An improved method of scanning monopulse forward looking imaging

Abstract

The application relates to an improved forward-looking imaging method of a single-pulse radar, and discloses an improved forward-looking imaging method of a single-pulse radar. The application aims to solve the problems of low azimuth resolution, large angle measurement error and blurred imaging of actual measured scene data in the prior art of forward-looking imaging based on a single-pulse amplitude comparison method. The process comprises the following steps: 1, continuously obtaining echo signals of target scattering points by a receiving antenna in scanning to establish sum signals and difference signals; 2, obtaining a new two-dimensional matrix; 3, obtaining pulse compression echo results of the sum channel and the difference channel; 4, constructing a sum-difference ratio of the three processed signals, then performing angle measurement by a phase comparison method to obtain deflection angles of the target scattering points, and obtaining gray values of target distance-pulse domains based on the deflection angles and distances between the target scattering points and the center of the antenna; transforming the distance-pulse domains into distance-azimuth domains; and 5, performing pulse-by-pulse non-coherent accumulation on the converted distance-azimuth domains to redraw a forward-looking image. The application is used in the field of radar imaging.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of radar imaging, in particular to an improved forward-looking imaging method of monopulse radar. BACKGROUND

[0002] According to the working characteristics of synthetic aperture radar, its working mode is to image the targets on the left and right sides of the platform motion direction, and cannot image the forward-looking area, thus having an inherent blind area. In the imaging of the front end of the airport during the landing of the airplane, the imaging of the target by the missile guidance, and the like, the forward-looking imaging is an extremely sensitive imaging field.

[0003] In the implementation of the forward-looking imaging, the original method is the real-beam imaging method, which has low resolution and short detection distance, and especially the azimuth resolution is restricted by the size of the antenna aperture. Later, with the in-depth study of the forward-looking imaging, scholars proposed other forward-looking imaging algorithms, such as the SIREV (Sight Image Enhanced Viewing), the bistatic SAR forward-looking imaging, the real-aperture imaging based on the convolution theory, and the monopulse imaging (MT).

[0004] The monopulse angle measurement technology can accurately estimate the angle information of the target, and in combination with the high-precision one-dimensional ranging information, the coordinates of the target scattering points can be obtained, and then the image of the target can be obtained, that is, the monopulse forward-looking imaging algorithm. In the forward-looking imaging area, the monopulse forward-looking imaging algorithm can better distinguish the targets with obvious features, and the imaging diagram will be clearer. In addition, the monopulse forward-looking imaging algorithm is simple, has no specific requirements for the motion trajectory of the carrying platform, is suitable for real-time imaging, and has considerable prospects in engineering applications.

[0005] The monopulse angle measurement is divided into the amplitude comparison method and the phase comparison method, and based on this, two monopulse forward-looking imaging algorithms are generated. The amplitude comparison method is to obtain the angle information according to the amplitude difference of the echo signals arriving at two receiving antennas, and the phase comparison method is to determine the angle information according to the phase difference of the received echo signals.

[0006] When the monopulse radar is used for imaging, the resolution is limited by the radar beam, and is generally not very high, so that the image quality is general. The scanning system can improve the resolution of the radar system. When the radar beam scans in the azimuth direction of the imaging area, the target will gradually enter the beam irradiation area, and then gradually exit. With the continuous scanning, the contour of the target scattering point will be obtained, so as to improve the image quality and make the detail information more abundant. The application of the scanning system to the monopulse radar can improve the azimuth resolution of the image and obtain a clearer forward-looking image.

[0007] In the acquisition of the measured data of the scene, after processing, the imaging result obtained by using the traditional monopulse forward imaging algorithm is blurred, and the feature points cannot be effectively extracted, because the measured scene is rich in a large amount of background noise, the azimuth resolution is reduced by using the amplitude comparison method, the angle measurement is inaccurate, and the measured scene image is distorted. Therefore, it is necessary to explore a monopulse algorithm capable of clearly imaging target data. SUMMARY

[0008] The purpose of the present application is to solve the problems of low azimuth resolution, large angle measurement error and blurred imaging of measured scene data in the prior art based on monopulse amplitude comparison method for forward imaging, and an improved scanning monopulse forward imaging method is provided.

[0009] The specific process of the improved scanning monopulse forward imaging method is as follows:

[0010] Step one, within the imaging time T, two receiving antennas scan the target area at intervals, and the scanning angular velocity of the antennas is ω, the scanning range is (θ min ,θ max ), there are N target scattering points P n , n = 1, 2, …, N in the space, the receiving antennas continuously obtain the target scattering point echo signals in the scanning, and the sum signal and the difference signal are established through the sum and difference channels;

[0011] Step two, the established sum signal and difference signal are screened, and the signals with non-zero degrees in the beam center are removed to obtain a new two-dimensional matrix;

[0012] Step three, the new two-dimensional matrix obtained in step two is matched filtered and range migration corrected to obtain the pulse compression echo results of the sum and difference channels;

[0013] Step four, the signals processed in step three are constructed into a sum-difference ratio, and then the angle is measured by the phase comparison method to obtain the deflection angle θ n of the nth target scattering point P n , based on the deflection angle θ n and the center distance R n between the target scattering point and the antenna, the gray value of the target range-pulse domain is obtained; the range-pulse domain is transformed to the range-azimuth domain;

[0014] Step five, the converted range-azimuth domain is incoherently accumulated pulse by pulse to reduce the interference of noise, and the forward view image is redrawn.

[0015] The beneficial effects of the present application include:

[0016] 1. The present application is based on the scanning system monopulse forward imaging, which improves the azimuth resolution, improves the angle measurement progress and improves the image quality.

[0017] 2、The application filters echo data during pretreatment, saves calculation amount, avoids redundant angle measurement, and the algorithm is more simple and efficient.

[0018] 3、The angle measurement cumulative imaging by the phase comparison method adopted by the application can suppress clutter and noise, and also avoids multiple scattering points in the same range gate, and the problem of affecting estimation accuracy. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The method flowchart of the application is shown in the figure;

[0020] Figure 2 The single pulse imaging schematic diagram used in the application is shown in the figure;

[0021] Figure 3 The angle measurement schematic diagram by the phase comparison method is shown in the figure;

[0022] Figure 4 The SAR original reference diagram in Example 1 is shown in the figure;

[0023] Figure 5 The traditional single pulse forward-looking imaging effect diagram in Example 1 is shown in the figure;

[0024] Figure 6 The forward-looking imaging effect diagram of the method of the application in Example 1 is shown in the figure;

[0025] Figure 7 The SAR original reference diagram in Example 2 is shown in the figure;

[0026] Figure 8 The traditional single pulse forward-looking imaging effect diagram in Example 2 is shown in the figure;

[0027] Figure 9 The forward-looking imaging effect diagram of the method of the application in Example 2 is shown in the figure. DETAILED DESCRIPTION

[0028] Specific implementation mode one: the specific process of the improved scanning single pulse forward-looking imaging method is as follows:

[0029] Step one, in the imaging time T, two receiving antennas scan the target area at intervals, the scanning angular velocity of the antenna is ω, the scanning range is (θ min ,θ max ), there are N target scattering points P n ,n=1,2, …, N in the space, the receiving antenna continuously obtains the target scattering point echo signal in the scanning, and the sum signal and the difference signal are established through the sum and difference channels respectively;

[0030] Step 2: Filter the established sum and difference signals separately, remove signals with non-zero degrees at the beam center, and obtain a new two-dimensional matrix;

[0031] Step 3: Perform matched filtering and distance migration correction on the new two-dimensional matrix obtained in Step 2 to obtain the pulse compression echo results of the sum and difference channels;

[0032] Step 4: Construct a sum-difference ratio from the signal processed in Step 3, and then perform angle measurement using the phase comparison method (using formulas 9 and 10 of the phase comparison method principle) to obtain the nth target scattering point P. n deflection angle θ n (Formula 10), based on the deflection angle θ n and the distance R between the high-resolution target scattering point and the center of the antenna n (Measured), obtain the grayscale value of the target range-pulse domain; transform the range-pulse domain to the range-azimuth domain (range domain, pulse domain, and azimuth domain are coordinate axes that can be represented in the simulation image; the target range can be obtained from the time delay of the received echo; the number of radar transmitted pulses can be designed during simulation. The angle obtained through formula 10 can transform the pulse domain to the azimuth domain, which is equivalent to the transformation of the coordinate axes).

[0033] Step 5: Perform pulse-by-pulse incoherent accumulation on the converted range-azimuth domain to reduce noise interference and redraw the front view image.

[0034] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that: in step one, within the imaging time T, the two receiving antennas scan the target area at intervals. Let the scanning angular velocity of the antenna be ω, and the scanning range be (θ). min ,θ max There are N target scattering points P in space. n For n = 1, 2, ..., N, the receiving antenna continuously acquires the echo signal from the target scattering point during scanning, and establishes the sum and difference signals respectively after passing through the sum and difference channels; the specific process is as follows:

[0035] Assume the radar transmits a linear frequency modulated signal s(t);

[0036] s(t)=ρexp(j2πf c t) (1)

[0037] In the formula, j is the imaginary unit, j 2 =-1; f c ρ is the carrier frequency of the radar transmitted signal, t is time, and ρ is the signal amplitude.

[0038] The reflection coefficient of each target scattering point is σ n The distance between the target scattering point and the center of the antenna is R. n ;

[0039] Let the time required for one transmission and reception pulse be Δt, and the angle through which the receiving antenna scans be Δα;

[0040] Δα = Δt x ω (2)

[0041] During the scanning rotation of the receiving antenna, let the deflection angle of the beam center of the receiving antenna when it receives the echo signal of the target scattering point be α, and the deflection angle of the beam center of the receiving antenna when it receives the echo signal of the nth target scattering point Pn be αn, then the angle between the beam center and the nth target scattering point Pn is θn = αn - α. n n -α;

[0042] θn = αn - α (1) n n

[0043] The scanning of the receiving antenna is performed at an angle interval, and the deflection angle of the beam center when the antenna transmits a linear frequency modulation signal is α - Δα.

[0044] After the receiving antenna obtains the echo signal of the target scattering point, the echo signal of the target scattering point is passed through the sum signal channel to obtain the sum signal of the monopulse forward-looking scanning imaging radar as follows:

[0045]

[0046] After the receiving antenna obtains the echo signal of the target scattering point, the echo signal of the target scattering point is passed through the difference signal channel to obtain the difference signal of the monopulse forward-looking scanning imaging radar as follows:

[0047]

[0048] In the formula, F(α) is the sum beam antenna pattern of the antenna after modulation, F(α - Δα) is the difference beam antenna pattern of the antenna after modulation, and C is the physical speed of light. Σ Δ

[0049] The other steps and parameters are the same as in the first embodiment.

[0050] In the third embodiment, the sum signal and the difference signal obtained in step two are respectively screened to remove the signals with a non-zero beam center, and a new two-dimensional matrix is obtained.

[0051] Only the echo signals with a zero beam center are retained for processing. The screening process can use FFT to obtain the frequency spectrum, multiply by a rectangular window in the frequency domain to remove, and then perform IFFT transformation to the time domain. In the formula, FFT and IFFT respectively represent Fourier transform and inverse Fourier transform.

[0052] The specific process is as follows:

[0053] ​​​​​The echo signal of each scan is recorded, and the interval of the antenna scanning angle is usually set to correspond to the number of transmitted pulses within the imaging time. Within the scanning range (θ min ,θ max ), the monopulse forward-looking scanning radar and difference signals obtained in step one are respectively represented as two-dimensional matrices:

[0054] R Σ = s Σ (t i ,a j ) (5)

[0055] R Δ = s Δ (t i ,a j ) (6)

[0056] In the formula, t i is the sampling time interval, and a j is the antenna scanning angle corresponding to different pulse numbers;

[0057] The two-dimensional matrices of the sum and difference signals are respectively subjected to Fourier transform, and the signals are filtered in the frequency domain (multiplied by a rectangular window in the frequency domain) after the Fourier transform. The echo signals with a beam center of zero degrees are screened out, and the echo signals with a beam center of non-zero degrees (false scattering points) are removed. Then, the echo signals with a beam center of zero degrees are subjected to inverse Fourier transform to the time domain to obtain new two-dimensional matrices R Σ1 and R Δ1 .

[0058] The other steps and parameters are the same as those in specific implementation mode one or two.

[0059] Specific implementation mode four: different from one of the specific implementation modes one to three is that the new two-dimensional matrix obtained in step two is subjected to matched filtering and range migration correction in step three to obtain the pulse compression echo results of the sum and difference channels. The specific process is as follows:

[0060] A matching function H1 is constructed, and the new two-dimensional matrices R ∑1 and R Δ1 obtained in step two are subjected to matched filtering (R ∑1 is multiplied by the matching function H1, and R Δ1 is multiplied by the matching function H1);

[0061] Range compression in the range Doppler algorithm is adopted to obtain high resolution in the range direction. The echo signals of the new two-dimensional matrices R ∑1 and R Δ1 obtained in step two are compensated by selecting the matching function H1, that is, matched filtering;

[0062] The matching function H1 is:

[0063] H1 = exp(j4π / C x 9(-v x t s ) x (f r +F C )) (7)

[0064] where C is the physical light speed, v is the radar platform moving speed, t s is the slow time sampling sequence, f r is the signal sampling frequency, and F C is the signal carrier frequency;

[0065] When multiple scattering points are located in the same range cell, after receiving the signal echo, the scattering points will move with the radar platform rotation, which not only affects the resolution of the scattering points in the range direction, but also seriously interferes with the subsequent angle measurement, causing the angle measurement accuracy to decrease. At this time, the compensation function H2 needs to be multiplied to correct the range migration of the echo, and to reduce the error caused by the range migration of the scattering points to the angle measurement;

[0066] The compensation function H2 is constructed to correct the range migration of the echo signal after the matching filtering process (the echo signal after the matching filtering process is multiplied by the matching function H2), so as to reduce the error caused by the range migration of the scattering points to the angle measurement;

[0067] The compensation function H2 is:

[0068] H2 = exp(jπ x K r x t r 2 ) (8)

[0069] where K r is the frequency modulation slope, and t r is the fast time sampling sequence.

[0070] The other steps and parameters are the same as one of the first to third embodiments.

[0071] Embodiment five: The embodiment is different from one of the first to fourth embodiments in that: in the step four, the signal processed in the step three is constructed and the difference ratio is obtained, and then the angle measurement is performed by the ratio phase method (using the ratio phase method principle formula 9, 10) to obtain the deflection angle θ n (formula 10) of the nth target scattering point P n based on the deflection angle θ n and the high-resolution target scattering point and the center distance R n of the antenna (measured), the gray value in the range-pulse domain is obtained; the range-pulse domain is transformed into the range-azimuth domain; the specific process is as follows:

[0072] Figure 3The single pulse phase comparison method is shown in the schematic diagram, and the distance between the beam centers of two antennas is d, and the nth target scattering point P n The deflection angle is θ n Under the far field condition, the wave path difference of the target echo reaching two beams can be approximated as:

[0073] ΔR = d x sin θ n (9)

[0074] The nth target scattering point P n The deflection angle is θ n is:

[0075]

[0076] In the formula, λ is the wavelength; is the phase difference of the scattering point;

[0077] After obtaining the deflection angle of each scattering point of the target, the range-pulse domain pattern is transformed into the range-azimuth domain, and the specific process is as follows:

[0078] The pixel unit of the range-pulse domain is set as The pixel unit of the range-azimuth domain is set as The pixel unit of the range-pulse domain is transformed into the pixel unit of the range-azimuth domain

[0079] Where b represents the azimuth resolution unit sequence, and the expression is:

[0080]

[0081] In the formula, θ0 is the angle when the azimuth resolution unit sequence is 0, β is the antenna beam width, M is a constant, M is generally an integer (used to change the azimuth interval of the image), is the number of distance sampling points, and m is the number of radar transmission pulses,

[0082] is the angle of the corresponding resolution unit.

[0083] The other steps and parameters are the same as those in the first to fourth embodiments.

[0084] Embodiment six: the embodiment is different from one of the first to fifth embodiments, and the converted range-azimuth domain is accumulated pulse by pulse in step five, the noise interference is reduced, and the forward-looking image is redrawn; the specific process is as follows:

[0085]

[0086] In the formula, is the distance between the beam centers of two antennas ​The imaging result after the secondary accumulation, is the gray information of the scattering point in the initial pulse sequence in the range-pulse domain, is the gray information of the scattering point in the initial pulse sequence in the range-azimuth domain, is the gray information of the scattering point in the m-th sequence in the range-azimuth domain. is the gray information of the scattering point in the m-th sequence in the range-azimuth domain.

[0087] The other steps and parameters are the same as those in the first to fifth embodiments.

[0088] The beneficial effects of the present application are verified by the following examples.

[0089] Example 1

[0090] This example aims to compare the imaging effects of the present application and the conventional single-pulse forward-looking imaging on a target scene.

[0091] The radar parameters are designed as shown in Table 1.

[0092] Table 1 Radar system parameters

[0093]

[0094] After screening the radar echoes, there are 150 pulses of the echoes with the beam center of zero degrees, and only these pulses are subjected to imaging processing. After the angle measurement by the phase comparison method, the accumulated imaging number is 20.

[0095] Example 2

[0096] This example aims to compare the imaging effects of the present application and the conventional single-pulse forward-looking imaging on a target scene.

[0097] The system parameters in Table 1 are still used, except that the number of transmitted pulses is changed.

[0098] Obviously, the image quality obtained by the imaging method of the present application on the actually measured target scene data is much higher than that of the conventional single-pulse forward-looking imaging. Not only the azimuth resolution is improved, but also the noise and clutter suppression effects are obvious. The target features are more obviously characterized, and the target details are greatly preserved.

[0099] The present application also has other various embodiments. Those skilled in the art can make various corresponding changes and modifications according to the present application without departing from the spirit and essence of the present application, and these corresponding changes and modifications should all belong to the protection scope of the claims attached to the present application.

Claims

1. An improved scanning single-pulse forward-looking imaging method, characterized in that: The specific process of the method is as follows: Step 1: Within the imaging time T, the two receiving antennas scan the target area at intervals. Let the scanning angular velocity of the antennas be ω, and the scanning range be (θ). min ,θ max There are N target scattering points P in space. n n = 1, 2, ..., N, the receiving antenna continuously obtains the echo signal of the target scattering point during scanning, and establishes the sum signal and the difference signal respectively after passing through the sum and difference channels; Step 2: Filter the established sum and difference signals separately, remove signals with non-zero degrees at the beam center, and obtain a new two-dimensional matrix; Step 3: Perform matched filtering and distance migration correction on the new two-dimensional matrix obtained in Step 2 to obtain the pulse compression echo results of the sum and difference channels; Step 4: Construct a sum-difference ratio from the signal processed in Step 3, and then perform phase comparison angle measurement to obtain the nth target scattering point P. n deflection angle θ n Based on the deflection angle θ n and the distance R between the target scattering point and the center of the antenna n Obtain the grayscale value of the target distance in the pulse domain; Transform the range-pulse domain to the range-azimuth domain; Step 5: Perform pulse-by-pulse incoherent accumulation on the converted range-azimuth domain to reduce noise interference and redraw the front view image; In step one, within the imaging time T, the two receiving antennas scan the target area at intervals. Let the scanning angular velocity of the antennas be ω, and the scanning range be (θ). min ,θ max There are N target scattering points P in space. n For n = 1, 2, ..., N, the receiving antenna continuously acquires the echo signal from the target scattering point during scanning, and establishes the sum and difference signals respectively after passing through the sum and difference channels; the specific process is as follows: Assume the radar transmits a linear frequency modulated signal s(t); s(t)=ρexp(j2πf c t) (1) In the formula, j is the imaginary unit, j 2 =-1; f c The carrier frequency of the radar transmission signal is t, and time is t. ρ is the signal amplitude; The reflection coefficient of each target scattering point is θ n The distance between the target scattering point and the center of the antenna is R. n ; Let Δt be the time required to complete one transmit and receive pulse, and Δα be the angle through which the receiving antenna scans. Δα=Δt×ω (2) During the scanning rotation of the receiving antenna, if the deflection angle of the antenna beam center is α when the receiving antenna receives the echo signal from the target scattering point, then the nth target scattering point P... n The angle between the beam center and the direction of the beam is θ n -α; θ n For the nth target scattering point P n Deflection angle; The receiving antenna scans at angular intervals, and the deflection angle of the beam center when the antenna transmits a linear frequency modulated signal is α-Δα. After the receiving antenna obtains the target scattering point echo, it passes the target scattering point echo through the signal channel to obtain the single-pulse forward-looking imaging radar signal: After the receiving antenna obtains the target scattering point echo, the target scattering point echo is passed through the difference signal channel to obtain the single-pulse forward-looking imaging radar difference signal: In the formula, F ∑ The antenna pattern after modulation and the beam pattern are shown in Figure F. Δ The image shows the radiation pattern of the differential beam antenna after modulation, where C is the physical speed of light.

2. The improved scanning single-pulse forward-looking imaging method according to claim 1, characterized in that: In step two, the established sum and difference signals are filtered separately to remove signals with non-zero degrees at the beam center, resulting in a new two-dimensional matrix; the specific process is as follows: Within the scanning range (θ) min ,θ max Within this, the sum and difference signals of the single-pulse forward-looking imaging radar obtained in step one are represented as two-dimensional matrices: R ∑ =s ∑ (t i ,a j ) (5) R Δ =s Δ (t i ,a j ) (6) In the formula, t i a is the sampling time interval. j Antenna scanning angles corresponding to different pulse numbers; The sum and difference signal two-dimensional matrices are subjected to Fourier transforms respectively. After the Fourier transform, the signals are filtered in the frequency domain to select echo signals with a beam center of zero degrees and discard echo signals with a beam center of non-zero degrees. Then, the echo signals with a beam center of zero degrees are subjected to an inverse Fourier transform to the time domain to obtain a new two-dimensional matrix R. ∑1 and R Δ1 .

3. An improved scanning single-pulse forward-looking imaging method according to claim 2, characterized in that: In step three, matched filtering and distance migration correction are performed on the new two-dimensional matrix obtained in step two to obtain the pulse compression echo results of the sum and difference channels; the specific process is as follows: Construct a matching function H1 for the new two-dimensional matrix R obtained in step two. ∑1 and R Δ1 Perform matched filtering; The matching function H1 is: H1=exp(j4π / C×9(-v×t s )×(f r +F C )) (7) In the formula, C is the physical speed of light, v is the speed of the radar platform, and t is the speed of light. s For slow-time sampling sequences, f r F is the signal sampling frequency. C For signal carrier frequency; Construct a compensation function H2 to perform distance migration correction on the echo signals after matched filtering; The compensation function H2 is: H2=exp(jπ×K r ×t r 2 ) (8) In the formula, K r For the frequency modulation slope, t r It is a fast-time sampling sequence.

4. An improved scanning single-pulse forward-looking imaging method according to claim 3, characterized in that: In step four, the signal processed in step three is used to construct a sum-difference ratio, and then the phase comparison method is used to measure the angle to obtain the nth target scattering point P. n deflection angle θ n Based on the deflection angle θ n and the distance R between the target scattering point and the center of the antenna n Obtain the grayscale value of the target in the range-pulse domain; transform the range-pulse domain to the range-azimuth domain; the specific process is as follows: Let the distance between the centers of the two antenna beams be d, and the nth target scattering point P n The deflection angle is θ n Under far-field conditions, the path difference between the target echo arriving at the two beams can be approximated as: ΔR=d×sinθ n (9) The nth target scattering point P n The deflection angle is θ n for: In the formula, λ is the wavelength; The phase difference at the scattering points; After obtaining the deflection angles of each scattering point of the target, the range-pulse domain graph is transformed to the range-azimuth domain. The specific process is as follows: The pixel unit in the distance-pulse domain is set as The pixel unit in the distance-azimuth domain is set as pixel units in the distance-pulse domain Pixel units transformed to the range-orientation domain Where b represents the azimuth resolution element number expression is: In the formula, θ0 is the angle when the azimuth resolution element number is 0, β is the antenna beamwidth, and M is a constant, which takes integer values. m represents the number of range sampling points, and m represents the number of radar pulses transmitted. The angle of the corresponding resolution unit.

5. An improved scanning single-pulse forward-looking imaging method according to claim 4, characterized in that: In step five, pulse-by-pulse incoherent accumulation is performed on the converted range-azimuth domain to reduce noise interference, and the front view image is redrawn; the specific process is as follows: In the formula, For the process Imaging results after multiple accumulations This represents the grayscale information of the scattering points in the range-pulse domain for the initial pulse sequence. This represents the grayscale information of the scattering points of the initial pulse sequence in the range-azimuth domain. For the sequence number m, the first... Gray-scale information of scattering points of each distance resolution unit.

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