JP2024048892A - Synthetic aperture radar system and download determination method - Google Patents

Abstract

[Problem] To achieve both reduction in the amount of downloaded data and reduction in calculation load, and realize highly accurate target detection. [Solution] The system includes a thinning processing unit 120 that thins out received data of a synthetic aperture radar (SAR) in the azimuth direction for each synthetic aperture time and outputs a predetermined number of received data, a range compression unit 121 that executes range compression processing on the predetermined number of received data, a target presence/absence determination unit 125 that determines the presence or absence of a target in a narrow-sized block by comparing the feature amount of a predetermined-sized block divided in the range direction of data D1 after range compression with the feature amount of a narrow-sized block included in the predetermined-sized block, and a compilation determination unit 126 that determines whether or not the SAR received data can be downloaded based on the result of comparing the total number of targets in the SAR received data with a threshold value. [Selected Figure] Figure 4

Description

The present invention relates to synthetic aperture radar systems, and in particular to technology for downloading data.

Synthetic Aperture Radar (SAR) is an active sensor that irradiates microwaves and measures the intensity of their reflection to image an object. Microwaves have a much longer wavelength than visible light or near-infrared light, and have the ability to penetrate clouds and fog. Therefore, by mounting SAR on a mobile platform such as a satellite or aircraft, it is possible to obtain various data on the earth's surface and sea surface even at night or in bad weather.

One of the main uses of SAR is ocean surveillance to check for the presence of ships and other objects. However, much of the SAR data used in ocean surveillance is photographic data of vast expanses of ocean where no ships or other objects exist. Downloading such SAR data to the ground to check for the presence of ships and other objects is extremely inefficient.

In addition, the amount of data is increasing rapidly as SAR resolution improves and the observation width expands, and the amount of data downloaded (DL) from the satellite to the ground is also increasing rapidly, making it difficult to download all the data due to line capacity constraints.

Patent Document 1 discloses an example of a method for reducing DL data from satellites. According to Patent Document 1, image analysis is performed within the satellite, and the amount of DL data is reduced by downloading only image data of the area of interest (for example, image parts without clouds, image parts including potential ships).

JP 2021-119693 A

However, the system disclosed in Patent Document 1 mainly analyzes images captured by an optical sensor based on the physical characteristics of the target, and downloads only data that includes identified targets. Therefore, Patent Document 1 does not disclose a system that is premised on SAR.

As is well known, SAR requires range compression and azimuth compression to generate images. Azimuth compression processing is performed on image data after range compression, which imposes a large computational load and requires a large memory capacity. On-board processing on a satellite is generally severely restricted by the computational power and memory capacity of the CPU (Central Processing Unit), so it is desirable to reduce the processing load and the required memory capacity.

For this type of SAR, the DL data reduction method based on image analysis, such as that in Patent Document 1, cannot be applied as is. Furthermore, Patent Document 1 does not even recognize the issues of reducing the processing load and memory requirements in SAR.

The object of the present invention is to provide a synthetic aperture radar system, a download determination method, a satellite, and a program that achieves both a reduction in the amount of downloaded data and a reduction in the computational load, and realizes highly accurate target detection.

According to a first aspect of the present invention, a synthetic aperture radar system mounted on a mobile platform has a thinning processing unit that thins out synthetic aperture radar (SAR) reception data shown in the azimuth direction and range direction for each synthetic aperture time in the azimuth direction to output a predetermined number of pieces of reception data, a range compression unit that performs range compression processing on the predetermined number of pieces of reception data, a target presence/absence determination unit that determines the presence or absence of a target in the narrow-sized block by comparing the features of a predetermined-sized block into which the range-compressed data is divided in the range direction with the features of a narrow-sized block included in the predetermined-sized block, and a counting determination unit that determines whether or not the SAR reception data can be downloaded based on the comparison result between the total number of targets in the SAR reception data and a predetermined number.
According to a second aspect of the present invention, a method for determining whether to download SAR reception data in a synthetic aperture radar (SAR) system mounted on a mobile platform includes a data processing unit of the SAR system that thins out synthetic aperture radar (SAR) reception data indicated in the azimuth direction and range direction for each synthetic aperture time in the azimuth direction to output a predetermined number of pieces of reception data, performs range compression processing on the predetermined number of pieces of reception data, and compares the features of a predetermined-size block divided into the range direction of the range-compressed data with the features of a narrow-size block included in the predetermined-size block to determine the presence or absence of a target in the narrow-size block, and determines whether or not to download the SAR reception data based on the comparison result between the total number of targets in the SAR reception data and the predetermined number.
According to a third aspect of the present invention, a satellite having a synthetic aperture radar (SAR), a memory unit for storing SAR reception data, a communication unit for downloading data, and a data processing unit, the data processing unit thins out SAR reception data indicated in the azimuth direction and range direction for each synthetic aperture time in the azimuth direction to output a predetermined number of pieces of reception data, performs range compression processing on the predetermined number of pieces of reception data, and determines the presence or absence of a target in the narrow range by comparing the features of a predetermined-sized block divided into the range direction of the data after range compression with the features of a narrow-sized block included in the predetermined-sized block, determines whether or not to download the SAR reception data based on the comparison result between the total number of targets in the SAR reception data and the predetermined number, and reads out the SAR reception data that has been permitted to be downloaded from the memory unit and downloads it via the communication unit.
According to a fourth aspect of the present invention, a program that causes a computer to function as a download determination device in a synthetic aperture radar system mounted on a mobile platform implements in the computer a function of thinning out synthetic aperture radar (SAR) reception data, which is indicated in the azimuth direction and the range direction, in the azimuth direction for each synthetic aperture time to output a predetermined number of pieces of reception data, a function of performing range compression processing on the predetermined number of pieces of reception data, a function of determining the presence or absence of a target in a narrow-sized block by comparing the features of a predetermined-sized block into which the range-compressed data is divided in the range direction with the features of a narrow range contained in the predetermined-sized block, and a function of determining whether or not the SAR reception data can be downloaded based on a comparison result between the total number of targets in the SAR reception data and a predetermined number.

The present invention makes it possible to reduce both the amount of downloaded data and the computational load, enabling highly accurate target detection.

FIG. 1 is a schematic diagram for explaining the positional relationship between a satellite-mounted SAR system and a target. FIG. 2 is a waveform diagram for explaining range compression of the SAR. FIG. 3 is a schematic diagram showing an example of an image after range compression for explaining a target counting method in the SAR system according to the present invention. FIG. 4 is a block diagram illustrating the functional configuration of a SAR system according to an embodiment of the present invention. FIG. 5 is a schematic diagram showing an example of an image expressed by the number of data in the range direction and the azimuth direction in the SAR system according to this embodiment. FIG. 6 is a schematic diagram showing an example of a determination region in the image shown in FIG. FIG. 7 is a schematic diagram showing an example of a background region in the image shown in FIG. FIG. 8 is a schematic diagram showing the overlap of the determination region and the background region in the image shown in FIG. FIG. 9 is a graph showing power distribution in a determination region and a background region for explaining a target presence/absence determination according to this embodiment. FIG. 10 is a schematic diagram showing an example of target counting to explain the DL data determination method according to this embodiment. FIG. 11 is a flowchart showing an example of a DL data determination method according to this embodiment. FIG. 12 is a schematic diagram for explaining a case where a squint angle exists in the positional relationship between a satellite-mounted SAR system and a target. FIG. 13 is a graph illustrating the relationship between the squint angle and the magnitude of range migration. FIG. 14 is a graph illustrating the relationship between the squint angle and the number of integrated pixels in the azimuth direction.

<Overview of the embodiment>
According to one embodiment of the present invention, the received data of a synthetic aperture radar (SAR) is thinned out for each synthetic aperture time and range compression processing is performed. The presence or absence of a target is determined by comparing the feature amounts of the background area and the judgment area in the range compressed data, and the total number of targets can be counted. Only SAR received data in which the total number of targets exceeds a predetermined value can be downloaded.

By thinning out the received data for each synthetic aperture time, only the data necessary to find the target can be retained, reducing the computational load. The presence or absence of a target is determined using the features of the range-compressed data, so the number of targets can be counted without azimuth compression. Azimuth compression requires accessing memory in all directions in two dimensions, which places a heavy computational load on the system and increases processing time. Therefore, being able to count the number of targets without azimuth compression is particularly advantageous for mobile platforms with limited computational resources and capabilities.

Furthermore, according to this embodiment, only SAR reception data in which the number of targets is greater than a threshold is downloaded. This allows the amount of downloaded data to be significantly reduced, and the problem of DL line constraints to be resolved. Also, since only valid SAR reception data is downloaded, targets can be detected with high accuracy in the required range on the ground.

1. Target Counting Principle As illustrated in FIG. 1, the SAR system according to the present invention is mounted on a mobile platform 10 and detects a target 20 on the ground. The mobile platform 10 is, for example, an artificial satellite or an aircraft, and is assumed to be moving in a direction of travel 30 (azimuth direction). The mobile platform 10 is provided with an antenna 100 of the SAR system, which transmits pulse waves to the ground at a fixed interval PRI (Pulse Repetition Interval) and receives the reflected waves. The time during which the SAR system can observe the target 20 is the synthetic aperture time T. Therefore, the SAR system detects the target 20 in the azimuth direction for the synthetic aperture time T (-T/2 to +T/2 in FIG. 1). For the convenience of the following explanation, it is assumed that the slant range distance between the antenna 100 and the target 20 is shortest at time t ₀ , which is the center of the synthetic aperture time T. Time t ₀ is taken as a reference point, and the distance increases as the distance from the reference point increases.

2, the antenna 100 radiates pulse waves (N=2) at regular intervals PRI while moving in the azimuth direction. The radiated pulse waves are reflected by the target 20, and the reflected waves are received by the antenna 100. As described above, pulse waves (N=2) are radiated at time t ₀ =0 at the reference point, time t _-1 T/4 before the reference point, time t _-2 T/2 before the reference point, time t ₁ T/4 after the reference point, and time t ₂ T/2 after the reference point.

When a pulse wave is emitted at each time point, the time that elapses from the time of emission until the reflected wave is received is shortest at time point _t0 , and the time that elapses increases as the distance from the reference point _t0 increases. That is, the movement of the antenna 100 changes the slant range distance to the target 20 (range migration), and this changes the time that elapses until the arrival of the reflected wave. Therefore, the received data 200 is range compressed, and the presence of the target 20 can be detected as a range migration line 201 from the range-compressed data D1.

In existing methods, azimuth compression is performed on the range-compressed data D1, and the number of targets is counted from the azimuth-compressed data. Azimuth compression requires accessing memory in all directions in two dimensions, which increases costs and risks for satellites with limited computing resources and capabilities.

In contrast, according to this embodiment, targets are detected by extracting data in the azimuth direction from the range-compressed data D1 at a period equal to the synthetic aperture time T. The principle of target counting will be explained below with reference to FIG. 3.

As shown diagrammatically in FIG. 3, in the range-compressed image in the range and azimuth directions, the target is shown as a range migration line 201 of synthetic aperture time T. Therefore, if the range-compressed image is extracted in the azimuth direction for each synthetic aperture time T (and other image portions are thinned out), the targets present in the range-compressed image can be detected as shown by black circles. By counting the number of black circles in this way, the total number of targets present in the range-compressed image can be obtained.

The target counting method described above can be used to determine which images to download. Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the components described in the following embodiments are merely examples, and are not intended to limit the technical scope of the present invention to those alone.

2. One embodiment As illustrated in Fig. 4, a SAR system according to one embodiment of the present invention has a transmitting unit 101 and a receiving unit 102. The transmitting unit 101 transmits pulse waves from an antenna 100 at a constant interval PRI. The transmitted pulse waves are reflected from an observation surface, and a portion of the transmitted pulse waves reaches the antenna 100. The receiving unit 102 receives the reflected waves and outputs SAR reception data. The SAR reception data is reception data 200 illustrated in Fig. 2, and is stored in a SAR reception data storage unit 103.

The SAR system further comprises a DL determination unit 104, a control unit 105, and a communication unit 106. The DL determination unit 104 determines whether or not the SAR reception data can be downloaded, as described below. As described above, the SAR reception data forms an image consisting of a received pulse waveform in a two-dimensional space consisting of the range direction and the azimuth direction. The control unit 105 reads out the SAR reception data that has been determined to be valid for download from the SAR reception data storage unit 103, and downloads it to the ground station via the wireless communication unit 106. The functional configuration and operation of the DL determination unit 104 will be described below.

<Download Judgment>
4, the DL determination unit 104 has a thinning processing unit 120 and a range compression unit 121. The thinning processing unit 120 sequentially reads out the SAR reception data from the SAR reception data storage unit 103, thins out the data in the azimuth direction at a period of the synthetic aperture time T, and then outputs only the remaining data for one line in the range direction to the range compression unit 121. The range compression unit 121 obtains a correlation between the reception data and a reference function, and outputs range-compressed data D1 to the data extraction unit 122.

The data extraction unit 122 extracts judgment region data D2 and background region data D3 from the range-compressed data D1 and outputs them to a judgment feature amount calculation unit 124 and a background feature amount calculation unit 125. As will be described later, the size of the judgment region data D2 in the azimuth direction is adjusted in accordance with the squint angle θ _sq .

As described below, the judgment feature calculation unit 124 calculates the feature of each judgment block from the judgment region data D2, and the background feature calculation unit 125 calculates the feature of each background block from the background region data D3. A background block is a data range of a predetermined size, and a judgment block is a narrow data range contained in a background block.

The target presence/absence determination unit 126 compares the feature amount of the background block with the feature amount of the judgment block contained in the background block, and determines whether or not a target is present in the judgment block. In doing so, the expected size of the target can be used as reference information. The above-mentioned process is repeated for the data D1 output sequentially from the range compression unit 121 for each line in the range direction.

The counting and determining unit 126 counts the number of determination blocks in which it is determined that a target exists across the entire image of the SAR reception data, and if the total number of targets counted is greater than a threshold, it determines that the image (SAR reception data) is valid for download and notifies the control unit 105. SAR reception data in which the total number of targets is less than the threshold is determined to be invalid for download. Only SAR reception data determined to be valid for download in this way is downloaded to the ground via the wireless communication unit 106.

The target presence/absence determination in the DL determination unit 104 may be performed for each line of data D1 in the range direction, or may be performed for each of the data obtained by dividing one line of data D1 into two or more parts. Dividing the data D1 can reduce the memory capacity required for processing.

Furthermore, the functions of the DL determination unit 104 and the control unit 105 can be realized by executing a program stored in a memory (not shown) on a CPU or processor.

3. Example of Download Judgment Hereinafter, a download judgment in the case where the SAR system according to this embodiment is mounted on a satellite and used for ocean monitoring will be described with reference to FIGS.

First, as shown in Fig. 5, the SAR reception data is represented by a two-dimensional coordinate system with the azimuth direction as the vertical axis and the range direction as the horizontal axis. As described above, the antenna 100 transmits pulse waves at regular intervals PRI while moving. Therefore, the vertical axis represents the number of times pulse transmission is performed at regular intervals PRI while moving in the azimuth direction, and this number represents the number of data in the azimuth direction. N _az corresponds to the moving distance or time in the azimuth direction to obtain the SAR reception data. N ^d _az is the number of pulse transmissions in the azimuth direction corresponding to the synthetic aperture time T, and N ^d _az = T/PRI.

The horizontal axis represents the number of pulses reflected by the target and sampled at a fixed time, and therefore represents the number of data points in the range direction. _Nrg corresponds to the elapsed time or distance of the detectable reflected pulse for each pulse transmission.

5, the number of pulses in the azimuth direction indicates the number of pieces of data in the direction of travel (azimuth direction) of the antenna 100, and the number of pieces of data in the range direction indicates the number of pieces of data in the direction (range direction) of the pulses emitted from the antenna 100. Therefore, any unit section PXL (1 x 1 pulse) can be expressed as a coordinate by the number of pieces of data, and by receiving a reflected pulse from that position, the received data for that position can be obtained. In the example of FIG. 5, N _az = 30,000, N ^d _az = 10,000, and N _rg = 20,000.

<Determination area>
As illustrated in FIG. 6, according to this embodiment, data is thinned out in the azimuth direction at a period of the synthetic aperture time T, and the judgment area data D2 is extracted from the remaining data for one line in the range direction. In this example, the number of data in the azimuth direction of the judgment area data D2 is N ^t _az . The judgment area data D2 is divided into units of judgment blocks Rt. The judgment block Rt is a rectangular area (narrow-sized block) having a size of the number of data N ^t _rg in the range direction and the number of data N ^t _az in the azimuth direction. However, as will be described later, N ^t _az in the azimuth direction is determined depending on the squint angle θ _sq . The judgment block Rt is used to calculate a judgment feature for determining the presence or absence of a ship, which is a monitored object. An arbitrary judgment block is expressed by coordinates (i _t , j _t ), and the judgment feature of the judgment block is expressed as μ(i _t , j _t ).

<Background area>
As illustrated in Fig. 7, according to this embodiment, data is thinned out in the azimuth direction at a period of the synthetic aperture time T, and background region data D3 is extracted from the remaining data for one line in the range direction. In this example, the background region data D3 is divided into units of background blocks Rs _{. The background blocks Rs are rectangular regions (blocks of a predetermined size) having a size of the number of data Nsrg in} ^the range direction and the number of data ^Nsaz in the azimuth direction. The background blocks Rs are used to calculate background features for determining the presence or absence of a ship, which is a monitored object. An arbitrary background block is expressed by coordinates (i _s , _js ), and the determination feature of the background block is expressed as _σ0s (i ^s _{, js} ).

The determination region data D2 may be a partial region included in the background region data D3. The size and positional relationship of the background block Rs and the determination block Rt are as shown in FIG. 6. That is, the background block Rs has a size that includes a plurality of determination blocks Rt, and has the relationship ^Ntaz << ^Nsaz _and ^Ntrg << ^Nsrg . That _{is ,} each background block Rs of _a predetermined size includes a plurality of narrow-sized determination blocks Rt.

As an example, each background block Rs has _a predetermined size of ^Nsrg = 500 in the range direction ^{and Nsaz} ₌ 10 in the azimuth direction, and each judgment block Rt included in each background block Rs has a narrow size of ^Ntrg = ₁ in the range direction _{and Ntaz} ⁼ 3 in the azimuth direction. In this example, as illustrated in Fig. 6, the multiple judgment blocks Rt included in each background block Rs are arranged so as to be unevenly distributed on one side in the azimuth direction within the background block Rs.

When searching for a ship (target) floating on the sea surface, it is assumed that the sea surface serves as a background where the change in reception strength is gradual, and that the reception strength in the narrow area where the target is present is significantly higher than that of the sea surface. In this example, by introducing CFAR (constant probability of false alarm), it becomes possible to adaptively respond to the conditions on the sea surface. In other words, a relatively wide area is set as the background block Rs, and the reception strength of each judgment block Rt contained in the background block Rs is normalized using its average reception strength.

<Target presence/absence determination>
For the sake of simplicity, each background block Rs is assumed to include three judgment blocks Rt as shown in Fig. 8. The feature amount of the background block Rs is ^assumed to be _σ0s (i _s , j _s ), and the feature amount of the judgment block Rt included in the background block Rs is assumed to be _μt (i _t , _jt ).

The feature value σ ₀ ^s (i _s , j _s ) of the background block Rs is a feature value of sea surface scattering of the predetermined region. In this example, the feature value σ ₀ ^s (i _s , j _s ) of the background block Rs (i _s , j _s ) is calculated by the following formula:
_σ0s ⁽ _i , _js )=a _s · _μs ( _i , _js )+b _s · _σs ( _i , _js ).
Here, μ _s (i _s , j _s ) is the average of the received power, σ _s (i _s , j _s ) is its standard deviation, and a _s and b _s are adjustable coefficients.

By comparing the feature value μ _t (i _t , j _t ) of each judgment block Rt (i _t , j _t ) with the feature value σ ₀ ^s (i _s , _js ) of the background block Rs (i _s , _js ), it is possible to determine whether or not a target ship is present in that judgment block Rt. This will be described in detail with reference to FIG. 9.

In Fig. 9, it is determined whether the feature value _μt (i _t , _jt ) of each judgment block Rt(i _t , _jt ) is greater than the feature value _σ0s (i _s , _js ) of the background block ^Rs (i _s , _js ) in which the judgment block Rt is included. If _μt (i _t , _jt ⁾ > _σ0s (i _s , _js ), it is determined that a target is present, and if _μt (i _t , _jt )<= _σ0s (i ^s _{, js} ), it is determined that a target is not present. In the example shown in Fig. 9, only the judgment block Rt(3) of the background blocks Rs is determined to have a target.

However, when it is determined that a target is present in a certain decision block Rt(p), target presence/absence determination is skipped for ^a certain number n= ^Nskiprg of subsequent decision blocks Rt(p+1) to Rt(p+n). This skip number n _{=Nskiprg can} be set depending on the expected size of the target. Considering the case where it is not possible to determine whether there is one ship or multiple ships crowded together due to the resolution of the SAR system, counting when it is determined that there is a target due to _crowding is not performed for the skip number n= ^Nskiprg . Therefore, in this skip section, it is determined that there is no ^target even if _μt (i _t , _jt )> _σ0s (i _s , _js ).

In this way, target presence/absence determination is performed based on the amount of data for one line in the range direction (in this example, the amount of data for one line of background region D3), and the number of targets for each line can be tallied. In this example, since the determination region D2 is included in the background region D3, the presence/absence of a target can be determined using one line of data for the background region D3, making it possible to reduce processing time and memory usage.

<Download availability determination>
10, the number of targets _Nt (1) to _Nt ( _Naz / ^Ndaz ) in the judgment region D2 for each synthetic aperture time T is tallied and then tallied in the azimuth direction to obtain _the total number of targets ^Nttl _t in the image of the SAR reception data. In this example, a threshold value ^Nusr _t is used, and if the total number of targets ^Nttl _t > ^Nusr _t , downloading of the SAR reception data of the image is judged to be valid, and otherwise is judged to be invalid. The threshold value ^Nusr _t is, for example, about 100.

The DL determination function by the DL determination unit 104 and the DL control function by the control unit 105 described above can be realized by a data processing unit (CPU or processor) executing a program. The control flow by the data processing unit will be described below with reference to FIG.

11, when the DL determination unit 104 inputs the received data from the SAR received data storage unit 103 (operation 301), the thinning processing unit 120 skips (N ^d _az -N ^s _az +1) pieces of received data to execute thinning processing for each synthetic aperture time T (operation 302). Next, the range compression unit 121 performs range compression on the N ^s _az pieces of data remaining (read) after the thinning processing (operation 303). As a result of the range compression, data D1 for one line in the range direction is output to the data extraction unit 122.

The data extraction unit 122 extracts N ^t _az of judgment region data D2 and N ^s _az of background region data D3 from the range-compressed data D1 (operation 304; see Figs. 6 and 7). The background feature amount calculation unit 124 sequentially calculates background feature amounts σ ₀ ^s for each background block Rs from the N ^s _az of background region data D3, and the judgment feature amount calculation unit 123 sequentially calculates judgment feature amounts μ _t for each judgment block Rt from the N ^t _az of judgment region data D2 (operation 305).

As illustrated in FIG. 9 , the target presence/absence determination unit 125 compares the sequentially calculated background feature amount σ ₀ ^s for each background block Rs with the determination feature amount μ _t of each determination block included in each background block Rs to determine the presence or absence of a target (operation 306).

When the above target presence/absence determination process using N ^s _az worth of data (operations 302 to 306) is completed, it is determined whether or not processing of the entire image has been completed (operation 307). If there is still received data to be processed (NO in operation 307), the process returns to the thinning process in operation 302, and operations 303 to 306 are repeated for the next line of N ^s _az worth of data.

When the processing of the entire image is completed (YES in operation 307), the counting and determining unit 126 counts the number of targets in the range direction and the azimuth direction, and calculates the total number of targets ^Nttl _t (operation 308), as shown in Fig. 10. If the total number of targets ^Nttl _t > ^Nusr _t (YES in operation 309), the counting and determining unit 126 stores the SAR reception data of the image as download valid (operation 310). Otherwise (NO in operation 309), the data is not stored as download invalid.

In this way, the control unit 105 downloads only the SAR reception data that is determined to be valid for download to the ground station via the wireless communication unit 106.

5. Range Migration and Squint Angle In FIG. 6, the number of data N ^t _az in the azimuth direction of the judgment area data D2 is determined depending on the squint angle θ _sq .

12, it is assumed that the antenna 100a mounted on the mobile platform 10 has a squint angle θ _sq ≠ 0. In this case, it becomes necessary to change the number of pieces of data N ^t _az in the azimuth direction of the judgment region data D2 according to the magnitude of the squint angle θ _sq .

As shown in Fig. 13, if the squint angle θ _sq = 0, the range migration is small (range migration line 201a). Therefore, the number of data N ^t0 _az in the azimuth direction of the judgment area data D2 can be made large. On the other hand, if the squint angle θ _sq > 0 is large (range migration line 201b), if the number of data N ^t _az is made large, the range without a target signal will also be added to the integration of the reception strength, and the detection accuracy will deteriorate significantly. For this reason, if the squint angle θ _sq > 0 is large, it is necessary to make the number of data N ^t _az small in accordance with the range migration line 201b.

14 is a graph showing the change in the number of integrated pixels (number of data) in the azimuth direction with respect to the squint angle θsq. ^{N t} _az can be determined by the following formula:
N ^t _az =N ^t0 _az ·cos θ _sq +1.
As shown in this graph, by reducing the number of data N ^t _az in response to the squint angle θ _sq >0, it is possible to avoid deterioration in the accuracy of the average intensity of the decision block Rt.

6. Effects As described above, according to this embodiment, by thinning out the received data for each synthetic aperture time, it is possible to leave the data necessary to find the target, and to reduce the computational load. Since the presence or absence of a target is determined using the feature amount of the data after range compression, it is possible to count the number of targets without using azimuth compression, which has a large processing load. Therefore, this is particularly advantageous for mobile platforms with limited computational resources and capabilities.

In addition, according to this embodiment, only SAR reception data in which the number of targets is greater than a threshold is downloaded. This allows the amount of downloaded data to be significantly reduced, and the problem of DL line constraints to be resolved. Since only valid SAR reception data is downloaded, targets can be detected with high accuracy in the required range on the ground.

In particular, since only data that is useful for marine monitoring needs to be downloaded, the problem of line constraints can be fundamentally overcome, data can be acquired over a wider area with higher resolution, and the quality of marine monitoring can be drastically improved. In addition, as DL constraints are relaxed, it can be allocated to the transmission of other mission data.

7. Supplementary Notes A part or all of the above-described embodiment and examples may be described as the following supplementary notes, but the present invention is not limited to these.
(Appendix 1)
1. A synthetic aperture radar system mounted on a mobile platform, comprising:
a thinning processing unit that thins out synthetic aperture radar (SAR) reception data indicated in the azimuth direction and the range direction for each synthetic aperture time in the azimuth direction to output a predetermined number of reception data;
a range compression unit that executes a range compression process on the predetermined number of pieces of received data;
a target presence/absence determination unit that determines the presence or absence of a target in a narrow-sized block by comparing a feature amount of a predetermined-sized block into which the range-compressed data is divided in the range direction with a feature amount of a narrow-sized block included in the predetermined-sized block;
a counting and determining unit that determines whether or not the SAR reception data can be downloaded based on a comparison result between the total number of the targets in the SAR reception data and a predetermined number;
A synthetic aperture radar system having:
(Appendix 2)
the target presence/absence determination unit has a data extraction unit that extracts a background region divided into blocks of the predetermined size and a determination region divided into blocks of the narrow size from the range compressed data,
2. The synthetic aperture radar system according to claim 1, wherein the data extraction unit sets a smaller number of pieces of data in the azimuth direction of the narrow-sized block as the squint angle increases.
(Appendix 3)
3. The synthetic aperture radar system of claim 1, wherein the target presence/absence determination unit compares a background feature consisting of an average intensity and standard deviation of the blocks of the predetermined size with a judgment feature consisting of the average intensity of the blocks of the narrow size, and determines that a target is present in the narrow size block when the judgment feature of the blocks of the narrow size is greater than the background feature of the blocks of the predetermined size.
(Appendix 4)
3. The synthetic aperture radar system according to claim 1, wherein, when the target presence determination unit determines that a target is present in one narrow-sized block, the target presence determination unit does not perform target presence determination for a predetermined number of narrow-sized blocks following the one narrow-sized block depending on the expected size of the target.
(Appendix 5)
A method for determining download of SAR reception data in a synthetic aperture radar (SAR) system mounted on a mobile platform, comprising:
A data processing unit of the SAR system,
thinning out synthetic aperture radar (SAR) reception data indicated in the azimuth direction and range direction for each synthetic aperture time in the azimuth direction to output a predetermined number of reception data;
performing a range compression process on the predetermined number of pieces of received data;
determining whether or not a target is present in a narrow-sized block by comparing a feature amount of a block of a predetermined size obtained by dividing the data into range directions and a feature amount of a narrow-sized block included in the block of the predetermined size;
determining whether or not to download the SAR reception data based on a comparison result between the total number of the targets in the SAR reception data and a predetermined number;
Download determination method.
(Appendix 6)
A download determination method as described in Appendix 5, which extracts a background area divided into blocks of the specified size and a determination area divided into blocks of the narrow size from the range compressed data, and sets the number of data in the azimuth direction of the narrow size blocks to be smaller as the squint angle becomes larger.
(Appendix 7)
The download determination method according to claim 5 or 6, further comprising: comparing a background feature consisting of an average intensity and standard deviation of the blocks of the predetermined size with a determination feature consisting of an average intensity of the blocks of the narrow size; and determining that a target exists in the block of the narrow size when the determination feature of the block of the narrow size is greater than the background feature of the block of the predetermined size.
(Appendix 8)
A download determination method as described in Appendix 5 or 6, in which, when it is determined that a target exists in a narrow-sized block, no determination of the presence or absence of a target is made for a predetermined number of narrow-sized blocks following the narrow-sized block, depending on the expected size of the target.
(Appendix 9)
A satellite having a synthetic aperture radar (SAR), a memory unit for storing SAR received data, a communication unit for downloading data, and a data processing unit,
The data processing unit:
thinning out the SAR reception data indicated in the azimuth direction and the range direction for each synthetic aperture time in the azimuth direction to output a predetermined number of reception data;
performing a range compression process on the predetermined number of pieces of received data;
determining whether or not a target is present in the narrow range by comparing the feature amount of a block of a predetermined size into which the range-compressed data is divided in the range direction with the feature amount of a narrow block included in the block of the predetermined size;
determining whether or not to download the SAR reception data based on a comparison result between the total number of targets in the SAR reception data and a predetermined number;
reading the SAR reception data that is permitted to be downloaded from the storage unit, and downloading the data through the communication unit;
satellite.
(Appendix 10)
The satellite described in Appendix 7, wherein the data processing unit extracts a background region divided into blocks of the predetermined size and a judgment region divided into blocks of the narrow size from the range-compressed data, and sets a smaller number of data in the azimuth direction of the narrow size blocks as the squint angle increases.
(Appendix 11)
The satellite described in Appendix 9 or 10, wherein the data processing unit compares a background feature consisting of an average intensity and standard deviation of the blocks of the predetermined size with a judgment feature consisting of an average intensity of the blocks of the narrow size, and judges that a target is present in the narrow size block when the judgment feature of the blocks of the narrow size is greater than the background feature of the blocks of the predetermined size.
(Appendix 12)
A satellite as described in Appendix 9 or 10, wherein when the data processing unit determines that a target is present in one narrow-sized block, it does not perform target presence/absence determination for a predetermined number of narrow-sized blocks following the one narrow-sized block, depending on the expected size of the target.
(Appendix 13)
A program for causing a computer to function as a download determination device in a synthetic aperture radar system mounted on a mobile platform, comprising:
a function of thinning out synthetic aperture radar (SAR) reception data indicated in the azimuth direction and the range direction for each synthetic aperture time in the azimuth direction to output a predetermined number of reception data;
a function of executing a range compression process on the predetermined number of pieces of received data;
a function of determining whether or not a target is present in a narrow-sized block by comparing a feature amount of a block of a predetermined size into which the range-compressed data is divided in the range direction with a feature amount of a narrow range included in the block of the predetermined size;
a function of determining whether or not to download the SAR reception data based on a result of comparing the total number of the targets in the SAR reception data with a predetermined number;
A program for realizing the above on the computer.
(Appendix 14)
The program described in Appendix 13 further implements in the computer a function of extracting a background region divided into blocks of the specified size and a judgment region divided into blocks of the narrow size from the data after range compression, and setting the number of data in the azimuth direction of the narrow size blocks to a smaller number as the squint angle increases.
(Appendix 15)
The program described in Appendix 13 or 14, further implementing in the computer a function of comparing a background feature consisting of the average intensity and standard deviation of the blocks of the predetermined size with a judgment feature consisting of the average intensity of the blocks of the narrow size, and determining that a target exists in the block of the narrow size when the judgment feature of the block of the narrow size is greater than the background feature of the block of the predetermined size.
(Appendix 16)
The program described in Appendix 13 or 14 further implements in the computer a function of, when it is determined that a target is present in a narrow-sized block, not determining whether or not a target is present for a predetermined number of narrow-sized blocks following the narrow-sized block, depending on the expected size of the target.

The present invention can be applied to SAR systems installed on artificial satellites, aircraft, etc.

10 Mobile platform 20 Target 30 Direction of travel 100, 100a Antenna 101 Transmitter 102 Receiver 103 SAR received data storage 104 DL determination unit 105 Control unit 106 Wireless communication unit 120 Thinning out unit 121 Range compression unit 122 Data extraction unit 123 Determination feature amount calculation unit 124 Background feature amount calculation unit 125 Target presence/absence determination unit 126 Aggregation determination unit 200 Received data 201 Range migration line

Claims (10)

1. A synthetic aperture radar system mounted on a mobile platform, comprising:
a thinning processing unit that thins out synthetic aperture radar (SAR) reception data indicated in the azimuth direction and the range direction for each synthetic aperture time in the azimuth direction to output a predetermined number of reception data;
a range compression unit that executes a range compression process on the predetermined number of pieces of received data;
a target presence/absence determination unit that determines the presence or absence of a target in a narrow-sized block by comparing a feature amount of a predetermined-sized block into which the range-compressed data is divided in the range direction with a feature amount of a narrow-sized block included in the predetermined-sized block;
a counting and determining unit that determines whether or not the SAR reception data can be downloaded based on a comparison result between the total number of the targets in the SAR reception data and a predetermined number;
A synthetic aperture radar system having: the target presence/absence determination unit has a data extraction unit that extracts a background region divided into blocks of the predetermined size and a determination region divided into blocks of the narrow size from the range compressed data,
2. The synthetic aperture radar system according to claim 1, wherein the data extraction unit sets a smaller number of pieces of data in the azimuth direction of the narrow-sized block as the squint angle increases. The synthetic aperture radar system according to claim 1 or 2, wherein the target presence/absence determination unit compares a background feature consisting of the average intensity and standard deviation of the blocks of the predetermined size with a determination feature consisting of the average intensity of the blocks of the narrow size, and determines that a target is present in the blocks of the narrow size when the determination feature of the blocks of the narrow size is greater than the background feature of the blocks of the predetermined size. The synthetic aperture radar system according to claim 1 or 2, wherein when the target presence/absence determination unit determines that a target is present in one narrow-sized block, it does not perform target presence/absence determination for a predetermined number of narrow-sized blocks following the one narrow-sized block according to the expected size of the target. A method for determining download of SAR reception data in a synthetic aperture radar (SAR) system mounted on a mobile platform, comprising:
A data processing unit of the SAR system,
thinning out synthetic aperture radar (SAR) reception data indicated in the azimuth direction and range direction for each synthetic aperture time in the azimuth direction to output a predetermined number of reception data;
performing a range compression process on the predetermined number of pieces of received data;
determining whether or not a target is present in a narrow-sized block by comparing a feature amount of a block of a predetermined size obtained by dividing the data into range directions and a feature amount of a narrow-sized block included in the block of the predetermined size;
determining whether or not to download the SAR reception data based on a comparison result between the total number of the targets in the SAR reception data and a predetermined number;
Download determination method. The download determination method according to claim 5, which extracts a background region divided into blocks of the predetermined size and a determination region divided into blocks of the narrow size from the range-compressed data, and sets the number of data in the azimuth direction of the narrow size blocks to be smaller as the squint angle increases. A satellite having a synthetic aperture radar (SAR), a memory unit for storing SAR received data, a communication unit for downloading data, and a data processing unit,
The data processing unit:
thinning out the SAR reception data indicated in the azimuth direction and the range direction for each synthetic aperture time in the azimuth direction to output a predetermined number of reception data;
performing a range compression process on the predetermined number of pieces of received data;
determining whether or not a target is present in the narrow range by comparing the feature amount of a block of a predetermined size into which the range-compressed data is divided in the range direction with the feature amount of a narrow block included in the block of the predetermined size;
determining whether or not to download the SAR reception data based on a comparison result between the total number of targets in the SAR reception data and a predetermined number;
reading the SAR reception data that is permitted to be downloaded from the storage unit, and downloading the data through the communication unit;
satellite. The satellite according to claim 7, which extracts a background region divided into blocks of the predetermined size and a judgment region divided into blocks of the narrow size from the range-compressed data, and sets the number of data in the azimuth direction of the narrow size blocks to a smaller number as the squint angle increases. A program for causing a computer to function as a download determination device in a synthetic aperture radar system mounted on a mobile platform, comprising:
A function of thinning out synthetic aperture radar (SAR) reception data indicated in the azimuth direction and the range direction for each synthetic aperture time in the azimuth direction to output a predetermined number of reception data;
a function of executing a range compression process on the predetermined number of pieces of received data;
a function of determining whether or not a target is present in a narrow-sized block by comparing a feature amount of a block of a predetermined size into which the range-compressed data is divided in the range direction with a feature amount of a narrow range included in the block of the predetermined size;
a function of determining whether or not to download the SAR reception data based on a result of comparing the total number of the targets in the SAR reception data with a predetermined number;
A program for realizing the above on the computer. The program according to claim 9, further realizing in the computer a function for extracting a background region divided into blocks of the predetermined size and a judgment region divided into blocks of the narrow size from the range-compressed data, and for decreasing the number of data in the azimuth direction of the narrow size blocks as the squint angle increases.

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