JP6324108B2 - Synthetic aperture radar equipment - Google Patents

Description

  The present invention relates to a synthetic aperture radar apparatus that detects a moving target.

As a detection method of a moving target in a synthetic aperture radar apparatus, for example, a 2-pulse scancella, a 3-pulsescanner, and pulse Doppler processing disclosed in Non-Patent Document 1 below are known.
However, these detection methods have a problem that a low-speed moving target cannot be detected when the Doppler bandwidth of the clutter is wide.

As a method for enabling detection of a moving target even when the Doppler bandwidth of the clutter is wide, two or more antenna openings are arranged in the moving direction of the platform (hereinafter referred to as “along track direction”), and each antenna is arranged. A method of detecting a moving target by combining a signal received at an aperture and suppressing a stationary target signal is known.
As a method of arranging two antenna openings in the along track direction and using signals received by the two antenna openings, for example, a DPCA (Displaced Phase Center Antenna) method disclosed in Non-Patent Document 1 is known.
The DPCA method uses that signals received at two antenna apertures correspond to signals observed from the same position at different times, and the difference between signals received at the two antenna apertures. Clutter is suppressed by calculating.

As a system using three or more antenna apertures arranged in the along track direction and using signals received by the three or more antenna apertures, for example, a STAP (Space Time Adaptive Processing) system disclosed in Non-Patent Document 2 below. It has been known.
The STAP method is a method for detecting a moving target by applying a spatio-temporal filter to signals received at three or more antenna apertures.

In the DPCA system and the STAP system, as described above, it is necessary to arrange a plurality of antenna openings in the along track direction in advance.
On the other hand, in a radar apparatus mounted on an aircraft, the opening generally faces forward in the traveling direction. Further, the aperture of the radar apparatus is divided into left and right to have two reception channels in order to realize the monopulse system disclosed in Non-Patent Document 1 below.
Therefore, if it is possible to suppress clutter using two antenna apertures that are perpendicular to the direction of travel and aligned in the horizontal direction, it will be installed in an aircraft simply by updating the signal processing method. It is possible to use the existing radar device, and the detection performance of the moving target can be improved.

Non-Patent Document 3 below discloses a method in which the same idea as the above STAP method is applied using a plurality of antenna openings that are perpendicular to the traveling direction and aligned in the horizontal direction.
However, in this method, if the pulse integration time is lengthened, the signal of the movement target may straddle the range cell due to the influence of movement, and thus the signal may not be properly integrated.
For this reason, the method disclosed in Non-Patent Document 3 has a limited integration time of applicable pulses, and it is difficult to detect a moving target with a low signal-to-noise ratio (SNR). is there.
The method disclosed in Non-Patent Document 3 is effective when three or more antenna openings are mainly used.

Here, the principle of the DPCA method using two antenna openings will be described.
FIG. 6 is a conceptual diagram for explaining the principle of the DPCA method. In FIG. 6, the horizontal axis represents time, and the vertical axis represents the traveling direction of the platform.
Two antennas 1a and 1b are arranged in the traveling direction of the platform. FIG. 6 shows the positions of the antennas 1a and 1b at time η and the positions of the antennas 1a and 1b at time (η + η ₁₂ ). .
In the example of FIG. 6, the distance between the two antennas 1a and 1b is d [m], and the platform speed is V _r [m / s].
Therefore, in the example of FIG. 6, the positions of the two antennas 1a and 1b move along a straight line having a slope _Vr .

In this specification, “antenna position” and “antenna aperture position” are defined by the phase center position of transmission and reception. That is, in the case of a system in which the transmitting antenna is shared and only two receiving antennas are used, d is half the distance between the phase centers of the receiving antennas.
From FIG. 6, it can be seen that the time at which the antenna 1b arrives at the position of the antenna 1a at time η is η ₁₂ = d / V _r [sec] after the time η. That is, the signals observed by the two antennas 1a and 1b correspond to those observed from the same position with a time difference η ₁₂ = d / V _r .
Therefore, when the pulse repetition period (PRI) coincides with the time difference η ₁₂ (this relationship is established as a “DPCA condition”), the signal observed by the antenna 1a at the time η and the antenna 1b The signal observed at time (η + η ₁₂ ) corresponds to the signal observed at different time from the same place.
Therefore, in the signal observed by the antenna 1a and the signal observed by the antenna 1b, the stationary clutter signal coincides, but the phase of the moving target signal changes according to the distance change accompanying the movement.

Non-Patent Document 4 below discloses a method (SAR-DPCA method) in which the DPCA method is combined with observation by a synthetic aperture radar (SAR).
In this SAR-DPCA method, SAR images are reproduced from a signal observed by the antenna 1a and a signal observed by the antenna 1b.
In the two SAR images, the stationary clutter signals match, but the phase of the moving target signal changes during the time difference η ₁₂ , so the clutter is suppressed by the differential processing of the two SAR images, and the moving target is I try to detect it.

The “observation time” in the synthetic aperture radar is given by the time at which the antenna is located at the center of the synthetic aperture used for reproducing the image of the azimuth position.
Since the integration time can be extended by combining the SAR-DPCA method with synthetic aperture radar observation, it is possible to detect a moving target with a smaller signal-to-noise power ratio compared to the DPCA method described above. become.
However, similar to the above DPCA method, it is necessary to arrange a plurality of antenna openings side by side in the along track direction.

M.M. A. Richards, J.A. A. Scheer, W.M. A. Holm, “Principles Of Modern Radar, Basic Principles,” SciTech Publishing Inc. , 2010 J. et al. H. Ender, “Space-time processing for multichannel synthetic aperture radar,“ Electronics & Communication Engineering Journal, pp. 197 29-38, Feb. 1999 J. et al. T.A. Caldwell, T.W. D. Hale, “Space-Time Adaptive Processing for forward locking arrays,” Proceedings of the IEEE Radar Conference, 2004. , Pp. 514-519 C. Gierull, “Moving Target Detection with Along-Track SAR Interferometry,” Technical Report DRDC-OTTAWA-TR-2002-084, Defense Research & Development 200

  Since the conventional synthetic aperture radar apparatus is configured as described above, a method that applies the same concept as the STAP method using a plurality of antenna openings that are perpendicular to the traveling direction and aligned in the horizontal direction. If it is used, it is possible to use the existing radar apparatus mounted on the aircraft. However, in this method, if the pulse integration time is lengthened, the signal of the movement target may straddle the range cell due to the influence of movement, and thus the signal may not be properly integrated. For this reason, there is a problem that it is difficult to detect a moving target having a low signal-to-noise power ratio because the applicable pulse integration time is limited.

  The present invention has been made in order to solve the above-described problems, and has a low signal-to-noise power ratio even when using a plurality of antenna openings that are perpendicular to the traveling direction and aligned in the horizontal direction. An object of the present invention is to obtain a synthetic aperture radar device capable of detecting a moving target.

The synthetic aperture radar apparatus according to the present invention includes a transmitting unit that radiates a pulse signal to space, a plurality of antennas arranged in a direction orthogonal to the traveling direction of the platform, and radiated from the transmitting unit and then scattered by an observation target. When the scattered wave of the pulse signal is incident on a plurality of antennas, receiving means for detecting the received signals of the scattered waves incident on the plurality of antennas, measuring means for measuring the position and velocity of the platform, and measurement A radar image reproducing means for reproducing a synthetic aperture radar image from a plurality of received signals detected by the receiving means using the position and velocity measured by the means, and a radar image using the speed measured by the measuring means. line a plurality of synthetic aperture radar image reproduced by the reproducing means, without moving in the azimuth direction, the alignment of the range direction And a clutter suppression unit that suppresses clutter included in the plurality of synthetic aperture radar images that have been aligned by the alignment unit. The moving target detection unit suppresses the clutter by the clutter suppression unit. The moving target is detected from the synthetic aperture radar image.

According to the present invention, the alignment unit uses the speed measured by the measurement unit to align the plurality of synthetic aperture radar images reproduced by the radar image reproduction unit in the range direction without moving in the azimuth direction. The effect of being able to detect a moving target with a low signal-to-noise power ratio in the case of using a plurality of antenna openings that are perpendicular to the traveling direction and aligned in the horizontal direction. There is.

It is a block diagram which shows the synthetic aperture radar apparatus by Embodiment 1 of this invention. It is explanatory drawing which shows the geometry of the observation in the synthetic aperture radar apparatus by Embodiment 1 of this invention. It is explanatory drawing which shows the geometry of the observation in the synthetic aperture radar apparatus by Embodiment 1 of this invention. It is a conceptual diagram which shows two SAR images s ₁ (τ, η), s ₂ (τ, η). It is a block diagram which shows the synthetic aperture radar apparatus by Embodiment 2 of this invention. It is a conceptual diagram for demonstrating the principle of a DPCA system.

Embodiment 1 FIG.
1 is a block diagram showing a synthetic aperture radar apparatus according to Embodiment 1 of the present invention.
In FIG. 1, antennas 1a and 1b are arranged at intervals of d in a direction orthogonal to the traveling direction of the platform. The antenna 1a is used as a transmission / reception antenna and the antenna 1b is used as a reception antenna.
The transmitter 2 generates _a pulse signal and repeatedly outputs the pulse signal with _a repetition period Δη (repetition frequency F _a ). The transmitter 2 constitutes a transmission means.
The transmission / reception switch 3 outputs the pulse signal output from the transmitter 2 to the antenna 1a to radiate the pulse signal to the space, while the scattered wave of the pulse signal scattered by the observation target enters the antenna 1a. Then, the scattered wave is output to the receiver 4a.

When the receiver 4a radiates from the antenna 1a and then the scattered wave of the pulse signal scattered by the observation object enters the antenna 1a, the receiver 4a detects the received signal of the scattered wave incident on the antenna 1a and receives the received signal. By performing phase detection processing and A / D conversion processing on the signal, a digital reception signal indicating the amplitude and phase of the reception signal is output.
When the receiver 4b radiates from the antenna 1a and then the scattered wave of the pulse signal scattered by the observation object enters the antenna 1b, the receiver 4b detects the received signal of the scattered wave incident on the antenna 1b and receives the received signal. By performing phase detection processing and A / D conversion processing on the signal, a digital reception signal indicating the amplitude and phase of the reception signal is output. The receivers 4a and 4b constitute receiving means.
Although FIG. 1 shows an example in which the number of receivers 4 and the number of antennas 1 are the same, the number of receivers 4 may be smaller than the number of antennas 1. In that case, a single receiver may be connected to a plurality of antennas via a switch, and the signal may be received through a plurality of openings by switching the switch every time a pulse signal is transmitted and received.

The platform motion measurement unit 5 is composed of, for example, a position measuring device or a speedometer mounted on a platform such as an aircraft, and performs processing for measuring the position and speed of the platform. The platform motion measurement unit 5 constitutes a measurement unit.
The radar signal processing unit 6 is composed of, for example, a semiconductor integrated circuit on which a CPU is mounted, a one-chip microcomputer, or the like, and uses the platform position and speed measured by the platform motion measurement unit 5 to receive the receiver 4a. , 4b, a process of detecting a moving target from the digital received signal output is performed.

The SAR image reproduction unit 7a uses the platform position and velocity measured by the platform motion measurement unit 5 to perform a process of reproducing an SAR image (synthetic aperture radar image) from the digital reception signal output from the receiver 4a. .
The SAR image reproduction unit 7b performs a process of reproducing the SAR image from the digital reception signal output from the receiver 4b using the platform position and velocity measured by the platform motion measurement unit 5. The SAR image reproducing units 7a and 7b constitute radar image reproducing means.

  The range direction image registration unit 8 uses the platform speed measured by the platform motion measurement unit 5 to align the two SAR images reproduced by the SAR image reproduction units 7a and 7b in the range direction. The range direction image registration unit 8 constitutes an alignment unit.

The phase compensation unit 9 performs processing for compensating for the phase difference between the two SAR images that have been aligned by the range direction image registration unit 8.
The amplitude compensation unit 10 performs a process of compensating for the amplitude difference between the two SAR images that have been aligned by the range direction image registration unit 8.
The phase compensation unit 9 and the amplitude compensation unit 10 constitute compensation means.

The clutter suppression unit 11 performs a process of suppressing clutter (still target signal component) included in the two SAR images after the compensation processing by the phase compensation unit 9 and the amplitude compensation unit 10. Note that the clutter suppression unit 11 constitutes a clutter suppression unit.
The moving target detection / speed estimation unit 12 detects the moving target from the SAR image in which the clutter suppression unit 11 suppresses the clutter, and from the phase difference between the two SAR images after the compensation processing by the amplitude compensation unit 10, A process for estimating the velocity in the radar line-of-sight direction is performed. The movement target detection / speed estimation unit 12 constitutes movement target detection means.

In the example of FIG. 1, the antennas 1a and 1b, the transmitter 2, the transmission / reception switch 3, the receivers 4a and 4b, the platform motion measurement unit 5 and the radar signal processing unit 6 that are components of the synthetic aperture radar apparatus are dedicated. However, a part of the synthetic aperture radar device (for example, the radar signal processing unit 6) may be configured by a computer.
When a part of the synthetic aperture radar apparatus (for example, the radar signal processing unit 6) is configured by a computer, the SAR image reproducing units 7a and 7b, the range direction image registration unit 8, the phase compensation unit 9, the amplitude compensation unit 10, and the clutter. A program describing the processing contents of the suppression unit 11 and the moving target detection / speed estimation unit 12 may be stored in a memory of a computer, and the CPU of the computer may execute the program stored in the memory. .

Next, the operation will be described.
First, the observation geometry and signal model by the synthetic aperture radar apparatus of FIG. 1 will be described.
2 and 3 are explanatory views showing the geometry of observation in the synthetic aperture radar apparatus according to Embodiment 1 of the present invention.
In FIG. 2, the observation geometry is represented in two dimensions, and in FIG. 3, the observation geometry is represented in three dimensions.

The synthetic aperture radar apparatus of FIG. 1 uses two antennas 1a and 1b arranged side by side toward the front.
Further, in order to detect a moving target having a low signal-to-noise power ratio, as in the SAR-DPCA method disclosed in Non-Patent Document 4 above, a synthetic aperture radar is used for signals observed by two antennas 1a and 1b. The SAR image is generated by applying the above process, and the clutter is suppressed by the difference process between the two SAR images.
Since the antennas 1a and 1b are arranged side by side toward the front, the “observation time difference” between the two SAR images is the same as in the case of the SAR-DPCA method in which the antennas 1a and 1b are arranged in the traveling direction. Is not obvious.
In this specification, for simplification of description, the description will be made in the order of formulation in a two-dimensional problem using FIG. 2 and then expanding to three dimensions using FIG.

First, an observation signal from the synthetic aperture radar apparatus will be described.
Here, it is assumed that the two antennas 1a and 1b and the target to be observed exist in a two-dimensional plane.
In the example of FIG. 2, the opening of the antenna 1a exists at the position of the coordinates (0, 0) on the xy plane, and the opening of the antenna 1b exists at the position of the coordinates (0, d) on the xy plane. The position of the antenna aperture is defined by the phase center position of transmission / reception.
The two antennas 1a and 1b are assumed to be mounted on the same platform, but the platform may be a moving body such as an aircraft, an artificial satellite, or a vehicle. However, the moving body is not limited.
The two antennas 1a and 1b do not need to be mounted on the same platform, and may be mounted on different platforms. In this case, the platforms move side by side in parallel trajectories. It shall be.

It is assumed that the antenna 1a and the antenna 1b are arranged at intervals of d in the y-axis direction, and the antennas 1a and 1b are moving at a constant velocity V _r in the x-axis direction.
Further, it is assumed that the beam is directed in the direction of the squint angle θ _sq in the xy plane, and when reproducing the SAR image, the Doppler frequency f _ηc = 2f ₀ V _r sin / θ corresponding to the squint angle θ _sq is obtained. _It is assumed that _sq c (f ₀ is the center frequency of the transmission signal) is reproduced with the Doppler center.
The SAR image reproduction process may use, for example, a well-known technique disclosed in Non-Patent Document 5 below, but the ω-K algorithm may be suitable for SAR image reproduction in the squint direction. Are known.
The SAR image reproduced from the signal observed by the antenna 1a is s ₁ (τ, η), and the SAR image reproduced from the signal observed by the antenna 1b is s ₂ (τ, η).
However, τ represents the position in the range direction in units of time, and η represents the position in the azimuth direction in units of time.
[Non-Patent Document 5]
I. G. Cumming, F.M. H. Wong, “Digital Processing of Symptom Aperture Radar Data,” Artech House 2005

FIG. 4 is a conceptual diagram showing two SAR images s ₁ (τ, η) and s ₂ (τ, η).
Further, a spatial spectrum signal obtained by two-dimensional Fourier transform of the SAR image s ₁ (τ, η) is converted into S ₁ (f _τ , f _η ), and the SAR image s ₂ (τ, η) is converted into a two-dimensional Fourier transform. Let the spatial spectrum signal obtained be S ₂ (f _τ , f _η ).
Here, f _τ is a range frequency, and the range of the range frequency f _τ is −F _r / 2 ≦ f _τ ≦ F _r / 2 where the sampling frequency of the baseband signal is F _r .
Further, f _η is a Doppler frequency, and the range of the Doppler frequency f _η is −F _a / 2 ≦ f _η −f _ηc ≦ F _a / 2, where PRF (Pulse Repetition Frequency) is F _a .

Here, consider a stationary point target whose coordinates on the xy plane are at the position (x ₀ , R ₀ ).
In a synthetic aperture radar apparatus, a signal of a point target is generally designed so that the antenna position appears at the time closest to the point target regardless of the squint angle θ _sq (for example, Non-Patent Document 5). See).
As shown in FIG. 4, the SAR image s ₁ (τ, η) reproduced by observing the reflected signal from this point with the antenna 1a is (τ, η) = (2R ₀ / c, η _0). ) And the SAR image s ₂ (τ, η) reproduced by being observed by the antenna 1b is a position of (τ, η) = (2 (R ₀ −d) / c, η ₀ ). Appears in
As for the range position, the antenna position appears at the distance at the time closest to the point target.

In the example of FIG. 2, since the positions of the antenna 1a and the antenna 1b coincide with each other in the azimuth direction (x-axis direction), the time when both antennas are closest to the stationary point target is η ₀ = x ₀ / V Same for _r .
On the other hand, in the range direction (y-axis direction), the distance from each antenna 1a, 1b to the stationary point target is different due to the influence of the distance d between the two antennas 1a, 1b. In the SAR image of the antenna 1b, it appears at a position close by 2d / c.

ここで、２つの合成開口の中心と静止点目標をそれぞれ結ぶ線分は、図２に示すように、同一の直線に一致する位置関係となる。
このため、アンテナ１ａとアンテナ１ｂによる静止点目標の観測時刻差η₁₂は、下記の式（２）で与えられる。

By the way, in the SAR images s ₁ (τ, η) and s ₂ (τ, η) reproduced from the signals observed by the antennas 1a and 1b, the synthetic aperture set to obtain the image of the signal at the stationary point target. The coordinates x _c1 and x _c2 of the center position in the along track direction are given by the following equation (1).

Here, as shown in FIG. 2, the line segments connecting the centers of the two synthetic apertures and the stationary point target have a positional relationship that coincides with the same straight line.
Therefore, the observation time difference η ₁₂ of the stationary point target between the antenna 1a and the antenna 1b is given by the following equation (2).

ただし、ｖ_losは移動目標における視線方向の速度である。 Therefore, the speed moves at v bar (in the text of the description, “−” sign cannot be added on the letter because of electronic application, so it is written as “v bar”) For the movement target signal, a phase difference φ _tgt expressed by the following equation (3) is generated between the two SAR images s ₁ (τ, η) and s ₂ (τ, η).

However, v _los is the speed of the line of sight in the moving target.

Therefore, from the phase difference φ _tgt generated between the two SAR images s ₁ (τ, η) and s ₂ (τ, η), the velocity v _los in the line-of-sight direction at the moving target is determined according to the following equation (4). Can be estimated.

Next, it is formulated in a three-dimensional space using FIG.
The two antennas 1a and 1b are arranged in the y-axis direction, and the distance between them is d. In addition, it is assumed that the antennas 1a and 1b exist at a height of altitude h from the ground surface and are moving at a constant velocity _Vr in the x-axis direction.
First, the position p ₁ (η) bar of the antenna 1a at the time η (in the text of the description, because of the electronic application, a “−” symbol cannot be added on the character, so “p ₁ (η ) Bar ”), a virtual base line vector d (η ₁₂ ) bar directed to the position p ₂ (η + η ₁₂ ) bar of the antenna 1b after η ₁₂ seconds is expressed by the following equation (5). Define as follows.

In addition, the line-of-sight direction r _los hat (in the text of the description, “^” symbol cannot be added on the letter because of the electronic application, so it is expressed as “r _los hat”) Is defined as the following equation (6).

In Equation (6), θ _sq is a squint angle in the slant plane, which is an angle formed by the radar line-of-sight direction and the cross track direction. θ _n is an angle formed by the cross track direction and the vertically downward direction.
Here, the slant surface is a surface including the trajectory and the target of the platform, and the cross track direction is a direction perpendicular to the trajectory of the platform and included in the slant surface.
It should be noted that when the squint angle θ _sq in the slant plane is large, θ _n is significantly different from the value of the incident angle θ _{ink at} the beam center.
In the following formulation, it is assumed that θ _n is used rather than the incident angle θ _ink , so that θ _n is exclusively used below because θ _n can be developed with better visibility.

When extended to a three-dimensional problem, the difference from the two-dimensional case is that the baseline vector d (η ₁₂ ) bar and the line-of-sight direction r _los hat are generally values of η ₁₂ except when θ _n = 90 deg. Regardless, it does not match.
For 2-dimensional, eta ₁₂ when viewing direction r _los hat and baseline vector d (η ₁₂₎ bars to match, between two SAR images _{s 1 (τ, η),} s 2 (τ, η) However, in the three-dimensional problem, there is no matching condition, so an alternative condition is necessary.
In this case, when the component d _⊥ (η ₁₂ ) bar orthogonal to the line-of-sight direction r _los hat in the baseline vector d (η ₁₂ ) bar is minimized, the two SAR images s ₁ (τ, η), s ₂ ( Since τ, η) are considered to be the most consistent, it is reasonable to regard η ₁₂ that minimizes the orthogonal component d _⊥ (η ₁₂ ) bar as an observation time difference.

このことは、基線ベクトルｄ（η₁₂）バーにおける視線方向ｒ_losハットからのずれ分が、アロングトラック方向の成分を持たないことを意味している。よって、アロングトラック方向には、同じ位置から観測していることになるため、上記のＤＰＣＡ方式と同様な処理が成立し、このときの２つのＳＡＲ画像ｓ₁（τ，η），ｓ₂（τ，η）の観測時刻差は、式（１０）で与えられるη₁₂に相当する。
また、式（１１）は、基線ベクトルｄ（η₁₂）バーと視線方向ｒ_losハットが最も良く一致している場合でも、２次元の場合と違って、θ_n＝９０ｄｅｇの場合を除いて、完全には一致しないことを示している。
基線ベクトルｄ（η₁₂）バーのｙ軸成分及びｚ軸成分は、クラッタ抑圧性能の劣化要因であるが、小さい値に抑えることが可能である。

This means that the deviation from the line-of-sight direction r _los hat in the baseline vector d (η ₁₂ ) bar has no component in the along track direction. Therefore, since the observation is performed from the same position in the along track direction, the same processing as the DPCA method is established, and the two SAR images s ₁ (τ, η) and s ₂ ( The observation time difference of τ, η) corresponds to η ₁₂ given by equation (10).
Further, even when the baseline vector d (η ₁₂ ) bar and the line-of-sight direction r _los hat are the best match, the equation (11) is different from the two-dimensional case except for the case of θ _n = 90 deg. It indicates that they do not match completely.
The y-axis component and the z-axis component of the baseline vector d (η ₁₂ ) bar are factors that degrade the clutter suppression performance, but can be suppressed to a small value.

厳密には、この距離ｄ_||（η₁₂）バーはレンジに依存する値であり、地形の影響も受ける値である。
地形が既知であれば、レンジ毎に視線方向ｒ_losハットを求めることができるので、距離ｄ_||（η₁₂）バーもレンジ毎に計算することが可能である。
なお、地形が未知である場合は、例えば、特許文献１（特開２０１０−１７５３３０号公報）に開示されている方式によって、２画像の間の干渉画像から地形に起因する位相差を推定し、その位相差によって位相補償を行う等の対策を講じる必要がある。 Note that the distance d _|| (η ₁₂ ) bar of the line-of-sight direction component of the baseline vector d (η ₁₂ ) bar is expressed as the following equation (12). Before DPCA processing, distance compensation and phase compensation corresponding to this distance d _|| (η ₁₂ ) bar are necessary.

Strictly speaking, this distance d _|| (η ₁₂ ) bar is a value depending on the range, and is also a value affected by the terrain.
If the terrain is known, the line-of-sight direction r _los hat can be obtained for each range, and the distance d _|| (η ₁₂ ) bar can also be calculated for each range.
When the topography is unknown, for example, the phase difference caused by the topography is estimated from the interference image between the two images by the method disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 2010-175330). It is necessary to take measures such as phase compensation based on the phase difference.

ただし、ｖ_losは移動目標における視線方向の速度である。 Since the observation time difference η ₁₂ between the two SAR images s ₁ (τ, η) and s ₂ (τ, η) is given by Equation (10), the signal of the moving target moving at the speed v bar is A phase difference φ _tgt as shown in the following equation (13) is generated between the two SAR images s ₁ (τ, η) and s ₂ (τ, η).

However, v _los is the speed of the line of sight in the moving target.

Therefore, from the phase difference φ _tgt generated between the two SAR images s ₁ (τ, η) and s ₂ (τ, η), the velocity v _los in the line-of-sight direction at the moving target is determined according to the following equation (14). Can be estimated.

Next, the processing contents of the synthetic aperture radar apparatus will be specifically described.
The transmitter 2 generates a pulse signal and outputs the pulse signal to the repetitive transmission / reception switch 3 with a repetitive period Δη (repetitive frequency F _a ).
Each time the transmission / reception switch 3 receives a pulse signal from the transmitter 2, the transmission / reception switch 3 outputs the pulse signal to the antenna 1a to radiate the pulse signal to the space.
The pulse signal radiated from the antenna 1a into the space is scattered by a moving target or the like to be observed, and part of the scattered wave of the pulse signal is incident on the antennas 1a and 1b.

When a scattered wave of a pulse signal is incident from the antenna 1a, the receiver 4a detects the received signal of the scattered wave, and performs phase detection processing and A / D conversion processing on the received signal, thereby A digital reception signal indicating the amplitude and phase of the reception signal is output to the SAR image reproduction unit 7a.
When the scattered wave of the pulse signal is incident from the antenna 1b, the receiver 4b detects the received signal of the scattered wave and performs phase detection processing and A / D conversion processing on the received signal. A digital reception signal indicating the amplitude and phase of the reception signal is output to the SAR image reproduction unit 7b.

  The platform motion measuring unit 5 is composed of, for example, a position measuring device or a speedometer mounted on the platform, measures the platform position and velocity, and outputs the measurement results to the SAR image reproducing units 7a and 7b and Output to the range direction image registration unit 8.

When the SAR image reproduction unit 7a receives the digital reception signal from the receiver 4a, the SAR image s ₁ (τ, η) is obtained from the digital reception signal by using the platform position and velocity measured by the platform motion measurement unit 5. Play.
When the SAR image reproduction unit 7b receives the digital reception signal from the receiver 4b, the SAR image s ₂ (τ, η) is obtained from the digital reception signal by using the platform position and velocity measured by the platform motion measurement unit 5. Play.
Since the SAR image playback processing by the SAR image playback units 7a and 7b is a known technique, a detailed description thereof will be omitted. The SAR image reproduction process is disclosed in Non-Patent Document 5, for example.

When the SAR image reproducing units 7a and 7b reproduce the SAR images s ₁ (τ, η) and s ₂ (τ, η), the range direction image registration unit 8 measures the platform speed measured by the platform motion measuring unit 5. Are used to align the two SAR images s ₁ (τ, η) and s ₂ (τ, η) in the range direction.
That is, the range direction image registration unit 8 uses the distance d _|| (shown in the above equation (12) as the shift amount between the two SAR images s ₁ (τ, η) and s ₂ (τ, η). η ₁₂ ) Bar = dsin θ _n / cos θ _sq is calculated.
Then, the range direction image registration unit 8 moves the SAR image s ₂ (τ, η) reproduced by the SAR image reproduction unit 7 b in the range direction by a distance d _|| (η ₁₂ ) bar which is the shift amount. By doing so, the two SAR images s ₁ (τ, η) and s ₂ (τ, η) are aligned in the range direction.
However, with the movement of the SAR image s ₂ (τ, η), it is also necessary to perform a phase adjustment process for the movement.
In this phase adjustment process, the SAR image s ₂ (τ, η) reproduced by the SAR image reproducing unit 7b is two-dimensionally Fourier transformed to obtain a spatial spectrum signal S ₂ (f _τ , f _η ), and the spatial spectrum thereof. This can be realized by multiplying the signal S ₂ (f _τ , f _η ) by a phase function as shown in the following equation (15).

Here, the _two SAR images are aligned by moving the SAR image s ₂ (τ, η) reproduced by the SAR image reproducing unit 7b in the range direction, and the SAR image s in the azimuth direction. ₂ (τ, η) is not moved, but an observation time difference η ₁₂ as shown in the above equation (2) is generated between the antenna 1a and the antenna 1b.
The range direction image registration unit 8 performs inverse two-dimensional Fourier transform on the spatial spectrum signal S _{2, reg} (f _τ , f _η ) after the phase function multiplication, and the SAR image s _{2, reg} ( τ, η) is output to the phase compensation unit 9. Further, the SAR image s ₁ (τ, η) reproduced by the SAR image reproduction unit 7 a is output to the phase compensation unit 9.

The SAR image s ₁ (τ, η) output from the range direction image registration unit 8 and the SAR image s _{2, reg} (τ, η) after moving in the range direction are different in the gains of the receivers 4a and 4b. As a result, a bias error is added to the amplitude and phase.
Therefore, the phase compensation unit 9 and the amplitude compensation unit 10 determine the phase and the phase from the ratio between the SAR image s ₁ (τ, η) and the SAR image s _{2, reg} (τ, η) after moving in the range direction. An amplitude bias error is estimated to compensate for the bias error.
When the average value of the ratio of the SAR image s ₁ (τ, η) to the SAR image s _{2, reg} (τ, η) after moving in the range direction is ε ₁₂ , the average value ε _{12 of the} ratio is expressed by the following formula ( 16).

なお、式（１７）に示す位相及び振幅の補償処理は、受信チャネル間の位相差と振幅差を補償する処理である。したがって、比率の平均値ε₁₂を予め校正係数として計測して保存することができる場合には、式（１６）によって、観測毎に比率の平均値ε₁₂を推定する必要はない。
振幅補償部１０は、ＳＡＲ画像再生部７ａにより再生されたＳＡＲ画像ｓ₁（τ，η）と、位相及び振幅の補償処理後のＳＡＲ画像ｓ_2,cal（τ，η）をクラッタ抑圧部１１に出力する。 For the SAR image s _{2, reg} (τ, η) after moving in the range direction, the phase compensation unit 9 compensates a component corresponding to the phase of the average value ε ₁₂ of the ratio, and the amplitude compensation unit 10 corresponds to the amplitude. Compensate for components that
The phase and amplitude compensation processing by the phase compensation unit 9 and the amplitude compensation unit 10 is expressed by the following equation (17).

Note that the phase and amplitude compensation processing shown in Expression (17) is processing for compensating for the phase difference and amplitude difference between the reception channels. Therefore, when the average value ε _{12 of the} ratio can be measured and stored in advance as a calibration coefficient, it is not necessary to estimate the average value ε _{12 of the} ratio for each observation according to the equation (16).
The amplitude compensation unit 10 clarifies the SAR image s ₁ (τ, η) reproduced by the SAR image reproduction unit 7 a and the SAR image s _{2, cal} (τ, η) after the phase and amplitude compensation processing, as a clutter suppression unit 11. Output to.

When the clutter suppression unit 11 receives the SAR image s ₁ (τ, η) and the SAR image s _{2, cal} (τ, η) after the phase and amplitude compensation processing from the amplitude compensation unit 10, the following expression (18 as shown in), the difference s _out (tau their SAR image, eta) by calculating the suppresses clutter contained in their SAR image, a difference s _out of the two SAR images (tau, η) is output to the moving target detection / speed estimation unit 12 as a SAR image after clutter suppression.

When the movement target detection / speed estimation unit 12 receives the SAR image after clutter suppression from the clutter suppression unit 11, the movement target detection / speed estimation unit 12 detects the movement target from the SAR image after clutter suppression.
That is, the moving target detection / speed estimation unit 12 detects the target signal by detecting a signal whose intensity is higher than a preset detection threshold for the SAR image after clutter suppression. For the target detection process, any method widely used in radar signal processing may be used. For example, CFAR (Constant False Alarm Rate) disclosed in Non-Patent Document 1 may be used.
Finally, the moving target detection / speed estimation unit 12 outputs the SAR image s ₁ (τ, η) output from the amplitude compensation unit 10 and the SAR image s _{2, cal} (τ, η) after the phase and amplitude compensation processing. ) from the phase difference phi _tgt with estimates the radar line-of-sight direction of the velocity v _los in moving target according to the above equation (14).

As is apparent from the above, according to the first embodiment, the range direction image registration unit 8 uses the platform speed measured by the platform motion measurement unit 5 to reproduce by the SAR image reproduction units 7a and 7b. Since the SAR images s ₁ (τ, η) and s ₂ (τ, η) are aligned in the range direction, a plurality of the SAR images s ₁ (τ, η) and s ₂ (τ, η) are arranged at right angles to the traveling direction and in the horizontal direction. Even when the antenna aperture is used, it is possible to detect a moving target having a low signal-to-noise power ratio.
That is, synthetic aperture processing is performed on a signal observed using two antenna apertures arranged in a direction orthogonal to the traveling direction, two SAR images are observed, and the two SAR images are set in the range direction. By registering the two SAR images, adjusting the phase and amplitude, and then calculating the difference between the two SAR images to suppress the clutter, the Doppler bandwidth of the clutter The detection performance and speed estimation performance of a low-speed moving target with a low signal-to-noise power ratio when the signal is wide are improved.

Embodiment 2. FIG.
5 is a block diagram showing a synthetic aperture radar apparatus according to Embodiment 2 of the present invention. In the figure, the same reference numerals as those in FIG.
The autofocus processing unit 21 estimates the position and speed of the platform from the digital reception signals output from the receivers 4a and 4b, and uses the estimation results to generate two SAR images reproduced by the SAR image reproduction units 7a and 7b. Perform autofocus processing to adjust the focus. The autofocus processing unit 21 constitutes autofocus means.

The range / azimuth direction image registration unit 22 identifies the peak positions of the correlation functions in the two SAR images after the autofocus processing by the autofocus processing unit 21, and uses the peak positions of the correlation functions to determine the two SAR images. Perform alignment. The range / azimuth direction image registration unit 22 constitutes an alignment unit.
The terrain-considered phase compensator 23 estimates the phase difference generated by the influence of the terrain between the two SAR images that have been aligned by the range / azimuth direction image registration unit 22, and the phase difference Compensation processing to compensate for The topography-considered phase compensation unit 23 constitutes compensation means.

In the example of FIG. 5, the antennas 1a and 1b, the transmitter 2, the transmission / reception switch 3, the receivers 4a and 4b, the platform motion measurement unit 5 and the radar signal processing unit 6 which are constituent elements of the synthetic aperture radar apparatus are dedicated. However, a part of the synthetic aperture radar device (for example, the radar signal processing unit 6) may be configured by a computer.
When a part of the synthetic aperture radar apparatus (for example, the radar signal processing unit 6) is configured by a computer, the SAR image reproducing units 7a and 7b, the autofocus processing unit 21, the range / azimuth direction image registration unit 22, and the terrain-considering type A program describing the processing contents of the phase compensation unit 23, the amplitude compensation unit 10, the clutter suppression unit 11, and the moving target detection / speed estimation unit 12 is stored in a computer memory, and the CPU of the computer is stored in the memory. The program that is running should be executed.

Next, the operation will be described.
The platform motion measurement unit 5 measures the position and speed of the platform, and the measurement result generally includes a certain error.
The SAR image reproduction units 7a and 7b use the measurement results of the platform motion measurement unit 5 including a certain error, and use the SAR images s ₁ (τ, η) from the digital reception signals output from the receivers 4a and 4b. ), S ₂ (τ, η) is reproduced, and there are not a few phenomena that the SAR images s ₁ (τ, η), s ₂ (τ, η) are blurred.

Therefore, the autofocus processing unit 21 estimates the platform position and speed from the digital reception signals output from the receivers 4a and 4b, and uses the estimation results to reproduce the 2 reproduced by the SAR image reproduction units 7a and 7b. An autofocus process for automatically focusing the two SAR images is performed. Since the SAR image is formed by focusing the SAR image, an image with reduced blur can be obtained.
Note that the autofocus process is a known technique and will not be described in detail, but the autofocus process is disclosed in Non-Patent Document 6 below, for example.
[Non-Patent Document 6]
Kazuo Ouchi “Synthetic Aperture Radar for Remote Sensing” Tokyo Denki University Press

When the two SAR images s ₁ (τ, η) and s ₂ (τ, η) after the autofocus processing by the autofocus processing unit 21 are received, the range / azimuth direction image registration unit 22 receives the following equation (19 ), A cross-correlation function C (Δτ, Δη) between the SAR image s ₁ (τ, η) and the SAR image s ₂ (τ, η) is obtained.

When the range / azimuth direction image registration unit 22 obtains the cross-correlation function C (Δτ, Δη) of the SAR image s ₁ (τ, η) and the SAR image s ₂ (τ, η), the cross-correlation function C ( The peak position of (Δτ, Δη) is specified.
The peak position of the cross-correlation function C (Δτ, Δη) is given by a set of (Δτ, Δη) that gives the maximum absolute value of the cross-correlation function C (Δτ, Δη) as shown in the following equation (20). It is done.

When the range / azimuth direction image registration unit 22 identifies the peak position of the cross-correlation function C (Δτ, Δη), the peak position of the cross-correlation function C (Δτ, Δη) is used to determine the SAR image s ₁ (τ , Η) with respect to the SAR image s ₂ (τ, η) in the range direction τ 1 and the SAR image s ₂ (τ, η) with respect to the SAR image s ₂ (τ, η) in the azimuth direction. calculate.
The set of (Δτ, Δη) calculated by the equation (20) corresponds to the shift amount τ1 in the range direction and the shift amount η1 in the azimuth direction.

The range / azimuth direction image registration unit 22 calculates the shift amount τ1 in the range direction and the shift amount η1 in the azimuth direction, and moves the SAR image s ₂ (τ, η) by the shift amount τ1 in the range direction. The SAR image s ₂ (τ, η) is moved in the azimuth direction by the shift amount η ₁ to align the two SAR images s ₁ (τ, η), s ₂ (τ, η).
The range / azimuth direction image registration unit 22 includes the SAR image s ₁ (τ, η) after the autofocus processing by the autofocus processing unit 21 and the SAR image s _{2, reg} (τ) after moving in the range direction and the azimuth direction. , Η) is output to the terrain-considered phase compensator 23.

Although ignored in the first embodiment, the base line connecting the antenna openings has a component in the cross-track direction, so that a phase difference due to the topography is generated between the SAR images after registration. .
Therefore, the terrain-considered phase compensation unit 23 receives the SAR image s ₁ (τ, η) from the range / azimuth direction image registration unit 22 and the SAR image s _{2, reg} (τ, τ) after moving in the range direction and the azimuth direction. η), the phase difference generated due to the topography is estimated between the SAR image s ₁ (τ, η) and the SAR image s _{2, reg} (τ, η), and Compensation processing is performed to compensate for the phase difference.
Since the compensation process for compensating for the phase difference is a known technique, a detailed description thereof will be omitted, but the compensation process for compensating for the phase difference is disclosed in, for example, Patent Document 1 described above.

As is apparent from the above, according to the second embodiment, the autofocus processing unit 21 estimates the platform position and velocity from the digital reception signals output from the receivers 4a and 4b, and uses the estimation results. Since the autofocus process for automatically focusing the two SAR images reproduced by the SAR image reproducing units 7a and 7b is performed, the platform motion measuring unit 5 including a certain error is included. By using the measurement result, there is an effect that blur of the SAR images s ₁ (τ, η) and s ₂ (τ, η) in which blur is generated can be suppressed.

Further, according to the second embodiment, the range / azimuth direction image registration unit 22 uses the two SAR images s ₁ (τ, η), s ₂ (τ, τ) after the autofocus processing by the autofocus processing unit 21. Since the peak position of the correlation function in (η) is specified and the two SAR images are aligned using the peak position of the correlation function, it is included in the measurement result of the platform motion measurement unit 5. There is an effect that the disappearance of the clutter due to the error can be reduced.

Furthermore, according to the second embodiment, the terrain-considered phase compensation unit 23 is generated between the SAR image s ₁ (τ, η) and the SAR image s _{2, reg} (τ, η) due to the influence of the terrain. Since the phase difference is estimated and the phase difference is compensated, an effect of reducing the influence of the phase difference caused by the topography is obtained.

  The synthetic aperture radar apparatus of the second embodiment is equipped with the autofocus processing unit 21, but the synthetic aperture radar apparatus of the first embodiment (the synthetic aperture radar apparatus of FIG. 1) is the SAR image reproducing unit 7a. , 7b and the range direction image registration unit 8 may be mounted with an autofocus processing unit 21.

  Further, the synthetic aperture radar apparatus of the second embodiment is equipped with the terrain-considered phase compensation unit 23, but the synthetic aperture radar apparatus of the first embodiment (the synthetic aperture radar apparatus of FIG. 1) Instead of mounting the compensator 9, a terrain-considered phase compensator 23 may be mounted.

  In the present invention, within the scope of the invention, any combination of the embodiments, or any modification of any component in each embodiment, or omission of any component in each embodiment is possible. .

  1a, 1b Antenna, 2 Transmitter (transmission means), 3 Transmission / reception switch, 4a, 4b Receiver (reception means), 5 Platform motion measurement section (measurement means), 6 Radar signal processing section, 7a, 7b SAR image reproduction Part (radar image reproduction means), 8 range direction image registration part (positioning means), 9 phase compensation part (compensation means), 10 amplitude compensation part (compensation means), 11 clutter suppression part (clutter suppression means), 12 Moving target detection / speed estimation unit (moving target detection unit), 21 Autofocus processing unit (autofocus unit), 22 Range / azimuth direction image registration unit (positioning unit), 23 Topography-considered phase compensation unit (compensation unit) ).

Claims (6)

Transmitting means for emitting a pulse signal to space;
A plurality of antennas arranged in a direction perpendicular to the direction of travel of the platform;
Receiving means for detecting received signals of the scattered waves incident on the plurality of antennas when the scattered waves of the pulse signals scattered by the observation target after being radiated from the transmitting means are incident on the plurality of antennas. When,
Measuring means for measuring the position and velocity of the platform;
Radar image reproduction means for reproducing a synthetic aperture radar image from a plurality of received signals detected by the reception means using the position and velocity measured by the measurement means;
A positioning unit that performs positioning in the range direction without moving the plurality of synthetic aperture radar images reproduced by the radar image reproducing unit in the azimuth direction using the speed measured by the measuring unit;
Clutter suppression means for suppressing clutter contained in a plurality of synthetic aperture radar images aligned by the alignment means;
A synthetic aperture radar apparatus comprising: a moving target detection unit that detects a moving target from a synthetic aperture radar image in which clutter is suppressed by the clutter suppression unit. Transmitting means for emitting a pulse signal to space;
A plurality of antennas arranged in a direction perpendicular to the direction of travel of the platform;
Receiving means for detecting received signals of the scattered waves incident on the plurality of antennas when the scattered waves of the pulse signals scattered by the observation target after being radiated from the transmitting means are incident on the plurality of antennas. When,
Measuring means for measuring the position and velocity of the platform;
Radar image reproduction means for reproducing a synthetic aperture radar image from a plurality of received signals detected by the reception means using the position and velocity measured by the measurement means;
Positioning means for identifying peak positions of correlation functions in a plurality of synthetic aperture radar images reproduced by the radar image reproducing means and aligning the plurality of synthetic aperture radar images using the peak positions of the correlation functions When,
Clutter suppression means for suppressing clutter contained in a plurality of synthetic aperture radar images aligned by the alignment means;
A synthetic aperture radar apparatus comprising: a moving target detection unit that detects a moving target from a synthetic aperture radar image in which clutter is suppressed by the clutter suppression unit.   The moving target detection means detects a moving target from a synthetic aperture radar image in which clutter is suppressed by the clutter suppression means, and from a plurality of synthetic aperture radar images that have been aligned by the alignment means. The synthetic aperture radar apparatus according to claim 1, wherein a speed in a radar line-of-sight direction at the moving target is estimated.   Autofocus for estimating the position and velocity of the platform from a plurality of received signals detected by the receiving means, and using the estimation results to focus a plurality of synthetic aperture radar images reproduced by the radar image reproducing means 4. The apparatus according to claim 1, further comprising an autofocus unit that performs processing and outputs a plurality of synthetic aperture radar images after the autofocus process to the positioning unit. 5. Synthetic aperture radar device.   Compensation processing is performed to compensate for phase differences and amplitude differences of the plurality of synthetic aperture radar images that have been aligned by the alignment unit, and the plurality of synthetic aperture radar images after compensation processing are output to the clutter suppression unit. The synthetic aperture radar apparatus according to any one of claims 1 to 4, further comprising compensation means.   Compensation processing is performed by estimating a phase difference generated by the influence of topography between a plurality of synthetic aperture radar images that have been aligned by the positioning means, and compensating for the phase difference. 5. The synthetic aperture radar apparatus according to claim 1, further comprising a compensation unit that outputs a plurality of later synthetic aperture radar images to the clutter suppression unit. 6.

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