RU2709623C1 - Method of obtaining a three-dimensional image in a side-scan radar with synthesizing an antenna aperture - Google Patents

Abstract

FIELD: radar ranging and radio navigation.

SUBSTANCE: invention relates to radio vision systems and can be used to produce a three-dimensional radar image of scene objects with a lateral view with high resolution both in range and angle of azimuth, regardless of weather conditions and level of illumination. High range resolution is ensured by using broadband signals with linear-frequency modulation. High azimuth resolution is obtained by synthesizing the antenna aperture during movement of the radar when observing the scene objects for a long time from different perspectives. Elevation of objects is assessed by frequency scanning with a beam in a vertical plane, which ultimately enables to build a three-dimensional radar image. Platform of the radar is ground transport or low-flyer unmanned aerial vehicles. Objects of the scene are surrounding objects (sidewalks, bushes, trees, people, buildings, other vehicles, etc.). Technical result is achieved by combining partial signals obtained at different positions of the FP antenna as frequency scanning of the beam and addition to the flat image of the azimuth-range of the additional axis along height.

EFFECT: high information value of the radar image owing to additional extraction of information on the altitude profile of surrounding objects.

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Description

The invention relates to radio-vision systems and can be used to obtain a three-dimensional radar image of scene objects in a high-resolution side view, both in range and in azimuth angle, regardless of weather conditions and light level. Obtaining high resolution in range is provided by the use of broadband signals with linear frequency modulation. Obtaining high resolution in azimuth is realized by synthesizing the antenna aperture during the movement of the radar when observing scene objects for a long time from different angles. Assessing the height of objects is carried out using frequency scanning of the beam in a vertical plane, which ultimately allows you to build a three-dimensional radar image. As a radar platform, ground transport or low-flying unmanned vehicles are used. The objects of the scene are the objects surrounding the transport (sidewalks, bushes, trees, people, buildings, other vehicles, etc.).

In [1], a method is known for constructing a radar image in an automobile continuous-wave radar device with aperture synthesis. It is focused on detecting obstacles around the car and can be used for parking. However, in this case, the image is built on a plane in the range-azimuth coordinates, without the possibility of extracting information about the height of the object, which means that the restoration of a three-dimensional picture of the scene is not provided.

In article [2] and patents [3, 4] a method for frequency scanning by a beam of an antenna radiation pattern in a millimeter radar with a continuous frequency-modulated signal is presented. But the issue of synthesizing the aperture and building the final high-resolution three-dimensional image is not addressed.

In [5], a method for forming a three-dimensional image in a synthesized aperture radar is known. It is based on the fact that several receiving antenna elements spaced in height are used in the synthesis. The disadvantage of this method is that it complicates the structure of the system, increases its cost and dimensions.

Closest to the claimed invention is a method of constructing a radar image, described in the description of the invention under the name "Device radar station with a continuous linear frequency-modulated signal and aperture synthesis" [6] and which consists in the use of radar stations (RLS) with a continuous linear frequency -modulated signal to increase the stealth of the radar while simplifying the procedure for compensating the migration of the target signal in range and phase in the synthesis algorithm Ania aperture antenna. The specified result (building a radar image) is achieved by receiving a reflected signal to obtain a beat signal of the reflected signal with a probing signal, demodulating the beat signal using a reference reference signal that compensates for the migration of scene signals in range and phase over the aperture synthesis interval, after which the signals are compressed range and selection of signals of scene elements in a uniform grid of Doppler frequencies. The disadvantage of the prototype is the lack of options for constructing a signal processing algorithm designed to form a three-dimensional radar image of scene objects.

The task to which the proposed technical solution is directed is to increase the information content of the radar image by constructing a high-altitude profile of surrounding objects (performing three-dimensional mapping) while maintaining the relative simplicity of the radar device, small dimensions and low cost for the possibility of mass use in automotive systems and on the basis of unmanned aerial vehicles where dimensions and cost are important.

The solution to this problem is achieved by the fact that in the method of constructing a radar image in radar systems with a synthesized aperture of the side-view antenna, which consists in the fact that a broadband continuous signal with linear frequency modulation is emitted, a reflected signal from the target is received, it is multiplied with a reference signal on the mixer, passes a band-pass filter to isolate the beat signal, is digitized in an analog-to-digital converter and enters the digital processing unit for synthesizing apertures rounds of the antenna, in addition, the received signal is divided into different channels according to the elevation angle according to the law of frequency scanning by the beam of the antenna system in the vertical plane. Separate signal processing is performed in each channel at an elevation angle: the window function is superimposed on the signal in range and azimuth, a two-dimensional Fourier transform is implemented, digital azimuth diagram formation is performed for side view. Then, the received signals in each channel in elevation are multiplied by the corresponding two-dimensional reference function in azimuth, which takes into account the nonlinear change in range to the target by expanding in the Taylor series and storing the first three terms, after which the inverse Fourier transform in azimuth is performed to obtain a focused image in each channel by the corner of the place. At the final stage, the channels are combined in elevation by correspondingly projecting the resulting points into a single Cartesian coordinate system. To do this, knowing the oblique range and the target position in azimuth (Y coordinate), the azimuth angle is calculated by the target, then by multiplying the oblique range by the cosine of the elevation angle and the cosine of the azimuth angle, the distance to the target along the Earth’s surface (X coordinate) is estimated by multiplying the oblique range the height of the object (Z coordinate) is estimated at the sine of the elevation angle. As a result, a three-dimensional image of the surrounding space is formed in XYZ coordinates for ground or low-flying radar platforms, in which the color intensity of the image is proportional to the reflectivity of the targets.

A block diagram of a frequency scanning beam radar is shown in FIG. 1, on which the following notation is adopted: 1 - transmitting antenna (Per); 2 - power amplifier (PA); 3 - a signal generator with linear frequency modulation (LFM generator); 4 - receiving antenna (PR); 5 - low noise power amplifier (LNA); 6 - mixer (SM); 7 - band-pass filter (PF); 8 - amplifier with variable gain (UPA); 9 - analog-to-digital Converter (ADC); 10 - Digital Signal Processing Unit (Processing Unit).

The block diagram of the signal processing algorithm is shown in FIG. 2, on which is indicated: 11 - the formation of K channels of the received signal by elevation; 12 - overlay on the signal window function in range and azimuth; 13 - two-dimensional Fourier transform for fast (along range) and slow (along azimuth) time; 14 is a digital azimuthal charting for a side view; 15 - multiplication of the signal by the reference function in azimuth; 16 is the inverse Fourier transform of slow time; 17 - the combination of channels in elevation to form the final three-dimensional radar image.

Detailed description of the method.

The method is based on the idea of extracting information about the height of objects in a continuous-wave radar with a synthesized aperture due to the frequency beam scanning (ChKL) of the radiation pattern. The principle of CCL is based on the fact that with a change in the frequency of the reference oscillator, the electric distance between the emitters equivalently changes. This leads to a change in phase shifts between them and, as a result, to the formation of a certain slope of the phase front of the antenna array. As a result, the main lobe of the antenna beam is deflected by a predetermined angle.

A functional diagram of the radar is shown in FIG. 1. The signal generator with linear frequency modulation generates a continuous broadband signal of short duration with a periodic linear change in frequency (one-sided saw). Having passed the power amplifier (PA), it is radiated by a transmitting antenna (Per). The transmitting antenna is an antenna array that forms a narrow beam in the vertical plane and wide in the horizontal. In this case, due to a change in the frequency of the LFM signal, the beam of the transmitting antenna changes its orientation in the vertical plane, scanning in a given sector of angles vertically. The emitted signal with a modulation period T and a band Δƒ is recorded as a function of time

k=Δƒ/T - крутизна ЛЧМ модуляции, ƒ₀ - несущая частота.Where

k = Δƒ / T is the slope of the LFM modulation, ƒ ₀ is the carrier frequency.

The signal reflected from the target at a range of R ₀ is fed to the input of the receiving antenna (Pr) and is described by the following expression:

- временная задержка сигнала.where σ characterizes the reflective characteristics of the target,

- time delay of the signal.

The received signal passes through a low-noise power amplifier (LNA) and enters the mixer (SM). In the mixer, the emitted and received signals are multiplied, the output signal is the difference and total frequencies. From the output of the mixer, the signal goes to a band-pass filter (PF), which performs filtering of the difference frequency signal. We get a beat signal:

where A is the equivalent amplitude factor.

This signal is amplified in a variable gain amplifier (VGA), digitized by an analog-to-digital converter (ADC) with a frequency of ƒ, and enters the processing unit. Samples of the digitized signal are recorded as

where Δt = 1 / ƒ _s is the time discretization step.

The receiving antenna array in the vertical plane forms a narrow beam, the position of which changes in elevation with a change in the signal frequency. In the horizontal plane, it has M channels. Thus, the processing unit receives M beat signals, which can be represented by a data matrix of size [M, N], where N is the number of points of the digitized beat signal for one period of signal modulation. It should be noted that the value of M is usually small, because in azimuth it is necessary to provide a wide MD for the possibility of synthesizing a large aperture. In the processing unit, each period of the received beat signal is divided into K time intervals. It turns out that each segment of P = N / K points corresponds to a certain position of the beam in the vertical plane (elevation angle). The choice of the K value depends on a compromise between the relative constancy of the beam position in elevation over the duration of one partial time interval equal to τ = N / (K) _s ) and the desired preservation of an acceptable range resolution, which is equal to δr≈K⋅c / ( 2Δƒ). As a result, we obtain a three-dimensional matrix of values of the digitized signal with the dimension [M, P, K]. A window function is superimposed on this signal matrix to reduce the level of the side lobes of the signal response. Then, in each of the K channels, according to the elevation angle, the Fourier transform is performed in time (along M rows of the signal matrix) and in azimuth (along P columns). As a result, for each of K sections in height (in elevation), a radar image is obtained with good resolution in range and relatively poor in azimuth.

When receiving a pack of S of such radar signals (images), in order to improve the azimuth resolution, the antenna aperture is synthesized for each channel in elevation. The synthesis algorithm used is the Range Doppler Algorithm (RDA). Since a burst of pulses is processed immediately, the beat signal is described by the two-dimensional function (1.1). To simplify the mathematical calculations, the signal envelope in azimuth is assumed to be rectangular in shape with an amplitude level equal to unity.

where t is the time in range (fast time), η is the time in azimuth (slow time), η _c is the time of the onset of zero Doppler; T _a is the synthesis time.

The signal delay depends on the slow time and is calculated by the formula:

where R (η) is the instantaneous slant range; R ₀ is the shortest distance between the radar and the target ν is the speed of the radar.

The geometry of the system is shown in FIG. 3.

The last exponent in (1.1) is known as the residual video phase (RVP). With a small beam width in azimuth and a small bevel angle, it can be neglected, and otherwise eliminated. After eliminating the residual video phase, we obtain the expression

In the RDA algorithm, at the first step, the Fourier transform is performed along the fast time; a signal of the form

Where

Then the Fourier transform is performed along the slow time and a two-dimensional signal is obtained in the frequency domain

It is seen from (1.2) that the image is compressed in range, while the peak of the Kotelnikov function changes its position from the azimuth (from the azimuthal frequency ƒ _η ). The dependence of the range shift on the azimuth is called Range Migration (RCM) and it is equal to:

With a small beam width in azimuth and a small angle of inclination, migration can be neglected, otherwise eliminated. To compress the signal in azimuth, it must be passed through a matched filter, which has the following frequency response:

The support function takes into account a nonlinear change in the range to the target by expanding into a Taylor series and preserving the first three terms [7].

As a result, we obtain a signal of the form:

The inverse Fourier transform along the slow time gives the final image of the i-th point target:

Expression (1.3) characterizes the two-dimensional response, which is the product of two Kotelnikov functions. The position of the maximum of the two-dimensional function along the X axis is proportional to the slant range to the target, and along the Y axis to the position at which zero Doppler was recorded (the radar was opposite the target). The last exponent has a constant phase, but it does not affect the absolute value of the response.

Obtaining a set of two-dimensional images "range-azimuth" for each position of the beam along the elevation angle, the resulting three-dimensional image "range-azimuth-height" is formed. In this case, the projection of the inclined range is carried out taking into account the knowledge of the central angular position of each beam, and the height of the reflected objects is equal to the height of the center of the beam for each range:

where R ₀ is the estimate of the slant range to the target; Y is the target position estimate in azimuth obtained from the image compressed after the synthesis of the aperture; γ is the center of the axis of the beam along the elevation at the current orientation of the beam in the vertical plane; Z is the height of the object.

The geometry of the target location is shown in FIG. 4.

Thus, having synthesized an aperture of estimated oblique range R ₀ to the target and its position in azimuth Y, its coordinate along the X axis and height Z is restored. As a result, from each reflector we get a point in the three-dimensional coordinate space XYZ, the amplitude of which is proportional to the reflective characteristic of the object .

The proposed method increases the information content of the radar image, compared with the system described in the prototype method, since it involves the use of antennas with frequency scanning of the beam with subsequent processing of the received signals, which allows to extract information about the altitude profile of surrounding objects, and therefore build a three-dimensional image. An increase in information content is achieved by combining partial signals received at different positions of the antenna beam as the beam is frequency-scanned and the azimuth-distance of the additional axis in height is added to the flat image in comparison with the prototype method, which provides construction only in the azimuth-range plane.

LITERATURE

1. EU patent No. 3264131 A1, IPC G01S 13/34. A vehicle radar for environmental detection

2. Shishanov S.V., Myakinkov A.V. Possibilities of applying the method of frequency swing of the beam of the antenna radiation pattern in the automobile radar // Information systems and technologies: reports of the international scientific and technical conference, 2014 - P. 42-43.

3. US patent No. 8362946, IPC G01S 13/89. Millimeter wave surface imaging radar system.

4. US patent No. 7019682, IPC G01S 13/89. Imaging millimeter wave radar system.

5. US patent No. 6741202, IPC G01S 13/90. Techniques for 3-dimensional synthetic aperture radar.

6. RF patent No. 266450, IPC G01S 13/90. The device of a radar station with a continuous linear frequency-modulated signal and aperture synthesis.

7. Aircraft radio-vision systems. Monograph. / Ed. G.S. Kondratenkova. - M.: “Radio Engineering”, 2015.

Claims (1)

A method of constructing a radar image in radar systems with a synthesized aperture of the side-view antenna, which consists in the fact that a broadband continuous signal with linear frequency modulation is emitted, a reflected signal from the target is received, it is multiplied with a reference signal at the mixer, a band-pass filter is used to extract the beat signal , is digitized in an analog-to-digital converter and enters the digital aperture synthesis synthesizing processing unit, characterized by the separation of the received si drove to different channels in elevation according to the law of frequency scanning by the beam of the antenna system in the vertical plane, in each channel in elevation, the window function is superimposed on the signal in range and azimuth, two-dimensional Fourier transform is performed, digital azimuth diagram formation is performed for side view, then the received signals in each channel along the elevation angle are multiplied by the corresponding two-dimensional reference function in azimuth, taking into account the nonlinear change in the distance to the target by filling in the Taylor series and saving the first three terms, then the inverse Fourier transform in azimuth is implemented to obtain a focused image in each channel in elevation, at the final stage, the channels are merged in elevation by projecting the resulting points into a single Cartesian coordinate system, for this Knowing the oblique range and target position in azimuth (Y coordinate), the azimuth angle is calculated by the target, then by multiplying the oblique range by the cosine of the elevation angle and cosines from the azimuth angle, the distance to the target along the Earth’s surface (X coordinate) is estimated, multiplying the oblique range by the sine of the elevation angle, the object’s height (Z coordinate) is estimated, as a result, a three-dimensional image of the surrounding space is formed in XYZ coordinates for ground or low-flying radar platforms, in which the intensity The color of the image is proportional to the reflectivity of the targets.

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