RU2790930C1 - Method for determining vertical intensity profile of optical turbulence in atmosphere - Google Patents

Abstract

FIELD: meteorology; atmospheric physics.

SUBSTANCE: invention can be used to determine the vertical intensity profile of optical turbulence in the atmosphere. Sensing of the atmosphere is performed using a lidar operating on the effect of backscatter amplification, and a route survey meter. At that, the lidar system consists of a lidar, an echo signal transceiver, an echo signal registration unit, and a computer. The route survey meter consists of a low power continuous wave laser and a receiver. The lidar sensing route has a horizontal section equal to the length of the route survey meter base, located between the lidar and the flat rotary mirror, and a vertical section extending upward from the rotary mirror. The route of the meter is located next to the horizontal section of the lidar route. The atmosphere is sensed in the vertical direction using the lidar. The echo signals received by the lidar transceiver enter the recording unit in the form of photoelectric pulses. The signals are analysed in the recording unit using a discriminator, and then converted into TTL-level signals, which enter the photon counter. The photon counter accumulates signals along the entire sensing route The accumulated information about the spatial distribution of the echo signals of the main and additional receiving channels is transmitted from the recording unit to the computer. The computer calculates the effect of turbulence on the average scattered light power at the receiver. Continuous laser radiation is propagated into a turbulent atmosphere and the light intensity is recorded in the form of electrical signals using the route survey meter receiver. The recorded signals are filtered, digitized and transferred to a computer, where the relative dispersion of light intensity fluctuations is calculated. Then, the structural characteristic of the refractive index is calculated using the Rytov formula and the vertical intensity profile of optical turbulence in the atmosphere is determined.

EFFECT: determination of the vertical intensity profile of optical turbulence in the atmosphere.

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Description

The invention relates to optical instrumentation and can be used in schemes for remote sensing of the intensity of optical turbulence in the atmosphere and to support the operation of adaptive optical systems of astronomical telescopes or aerospace tracking systems.

The main parameter characterizing the intensity of optical turbulence in the atmosphere is the structural characteristic of the refractive index C _n ² .

Known acoustic method for determining the structural characteristics of the temperature C _T ² from which you can determine C _n ² . The acoustic method is based on pulsating measurements of wind speed and temperature in the atmosphere using a bistatic sodar operating on the phenomenon of acoustic wave scattering by atmospheric small-scale turbulent inhomogeneities ( V.F. Kramar et al . Bistatic sodar for studying wind fields and turbulence characteristics in the surface and boundary layers atmosphere, patent RU 2735909 C1). The disadvantages of this method are a small sensing range of several hundred meters.

There is a known method for analyzing the differential jitter of two or four images of a star (DIMM - Differential Image Motion Monitor), built on the same lens ( Sarazin M., Roddier F. The ESO differential image motion monitor // Astron. Astrophys. 1990. V. 227, N 1. P. 294-300). The disadvantage of this method is the inability to obtain a vertical distribution of characteristics C _n ² , because DIMM way is integral.

An improved MASS-DIMM (MASS - Multi Aperture Scintillation Sensor) method is known, in which the DIMM device is supplemented with annular apertures for measuring the vertical distribution of the C _n ² characteristic ( Kornilov V., Tokovinin A., Vozyakova O., Zaitsev A., Shatsky N. , Potanin S., Sarazin M. MASS: a monitor of the vertical turbulence distribution // Proc. SPIE. 2003. V. 4839. P. 837-845). The disadvantage of this method is the low spatial resolution and insensitivity to the turbulent layer near the earth's surface.

A known method for determining the altitude profile C _n ² (z) by triangulation, when the correlation of the intensity of twinkling of two stars with a known angular distance between them (SCIDAR - SCIntillation Detection and Ranging) is measured. The SCIDAR method analyzes the cross-correlation function of a binary star ( Vernin J., Roddier F. Experimental determination of two-dimensional spatiotemporal power spectra of stellar light scintillation Evidence for a multilayer structure of the air turbulence in the upper troposphere // J. Opt. Soc. Am. 1973. V. 63. P. 270-273). The disadvantage of this method is the need to use a large telescope (at least 1.5 m) and the difficulty in finding double stars with a large angular separation and the required brightness.

There is a known method for determining the altitude profile C _n ² (z) by triangulation, when a double star is used to measure the cross-correlation function of wavefront slopes (SLODAR - SLOpe Detection And Ranging) using a Shack-Hartmann sensor ( Wilson RW SLODAR: measuring optical turbulence altitude with a Shack-Hartmann wavefront sensor // Mon. Not. R. Astron. Soc. 2002. V. 337, Iss. 1. P. 103-108). The disadvantage of this method is also the need to use a large telescope (at least 1.5 m) and the difficulty in finding double stars with a large angular separation and the required brightness.

The closest in technical essence to the proposed invention is a lidar method for remote measurement of the intensity of optical turbulence, in which the differential DIMM method is combined with the use of a laser reference star ( Gimmestad GG, Roberts DW, Stewart JM, Wood JW Development of a lidar technique for profiling optical turbulence / / Opt. Engin 2012. V. 51, N 10. P. 101713-1-101713-18). The prototype works as follows. The pulsed laser beam is focused at a given height using the telescope of the lidar transmitter, creating a bright artificial star. The lidar receiving system simultaneously registers four images of the star using a mask located in front of the receiving telescope. Due to turbulence, the images move relative to each other in time. By recording and analyzing the time series of relative displacements of an artificial star, one can estimate the intensity of optical turbulence in the atmosphere. The disadvantages of this method are a small sensing range (less than 2 km) and low efficiency due to the need to move the artificial star in height to obtain a profile of the structural characteristic of the refractive index C _n ² .

The technical result of the claimed invention is the determination of the vertical profile of the structural characteristic of the refractive index C _n ² .

The technical result of the invention is achieved by using a trace meter of the structural constant of the refractive index C _n ² , a turbulent lidar and a flat rotating mirror, due to which the lidar probing trace has a horizontal section equal to the length of the trace of the meter. Joint sounding of the atmosphere by lidar and a track meter is carried out as follows. The lidar sensing path is organized in such a way that at first the laser beam propagates parallel to the earth's surface for a distance equal to the path length of the path meter C _n ² , and then the lidar path is directed vertically upwards by a rotating flat mirror. The lidar transmitter sends laser pulses into the atmosphere and receives echoes. The echo signals of the main and additional receiving channels are recorded by photodetectors in the photon counting mode, then, in the form of electrical single-electron pulses, they enter the recording system, where they are accumulated. The work of the trace meter C _n ² is organized as follows. The tracer laser is next to the turbulent lidar, and the tracer receiver is next to the tilting mirror that directs the lidar sounding trace vertically upwards. The laser beam of continuous radiation of the path meter passes near the horizontal section of the lidar path. The receiver of the path measurer registers and digitizes electrical signals proportional to the intensity of laser radiation along the propagation path. The accumulated lidar echo signals and tracer signals are transferred to a computer, where they are jointly processed ( Razenkov I.A. Heuristic approach to determining the structural characteristic C _n ² from lidar data. // Optics of the atmosphere and ocean. 2022. V. 35. no. 03, pp. 195-204, DOI: 10.15372/AOO20220304). The result of the system operation is the determination of the altitude distribution of optical turbulence intensity with a high spatial resolution directly from the earth's surface.

The peculiarity of the method lies in the fact that for the first time a system operating on the effect of backscatter amplification is used for remote control of the intensity of turbulence. The backscattering enhancement effect occurs during double (forward and backward) propagation of optical radiation in a turbulent atmosphere ( Vinogradov A.G., Gurvich A.S., Kashkarov S.S., Kravtsov Yu.A., Tatarsky V.I. “Regularity increase in the backscattering of waves". Certificate of discovery No. 359. Priority of discovery: August 25, 1972 in terms of theoretical justification and August 12, 1976 in terms of experimental evidence of the pattern. State Register of Discoveries of the USSR // Bull. Inventions. 1989. No. 21 .) Analogues work on other principles and do not use the effect of backscatter amplification. Unlike analogues, the advantages of the system lie in the ability to control the intensity of turbulence with high spatial resolution directly from the earth's surface.

The achievement of the technical result in the proposed invention is ensured through the use of a turbulent lidar, a path meter for the structural constant of the refractive index C _n ² and specialized processing of the received information in real time.

. Вычисляют структурную характеристику коэффициента преломления согласно алгоритму (Разенков И.А. Эвристический подход к определению структурной характеристики C _n ² из лидарных данных. // Оптика атмосферы и океана. 2022. Т. 35. №03. С.195-204. DOI: 10.15372/AOO20220304):

, где: C _{n ,0} ² - значение структурной характеристики у поверхности земли, определяемое из измерений трассового измерителя; R - радиус приемо-передающей апертуры лидара;

- волновое число; λ - длина волны;

- масштаб Френеля.The lidar system for monitoring the intensity of turbulence in the atmosphere consists of a transceiver, an echo signal registration unit, and a computer. The lidar sounding path has a horizontal section and a vertical section. Lidar probes the atmosphere in the vertical direction. The echo signals received by the transceiver in the form of photoelectric pulses enter the registration unit, where they are analyzed by the discriminator, then the photoelectric pulses that have passed through the discriminator are converted into standard TTL-level signals. TTL-level signals arrive at the photon counter, which accumulates signals along the entire probing path. The accumulated information about the spatial distribution of the echo signals of the main P ₁ (z) and additional P ₂ (z) receiving channels, where z is the distance from the lidar, is transmitted from the registration unit to the computer. The accumulation time of echo signals in each measurement cycle is 1 min. The computer calculates the factor q(z) of the influence of turbulence on the average scattered light power at the receiver according to the algorithm:

. The structural characteristic of the refractive index is calculated according to the algorithm ( Razenkov I.A. Heuristic approach to determining the structural characteristic C _n ² from lidar data. // Optics of the atmosphere and ocean. 2022. V. 35. No. 03. P. 195-204. DOI: 10.15372/AOO20220304):

, where: C _{n ,0} ² - the value of the structural characteristic at the earth's surface, determined from the measurements of the track meter; R is the radius of the receiving-transmitting aperture of the lidar;

- wave number; λ - wavelength;

- Fresnel scale.

, где

- среднее значение интенсивности света. Затем вычисляют структурную характеристику коэффициента преломления согласно алгоритму (формула Рытова):

, где L - длина трассы.The path measurer of the structural characteristic C _n ² consists of a low-power cw laser and a receiver. The meter trace is located near the horizontal section of the turbulent lidar trace. Continuous laser radiation propagates in a turbulent atmosphere, the path meter receiver registers the light intensity I in the form of electrical signals, which are filtered, digitized and transmitted to a computer. Next, the relative dispersion of light intensity fluctuations is calculated according to the algorithm:

, Where

- average value of light intensity. Then the structural characteristic of the refractive index is calculated according to the algorithm (Rytov's formula):

, where L is the path length.

Figure 1 shows the scheme of sounding turbulent lidar in conjunction with the work of the path meter structural characteristics C _n ² . The lidar transceiver and pathfinder laser are located side by side. Also nearby are a flat rotating mirror, which directs the lidar track vertically upwards, and the receiver of the C _n ² track meter. The horizontal section of the lidar path and the distance between the laser and the receiver of the path measurer of the structural characteristic C _n ² are equal to the value L , which can be from 100 to 500 m.

The principle of operation of a turbulent lidar is based on the backscatter enhancement effect (BSR). The lidar track has a horizontal section L , the size of which coincides with the path length of the track measurer of the structural characteristic C _n ² . The path meter registers and accumulates a series of electrical signals proportional to the intensity of laser radiation I , which has passed through a turbulent path of length L . Electrical signals change randomly over time, and the dispersion of signal deviations is proportional to the intensity of turbulence. The result of the track meter operation is the relative dispersion of intensity fluctuations β ² , from which the intensity of optical turbulence is determined in the form of a characteristic C _{n ,0} ² at the earth's surface on the path L . The turbulent lidar and the path meter work synchronously. The parameter C _{n ,0} ² obtained from the track meter data is used as an input parameter to determine the height profile of the structural characteristic C _n ² (z) from the lidar data.

The present invention will improve the accuracy and reliability of control of the intensity of optical turbulence in the vertical direction and ensure the operation of adaptive optical systems of astronomical telescopes or aerospace tracking systems.

Claims (1)

A method for determining the vertical profile of the intensity of optical turbulence in the atmosphere, carried out by a lidar based on the effect of backscatter amplification, a path meter consisting of a low-power cw laser and a receiver, the lidar sounding path has a horizontal section equal in size to the length of the baseL path meter, the horizontal section is located between the lidar and a flat rotating mirror, which directs the lidar sounding path vertically upwards, the lidar probes the atmosphere in the vertical direction, the echo signals received by the transceiver in the form of photoelectric pulses enter the recording unit, where they are analyzed by the discriminator, then passed discriminator, photoelectric pulses are converted into standard TTL-level signals, which are fed to a photon counter, where signals are accumulated along the entire sounding path, the accumulated information on the spatial distribution of echo signals of the main and additional receiving channels is transmitted from the registration unit to a computer, where the influence factor is calculated turbulence to the average scattered light power at the receiver, the meter trace is located near the horizontal section of the turbulent lidar path, the trace meter receiver registers the intensity light intensityI in the form of electrical signals that are filtered, digitized and transmitted to a computer, the relative dispersion of light intensity fluctuations is calculated according to the algorithm

, then calculate the structural characteristic of the refractive index according to the algorithm

, Where

- wave number,

- wavelength, structural characteristic value

is an input parameter to the algorithm for restoring the height profile of the structural characteristic

from lidar data.

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