JP7351794B2 - Synthetic aperture radar system - Google Patents

Description

The present disclosure relates to a synthetic aperture radar system that is multi-channeled by a plurality of receivers.

SAR (Synthetic Aperture Radar) is a sensor mounted on artificial satellites, aircraft, etc. that can penetrate clouds and fog and observe the ground surface day and night. SAR emits radio waves toward the ground, receives scattered waves from objects on the ground, and images them.

In recent years, in SAR, there has been a demand for wider area and higher resolution of SAR images. In order to realize these, the SAR system described in Non-Patent Document 1 includes a synthetic aperture radar whose receiving system is multi-channeled by a plurality of receiving units.

Y.Okada, S.Nakamura, K.Iribe, Y.Yokota, M.Tsuji, M.Tsuchida, K.Hariu, Y.Kankaku, S.Suzuki, Y.Osawa, and M.Shimada "SYSTEM DESIGN OF WIDE SWATH , HIGH RESOLUTION, FULL POLARIMIETORIC L-BAND SAR ONBOARD ALOS-2", 2013 IEEE International Geoscience and Remote Sensing Symposium-IGARSS, 2013 IEEE International

The signal strength of a SAR image, which is an image generated by SAR, is mainly determined by the type of object to be observed. Therefore, in order to image many kinds of objects at once in the SAR system of Non-Patent Document 1, it is required to expand the dynamic range of the SAR image.

The SAR system of Non-Patent Document 1 generates a SAR image after performing AD (Analog to Digital) conversion on a received signal in a receiving section. AD conversion quantizes the level value of a received signal. The level of the received signal that can be AD converted is determined by the total input range and the reference level. The total input range is the width of signal levels that can be AD converted. Further, the reference level is the minimum signal level of the entire input range. If a signal outside the entire input range is AD-converted, the signal will not be AD-converted correctly and information about the amplitude or phase of the signal will be lost. Therefore, the dynamic range corresponds to the entire input range during AD conversion. Therefore, when the SAR system of Non-Patent Document 1 expands the dynamic range, it is conceivable to expand the entire input range.

However, expanding the total input range increases the amount of data and the resources required for signal processing, thus requiring a large-scale SAR system. For this reason, it has been difficult to expand the dynamic range and increase the signal strength of SAR images with a small-scale device configuration.

The present disclosure has been made in view of the above, and an object of the present disclosure is to obtain a synthetic aperture radar system that can expand the dynamic range and increase the signal strength of SAR images with a small-scale device configuration.

In order to solve the above-mentioned problems and achieve the objectives, the synthetic aperture radar system of the present disclosure includes an antenna that emits a transmitted wave and receives scattered waves from an observation target, and an analog signal of the scattered waves received by the antenna. a first receiver that converts the analog signal into a first digital signal at a first reference level; and a second receiver that converts the analog signal into a second digital signal at a second reference level different from the first reference level. It is equipped with a receiving section. Further, the synthetic aperture radar system of the present disclosure extracts a first signal portion whose signal level is not saturated from the first digital signal, and extracts a first signal portion whose signal level is not saturated from the second digital signal. a signal synthesis unit that extracts the second signal part and synthesizes the first signal part and the second signal part to generate a third digital signal; and an imaging process of the observation target based on the third digital signal. and an imaging processing unit that executes. Further, the synthetic aperture radar system of the present disclosure includes a transmission unit that converts the first digital signal and the second digital signal into analog signals and transmits the analog signals to the ground. The signal synthesis section and the imaging processing section are included in a ground processing section located on the ground, and the transmission section transmits the first digital signal and the second digital signal to the ground processing section.

According to the present disclosure, it is possible to expand the dynamic range and increase the signal strength of the SAR image with a small-scale device configuration.

A diagram showing the configuration of a SAR system according to Embodiment 1 A diagram showing the configuration of a ground processing unit included in the SAR system according to Embodiment 1. Flowchart showing the processing procedure of signal processing in the SAR system according to the first embodiment A diagram for explaining the concept of AD conversion processing executed by the SAR receiving unit according to the first embodiment A diagram for explaining AD conversion processing executed by the SAR receiving unit according to the first embodiment A diagram showing the configuration of a SAR system according to Embodiment 2 Flowchart showing the processing procedure of signal processing in the SAR system according to the second embodiment A diagram showing the configuration of a SAR system according to Embodiment 3 A diagram showing the configuration of a ground processing unit included in the SAR system according to Embodiment 3 Flowchart showing the processing procedure of signal processing in the SAR system according to the third embodiment A diagram showing a first example of a hardware configuration that realizes a signal processing unit included in the SAR according to Embodiments 1 to 3. A diagram illustrating a second example of a hardware configuration that implements a signal processing unit included in the SAR according to Embodiments 1 to 3.

Embodiments of a synthetic aperture radar system according to the present disclosure will be described in detail below based on the drawings.

Embodiment 1.
FIG. 1 is a diagram showing the configuration of the SAR system according to the first embodiment. The SAR system 100A, which is a synthetic aperture radar system, includes an SAR 10A and a ground processing section 20A.

SAR10A is installed on artificial satellites, aircraft, etc. The SAR 10A includes a signal processing section 15A, an antenna 2, and two independent transmission sections 5A and 5B. The signal processing section 15A includes a transmitting section 1 and two independent receiving sections 3 and 4. The transmitter 5A is connected to the receiver 3, and the transmitter 5B is connected to the receiver 4.

In the first embodiment, a case will be described in which the SAR 10A includes two receiving sections 3 and 4, but the SAR 10A may include three or more receiving sections. In this case as well, the SAR 10A includes the same number of transmitting sections as receiving sections.

The transmitter 1 generates a radiated radio wave signal and sends it to the antenna 2. The antenna 2 radiates the signal generated by the transmitter 1 into space as a transmission wave, which is a radio wave. Radio waves radiated from the antenna 2 are scattered by the observation target 30. The observation target 30 is an object on the ground.

The antenna 2 receives scattered waves, which are radio waves scattered by the observation target 30, as analog signals. Antenna 2 sends the received analog signal to receiving sections 3 and 4. The receiving units 3 and 4 perform AD conversion on the received analog signals. The reference level used in AD conversion by the receiving unit 3, which is the first receiving unit, is different from the reference level used in AD conversion, which is used by the receiving unit 4, which is the second receiving unit. That is, the receiving unit 3 converts the analog signal of the scattered wave received by the antenna 2 into a first digital signal at the first reference level. The receiving unit 4 converts the analog signal of the scattered wave received by the antenna 2 into a second digital signal at a second reference level. Even when the SAR 10A includes three or more receiving sections, the reference level for AD conversion is different for each receiving section.

The receiving section 3 sends the AD-converted digital signal to the transmitting section 5A, and the receiving section 4 sends the AD-converted digital signal to the transmitting section 5B. The transmission section 5A converts the digital signal from the reception section 3 into an analog signal and transmits it to the ground processing section 20A, and the transmission section 5B converts the digital signal from the reception section 4 into an analog signal and transmits it to the ground processing section 20A. to be transmitted.

The ground processing unit 20A converts the analog signals received from the transmission units 5A and 5B into digital signals, and images the observation target 30 using the digital signals. Note that the ground processing unit 20A may be arranged inside the SAR 10A.

FIG. 2 is a diagram showing the configuration of a ground processing unit included in the SAR system according to the first embodiment. The ground processing section 20A includes a receiving device 21, a signal combining section 22, and an imaging processing section 23.

The receiving device 21 has the function of an antenna that transmits and receives analog signals, and the function of a receiving section that reproduces a digital signal from the received analog signal. When analog signals from the transmission units 5A and 5B are received by an antenna (not shown) placed before the ground processing unit 20A, the analog signals are converted into digital signals in the reception unit and sent to the signal synthesis unit 22. Sent. In addition, in the following description, description of the process by which the transmission parts 5A and 5B convert a digital signal into an analog signal, and the process by which the receiving device 21 converts an analog signal into a digital signal may be omitted. That is, there are cases where the transmission sections 5A and 5B transmit the digital signal while the transmission sections 5A and 5B convert the digital signal into an analog signal and then transmit the signal. Further, the signals handled by the ground processing section 20A may be referred to as digital signals from the transmission sections 5A and 5B. The signal synthesis unit 22 determines that a portion of the digital signal whose signal level does not change over a specific period of time is saturated, and extracts a signal that is not saturated. The signal synthesis unit 22 generates a digital signal by synthesizing the extracted signals. The digital signal synthesized by the signal synthesis section 22 is the third digital signal. The imaging processing unit 23 executes imaging of the observation target 30 based on the digital signals synthesized by the signal synthesis unit 22.

FIG. 3 is a flowchart showing a signal processing procedure in the SAR system according to the first embodiment. The transmitter 1 generates a signal, and the antenna 2 radiates the signal generated by the transmitter 1 as a transmission wave, which is a radio wave, toward the observation target 30 on the ground (step S110).

The antenna 2 receives reflected waves scattered by the observation target 30 (step S120). The receiving unit 3 performs AD conversion on the analog signal of the reflected wave received by the antenna 2 using a unique reference level set in the receiving unit 3 (step S130-1). The receiving unit 4 AD converts the analog signal of the reflected wave received by the antenna 2 using a unique reference level set in the receiving unit 4 (step S130-2).

Here, the concept of AD conversion will be explained. FIG. 4 is a diagram for explaining the concept of AD conversion processing executed by the SAR receiving unit according to the first embodiment. Note that since the AD conversion processing executed by the reception units 3 and 4 is the same, the concept of the AD conversion processing executed by the reception unit 3 will be explained here. Further, here, a case will be described in which the receiving unit 3 performs AD conversion on an analog signal 510, which is an example of an analog signal.

The signal level of the analog signal 510 received by the antenna 2 changes over time. In the analog signal 510 shown in FIG. 4, the horizontal axis is time and the vertical axis is the signal level. The receiving unit 3 performs AD conversion processing by sampling the analog signal 510 received by the antenna 2 (st1) and quantizing the sampled signal (st2).

Sampling is a process of extracting pulsed discrete signals 520 from continuous analog signals 510 at specific time intervals T. During sampling, the receiving unit 3 extracts a discrete signal 520 having a reference level L0 or higher from the analog signal 510. In the discrete signal 520 shown in FIG. 4, the horizontal axis is time and the vertical axis is signal level.

Quantization is the process of applying analog input amplitude, which is a continuous quantity, to discrete values. In quantization, the sampled values are applied to the entire input range and reference level L0. The entire input range corresponds to the dynamic range D0. In the quantized signal 530 shown in FIG. 4, the horizontal axis is time and the vertical axis is the signal level.

Here, a specific AD conversion method in the first embodiment will be explained. FIG. 5 is a diagram for explaining AD conversion processing executed by the SAR receiving unit according to the first embodiment. Here, AD conversion processing when an analog signal exceeding the dynamic range D0 is input to the receiving sections 3 and 4 will be described.

In the analog signals 31A, 41A and digital signals 31D, 41D, and 70 shown in FIG. 5, the horizontal axis is time, and the vertical axis is the signal level. The analog signal 31A is a signal received by the receiving section 3 before AD conversion. The analog signal 41A is a signal received by the receiving unit 4 before AD conversion. The digital signal 31D is a signal after AD conversion by the receiving section 3. The digital signal 41D is a signal after AD conversion by the receiving section 4.

The receiving units 3 and 4 included in the SAR 10A of the first embodiment shift the reference levels between the reference levels and then perform AD conversion in parallel. FIG. 5 shows a case where the receiving section 3 performs AD conversion using a reference level L1, and the receiving section 4 performs AD conversion using a reference level L2 different from the reference level L1. The receiving unit 3 executes AD conversion using the entire input range D1 with the reference level L1 as the lowest value, and the receiving unit 4 executes AD conversion using the entire input range D2 with the reference level L2 as the lowest value. do. Since the reference level L1 and the reference level L2 are different values, the total input range D1 and the total input range D2 are different ranges.

In FIG. 5, the full input range D1 is set in the receiver 3 so that the signal level of the upper half of the analog signal 31A becomes the full input range, and the full input range D1 is set so that the signal level of the lower half of the analog signal 41A becomes the full input range. A case is shown in which the input range D2 is set in the receiving section 4.

The receiving unit 3 converts the analog signal 31A into a digital signal 31D by performing AD conversion using the reference level L1 and the entire input range D1. The receiving unit 4 converts the analog signal 41A into a digital signal 41D by performing AD conversion using the reference level L2 and the entire input range D2.

In the SAR 10A, the receiving units 3 and 4 perform AD conversion by changing the reference levels L1 and L2, so it is possible to obtain digital signals 31D and 41D of different levels. That is, since the receiving unit 3 executes AD conversion at the reference level L1, and the receiving unit 4 executes AD conversion at the reference level L2, the AD converted digital signal 31D and the AD converted digital signal The signal has a different level from that of 41D.

In this way, since the receiving sections 3 and 4 shift the reference levels L1 and L2, the receiving sections 3 and 4 can AD convert signals in different level ranges. Thereby, the receiving sections 3 and 4 can perform AD conversion on signals having a wider level range than when the reference levels are the same.

The digital signal 31D after AD conversion by the receiving unit 3 is converted into an analog signal by the transmitting unit 5A and transmitted to the ground processing unit 20A (step S140-1). The transmission unit 5B converts the digital signal 41D into an analog signal and transmits it to the ground processing unit 20A (step S140-2).

The signal synthesis unit 22 determines that a portion of the digital signal 31D from the transmission unit 5A in which the signal level does not change over a specific period of time is saturated, and extracts a signal that is not saturated. Furthermore, the signal synthesis section 22 determines that the signal in the portion of the digital signal 41D from the transmission section 5B whose signal level does not change over a specific period of time is saturated, and extracts the signal that is not saturated. The signal extracted from the digital signal 31D by the signal synthesis unit 22 is the first signal portion, and the signal extracted from the digital signal 41D is the second signal portion.

The signal synthesis unit 22 combines an unsaturated first signal portion of the digital signal 31D generated using the reference level L1 and an unsaturated first signal portion of the digital signal 41D generated using the reference level L2. A digital signal 70 is generated by adding the second signal portion that is not present. That is, the signal synthesis unit 22 synthesizes signals of unsaturated portions of the digital signals 31D and 41D (step S150).

The digital signal 70 has a dynamic range Dx that includes the entire input range D1 of the digital signal 31D and the entire input range D2 of the digital signal 41D. That is, the ground processing unit 20A can generate a digital signal 70 with a dynamic range Dx that is an expanded dynamic range of the analog signal received by the SAR 10A. For example, the total input ranges D1 and D2 and the reference levels L1 and L2 are set in the receiving sections 3 and 4 so that the dynamic range Dx is the sum of the total input range D1 and the total input range D2. For example, total input ranges D1, D2 and reference levels L1, L2 are set for the receiving units 3, 4 so as to cover all signal levels of analog signals inputted beyond the dynamic range.

In addition, even when SAR10A is equipped with three or more receiving sections, the dynamic range of the analog signal received by SAR10A can be expanded by combining the unsaturated parts of the digital signals AD-converted by each receiving section. A digital signal 70 having a dynamic range Dx can be generated.

The imaging processing unit 23 executes imaging processing of the observation target 30 based on the digital signal 70 (step S160).

Here, a method for the signal synthesis unit 22 to extract a non-saturated digital signal will be described. For example, the signal synthesis unit 22 extracts digital signals having a signal level other than the threshold value as a non-saturated digital signal based on a preset threshold value of the signal saturation level. In other words, the signal synthesis unit 22 extracts a digital signal whose signal saturation level is less than the threshold value as a non-saturated digital signal.

In addition, the signal synthesis unit 22 monitors the time-differentiated value of the signal level, and determines the saturation start time and saturation end time when the absolute value of the time-differentiated value exceeds a specific value. It may be possible to extract digital signals that are not present. The value obtained by time-differentiating the signal level of the digital signal 31D by the signal synthesis section 22 is the first value, and the value obtained by time-differentiating the signal level of the digital signal 41D by the signal synthesis section 22 is the second value.

Further, the signal synthesis unit 22 determines that a digital signal whose time-differentiated value of the signal level becomes 0 for a specific time period is saturated, and extracts digital signals other than the saturated digital signal as non-saturated digital signals. You may.

Note that the method for extracting the unsaturated digital signal is not limited to the method described above, and the signal synthesis unit 22 can extract the unsaturated digital signal using any method that can extract the unsaturated digital signal. May be extracted.

Here, a comparative SAR system that expands the dynamic range without changing the reference level will be described. In the case of the SAR system of the comparative example, since the total input range of the receiving section of the SAR of the comparative example is expanded, the resources required for signal processing in the subsequent stage of the receiving section increase due to the expansion of the total input range of the receiving section. do. That is, the resources required for AD conversion processing by the SAR receiving unit of the comparative example increase. The resources here include the number of FPGAs (Field-Programmable Gate Arrays) that perform signal processing, calculation time required for signal processing, power consumption required for signal processing, and the like. Furthermore, in the case of the SAR system of the comparative example, the total input range of the SAR receiving section of the comparative example is expanded, so the amount of data increases. This increase in the amount of data may result in operational constraints such as data transmission from SAR mounted on a satellite or the like to a ground processing unit.

On the other hand, in the first embodiment, the SAR 10A performs AD conversion using different reference levels L1 and L2, so the SAR 10A generates a digital signal 70 with a dynamic range Dx that expands the dynamic range of the received analog signal. be able to. That is, the SAR 10A does not require an increase in resources or data amount, and therefore can generate the digital signal 70 with a wide dynamic range Dx without increasing hardware.

In this way, in the first embodiment, in the SAR system 100A, the analog signal 31A received by the antenna 2 is AD converted into the digital signal 31D at the reference level L1 of the receiving section 3, and the analog signal 41A received by the antenna 2 is converted into the digital signal 31D by the receiving section. AD conversion is performed to a digital signal 41D at a reference level L2 of 4. Then, the SAR system 100A extracts a signal portion whose signal level is not saturated from among the digital signals 31D and 41D, and synthesizes the extracted signal portions to generate a digital signal 70. As a result, the SAR system 100A can increase the signal strength of SAR images by expanding the dynamic range during reception to the dynamic range Dx with a small-scale device configuration while suppressing increases in data volume and hardware scale. becomes.

Embodiment 2.
Next, Embodiment 2 will be described using FIGS. 6 and 7. In the second embodiment, the SAR includes a plurality of antennas, and the plurality of antennas radiate radio waves and receive radio waves scattered by the observation target 30.

FIG. 6 is a diagram showing the configuration of the SAR system according to the second embodiment. Among the components in FIG. 6, components that achieve the same functions as the SAR system 100A of the first embodiment shown in FIG.

The SAR system 100B includes a SAR 10B and a ground processing section 20A. The SAR 10B includes multiple antennas. In the second embodiment, a case will be described in which the SAR 10B includes two antennas 7 and 8, but the SAR 10B may include three or more antennas. Further, the SAR 10B includes a signal processing section 15B and transmission sections 5A and 5B.

The signal processing section 15B includes a transmitting section 1, receiving sections 3 and 4, and a phase correcting section 9. When the SAR system 100B uses a plurality of antennas 7 and 8, the phases of the received signals are shifted depending on the positions of the antennas 7 and 8. For this reason, the signal processing section 15B includes a phase correction section 9 that corrects the phase.

The phase correction section 9 is connected to the antennas 7 and 8 as well as to the reception sections 3 and 4. Note that in the SAR 10B, the phase correction section 9 is connected before the reception sections 3 and 4, but the phase correction section 9 may be arranged after the reception sections 3 and 4. In this case, the phase correction section 9 is connected to the reception sections 3, 4 and the transmission sections 5A, 5B, and corrects the phase of the digital signal after AD conversion in the reception sections 3, 4, and the transmission sections 5A, 5B. Send to 5B. Further, the phase correction section 9 may be connected upstream of the ground processing section 20A. In this case, the phase correction section 9 is connected between the antenna on the ground and the ground processing section 20A, and sends the phase-corrected digital signal to the ground processing section 20A.

When the phase correction unit 9 is connected to the antennas 7 and 8, the received signals corresponding to the scattered waves received by the antennas 7 and 8 are analog signals received by the antennas 7 and 8. When the phase correction section 9 is connected to the reception sections 3 and 4 and the transmission sections 5A and 5B, the received signals corresponding to the scattered waves received by the antennas 7 and 8 are digital signals AD converted by the reception sections 3 and 4. It is. When the phase correction unit 9 is connected upstream of the ground processing unit 20A, the received signals corresponding to the scattered waves received by the antennas 7 and 8 are transmitted from the transmission units 5A and 5B, and are digital signals sent to the phase correction unit 9. It's a signal.

The transmitter 1 generates a radio wave signal to be radiated and sends it to antennas 7 and 8. The antennas 7 and 8 radiate the signals generated by the transmitter 1 into space as transmission waves, which are radio waves.

The antennas 7 and 8 receive scattered waves, which are radio waves scattered by the observation target 30, as analog signals. Antennas 7 and 8 send the received analog signals to phase correction section 9. The phase correction unit 9 corrects the phases of the analog signals received by the antennas 7 and 8 to a phase corresponding to the arrangement positions of the antennas 7 and 8. The phase corrector 9 sends the phase corrected analog signal to the receivers 3 and 4. That is, the phase correction section 9 corrects the phase of the analog signal received by the antenna 7 and sends it to the reception section 3, and corrects the phase of the analog signal received by the antenna 8 and sends it to the reception section 4.

FIG. 7 is a flowchart showing a signal processing procedure in the SAR system according to the second embodiment. Note that, among the processes described in FIG. 7, the description of processes similar to those described in FIG. 3 will be omitted.

The transmitter 1 generates a signal. The antenna 7 emits the signal generated by the transmitter 1 as a transmission wave, which is a radio wave, toward the observation target 30 on the ground (step S210-1), and the antenna 8 emits the signal, which is generated by the transmitter 1, as a radio wave. It radiates toward the observation target 30 on the ground as a transmission wave (step S210-2).

The antenna 7 receives the reflected waves scattered by the observation target 30 (step S220-1), and the antenna 8 receives the reflected waves scattered by the observation target 30 (step S220-2). The phase correction unit 9 corrects the phase of the analog signal of the reflected wave received by the antennas 7 and 8 to a phase corresponding to the arrangement position of the antennas 7 and 8 (step S230).

The receiving unit 3 AD converts the phase-corrected analog signal received by the antenna 7 using a unique reference level L1 set in the receiving unit 3 (step S240-1). The receiving unit 4 AD converts the phase-corrected analog signal received by the antenna 8 using a unique reference level L2 set in the receiving unit 4 (step S240-2).

The digital signal AD-converted by the receiving unit 3 is converted into an analog signal by the transmitting unit 5A and transmitted to the ground processing unit 20A (step S250-1), and the digital signal AD-converted by the receiving unit 4 is transmitted. The unit 5B converts it into an analog signal and transmits it to the ground processing unit 20A (step S250-2).

The receiving device 21 receives analog signals from the transmission units 5A and 5B using an antenna, and reproduces a digital signal from the analog signal using the receiving unit. The receiving device 21 sends the reproduced digital signal to the signal synthesizing section 22. The signal synthesizing section 22 determines that the digital signal in the portion where the signal level does not change over a specific period of time out of the digital signal from the transmission section 5A is saturated, and extracts the digital signal that is not saturated. The signal synthesis unit 22 determines that the digital signal in the portion where the signal level does not change over a specific period of time out of the digital signal from the transmission unit 5B is saturated, and extracts the digital signal that is not saturated. The signal synthesis section 22 synthesizes the extracted digital signals. That is, the signal synthesis section 22 synthesizes the signal extracted from the digital signal of the transmission section 5A and the signal extracted from the digital signal of the transmission section 5B (step S260).

The digital signal generated by the signal synthesis unit 22 through synthesis is the same digital signal as the digital signal 70 described in the first embodiment. The imaging processing unit 23 executes imaging processing of the observation target 30 based on the digital signals synthesized by the signal synthesis unit 22 (step S270).

Here, the phase correction process by the phase correction section 9 will be explained. The phase correction unit 9 detects the time-based phase of the analog signals received by the antennas 7 and 8, and calculates the average value of the time-based phase of each analog signal. The phase correction unit 9 corrects the phase of the analog signal received by the antennas 7 and 8 by changing the phase of the analog signal received by the antennas 7 and 8 to the average value of each time.

Note that the phase correction unit 9 may detect a change in the phase of the analog signal received by the antennas 7 and 8 during a specific period, and calculate the median value of the phase during this specific period. In this case, the phase correction unit 9 determines that each median value of the phase of the antennas 7 and 8 at the specific time is the average value of the median value of the phase of the antenna 7 at the specific time and the median value of the phase of the antenna 8 at the specific time. Correct the phase so that

Further, the phase correction unit 9 adjusts the antennas so that the phase of one of the analog signals received by the antennas 7 and 8 is the same as the phase of the other, based on the phase difference expected from the distance between the antennas 7 and 8. The phase of at least one of the received signals at 7 and 8 may be corrected. The phase difference expected from the distance between the antennas 7 and 8 may be calculated by the phase correction section 9, or may be calculated by another device and set in the phase correction section 9.

Further, the phase correction unit 9 may correlate the analog signals received by the antennas 7 and 8 using a reference function, and record the time at which the degree of correlation reaches a peak (hereinafter referred to as peak time). In this case, the phase correction unit 9 executes the phase correction process using the recorded time, that is, the peak time at the antennas 7 and 8. The phase correction unit 9 may correct the phase so that, for example, the average time of the peak times at the antennas 7 and 8 becomes the peak time at the antennas 7 and 8. Further, the phase correction unit 9 may correct the phase so that the peak time of one of the antennas 7 and 8 becomes the same as the peak time of the other.

Note that the phase correction method is not limited to the method described above, and the phase correction section 9 may detect the phase of an analog signal or digital signal and correct the phase using any method that can correct the phase. good.

As described above, according to the second embodiment, since the SAR system 100B includes the phase correction section 9, even in a system having a plurality of antennas such as the antennas 7 and 8, the dynamic range Dx can be improved with a small-scale device configuration. It becomes possible to increase the signal strength of the SAR image by enlarging the SAR image.

Embodiment 3.
Next, Embodiment 3 will be described using FIGS. 8 and 9. In Embodiment 3, the SAR extracts unsaturated digital signals from the digital signals, synthesizes them, and then transmits them to the ground processing unit.

FIG. 8 is a diagram showing the configuration of the SAR system according to the third embodiment. Among the components in FIG. 8, components that achieve the same functions as the SAR system 100A of the first embodiment shown in FIG. 1 are denoted by the same reference numerals, and redundant explanation will be omitted.

The SAR system 100C includes a SAR 10C and a ground processing section 20C. The SAR 10C includes an antenna 2, a signal processing section 15C, and a transmission section 5C. The signal processing section 15C includes a transmitting section 1, receiving sections 3 and 4, and a signal combining section 6.

The signal synthesis section 6 is connected to the reception sections 3 and 4, synthesizes the digital signals AD-converted by the reception sections 3 and 4, and sends the synthesized signal to the transmission section 5C. The transmission section 5C transmits the digital signal synthesized by the signal synthesis section 6 to the ground processing section 20C. The ground processing unit 20C images the observation target 30 using the digital signal received from the transmission unit 5C.

FIG. 9 is a diagram showing the configuration of a ground processing section included in the SAR system according to the third embodiment. The ground processing section 20C includes an imaging processing section 23. That is, the ground processing section 20C does not have the signal combining section 22, compared to the ground processing section 20A. The imaging processing unit 23 executes imaging of the observation target 30 based on the digital signal received from the transmission unit 5C.

FIG. 10 is a flowchart showing a signal processing procedure in the SAR system according to the third embodiment. Note that, among the processes described in FIG. 10, descriptions of processes similar to those described in FIG. 3 will be omitted.

The antenna 2 radiates the signal generated by the transmitter 1 as a transmission wave, which is a radio wave, toward the observation target 30 on the ground (step S310). The antenna 2 receives reflected waves scattered by the observation target 30 (step S320).

The receiving unit 3 performs AD conversion on the analog signal of the reflected wave received by the antenna 2 using a unique reference level L1 set in the receiving unit 3 (step S330-1). The receiving unit 4 AD converts the analog signal of the reflected wave received by the antenna 2 using the unique reference level L2 set in the receiving unit 4 (step S330-2).

The signal synthesis unit 6 synthesizes the digital signals AD-converted by the reception units 3 and 4 (step S340). The transmission section 5C converts the digital signal synthesized by the signal synthesis section 6 into an analog signal and transmits it to the ground processing section 20C (step S350). When receiving the analog signal, the receiving device 21 converts it into a digital signal, and the imaging processing unit 23 uses the digital signal to perform imaging processing of the observation target 30 (step S360). In this way, in the SAR system 100C, compared to the SAR system 100A, the SAR 10C combines digital signals and then transmits the digital signals to the ground processing unit 20C.

As described above, according to the third embodiment, since the SAR 10C in the SAR system 100C includes the signal synthesis section 6, the synthesis processing of digital signals can be performed even on the orbit of the SAR 10C.

Here, the hardware configuration of the signal processing units 15A to 15C will be explained. FIG. 11 is a diagram illustrating a first example of a hardware configuration that implements a signal processing unit included in the SAR according to the first to third embodiments. FIG. 12 is a diagram illustrating a second example of a hardware configuration that implements the signal processing unit included in the SAR according to the first to third embodiments. Note that since the signal processing units 15A to 15C have similar hardware configurations, the hardware configuration of the signal processing unit 15A will be described here.

The signal processing unit 15A can be realized by the processor 501, memory 502, and interface 504 shown in FIG. The processor 501 is a CPU (Central Processing Unit, also referred to as a central processing unit, processing unit, arithmetic unit, microprocessor, microcomputer, DSP (Digital Signal Processor)), system LSI (Large Scale Integration), or the like. The memory 502 is a RAM (Random Access Memory), a ROM (Read Only Memory), or the like.

The signal processing unit 15A is realized by the processor 501 reading and executing a computer-executable signal processing program stored in the memory 502 to execute the operations of the signal processing unit 15A. The signal processing program, which is a program for executing the operation of the signal processing section 15A, can also be said to cause a computer to execute the procedure or method of the signal processing section 15A.

The signal processing program executed by the signal processing section 15A has a module configuration including a transmitting section 1 and receiving sections 3 and 4, which are loaded onto the main storage device; generated.

The memory 502 is used as temporary memory when the processor 501 executes various processes. Furthermore, the memory 502 stores AD-converted digital signals and the like. Interface 504 receives analog signals from antenna 2 and transmits digital signals to transmission units 5A and 5B.

The signal processing program may be provided as a computer program product by being stored in a computer readable storage medium in an installable or executable format file.

Note that the processor 501 and memory 502 shown in FIG. 11 may be replaced with the processing circuit 503 shown in FIG. 12. The processing circuit 503 is, for example, a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or a combination thereof. Applicable. Note that some of the functions of the signal processing unit 15A may be realized by dedicated hardware, and some may be realized by software or firmware.

The configurations shown in the embodiments above are merely examples, and can be combined with other known techniques, or can be combined with other embodiments, within the scope of the gist. It is also possible to omit or change part of the configuration.

1 Transmission section, 2, 7, 8 Antenna, 3, 4 Receiving section, 5A, 5B, 5C Transmission section, 6 Signal combining section, 9 Phase correction section, 10A to 10C SAR, 15A to 15C Signal processing section, 20A, 20C Ground processing unit, 21 Receiving device, 22 Signal synthesis unit, 23 Imaging processing unit, 30 Observation target, 31A, 41A, 510 Analog signal, 31D, 41D, 70 Digital signal, 100A to 100C SAR system, 501 Processor, 502 Memory , 503 processing circuit, 504 interface, 520 discrete signal, 530 quantized signal, D1, D2 full input range, Dx dynamic range, L0 to L2 reference level.

Claims (6)

an antenna that emits transmitted waves and receives scattered waves from the observation target;
a first receiving unit that converts the analog signal of the scattered wave received by the antenna into a first digital signal at a first reference level;
a second receiving unit that converts the analog signal into a second digital signal at a second reference level different from the first reference level;
extracting a first signal portion whose signal level is not saturated from the first digital signal; extracting a second signal portion whose signal level is not saturated from the second digital signal; a signal synthesis unit that synthesizes the first signal portion and the second signal portion to generate a third digital signal;
an imaging processing unit that performs imaging processing of the observation target based on the third digital signal;
a transmission unit that converts the first digital signal and the second digital signal into analog signals and transmits them to the ground;
Equipped with
The signal synthesis unit and the imaging processing unit are included in a ground processing unit located on the ground,
the transmission unit transmits the first digital signal and the second digital signal to the ground processing unit;
A synthetic aperture radar system characterized by : The signal combining section includes:
Extracting a portion of the first digital signal in which the signal level does not change for a specific time as the first signal portion;
extracting a portion of the second digital signal in which the signal level does not change for a specific time as the second signal portion;
The synthetic aperture radar system according to claim 1 , characterized in that: The signal combining section includes:
Extracting a portion of the first digital signal whose signal level is less than a threshold as the first signal portion,
extracting a portion of the second digital signal whose signal level is less than the threshold as the second signal portion;
The signal combining section includes:
Extracting a portion of the first digital signal in which the signal level does not change for a specific time as the first signal portion;
extracting a portion of the second digital signal in which the signal level does not change for a specific time as the second signal portion;
The synthetic aperture radar system according to claim 1 , characterized in that: The signal combining section includes:
A first value obtained by time-differentiating the signal level of the first digital signal is monitored, and when the absolute value of the first value becomes a specific value or more, the saturation start time and saturation of the first digital signal are determined. Determining that it is the end time and extracting the first signal portion,
A second value obtained by time-differentiating the signal level of the second digital signal is monitored, and when the absolute value of the second value exceeds a specific value, the saturation start time and saturation of the second digital signal are determined. determining that it is the end time and extracting the second signal portion;
The signal combining section includes:
Extracting a portion of the first digital signal in which the signal level does not change for a specific time as the first signal portion;
extracting a portion of the second digital signal in which the signal level does not change for a specific time as the second signal portion;
The synthetic aperture radar system according to claim 1 , characterized in that: The signal combining section includes:
Extracting a portion of the first digital signal other than a portion where a first value obtained by time-differentiating the signal level of the first digital signal is 0 for a specific time or more as the first signal portion;
Extracting a portion of the second digital signal other than a portion where a second value obtained by time-differentiating the signal level of the second digital signal is 0 for a specific time or more as the second signal portion;
The signal combining section includes:
Extracting a portion of the first digital signal in which the signal level does not change for a specific time as the first signal portion;
extracting a portion of the second digital signal in which the signal level does not change for a specific time as the second signal portion;
The synthetic aperture radar system according to claim 1 , characterized in that: a first digital signal obtained by converting an analog signal of a reflected wave received from an observation target by an antenna that emits a transmitted wave at a first reference level; and a second reference level for converting the analog signal to a second reference level different from the first reference level. a receiving device that receives a second digital signal converted by the level;
a signal synthesis unit that generates a third digital signal by synthesizing a first signal portion extracted from the first digital signal and a second signal portion extracted from the second digital signal;
an imaging processing unit that performs imaging processing of the observation target based on the third digital signal;
a transmission unit that converts the first digital signal and the second digital signal into analog signals and transmits them to the ground;
Equipped with
The receiving device, the signal combining unit, and the imaging processing unit are included in a ground processing unit located on the ground,
The transmission unit transmits the first digital signal and the second digital signal to the receiving device of the ground processing unit.
A synthetic aperture radar system characterized by :

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