JP5017786B2 - Radar equipment - Google Patents

Description

  The present invention relates to a radar apparatus that is mounted on a mobile platform such as an aircraft or a satellite and obtains a high-resolution image of the ground surface, the sea surface, etc. by performing synthetic aperture processing of received signals.

  A conventional synthetic aperture radar synthesizes a plurality of reception antenna beams by synthesizing the reception signals of a plurality of reception antenna beams in a radar device mounted on a platform, and then reproducing the image using a so-called range Doppler algorithm. By processing, the signal band is expanded and the separation performance between the stationary object and the low-speed moving target is improved (for example, see Patent Document 1).

JP-A-6-118166 (FIG. 1, paragraphs 10 to 14)

  The range Doppler algorithm achieves high resolution in the range direction by pulse compression means, and separates the target in the azimuth direction by azimuth compression means taking into account the temporal change of the Doppler frequency of the static target, thereby generating a high resolution image of the static target. Obtainable. However, in the range Doppler algorithm, when the synthetic aperture angle is large and the range sampling interval is small, the Doppler spreads in the azimuth direction by the azimuth FFT before azimuth compression, and an error occurs in the range movement compensation. Therefore, there is a limitation on the synthetic opening time.

  On the other hand, a Polar Format algorithm is known as a method for reproducing an image with relatively few restrictions on the synthetic aperture time (see, for example, Patent Document 2). In the Polar Format algorithm, the output signal after range movement compensation is first placed on polar coordinates. Further, the signal on the grid point of the orthogonal coordinate is interpolated from the signal on the polar coordinate by signal interpolation. As a result, the range movement compensation can be performed more accurately than the range Doppler algorithm by performing IFFT on signals on lattice points that are equally spaced on the orthogonal coordinates, and there are relatively few restrictions on the synthetic aperture time.

JP 2003-90880 A

  However, the conventional radar apparatus using the Polar Format algorithm only considers processing using a single received beam, and does not combine multiple received antenna beams. Therefore, it is desirable to improve the azimuth resolution of the reproduced image by synthesizing multiple receive antenna beams to generate a wider-band received signal with the same synthetic aperture time as that of a single receive beam. It was rare.

  The present invention has been made to solve the above-described problems, and an object thereof is to improve the azimuth resolution by synthesizing the reception signals of a plurality of reception antenna beams.

  The radar apparatus according to the present invention includes a beam forming unit that forms a multi-beam from a plurality of received signals obtained from a plurality of antennas whose center directions of the received beam are different in the azimuth direction, and a multi-beam obtained by the beam forming unit. Spectrum synthesizing means for performing spectrum synthesizing using, and image reproducing means for performing image reproducing processing using a Polar Format algorithm based on a signal obtained by performing inverse Fourier transform on the synthesized signal of the spectrum synthesizing means.

  According to the present invention, by combining multi-beams before processing of the Polar Format algorithm, it is possible to widen the received signal and improve the azimuth resolution.

Embodiment 1. FIG.
Embodiment 1 according to the present invention will be described below with reference to the drawings.
FIG. 1 is a diagram showing a mobile platform 100 equipped with a radar device 50 according to the first embodiment. In the figure, the z-axis is the vertical direction.

The mobile platform 100 is composed of an aircraft or an artificial satellite, and moves at a velocity V on the ground surface or in the space above the sea surface. In the example of the figure, an aircraft moving the mobile platform 100 in a direction parallel to the aircraft axis (x axis) is shown. The radar apparatus 50 is disposed below the mobile platform 100 (on the ground surface side), and acquires a two-dimensional image of the ground surface 60 in a band shape as the mobile platform 100 moves. The radar apparatus 50 has a predetermined off-nadir angle θ _ofn with respect to the z-axis and _{looks down} at the observation center C of the ground surface 60 from the antenna aperture center P. Further, the radar device 50 has a _squint angle θ _sqti (i = 1, 2,...) With respect to the x axis, and forms a plurality of receiving antenna beams.

  FIG. 2 shows the configuration of the radar apparatus according to the first embodiment. The radar device 50 includes an array antenna 1, a receiver 2, a beam forming unit 3, a shake compensation unit 4, an FFT unit 5, a spectrum synthesis unit 6, an azimuth IFFT unit 7, an image reproduction unit 8, and a display unit 20. The Since this embodiment has a feature in the processing of the reception system, the illustration and description of the transmission system including the transmitter are omitted.

  In FIG. 2, the array antenna 1 includes a plurality of antennas that receive reflected pulses from a reflector. Each antenna is arranged in the azimuth direction (azimuth direction), and the center of the reception beam of each antenna is different from each other in the azimuth direction. The array antenna 1 radiates (transmits) a high-frequency pulse signal to space and collects (receives) echo signals reflected by a reflector. The illustration of the transmitting antenna is omitted.

  The receiver 2 frequency-converts an RF (Radio Frequency) signal received by the array antenna 1 into a video signal, and outputs a reception signal converted into a digital signal for each range bin by A / D conversion. A plurality of receivers 2 are connected to each antenna of the array antenna 1 to constitute a receiver group.

The beam forming means 3 receives the output of each receiver 3 constituting the receiver group as an input, forms a multi-beam, and outputs each formed beam. The mobile platform 100 is equipped with an inertial device (not shown).
The motion compensation means 4 calculates the perturbation amount of the movement trajectory accompanying the motion of the mobile body 100 using the position information and attitude information of the mobile body platform 100 output from the inertial device, and is output from the beam forming means 3. Compensate each beam.

  The FFT means 5 receives the output of the fluctuation compensation means 4 and performs FFT (Fourier transform) processing in the hit direction of the reflected signal from the ground surface for each beam. The spectrum synthesizing unit 6 receives the processing result of the FFT unit 5 for each beam and synthesizes the signal spectrum for each beam to obtain a synthesized signal having a signal band for the beam.

The azimuth IFFT means 7 receives the output signal of the synthesized signal obtained by the spectrum synthesizing means, performs IFFT (inverse Fourier transform) processing in the hit direction in the azimuth direction, and returns the signal to the time domain. The image reproduction means 8 receives the output of the azimuth IFFT means 7 and performs image reproduction processing according to the Polar format algorithm.
The display means 20 inputs the output image reproduced by the image reproduction means 8 and outputs a radar image.

The operation will be described below.
In FIG. 2, when the received signal for the mth transmission pulse in the nth element antenna is represented by s (n, m), the output of the beam forming means 3 is S1 (k, m) in the equation (1). expressed. Here, λ is the transmission wavelength of the transmission wave output from the array antenna 1.

  Next, the output of the beam forming means 3 is input to the fluctuation compensating means 4. The fluctuation compensation unit 4 compensates the output of the beam forming unit 3 when the moving platform 100 performs an ideal linear motion based on the phase compensation amount Δ according to the equation (2).

  FIG. 3 is a diagram illustrating the relationship between the movement trajectory of the array antenna 1 and the phase compensation amount Δ, which is processed by the fluctuation compensation unit 4. The broken line in the figure shows the actual movement locus when the mobile platform 100 is shaken. On the other hand, a solid line shows a movement locus when an ideal linear motion is assumed.

For convenience of explanation, the Euler angles of the airframe of the mobile platform 100 obtained from the inertial device with reference to the airframe axis coordinate system are α, β, γ, and the direction of the airframe axis in the NED coordinate system (north reference coordinate system) , Ψ, θ. FIG. 4 illustrates the definition of these coordinate systems.
In the figure, as shown in FIG. 4 (a), the center of gravity at the start of platform observation is the origin, north is positive on the x axis, east is positive on the y axis, and vertically downward is positive on the z axis so as to form a right-handed system. Coordinate system.
Also, the aircraft axis coordinate system is a coordinate transformation of the NED coordinate system such that the nose direction (speed vector direction for convenience) of the mobile platform 100 is positive of the x axis, and Euler angles α, β, and γ are This is a coordinate system defined as shown in FIG.

Further, the inertial device and the center of the mechanical aperture for each beam of the array antenna 1 are known with respect to the center of gravity of the body of the mobile platform 100, and these coordinates are represented by (Xs, Ys, Zs) ^T , (Xa , Ya, Za) ^T.
Since the position of the airframe of the mobile platform 100 obtained by the inertial device is a coordinate based on the inertial device, first, it is converted into the coordinate based on the antenna position according to the equations (3) to (9) of Equation 3.

  The solid line in FIG. 3 shows the locus based on the antenna position, and this locus represents the locus when it is assumed that constant-velocity linear motion is performed while maintaining the initial speed from the period coordinates based on the antenna position. A broken line represents an actual antenna position accompanied by perturbation due to the movement of the mobile platform 100.

  It is assumed that the coordinates on the broken line are A (X1, Y1, Z1) at the sampling time tk. The intersection point between the straight line connecting the coordinates O (Xct, Yct, Zct) of the imaging center and the point A and the ideal trajectory of the aircraft is shown as an intersection point B (X2, Y2, Z2). The phase compensation amount Δ can be defined as Δ = 2πΔr / λ by the distance difference Δr between the intersection B and the point A (X1, Y1, Z1). That is, these distances are defined as r0 (t) and r1 (t), and the compensation calculation unit obtains a difference between these distances as shown in Expression (10), and Δr and Δ are calculated. Based on the calculated Δ and Expression (2), the output of the beam forming unit 3 is subjected to vibration compensation in the vibration compensation unit.

  Next, in the FFT means 5, S2 (k, m), which is a signal for each beam, is Fourier-transformed in the hit direction as shown in Expression (11) of Equation 4 for each beam output. As a result, a Doppler spectrum S3 (k, l) for each beam is obtained.

The spectrum synthesizing means 6 synthesizes the spectrum for the number of beams from the Doppler spectrum S3 (k, l) for each beam obtained as a result, and the band is expanded by the number of beams according to the equation (12) of Formula 5. A synthetic spectrum S4 (i) is obtained.

Here, a method of synthesizing the Doppler spectrum S3 (k, l) for each beam with S4 (i) will be described with reference to FIG. FIG. 5 shows the case of two beams. For the sake of explanation, the squint angle of beam 1 is θsqt1, and the squint angle of beam 2 is θsqt2. It is assumed that the relationship θsqt1 <90 ° <θsqt2 holds.
At this time, assuming that the spectrum centers for each beam are fb1 and fb2, the relations of fb1> 0 and fb2 <0 are established from Equations (13) to (16) of Equation 6.

In FIG. 5, (1a) represents the spectrum when S (k, 1) which is the signal of the beam 1 is Fourier-transformed in the hit direction. Since the signal band is from 0 to PRF from the spectrum center fb1> 0, the spectrum component outside the band indicated by the broken line in (1a) appears as a folded signal.
Similarly, (2a) is a spectrum when S (k, 2), which is the signal of beam 2, is Fourier-transformed in the hit direction.
The center frequency of the signal is in the interval from -PRF to 0, but the observed signal appears as a folded signal as indicated by the solid line (2a).

The spectrum synthesizing means 6 first performs a restoration process as shown in (1b) and (2b) so that the spectrum for each beam does not turn up. This restoration process requires twice as much signal bandwidth (2 PRF signal bandwidth).
Next, the restored spectrum is synthesized as shown in (c) in consideration of the center frequency of the spectrum of each beam. Then, the signal band for 2PRF is obtained. By this synthesis process, an effect equivalent to that obtained when a double signal band is obtained with one beam can be obtained.

  Next, the spectrum synthesized by the spectrum synthesizing means 6 is inverse Fourier transformed by the equation (17) by the azimuth IFFT means 7, and the beam output after the spectrum synthesis is returned to the time domain, thereby obtaining one beam. A high-band beam output equivalent to the case can be obtained. Thereby, high resolution in the azimuth direction can be realized.

The image reproduction means 8 performs image reproduction processing according to a normal Polar-format algorithm using the beam processed by the azimuth IFFT means 7 as a reception signal.
The Polar-format algorithm is one of synthetic aperture radar image reproduction algorithms. In this processing, as shown in FIG. 6, the received signal obtained by observation of the mobile platform 100 is mapped to a sector-shaped area in a polar coordinate system of a virtually obtained spatial frequency. Here, assuming that the coordinate point in polar coordinates is (Kr, θp), Kr is a distance from the platform, and θp is an angle formed with the x axis when the imaging center is viewed from the platform.
This is interpolated into rectangular coordinates indicated by the hatched area in the figure, thereby obtaining a two-dimensional image that realizes phase compensation.
As the spatial frequency coordinate system, for example, a polar coordinate system constituted by a distance from the platform and an angle formed with the x axis when the imaging center is viewed from the platform is used.
As the rectangular coordinate system indicated by the hatched area, for example, a range axis (PC direction) and an azimuth axis (x-axis direction) are used.

  Details of the Polar format algorithm are introduced in, for example, “WG Carrara, RS Goodman, RM Majewski,“ Spotlight Synthetic Aperture Radar ”Artec House, 1995, p97” and the like. Therefore, the description of the detailed processing of the Polar format algorithm is omitted here.

  In this embodiment, the received signal obtained by multi-beams is subjected to spectrum synthesis after compensating for the motion of the airframe, so that an effect equivalent to that obtained when a double signal band is obtained by one beam can be obtained. After spectrum synthesis, after performing inverse Fourier transform, the image is reproduced using the Polar format algorithm, so that errors are less likely to occur in range movement compensation.

Embodiment 2. FIG.
FIG. 7 shows the configuration of the radar apparatus according to the second embodiment. Here, only the part which makes this invention the summary is demonstrated.
In FIG. 7, the spectrum simple synthesizing means 9 receives the output of the FFT means 5 for each beam and synthesizes the signal spectrum for each beam to obtain a signal band for the beam.
Other configurations are the same as those described in FIG. 2 of the first embodiment.

A principle diagram for synthesizing the Doppler spectrum for each beam will be described with reference to FIG.
FIG. 8 shows the case of two beams. For convenience of explanation, it is assumed that the squint angle of the beam 1 is θsqt1, and the squint angle of the beam 2 is θsqt2. It is assumed that the relationship θsqt1 <90 ° <θsqt2 holds. At this time, assuming that the spectral centers for each beam are fb1 and fb2, the relations of fb1> 0 and fb2 <0 are established according to the equations (18) to (20) in Equation 8. Furthermore, the frequency with respect to the center of the beam 1 and the beam 2 is set to fb.

  In FIG. 8, (1a) represents a spectrum when S (k, 1) which is the signal of the beam 1 is Fourier-transformed in the hit direction. Since the signal band is from 0 to PRF from the spectrum center fb1> 0, the spectrum component outside the band indicated by the broken line in (1a) appears as a folded signal.

  Similarly, (2a) is a spectrum when S (k, 2), which is the signal of beam 2, is Fourier-transformed in the hit direction. The center frequency of the signal is in the interval from -PRF to 0, but the observed signal appears as a folded signal as shown by the solid line in (2a).

In the simple spectrum synthesizing unit 9, first, frequency arrangement conversion is performed as shown in (1b) and (2b) with reference to the center frequency fb of the two beams derived from the equation 8 for each beam.
Next, the spectrum after the arrangement conversion of each beam is combined around fb as shown in FIG. Then, the signal band for 2PRF is obtained.
By this synthesis process, an effect equivalent to that obtained when a double signal band is obtained with one beam can be obtained.

  Compared to Embodiment 1, in this embodiment, spectrum synthesis can be performed relatively easily. In addition, a double signal band is not required at the stage of the layout conversion process. However, in the present embodiment, since the folding of the spectrum is allowed, the front-rear relation of the spectrum is broken, so that a virtual image may appear as a reproduced image.

  In this embodiment, frequency synthesis is allowed at the time of spectrum synthesis to perform synthesis processing. Compared to the first embodiment, there is less processing for expanding the signal band to the number of beams, and the memory of the computer can be saved.

Embodiment 3 FIG.
FIG. 9 shows the configuration of the radar apparatus according to the third embodiment. About the same code | symbol as FIG. 1 of Embodiment 1, the same thing is shown.
In FIG. 9, the shake compensation unit 10 calculates the amount of perturbation of the movement trajectory accompanying the body shake from the position information of the machine body output from the inertial device, and compensates the output of the beam forming unit 3.
In the first embodiment, when performing shake compensation, compensation is performed in consideration of the mechanical aperture center position of the receiving antenna. However, in the present embodiment, the mechanical aperture center of the array antenna 1 in Equation 3 may be replaced with the phase center of the array antenna 1 in consideration of the phase center for each beam calculated in advance. As a result, the compensation amount calculation unit obtains the phase compensation amount Δ = 2πΔr / λ using Δr obtained according to Equation 3, and then the beam when the body performs an ideal linear motion in the shake compensation unit. Compensate the output of the forming means.

The operation will be described below.
FIG. 10 is a diagram showing the relationship of the phase compensation amount Δ processed by the fluctuation compensation means 10.
The broken line in the figure shows the actual movement trajectory accompanied by airframe shaking. On the other hand, a solid line means a movement trajectory of the movement platform 100 when an ideal linear motion is assumed.
It is assumed that the phase center of the antenna is known in advance and this coordinate is set as (Xp, Yp, Zp) ^T , and the compensation process is (Xa, Ya, Za) in the first embodiment. the ^T (Xp, Yp, Zp) is replaced by ^T, it is possible to obtain the phase compensation amount Δ by the same process.

  According to this embodiment, the motion compensation of the airframe of the mobile platform 100 is performed not by the center of the mechanical aperture of the beam of the array antenna 1 but by the phase center reference, so that accurate motion compensation is performed compared to the center of the mechanical aperture position. it can.

Embodiment 4 FIG.
FIG. 11 shows a configuration of a radar apparatus according to the fourth embodiment. Here, only the part which makes this invention the summary is demonstrated.
In FIG. 11, reference numeral 11 denotes a shake compensation unit that calculates the perturbation amount of the trajectory accompanying the shake of the aircraft from the position information of the aircraft output from the inertial device, and compensates the output of the beam forming unit. However, in the embodiments so far, the Euler angles of the airframe obtained from the inertial device are directly used. However, considering the fact that there is an error in these, the Euler angles are first expressed by Equation (21). Smoothing is performed according to (23), and the motion compensation of the aircraft is performed from the smoothed Euler angle.

  According to this embodiment, when performing motion compensation of the airframe of the mobile platform 100, considering that there is an error in the Euler angle of the airframe obtained from the inertial device, the Euler angle is first smoothed and the smoothed Euler angle is obtained. To compensate for the motion of the aircraft.

It is a figure which shows the mobile platform carrying the radar apparatus by Embodiment 1 of this invention. BRIEF DESCRIPTION OF THE DRAWINGS It is a block diagram of the radar apparatus which shows Embodiment 1 of this invention. It is a principle figure of the fluctuation compensation means in Embodiment 1 of this invention. It is explanatory drawing of the coordinate system which defines the Euler angle and nose direction which are used with the shake compensation means in Embodiment 1 of this invention. It is explanatory drawing of the spectrum synthetic | combination means in Embodiment 1 of this invention. It is a conceptual diagram of coordinate conversion of Polar format algorithm. It is a block diagram of the radar apparatus which shows Embodiment 2 of this invention. It is explanatory drawing of the spectrum simple synthetic | combination means in Embodiment 2 of this invention. It is a block diagram of the radar apparatus which shows Embodiment 3 of this invention. It is a principle figure of the fluctuation compensation means in Embodiment 3 of this invention. It is a block diagram of the radar apparatus which shows Embodiment 4 of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Array antenna, 2 Receiver, 3 Beam formation means, 4 Shake compensation means, 5 FFT means, 6 Spectrum synthesis means, 7 IFFT means, 8 Image reproduction means, 9 Spectrum simple synthesis means, 10 Shake compensation means, 11 Shake compensation means 20 Display means.

Claims (4)

Beam forming means for forming a multi-beam from a plurality of reception signals obtained from a plurality of antennas having different center directions of the reception beam in the azimuth direction;
For the multi-beam obtained by the beam forming means, the shake compensation means for compensating for the shake caused by the mounted mobile platform,
For the multi-beams obtained by the beam forming means, the center frequency of the Doppler spectrum for each beam is calculated from the direction cosine of each beam squint angle, and the folding of the spectrum for each beam occurs in consideration of the sign of the center frequency. In order to avoid this, the spectrum of the beam whose center frequency is positive is restored to the position where the center frequency is positive by restoring the omission due to aliasing, and the spectrum of the beam where the center frequency is negative is restored. Then, restore the spectrum by placing it at a position where the center frequency is negative, and place the spectrum of the beam placed at a position where the center frequency is positive with respect to the restored spectrum at a position where the center frequency is negative. Spectrum synthesis means for performing spectrum synthesis of the spectrum of the arranged beam;
Based on a signal obtained by performing an inverse Fourier transform on the synthesized signal of the spectrum synthesizing unit, an image reproducing unit that performs an image reproducing process using a Polar Format algorithm;
A radar apparatus comprising:   The shake compensation means compensates the amount of movement of the antenna aperture center due to the shake of the mobile platform on which the radar apparatus is mounted with respect to the multi-beam obtained by the beam forming means, and the compensated multi-beam is spectrally converted. The radar apparatus according to claim 1, wherein the radar apparatus is supplied to a synthesizing unit.   The fluctuation compensation means compensates for the phase change at the center of the antenna aperture accompanying the fluctuation of the mobile platform on which the radar apparatus is mounted with respect to the multi-beam obtained by the beam forming means, and the compensated multi-beam spectrum The radar apparatus according to claim 1, wherein the radar apparatus is supplied to a synthesizing unit.   2. The shake compensation means includes an inertial device that measures the motion of a moving platform using a coordinate system expressed by an Euler angle, and smoothes a measured value of the Euler angle by the inertial device. The radar apparatus described.

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