JP7160268B2 - Synthetic aperture radar signal processing device and synthetic aperture radar signal processing program - Google Patents

Description

The present disclosure relates to synthetic aperture radar technology that achieves both high signal-to-noise ratio and high resolution.

Synthetic aperture radar technology requires resolution of the following tradeoffs. As a first trade-off, in order to achieve a high signal-to-noise ratio, it is necessary to increase the gain of the antenna, that is, increase the aperture length, but the synthetic aperture reduces the azimuth resolution. As a second trade-off, a broadband chirped pulse is required to achieve high range resolution, but the signal-to-noise ratio is degraded due to the beam tilt caused by the imperfection of the directivity of the antenna. put away.

Patent No. 6080783 Patent No. 6072262 Patent No. 3332000 specification

N. Gebert et al. , "ULTRA WIDE SWATH IMAGING WITH MULTI-CHANNEL SCANSAR", IGARSS 2008-2008, IEEE International Geoscience and Remote Sensing Symposium.

Patent Documents 1 to 3 and Non-Patent Document 1 disclose methods for achieving both a high signal-to-noise ratio and high resolution in synthetic aperture radar technology. In Patent Documents 1 to 3, chirped pulses are irradiated in random directions to widen the observation range of the spotlight mode. In Non-Patent Document 1, a chirped pulse is scanned in the range direction and the azimuth direction to realize Scan-SAR, TOPS-SAR, and the like. However, in Patent Documents 1 to 3 and Non-Patent Document 1, in order to intentionally change the direction of the irradiation beam, a mechanical or electrical mechanism for controlling the direction of the irradiation beam is required. It becomes complicated and the system becomes expensive.

Therefore, in order to solve the above problems, the present disclosure achieves both a high signal-to-noise ratio and high resolution in synthetic aperture radar technology, and does not require a mechanical or electrical mechanism for controlling the direction of an irradiation beam. The purpose is to realize circuit simplification and system cost reduction.

In order to solve the above problems, the beam tilt caused by the directional characteristics of the antenna is actively used. That is, in irradiating the chirped pulse, the direction of the irradiation beam is changed in the azimuth direction according to the frequency of the irradiation beam according to the inherent directivity characteristics without performing mechanical or electronic directivity control. Of the entire chirped pulse irradiated at each azimuth position, only a partial section of the chirped pulse irradiated toward the observation target position is considered in the correlation processing between the reflected signal and the reference signal.

Specifically, the present disclosure provides a chirped pulse generator for generating a chirped pulse for range compression of a synthetic aperture radar, and a mechanical and electronic orientation for irradiating the chirped pulse generated by the chirped pulse generator. An antenna unit that changes the direction of the irradiation beam in the azimuth direction according to the frequency of the irradiation beam according to the inherent directional characteristics without performing control, a reflected signal receiving unit that receives the reflected signal reflected from the target, A synthetic aperture radar device comprising:

According to this configuration, since the beam tilt caused by the directivity of the antenna is actively used in irradiating the chirped pulse, the observable azimuth time is increased for any observation target position. Therefore, even if an attempt is made to increase the gain of the antenna, that is, to increase the aperture length in order to achieve a high signal-to-noise ratio, the azimuth resolution can be prevented from deteriorating due to the synthetic aperture. In addition, since the direction of the irradiation beam is changed in the azimuth direction according to the frequency of the irradiation beam according to the inherent directivity characteristics without mechanical or electronic directivity control, the circuit is simplified and the system cost is low. can be realized.

Further, according to the present disclosure, the chirped pulse generation unit controls the center frequency for each azimuth position to generate a chirped pulse so that the irradiation beam emitted by the antenna unit is directed to an observation area having an azimuth direction. and performing synthetic aperture radar observation using a spotlight mode.

According to this configuration, circuit simplification and system cost reduction can be achieved when using the spotlight mode with a limited observation range.

Further, according to the present disclosure, the chirped pulse generation unit generates a chirped pulse having a shape that does not depend on the azimuth position, thereby changing the direction of the irradiation beam in the azimuth direction and not changing the direction of the irradiation beam in the azimuth direction. A synthetic aperture radar apparatus characterized by performing synthetic aperture radar observation using a strip mode in which an observable azimuth time is longer than that of a conventional synthetic aperture radar.

According to this configuration, it is possible to achieve high azimuth resolution, circuit simplification, and system cost reduction when using the strip mode that widens the observation range.

Further, in the present disclosure, the chirped pulse generation unit generates a chirped pulse having a shape that does not depend on the azimuth position and having a center frequency that causes a squint in the azimuth direction of the irradiation beam irradiated by the antenna unit. By changing the direction of the irradiation beam in the azimuth direction, compared to when the direction of the irradiation beam is not changed in the azimuth direction, it is possible to observe the azimuth in a longer azimuth time. It is a synthetic aperture radar device characterized by this.

According to this configuration, it is possible to achieve high azimuth resolution, circuit simplification, and system cost reduction when using the squint mode in the diagonally forward or diagonally rearward direction.

In addition, the present disclosure is directed to the reflected signal received by the reflected signal receiving unit of the synthetic aperture radar device described above and the chirped pulse generated by the chirped pulse generating unit reflected from the observation target position for each azimuth position. A synthetic aperture radar signal processing apparatus characterized by executing correlation processing between a reference signal selected only for a partial interval expected to be

In addition, the present disclosure is directed to the reflected signal received by the reflected signal receiving unit of the synthetic aperture radar device described above and the chirped pulse generated by the chirped pulse generating unit reflected from the observation target position for each azimuth position. A synthetic aperture radar signal processing program for causing a computer to execute a correlation process between a reference signal selected only for a partial section expected to be

According to these configurations, of the entire section of the chirped pulse irradiated at each azimuth position, only a partial section of the chirped pulse irradiated toward the observation target position is the time between the reflected signal and the reference signal. Considered in correlation processing in the domain or frequency domain. Then, for a reflected signal reflected from a certain observation target position, in the present disclosure, the receivable range time is shorter than the conventional technique, but the receivable azimuth time is longer, so the total receivable time does not decrease. Therefore, even if an attempt is made to broaden the chirped pulse band in order to achieve high range resolution, the signal-to-noise ratio can be prevented from deteriorating due to the beam tilt.

Further, in the present disclosure, among the reflected signal reflected from the observation target position and the chirped pulse generated by the chirped pulse generation unit, only a partial section expected to be reflected from the observation target position for each azimuth position is a synthetic aperture radar signal processing apparatus characterized by executing correlation processing in the time domain between a reference signal selected from

Further, in the present disclosure, among the reflected signal reflected from the observation target position and the chirped pulse generated by the chirped pulse generation unit, only a partial section expected to be reflected from the observation target position for each azimuth position is a synthetic aperture radar signal processing program for causing a computer to execute correlation processing in the time domain between a selected reference signal and

According to these configurations, of the entire section of the chirped pulse irradiated at each azimuth position, only a partial section of the chirped pulse irradiated toward the observation target position is the time between the reflected signal and the reference signal. Considered in region correlation processing. Then, for a reflected signal reflected from a certain observation target position, in the present disclosure, the receivable range time is shorter than the conventional technique, but the receivable azimuth time is longer, so the total receivable time does not decrease. Therefore, even if an attempt is made to broaden the chirped pulse band in order to achieve high range resolution, the signal-to-noise ratio can be prevented from deteriorating due to the beam tilt. Also, although the calculation time is long, the image accuracy is high.

In this way, the present disclosure eliminates the need for a mechanical or electrical mechanism for controlling the direction of the irradiation beam while achieving both a high signal-to-noise ratio and high resolution in synthetic aperture radar technology, and simplifies the circuit. And the cost of the system can be reduced.

FIG. 2 illustrates a chirped pulse waveform and antenna directivity of the present disclosure; 1 is a diagram showing a configuration diagram of a conventional spotlight mode; FIG. FIG. 3 illustrates a strip mode of the present disclosure; FIG. 12 illustrates a squint mode of the present disclosure; 1 is a diagram showing the configuration of a synthetic aperture radar system of the present disclosure; FIG. FIG. 2 illustrates the processing of the synthetic aperture radar system of the present disclosure; FIG. 4 is a diagram showing frequency characteristics of antenna directivity of the present disclosure; FIG. 4 is a diagram showing frequency characteristics of antenna directivity of the present disclosure; FIG. 4 is a diagram showing the time variation of the chirp frequency of the present disclosure; FIG. 4 is a diagram showing changes in antenna directivity over time according to the present disclosure; FIG. 4 illustrates the definitions of target direction and distance of the present disclosure; FIG. 4 is an illustration of azimuth times at which reflected signals of the present disclosure are present; FIG. 4 is an explanatory diagram of the range time during which the chirped pulse of the present disclosure is directed toward the target; [0014] Fig. 4 illustrates a reflected signal and a reference signal of the present disclosure; FIG. 11 illustrates a point extension function of the present disclosure versus range-wise distance from target; FIG. 10 illustrates a point extension function of the present disclosure versus azimuth distance from target;

Embodiments of the present disclosure will be described with reference to the accompanying drawings. The embodiments described below are examples of implementing the present disclosure, and the present disclosure is not limited to the following embodiments.

(Basic principle of the synthetic aperture radar system of the present disclosure)
The chirped pulse waveform and antenna directivity of the present disclosure are shown in FIG. The present disclosure makes positive use of the beam tilt caused by the directional characteristics of the antenna. That is, in irradiating the chirped pulse, the direction of the irradiation beam is changed in the azimuth direction according to the frequency of the irradiation beam according to the inherent directivity characteristics without performing mechanical or electronic directivity control. In the upper part of FIG. 1, the chirped pulse sweeps in frequency from a minimum frequency f ₀ −B _W /2 through a center frequency f ₀ to a maximum frequency f ₀ +B _W /2 over a range time 0≦τ≦τ ₀ . .

An array antenna and a slot antenna are applied as the antenna section A in the middle part of FIG. 1 and the bottom part of FIG. Each antenna element of the antenna section A is arranged in the azimuth direction x.

When the chirped pulse sweeps the center frequency f ₀ , each antenna element of antenna section A is excited in phase, and the illuminating beam of antenna section A points in the front direction y. When the chirped pulse sweeps through the minimum frequency f ₀ −B _w /2, the antenna elements of antenna section A are not excited in phase and the illuminating beam of antenna section A is directed φ _min oriented <0. When the chirped pulse sweeps through the maximum frequency f ₀ +B _w /2, the antenna elements of antenna section A are not excited in phase and the illuminating beam of antenna section A is directed in a direction φ _max > Point to 0. Thus, in irradiating the chirped pulse, the direction φ of the irradiation beam is changed to the azimuth direction x in accordance with the frequency f of the irradiation beam according to the inherent directivity characteristics without mechanical or electronic directivity control. Let

FIG. 2 shows a block diagram of a conventional spotlight mode. In the upper stage, after generating a chirped pulse having a shape that does not depend on the azimuth position x, the antenna driving section 13 (connected by a rotary joint or the like) mechanically controls the orientation, thereby controlling the direction of the irradiation beam emitted by the antenna section A. The azimuth direction x is directed to a certain observation area, and synthetic aperture radar observation is performed using the spotlight mode. In the lower stage, after generating a chirped pulse having a shape that does not depend on the azimuth position x, the azimuth of the irradiation beam emitted by the antenna section A is controlled electronically by the phase shifter and the distributor of the antenna section A. A synthetic aperture radar observation is performed using the spotlight mode with the direction x pointing to a certain observation area.

A block diagram of the spotlight mode of the present disclosure is shown in FIG. By generating a chirped pulse by controlling the center frequency for each azimuth position x, the azimuth direction of the irradiation beam emitted by the antenna part A is adjusted according to the inherent directivity characteristics without mechanical or electronic directivity control. Synthetic aperture radar observations are performed using the spotlight mode with x pointed at a certain observation area. Note that FIG. 5 is applicable to both strip and squint modes.

In this way, when using the spotlight mode with a limited observation range, circuit simplification and system cost reduction can be achieved.

A strip mode of the present disclosure is shown in FIG. By generating a chirped pulse having a shape that is independent of azimuth position x, the observable azimuth time is reduced compared to changing the direction of the illumination beam in azimuth direction x and not changing the direction of the illumination beam in azimuth direction x. Synthetic aperture radar observations are carried out using the strip mode in which .

The upper part of FIG. 3 shows the strip mode without prior art beam tilt. First, when the synthetic aperture radar system S is positioned at the azimuth position x _st,i and the chirped pulse sweeps from frequency f ₀ −B _w /2 to f ₀ +B _w /2, the target T is acquired is beginning to be Then, when the synthetic aperture radar system S is positioned at the azimuth position x _st,f and the chirped pulse sweeps from frequency f ₀ −B _w /2 to f ₀ +B _w /2, the target T is It's out of range. Here, since the azimuth direction range x _st,i to x _{st, f} is shorter than in the present disclosure described later, the observable azimuth time is shorter than in the present disclosure described later, and the azimuth resolution is Lower than disclosed.

The lower part of FIG. 3 shows the strip mode with the beam tilt of the present disclosure. First, the target T is beginning to be acquired when the synthetic aperture radar system S is positioned at the azimuth position x' _st,i and the chirped pulse sweeps through the frequency f ₀ -B _w /2. Next, when the synthetic aperture radar system S is positioned at the azimuth position x' _st,f and the chirp pulse sweeps through the frequency f ₀ +B _w /2, the target T is out of acquisition range. Here, the azimuth position x' _st,i is behind the azimuth position x _st,i and the azimuth position x' _st,f is ahead of the azimuth position x _st,f . Therefore, since the azimuth direction range _x'st,i to _x'st,f is longer than in the prior art, the observable azimuth time is longer and the azimuth resolution is higher than in the prior art.

In this way, when using the strip mode that widens the observation range, it is possible to achieve high azimuth resolution, circuit simplification, and system cost reduction.

The squint mode of the present disclosure is illustrated in FIG. By generating a chirped pulse having a shape that does not depend on the azimuth position x and having a center frequency that causes a squint in the azimuth direction x of the irradiation beam irradiated by the antenna part A, the direction of the irradiation beam is oriented in the azimuth direction x. Synthetic aperture radar observation is performed using the Squint mode in which the observable azimuth time is longer than when the direction of the irradiation beam is changed in the azimuth direction x.

Here, when using the squint mode in the oblique front, the center frequency is changed from f ₀ in FIGS. ₁ and ₃ to f ₀ ' in FIG. and shift. On the other hand, when using the squint mode in the obliquely rearward direction, the center frequency is changed from f ₀ in FIGS. ₁ and ₃ to f ₀ ′ in FIG. and shift.

The top of FIG. 4 shows the squint mode without prior art beam tilt. First, when the synthetic aperture radar system S is positioned at the azimuth position x _sq,i and the chirped pulse sweeps the frequency f ₀ ′−B _W /2 to f ₀ ′+B _W /2, the target T is , is beginning to be captured. Then, when the synthetic aperture radar system S is positioned at the azimuth position x _sq,f and the chirped pulse sweeps from frequency f ₀ ′−B _W /2 to f ₀ ′+B _W /2, the target T has been captured. Here, since the azimuth direction range x _sq,i to x _sq,f is shorter than in the present disclosure described later, the observable azimuth time is shorter than in the present disclosure described later, and the azimuth resolution is Lower than disclosed.

The lower part of FIG. 4 shows the squint mode with the beam tilt of the present disclosure. First, the target T is beginning to be acquired when the synthetic aperture radar system S is positioned at the azimuth position x′ _sq,i and the chirped pulse sweeps through the frequency f ₀ ′−B _W /2. Next, when the synthetic aperture radar system S is located at the azimuth position x′ _sq,f and the chirped pulse sweeps through the frequency f ₀ ′+B _w /2, the target T is out of acquisition range. . Here, the azimuth position x′ _sq,i is behind the azimuth position x _sq,i and the azimuth position x′ _sq,f is ahead of the azimuth position x _sq,f . Therefore, since the azimuth direction range x' _sq,i to x' _sq,f is longer than that of the prior art, the observable azimuth time is longer than that of the prior art, and the azimuth resolution is higher than that of the prior art.

In this way, when using the squint mode in the diagonally forward or diagonally backward direction, it is possible to achieve high azimuth resolution, circuit simplification, and system cost reduction.

(Configuration and processing of the synthetic aperture radar system of the present disclosure)
FIG. 5 shows the configuration of the synthetic aperture radar system of the present disclosure. The processing of the synthetic aperture radar system of the present disclosure is illustrated in FIG. A synthetic aperture radar system S includes a synthetic aperture radar device 1 and a synthetic aperture radar signal processing device 2 . A synthetic aperture radar apparatus 1 is composed of a chirp pulse generator 11, an antenna section A, and a reflected signal receiver 12, and is mounted on an artificial satellite, aircraft, or the like. The synthetic aperture radar signal processing device 2 comprises a correlation processing execution unit 21 and is mounted on an artificial satellite, aircraft, ground equipment, etc., and the synthetic aperture radar signal processing program shown in the lower column of FIG. 6 is installed in the computer. It is realized by

First, the synthetic aperture radar apparatus 1 positively utilizes beam tilt caused by the directivity characteristics of the antenna. In other words, in irradiating the chirped pulse, the synthetic aperture radar apparatus 1 does not perform mechanical or electronic orientation control, but according to the inherent directivity characteristics, changes the direction φ of the irradiation beam according to the frequency f of the irradiation beam. Change in the azimuth direction x.

The chirped pulse generator 11 generates a chirped pulse for range compression of the synthetic aperture radar (step S1). When irradiating a chirped pulse for range compression of a synthetic aperture radar, the antenna section A does not perform mechanical or electronic directivity control, but according to its inherent directivity characteristics, changes the direction of the irradiation beam to the frequency of the irradiation beam. It is changed in the azimuth direction x according to f (step S2). The reflected signal receiving unit 12 receives the reflected signal reflected from the target T by the antenna unit A (step S3). The synthetic aperture radar device 1 will be described with reference to FIGS. 7 to 10. FIG. 7-10 apply to strip mode, but are applicable to both spotlight and squint modes if the center frequency is changed.

The antenna directivity frequency characteristics of the present disclosure are shown in FIGS. The direction φ of the irradiation beam is represented by Equation 1 with respect to the frequency f of the irradiation beam. However, a>0. Note that a<0 may be satisfied, the frequency characteristic of the antenna directivity does not necessarily pass through the origin (ff ₀ =0, φ=0), and the frequency characteristic does not necessarily have to be linear. good.

When the chirped pulse sweeps the frequency f ₀ −B _W /2 to f ₀ , the irradiation beam of the antenna section A is oriented in a direction φ<0 deviated from the front direction y. When the chirped pulse sweeps the frequency f ₀ to f ₀ +B _W /2, the irradiation beam of the antenna section A is oriented in a direction φ>0 deviated from the front direction y. It is assumed that the half width of the irradiation beam of the antenna section A is η, and that the irradiation beam intensity of the antenna section A is uniform within the irradiation beam width of the antenna section A. For the sake of simplicity, the irradiation beam intensity of the antenna section A is assumed to be uniform within the irradiation beam width of the antenna section A. However, in reality, it is not uniform within the irradiation beam width of the antenna section A. may be

FIG. 9 shows the time variation of the chirp frequency of the present disclosure. The frequency f of the chirped pulse is expressed as Equation 2 with respect to the range time τ. It should be noted that the time change of the chirp frequency may be upward or downward, and does not necessarily have to be a linear time change.

FIG. 10 shows changes in antenna directivity over time according to the present disclosure. The direction φ of the irradiation beam is expressed as Equation 3 with respect to the range time τ. The change in the antenna directivity over time may be upward or downward, and may not necessarily be a linear change over time.

Next, the synthetic aperture radar signal processing device 2 refers to only a partial section of the chirped pulse irradiated toward the observation target position, out of the entire section of the chirped pulse irradiated at each azimuth time t, as a reflected signal. Considered in correlation processing between signals.

The correlation processing execution unit 21 executes correlation processing between the reflected signal and the reference signal (step S11). The synthetic aperture radar signal processing device 2 will be described with reference to FIGS. 11 to 14. FIG. 11-14 apply to strip mode, but are applicable to both spotlight and squint modes if the center frequency is changed.

Specifically, the correlation processing execution unit 21 expects that the reflected signal received by the reflected signal receiving unit 12 and the chirped pulse generated by the chirped pulse generating unit 11 are reflected from the observation target position for each azimuth position x. and a reference signal that selects only a part of the interval to be correlated.

In the present embodiment, the correlation processing execution unit 21 expects the reflected signal reflected from the observation target position and the chirp pulse generated by the chirp pulse generation unit 11 to be reflected from the observation target position for each azimuth position x. and a reference signal that selects only a part of the interval from the time domain. That is, in the present embodiment, the correlation processing execution unit 21 executes the time domain correlation processing between the reflected signal and the reference signal, thereby increasing the image accuracy although the calculation time is long. Here, as a modification, the correlation processing execution unit 21 may significantly shorten the calculation time by executing the correlation processing in the frequency domain between the reflected signal and the reference signal.

The definitions of target direction and distance of the present disclosure are shown in FIG. Let the position of the synthetic aperture radar system S be (x(t), 0) and the position of the target T be (X, Y). The position x(t) in the azimuth direction of the synthetic aperture radar system S, the direction θ(t) of the target T viewed from the synthetic aperture radar system S, and the distance R(t) from the synthetic aperture radar system S to the target T are given by Equation 4: is represented as where v is the velocity of the synthetic aperture radar system S;

つまり、合成開口レーダシステムＳから見たターゲットＴの方向θ（ｔ）が、以下の条件を満たせばよい：（１）アンテナ部Ａの照射ビームが方向φ_ｍｉｎ＝－ａＢ_Ｗ／２を指向するときの、アンテナ部Ａの照射ビームの外縁の方向φ＝φ_ｍｉｎ－η＝－ａＢ_Ｗ／２－ηと比べて、方向θ（ｔ）がレンジ方向ｙ寄りであること、（２）アンテナ部Ａの照射ビームが方向φ_ｍａｘ＝＋ａＢ_Ｗ／２を指向するときの、アンテナ部Ａの照射ビームの外縁の方向φ＝φ_ｍａｘ＋η＝＋ａＢ_Ｗ／２＋ηと比べて、方向θ（ｔ）がレンジ方向ｙ寄りであること。 An illustration of the azimuth time at which the reflected signal of the present disclosure is present is shown in FIG. The existence range of the target T observable at each azimuth time t is represented by Equation 5.

In other words, the direction θ(t) of the target T viewed from the synthetic aperture radar system S should satisfy the following conditions: (1) The radiation beam of the antenna section A points in the direction φ _min =−aB _W /2. (2) the direction θ (t) is closer to the range direction y than the direction φ=φ _min −η=−aB _W /2−η of the outer edge of the irradiation beam of the antenna unit A when When the irradiation beam of A is directed in the direction φ _max =+aB _W /2, compared with the direction φ=φ _max +η=+aB _W /2+η of the outer edge of the irradiation beam of the antenna part A, the direction θ(t) is within the range It should be closer to the direction y.

Substituting the second formula of formula 4 into formula 5, the azimuth time t at which the reflected signal from the target T exists is expressed as formula 6. Here, the left side of Equation 6 corresponds to the azimuth time t _i in FIG. 14, and the right side of Equation 6 corresponds to the azimuth time t _f in FIG.

In this way, since the beam tilt caused by the directivity of the antenna is actively used in irradiating the chirped pulse, the observable azimuth time t is increased for any observation target position. Therefore, even if an attempt is made to increase the gain of the antenna, that is, to increase the aperture length in order to achieve a high signal-to-noise ratio, the azimuth resolution can be prevented from deteriorating due to the synthetic aperture. In addition, since the direction φ of the irradiation beam is changed in the azimuth direction x according to the frequency f of the irradiation beam according to the inherent directivity characteristics without performing mechanical and electronic directivity control, the circuit and system can be simplified. price reduction can be realized.

つまり、合成開口レーダシステムＳから見たターゲットＴの方向θ（ｔ）が、以下の条件を満たせばよい：（１）アンテナ部Ａの照射ビームが周波数ｆのチャープの過程でターゲットＴに向き始めるときの、アンテナ部Ａの照射ビームの外縁の方向φ（τ_ｉ（ｔ））＋ηと比べて、方向θ（ｔ）が同じである、又は、方向θ（ｔ）がφ（０）－ηとφ（０）＋ηとの間にあること、（２）アンテナ部Ａの照射ビームが周波数ｆのチャープの過程でターゲットＴに向き終わるときの、アンテナ部Ａの照射ビームの外縁の方向φ（τ_ｆ（ｔ））－ηと比べて、方向θ（ｔ）が同じである、又は、方向θ（ｔ）がφ（τ_０）－ηとφ（τ_０）＋ηとの間にあること。 FIG. 13 shows an explanatory diagram of the range time during which the chirped pulse of the present disclosure is irradiated toward the target T. As shown in FIG. A range time τ at which the target T can be observed at each azimuth time t is expressed as in Equation 7.

In other words, the direction θ(t) of the target T viewed from the synthetic aperture radar system S should satisfy the following conditions: (1) The radiation beam of the antenna section A begins to face the target T in the process of chirp of frequency f. The direction θ(t) is the same as the direction φ(τ _i (t)) + η of the outer edge of the irradiation beam of the antenna part A at the time, or the direction θ(t) is φ(0)−η and φ(0)+η, and (2) the direction φ( The direction θ(t) is the same as τ _f (t))−η, or the direction θ(t) is between φ(τ ₀ )−η and φ(τ ₀ )+η .

Substituting Equation 3 into Equation 7, the range time τ at which the reflected signal from the target T exists is expressed as Equation 8. Here, the left side of Equation 8 corresponds to range time τ _i (t) in FIGS. 13 and 14, and the right side of Equation 8 corresponds to range time τ _f (t) in FIGS.

The reflected and reference signals of the present disclosure are shown in FIG. A reflected signal F(t, τ) reflected from the target T is represented by Equation (9). The reference signal to be correlated with the reflected signal F(t, τ) is determined based on the following theoretical formula for the reflected signal F(t, τ).

ここで、ビームチルトがない場合とビームチルトがある場合とを比較する。 However, the value range of the azimuth time t of the reflected signal F(t, τ) is represented by Equation 10 based on Equation 6. Then, the value range of the range time τ of the reflected signal F(t, τ) is represented by Equation 11 based on Equation 8 and the delay time of electromagnetic wave propagation.

Here, the case without beam tilt and the case with beam tilt are compared.

First, when there is no beam tilt, the correlation between the reflected signal and the reference signal is calculated over the entire section of the chirped pulse irradiated toward the target T, out of the entire section of the chirped pulse irradiated at each azimuth time t. Consider in processing.

Specifically, in the azimuth time t _i /t _f where the irradiation beam of the antenna part A starts/finishes toward the target T, the range time range τ _if ≤ τ ≤ τ _if + τ ₀ (where τ _if is the electromagnetic wave propagation delay time), over the entire range time range τ _if ≤ τ ≤ τ _if + τ ₀ are considered in the correlation processing between the reflected signal and the reference signal. Then, at the azimuth time _tc where the synthetic aperture radar system S approaches the target T closest, within the range time range _τc ≦τ≦ _τc + _τ0 (where _τc is the delay time of electromagnetic wave propagation), Over the entire range time range τ _c ≤ τ ≤ τ _c + τ ₀ is considered in the correlation process between the reflected signal and the reference signal.

In FIG. 14, the azimuth time width in which the reflected signal exists when there is no beam tilt and the azimuth time width in which the reflected signal exists when there is beam tilt are shown to have the same length. However, in reality, the azimuth time width in which the reflected signal exists when there is no beam tilt is shorter than the azimuth time width in which the reflected signal exists when there is beam tilt. The area of the signal is not greater without beam tilt than without beam tilt.

On the other hand, when there is a beam tilt, only a partial section of the chirped pulse irradiated toward the target T, out of the entire section of the chirped pulse irradiated at each azimuth time t, is between the reflected signal and the reference signal. are considered in the correlation processing of

Specifically, at the azimuth time t _i at which the irradiation beam of the antenna part A begins to face the target T, part of the range time range _{max { τ i} (t _i ), τ _if } ≤ τ ≤ min { τ _f (t _i ), τ _if + τ ₀ } (see FIG. 13 and equations 8 and 11) only in the correlation process between the reflected signal and the reference signal Consider. Then, at the azimuth time t _c where the synthetic aperture radar system S approaches the target T at the closest time, max _{{ τ i (} t _{c )} , τ _c } ≤ τ ≤ min {τ _f (t _c ), τ _c + τ ₀ } (see Fig. 13 and Equations 8 and 11) are only considered in the correlation process between the reflected signal and the reference signal. Furthermore, at the azimuth time tf when the irradiation beam of the antenna part A ends toward the target T, a part of the range time range τ _if ≤ τ ≤ τ _if + ₀ , max {τ _i (t _{f )} , Only τ _if }≦τ≦min{τ _f (t _f ), τ _if +τ ₀ } (see FIG. 13 and Equations 8 and 11) are considered in the correlation process between the reflected signal and the reference signal.

In this way, of the entire chirped pulse irradiated at each azimuth time t, only a partial section of the chirped pulse irradiated toward the target T is the time domain or frequency between the reflected signal and the reference signal. Considered in region correlation processing. As for the reflected signal reflected from the target T, in the present disclosure, the receivable range time τ is shorter than the conventional technique, but the receivable azimuth time t is longer, so the total receivable time does not decrease. Therefore, even if an attempt is made to broaden the chirped pulse band in order to achieve high range resolution, the signal-to-noise ratio can be prevented from deteriorating due to the beam tilt.

相関Ｓは、Ｐ：（Ｘ、Ｙ）とＰ’：（Ｘ’、Ｙ’）とが近ければ、大きな値をとると期待され、Ｐ：（Ｘ、Ｙ）とＰ’：（Ｘ’、Ｙ’）とが遠ければ、小さな値をとると期待される。 (Resolution evaluation of the synthetic aperture radar system of the present disclosure)
First, correlation processing between the reflected signal and the reference signal will be described. Let F _{(X, Y)} (t, τ) be the reflected signal from a certain target P: (X, Y), and F be the reflected signal from another target P': (X', Y'). _{Let (X', Y')} be (t, τ). A correlation S between the reflected signal F _{(X, Y)} (t, τ) and the reflected signal F _{(X', Y')} (t, τ) is represented by Equation 12.

The correlation S is expected to take a large value if P: (X, Y) and P': (X', Y') are close, and P: (X, Y) and P': (X', Y′) is expected to take a small value.

相関Ｓは、Ｐ_１、Ｐ_２、・・・、Ｐ_ｎの中にＰにごく近い点があれば、大きな値をとると期待され、Ｐ_１、Ｐ_２、・・・、Ｐ_ｎの中にＰにごく近い点がなければ、小さな値をとると期待される。そこで、観測対象位置Ｐ：（Ｘ、Ｙ）におけるターゲットの有無を画像化するために、観測対象位置Ｐ：（Ｘ、Ｙ）が関わる相関Ｓの大きさを画像の階調数に対応付ける。 Correlation processing between the reflected signal and the reference signal is performed using the above principle. _Reflected signals reflected from a plurality of targets P ₁ : (X ₁ , Y ₁ ), _{P 2} : ( _{X 2} , Y ₂ ), . is the sum of individual reflected signals, and the reference signal expected to be reflected from the observation target position P: (X, Y) is assumed to be F _{(X, Y)} (t, τ). A correlation S between the reflected signal R(t, τ) and the reference signal F _{(X, Y)} (t, τ) is represented by Equation 14.

_The correlation S is expected to take _{a large value if there is a point in P 1 , P 2 ,} . is expected to take a small value if there are no points very close to P. Therefore, in order to visualize the presence or absence of the target at the observation target position P: (X, Y), the magnitude of the correlation S related to the observation target position P: (X, Y) is associated with the number of gradations of the image.

Next, the resolution evaluation of the synthetic aperture radar system will be described. In the above correlation processing, the correlation S involving P: (X, Y) and P': (X', Y') is P: (X, Y) and P': (X', Y') is assumed to decrease rapidly with increasing distance. Therefore, the extent to which the correlation S rapidly decreases, that is, the resolution of the synthetic aperture radar system S is evaluated.

なお、ターゲットのアジマス方向位置を０としたが、一般性は失われない。ターゲットのアジマス方向位置をＸとすれば、アジマス時間ｔをＸ／ｖだけシフトすればよい。 A reflected signal reflected from the target (0, Y) is represented by Equation 15, and a reference signal expected to be reflected from the observation target position (ΔX, Y+ΔY) is represented by Equation 16.

Note that the azimuth direction position of the target is set to 0, but generality is not lost. If the azimuth direction position of the target is X, the azimuth time t should be shifted by X/v.

A correlation S between the reflected signal F(t, τ) and the reference signal G(t, τ) is expressed as Equation 17. Here, ΔR(t), R(t) and R'(t) are expressed as in Equation 18. Note that the correlation S shown in Equation 17 corresponds to the point extension function shown in FIGS.

A correlation S between the reflected signal F(t, τ) and the reference signal G(t, τ) is calculated as in Equation 19. However, A(t) and B(t) are expressed as in Equations 20 and 21. The integration range of the azimuth time t in Equation 19 extends over the azimuth time t satisfying B(t)>A(t). Furthermore, θ(t) and θ′(t) are expressed as in Equation 22.

The point extension function of the present disclosure versus range direction distance from target is shown in FIG. The point extension function of the present disclosure versus azimuth distance from target is shown in FIG. In calculating the point spread function, the center frequency _f0 is 9.25 GHz, the chirped pulse frequency width _BW is 800 MHz, the chirped pulse time width _τ0 is 1.0 μs, the beam width 2η is 1.5 deg, and the beam Assume that the tilt change speed is 1.0 deg/100 MHz, the speed v of the synthetic aperture radar system S is 100 m/s, and the range direction distance of the target T is 15.0 km.

Referring to FIG. 15, it can be seen that comparable range resolution is obtained with the beam tilt of the present disclosure as compared to the prior art without beam tilt. That is, with the beam tilt of the present disclosure, the range time τ over which the reflected signal from the target T is present is reduced compared to the prior art without the beam tilt, so the frequency band of the reflected signal from the target T is is decreased, the azimuth time t during which the reflected signal from the target T exists increases, so the same range resolution can be obtained without decreasing the total time during which the reflected signal from the target T exists. Then, in the correlation processing between the reflected signal and the reference signal, among the chirped pulses irradiated at each azimuth time t, compared to the case where all of the chirped pulse time width τ ₀ is considered, the irradiation toward the target T In some cases, high signal-to-noise ratios can be achieved.

Referring to FIG. 16, it can be seen that higher azimuth resolution is obtained with the beam tilt of the present disclosure compared to the prior art without beam tilt. In other words, compared to the case without beam tilt in the prior art, when there is the beam tilt of the present disclosure, since the beam tilt caused by the directivity characteristics of the antenna is actively used in irradiating the chirped pulse, any observation The observable azimuth time t also increases for the target position. Therefore, even if an attempt is made to increase the gain of the antenna, that is, to increase the aperture length in order to achieve a high signal-to-noise ratio, the azimuth resolution can be prevented from deteriorating due to the synthetic aperture.

A synthetic aperture radar device, a synthetic aperture radar signal processing device, and a synthetic aperture radar signal processing program according to the present disclosure control the direction of an irradiation beam while achieving both a high signal-to-noise ratio and high resolution in synthetic aperture radar technology. No mechanical or electrical mechanism is required, so circuit simplification and system cost reduction can be achieved.

S: Synthetic aperture radar system A: Antenna unit T: Target 1: Synthetic aperture radar device 2: Synthetic aperture radar signal processing device 11: Chirp pulse generation unit 12: Reflected signal reception unit 13: Antenna drive unit 21: Correlation processing execution unit

Claims (8)

a chirped pulse generator for generating a chirped pulse for range compression of a synthetic aperture radar;
In irradiating the chirped pulse generated by the chirped pulse generator, the direction of the irradiation beam is changed in the azimuth direction according to the frequency of the irradiation beam according to the inherent directivity characteristics without mechanical or electronic directivity control. Antenna part to be changed,
a reflected signal receiver that receives a reflected signal reflected from the target;
A synthetic aperture radar signal processing device used in a synthetic aperture radar device comprising
By changing the direction of the irradiation beam in the azimuth direction, setting the observable azimuth time of the observation target position longer than when the direction of the irradiation beam does not change in the azimuth direction,
Select only a partial section expected to be reflected from the observation target position for each azimuth position from all sections of the reflected signal received by the reflected signal receiver and the chirped pulse generated by the chirped pulse generator. A synthetic aperture radar signal processing apparatus, characterized by executing correlation processing between a reference signal obtained by the above method and a reference signal. The observable azimuth time of the observation target position is set based on the observation target position, the speed of the synthetic aperture radar device, and the directional change width and beam width of the irradiation beam. Synthetic aperture radar signal processor. The total time width of the chirped pulse, the direction of the observation target position as seen from the synthetic aperture radar device, the direction change width of the irradiation beam, and the beam for the partial section expected to be reflected from the observation target position for each azimuth position 3. The synthetic aperture radar signal processing apparatus according to claim 1, wherein the setting is made based on the width. 4. The synthetic aperture radar signal processing apparatus according to claim 1 , wherein said correlation processing between said reflected signal and said reference signal is performed in the time domain . After controlling the center frequency for each azimuth position, a chirped pulse is generated so that the azimuth direction of the irradiation beam is directed to a certain observation area. The synthetic aperture radar signal processing apparatus according to any one of claims 1 to 4, wherein observation is performed. Synthetic aperture radar signal processing according to any one of claims 1 to 4, characterized in that synthetic aperture radar observation is performed using a strip mode by generating a chirped pulse having a shape independent of the azimuth position. Device. Synthetic aperture radar observation is performed using the squint mode by generating a chirped pulse that has a shape that does not depend on the azimuth position and that has a center frequency that causes a squint in the azimuth direction of the irradiation beam. The synthetic aperture radar signal processing apparatus according to any one of claims 1 to 4, wherein A synthetic aperture radar signal processing program for causing a computer to execute each processing step executed by the synthetic aperture radar signal processing apparatus according to any one of claims 1 to 7.

Images