Synthetic aperture radar system

Abstract

To provide a synthetic aperture radar system capable of improving spatial resolution at an observation point by repeating the transmission, reflection and reception of a microwave between the observation point and a satellite two or more times.SOLUTION: A synthetic aperture radar system generates image data related to a point scatterer on the surface of the earth on the basis of a radar signal generated by a radar signal generation device and a radar signal based on a radar wave reflected on the surface of the earth using a timing when generating the radar signal by the radar signal generation device by repeating two or more times the sequence of generating a radar signal at a predetermined timing by the radar signal generation device in a satellite, generating the radar wave on the basis of the radar signal to irradiate the surface of the earth, receiving the radar wave reflected on the surface of the earth in the satellite, amplifying the radar wave to irradiate the surface of the earth again and be reflected on the surface of the earth again and receiving the radar wave reflected on the surface of the earth in the satellite again.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a synthetic aperture radar system. [Background technology]

[0002] Synthetic aperture radar satellites, equipped with synthetic aperture radar (SAR), can grasp the situation on the Earth's surface over a wide area. Synthetic aperture technology transmits microwaves from the satellite's radar antenna, receives backscattered waves from the ground along its orbit, and recombines the signals to achieve a resolution equivalent to that achieved by using an antenna with a larger aperture than the onboard antenna. Furthermore, in the direction perpendicular to the orbital direction (range direction), a signal called a chirp signal, whose frequency changes over time, is used to identify the Earth's position. The bandwidth of this chirp signal determines the correlation width, which in turn determines the resolution in the range direction. Meanwhile, the resolution in the azimuth direction achieved by synthetic aperture technology utilizes frequency changes due to the Doppler effect, which is determined by the satellite's speed.

[0003] For example, Non-Patent Document 1 below discloses a technology for generating a synthetic aperture radar image by transmitting radar waves from one of two moving bodies moving on orbit and receiving the radar waves reflected by the ground at the other moving body. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] “Bistatic SAR System and Signal Processing Technology”,Robert Wang,Yunkai Deg Springer,2018 Summary of the Invention [Problem to be solved by the invention]

[0005] As mentioned above, the resolution of a synthetic aperture radar depends on the magnitude of the frequency change of the transmitted microwaves, or the so-called frequency bandwidth, in both the azimuth and range directions. Generally, there is a positive correlation between bandwidth and resolution, and it is necessary to increase the bandwidth to achieve high resolution.

[0006] However, the bandwidth in the range direction is limited by the Radio Law, and the azimuth direction is limited by the Doppler effect, which depends on the satellite's ground speed, so the resolution is also currently limited by these factors.

[0007] Therefore, since the bandwidth of the microwaves used by the synthetic aperture radar satellite is restricted by the ground speed or the Radio Law, there is a problem that the resolution of the synthetic aperture radar cannot be improved.

[0008] The present invention has been made in view of the above circumstances, and aims to provide a synthetic aperture radar system that can achieve high spatial resolution by equivalently widening the frequency band and improving the time resolution. [Means for solving the problem]

[0009] A synthetic aperture radar system according to the present invention is a synthetic aperture radar system provided in a mobile body and including a radar signal generating device, a radar wave transmitting unit, a radar wave receiving unit, a radar signal amplifying device, a radar signal mixing device, and a radar signal processing device, the synthetic aperture radar system including: a radar signal generating device that generates a radar signal at a predetermined timing; a radar wave transmitting unit that generates radar waves based on the radar signal and irradiates the generated radar waves from the mobile body onto the ground surface; a radar wave receiving unit that receives the radar waves transmitted from the radar wave transmitting unit and reflected by the ground surface and converts the received radar waves into radar signals; and a radar signal processing unit that receives the converted radar signals. a radar signal mixer that mixes the amplified signal with the radar signal generated by the radar signal generator to generate a composite radar signal; and a radar signal processor that generates image data relating to point scatterers on the ground surface based on the radar signal based on the radar waves that have been reflected multiple times by the ground surface and the radar signal generated by the radar signal generator, wherein the radar wave transmitter generates the radar wave based on the composite radar signal generated by the radar signal mixer and irradiates the generated radar wave onto the ground surface.

[0010] According to this configuration, the mobile object generates radar waves from the generated radar signal and irradiates them onto the ground surface. The mobile object receives the radar waves reflected by the ground surface, amplifies the radar waves, and irradiates them onto the ground surface again. By repeating this operation multiple times, the bandwidth is equivalently expanded. As a result, the time resolution is improved, and the resolution of the image constructed using the radar waves is also improved. [Effects of the Invention]

[0011] According to the present invention, the frequency band of the transmitted microwave is equivalently broadened, thereby improving the time resolution and, as a result, the spatial resolution. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is an explanatory diagram of a synthetic aperture radar system according to a first embodiment. [Figure 2] FIG. 2 is a diagram illustrating the internal configuration of a satellite according to the first embodiment. [Figure 3] 2 is an explanatory diagram showing how radar waves are repeatedly reflected and returned between a satellite and an observation point according to the first embodiment. FIG. [Figure 4] 1 is a diagram for explaining the principle of the present invention according to a first embodiment. [Figure 5] FIG. 10 is an explanatory diagram of a synthetic aperture radar system according to a second embodiment. [Figure 6] FIG. 10 is a diagram illustrating the internal configuration of a main satellite according to a second embodiment. [Figure 7] FIG. 10 is a diagram illustrating the internal configuration of a slave satellite according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] A synthetic aperture radar system according to an embodiment of the present invention will be described below with reference to the drawings. The synthetic aperture radar system according to this embodiment generates a radar signal on a satellite 1, uses the radar signal to generate radar waves, transmits the waves, receives the waves reflected by the Earth's surface, and then amplifies and retransmits them repeatedly, thereby increasing the travel length of the radar waves and achieving high resolution (Example 1). [Example]

[0014] FIG. 1 is an explanatory diagram showing the overall structure of a synthetic aperture radar system using a satellite 1 according to Example 1 of this embodiment. In FIG. 5, V1 indicates the speed of the satellite 1. FIG. 2 is a diagram showing the internal configuration of the satellite 1, which includes a radar signal generator 2, a radar wave transmitter 3, a radar wave transmitting antenna 4, a radar wave receiving antenna 5, a radar wave receiving device 6, a radar signal amplifier 7, and a radar signal mixer 8. The radar wave transmitter 3 and the radar wave transmitting antenna 4 form a radar wave transmitting unit according to the present invention, and the radar wave receiving antenna 5 and the radar wave receiving device 6 form a radar wave receiving unit according to the present invention. Here, radar waves are, for example, high-frequency, high-power microwaves irradiated onto the Earth's surface, and a radar signal refers to a signal that modulates the radar waves.

[0015] A satellite 1 shown in Fig. 1 generates a radar signal using a chirp signal generated at a predetermined timing by a radar signal generator 2 shown in Fig. 2. From the generated radar signal, a radar wave transmitter 3 generates radar waves that are irradiated onto the Earth's surface. The radar waves are transmitted from a radar wave transmitting antenna 4. The transmitted radar waves undergo frequency changes due to the Doppler effect.

[0016] The radar waves transmitted from the radar wave transmitting antenna 4 are reflected at the observation point (ground) and received by the radar wave receiving antenna 5 of the satellite 1. At this time, frequency changes occur due to the Doppler effect both during transmission and reception. This change is the same as in normal SAR observation. The received radar waves are downconverted by the radar wave receiving device 6, and the radar signal after reflection by the ground is extracted. The extracted radar signal is amplified by the radar signal amplifier 7. The amplified radar signal is combined with the chirp signal of the radar signal generated by the radar signal generator 2 by the radar signal mixer 8 to generate a combined radar signal. The radar waves generated in a phase-continuous state by the radar wave transmitting device 3 based on the combined radar signal are upconverted and transmitted from the radar wave transmitting antenna 4. The radar signal processing device 9 also extracts components that have been reflected a predetermined number of times from the radar signal extracted from the received radar waves using timing calculated based on the time when the radar signal was generated by the radar signal generating device 2, converts the extracted components into two-dimensional data, and reconstructs an image using a reference signal generated based on the data and the generated chirp signal. This reconstruction process in the radar signal processing device 9 can be performed inside the satellite 1 and the results can be transmitted to the ground, or the results can be transmitted to the ground and then reconstructed. In the latter case, the radar signal processing device 9 is installed on the ground.

[0017] Furthermore, in the above technology, a radar signal, which is a baseband component, is extracted from the received radar wave. The key point of this embodiment is to increase the path length of the radar wave by repeatedly reflecting the radar signal while maintaining its phase, thereby equivalently broadening the frequency bandwidth of the chirp signal that constitutes the radar signal. Therefore, it is also possible to directly amplify the received radar wave without extracting the radar signal, which is a baseband component, from the radar wave, mix it with a radar wave generated by a radar wave transmitter 3 based on the radar signal output from the radar signal generator 2, and transmit it from the radar wave transmitting antenna 4. Furthermore, when amplifying the received radar wave or the radar signal extracted from the received radar wave for retransmission, it is also possible to sample the radar wave or signal at a high rate and digitally process it while maintaining the phase information.

[0018] The technical feature of this embodiment is that it is a synthetic aperture radar system that uses radar waves that have traveled back and forth between a satellite and the ground multiple times. However, detailed explanation of the technology common to ordinary synthetic aperture radars that receive and process backscattered microwaves emitted by a satellite onto the ground surface will be omitted.

[0019] The following describes how this embodiment increases the path length of the radar wave, thereby widening the bandwidth and improving the resolution.

[0020] First, if the radar aperture length of satellite 1 is D, it is known that the resolution in the azimuth direction of a normal synthetic aperture radar satellite is D / 2.

[0021] FIG. 3 is a diagram illustrating the principle of the present invention using Example 1. The key point of the present invention is that radar waves irradiated to an observation point and reflected are reflected back, and then phase-continuous radar waves are irradiated multiple times at the same observation point. In the following, the number of repetitions is assumed to be N. In ordinary SAR technology, analysis is performed under the assumption that the satellite's position does not change from the time the radar wave is transmitted until the reflected wave is received, which is called the Stop and Go Model. However, in the present invention, since there are N repetitions, it is necessary to take into account the change in the satellite's position from the time the radar wave is transmitted until it is received. Here, the time when the radar wave is first transmitted is defined as t0, and the time when the radar wave is received by satellite 1 immediately after the i-th reflection is defined as t1. i In addition, for i=1,2,...,N-1, t i corresponds to the time when the radar wave is returned from satellite 1 for the i-th time. Here, V is the velocity (y direction) of satellite 1, h is the altitude of satellite 1, (x, y, 0) is the coordinate of the observation point, (0, y0, h) is the position of satellite 1 at time 0, and (t) is the time when the radar wave is transmitted from satellite 1 at time t i is sent at time t i+1 The travel length of the radar wave until it is received is l i Then,

number

number

number

number

number

number

number

number

number

number

number

number

number

number

number

number

number

number

number

number

number

number

number

number

[0022] In the above embodiment, a case where radar waves are transmitted and received by one satellite has been described, but the present invention is not limited to this example.

[0023] Furthermore, the actual image generation can be performed within the satellite or the data can be transmitted to the ground for processing, and the location and scope of the processing can be arbitrary.

[0024] In the above-described embodiment, the mobile object is a satellite, but the present invention is not limited to this example. The mobile object may be an airplane or a vehicle that travels on the ground.

[0025] Although Example 1 has been described in detail with reference to the drawings as an example of an embodiment of the present invention, the specific configuration is not limited to these examples, and various design changes can be made within the scope of the gist of the present invention. Also, although microwaves are used for communication between satellites, any electromagnetic wave that can transmit radar signals and has a Doppler effect during transmission, such as light, may be used.

[0026] In the first embodiment, the radar waves received by the satellite 1 are extracted based on the timing of chirp signal generation. However, it is also possible to transmit the time series data of the received radar waves and the time data of the timing of chirp signal generation to the ground and perform extraction processing on the ground.

[0027] In the first embodiment, a case where radar waves are transmitted and received using one satellite has been described, but the present invention is not limited to this example. An example using two satellites will be described below. [Example]

[0028] FIG. 5 is an explanatory diagram showing the overall configuration of a synthetic aperture radar system using a primary satellite 14 and a secondary satellite 15 according to Example 2 of this embodiment. In FIG. 5, V1 indicates the speed of the primary satellite 14, and V2 indicates the speed of the secondary satellite 15. FIG. 6 is a diagram showing the internal configuration of the primary satellite 14, which includes a radar signal generator 2, a radar wave transmitter 3, a radar wave transmitting antenna 4, a radar signal receiving antenna 16, a radar signal receiver 17, a radar signal amplifier 7, a radar signal mixer 8, and a radar signal processor 9. The radar signal receiving antenna 16 and the radar signal receiver 17 form a radar signal receiver according to the present invention, and the radar wave transmitter 3 and the radar wave transmitting antenna 4 form a radar wave transmitter according to the present invention. FIG. 7 is a diagram showing the internal configuration of the secondary satellite 15, which includes a radar wave receiving antenna 5, a radar wave receiver 6, a radar signal transmitter 18, and a radar signal transmitting antenna 19. The radar wave receiving antenna 5 and the radar wave receiving device 6 constitute a radar wave receiving section according to the present invention, and the radar signal transmitting device 18 and the radar signal transmitting antenna 19 constitute a radar signal transmitting section according to the present invention.

[0029] Here, as in the first embodiment, the radar wave refers to, for example, a high-frequency, high-power microwave irradiated onto the ground, and the radar signal refers to a signal that modulates the radar wave. The difference from the first embodiment is that a function has been added to transmit the radar signal from the secondary satellite to the primary satellite using a low-power microwave.

[0030] The main satellite 14 shown in Fig. 5 generates a radar signal using a chirp signal generated at a predetermined timing by the radar signal generator 2 shown in Fig. 6. The generated radar signal is amplified into microwaves by the radar wave transmitter 3 to generate radar waves that are irradiated onto the ground. The radar waves are transmitted from the radar wave transmitting antenna 4. The transmitted radar waves undergo frequency changes due to the Doppler effect.

[0031] The radar waves transmitted from the radar wave transmitting antenna 4 of the primary satellite 14 are reflected at the observation point (ground) and received by the radar wave receiving antenna 5 (shown in FIG. 7) of the secondary satellite 15 (shown in FIG. 5). At this time, a frequency change occurs due to the Doppler effect both during transmission and reception. This change is the same as in normal SAR observation. The received radar waves are down-converted by the radar wave receiving device 6, and the radar signal after reflection on the ground is extracted. The extracted radar signal is converted to a low-power microwave by the radar signal transmitting device 18 and transmitted from the radar signal transmitting antenna 19 toward the primary satellite 14. The primary satellite 14 receives the radar signal transmitted from the secondary satellite 15 by the radar signal receiving antenna 16 and extracts the radar signal by the radar signal receiving device 17. The extracted radar signal is amplified by the radar signal amplifier 7. The amplified radar signal is mixed with the chirp signal of the radar signal generated by the radar signal generator 2 in the radar signal mixer 8 to generate a mixed radar signal. The radar wave transmitter 3 upconverts the mixed radar signal to generate radar waves, which are then transmitted from the radar wave transmitter antenna 4. The radar signal processor 9 extracts components reflected a predetermined number of times from the radar signal extracted by the radar signal receiver 17 using the timing at which the radar signal was generated by the radar signal generator 2, converts the extracted components into two-dimensional data, and reconstructs an image using a reference signal generated based on the extracted data and the generated chirp signal. This reconstruction process in the radar signal processor 9 can be performed inside the main satellite 14 and the results can be transmitted to the ground, or the process can be performed after transmission to the ground. In the latter case, the radar signal processor 9 is installed on the ground.

[0032] In the second embodiment, two satellites are used, with radar waves transmitted from the primary satellite 14, and the secondary satellite 15 receives the reflected waves from the observation point, and then transmits the signal back to the primary satellite 14. By repeating this process N times, it is possible to obtain the same effect as in the first embodiment. Note that the frequency of the communication between satellites does not need to be the same as that of the radar waves, and it is possible for the satellite receiving the reflected waves to up-convert or down-convert so that the Doppler effect during relaying is preserved, thereby avoiding interference.

[0033] The direction in which high resolution can be obtained due to the Doppler effect differs depending on the orbits of the primary satellite 14 and the secondary satellite 15. As a result, by performing the processing of Example 1 on the primary satellite 14 and the processing of Example 2 on the secondary satellite 15, high-resolution data due to the Doppler effect can be obtained as two independent directional components. This makes it possible to generate a two-dimensional image with high resolution due to the Doppler effect. Nonlinear processing may be required in two-dimensional image analysis. The satellite that transmits radar waves to the ground may be a geostationary satellite. Furthermore, although microwaves are used for communication between satellites, any electromagnetic wave that can transmit radar signals and has a Doppler effect during transmission may be used, such as light.

[0034] The synthetic aperture radar system of this embodiment can broaden the bandwidth of microwaves emitted or received. As a result, it has the advantage of exceeding the resolution limits of conventional SAR technology. Furthermore, when a secondary satellite 15 is used, the secondary satellite 15 transmits to the primary satellite 14 a signal (radar signal) with the same waveform as that generated by a conventional synthetic aperture radar. Since it is sufficient for the radar signal to reach the primary satellite 14, a high-output transmitter for transmitting radar waves to the Earth's surface is not required. This is also an important feature and feature of this embodiment. Therefore, it has the advantage of requiring less power and being implemented with a relatively low-cost small satellite. Furthermore, a technique for improving SAR resolution through multiple observations using multiple satellites or the same satellite is known as "interferometric SAR." However, while the principle of "interferometric SAR" is to improve resolution by increasing the distance between multiple observation points, thereby equivalently expanding the aperture length, this invention differs from this. It improves resolution by increasing the path length of the radar wave through multiple reflections in a short period of time, thereby equivalently expanding the frequency bandwidth. Therefore, it can be said that this invention is fundamentally different.

[0035] In the above-described embodiment, the mobile object is a satellite, but the present invention is not limited to this example. The mobile object may be an airplane, and a similar synthetic aperture radar system can be implemented using two airplanes. The multiple mobile objects may also be a combination of a satellite and an airplane. Furthermore, in the present invention, the mobile object may be a vehicle traveling on the ground, and the multiple mobile objects may also be a combination of a satellite or an airplane and a vehicle. [Industrial Applicability]

[0036] By repeatedly receiving and retransmitting radar waves reflected from observation points above the observation area, high-resolution images can be captured, making it suitable for high-precision Earth observation. [Explanation of symbols]

[0037] 1 satellite (mobile) 2. Radar signal generator 3. Radar wave transmitter (radar wave transmitter) 4. Radar wave transmitting antenna (radar wave transmitting section) 5. Radar wave receiving antenna (radar wave receiving section) 6. Radar wave receiving device (radar wave receiving section) 7. Radar signal amplifier 8 Radar signal mixer 9. Radar signal processing device 10 Radar wave received after one reflection 11 Radar wave received after N(=5) reflections 12 The extracted portion of a radar wave reflected earlier than the intended radar reflection 13 Extracted portion of a radar wave reflected later than the intended radar reflection 14 Main satellite (mobile) 15 Secondary satellite (second mobile body) 16 Radar signal receiving antenna (radar signal receiving section) 17 Radar signal receiving device (radar signal receiving unit) 18 Radar signal transmitter (radar signal transmitter) 19 Radar signal transmitting antenna (radar signal transmitting section)

Claims

1. A synthetic aperture radar system in a mobile body, comprising a radar signal generating device, a radar wave transmitting unit, a radar wave receiving unit, a radar signal amplifying device, a radar signal mixing device, and a radar signal processing device, a radar signal generating device that generates a radar signal at a predetermined timing; a radar wave transmitter that generates a radar wave based on the radar signal and irradiates the generated radar wave onto the ground surface; a radar wave receiving unit that receives the radar waves transmitted from the radar wave transmitting unit and reflected by the ground surface and converts the received radar waves into a radar signal; a radar signal amplifier that amplifies the converted radar signal; a radar signal mixer that mixes the radar signal amplified by the radar signal amplifier with the radar signal generated by the radar signal generator to generate a composite radar signal; a radar signal processing device that generates image data relating to point scatterers on the ground surface based on a radar signal generated by the radar signal generating device and based on the radar wave that has been reflected multiple times by the ground surface; Equipped with the radar wave transmitter generates the radar wave based on the composite radar signal generated by the radar signal mixer, and irradiates the generated radar wave onto the ground surface.

2. the radar wave receiving unit is provided in a second moving body separate from the moving body instead of the moving body, the second moving body is equipped with a radar signal transmitting device, the moving object further includes a radar signal receiving unit, the radar signal transmitting device extracts a radar signal from the radar wave received by the radar wave receiving unit provided in the second moving body, and transmits the extracted radar signal to the moving body; the radar signal receiving unit receives the radar signal transmitted from the second moving object; the radar signal amplifier amplifies the radar signal received by the radar signal receiver; 2. The synthetic aperture radar system according to claim 1, wherein the radar signal mixer mixes the radar signal amplified by the radar signal amplifier and the radar signal generated by the radar signal generator to generate a composite radar signal.

3. 2. The synthetic aperture radar system according to claim 1, wherein the radar signal processing device is provided in a ground facility installed on the ground instead of in the mobile object.

Images