JP2011169869A - Apparatus for processing radar signal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a radar signal processing apparatus that forms an image without resolution deterioration or the like, even when a synthetic aperture time becomes long. <P>SOLUTION: The radar signal processing apparatus includes an azimuth reference function calculation unit 7 which calculates an azimuth reference function, based on moving route of a radar device and a coordinate of observation point, an azimuth direction dividing part 1 which partitions received signal in an azimuth direction, an azimuth Fourier transform part 2 which forms a received signal after azimuth Fourier transform, by performing Fourier transform in the azimuth direction for a divided received signal, a reference function multiplication part 3 which forms a received signal after phase compensation, by multiplying a computed azimuth reference function for the received signal after the azimuth Fourier transform, an azimuth inverse Fourier transform part 5 which forms a divided image, by performing azimuth direction inverse Fourier transform for the received signal after phase compensation, and an azimuth combination part 6 which forms a high resolution image by synthesizing the divided image. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a radar signal processing apparatus that obtains an image of an observation region by performing signal processing for improving resolution in a direction parallel to a moving direction on a received signal from a radar apparatus.

  Synthetic Aperture Radar (SAR) is a radar that is mounted on a moving body such as an aircraft or a satellite and observes the ground surface. By performing signal processing on the received signal received during movement, it is possible to improve the angular resolution and obtain an image of the ground surface to be observed.

  The sub-aperture method is one of the image reconstruction algorithms using Synthetic Aperture Radar (SAR), and all Doppler bands in received data, such as spotlight observation and sliding spotlight observation, are pulse repetition frequency (PRF). It is an algorithm for processing larger observation data.

  This sub-aperture method is used as a regeneration process for the commercial satellite TerraSAR-X developed by the German Aerospace Center. For example, Non-Patent Document 1 describes the method.

  The feature of the sub-aperture method is that after the received data is divided into a range where the Doppler band does not exceed the PRF (sub-aperture), reproduction processing is performed for each sub-aperture unit, and the spectral analysis method (SPECAN: Spectral Analysis) is performed at the final stage. Using this method, the sub-apertures are combined to form a high-resolution image.

  In the sub-aperture method, a chirp scaling algorithm (CSA) is used for reproduction processing. In this CSA processing, it is assumed that the platform trajectory is a constant velocity linear motion, and the slant range R [η] between the observation point and the radar becomes a hyperbolic type as in the following equation (1). It is assumed that.

Here, R ₀ is the closest distance to the radar, η is the azimuth time based on the time that is the closest distance, and V _r is the platform speed. At this time, the reference function _Haz at the time of azimuth compression in the azimuth frequency region (Doppler frequency region) is performed by the following equation (2).

J. Mittermayer et al., “Spotlight SAR Data Processing Using the Frequency Scaling Algorithm”, IEEE Trans. On Geosc. And Remote Sensing, Vol.37, No.5, pp. 2198-2214, Sept. 1999.

  The azimuth reference function of equation (2) is premised on that equation (1) holds. Until now, this algorithm has been used in an environment with a short synthetic aperture length in which the slant range between the observation point of equation (1) and the radar is a hyperbolic type, such as an aircraft-mounted SAR and a satellite-mounted SAR having a high transmission frequency. It was. However, since the azimuth resolution is approximately inversely proportional to the transmission frequency, in the case of the low transmission frequency SAR, it is necessary to increase the synthetic aperture time in order to obtain the same azimuth resolution. When the synthetic aperture time is large, the orbit is curved in the satellite environment, so the slant range locus deviates from the hyperbolic shape, and the azimuth compression processing according to equation (2) causes degradation of resolution and sidelobe characteristics. was there.

  The present invention has been made to solve the above-described problems, and even when the synthetic aperture time becomes long, by performing an appropriate azimuth compression process, there is no degradation in resolution or sidelobe characteristics. An object of the present invention is to obtain a radar signal processing device capable of generating an image.

  The radar signal processing device according to the present invention divides the received signal in the azimuth direction based on the movement path of the radar device and the coordinates of the observation point in the observation target area and the azimuth reference function calculation unit that calculates the azimuth reference function. An azimuth direction division unit; a first azimuth Fourier transform unit that generates a reception signal after azimuth Fourier transform by performing Fourier transform in the azimuth direction on the reception signal divided by the azimuth direction division unit; A reference function multiplication unit that generates a phase-compensated reception signal by multiplying the reception signal after azimuth Fourier transform by the azimuth reference function calculated by the azimuth reference function calculation unit; An azimuth inverse Fourier transform unit that generates a divided image by performing Fourier transform, and the divided image By synthesizing those and a azimuth coupling unit generating a high resolution image.

  The radar signal processing apparatus according to the present invention can generate an image having no resolution degradation or sidelobe characteristic degradation by performing appropriate azimuth compression processing even when the synthetic aperture time becomes long.

It is a block diagram which shows the structure of the radar signal processing apparatus which concerns on Embodiment 1 of this invention. It is a block diagram which shows the structure of the slant range locus | trajectory calculation part of the radar signal processing apparatus which concerns on Embodiment 1 of this invention. It is a figure for demonstrating the outline | summary of the sub-aperture method of the radar signal processing apparatus which concerns on Embodiment 1 of this invention. It is a figure which shows the range of the azimuth reference signal of the radar signal processing apparatus which concerns on Embodiment 1 of this invention. It is a block diagram which shows the structure of the azimuth reference function calculation part of the radar signal processing apparatus concerning Embodiment 2 of this invention. It is a figure which shows the range of the azimuth reference signal of the radar signal processing apparatus which concerns on Embodiment 2 of this invention.

  Hereinafter, a preferred embodiment of a radar signal processing apparatus of the present invention will be described with reference to the drawings.

Embodiment 1 FIG.
A radar signal processing apparatus according to Embodiment 1 of the present invention will be described with reference to FIGS. FIG. 1 is a block diagram showing a configuration of a radar signal 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.

  In FIG. 1, a radar signal processing apparatus according to Embodiment 1 of the present invention includes an azimuth direction dividing unit 1, an azimuth Fourier transform unit 2, a reference function multiplication unit 3, an azimuth scaling unit 4, and an azimuth inverse Fourier transform. A unit 5, an azimuth coupling unit 6, and an azimuth reference function calculation unit 7 are provided.

  The azimuth reference function calculation unit 7 includes a slant range trajectory calculation unit 71, an azimuth reference signal calculation unit 72, an azimuth range setting unit 73, and an azimuth Fourier transform unit 74.

  FIG. 2 is a block diagram showing the configuration of the slant range trajectory calculation unit of the radar signal processing apparatus according to Embodiment 1 of the present invention.

  In FIG. 2, the slant range trajectory calculation unit 71 includes a ground observation point setting unit 711, a polynomial approximation coefficient calculation unit 712, a polynomial approximation coefficient storage unit 713, and a slant range trajectory restoration unit 714.

  Next, the operation of the radar signal processing apparatus according to the first embodiment will be described with reference to the drawings.

  FIG. 3 is a diagram for explaining the outline of the sub-aperture method of the radar signal processing apparatus according to Embodiment 1 of the present invention. FIG. 4 is a diagram showing the range of the azimuth reference signal of the radar signal processing apparatus according to Embodiment 1 of the present invention.

  In general, a synthetic aperture radar (SAR) performs signal processing for increasing the resolution in both the range direction and the azimuth direction. However, since the first embodiment of the present invention relates to signal processing for increasing the resolution in the azimuth direction, the resolution increasing processing in the range direction, specifically, the pulse compression processing is not particularly described. In practice, a generally known method may be used.

  The radar signal processing apparatus according to the first embodiment irradiates a wave to an observation area while moving, and receives a wave scattered in the observation area and is parallel to the movement direction with respect to the received signal. An image of the observation region is obtained by performing signal processing for improving resolution in various directions, and the signal processing similar to the conventional sub-aperture method is performed except that the azimuth reference function is calculated by the azimuth reference function calculation unit 7. Do.

  First, the azimuth direction dividing unit 1 divides the received signal in the azimuth direction as shown in FIG. As shown in FIG. 3, the Doppler frequency of the received signal changes as the azimuth time elapses, and the observation time as a whole has a wide Doppler frequency band, which is wider than the pulse repetition frequency (PRF). However, the division in the azimuth direction prevents the Doppler frequency band of each division from exceeding the PRF.

  The azimuth Fourier transform unit 2 performs Fourier transform in the azimuth direction for each divided aperture. The reference function multiplication unit 3 multiplies the received signal after the azimuth Fourier transform by the azimuth reference function calculated by the azimuth reference function calculation unit 7.

  The azimuth scaling unit 4 performs phase compensation necessary for azimuth scaling, and the azimuth inverse Fourier transform unit 5 performs inverse Fourier transform to generate a low-resolution image for each divided aperture.

  Further, the azimuth coupling unit 6 generates one high-resolution image by synthesizing the low-resolution images obtained for each divided aperture. In the previous Non-Patent Document 1, synthesis is performed using a spectrum analysis method (SPECAN).

  The difference between the first embodiment of the present invention and the prior art is that the azimuth reference function is directly derived from the slant range R [η] calculated from the ground observation point and the trajectory obtained from the irradiation direction and attitude of the radar to compress the azimuth. It is used for processing. By doing so, it is possible to create an azimuth reference function suitable for a true Doppler change, and thus it is possible to suppress resolution degradation and sidelobe characteristic degradation.

  The procedure for creating a reference function in the azimuth reference function calculation unit 7 is as follows.

  The reference function (matched filter) used at the time of azimuth compression is phase modulation by Doppler change in the azimuth frequency axis. Therefore, the slant range trajectory calculation unit 71 creates a slant range trajectory R [τ, η] of the target at the range time τ. A specific creation method will be described later.

  Using the created slant range locus R [τ, η], the azimuth reference signal calculation unit 72 creates an azimuth reference signal h [τ, η] by Equation (3).

  The azimuth reference signal h [τ, η] is extracted by the azimuth width set by the azimuth range setting section 73 and is subjected to Fourier transform in the azimuth direction by the azimuth Fourier transform section 74, thereby obtaining phase modulation due to Doppler change in the azimuth frequency axis. This is the azimuth reference function represented by the following equation (4).

  Next, a method for setting the azimuth width by the azimuth range setting unit 73 will be described.

  In the sub-aperture method, as shown in FIG. 3, reproduction processing is performed in units of aperture division based on the PRF in the azimuth direction. At this time, since the azimuth reference function width required for each sub-aperture needs to satisfy the entire Doppler width in the received signal in the sub-aperture, the time width T_ref wider than the sub-aperture width T_sub as shown in FIG. Slant range change is required. Therefore, the azimuth range of the time width T_ref is set by the azimuth range setting unit 73.

  The reference function multiplication unit 3 multiplies the received signal by a reference function on the azimuth frequency axis as azimuth compression processing. The frequency increment in the azimuth direction of the image data immediately before azimuth compression is 1 / T_sub. However, when an azimuth reference function is created with a time width T_ref, it becomes a reference function in increments of 1 / T_ref. Therefore, in order to adjust to the increment of 1 / T_sub, the reference function is interpolated to obtain the value of the azimuth reference function of 1 / T_sub.

  The slant range trajectory calculation unit 71 obtains the slant range trajectory R [τ, η] of the observation point at the range time τ for each range.

  The slant range trajectory calculation unit 71 calculates the slant range trajectory in the following procedure.

  (1) The ground observation point setting unit 711 obtains a ground observation point in the beam center direction at the observation center time. The obtained observation points are about the beam irradiation range in the range direction, and a plurality of points are calculated at approximately equal off-nadir angle intervals.

(2) The polynomial approximation coefficient calculation unit 712 calculates the slant range trajectory R _m [η] from each observation point position set by the ground observation point setting unit 711 and the radar device position for each hit obtained from the trajectory information. Coefficients of polynomial approximation (a _{n, m} , a _{n−1, m} ,..., A _{0, m} ) are calculated by the least square method for the relationship between the time and the slant range.

(3) Further, in the polynomial approximation coefficient calculation unit 712, the obtained polynomial approximation coefficient of the slant range trajectory (5 in the case of fourth order approximation) is related to the slant range R ₀ to the observation point for each coefficient of the same dimension. Coefficients of polynomial approximation (C _{n, m} , C _{n−1, m} ,..., C _{0, m} ) are calculated by the least square method. Since the change of the coefficient due to the range is not large, a second-order approximation is sufficient. However, it is necessary to match the slant range to the observation point used for coefficient calculation according to the range to which the migration correction is performed in the RCMC (Range Cell Migration Compensation) process during playback processing (0 Doppler range or observation center range). There is.

  In the reproduction process, the coefficients (1) to (3) are calculated in advance and stored in the polynomial approximation coefficient storage unit 713. Then, when creating the azimuth reference function, the slant range trajectory restoration unit 714 restores the slant range trajectory for each range.

  In the above description, the radar apparatus transmits and receives radio waves. However, the radar signal processing apparatus according to Embodiment 1 of the present invention is not particularly limited in the types of waves transmitted and received by the radar apparatus. For example, the present invention can be similarly applied to a radar apparatus using sound waves or laser light.

  According to the radar signal processing device according to the first embodiment, by using the reference function derived from the slant range trajectory, even when the synthetic aperture time becomes long, the appropriate azimuth compression processing is performed. Thus, it is possible to generate an image with no resolution degradation or sidelobe characteristic degradation.

Embodiment 2. FIG.
A radar signal processing apparatus according to Embodiment 2 of the present invention will be described with reference to FIGS. FIG. 5 is a block diagram showing the configuration of the azimuth reference function calculation unit of the radar signal processing apparatus according to Embodiment 2 of the present invention. FIG. 6 is a diagram showing the range of the azimuth reference signal of the radar signal processing apparatus according to Embodiment 2 of the present invention.

  In the radar signal processing device according to the first embodiment, the frequency increment in the azimuth direction of the image data immediately before azimuth compression is 1 / T_sub. However, when the azimuth reference function is created with the necessary slant range width T_ref, the reference function becomes a step 1 / T_ref. For this reason, an interpolation process is required to match the 1 / T_sub step. Therefore, in the second embodiment, the process of matching the reference function is made efficient.

  The radar signal processing apparatus according to the second embodiment is different from the radar signal processing apparatus according to the first embodiment only in the configuration of the azimuth reference function calculation unit 7A, and the other parts have the same configuration. Have.

  In FIG. 5, the azimuth reference function calculation unit 7A includes a slant range locus calculation unit 71, an azimuth reference signal calculation unit 72, an azimuth range setting unit 73, an azimuth Fourier transform unit 74, an azimuth range expansion unit 75, and an azimuth range. A reference function thinning unit 76 is provided.

  In the azimuth range setting unit 73, setting the azimuth width T_ref necessary as the azimuth reference function is the same as in the first embodiment, but in the azimuth reference function calculation unit 7A in FIG. Extend azimuth width.

  Specifically, an area for the azimuth reference function width T_ref2 = N * T_sub that is an integral multiple of the sub-aperture width T_sub is secured, and an azimuth reference function is created with 0 outside the area of T_ref. Thereby, the increment of the azimuth reference function becomes 1 / (N * T_sub).

  The azimuth reference function thinning unit 76 thins out every N reference functions obtained by the FFT processing. As a result, the received signal after the azimuth Fourier transform and the azimuth increment of the azimuth reference function coincide with each other at 1 / T_sub. Therefore, azimuth compression can be efficiently performed only by multiplying both without interpolation processing.

  1 azimuth direction division unit, 2 azimuth Fourier transform unit, 3 reference function multiplication unit, 4 azimuth scaling unit, 5 azimuth inverse Fourier transform unit, 6 azimuth coupling unit, 7 azimuth reference function calculation unit, 7A azimuth reference function calculation unit, 71 Slant range trajectory calculation unit, 72 azimuth reference signal calculation unit, 73 azimuth range setting unit, 74 azimuth Fourier transform unit, 75 azimuth range expansion unit, 76 azimuth reference function thinning unit, 711 ground observation point setting unit, 712 polynomial approximation coefficient calculation Part, 713 polynomial approximation coefficient storage part, 714 slant range trajectory restoration part.

Claims (5)

An azimuth reference function calculation unit that calculates an azimuth reference function based on the movement path of the radar device and the coordinates of the observation point in the observation target area;
An azimuth direction dividing unit for dividing the received signal in the azimuth direction;
A first azimuth Fourier transform unit that generates a reception signal after azimuth Fourier transform by performing a Fourier transform in the azimuth direction on the reception signal divided by the azimuth direction division unit;
A reference function multiplier for generating a phase-compensated received signal by multiplying the received signal after azimuth Fourier transform by the azimuth reference function calculated by the azimuth reference function calculator;
An azimuth inverse Fourier transform unit that generates a divided image by performing an azimuth direction inverse Fourier transform on the received signal after phase compensation;
A radar signal processing apparatus comprising: an azimuth coupling unit that generates a high-resolution image by synthesizing the divided images. The azimuth reference function calculation unit
A slant range trajectory calculation unit for calculating a slant range trajectory based on the movement path of the radar device and the coordinates of the observation point in the observation target area;
An azimuth reference signal calculating unit for calculating an azimuth reference signal having a phase determined by the slant range locus;
The radar signal processing apparatus according to claim 1, further comprising: a second azimuth Fourier transform unit that calculates an azimuth reference function by performing a Fourier transform on the azimuth reference signal in the azimuth direction. The azimuth reference function calculation unit
An azimuth range setting unit that sets the width in the azimuth direction of the azimuth reference signal input to the second azimuth Fourier transform unit so that the azimuth reference function has the same frequency band as the Doppler frequency band of the received signal in the division The radar signal processing apparatus according to claim 2, further comprising: The azimuth reference function calculation unit
An azimuth range expansion unit that expands the width in the azimuth direction set by the azimuth range setting unit and sets the azimuth reference signal of the expansion range to 0;
The radar signal processing apparatus according to claim 3, further comprising: an azimuth reference function thinning unit that thins out the azimuth reference function calculated by the second azimuth Fourier transform unit in the azimuth direction. The slant range trajectory calculation unit
A ground observation point setting unit for setting a ground observation point in the beam center direction at the observation center time;
A polynomial approximation coefficient calculation unit for calculating a coefficient of a polynomial approximation function that approximates a slant range locus with a function related to the range time and the azimuth time;
A polynomial approximation coefficient storage unit for storing coefficients of the polynomial approximation function;
The radar signal processing apparatus according to claim 2, further comprising a slant range trajectory restoration unit that restores a slant range trajectory for each range when calculating the azimuth reference function.

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