WO2025191739A1 - Elevation analysis device and elevation analysis method - Google Patents

Abstract

The present invention comprises: a DEM difference processing unit 14 that calculates the difference between interference SAR data and elevation data produced by a digital elevation model, thereby acquiring differential interference SAR data in which uncorrelated noise and a displacement component resulting from ground surface deformation are reduced; a filter processing unit 15 that performs high-pass filter processing on the differential interference SAR data, thereby acquiring phase difference data in which atmospheric error and satellite orbital stripe error are further reduced from the differential interference SAR data; and an elevation change calculation unit 16 that, from the phase difference data thus acquired, calculates ground surface elevation change data by using known information relating to ground surface observation performed by a synthetic aperture radar. The elevation change data is calculated by using phase difference data in which various kinds of noise are reduced from the interference SAR data, thereby making it possible to accurately measure, by using interference SAR data obtained through a repeat pass observation technique, an elevation difference relating to a phenomenon in which the ground surface, for example, changes greatly in a relatively short period.

Description

Elevation analysis device and elevation analysis method

The present invention relates to an elevation analysis device and an elevation analysis method, and is particularly suitable for use in technology that uses interferometric SAR to measure elevation differences on the Earth's surface.

Elevation monitoring is the process of monitoring changes in the terrain using Geographic Information Systems (GIS) and remote sensing technology, and is used in a wide range of fields, including the following: Elevation monitoring helps to understand terrain changes in real time or collect data periodically, allowing appropriate measures to be taken.
○ River basin flood control (understanding sediment movement and deposition volume, monitoring the status of sabo dams and sabo dams, etc.)
Measurement of the amount and extent of sediment movement during landslides Monitoring of embankments Measurement of topographical deformation during large-scale collapses of dams, slopes, mines, etc.

Technologies for monitoring elevation include terrestrial surveying, airborne laser surveying, differential measurements using digital elevation models (DEMs), and change measurement using interferometric SAR (InSAR) using satellite-based SAR (Synthetic Aperture Radar). Terrestrial surveying and airborne laser surveying are relatively accurate, but have the disadvantages of difficulty in surveying wide areas and limited survey frequency due to their high cost. Measurements using DEM differentials are low-cost and can cover wide areas if using existing DEMs that have already been measured and published, but timely data acquisition is difficult and they are not suitable for real-time measurements.

As interferometric SAR is a satellite-based measurement, it can cover a wide area and has a high degree of accuracy in detecting changes in millimeters or centimeters, but it cannot be used for phenomena such as those mentioned above, where the Earth's surface undergoes major changes. In contrast, a technique is known for calculating elevation using SAR displacement over time (time-series differential interferometry analysis) (see, for example, Non-Patent Document 1). Non-Patent Document 1 discloses a method for analyzing data captured by SAR to calculate the elevation of the target area on a surface-by-surface basis, then analyzing the image data after sediment movement to calculate the surface height, and then calculating the amount of sediment movement by performing a differential analysis of these two surface height data.

However, performing time-series differential interferometry analysis requires a large amount of SAR image data, and it takes a long time to gather all the data. Furthermore, during the period covered by the time-series analysis, there must be no changes in the ground surface due to phenomena other than those mentioned above, making it difficult to accurately calculate sediment movement in areas where changes are likely to occur.

Technology has also been proposed for calculating elevation by combining SAR with DEM or a digital terrain model (DTM: Digital Terrain Model) (see, for example, Patent Documents 1 and 2). Patent Document 1 discloses that for N (N>20) images acquired using SAR that can be used over many years, N-1 differential interferograms are formed in relation to the main image with a vertical accuracy of better than 50 meters. The differential interferograms are created using an existing DEM, subtracting the influence of the terrain from the phase difference of each pixel.

Patent Document 1 also explains that when a satellite is equipped with two radar antennas, one for transmitting and one for receiving, which irradiate the same surface area, and are located at distances ρ and ρ+Δρ from a point on the ground, respectively, the measured value of the phase difference Φq relative to the altitude q of that point on the ground can be extrapolated with an accuracy of a few meters using the following equation (a): where λ is the wavelength of the radar wave, B is the baseline length, which is the distance between two satellites projected perpendicular to the line of sight, and θ is the angle of incidence of the radar wave irradiated from the satellite that is offset from the perpendicular to the ground surface.

Patent document 1 also explains that by creating a temporal series of interferometer phase for each pixel of a differential interferogram selected based on the statistical properties of reflectance, and then calculating the phase linear component a (the slope a of the line in the equation for least squares Φ = aB + C (C is a constant)) for each temporal series relative to the baseline, the elevation difference Δq at a point can be calculated using the following equation (b):

The terrain change extraction device described in Patent Document 2 generates an interference fringe image by processing two sets of SAR reconstructed images acquired from a single satellite, while also generating a DTM from the interference fringe image and two sets of orbital data, and generating a pseudo interference fringe image for the same area using the DTM and two sets of orbital data, assuming no terrain change. The interference fringe image and pseudo interference fringe image generated in this way are then processed to calculate the amount of terrain change, and the amount of geometric distortion is then calculated from the amount of terrain change and DTM, and a geometric transformation is performed.

Special Publication No. 2003-500658 Japanese Unexamined Patent Publication No. 8-15426

2022.11.01 "NEC and others develop technology to monitor elevation using SAR data - managing river sediment movement," Internet search [URL: https://uchubiz.com/article/new9328/]

Technologies for measuring elevation using interferometric SAR include bistatic observation technology, which measures the difference between SAR images taken simultaneously of the same location by two satellites, and repeat-pass observation technology, which measures the difference between SAR images taken twice of the same location by one satellite. The former allows for highly accurate measurements, but requires a dedicated satellite. The latter does not require a dedicated satellite, but because the two observations are not simultaneous, atmospheric errors, surface movement, and reduced coherence can significantly reduce measurement accuracy.

Patent Document 1 discloses that it is possible to calculate the elevation difference Δq as shown in equation (b), but does not disclose how to reduce the effects of atmospheric errors, surface movement, and reduced coherence when using repeat-pass observation technology. As a result, it is difficult to accurately measure elevation difference using the technology described in Patent Document 1.

The present invention was developed to solve these problems, and aims to use interferometric SAR data obtained through repeat-pass observation technology to enable accurate measurement of elevation differences related to phenomena in which the earth's surface undergoes significant changes over a relatively short period of time.

In order to solve the above-mentioned problems, the present invention calculates the difference between interferometric SAR data generated by a specified interferometric processing of data observed by a synthetic aperture radar mounted on a satellite and elevation data obtained from a digital elevation model, thereby obtaining differential interferometric SAR data in which displacement components due to surface movement and uncorrelated noise have been reduced. Then, by performing high-pass filtering on the differential interferometric SAR data, phase difference data is obtained from the differential interferometric SAR data in which atmospheric errors and satellite orbital fringe errors have been further reduced. Then, from the phase difference data obtained in this way, data on changes in the elevation of the earth's surface are calculated using known information regarding earth's surface observations by synthetic aperture radar.

It is possible to calculate topography from interferometric SAR data generated based on observation data from synthetic aperture radar, but when interferometric SAR data is generated using repeat-pass observation technology, various types of noise, such as atmospheric errors, surface movements, satellite orbital fringe errors, and uncorrelated noise, are included in the interferometric SAR data due to changes in conditions between the two observations of the same location. In contrast, according to the present invention configured as described above, phase difference data is generated from the interferometric SAR data in which the various types of noise mentioned above have been reduced, and elevation change data is calculated from this phase difference data using known information. Therefore, interferometric SAR data generated using repeat-pass observation technology can be used to accurately measure elevation differences related to phenomena in which the earth's surface changes significantly over a relatively short period of time.

FIG. 1 is a diagram illustrating an example of a network configuration of an analysis system according to an embodiment of the present invention. 1 is a block diagram showing an example of the functional configuration of an elevation analysis device according to an embodiment of the present invention; FIG. 10 is a diagram schematically illustrating the relationship between the baseline length and altitude changes that can be detected within a dynamic range of the phase difference of 2π or less. FIG. 10 is a diagram showing a partially enlarged view of the sensitivity of CSK. FIG. 1 is a diagram showing a schematic diagram of the baseline length for multiple SAR images obtained when multiple observations are performed using the TSX.

An embodiment of the present invention will now be described with reference to the drawings. Figure 1 is a diagram showing an example of the network configuration of an analysis system 100 according to this embodiment. The analysis system 100 of this embodiment comprises a satellite base station 50 that receives satellite data from a single satellite S flying in a satellite orbit in space, a satellite database 51 that records the satellite data, a DEM database 52 that stores elevation data (e.g., DEM data) based on a digital elevation model, and an elevation analysis device 10 that analyzes the satellite data recorded in the satellite database 51.

Satellite S is equipped with a synthetic aperture radar (SAR) and photographs points on Earth from a pre-specified orbit, transmitting satellite data including the captured ground images (hereinafter referred to as SAR images or SAR data) to satellite base station 50. SAR photographs the same point multiple times at different times, and each time transmits satellite data including the SAR image to satellite base station 50.

Satellite base station 50 receives satellite data from satellite S and records it in satellite database 51. Satellite database 51 is connected to a communications network N such as the Internet. A DEM database 52 is also connected to communications network N. DEM data generated in advance is stored in DEM database 52.

The elevation analysis device 10 acquires satellite data stored in the satellite database 51 and DEM data pre-stored in the DEM database 52 via the communications network N, and uses this acquired data to perform processing related to the calculation of elevation differences (elevation changes) caused by phenomena that cause significant changes in the earth's surface over a relatively short period of time. In this embodiment, the phenomena targeted for elevation difference calculation include, for example, watershed flood control, landslides, embankments, and large-scale collapses, which are phenomena that cause elevation changes of approximately 1 m to several tens of meters over a short period of time.

FIG. 2 is a block diagram showing an example of the functional configuration of an elevation analysis device 10 according to this embodiment. As shown in FIG. 2, the elevation analysis device 10 according to this embodiment has, as its functional configuration, a satellite data acquisition unit 11, an interferometric SAR image generation unit 12, a DEM data acquisition unit 13, a DEM difference processing unit 14, a filter processing unit 15, and an elevation change calculation unit 16.

The above-mentioned functional blocks 11 to 16 execute the processes described below through the cooperation of hardware and software. For example, the processes of the above-mentioned functional blocks 11 to 16 are executed by the operation of a program stored in a storage medium such as RAM, ROM, a hard disk, or semiconductor memory under the control of a microcomputer comprising a CPU, RAM, ROM, etc. In addition to the microcomputer, a DSP (Digital Signal Processor) etc. may also be included.

Note that the physical configuration described here is an example and does not necessarily have to be an independent configuration. For example, the elevation analysis device 10 may be equipped with an LSI (Large-Scale Integration) that integrates a CPU with RAM and ROM. Furthermore, although this embodiment describes a case where the elevation analysis device 10 is configured with a single computer, the elevation analysis device 10 may also be realized by combining multiple computers.

The satellite data acquisition unit 11 acquires satellite data stored in the satellite database 51. In this embodiment, repeat-pass observation technology is used, and the satellite data acquisition unit 11 acquires satellite data relating to two images taken from among the multiple satellite data generated by multiple SAR images for the area of interest for which the elevation difference is to be calculated.

The satellite data acquired by the satellite data acquisition unit 11 includes, in addition to SAR images, known information related to SAR observation of the Earth's surface as metadata. The metadata includes, for example, the wavelength λ of the radar wave, the baseline length _B⊥ , the off-nadir angle θ, the tilt distance r, the satellite position, the satellite speed, the satellite movement direction, the satellite time, and geographic information of the observation area. The baseline length _B⊥ is the distance between the observation points in two observations by one satellite S. The off-nadir angle θ is the angle of incidence of the radar wave emitted from the SAR, which is offset from the perpendicular to the Earth's surface. The tilt distance r is the distance from the antenna of the satellite S to the observation point on the Earth's surface.

The interferometric SAR image generation unit 12 generates an interferometric SAR image (interferometric SAR data) by performing a predetermined interference process that causes interference between the two SAR images acquired by the satellite data acquisition unit 11. Known processes related to interferometric SAR can be used as the predetermined interference process.

Interferometric SAR is a technology that uses the phase of radar waves to obtain images. Two observations are made of the same location on the Earth's surface to generate two SAR images (phase images), which are then interfered with to obtain the difference, making it possible to obtain slight distance differences as phase difference information. The image generated by interfering the two SAR images in this case is an interferometric SAR image.

While typical interferometric SAR can capture the movement of the Earth's surface that occurs between two observation periods as a change in the distance between the satellite and the Earth's surface in millimeters or centimeters, this embodiment does not aim to capture such slight displacements of the Earth's surface. The interferometric SAR image generator 12 only needs to generate an interferometric SAR image by interfering two SAR images, and does not need to calculate the phase difference (φ _disp , described below) corresponding to the slight displacement of the Earth's surface. However, the generated interferometric SAR image can be considered a phase difference image that contains information on the phase difference associated with surface deformation that is different from the phenomenon of the elevation difference calculation target.

The following equation (1) shows multiple elements included in the phase difference component Δφ of the interferometric SAR image generated by the interferometric SAR image generation unit 12. φ _atm is the atmospheric error, φ _orb is the orbital fringe error of satellite S, φ _disp is the displacement component due to surface movement, φ _topo is the DEM differential phase, φ _noise is uncorrelated noise, and 2kπ is the phase integer value bias.

Atmospheric conditions change between the two observations of the SAR images, and the phase delay of the radar wave changes depending on the changed atmospheric conditions. Therefore, even if there is no change on the Earth's surface between the two observations, if there is a change in the atmospheric conditions, a phase difference will occur between the two SAR images. This phase difference is included in the phase difference component Δφ of the interferometric SAR image as the atmospheric error φ _atm .

Furthermore, since the orbit of satellite S is not completely identical when the SAR images are observed twice, even if there is no change on the Earth's surface between the two observation times, a phase difference occurs between the two SAR images due to the difference in orbit. This appears as striped noise in the interferometric SAR image. Note that since the orbit of satellite S is known, it is possible to estimate the orbital fringe error φ _orb based on the orbital information and remove it to some extent. However, complete removal is difficult, and the orbital fringe error φ _orb is included in the phase difference component Δφ of the interferometric SAR image.

As described above, the displacement component φ _disp due to ground surface movement is the desired information in the case of general interferometric SAR, but in this embodiment, it is treated as noise information. In contrast, the desired information in this embodiment is the DEM differential phase φ _topo . The DEM differential phase φ _topo is information indicating the phase difference from the topography shown in the DEM data, as will be described later. Uncorrelated noise φ _noise is noise that occurs randomly.

The phase integer value bias 2kπ can be considered an error related to the so-called phase unwrapping problem. The phase value obtained in interferometric SAR is a fraction of the slope distance r from the antenna of satellite S to the observation point on the Earth's surface divided by the wavelength λ of the radar wave, so the obtained phase value is folded (wrapped) into the range of {-π, π}. As a result, phases with a large dynamic range have an uncertainty value that is an integer multiple of 2π. In other words, it is not possible to know how many 2π cycles k of the radar wave there were, only the position within the last cycle. While it is possible to estimate the value of cycle k using a specified unwrapping process, this is not always an accurate estimate, and this is included as noise in the phase difference component Δφ of the interferometric SAR image.

As shown in equation (1), the phase difference component Δφ of the interferometric SAR image generated by the interferometric SAR image generator 12 contains, in addition to the desired DEM differential phase φ _topo , atmospheric error φ _atm , orbital fringe error φ _orb , displacement component φ _disp due to ground surface movement, uncorrelated noise φ _noise , and phase integer value bias 2kπ. In this embodiment, as described in detail below, processing is performed to remove or reduce these noises, thereby enabling accurate calculation of changes in ground surface elevation from the phase difference component Δφ' in which various noises have been reduced.

The DEM data acquisition unit 13 acquires DEM data that has been stored in advance in the DEM database 52. The DEM data acquired here is elevation data for the area of interest for which the elevation difference is to be calculated, and includes data for the same area of interest contained in the SAR data acquired by the satellite data acquisition unit 11. Note that it is not essential that the area of the SAR data acquired by the satellite data acquisition unit 11 and the area of the DEM data acquired by the DEM data acquisition unit 13 exactly match; it is sufficient that the area of interest is included in both data. Note that both the SAR data and the DEM data are accompanied by geographical information as metadata, and it is possible to identify the area of interest using this geographical information, for example.

The DEM difference processing unit 14 corresponds to the difference processing unit in the claims, and acquires differential interferometric SAR data (hereinafter also referred to as a differential interferometric SAR image) in which the displacement component φ _disp due to ground surface movement and uncorrelated noise φ _noise have been reduced by calculating the difference between the interferometric SAR data generated by the interferometric SAR image generation unit 12 and the DEM data acquired by the DEM data acquisition unit 13. At this time, the DEM difference processing unit 14 geocodes the interferometric SAR data and the DEM data to align them, and then calculates the difference.

DEM data is existing elevation data from a numerical model generated at a point in the past by airborne laser surveying or the like, and is data that represents the terrain by arranging elevation values in a grid pattern, for example. In contrast, interferometric SAR data is data that represents slight displacements of the earth's surface at two observation points in two SAR images obtained from the satellite database 51 as a phase difference. Because the phase difference is information that corresponds to the distance difference, it is possible to obtain differential interferometric SAR data that corresponds to the distance difference from existing elevation data by calculating the difference between the interferometric SAR data and the DEM data.

Here, by shortening the time interval between the two observations in which the two SAR images are taken, it is possible to reduce the possibility of ground deformation occurring during that time. In this case, the differential interferometric SAR data generated by the DEM subtraction processor 14 can be said to be in a state in which the displacement component φ _disp due to ground deformation has been reduced. This differential interferometric SAR data is in a state in which not only the displacement component φ _disp but also the uncorrelated noise φ _noise has been reduced, but other noise remains.

The filter processing unit 15 performs high-pass filtering on the differential interferometric SAR data generated by the DEM differential processing unit 14, thereby obtaining phase difference data from the differential interferometric SAR data in which the atmospheric error φ _atm and the orbital fringe error φ _orb of the satellite S have been further reduced. In particular, the filter processing unit 15 of this embodiment performs low-pass filtering on the differential interferometric SAR data, and then subtracts the data obtained thereby from the original differential interferometric SAR data before the low-pass filtering.

That is, the filter processing unit 15 first performs low-pass filtering on the differential interferometric SAR image to extract smoothly varying components from the differential interferometric SAR image. The extracted smoothly varying components are then removed from the original differential interferometric SAR image. By applying a high-pass filter in this manner, it is possible to significantly reduce the atmospheric error φ _atm and the orbital fringe error φ _orb , which have components in the low-frequency region.

Since the atmospheric error φ _atm and the orbital fringe error φ _orb occur on the order of several hundred meters to several kilometers, it is preferable to perform filtering with a window size that matches their size. This window size is larger than the region of interest for which the elevation difference is to be calculated, so it does not significantly affect the phase information of the region of interest to be calculated.

The altitude change calculation unit 16 uses known information regarding the SAR observation of the Earth's surface to calculate altitude change data of the Earth's surface from the phase difference data generated by the filter processing unit 15. The known information is information included as metadata in the satellite data acquired by the satellite data acquisition unit 11, and includes the wavelength λ of the radar wave, the baseline length B _⊥ , the off-nadir angle θ, and the tilt distance r. Using this known information, the altitude change calculation unit 16 calculates the altitude change dh using the following equation (2):

By performing the processing up to the filter processing unit 15, it is possible to significantly reduce the atmospheric error φ _atm , the orbital fringe error φ _orb , the displacement component φ _disp due to ground movement, and the uncorrelated noise φ _noise contained in the phase difference component Δφ of the interferometric SAR image in the above-mentioned equation (1). As for the remaining phase integer value bias 2kπ, by performing SAR observation under conditions such that the phase difference data generated by the filter processing unit 15 is data within a range that does not require phase unwrapping, it is possible to avoid the phase unwrapping process, which is difficult to estimate, and to ignore the phase integer value bias 2kπ. This makes it possible to use the above-mentioned equation (2).

For this reason, in this embodiment, the sensitivity of the elevation change dh, which depends on the baseline length _B⊥ as shown in equation (2), is theoretically calculated for each satellite S, and observation conditions are found that make it unnecessary to perform phase unwrapping when detecting the elevation change dh of the magnitude related to the phenomenon of the elevation difference calculation target. The sensitivity of the elevation change dh here means that it is possible to detect the elevation change dh of the magnitude related to the phenomenon of the elevation difference calculation target within a range in which the dynamic range of the phase difference is 2π or less (a range not exceeding {-π, π}).

Figure 3 is a diagram showing the relationship between the baseline length _B⊥ and the altitude change dh that can be detected within a dynamic range of the phase difference of 2π or less. In Figure 3, the horizontal axis represents the baseline length _B⊥ and the vertical axis represents the altitude change dh. The figure shows the ranges for each type of satellite in which the altitude change dh can be detected within a dynamic range of the phase difference of 2π or less. Range 31 indicates the sensitivity when the satellite S is Sentinel-1 (C-band, wavelength λ = 5.6 cm), range 32 indicates the sensitivity when the satellite S is ALOS-2 (L-band, wavelength λ = 24 cm), range 33 indicates the sensitivity when the satellite S is TSX (X-band, wavelength λ = 3.1 cm), and range 34 indicates the sensitivity when the satellite S is CSK (X-band, wavelength λ = 3.1 cm).

In Figure 3, the minimum value _dhmin and maximum value _dhmax of the altitude change dh that can be detected within a dynamic range of the phase difference of 2π or less were calculated according to the following equation (3) derived from equation (2). The minimum value _dhmin of the altitude change dh shown in equation (3) is based on the case where coherence is low. The minimum value _dhmin when coherence is high is calculated using equation (4).

Based on the sensitivity calculation results shown in Figure 3, SAR observation is performed under the condition of a combination of satellite S and baseline length _B⊥ that has sensitivity to the amount of elevation change related to the phenomenon of the elevation difference calculation target within a range in which the dynamic range of the phase difference is 2π or less. Then, the DEM difference processing unit 14 generates differential interferometric SAR data using interferometric SAR data generated from data observed under such conditions.

In Figure 3, the sensitivity of Sentinel-1, shown in range 31, indicates that it cannot detect an elevation change dh of approximately 12 m or less in a range where the baseline length _B⊥ is 100 m or less (the sensitivity when the baseline length _B⊥ is 100 m is _dhmin = approximately 12 m, _dhmax = approximately 57 m). For example, when the elevation change amount related to the phenomenon of the elevation difference calculation target is on the order of several meters, Sentinel-1 is not suitable (does not satisfy the conditions) because it does not have the sensitivity to this elevation change dh of approximately several meters. Sentinel-1 cannot be said to be preferable also from the viewpoint that the range of usable baseline length _B⊥ is narrow.

The sensitivity of ALOS-2 shown in range 32 indicates that it cannot detect an elevation change dh of approximately 10 m or less in a range where the baseline length _B⊥ is 500 m or less (the sensitivity when the baseline length _B⊥ is 500 m is _dhmin = approximately 10 m, _dhmax = approximately 44 m). For example, if the elevation change related to the phenomenon of the elevation difference calculation target is on the order of a few meters, ALOS-2 would also be unsuitable because it would not be sensitive to this elevation change dh of approximately a few meters.

The sensitivity of TSX shown in range 33 indicates that it is unable to detect an elevation change dh of approximately 2 m or less in the range where the baseline length _B⊥ is 300 m or less (the sensitivity when the baseline length _B⊥ is 300 m is _dhmin = approximately 2 m, _dhmax = approximately 8 m). For example, when the elevation change amount related to the phenomenon of the elevation difference calculation target is on the order of several meters (but 2 m or more), TSX is deemed appropriate (meets the conditions) because it has sensitivity to elevation change dh of approximately several meters.

The sensitivity of CSK shown in range 34 indicates that it is unable to detect an elevation change dh of approximately 2 m or less in the range where the baseline length _B⊥ is 600 m or more and up to a predetermined value (1000 m not shown in the figure) (when the baseline length _B⊥ is 600 m, the sensitivity is _dhmin = approximately 2 m, _dhmax = approximately 5 m). For example, if the elevation change amount related to the phenomenon of the elevation difference calculation target is on the order of several meters (but 2 m or more), CSK is also appropriate because it has sensitivity to elevation changes dh of approximately several meters.

From the above, it can be seen that when the amount of elevation change related to the phenomenon of the elevation difference calculation target is on the order of a few meters, it is appropriate to perform observations using the TSX or CSK X-band satellite S. In other words, if SAR observations are performed using the X-band satellite S so as to fall within the range of the baseline length _B⊥ shown in Figure 3, it is possible to detect the elevation change dh of the elevation difference calculation target within a dynamic range of the phase difference of 2π or less, and it becomes possible to ignore the phase integer value bias 2kπ.

Figure 4 is a diagram showing a partially enlarged view of the sensitivity of the CSK shown in Figure 3. When the elevation change amount related to the phenomenon of the elevation difference calculation target is 2 m or more and 8 m or less, the sensitivity of the elevation change dh is too high when the baseline length _B⊥ is 100 _m or less, while the sensitivity of the elevation change dh is somewhat insufficient when the baseline length _B⊥ is 300 m or more. Therefore, when using CSK to calculate the above-mentioned elevation change amount as the elevation difference calculation target, it is preferable to perform SAR observation so that the baseline length B⊥ is in the range of 100 m or more and 300 m or less. A similar situation can be said when using TSX.

It goes without saying that if the amount of change in altitude of the altitude difference calculation target changes, the suitable combination of satellite S and baseline length _B⊥ will change accordingly.

One of the requirements for satisfying the condition that the dynamic range of the phase difference is sensitive to the amount of elevation change of the elevation difference calculation target within 2π or less is to use two SAR images with an appropriate baseline length _B⊥ . In other words, it is important to obtain satellite data related to two appropriate observations from the satellite data related to multiple observations stored in the satellite database 51.

Figure 5 is a diagram showing the baseline length _B⊥ for multiple SAR images obtained when multiple observations are made using the TSX. In Figure 5, the horizontal axis is the time axis, the vertical axis is the separation distance from the reference orbit to the actual observation position, and each circle indicates the separation distance at each observation point. The difference in separation distance between two circles indicates the baseline length _B⊥ for the two observations.

In this embodiment, the satellite data acquisition unit 11 selects and acquires one pair of satellite data whose baseline length _B⊥ satisfies the above-mentioned condition from among the multiple satellite data corresponding to the multiple circles shown in Figure 5. With this as the first condition, as described above, in this embodiment, one pair of satellite data is acquired that also satisfies the second condition relating to the short time interval between two observations. If the time interval between two observations is short, there is a low possibility that ground movement has occurred during that time, so it is possible to prevent or minimize the inclusion of the ground movement displacement component _φdisp in the phase difference component Δφ of the interferometric SAR image.

For example, if there are multiple combinations of satellite data that satisfy the first condition, it is possible to acquire one pair of satellite data with the shortest time interval from among them. Alternatively, if there are multiple combinations of satellite data that satisfy the first condition, it is possible to acquire one arbitrary pair of satellite data from among those pairs whose time interval is less than the threshold value.

As explained in detail above, in this embodiment, the difference between the interferometric SAR data and elevation data from the digital elevation model is calculated to obtain differential interferometric SAR data in which the displacement component φ _disp due to ground movement and the uncorrelated noise φ _noise have been reduced, and then high-pass filtering is performed on the differential interferometric SAR data to obtain phase difference data Δφ' (=φ _topo ) from the differential interferometric SAR data in which the atmospheric error φ _atm and the orbital fringe error φ _orb of the satellite S have been further reduced. Then, from the phase difference data Δφ' obtained in this way, the elevation change dh of the ground surface is calculated using equation (2) using known information regarding ground surface observation by SAR.

In this embodiment, which is configured in this manner, phase difference data Δφ' is generated from the interferometric SAR data, with the various types of noise described above reduced, and altitude change dh is calculated from this phase difference data Δφ' using known information.As a result, interferometric SAR data obtained using repeat-pass observation technology can be used to accurately measure altitude change dh related to phenomena in which the earth's surface changes significantly, for example in units of meters, over a relatively short period of time.

Furthermore, in this embodiment, differential interferometric SAR data is acquired using satellite data observed under conditions of a baseline length _B⊥ that is sensitive to the amount of elevation change of the elevation difference calculation target within a dynamic range of the phase difference of 2π or less, which eliminates the need for phase unwrapping, which generally involves estimation errors, and makes it possible to essentially ignore the phase integer value bias 2kπ, thereby improving the detection accuracy of the elevation change dh.

In the above embodiment, in some cases, SAR observation may be performed under conditions including a phase integer value bias of 2kπ, and the altitude change dh may be calculated from equation (2) after phase unwrapping. For example, if there is no combination of satellite S and baseline length _B⊥ that completely covers the altitude change amount that is the altitude difference calculation target within a range of 2π, SAR observation may be performed using a combination of satellite S and baseline length _B⊥ that covers the altitude change amount that is the altitude difference calculation target within a range of 4π, and the altitude change dh may be calculated from equation (2) after phase unwrapping. Even in this case, the range estimated by phase unwrapping can be narrower, thereby reducing the occurrence of estimation errors compared to when normal phase unwrapping is performed.

Furthermore, in the above embodiment, an adaptive filter processing unit may be provided between the filter processing unit 15 and the altitude change calculation unit 16, and may further perform processing to extract or remove specific frequency components, for example. This makes it possible to further reduce uncorrelated noise φ _noise that may remain even after processing by the interferometric SAR image generation unit 12. For example, if the SAR image includes a water area such as an ocean or lake, interference in the SAR image is unlikely to occur in that area, and therefore the noise appears as a grainy texture. This can be reduced by adaptive filter processing.

Furthermore, in the above embodiment, a Gaussian filter may be applied to the image representing the amount of elevation change downstream of the elevation change calculation unit 16 to remove fine noise.

Furthermore, in the above-described embodiment, it is also possible to calculate the elevation change dh ₁ using two SAR images and DEM data observed at a certain point in time, and to calculate the elevation change dh 2 using two SAR images and DEM data observed at a different point in time, and then calculate the difference _{dh 2-1 =} dh ₂ - dh ₁ between the two elevation changes dh ₁ and dh _2. For example, it is also possible to calculate the elevation change dh ₁ using two SAR images and DEM data observed before the occurrence of a phenomenon that is the target of the elevation difference calculation, and to calculate the elevation change dh ₂ using two SAR images and DEM data observed after the occurrence of the phenomenon, and then calculate the difference dh _2-1 therebetween.

By doing this, it is possible to measure the elevation change dh _2-1 between any two different points in time. Another advantage is that it is possible to offset any errors in the elevation values contained in the DEM data itself. Even when measuring elevation change dh _2-1 in this way, only four SAR images (two pairs) are required, making it possible to calculate elevation change dh _2-1 with far fewer SAR images than when performing time-series differential interferometry analysis as in Non-Patent Document 1.

Furthermore, the above-described embodiments are merely examples of specific ways of implementing the present invention, and should not be interpreted as limiting the technical scope of the present invention. In other words, the present invention can be implemented in various forms without departing from its gist or main features.

10 Elevation analysis device 11 Satellite data acquisition unit 12 Interferometric SAR image generation unit 13 DEM data acquisition unit 14 DEM difference processing unit (difference processing unit)
15 Filter processing unit 16 Elevation change calculation unit 51 Satellite database 52 DEM database S Satellite

Claims (7)

a differential processing unit that calculates the difference between interferometric SAR data generated by a predetermined interferometric processing of data observed by a synthetic aperture radar mounted on a satellite and elevation data obtained by a digital elevation model, thereby obtaining differential interferometric SAR data in which displacement components due to ground surface deformation and uncorrelated noise have been reduced;
a filter processing unit that performs high-pass filtering on the differential interferometric SAR data to obtain phase difference data from the differential interferometric SAR data in which atmospheric errors and orbital fringe errors of the satellite have been further reduced;
and an altitude change calculation unit that calculates altitude change data of the earth's surface from the phase difference data using known information regarding the earth's surface observation by the synthetic aperture radar. The elevation analysis device described in claim 1, characterized in that the filter processing unit performs low-pass filtering on the differential interferometric SAR data and subtracts the data obtained thereby from the original differential interferometric SAR data before the low-pass filtering. The known information includes a baseline length, which is the distance between observation points in two observations by one satellite;
The elevation analysis device according to claim 1, characterized in that the differential processing unit acquires the differential interferometric SAR data using data observed under conditions of a baseline length that is sensitive to the amount of elevation change of the elevation difference calculation target within a range in which the dynamic range of the phase difference indicated by the phase difference data is 2π or less. The known information includes a baseline length, which is the distance between observation points in two observations by one satellite;
2. The elevation analysis device according to claim 1, wherein the differential processing unit acquires the differential interferometric SAR data using two observation data that satisfy a first condition that the baseline length is sensitive to the amount of elevation change of the elevation difference calculation target within a range in which the dynamic range of the phase difference indicated by the phase difference data is 2π or less, and a second condition that the time interval between the two observations is short. using the interferometric SAR data generated based on data observed at a certain point in time by the synthetic aperture radar and the altitude data, by performing processing by the difference processing unit, the filter processing unit, and the altitude change calculation unit to calculate first altitude change data;
calculating second altitude change data by performing processing by the difference processing unit, the filter processing unit, and the altitude change calculation unit using the interferometric SAR data generated based on data observed by the synthetic aperture radar at a time different from the certain time point;
5. The altitude analysis device according to claim 1, wherein the altitude change calculation unit further calculates a difference between the first altitude change data and the second altitude change data. a first step in which a differential processing unit of a computer calculates the difference between interferometric SAR data generated by a predetermined interferometric processing of data observed by a synthetic aperture radar mounted on a satellite and elevation data obtained by a digital elevation model, thereby obtaining differential interferometric SAR data in which displacement components due to ground surface deformation and uncorrelated noise have been reduced;
a second step in which a filter processing unit of the computer performs high-pass filtering on the differential interferometric SAR data to obtain phase difference data from the differential interferometric SAR data in which atmospheric errors and orbital fringe errors of the satellite have been further reduced;
and a third step in which the altitude change calculation unit of the computer calculates altitude change data of the earth's surface from the phase difference data using known information regarding ground surface observation by the synthetic aperture radar. The known information includes a baseline length, which is the distance between observation points in two observations by one satellite;
The elevation analysis method according to claim 6, characterized in that the differential processing unit acquires the differential interferometric SAR data using data observed under conditions of a baseline length that is sensitive to the amount of elevation change of the elevation difference calculation target within a range in which the dynamic range of the phase difference indicated by the phase difference data is 2π or less.