RU2792964C2 - Method for generation of radar images in radar station with synthesized antenna aperture with pre-focusing and device implementing it - Google Patents

Abstract

FIELD: radiolocation.

SUBSTANCE: invention relates to radiolocation; it can be used in radio engineering systems mounted on mobile objects to obtain radar images (hereinafter – RI) during remote probing of a ground or water surface. A method is claimed, consisting in probing of a ground or water surface, reception, digitization, and compression by range of reflected signals, formation of a two-dimensional range portrait of a viewing zone based on line-by-line memorization during synthesizing aperture of signals compressed by range in each probing period, formation of samples of counting a trajectory signal from the formed two-dimensional range portrait in accordance with migration rules of point reflectors for each resolution element of the generated RI, focusing, and calculation of brightness values for each resolution element of RI by summing counting in the sample. At the same time, before formation of the two-dimensional range portrait of the viewing zone, readings of an onboard inertial-navigation system on speed, height, and planned coordinates are obtained, using which statistic characteristics of trajectory instability of a radar carrier flight are calculated, and, depending on their values, optimal synthesizing time is selected from table data in memory of an onboard computer, which is used in RI generation. A device for implementation of the method is also claimed.

EFFECT: increase in the quality of generated RI in conditions of trajectory instability of a carrier flight due to automatic selection of optimal time of synthesizing antenna aperture, carried out based on preliminary analysis of statistic characteristics of trajectory instability, without significant increase in computing costs.

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Description

The invention relates to radar and can be used in radio systems installed on mobile objects to obtain radar images (RLI) in the process of remote sensing of the earth (water) surface.

A known method (analogue) of automatic focusing of radar images to a minimum of entropy [School L.A. Air reconnaissance radar systems, interpretation of radar images. Moscow: VVIA im. prof. NOT. Zhukovsky, 2008. S. 214-225], in which the automatic selection of radar carrier flight speed and synthesis time, providing the best resolution of radar images, is carried out by forming a set of radar images of the same terrain for different carrier flight speeds and aperture synthesis time within a priori specified intervals, calculation of the entropy of the received images, selection of the image with the minimum entropy, which is transmitted to the consumer for further processing.

The disadvantage is that the implementation of the method is associated with high computational costs due to the need to generate a large number of radar images for different values of the carrier flight speed and synthesis time, which reduces the rate of delivery of the generated radar images to the consumer.

A known method (analogue) autofocusing radar, based on the compensation of phase fluctuations using micronavigation systems [Kondratenko G.S. Frolov A.Yu. radio vision. Radar systems for remote sensing of the earth M.: Radiotekhnika, 2005. S. 205-213]. The method includes measuring the speed and trajectory of the phase center of the antenna using micronavigation systems, evaluating the law of change in the phase of the trajectory signal, forming a complex phase multiplier, the multiplication of which by the trajectory signal provides compensation for phase fluctuations.

The disadvantage is that the trajectory of the phase center of the antenna is calculated with limited accuracy, as a result of which only partial compensation of phase fluctuations occurs. This makes it necessary to carry out the stage of additional focusing when forming detailed radar images. In addition, the implementation of this method requires the placement of additional high-precision navigation sensors on board the radar carrier.

A device (analogue) that performs the formation, followed by recovery (focusing) radar images [Patent RU 2624460 C1, IPC G01S 13/89, publ. 01/27/2016], including: a receiver, an information storage unit, a unit for accurate navigation information about geographic coordinates, a unit for determining the spatial coordinates of the phase center of the antenna, a unit for relief databases, a matched filter, a corrective filter, a unit for providing a given view type, a unit for generating RLI.

The disadvantage of the device is that the device does not work at a low signal-to-noise ratio and in the presence of significant phase errors in the trajectory signal.

The closest in essence is a method for generating detailed radar images (prototype) [Patent RU 2710961 C1, IPC G01S 13/90, publ. January 14, 2020], based on surface probing, receiving, digitizing and compressing the range of reflected signals, forming a two-dimensional range portrait of the view area based on line-by-line memorization during the time of synthesizing the aperture of the compressed signals in each period of sounding, forming samples of samples of the trajectory signal from the generated two-dimensional range portrait in accordance with the laws of migration of point reflectors for each resolution element of the generated radar image, focusing and calculating the brightness values of each radar image resolution element by summing the readings in the sample.

The closest in essence is a device for generating detailed radar images (prototype) [Patent RU 2710961 C1, IPC G01S 13/90, publ. 01/14/2020], including a reference oscillator, a frequency modulator, a transmitter, a transmitting antenna, a receiving antenna, a receiving device, an analog-to-digital converter, an interpolator, a spectrum calculator, a memory device, a signal sampling device, a multiplier, a reference signal shaper, a demodulator , adder, display device, control device.

The disadvantage of the method and device is that the quality of the generated radar images depends on the duration of the antenna aperture synthesis time, which is manually selected by the decoder operator based on his own experience, taking into account the type and nature of the flight of the radar carrier, as well as weather conditions. This is due to the fact that in the presence of trajectory instabilities in the flight of the radar carrier, an increase in the synthesis time over a certain optimal value does not lead to an improvement in the radar image, but, on the contrary, causes a violation of the coherence of the signals, which, in turn, leads to defocusing of the radar images and a deterioration in their quality. .

The technical result of this invention is to improve the quality of the generated radar images under conditions of trajectory flight instabilities of the carrier due to automatic selection of the optimal time for synthesizing the antenna aperture, carried out on the basis of a preliminary analysis of the statistical characteristics of trajectory instabilities, without a significant increase in computational costs.

The technical result is achieved by the fact that in the known method, which consists in probing the surface, receiving, digitizing and compressing the range of the reflected signals, forming a two-dimensional range portrait of the view area based on line-by-line memorization during the synthesis time of the aperture of the compressed in range signals in each period of sounding, forming samples of samples of the trajectory signal from the generated two-dimensional range portrait in accordance with the laws of migration of point reflectors for each resolution element of the generated radar image, focusing and calculating the brightness values for each radar image resolution element by summing the samples in the sample, according to the invention, additionally before forming a two-dimensional range portrait of the view area receive indications of the onboard inertial navigation system for speed, altitude and planned coordinates, using which the statistical characteristics of the trajectory instabilities of the radar carrier flight are calculated and, depending on their values, the optimal synthesis time is determined.

The technical result is achieved by the fact that in a known device containing a series-connected spectrum calculator, a memory device, a signal sampling device, a multiplier, an adder, as well as a control device and a reference signal generator, in which the first output of the control device is connected to the second input of the signal sampling device, its second output through the reference signal shaper is connected to the second input of the multiplier, and the second output of the signal sampling device is connected to the second input of the storage device; additionally connected in series are a white Gaussian noise sampler, a set of shaping filters, a signal conditioning device, a switch, and a second storage device , an inertial navigation system, a calculator and a second control device, wherein the output of the switch is connected to the input of the spectrum calculator, the first and second outputs of the calculator are connected to the input of the second storage device and the second input of the control device, respectively, and the first, second and third inputs are connected to the outputs of the second storage device, the adder and the inertial navigation system, respectively, in addition, the first, second, third, fourth and fifth outputs of the second control device are connected to the second input of the switch, the first input of the control device, the fourth input of the calculator, as well as the inputs of the white Gaussian sampler noise and a set of shaping filters, respectively.

The essence of the method lies in the fact that, in addition to the formation of a two-dimensional range portrait of the view area, the optimal synthesis time is selected, during which the statistical characteristics of the trajectory instabilities of the radar carrier flight (root-mean-square deviation (RMS) and correlation time) are calculated based on the readings of the onboard inertial navigation systems (INS) with subsequent selection on their basis of the optimal value of the synthesis time from pre-formed tabular values.

The procedure for choosing the optimal synthesis time includes two stages. At the first stage, in advance of performing a radar survey, the values of the optimal synthesis time are calculated for various intensities and correlation times of the trajectory instabilities of the carrier, determined by the root-mean-square deviation of its flight from the rectilinear uniform flight along the planned coordinates, height and speed, as well as the width of the spectrum of their fluctuations. The calculated values of the optimal synthesis time are entered into the memory of the on-board computer in the form of tabular data. At the second stage, in the process of radar survey, based on the information received from the ANN, estimates of the statistical characteristics of the trajectory instabilities of the carrier are calculated and, in accordance with the estimates obtained, the value of the optimal synthesis time is selected from the memory of the onboard computer, which is used in the formation of radar images.

The calculation of tabular values of the optimal synthesis time is carried out using the prototype method by generating radar images of a test scene (Fig. 1), consisting of point reflectors with known coordinates characteristic of the intended observation conditions, followed by calculating the average width of the response of point reflectors along the track range, and represents is the following sequence of actions:

calculation of the impulse responses of the shaping filters based on the initial data for various statistical characteristics of the trajectory instabilities in the flight of the radar carrier;

formation of implementations of trajectory instabilities with different intensity and correlation time according to the planned coordinates, height and flight speed of the carrier by passing white Gaussian noise through shaping filters;

generating a signal at the output of the receiving device, taking into account the presence of trajectory instabilities of varying intensity and time of their correlation, and then obtaining on its basis a radar image of a test scene using the prototype method;

calculation of the average width of the response of point reflectors to radar data (resolution) in terms of the track range;

calculation of the optimal synthesis time based on the obtained values of the track range resolution for different values of the synthesis time, RMS of the trajectory flight instabilities of the radar carrier and their spectrum width.

The calculation of the impulse responses of the shaping filters is carried out in accordance with the expression

- оператор обратного преобразования Фурье; H(ƒ)=A(ƒ)ехр(jϕ(ƒ)) - частотная характеристика фильтра;

ϕ(ƒ)=-πƒ/Δƒ - линейная фазо-частотная характеристика, обеспечивающая постоянство групповой задержки фильтра;

- спектральная плотность мощности траекторных нестабильностей; Re{ƒ(x)} и

- операторы выделения действительной части и преобразования Фурье функции ƒ{x) соответственно;

- автокорреляционная функция (АКФ) траекторных нестабильностей, определяемая заданными значениями СКО σ и ширины спектра Δƒ по соответствующим координатам, высоте или скорости, усредненные экспериментальные значения которых полученные для различных типов летательных аппаратов представлены в таблице (фиг. 2).Where

- inverse Fourier transform operator; H(ƒ)=A(ƒ)exp(jϕ(ƒ)) - frequency response of the filter;

ϕ(ƒ)=-πƒ/Δƒ - linear phase-frequency characteristic, ensuring the constancy of the group delay of the filter;

- power spectral density of trajectory instabilities; Re{ƒ(x)} and

- operators of extracting the real part and the Fourier transform of the function ƒ(x), respectively;

- autocorrelation function (ACF) of trajectory instabilities, determined by the given values of RMS σ and spectral width Δƒ for the corresponding coordinates, height or speed, the average experimental values of which obtained for various types of aircraft are presented in the table (Fig. 2).

Formation of realizations of trajectory instabilities M(t) according to the corresponding coordinates and flight speed of the carrier is carried out by filtering uncorrelated white Gaussian noise with unit dispersion by a shaping filter with an impulse response (1)

where * is the convolution operation.

The formation of the signal at the output of the receiving device, taking into account the presence of trajectory instabilities of varying intensity and time of their correlation, is carried out, for example, by conducting full-scale experiments or for a radar with continuous frequency-modulated radiation, it is calculated using the expression [Kupryashkin I.F., Likhachev V.P. , Ryazantsev L.B. Small-sized multifunctional radar stations with continuous frequency-modulated radiation. Monograph. - M.: Radio engineering, 2020. - 280 p. pp. 64-67]

A_i - сигнал на выходе приемного устройства РЛС и его амплитуда для i-го точечного отражателя тестовой сцены; j - мнимая единица; с - скорость света в свободном пространстве; ƒ_o - начальная частота зондирующего сигнала с линейной частотной модуляцией; μ=T_м/Δƒ_c; Т_м, Δƒ_с - период модуляции и ширина спектра зондирующего сигнала;

- закон изменения наклонной дальности; x_0i, y_0i - координаты i-го точечного отражателя относительно центра кадра тестовой сцены; h_н, V_н - высота и скорость полета носителя РЛС; k=0, 1, …, N_k-1; N_k=int[T_с/T_м] - общее количество зондирований в течение интервала синтезирования апертуры с длительностью Т_с; t∈[0,T_с]; t_м=frac[t/T_м] - время в пределах отдельного периода модуляции зондирующего сигнала; int[x] и frac[x] - целая и дробная часть х соответственно; M_h(t), M_x(t), M_v(t) - реализации траекторных нестабильностей носителя РЛС по высоте, горизонтальной дальности и скорости соответственно, сформированные с использованием процедуры (2).Where

A _i - signal at the output of the radar receiver and its amplitude for the i-th point reflector of the test scene; j - imaginary unit; c is the speed of light in free space; ƒ _o - initial frequency of the probing signal with linear frequency modulation; μ=T _m /Δƒ _c ; T _m , Δƒ _with - the modulation period and the width of the spectrum of the probing signal;

- law of change of slant range; x _0i , y _0i - coordinates of the i-th point reflector relative to the center of the test scene frame; h _n , V _n - altitude and flight speed of the radar carrier; k=0, 1, ..., N _k -1; N _k =int[T _c /T _m ] - the total number of probing during the interval of synthesizing the aperture with a duration of T _with ; t∈[0,T _with ]; t _m =frac[t/T _m ] - time within a single modulation period of the probing signal; int[x] and frac[x] - integer and fractional parts of x, respectively; M _h (t), M _x (t), M _v (t) - realizations of trajectory instabilities of the radar carrier in height, horizontal range and speed, respectively, formed using procedure (2).

Obtaining a radar image of a test scene can be based on the direct convolution procedure, which is implemented in the prototype method and, in the general case, is a matched filtering operation for each m, n-th resolution element within the frame of the generated radar image:

- закон изменения дальности до точечного отражателя, расположенного в m,n-м элементе разрешения; x_0m=x_б+mΔх, y_0n=nΔy-0,5L_y - координаты элемента разрешения РЛИ по горизонтальной и путевой дальности соответственно; х_б - ближняя граница кадра РЛИ; Δх и Δу - расстояние между центрами элементов разрешения по горизонтальной и путевой дальности соответственно; m=0, 1, … N_x-1, n=0, 1, … N_y-1; N_x=int[L_x/Δy]; L_x и L_v - линейные размеры формируемого кадра РЛИ по горизонтальной и путевой дальности соответственно.where S _on (t, R _{m, n} ) is a reference signal, which is a signal with a unit amplitude, the phase of which corresponds to the phase of the demodulated echo signal of a point reflector located in the m, n-th resolution element of the view area;

- the law of changing the range to a point reflector located in the m, n-th element of resolution; x _0m =x _b +mΔх, y _0n =nΔy-0.5L _y - coordinates of the radar image resolution element in horizontal and ground range, respectively; х _b - near border of the radar image frame; Δх and Δу are the distance between the centers of resolution elements in horizontal and ground range, respectively; m=0, 1, … N _x -1, n=0, 1, … N _y -1; N _x =int[L _x /Δy]; L _x and L _v are the linear dimensions of the formed radar image frame in horizontal and track range, respectively.

In practice, less resource-intensive methods of forming radar images are also used, based on the procedures of harmonic analysis, fast convolution, including correction of migration of point reflectors along the range channels, and others.

Calculation of the average width of the turnoff of point reflectors on the radar image according to the track range is carried out at a given level, usually equal to minus 3 dB from the maximum. If it is necessary to take into account the influence of side lobes on the expansion of the response of a point reflector, the level value can be reduced to minus 10 dB from the maximum. In FIG. Figure 3 shows examples of the calculated dependences of the width of the average response of point reflectors δl on the synthesis time for various values of the standard deviation of trajectory instabilities in velocity σ _v . So at σ _v =0.12 m/s, the T _opt value will be 2 s, and at σ _v = 0.24 m/s, the T _opt value will be 1.2 s.

Thus, the choice of the optimal synthesis time provides an increase in the quality of the formed radar images under the conditions of trajectory instabilities of the carrier flight due to a preliminary analysis of the statistical characteristics of trajectory instabilities, without a significant increase in computational costs.

In FIG. 4 shows a block diagram of a device for implementing a method for generating radar images in a radar with a synthetic aperture antenna with pre-focusing.

The device can be implemented using well-known radio elements produced by the industry.

The device implementing the method includes: a radar imaging device, consisting of the first memory device 1, the signal sampling device 2, the first control device 3, the spectrum calculator 4, the multiplier 5, the reference signal shaper 6, the adder 7, and also the switch 8, inertial navigation system 9, signal conditioning device 10, second storage device 11, calculator 12, second control device 13, set of shaping filters 14, white Gaussian noise sampler 15.

The switch 8 is designed to switch the source of the input radar signal for the radar imaging device 3 depending on the stage of operation of the device. Can be made on the basis of digital multiplexers K555KP11 [Shilo V.L. Popular Digital Circuits: A Handbook. Moscow: Radio and communication, 1987. 352 p. S. 147].

The inertial navigation system 9 is designed to obtain highly accurate estimates of the speed, planned coordinates and flight altitude of the radar carrier and can be implemented on the modules of the satellite navigation system or autonomous devices based on measuring the acceleration and angular velocities of the carrier [Kupryashkin I.F., Likhachev V. P., Ryazantsev L.B. Small-sized multifunctional radars with continuous frequency-modulated radiation. Monograph. M.: Radiotehnika, 2020. 280 p. S. 203-209].

The signal conditioning device 10 is designed to generate a test scene signal at the output of the radar receiver in accordance with expression (3). It can be performed, for example, based on the VM14Ya 1892 processor or similar ones [https://multicore.ru/processors-multicore/1892vm14ja].

The storage device 11 is designed to store, provide access for reading and writing tabular values containing information about the synthesis time, providing the minimum width of the point reflector responses for different values of intensity and correlation time of trajectory instabilities. Can be performed on electrically reprogrammable memory devices [Ugryumov E.P. Digital circuitry. St. Petersburg: BHV-Peterburg, 2007. 800 p. S. 269-287].

Calculator 12 is designed to calculate the average width of the response of point reflectors at the first stage of device operation, as well as to calculate the values of standard deviations and the correlation time of trajectory instabilities at the second stage of device operation. It can be made on the basis of a microcontroller of the STM32 or AVR32 series [Kupryashkin I.F., Likhachev V.P., Ryazantsev L.B. Small-sized multifunctional radars with continuous frequency-modulated radiation. Monograph. M.: Radiotehnika, 2020. 280 p. S. 187].

The second control device 13 is designed to generate control signals for the switch 8, trigger signals for the radar imaging device, white Gaussian noise sampler 15 and filter selection from a set of shaping filters 14. It can be performed, for example, on the basis of a programmable logic device [Ugryumov E. P. Digital circuitry. St. Petersburg: BHV-Peterburg, 2007. 800 p. S. 543-544].

A set of shaping filters 14 is designed to form the implementation of samples of trajectory instabilities with the required intensity and correlation time in accordance with expression (2). It can be made in the form of filters with a finite impulse response, implemented, for example, on programmable logic integrated circuits [Ugryumov E.P. Digital circuitry. St. Petersburg: BHV-Peterburg, 2007. 800 p. S. 553-559].

The white Gaussian noise sampler 15 is designed to obtain signal samples with a random distribution of amplitudes according to the normal law with unit dispersion. Can be performed, for example, on the basis of a software generator [Lyons R. Digital signal processing: Second edition. M.: Binom-Press LLC, 2006. 656 p. S. 509-511].

The operation of the radar imaging device corresponds to the description given in the prototype.

The operation of the device that implements the method does not differ from the operation of the prototype except for the following. At the first stage, in advance of performing a radar survey, for example, by a signal from the operator, the device calculates the values of the optimal synthesis time for various intensities and correlation times of the trajectory instabilities of the carrier. To do this, the switch 8, by a signal from the second control device 13, connects the output of the signal forming device 10 to the input of the radar imaging device, the operation of which does not differ from the prototype, after which the signal from the second control device 13 starts the white Gaussian noise sampler 15, the signal from the output which is fed to the input of a set of shaping filters 14. The impulse responses of shaping filters are pre-calculated for different intensities and correlation times of trajectory instabilities in accordance with expression (1). By a signal from the second control device 13, a filter is selected from a set of shaping filters 14, thereby forming the implementation of the samples of trajectory instabilities with the required intensity and spectral width in accordance with expression (2). Based on the generated samples of trajectory instabilities, the signal generating device 10 generates a signal corresponding to the signal at the output of the test scene receiving device in the presence of trajectory instabilities in accordance with expression (3). Based on the generated signal coming through the switch 8, the radar imaging device generates radar images of the test scene for various values of the aperture synthesis time and various intensities of trajectory instabilities. The generated radar images are sent to the computer 12 to calculate the average width of the response of point reflectors and determine the time value at which their minimum width is ensured. The obtained values of the synthesis time, providing the minimum width of the responses of a point reflector for different values of intensity and correlation time of trajectory instabilities in the form of tabular values are recorded in the memory 11.

In the process of radar shooting, for example, on a signal from the operator, the second control device 13 sends a signal to the switch 8, which connects the output of the radar receiver to the input of the radar imaging device. The readings of the coordinates, height and speed of the radar carrier during flight from the inertial navigation system 9 are fed to the calculator 12, in which the values of standard deviations and the correlation time of trajectory instabilities are calculated. In accordance with the calculated values, the optimal synthesis time is selected from a table stored in the memory device 11, which is transmitted to the first control device 3 of the radar imaging device. Next, the radar imaging device, based on the selected value of the optimal synthesis time, generates a radar image, which is transmitted to the consumer.

Claims (2)

1. A method for generating radar images in a radar with a synthesized aperture antenna with preliminary focusing, which consists in probing the earth (water) surface, receiving, digitizing and compressing the range of the reflected signals, forming a two-dimensional range portrait of the view area based on line-by-line memorization during the aperture synthesis time compressed in range in each period of signal probing, sampling of the trajectory signal samples from the generated two-dimensional range portrait in accordance with the laws of migration of point reflectors for each resolution element of the generated radar image, focusing and calculating the brightness values for each radar image resolution element by summing the samples in the sample, which differs by the fact that before performing a radar survey, the values of the optimal synthesis time are calculated for various intensities and correlation times of the trajectory instabilities of the carrier, determined by the root-mean-square deviation (RMS) of its flight from a rectilinear uniform flight along the planned coordinates, height and speed, as well as the width of the spectrum of their fluctuations, for which form radar images of a test scene consisting of point reflectors with known coordinates characteristic of the expected observation conditions, calculate the impulse responses of the shaping filters based on the initial data for various statistical characteristics of the trajectory instabilities of the radar carrier flight, form realizations of trajectory instabilities with different intensity and time correlations in plan coordinates, height and flight speed of the carrier by passing white Gaussian noise through shaping filters, form a signal at the output of the receiving device, taking into account the presence of trajectory instabilities of different intensity and time of their correlation, and based on it a radar image (RLI) of the test scene is obtained, calculate the average width of the response of point reflectors to radar data over the path range, calculate the synthesis time based on the obtained values of the resolution over the path range for different values of the synthesis time, the RMS of the trajectory instabilities in the flight of the radar carrier and their spectrum width, and take the antenna aperture as the optimal synthesis time synthesis time, which provides the minimum width of the point reflector responses for different values of the intensity and correlation time of trajectory instabilities, the calculated values of the optimal synthesis time are entered into the memory of the onboard computer and in the form of tabular data, then in the process of radar survey, before the formation of a two-dimensional range portrait of the view area, additional readings are obtained from the onboard of the inertial navigation system in terms of speed, altitude and planned coordinates, using which the statistical characteristics of the trajectory instabilities of the flight of the radar carrier are calculated and, depending on their values, the optimal synthesis time is selected from the tabular data stored in the memory of the onboard computer, which is used in the formation of the radar image. 2. A device for generating radar images in a radar with a synthesized aperture antenna with preliminary focusing, containing a spectrum calculator connected in series, a first memory device, a signal sampling device, a multiplier, an adder, as well as a first control device and a reference signal shaper, in which the first output of the first device control device is connected to the second input of the signal sampling device, its second output through the reference signal shaper is connected to the second input of the multiplier, and the second output of the signal sampling device is connected to the second input of the first memory device, characterized in that it additionally contains serially connected white Gaussian samplers noise, a set of shaping filters, a signal conditioning device, a switch, as well as a second storage device, an inertial navigation system, a calculator and a second control device, while the output of the switch is connected to the input of the spectrum calculator, the first and second outputs of the calculator are connected to the input of the second memory device and the second input of the first control device, respectively, and the first, second and third inputs are connected to the outputs of the second storage device, adder and inertial navigation system, respectively, in addition, the first, second, third, fourth and fifth outputs of the second control device are connected to the second input of the switch , the first input of the first control device, the fourth input of the calculator, as well as the inputs of the white Gaussian noise sampler and the set of shaping filters, respectively.

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