JP4749903B2 - Radar image processing device - Google Patents

Description

  This invention reproduces a focused, high-resolution radar image with high-precision phase retention by accurately calculating the distance difference between the reference trajectory and the point on the ground surface and compensating the received signal. The present invention relates to a radar image processing apparatus.

  The most common scanning method for radar observation is a strip mode. This scanning method is a method in which a beam is irradiated from a fixed antenna and is elongated along the movement of the platform. Here, the platform indicates a moving body such as an aircraft equipped with a radar. In the image reproduction process of reproducing a radar image from the received signal acquired by the scanning method, the process is performed assuming that the antenna trajectory is a straight line. Hereinafter, the antenna trajectory used in this image reproduction process is referred to as a reference trajectory. However, in reality, the orbit of the antenna is not a straight line due to the influence of wind at the time of observation. For this reason, a shake compensation process for compensating the received signal according to the difference between the actual trajectory and the reference trajectory is required.

  In a conventional radar image processing apparatus for performing image reproduction processing on the received signal obtained by the scanning method, the difference between the actual trajectory and the reference trajectory is calculated in the direction perpendicular to the reference trajectory, and the difference is calculated. The received signal was compensated based on (see, for example, Non-Patent Document 1).

  This compensation processing will be described using the radar observation geometry shown in FIG. In FIG. 9, an actual trajectory 71 represents an actual antenna trajectory, and a reference trajectory 72 is a linear antenna trajectory used in image reproduction. The swath center 74 indicates the center of the beam observation width (swath width), and the compensation reference point 73 indicates a reference point for calculating the difference between the actual trajectory 71 and the reference trajectory 72.

  In the conventional radar image processing apparatus, for each pulse transmission / reception position, a point in the plane orthogonal to the reference trajectory 72 and on the swath center of the observation region is set as the compensation reference point 73. Then, a difference between the distance of the actual trajectory 71 and the reference trajectory 72 with respect to the reference point 73 is obtained, and based on this distance difference, the received signal obtained at the pulse transmission / reception position in the same plane as the reference point 73 is obtained. Compensate. Also, a method has been proposed in which the reference trajectory 72 is set as a piecewise straight line segment, and the difference between the reference trajectory 72 and the actual trajectory 71 relative to the reference point is calculated to compensate the received signal (for example, Non-Patent Document 2).

W.G.Carrara, R.S.Goodman, R.M.Majewski, Spotlight Synthetic Aperture Radar signal processing algorithms, Artech House, 1995, p.67. S.N.Madsen, H.A.Zebker, J.Martin, “Topographic Mapping Using Radar Interferometry: processing techniques,” IEEE Transactions on Geoscience and remote sensing, vol.31, no.1, January 1993.

  The radar image processing apparatus uses a change in the distance between a point on the ground surface and an antenna that is generated by the movement of the platform, compresses the received signal in the azimuth direction, and obtains an image at that point. Therefore, in the shake compensation process, the difference in distance between the actual trajectory and the reference trajectory with respect to a point on the ground surface must be calculated in consideration of the change in distance according to the movement of the platform. The change in the difference in distance due to the movement of the platform becomes larger as the high-resolution radar having a larger azimuth beam width has a longer moving distance of the platform required for the synthetic aperture. Further, the manner of the change varies depending on the position of a point on the observation target region. The conventional radar image processing apparatus compensates the distance difference in the plane perpendicular to the reference trajectory as a compensation amount without considering the distance change according to the movement of the platform and the position of the scattering point. However, there is a problem that the resolution of the reproduced image is deteriorated. This problem is particularly significant in systems with high resolution and wide observation width in the azimuth direction. Even in the method of setting the reference signal as a piecewise line segment and reducing the difference between the actual trajectory and the reference trajectory to reduce the compensation error, the compensation error is large at the end of the line segment, and Since the error becomes discontinuous at the connection portion, there is a problem that a discontinuous portion is generated in the reproduced image. The compensation error in these conventional methods is a large measurement error when measuring the three-dimensional shape of the ground surface based on the phase difference between the two reproduced images observed with two antennas installed at different positions. There was a problem of causing.

  The present invention has been made to solve the above-described problems, and its purpose is to calculate the distance difference between the reference trajectory and the actual trajectory with respect to a point on the ground surface with high accuracy and compensate the received signal. A radar image processing apparatus capable of reproducing a focused, high-resolution radar image having a highly accurate phase is obtained.

A radar image processing apparatus according to the present invention includes an observation parameter storage unit that stores at least a radar wave wavelength as an observation parameter, a reception data storage unit that stores a reception signal obtained by radar observation, and a platform at the time of radar observation. From the trajectory information storage unit that stores trajectory information and the trajectory information of the platform stored in the trajectory information storage unit, a linear trajectory that minimizes the difference from the actual trajectory is obtained and set as a reference trajectory. The received signal stored in the received data storage unit is divided into a plurality of sections, and the position of the center point in a certain section defined on the received signal is h-th in the azimuth direction and k-th in the range direction. When present, the land from an antenna position where the h-th pulse transmission and reception, in the range distance r [k] in the irradiation direction of the beam Set the point on the surface as a reference point compensation, defining a synthetic aperture centered on the antenna position, in each pulse transmission and reception position during the synthetic aperture, the reference point of the compensation antenna positions on the real track A first distance and a second distance between the antenna position on the reference trajectory and the compensation reference point are obtained, respectively, and a distance change that is a difference between the first and second distances due to the shaking of the platform And a shake compensation unit that compensates the received signal based on the wavelength and distance change, the shake compensation unit calculates a track difference between the reference track and the actual track, and the calculated track difference, A trajectory difference determination unit that compares with a predetermined threshold value, and when the trajectory difference exceeds the threshold value, a piecewise linear shape is used so that the trajectory difference between the reference trajectory and the actual trajectory becomes small. Gauge to set reference trajectory It is intended to include a partitioning portion.

  The radar image processing apparatus according to the present invention calculates the difference in distance between the reference trajectory and the actual trajectory with respect to a point on the ground surface with high accuracy and compensates the received signal, thereby achieving a focused, high resolution and high phase accuracy. There is an effect that the held radar image can be reproduced.

Embodiment 1 FIG.
A radar image processing apparatus according to Embodiment 1 of the present invention will be described with reference to FIGS. 1 is a block diagram showing a configuration of a radar image processing apparatus according to Embodiment 1 of the present invention. In the following, in each figure, the same reference numerals indicate the same or corresponding parts.

  1, the radar image processing apparatus according to the first embodiment includes an observation parameter storage unit 1, a received data storage unit 2, a trajectory information storage unit 3, an image storage unit 4, and a control unit 10. It has been.

  The observation parameter storage unit 1 stores parameters in radar observation, such as radar wave wavelength, frequency, polarization, and range gate set distance. The reception data storage unit 2 stores reception signals obtained by radar observation. The trajectory information storage unit 3 stores the trajectory of the platform at the time of radar observation. The image storage unit 4 stores a radar image obtained through an image reproduction process, which is an output of the present apparatus. The control unit 10 controls the input / output of data from each storage unit and the exchange of data between parts constituting the apparatus described later.

  The control unit 10 includes a fluctuation compensation unit 20, a range compression unit 31, an azimuth compression unit 32, and a partition integration unit 40. In addition, the fluctuation compensation unit 20 includes a reference trajectory setting unit 21, a compensation point setting unit 22, and a compensation processing unit 23. The fluctuation compensation unit 20 calculates the deviation of the trajectory due to the platform fluctuation and compensates the received signal. . The range compressor 31 compresses the signal waveform of the received signal in the range direction. The azimuth compression unit 32 compresses the waveform in the azimuth direction of the received signal. Further, the partition integration unit 40 includes an averaging processing unit 41 and a synthesis unit 42, and integrates the sections that have been reproduced.

  Next, the operation of the radar image processing apparatus according to the first embodiment will be described with reference to the drawings. FIG. 2 is a flowchart showing the operation of the radar image processing apparatus according to Embodiment 1 of the present invention. FIG. 3 is a diagram showing the radar observation geometry of the radar image processing apparatus according to Embodiment 1 of the present invention.

  First, in step 101, the control unit 10 extracts stored data from the observation parameter storage unit 1, the reception data storage unit 2, and the orbit information storage unit 3.

  Next, in step 102, the range compressor 31 performs a range compression process based on these pieces of information. This range compression process is, for example, as shown in Non-Patent Document 3 (Kazuo Ouchi “Basics of Synthetic Aperture Radar for Remote Sensing”, Tokyo Denki University Press, 2004), page 131. This is a technique for compressing a pulse width by performing a matched filtering process on a received signal using a pulse waveform transmitted by adding. Further, in the delamp (meaning to eliminate the tilt) processing method as shown on page 191 of Non-Patent Document 3, for example, the data in the range direction is Fourier-transformed to reflect the reflection from the scatterer. It is a technology that accumulates in a place corresponding to the position of the body.

  Next, in step 103, the control unit 10 inputs various data to the fluctuation compensation unit 20, and the reference trajectory setting unit 21 determines, for example, a straight line that minimizes the difference from the actual trajectory from the trajectory information of the platform. A trajectory is obtained and set as a reference trajectory.

  Next, in step 104, the compensation point setting unit 22 first divides the received signal into several sections. Then, a point on the ground surface corresponding to the center of these sections is obtained, and this point is set as a compensation reference point.

  Here, reference points for compensation will be described with reference to FIG. First, it is assumed that the position of the center point in a certain section (not shown) defined on the received signal is h-th in the azimuth direction and k-th in the range direction in the received signal. In this case, a point on the ground surface at a range distance r [k] in the beam irradiation direction from the position Pac50 of the antenna that has transmitted and received the h-th pulse is set as a compensation reference point P52.

  Next, in step 105, the compensation processing unit 23 first determines a synthetic aperture L51 centered on the antenna position Pac50. At each pulse transmission / reception position in the opening, the distance PPa (h ′) between the antenna position Pa (h ′) 56 on the actual trajectory 54 and the reference point P52, and the antenna position Pi (h ′) on the reference trajectory 55. ) The distance PPi (h ′) between 57 and the reference point P50 is obtained in consideration of the movement of the platform. Then, the distance change Δr due to the platform shake is calculated from the equation (1).

  Next, in step 106, the compensation processing unit 23 generates a signal represented by, for example, Expression (2), and performs shake compensation by multiplying the h′th received signal in the azimuth direction.

  Here, λ in equation (2) is the wavelength of the radar wave, and is stored in the observation parameter storage unit 1. Further, although the shake compensation is performed by multiplying the equation (2), other equations may be used.

  Next, in step 107, the azimuth compression unit 32 performs range migration compensation, range conversion, or the like on the received signal after the shake compensation processing, as shown on pages 176 to 190 of Non-Patent Document 3, for example. Compensation processing such as skew correction is performed, and azimuth compression processing is performed. Further, processing such as secondary range compression is performed in accordance with the received signal.

  Next, in step 108, the control unit 10 inputs the image reproduced through the motion compensation, range compression, and azimuth compression to the segment integration unit 40. Then, the section integration unit 40 cuts out a section centered on the compensation reference point.

  Next, in step 109, the averaging processing unit 41 averages the signals at positions that are overlapped and separated from the compensation points between adjacent sections.

  Next, in step 110, the composition unit 42 integrates the sections into a single image.

  In step 111, the control unit 10 outputs the integrated image to the image storage unit 4 as one reproduced image.

  As described above, the compensation amount of fluctuation varies depending on the position of the reference point on the observation area, and the distance between the antenna and the reference point according to the movement of the platform is considered. Since the compensation amount is calculated and compensated, the compensation error can be reduced and a focused high-resolution reproduced image can be obtained.

  Further, since highly accurate compensation is performed, it is possible to realize an image reproduction process with high phase holding performance without damaging the phase information of the received signal. Further, since the received signal is divided into several sections, the processing time required for image reproduction and the accuracy of compensation can be controlled according to the size of the sections. In addition, since signals at positions away from the compensation points are averaged between adjacent sections, compensation errors at positions away from the compensation points can be suppressed, and discontinuities between the sections can be suppressed. . Further, the averaging process can suppress an error due to noise included in the phase.

  Although the first embodiment has been described on the assumption that the range compression process is performed before the shake compensation, the shake compensation process may be performed before the azimuth compression process. Further, although the first embodiment has been described on the assumption that the range Doppler image reproduction algorithm is used as the image reproduction processing algorithm, polar format, deramp processing, step processing, wk algorithm, chirp scaling algorithm, etc. Other image reproduction algorithms may be used.

Embodiment 2. FIG.
A radar image processing apparatus according to Embodiment 2 of the present invention will be described with reference to FIGS. FIG. 4 is a block diagram showing the configuration of the radar image processing apparatus according to Embodiment 2 of the present invention.

  In the first embodiment, the reference trajectory is set to be a straight line. However, in this second embodiment, the change of the antenna trajectory is large. When the difference becomes larger, a piecewise straight line is set as the reference trajectory.

  In FIG. 4, the same reference numerals as those in FIG. 1 denote the same or corresponding parts. The motion compensation unit 20 includes a reference trajectory setting unit 21, a compensation point setting unit 22, a compensation processing unit 23, a trajectory difference determination unit 24, and a trajectory segmentation unit 25.

  The trajectory difference determination unit 24 obtains the difference between the reference trajectory set by the reference trajectory setting unit 21 and the actual trajectory. The trajectory segmentation unit 25 sets a piecewise linear reference trajectory when the difference between the reference trajectory and the actual trajectory is greater than a predetermined threshold.

  Next, the operation of the radar image processing apparatus according to the second embodiment will be described with reference to the drawings. FIG. 5 is a flowchart showing the operation of the radar image processing apparatus according to the second embodiment of the present invention. FIG. 6 is a diagram for explaining the operation of the radar image processing apparatus according to the second embodiment of the present invention.

  The processing in steps 201 to 203 and steps 207 to 214 is the same as that in steps 101 to 111 in the first embodiment.

  In step 204, the trajectory difference determination unit 24 calculates the difference d between the reference trajectory set by the reference trajectory setting unit 21 and the actual trajectory.

  Next, in step 205, the trajectory difference determination unit 24 compares the calculated trajectory difference d with a predetermined threshold value Δ. If d> Δ, the processing of the trajectory segmentation unit 25 (step Go to 206). On the other hand, in other cases, the process proceeds to the processing of the compensation point setting unit 22 (step 207).

  Next, in step 206, the trajectory segmentation unit 25 sets a piecewise linear reference trajectory when the difference d between the reference trajectory and the actual trajectory exceeds a threshold value Δ. This will be described with reference to FIG. FIG. 6 shows a case where the fluctuation of the airframe is large and a difference between the actual track 61 and the reference track is large when one linear reference track is defined from the actual track 61. In this case, the trajectory segmentation unit 25 sets a piecewise linear reference trajectory 62 as shown in FIG.

  As described above, since the reference trajectory is set piecewise so as to reduce the difference from the actual trajectory, it is possible to reduce the compensation error generated in the shake compensation processing.

  Further, since the averaging process is performed, it is possible to suppress the discontinuity of the post-reproduction image signal at the end of the piecewise orbit, which is a problem when the reference orbit is divided.

Embodiment 3 FIG.
A radar image processing apparatus according to Embodiment 3 of the present invention will be described with reference to FIGS. FIG. 7 is a block diagram showing a configuration of a radar image processing apparatus according to Embodiment 3 of the present invention.

  In each of the above embodiments, the compensation reference point is determined on the assumption that the ground surface is flat. However, in this third embodiment, an elevation map of the ground surface is added as an input, The reference point for compensation is determined based on the map.

  In FIG. 7, the radar image processing apparatus according to the third embodiment is further provided with a reference DEM storage unit 5.

  The reference DEM storage unit 5 stores a three-dimensional model (DEM: Digital Elevation Model) (three-dimensional terrain data) of the observation target region. The three-dimensional model stored here is an elevation map generally provided as a three-dimensional numerical map, a three-dimensional model obtained by an interferometric synthetic aperture radar, or the like.

  Next, the operation of the radar image processing apparatus according to the third embodiment will be described with reference to the drawings. FIG. 8 is a flowchart showing the operation of the radar image processing apparatus according to Embodiment 3 of the present invention.

  The processes of steps 301 to 306 and steps 309 to 315 are the same as steps 201 to 206 and steps 208 to 214 of the second embodiment, respectively.

  In step 307, the control unit 10 reads the reference DEM from the reference DEM storage unit 5 and inputs it to the compensation point setting unit 22.

  Next, in step 308, the compensation point setting unit 22 determines a reference point for compensation on the input DEM.

  Thus, since the reference point for compensation is determined in consideration of the undulations on the ground surface, an accurate distance between the antenna and the reference point can be calculated, and accurate compensation can be performed. As a result, a focused and high-resolution image can be obtained even for terrain with large undulations. In addition, since the error of the shake compensation according to the undulation of the ground surface can be suppressed, it is possible to realize an image reproduction process with high phase holding performance.

It is a block diagram which shows the structure of the radar image processing apparatus which concerns on Embodiment 1 of this invention. It is a flowchart which shows operation | movement of the radar image processing apparatus which concerns on Embodiment 1 of this invention. It is a figure which shows the geometry of the radar observation of the radar image processing apparatus which concerns on Embodiment 1 of this invention. It is a block diagram which shows the structure of the radar image processing apparatus which concerns on Embodiment 2 of this invention. It is a flowchart which shows operation | movement of the radar image processing apparatus which concerns on Embodiment 2 of this invention. It is a figure for demonstrating operation | movement of the radar image processing apparatus which concerns on Embodiment 2 of this invention. It is a block diagram which shows the structure of the radar image processing apparatus which concerns on Embodiment 3 of this invention. It is a flowchart which shows operation | movement of the radar image processing apparatus which concerns on Embodiment 3 of this invention. It is a figure which shows the geometry of the radar observation of the conventional radar image processing apparatus.

Explanation of symbols

  1 observation parameter storage unit, 2 received data storage unit, 3 trajectory information storage unit, 4 image storage unit, 5 reference DEM storage unit, 10 control unit, 20 motion compensation unit, 21 reference trajectory setting unit, 22 compensation point setting Unit, 23 compensation processing unit, 24 orbital difference determination unit, 25 orbit segmentation unit, 31 range compression unit, 32 azimuth compression unit, 40 division integration unit, 41 averaging processing unit, 42 synthesis unit.

Claims (4)

An observation parameter storage unit for storing at least a radar wave wavelength as an observation parameter;
A reception data storage unit for storing a reception signal obtained by radar observation;
A trajectory information storage for storing the trajectory information of the platform during radar observation;
From the trajectory information of the platform stored in the trajectory information storage unit, a linear trajectory that minimizes the difference from the actual trajectory is set as a reference trajectory,
The received signal stored in the received data storage unit is divided into a plurality of sections, and the position of the center point in a certain section defined on the received signal is h-th in the azimuth direction and k-th in the range direction. When, from the antenna position the h-th pulse transmission and reception, and set the point on the ground surface in the range distance r [k] in the irradiation direction of the beam as a reference point compensation,
A synthetic aperture centered on the antenna position is defined, and at each pulse transmission / reception position in the synthetic aperture, a first distance between the antenna position on the actual trajectory and the compensation reference point, and the antenna position on the reference trajectory A second distance from the compensation reference point is obtained;
Calculating a distance change which is a difference between the first and second distances due to the shaking of the platform;
A fluctuation compensator for compensating the received signal based on the wavelength and distance change,
The shake compensation unit
Calculating a trajectory difference between the reference trajectory and the actual trajectory, and comparing the calculated trajectory difference with a predetermined threshold;
A trajectory segmentation unit that sets a piecewise linear reference trajectory so that the trajectory difference between the reference trajectory and the actual trajectory becomes smaller when the trajectory difference exceeds the threshold value. A radar image processing apparatus. Radar image processing apparatus 請 Motomeko 1 wherein you characterized further by comprising an integrated part of the partition to create one image by combining each compartment after image reproduction for each partition. Integration of the compartment, radar image processing apparatus 請 Motomeko 2 wherein you comprising the averaging process unit for averaging the portion overlapping the adjacent compartments. A reference DEM storage unit for storing 3D terrain data;
The radar according to any one of claims 1 to 3, wherein the fluctuation compensation unit sets a reference point for compensation on the three-dimensional terrain data stored in the reference DEM storage unit. Image processing device.

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