WO2015129842A1

WO2015129842A1 - Radar device

Info

Publication number: WO2015129842A1
Application number: PCT/JP2015/055747
Authority: WO
Inventors: 田川哲也
Original assignee: 株式会社次世代技術研究所
Priority date: 2014-02-27
Filing date: 2015-02-27
Publication date: 2015-09-03

Abstract

Conventional methods are dependent upon the off-nadir angle and the state and contour of the surface of the ground, and cannot be applied to ground surfaces in the direction directly below a radar, or in a direction close thereto, thus limiting application to instances in which the off-nadir angle is relatively large. Additionally, because of a failure to take attenuation of radio waves by water vapor, rain, and the like into consideration, the value of the radar scatter cross section computed for the surface of the ground is smaller, by the equivalent of attenuation of the radio waves on the propagation path between the radar and the surface of the ground. According to the present invention, by applying the mean value theorem for integrals in a radar equation representing surface echo, the radar scatter cross section of the ground surface per unit area, and the amount of attenuation on the propagation path, can be calculated even in the direction directly below a radar or in a direction close thereto, whether on sea or land. Additionally, by carrying out antenna beam scanning to vary the antenna gain irradiating a given range cell, it is possible to improve the spatial resolution within a range cell.

Description

Radar equipment

The present invention relates to a radar apparatus for measuring a radar scattering cross section on the ground surface.

In radar observation, scattered waves (ground clutter) from the ground surface superimposed on the reflected waves from the target (aircraft, ships, weather targets such as rain and clouds) may become a problem due to interference. For example, in Patent Document 1, an antenna beam is scanned in the elevation direction for the purpose of reducing sea surface clutter superimposed on a reflected wave from a marine vessel observed by an onboard radar, and on the same range cell as the vessel. The estimated value of radar scattering cross section at sea level is obtained.

Although this method can estimate the radar scattering cross section of a horizontal surface such as the sea surface, it is difficult to apply it to an uneven ground surface such as land. At the time of calculating the radar scattering cross section of the undulating ground surface, there is also a method of correcting the antenna gain using ground surface altitude information by interferometry as disclosed in Patent Document 2.

The conventional method depends on the off-nadir angle and the undulations / states of the ground surface, cannot be applied to the ground surface in the direction directly below the radar and in the vicinity thereof, and the application is limited when the off-nadir angle is relatively large. In addition, since the attenuation of radio waves due to water vapor, rain, or the like is not taken into account, the calculated value of the radar scattering cross section of the ground surface is reduced by the amount of radio wave attenuation in the propagation path between the radar and the ground surface. The method of Patent Document 2 also has a problem that its use is limited to a synthetic aperture radar that can perform interferometry processing.

In order to solve the above-mentioned problems, the first invention applies a theorem of the average value of integrals in the radar equation representing the surface echo, so that the radar can be located directly below the radar or in the vicinity thereof, regardless of sea or land. The radar scattering cross section of the ground surface per unit area and the attenuation in the propagation path are obtained. Further, it is possible to improve the spatial resolution in the range cell by changing the antenna gain for irradiating the same range cell by antenna beam scanning. Also, the coordinates of the region emphasized by the antenna gain are obtained.

The second invention is based on the radar scattering cross section of the ground surface with improved spatial resolution in the range cell. It is possible to collate and estimate the error contained in the antenna gain.

The third invention calculates a ground surface echo based on the radar scattering cross section of the ground surface per unit area estimated for each distance and each beam scanning, and calculates the ground surface from the total received power at a certain distance and beam scanning. By subtracting the echo component, it is possible to reduce the ground surface clutter.

According to the present invention, the radar scattering cross section of the ground surface per unit area and the propagation regardless of whether it is in the vicinity of the radar or in the vicinity thereof, or in the vicinity of the radar, or when the radio wave is attenuated in the propagation path. The amount of attenuation in the path can be determined. In addition, the spatial resolution in the range cell can be improved.

Hereinafter, embodiments of the present invention will be described with reference to equations and drawings.
The radar equation representing the surface echo intensity Ps is represented by the following equation (1). The positional relationship between the radar and the ground surface, and the coordinate system are shown in FIG. 1 (a radar equation representing ground surface echo is shown in Non-Patent Document 1, etc.).

Where Ps (r, k) is the ground echo (mW) received from the range bin at the distance r, k is the number assigned for each antenna beam direction, Pt is the power (mW) transmitted from the radar, I (r , k) is determined by antenna gain (transmission: Gt, k reception: Gr, k), transmission pulse waveform u, radar scattering cross section σ0 of the ground surface, and radio wave attenuation A in the propagation path between the radar and the ground surface. The integral value, r ′, is the distance from the radar to a position on the ground surface, and S is the ground surface at the same distance as the range bin at the position of distance r. The antenna gain is expressed as a function of the incident angle θ and the azimuth angle φ, and any of a model based on a Gaussian function and a sinc function and an actually measured antenna gain can be used. A model formula based on the sinc function is shown in Patent Document 1 and the like. As the transmission pulse waveform u, any of a rectangular function, a model based on a Gaussian function or a sinc function, or an actually measured pulse waveform can be used. In the case of a Gaussian function, it is expressed by the equation (2).

Here, c is the speed of light, and t is the -6 dB width of the transmission / reception pulse.
The relationship between the range cell and the antenna gain when a rectangular pulse waveform is assumed as the transmission pulse waveform u can be illustrated as shown in FIG. (A resolution function may be used as the transmission pulse waveform u.) In the range cell observed near the center of the antenna gain, the gradient of the antenna gain is small, and the ground surface of the range cell is irradiated almost equally and the ground surface echo is received. . In the range cells on both sides, the antenna gain gradient is relatively large, and the area inside the antenna gain is irradiated more strongly, and the ground surface echo emphasized on the black dot (●) side in the range cell is received. When the beam direction is changed by the antenna beam scanning in the direction of the up / down / left / right arrows, even in the same range cell, the intensity of the irradiation area is generated by the gradient of the antenna gain, and a partially enhanced ground surface echo is received. . The spatial resolution of ground echoes received by the radar is determined by the radar range resolution and antenna gain. Even if the range cells determined by the radar range resolution are the same, the spatial resolution of the ground surface echo can be substantially improved by changing the gradient of the antenna gain by scanning the antenna beam.

For I (r, k) in Equation 1, since the functions included in the integration symbol can be integrated and both are functions larger than 0 (the sign of ± does not change in the integration interval), the average value of the integration The theorem (mean value theorem for integration) can be applied, and Equation 3 is obtained.

In Equation 3, Equation 4 is used to obtain the radar scattering cross-sectional area σ0A of the ground surface per unit area obtained by weighted averaging within the integration range S with the antenna gains Gt, k and Gr, k.

Equation 5 is obtained from Equation 1, Equation 3, and Equation 4.

When obtaining Equation (3) from I (r, k) of Equation (1), after giving the model equation (Equation 6) and the calculated or observed values for σ0 (θ, φ) and A (θ, φ), θt , k, φt, k and θr, k, φr, k need to be obtained (subscript t means transmit, r means initial of receive). θt, k, φt, k and θr, k, φr, k are variables indicating the position on the polar coordinate together with the distance r ′, and are coordinates representing an emphasized region as indicated by a black dot (●) in FIG. I can say that.

The simplest model is f (θ, φ) = 1 (true number). As f (θ, φ), a theoretical value based on an electromagnetic wave scattering model (depending on the classification of the ground surface such as the sea surface, forests, and deserts), an experimental model value, and an observation value obtained by a scatterometer or the like can be used. As shown in Patent Document 4, A (θ, φ) depends on the frequency of radio waves to be transmitted and received, and the amount of attenuation (difference in attenuation between single frequencies and attenuation between multiple frequencies) causes precipitation. There is a method for estimating the quantity and the like. Rainfall is non-uniform spatially, and the present invention, which can improve the spatial resolution even by scanning the antenna beam even in the same range cell, estimates and compensates for non-uniformity such as precipitation. Effective above. For example, in order to quantify non-uniformity, σ0 (θ, φ) within the same range cell is used for a plurality of values of σ0 (θ, φ) A (θ, φ) obtained by scanning the antenna beam. Calculate statistics such as variance of A (θ, φ). When obtaining non-uniformity in a wider area, the received data is oversampled in the direction of the distance r, and statistical values such as variance are calculated including adjacent range cells. By changing the beam position as shown by the up / down / left / right arrows in FIG. Therefore, it is possible to obtain three-dimensional non-uniformity by using the present invention.

FIG. 3 shows the relationship between the antenna beam, the resolution cell, and the θr, k value of Equation 3. FIG. 3 illustrates the case where the antenna beam on the receiving side is scanned in the θ direction. However, it is possible to scan in any direction of (θ, φ), and also in the case of scanning both the transmitting and receiving antenna beams. It is the same. FIG. 3A shows the magnitude relationship between the antenna gain in the beam direction k−1, k, k + 1 and the θr, k−1, θr, k, θr, k + 1 values of Equation 3. The beam in the k-1 direction has a smaller θ value than the beam in the k direction. Conversely, the beam in the k + 1 direction has a larger θ value than the beam in the k direction.
FIG. 3B illustrates a case where the value of σ0 (θ, φ) A (θ, φ) has a slope that rises to the right with respect to θ. Applying the integral mean value theorem to I (r, k) in Equation 1 to obtain Equation 3, the effect of the value of σ0 (θ, φ) A (θ, φ) simultaneously with the antenna gain is θt , k, φt, k, θr, k, φr, k. As shown in FIG. 3B, when θ has a slope that rises to the right, θr, k is compared to the case where σ0 (θ, φ) A (θ, φ) = 1 in FIG. The magnitude relationship such as the value is as illustrated in FIG.
While calculating the gradient of σ0 (θ, φ) A (θ, φ) by iterative calculation, the calculation accuracy of σ0 (θ, φ) A (θ, φ) can be improved. The flowchart is shown in FIG. If σ0 (θ, φ) A (θ, φ) is obtained for the lattice point (i, j) in FIG. 3 (c), its coordinates change by repeated calculation. The convergence determination is performed based on the residual norm or the upper limit of the number of iterations. Method that uses theoretical values, model values, and observed values as f (θ, φ) but does not use iterative calculation is set as Method 1, and θt, k, φt, k, θr calculated based on f (θ, φ) , k, φr, k are used in the calculation of σ0 (θ, φ) A (θ, φ). Method 2 is a method for obtaining σ0 (θ, φ) A (θ, φ) by increasing the accuracy of θt, k, φt, k and θr, k, φr, k by iterative calculation. The coordinate values obtained by this calculation are (r ′, θt, k, φt, k) and (r ′, θr, k, φr, k) on the polar coordinates.
The beam direction represented as (m, n) beam direction can be set while overlapping in any direction. The transmission antenna beam and the reception antenna beam may have different antenna gains and directions. The present invention is not limited to the case where the transmission and reception antennas are at the same position as shown in FIG. Can be applied.

Equation 5 can be expressed as Equation 7 using the coefficient wi, j. (i, j) is the lattice point in FIG. 3C, and is determined by the antenna beam direction, the oversampling position of the data in the range direction, and the like. The value of σ0 (θ, φ) A (θ, φ) at the position of the black point (●) in FIG. 3C can also be considered to be included in the lattice point (i, j) as a part of the region including the point. .

For the term A representing the attenuation, particularly when focusing on the attenuation before and after rainfall (assuming A ′), if the ratio is taken as in Equation 8, the observed value Ps of the ground surface echo without including the variable in Equation 5 It can be represented by (r, k). The case where the amount of attenuation due to water vapor or the like is obtained can be similarly expressed.

In the case of heavy rain or the like, the rain echo is received to the same extent or stronger than the echo from the ground surface. As a method for separating the echo from the ground surface and the rain echo, there is Patent Document 3, but this method has a problem that the attenuation on the propagation path of the radio wave is not considered. If the reflected wave (rain echo, etc.) from the target received in the beam direction k is Pr (r, k), the total received power P (r, k) and the ground surface echo Ps (r, k) It can be expressed as shown in Equation 9. It is assumed that Pr (r, k) and P (r, k) vary with time.

In Patent Document 1, clutter is reduced by using data obtained with a plurality of beams that irradiate one range cell, using the σ0 value obtained regardless of the intensity of the irradiation region (Equation 11 in Patent Document 1). ) In the method of the present invention, the intensity of the irradiation area depending on the antenna gain of each beam is used. Therefore, when the clutter mixed from the beam in the k direction is reduced in Equation 9, the beam from the k direction is used. Use mixed ground surface echoes (determined by σ0A and I ′ (r, k)).

Based on the principle described above, the first embodiment of the present invention (sea surface echo on the sea surface) will be described. The case of the land surface echo on the land surface will also be described later in the second embodiment.
Assuming a flat sea surface, Equation 10 holds geometrically with respect to r ′, θ, φ in FIG. The sea surface along the earth's spherical surface can also be expressed by a simple formula.

From this equation, it can be seen that the integral variables of Equations 1, 3 and 4 are θ and φ (where hs is the altitude from the point directly below the radar).
FIG. 5 is a configuration diagram of the radar apparatus according to the first and second embodiments of the present invention. A pulsed radio wave 11b is transmitted from the transmitter 13b through the transmission antenna 12b, and the reflected wave 11a is received through the reception antenna 12a. This operation is performed for each polarization and each transmission / reception frequency. The transmission / reception polarization is selected from horizontal / vertical polarization and right / left-hand circular polarization. The frequency of radio waves to be transmitted / received is selected so that the attenuation characteristics due to water vapor, precipitation, etc. are different. The receiver 13a receives the received wave and the reference signal 16 from the transmitter. The transmitting / receiving antenna beam is formed by mechanical scanning, electrical scanning, digital beam forming, or the like. The -6 dB width of the transmission / reception pulse in Equation 2 can be defined by the -6 dB width of the BPF output pulse waveform when the transmission pulse is input to the bandpass filter (BPF) of the receiver. Radio wave transmission / reception and beam direction control are performed by the controller 15, and the signal processing unit 14 performs data collection, coordinate determination, database reference / calculation.
As shown in FIG. 3C, the antenna beam scans in the direction of coordinates (m, n) in a manner that can overlap with the adjacent antenna footprint. The signal processing device 14 accumulates data for each beam direction and each range (including oversampling in the range direction). Radar position is obtained by positioning satellite data, inertial navigation system, etc.
A flowchart of the processing in the signal processing device 14 is shown in FIG.

Next, a second embodiment of the present invention will be described.
In the case of an echo from the ground surface with unevenness like the land surface, it is necessary to correct tens (in the case of the sea surface) according to the unevenness. Even if there are irregularities along the earth's spherical surface, it can be expressed by a simple formula. If the height (Gh) from the plane is used as shown in FIG. 2 of Patent Document 2 (corresponding to θ1 → θ and θ2 → θ ′ in the figure), θ in Equation 10 becomes θ ′ in Equation 11. Here, h's = hs-h, h is the height from the spheroid model such as the Earth's geoid or GRS80 ellipsoid to the plane of the figure.

DEM (degital Elevation Model) data is used as unevenness data on the ground surface. The unevenness of the ground surface is divided into slices for each range cell, and the portion where the ground surface echo is generated is specified including the region around the range cell.
In the flowchart of FIG. 6, the range bin in which the ground surface echo is mixed is specified by the DEM data from the data received for each beam direction and for each range bin. Next, θt, k, φt, k, θr, k, φr, k are obtained based on the model value and the observed value of σ0 (θ, φ) A (θ, φ). When σ0A is obtained by Method 1 or Method 2, the σ0 value changes depending on the sea, river, desert, forest, and ice surface, so it is possible to detect deviations in the antenna beam direction compared to the database of ground surface classification (covering conditions). . The deviation of the antenna beam direction is caused by an electrical / mechanical error of the antenna or an attitude change of the aircraft / satellite. When a deviation in the antenna beam direction is detected, the antenna gain for correcting the deviation is corrected, and σ0A is calculated again.
When calculating the attenuation amount A ′ due to precipitation, water vapor, etc., it is calculated by Equation 8.
If there are a plurality of polarizations and a plurality of frequencies, the above processing is repeated for the combination.

The industrial applicability of the present invention is useful for measuring radar scattering cross sections on the ground surface and attenuation in propagation paths, mapping the ground surface and reducing ground clutter.

It is a figure which shows the positional relationship of the radar mounted in the aircraft and the artificial satellite, the ground surface, and the target body. It is a figure which shows the relationship between the radar scattering cross section of a ground surface, a range cell, and an antenna gain. It is a figure which shows the relationship between an antenna beam, a range cell, (theta) r, k value of several 3, etc. FIG. It is a flowchart explaining the calculation method of the radar scattering cross section (with attenuation) of the ground surface. It is a block diagram of the radar apparatus in Embodiment 1, 2 of this invention. It is a flowchart of the process in a signal processing apparatus.

1: Radar equipment
2: Antenna beam direction 3: Incident angle 4: Azimuth angle 5: Target body 6: Ground surface at the same distance as the target body (shaded area)
11: Radio waves to be transmitted and received (depending on polarization and frequency)
12: Transmitting and receiving antenna (mechanical scanning / electrical scanning)
13: Transmitter / receiver 14: Signal processing device 15: Controller for controlling each part 16: Reference signal input from transmitter to receiver

Patent 4481078 "Radar equipment" Patent 3660989 “Synthetic Aperture Radar Antenna Pattern Correction Method” US 8138962 METHOD FOR PROCESSING MEASURED VERTICAL PROFILES OF THE POWER OF THE ECHOES RETURNED FOLLOWINGA TRANSMISSION OF RADAR SIGNALS Patent No. 3408943 “Dual-frequency measurement method and multi-frequency radar device”

Claims

Means for transmitting and receiving radio waves by scanning an antenna beam;
In a radar apparatus comprising means for measuring a received signal for each distance and each beam scan,
Means for estimating the radar scattering cross section of the ground surface per unit area by the theorem of the mean value of the integral;
A radar apparatus comprising means for changing an antenna gain for each range cell by the beam scanning.
2. The radar apparatus according to claim 1, further comprising means for calculating coordinates representing a position at which a ground surface echo is generated for each distance and for each beam scanning by calculating a coordinate value according to an integral average value theorem. Radar equipment.
3. The radar device according to claim 1, wherein the radar scattering cross section is compared with a radar scattering cross section by a model depending on an observed value or a ground surface state, and is included in the antenna gain. A radar apparatus comprising means for estimating an error.
2. The radar apparatus according to claim 1, wherein a ground surface echo is calculated based on the radar scattering cross section estimated for each distance and each beam scanning, and the ground surface echo is calculated from a certain distance and the total received power at the time of the beam scanning. A radar apparatus comprising means for reducing ground clutter by subtracting.
A radar apparatus comprising means for estimating non-uniformity such as precipitation based on the radar scattering cross section of the radar apparatus according to claim 1.