexing processing, and code demodulation processing are not performed, and can be processed by image reproduction processing.
The above is the principle of data amount reduction by code modulation processing, multiplexing processing, and code demodulation processing.

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

In step S<b>101 , the control unit 106 outputs pulse signal parameters stored in the control unit 106 to the excitation unit 107 using the control signal 30 . A pulse signal parameter is, in particular, one of a combination of parameters such as pulse width, bandwidth, chirp rate, or time interval between pulses.

In step S<b>102 , the excitation unit 107 receives the control signal 30 and repeatedly generates the pulse signal 31 indicated by the control signal 30 . Then, excitation section 107 outputs pulse signal 31 to transmission section 108, which is an amplification section. The excitation unit 107 outputs part or all of the generated pulse signals to the reception unit 130 . Here, it is assumed that the timing of outputting the pulse signal 31 to the receiving section 130 is before the receiving section 130 outputs the pulse signal 31 to the transmitting section 108, but the timing is not limited to this. The excitation section 107 may transmit pulse signals to the transmission section 108 and the reception section 130 at the same time, or may transmit the pulse signal 31 to the reception section 130 after outputting all pulse signals to the transmission section 108 .

In step S103, upon receiving 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 sends the amplified pulse signal to the duplexer n (n=1). , 2, . . . , N).
Transmit/receive switch n (n=1, 2, . N).

In step S104, the transmitting/receiving antenna 120 irradiates the observation area with the pulse signal received from the transmitting/receiving switch n (n=1, 2, . . . , N). The irradiated pulse signal is specifically scattered on the ground surface, and part of the scattered wave returns to the transmitting/receiving antenna 120 as a reflected wave of the pulse signal. Antenna openings n (n=1, 2, . . . , N) of the transmitting/receiving antenna 120 receive the reflected wave as an RF signal and output it to the duplexer n (n=1, 2, . . . , N).

In step S105, upon receiving the RF signal from the transmitting/receiving antenna 120, the duplexer n (n=1, 2, . Output.

In step S106, when receiving the RF signal from the duplexer n (n=1, 2, . . . , N), the receiver n (n=1, 2, . . . , N) changes the frequency of the RF signal, Convert to baseband signal. A 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 a received signal 32 . Then, the receiving section n (n=1, 2, . . . , N) outputs the received signal 32 to the signal processing section 140 . In addition, the receiving section n (n=1, 2, . . . , N) converts the pulse signal 31 received from the excitation section 107 into a digital signal and uses it as a transmission signal 33 . Then, the receiving section n (n=1, 2, . . . , N) outputs the transmission signal 33 to the signal processing section 140 .

In step S107, the range Fourier transform unit 40 of the signal processing unit 140 ranges Fourier transforms each of the plurality of received signals 32 received by the plurality of antenna apertures n (n=1, 2, . . . , N). Transform to the frequency domain and output as a plurality of transformed received signals 61 .
The code modulation section 41 of the signal processing section 140 multiplies each of the plurality of converted reception signals 61 by each of the plurality of code modulation codes 332 having orthogonality to each other, and outputs the result as a plurality of modulated reception signals 62 . Note that the code modulation unit 41 multiplies each of the plurality of converted received signals 61 by a reference signal 331 for performing dechirping processing for canceling the chirp rate, and multiplies each of the plurality of code modulation codes 332 to obtain a different signal. It outputs a plurality of modulated received signals 62 having chirp rates. The reference signal 331 is a signal having a complex conjugate relationship with each of the multiple transmission signals 33 corresponding to the multiple reception signals 32 .
The multiplexing unit 43 multiplexes the plurality of modulated reception signals 62 and records them in the data recording unit 150 as a multiplexed signal 34 .

Range Fourier transform unit 40 converts reception signal n (n=1, 2, . . . , N) output from reception unit n (n=1, 2, . , N) 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 a reference signal n (n=1, 2, . . . , N). The code modulation multiplier n (n=1, 2, . . . , N) of the code modulation unit 41 converts the reference signal recorded in the recording device 42 into the received signal n (n=1, 2, . . . , N) in the frequency domain. n (n=1, 2, . . . , N) and the code-modulation code C _nm (n=1, 2, . . . , N) are multiplied in frequency ^space . By this multiplication, received signals n (n=1, 2, . The multiplexing section 43 of the signal processing section 140 outputs the multiplexed signal 34 to the data recording section 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, . 32 is output to the code modulation unit 41 .
In this embodiment, it is assumed that the reference signal 331 having a complex conjugate relationship with the transmission signal 33 and N code modulation codes C _nm (n=1, 2, . . . , N) are stored in the recording ^device 42. do. Therefore, the range Fourier transform unit 40 Fourier transforms the transmission signal 33 received from the receiving unit 130 in the range direction, and outputs the result to the recording device 42 .
The recording device 42 generates and holds a reference signal 331 having a complex conjugate relationship of the range-Fourier-transformed digital transmission signal.

The recording device 42 outputs the reference signal and the code modulation code C _nm (n=1, 2, . . . , N) to the code ^modulation section 41 . Specifically, the code modulation unit 41 uses the code modulation multiplier 1 to multiply the signal output from the reception unit 1 by the reference signal and the code modulation code C ₁ ^m in this order. Also, the code modulation unit 41 uses the code modulation multiplier 2 to multiply the signal output from the reception unit 2 by the reference signal and the code modulation code C ₂ ^m in this order. Then, the code modulation unit 41 uses the code modulation multiplier N to multiply the signal output from the received signal from the reception unit N by the reference signal and the code modulation code C _N ^m in this order. The code modulation section 41 outputs the received signal n (n=1, 2, . . . , N) after code modulation processing to the multiplexing section 43 .
The multiplexer 43 multiplexes the code-modulated received signal n (n=1, 2, . Specifically, the multiplexing unit 43 synthesizes received signals n (n=1, 2, . 34 is output to the data recording unit 150 .

In step S108, upon receiving the multiplexed signal 34, the data recording section 150 records the multiplexed signal 34 and outputs part or all of the multiplexed signal 34 to the data transfer section 101 at a desired time.

In step S<b>109 , the data transfer unit 101 receives the multiplexed signal 34 , maps the multiplexed signal 34 to a radio frequency signal, and irradiates the radio frequency signal to the signal demodulation unit 102 .

In step S 110 , the demodulator 201 of the signal demodulator 102 demodulates the multiplexed signal 34 and outputs the demodulated multiplexed signal 63 .
Further, the code demodulator 202 of the signal demodulator 102 multiplies the demodulated multiplexed signal 63 by each of a plurality of code demodulation codes 221 each having a complex conjugate relationship with each of the plurality of code modulation codes 332. An image spectrum 36 consisting of a plurality of spectrums is output. Specifically, the signal demodulation unit 102 receives the multiplexed signal 34 and converts the multiplexed signal 34 into a code demodulation ^{code C nd} ₍ n=1, 2, . . . , N) and outputs the image spectrum n (n=1, 2, . . . , N) to the image reproduction unit 103 .

Here, the processing content of the signal demodulator 102 will be specifically described.
The demodulator 201 of the signal demodulator 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 . Demodulator 201 then outputs demodulated multiplexed signal 34 to code demodulator 202 .
The recording device 203 code-demodulates the code-demodulated code C _nd (n=1, 2, . . . ^, N) having a complex conjugate relationship with the code-modulated ^code C _nm (n=1, 2, .., N). Output to the unit 202 . More specifically, the code demodulator 202 multiplies the multiplexed signal 34 by the code demodulation code C ₁ ^d in the frequency space using the code demodulation multiplier 1 . Further, the code demodulator 202 multiplies the multiplexed signal 34 by the code demodulation code C ₂ ^d in the frequency space using the code demodulation multiplier 2 . Further, the code demodulator 202 multiplies the multiplexed signal 34 by the code demodulation code C _N ^d using the code demodulation multiplier N in the frequency space. Code demodulator multiplier n (n=1, 2, . . . , N) of code demodulator 202 generates image spectrum n (n=1, 2, . , . . . , N) to the image reproduction unit 103 .

In step S111, the image reproducing unit 103 uses the image spectrum 36 to reproduce an image. Specifically, the image reproduction unit 103 receives an image spectrum n (n=1, 2, . Play 37.
The display device 104 displays the SAR image 37 reproduced by the image reproduction unit 103 on the display.

***Other Configurations***
<Modification 1>
FIG. 7 is a configuration diagram showing a synthetic aperture radar system 100 according to a modification of the present embodiment. FIG. 8 is a configuration diagram showing signal processing section 140 according to a modification of the present embodiment.
In step S102 of the present embodiment, it is assumed that excitation section 107 transmits pulse signal 31 to reception section 130 and reference signal 331 is generated in signal processing section 140 . However, it is not necessary to generate the reference signal 331 as described above. 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 section 107 does not transmit the pulse signal 31 to the reception section 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 this embodiment, the range Fourier transform unit 40, the code modulation unit 41, the multiplexing unit 43, and the recording device 42, which are components of the signal processing unit 140, and the demodulator 201, which is a component of the signal demodulation unit 102, are described. The code demodulator 202 and the recording device 203 are implemented by a dedicated electronic circuit 909 . However, these functional elements may be implemented in software.

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

The synthetic aperture radar system 100 includes the code modulator 41, the multiplexer 43, the recorder 42, the demodulator 201, the code demodulator 202, and the recorder 203 as functional elements. These functional elements are called functional elements of the synthetic aperture radar system 100 .

Each functional element of the synthetic aperture radar system 100 is realized by software. Note that 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 implements the function 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).

Auxiliary storage device 922 is a storage device that stores data. A specific example of the auxiliary storage device 922 is an HDD. The auxiliary storage device 922 may be a portable storage medium such as an SD (registered trademark) memory card, CF, NAND flash, flexible disk, optical disk, compact disk, Blu-ray (registered trademark) disk, or DVD. Note that 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 Versatile Disk.

The input interface 930 is a port connected to an input device such as a mouse, keyboard, or touch panel. The input interface 930 is specifically 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. The output interface 940 is specifically a USB terminal or an HDMI (registered trademark) (High Definition Multimedia Interface) terminal. The display is specifically an LCD (Liquid Crystal Display).

Memory 921 and communication device 950 are the same as those described in FIG.

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

Synthetic aperture radar system 100 may include multiple processors in place of processor 910 . These multiple processors share the execution of the synthetic aperture radar program. Each processor, like processor 910, is a device that executes a synthetic aperture radar program.

The data, information, signal values and variable values used, processed or output by the synthetic aperture radar program are stored in memory 921 , secondary storage 922 , registers or cache memory within processor 910 .

The “unit” or “apparatus” of each functional element of the synthetic aperture radar system 100 may be read as “processing”, “procedure” or “process”. In addition, "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 recording program". .

The synthetic aperture radar program causes the computer to execute each process, each procedure, or each process, where the above "section" is read as "processing,""procedure," or "step." Also, the synthetic aperture radar method corresponds to each procedure in which the above-mentioned "part" is read as "procedure".
The synthetic aperture radar program may be stored and provided on a computer-readable recording medium. The synthetic aperture radar program may also be provided as a program product.

Each of the processor and electronic circuitry is also called processing circuitry. That is, the function of each functional element of the synthetic aperture radar system 100 is realized by processing circuitry.

***Description of the effects of the present embodiment***
A synthetic aperture radar system that employs the multi-beam SAR method can achieve widening and high 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 section and the data transfer rate, increasing system resources and costs. In order to prevent increases in system resources and costs while maintaining observation performance, it is necessary to consider methods to reduce the amount of data.
In the synthetic aperture radar system according to the present embodiment, the signal processing unit multiplexes received signals of reflected waves obtained from a plurality of antenna apertures after code modulation processing, thereby reducing the amount of data. Also, the signal demodulator performs code demodulation processing on the multiplexed received signal to generate an image spectrum corresponding to the antenna numerical aperture. 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 signals received by the N antenna apertures of the transmitting and receiving antennas into digital received signals in the frequency space, and multiplexes the signals after code modulation processing. Reduce data volume by implementing Then, the signal demodulator generates image spectra corresponding to the N received signals through 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 the multi-beam SAR method.

In the first embodiment described above, 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 of the above-described embodiment. The functional blocks of the synthetic aperture radar system may have any configuration as long as they can realize the functions described in the above embodiments. Also, the synthetic aperture radar system may be a system consisting of a plurality of devices instead of a single device.
Moreover, it is also possible to combine a plurality of portions of the first embodiment. Alternatively, one portion of this embodiment may be implemented. In addition, this embodiment may be implemented as a whole or partially in any combination.
That is, in Embodiment 1, it is possible to freely combine each embodiment, modify any component of each embodiment, or omit any component from each embodiment.

The above-described embodiments are essentially preferable examples, and are not intended to limit the scope of the invention, the scope of applications of the invention, or the scope of applications of the invention. Various modifications can be made to the above-described embodiments as required.

30 control signal 31 pulse signal 32 reception signal 33 transmission signal 34 multiplexed signal 35 radio frequency signal 36 image spectrum 37 SAR image 40 range Fourier transform section 41 code modulation section 42 recording device 43 Multiplexer 61 Converted received signal 62 Modulated received signal 63 Demodulated multiplexed signal 100 Synthetic aperture radar system 101 Data transfer unit 102 Signal demodulator 103 Image reproducer 104 Display 106 Control unit 107 Excitation section 108 Transmission section 109 Transmission/reception switch 120 Transmission/reception antenna 130 Reception section 140 Signal processing section 150 Data recording section 200 Observation target 201 Demodulator 202 Code demodulation section 203 Recording device 221 Code demodulation Code, 331 reference signal, 332 code modulation code, 909 electronic circuit, 910 processor, 921 memory, 922 auxiliary storage device, 930 input interface, 940 output interface, 950 communication device.

Claims (6)

a range Fourier transform unit for converting each of a plurality of received signals received by a plurality of antenna apertures into a frequency domain by range Fourier transforming them, and outputting them as a plurality of transformed received signals;
a code modulation unit that multiplies each of the plurality of transformed received signals by each of a plurality of code modulation codes having orthogonality to each other and outputs a plurality of modulated received signals;
A synthetic aperture radar system comprising: a multiplexing unit that multiplexes the plurality of modulated received signals and records them in a data recording unit as a multiplexed signal. The synthetic aperture radar system comprises:
a demodulator that demodulates the multiplexed signal and outputs it as a demodulated multiplexed signal;
A code demodulator that outputs an image spectrum composed of a plurality of spectra obtained by multiplying the demodulated multiplexed signal by each of a plurality of code demodulation codes each having a complex conjugate relationship with each of the code modulation codes. and,
2. The synthetic aperture radar system according to claim 1, further comprising an image reproducing section that reproduces an image using the image spectrum. The code modulation unit is
multiplying each of the plurality of transformed received signals by a reference signal for performing dechirping processing to cancel a chirp rate and multiplying each of the plurality of encoded modulation codes, and multiplying each of the plurality of modulated received signals with chirp rates different from each other; 3. The synthetic aperture radar system according to claim 1, wherein the synthetic aperture radar system outputs a signal. The code modulation unit is
4. The synthetic aperture radar system according to claim 3, wherein each of said plurality of transformed received signals is multiplied by said reference signal having a complex conjugate relationship with each of said plurality of transmitted signals corresponding to said plurality of received signals. In a synthetic aperture radar method for a synthetic aperture radar system,
A range Fourier transform unit transforms each of a plurality of received signals received by a plurality of antenna apertures into a frequency domain by range Fourier transforming them, and outputs them as a plurality of transformed received signals,
a code modulation unit that multiplies each of the plurality of transformed received signals by each of a plurality of code modulation codes that are orthogonal to each other, and outputs the result as a plurality of modulated received signals;
A synthetic aperture radar method in which a multiplexing unit multiplexes the plurality of modulated received signals and records them in a data recording unit as a multiplexed signal. In the synthetic aperture radar program of the synthetic aperture radar system,
Range Fourier transform processing for transforming each of a plurality of received signals received by a plurality of antenna apertures into a frequency domain by range Fourier transforming and outputting as a plurality of transformed received signals;
a code modulation process of multiplying each of the plurality of transformed received signals by each of a plurality of code modulated codes having orthogonality to each other and outputting as a plurality of modulated received signals;
A synthetic aperture radar program for causing a computer to execute a multiplexing process of multiplexing the plurality of modulated received signals and recording them in a data recording unit as a multiplexed signal.

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