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Unfortunately, so far the lidar measurement errors are large. Instrumental methods for their reduction areconsidered.
Теги: lidar methods of the reduction lidar measurement errors лидар методы снижения погрешности лидарных измерений
Almost 30 years separate us from the occurrence of the first books devoted to the laser remote probing of environment [1, 2]. Later, reviews and books which cover the development of lidar equipment have occurred, for example [3, 4]. It seemed that the advantages and disadvantages of lidar probing technologies were well known. But new approaches to the formulation of lidar equation [5, 6] are planned and it should lead us to the creation of improved lidars. Let us consider several of them.
The mobile lidar complex for the remote control of atmosphere has been developed [7]. It contains the platform and following components installed on it: solid-state laser, two TEA (Trancversly Excited Atmospheric) CО2 lasers, telescope, two-coordinate mirror scanner, optical systems of radiation transmission, detecting spectral system; synchronization system, interface unit which includes: drive control unit and unit of accumulation of the data received from sensors, control computing unit, solid-state laser is executed by two-channel adjusting to Ti: Sapphire with the units of Nd emitters; every TEA CО2-laser is equipped with the pair of identical, optically interconnected CО2-lasers (heterodyne and injection), exit connected with the entrance of TEA CО2-laser which jointly form two-channel heterodyne lidar and the optical system of radiation transmission of every channel of two-channel heterodyne lidar consists of: connected with the exit of TEA CО2-laser, first part of the optical path which is connected with the optical key-modulator, second part of optical path, bottom part of telescopic radiation beam expander, half of telescope and scanner lens. The complex has great capabilities but it is quite bulky. In many cases the part of tasks can be solved by simpler device which is suggested by authors [8].
Simplification of the construction of lidar system for atmospheric air quality control is performed at the expense of use of the principle of combination light scattering which allows using only one laser for the determination of the degree of atmospheric air pollution with the molecules of saturated hydrocarbons in the atmosphere above the industrial zone. The lidar system for atmospheric air quality control is constructed on the basis of the principle of combination light scattering. The lidar system consists of the platform with installed on it solid-state laser emitter on yttrium-aluminum garnet with neodymium operating under the conditions of the third harmonic at the wave length of 355 nm, optical system of laser radiation transmission, detecting telescope, unit for data accumulation and control computing unit.
The optical system of laser radiation transmission additionally contains the refractive prism which directs the laser radiation into the studied space area. Detecting telescope is executed in the form of Newton telescope with spherical mirror and lens objective and the unit of data accumulation contains interference light filter for selection of the region of combination light scattering by the studied molecules with lines of hydrocarbon molecules, photoelectric multiplier, scale amplifier, analog-to-digital converter and data acquisition board.
Molecules of saturated hydrocarbons refer to the main pollutants above the industrial enterprise. Therefore, it is not reasonable to use complex equipment for the multi-component analysis of air space above the studied area and it is sufficient to use the lidar system, which is simple in structure, for the determination of degree of atmospheric air pollution with the molecules of saturated hydrocarbons only.
The principle of laser remote probing by the method of combination light scattering consists in the registration of laser radiation, which is combinationally scattered by the molecules of saturated hydrocarbons in the direction of 180° with frequency shift specifically typical for these molecules and determined by their vibration bands.
The optical scheme of the lidar system for atmospheric air quality control is specified in Fig.1. The system functions in the following manner: pulse of the solid-state laser emitter on yttrium-aluminum garnet with neodymium operating under the conditions of the third harmonic at the wave length of 355 nm is directed into the studied space area (target) above the industrial zone. Part of laser radiation is used for the generation of reference signal which sets the time reference and its amplitude sets the energy of laser pulse. Backscattered radiation of combination light scattering by studied molecules is accumulated by the telescope of Newton type, focused through the lens objective passing through the interference light filter on photocathode of photo-multiplier. Voltage pulse is transferred from photoelectric multiplier to the entrance of scale amplifier, then with the help of analog-to-digital converter through the data acquisition board, signal arrives to the computer where information is processed by the standard software and the signal is formed for its transmission through communication channels (Internet, wi-fi etc.).
In order to check the accuracy of measurements, estimations of power of the registered radiation of combination light scattering by the molecules of saturated hydrocarbons were performed on the basis of the lidar equation of combination light scattering by studied molecules.
Obtained results prove that for the set concentration of probed atoms Na and energy of combination scattering which is equal to the energy of 10 photons, the laser radiation wave length λL = 335 nm, which allows performing measurements during the minimum period of time with the laser pulse energy of 1 mJ, is the optimal variant for the detection of saturated hydrocarbons at the distances up to 6 km.
In the whole probing route with the increase of distance from 0.01 to 2 km, the measurement time increases approximately by 4 orders. If during the set time from the distance of R=2.0 km with the set number of pulses, which are transmitted to the atmosphere, the photoelectric multiplier registers more than 10 photons and we can speak about exceeding of molecules concentrations at this distance above the set level. Besides, obtained values of measurement time meet the requirement for the lidar operation speed.
The method provides high accuracy of measurements in the media where molecules of saturated hydrocarbons are the sources of pollution.
Lidar in the mode of sector scanning which is provided at the expense of platform rotation is installed in the industrial zone on the structure which is dominant by height. The lidar system for atmospheric air quality control is intended for the continuous control of content of gaseous saturated hydrocarbons in the studied space area. In case of detection of accidental release or exceeding of the level of maximum permissible concentrations, processing of the results in control computing unit and signal transmission by communication channels take place in real-time mode.
The lidar system for atmospheric air quality control allows performing the remote monitoring of air pollution via the measurement of energy of the pulse reflected from pollutants (target). However, in case of low degree of pollution and long distances to the target, the pulse energy decreases by 103–109 times and becomes commensurable with the noises of electronic devices. As a result, the useful signal might not be recognized in these noises. Thus, the disadvantage of the lidar system for atmospheric air quality control includes the low sensitivity to the small levels of pollution which decreases with the increase of distance to the target.
The specified disadvantage can be corrected by the other lidar system for air pollution control [9]. The lidar system for air pollution control (Fig.2) operates in the following manner. Pulse of laser radiation from the laser is directed towards the pollution area (target) by the rotating prism. The major part of radiation reaches the target and is scattered by it generating information on the level of pollution.
Determination of air pollution level is performed more accurately because the final signal is formed by two radiation fluxes, instead of one flux as it was executed by the authors [8]. Flux from the reflector exceeds the radiation backscattered by the target by one order or more. Therefore, the sensitivity of the suggested system is noticeably higher than the sensitivity of the system of authors [8]. In the particular case of implementation, if the industrial facility (nuclear power station, petroleum refinery, cement plant etc.) refers to the controlled space area with pollutants, the reflector is installed directly at this site, for example, on ventilation pipe. Then, the flux coming out of the pipe is the target. In case if the laser radiation pulse is directed towards the pipe shear, very small portion of radiation is backscattered. If the reflector is installed on the way of the laser radiation scattered by pollutants so that the larger portion of reflected radiation is directed into the detection telescope, then the scattered flux of laser radiation which reaches the detecting telescope 1 grows by many times providing higher sensitivity of the lidar system. Other case of implementation of suggested technical solution can be offered. The reflector is installed on the aircraft (including unmanned aircraft). The aircraft hovers above the most hazardous zone of the controlled space, rotating prism directs the laser radiation on the reflector and it directs the considerable portion of scattered laser radiation into the detecting telescope providing higher sensitivity of lidar system. The larger area of the reflector, the larger flux of scattered laser radiation directed into the detecting telescope. Growth of area of the reflector in the specified examples is limited by the strength of connection of the reflector with ventilation pipe or aircraft lifting force and potential to retain the ability of radiation direction into the detecting telescope in case of wind blasts and other atmospheric phenomena. Optimal shape of the reflector is paraboloid; the target is located in its focus. Flat reflector or corner reflector are acceptable in certain cases. The reflector can be equipped with the selective reflective coatings for the relevant scattering radiation wave lengths.
Efficiency of the suggested model can be demonstrated on the basis of the following example. Laser based on yttrium-aluminum garnet with neodymium and average radiation power of the second harmonic (wave length 0.532 μm) of 1 W serves as the radiation source. Pairs of iodine-131 above the radioactive object are the target. The object is located at the distance of 1 km from the lidar system. The power of radiation backscattered into the detecting telescope without reflector is usually 1–10 μW or less. In case of installation of the reflector with the diameter of 0.5 m near the shear of ventilation pipe, the power of backscattered radiation increases by several orders and reaches 1–10 mW depending on weather conditions and distance from the radiating laser, which allows determining the degree of pollution exceeding from maximum permissible concentrations with larger sensitivity.
Let us consider one more solution [10]. Its task consists in the remote monitoring of radioactive pollution of radiation-hazardous enterprises at the expense of measurement of frequency spectrum of the pulse reflected from the target. The essential conception of suggested solution is illustrated in Fig.3 which also contains the functional diagram of measuring instrument.
Instrument for the radiation level measurement [10] functions in the following manner. Pulse of laser radiation is directed towards the target from the laser by the rotating device. Small portion of radiation is drawn to the photodiode by the beam splitter-3 on the way to the target; this photodiode forms electrical synchronizing pulse which arrives on the entrance of oscillograph. The major portion of radiation reaches the target and is scattered by it generating the information on radiation level. Scattered radiation reaches the spherical mirror, is reflected and converged by the lens through the reflecting surface of rotating device in photo-multiplier forming the electrical pulse which is intensified by the amplifier. From the amplifier signal arrives to two parallel entrances: unit for the formation of light pulse spectrum (FLPS) and oscillograph. Signal from the amplifier in the oscillograph is transformed into the signal of range to the target. The unit for FLPS selects the spectrum of frequencies of electric pulse and transfers it to the entrance of the computer in order to determine the radiation level on the basis of spectrum parameters. For example, on-stream spectrum analyzers C4–25 or C4–8 can be used in the capacity of the unit for FLPS.
Instrument for the measurement of radiation level allows detecting even small degrees of pollution above the radiation-hazardous enterprises at long distance via the measurements of frequency spectrum of the pulse reflected from the target and not on the basis of pulse energy. It is known that the pulse energy at reference level is currently measured with the fourth place accuracy and frequency – with the fourteenth place accuracy. Therefore, registration of pulse frequency spectrum increases the sensitivity of radiation measuring instrument by several orders.
Determination of radiation level by the spectrum parameters can be performed, for example, in the following manner. By test, the instrument for measurement of radiation level is calibrated by the signal reflected from the source radiation, level of which is well known. For example, depending on the distance from the source or depending on the radiation level by the source with variable radiation level. Calibration is performed on the basis of variation of pulse frequency spectrum and currently the frequency is measured with the accuracy which is by ten orders better than the pulse energy. Having the calibration curve, the radiation level of unknown target is measured at quite small amplitudes of backscattered signal. In the paper [11] analyzers of the spectrum of optical range signals are offered. The weight of evidence suggests that their use in the structure of radiation level measuring instrument will make it possible to improve the efficiency of measuring device operation.
As a result of application of the suggested radiation level measuring instrument, the capability of remote monitoring of distant radiation-hazardous objects with low radiation levels emerges. Instrument is transformed from the analog device into the digital one.
The mobile lidar complex for the remote control of atmosphere has been developed [7]. It contains the platform and following components installed on it: solid-state laser, two TEA (Trancversly Excited Atmospheric) CО2 lasers, telescope, two-coordinate mirror scanner, optical systems of radiation transmission, detecting spectral system; synchronization system, interface unit which includes: drive control unit and unit of accumulation of the data received from sensors, control computing unit, solid-state laser is executed by two-channel adjusting to Ti: Sapphire with the units of Nd emitters; every TEA CО2-laser is equipped with the pair of identical, optically interconnected CО2-lasers (heterodyne and injection), exit connected with the entrance of TEA CО2-laser which jointly form two-channel heterodyne lidar and the optical system of radiation transmission of every channel of two-channel heterodyne lidar consists of: connected with the exit of TEA CО2-laser, first part of the optical path which is connected with the optical key-modulator, second part of optical path, bottom part of telescopic radiation beam expander, half of telescope and scanner lens. The complex has great capabilities but it is quite bulky. In many cases the part of tasks can be solved by simpler device which is suggested by authors [8].
Simplification of the construction of lidar system for atmospheric air quality control is performed at the expense of use of the principle of combination light scattering which allows using only one laser for the determination of the degree of atmospheric air pollution with the molecules of saturated hydrocarbons in the atmosphere above the industrial zone. The lidar system for atmospheric air quality control is constructed on the basis of the principle of combination light scattering. The lidar system consists of the platform with installed on it solid-state laser emitter on yttrium-aluminum garnet with neodymium operating under the conditions of the third harmonic at the wave length of 355 nm, optical system of laser radiation transmission, detecting telescope, unit for data accumulation and control computing unit.
The optical system of laser radiation transmission additionally contains the refractive prism which directs the laser radiation into the studied space area. Detecting telescope is executed in the form of Newton telescope with spherical mirror and lens objective and the unit of data accumulation contains interference light filter for selection of the region of combination light scattering by the studied molecules with lines of hydrocarbon molecules, photoelectric multiplier, scale amplifier, analog-to-digital converter and data acquisition board.
Molecules of saturated hydrocarbons refer to the main pollutants above the industrial enterprise. Therefore, it is not reasonable to use complex equipment for the multi-component analysis of air space above the studied area and it is sufficient to use the lidar system, which is simple in structure, for the determination of degree of atmospheric air pollution with the molecules of saturated hydrocarbons only.
The principle of laser remote probing by the method of combination light scattering consists in the registration of laser radiation, which is combinationally scattered by the molecules of saturated hydrocarbons in the direction of 180° with frequency shift specifically typical for these molecules and determined by their vibration bands.
The optical scheme of the lidar system for atmospheric air quality control is specified in Fig.1. The system functions in the following manner: pulse of the solid-state laser emitter on yttrium-aluminum garnet with neodymium operating under the conditions of the third harmonic at the wave length of 355 nm is directed into the studied space area (target) above the industrial zone. Part of laser radiation is used for the generation of reference signal which sets the time reference and its amplitude sets the energy of laser pulse. Backscattered radiation of combination light scattering by studied molecules is accumulated by the telescope of Newton type, focused through the lens objective passing through the interference light filter on photocathode of photo-multiplier. Voltage pulse is transferred from photoelectric multiplier to the entrance of scale amplifier, then with the help of analog-to-digital converter through the data acquisition board, signal arrives to the computer where information is processed by the standard software and the signal is formed for its transmission through communication channels (Internet, wi-fi etc.).
In order to check the accuracy of measurements, estimations of power of the registered radiation of combination light scattering by the molecules of saturated hydrocarbons were performed on the basis of the lidar equation of combination light scattering by studied molecules.
Obtained results prove that for the set concentration of probed atoms Na and energy of combination scattering which is equal to the energy of 10 photons, the laser radiation wave length λL = 335 nm, which allows performing measurements during the minimum period of time with the laser pulse energy of 1 mJ, is the optimal variant for the detection of saturated hydrocarbons at the distances up to 6 km.
In the whole probing route with the increase of distance from 0.01 to 2 km, the measurement time increases approximately by 4 orders. If during the set time from the distance of R=2.0 km with the set number of pulses, which are transmitted to the atmosphere, the photoelectric multiplier registers more than 10 photons and we can speak about exceeding of molecules concentrations at this distance above the set level. Besides, obtained values of measurement time meet the requirement for the lidar operation speed.
The method provides high accuracy of measurements in the media where molecules of saturated hydrocarbons are the sources of pollution.
Lidar in the mode of sector scanning which is provided at the expense of platform rotation is installed in the industrial zone on the structure which is dominant by height. The lidar system for atmospheric air quality control is intended for the continuous control of content of gaseous saturated hydrocarbons in the studied space area. In case of detection of accidental release or exceeding of the level of maximum permissible concentrations, processing of the results in control computing unit and signal transmission by communication channels take place in real-time mode.
The lidar system for atmospheric air quality control allows performing the remote monitoring of air pollution via the measurement of energy of the pulse reflected from pollutants (target). However, in case of low degree of pollution and long distances to the target, the pulse energy decreases by 103–109 times and becomes commensurable with the noises of electronic devices. As a result, the useful signal might not be recognized in these noises. Thus, the disadvantage of the lidar system for atmospheric air quality control includes the low sensitivity to the small levels of pollution which decreases with the increase of distance to the target.
The specified disadvantage can be corrected by the other lidar system for air pollution control [9]. The lidar system for air pollution control (Fig.2) operates in the following manner. Pulse of laser radiation from the laser is directed towards the pollution area (target) by the rotating prism. The major part of radiation reaches the target and is scattered by it generating information on the level of pollution.
Determination of air pollution level is performed more accurately because the final signal is formed by two radiation fluxes, instead of one flux as it was executed by the authors [8]. Flux from the reflector exceeds the radiation backscattered by the target by one order or more. Therefore, the sensitivity of the suggested system is noticeably higher than the sensitivity of the system of authors [8]. In the particular case of implementation, if the industrial facility (nuclear power station, petroleum refinery, cement plant etc.) refers to the controlled space area with pollutants, the reflector is installed directly at this site, for example, on ventilation pipe. Then, the flux coming out of the pipe is the target. In case if the laser radiation pulse is directed towards the pipe shear, very small portion of radiation is backscattered. If the reflector is installed on the way of the laser radiation scattered by pollutants so that the larger portion of reflected radiation is directed into the detection telescope, then the scattered flux of laser radiation which reaches the detecting telescope 1 grows by many times providing higher sensitivity of the lidar system. Other case of implementation of suggested technical solution can be offered. The reflector is installed on the aircraft (including unmanned aircraft). The aircraft hovers above the most hazardous zone of the controlled space, rotating prism directs the laser radiation on the reflector and it directs the considerable portion of scattered laser radiation into the detecting telescope providing higher sensitivity of lidar system. The larger area of the reflector, the larger flux of scattered laser radiation directed into the detecting telescope. Growth of area of the reflector in the specified examples is limited by the strength of connection of the reflector with ventilation pipe or aircraft lifting force and potential to retain the ability of radiation direction into the detecting telescope in case of wind blasts and other atmospheric phenomena. Optimal shape of the reflector is paraboloid; the target is located in its focus. Flat reflector or corner reflector are acceptable in certain cases. The reflector can be equipped with the selective reflective coatings for the relevant scattering radiation wave lengths.
Efficiency of the suggested model can be demonstrated on the basis of the following example. Laser based on yttrium-aluminum garnet with neodymium and average radiation power of the second harmonic (wave length 0.532 μm) of 1 W serves as the radiation source. Pairs of iodine-131 above the radioactive object are the target. The object is located at the distance of 1 km from the lidar system. The power of radiation backscattered into the detecting telescope without reflector is usually 1–10 μW or less. In case of installation of the reflector with the diameter of 0.5 m near the shear of ventilation pipe, the power of backscattered radiation increases by several orders and reaches 1–10 mW depending on weather conditions and distance from the radiating laser, which allows determining the degree of pollution exceeding from maximum permissible concentrations with larger sensitivity.
Let us consider one more solution [10]. Its task consists in the remote monitoring of radioactive pollution of radiation-hazardous enterprises at the expense of measurement of frequency spectrum of the pulse reflected from the target. The essential conception of suggested solution is illustrated in Fig.3 which also contains the functional diagram of measuring instrument.
Instrument for the radiation level measurement [10] functions in the following manner. Pulse of laser radiation is directed towards the target from the laser by the rotating device. Small portion of radiation is drawn to the photodiode by the beam splitter-3 on the way to the target; this photodiode forms electrical synchronizing pulse which arrives on the entrance of oscillograph. The major portion of radiation reaches the target and is scattered by it generating the information on radiation level. Scattered radiation reaches the spherical mirror, is reflected and converged by the lens through the reflecting surface of rotating device in photo-multiplier forming the electrical pulse which is intensified by the amplifier. From the amplifier signal arrives to two parallel entrances: unit for the formation of light pulse spectrum (FLPS) and oscillograph. Signal from the amplifier in the oscillograph is transformed into the signal of range to the target. The unit for FLPS selects the spectrum of frequencies of electric pulse and transfers it to the entrance of the computer in order to determine the radiation level on the basis of spectrum parameters. For example, on-stream spectrum analyzers C4–25 or C4–8 can be used in the capacity of the unit for FLPS.
Instrument for the measurement of radiation level allows detecting even small degrees of pollution above the radiation-hazardous enterprises at long distance via the measurements of frequency spectrum of the pulse reflected from the target and not on the basis of pulse energy. It is known that the pulse energy at reference level is currently measured with the fourth place accuracy and frequency – with the fourteenth place accuracy. Therefore, registration of pulse frequency spectrum increases the sensitivity of radiation measuring instrument by several orders.
Determination of radiation level by the spectrum parameters can be performed, for example, in the following manner. By test, the instrument for measurement of radiation level is calibrated by the signal reflected from the source radiation, level of which is well known. For example, depending on the distance from the source or depending on the radiation level by the source with variable radiation level. Calibration is performed on the basis of variation of pulse frequency spectrum and currently the frequency is measured with the accuracy which is by ten orders better than the pulse energy. Having the calibration curve, the radiation level of unknown target is measured at quite small amplitudes of backscattered signal. In the paper [11] analyzers of the spectrum of optical range signals are offered. The weight of evidence suggests that their use in the structure of radiation level measuring instrument will make it possible to improve the efficiency of measuring device operation.
As a result of application of the suggested radiation level measuring instrument, the capability of remote monitoring of distant radiation-hazardous objects with low radiation levels emerges. Instrument is transformed from the analog device into the digital one.
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