Range-gated viewing systems, which were at first used for object observation under poor visibility conditions, are used now also to determine distances to objects and obtain 3D images. A number of methods were suggested to determine distance. For instruments operating in real time, it is important to use fast methods. Range-intensity correlation methods are such methods because they use only two images. The methods work with rectangular-shaped illumination pulses and rectangular gate pulse shape. However, real pulses may differ from the rectangular ones that may introduce significant error into the distance measurement. To avoid the error one has to use a calibration dependence of the distance on the signal. The calibration curve depends not only on time profiles of the pulse and gate, but also on the gate duration and the difference between delay times of two images. Experimental search for optimal measurement condition through trial and error is a rather cumbersome problem. Therefore, we used numerical simulation and analytical estimates. The pulse time profile could be taken arbitrary. The gate time profile was assumed to be rectangular. The ratio of the energies received by the same pixel in two images depending on distance is used to build calibration curve. With given pulse shape, by varying gate time and delay time difference, we obtain calibration curves with different slope, different position on the distance scale and with different range of the determined distances. It appeared that the greater the slope of the curve, the narrower the range of measured distances and the lower the pixel energy. The conditions were found to get monotonous calibration curve with the highest possible return signal from all points on the distance determination interval.
A method for distance determination with the help of range-gated viewing systems suitable for the arbitrary shape of the illumination pulse is proposed. The method is based on finding the delay time at which maximum of the return pulse energy takes place. The maximum position depends on the pulse and gate durations and, generally speaking, on the pulse shape. If the pulse length is less than or equal to the gate duration, the delay time appropriate to the maximum does not depend on the pulse shape. At equal pulse and gate durations, there is a strict local maximum, which turns into a plateau when pulse is shorter than gate duration. A delay time appropriate to the strict local maximum or the far boundary of the plateau (where non-strict maximum is) is directly related to the distance to the object. These findings are confirmed by analytical relationships for trapezoid pulses and numerical results for the real pulse shape. To verify the proposed method we used a vertical wall located at different distances from 15 to 120m as an observed object. Delay time was changing discretely in increments of 5 ns. Maximum of the signal was determined by visual observation of the object on the monitor screen. The distance defined by the proposed method coincided with the direct measurement with accuracy 1- 2m, which is comparable with the delay time step multiplied by half of the light velocity. The results can be useful in the development of 3-D vision systems.
Data of aerosol measurements from October 2004 to December 2006 and a collection of back trajectories arriving at a
monitoring site at the measurement instants were used to reveal main aerosol source areas with the help of the residence
time analysis. Overall aerosol content in the vertical atmosphere column was measured in Minsk (Belarus) and Belsk
(Poland) with the help of multiwavelength aerosol lidar and sun-scanning spectral radiometer. 3D five day back
trajectories were calculated using wind field data provided by Republican GidroMeteoCenter of Republic Belarus. These
data were supplemented with the vertical velocity data and the wind field in the planetary boundary layer. It was found
that the most powerful aerosol source areas are beyond of Belarus and Poland territories. They are on the south-east,
south and south-west directions to monitoring stations. Revealed sources are close to the expert EMEP aerosol emission
data. On the average the south territories affect the most atmosphere conditions at the monitoring sites. About 60% of
aerosols in Minsk and about 50% of aerosols in Belsk have a transboundary origin.
We have elaborated and evaluated a new high-precision method of automated establishment of the position of an object. Unlike the method using laser theodolites, the desire object is defined not by a specular reflector, but a laser beam directed upwards (laser spotlight). In so doing, the photodetectores register the laser radiation scattered by the aerosol component of the atmosphere and propagating perpendicularly to the beam axis (side scattering). The intensities of the side scattering registered by the photodetectors have been estimated in terms of the current concepts as a function of the distance, laser radiation parameters, and the state of the atmosphere. The calculations have shown that despite the relatively small value of the side scattering intensity, it is possible to reliably register light signal from large distances with the use of sufficiently powerful lasers and high-sensitivity photodetectores. To evaluate the proposed method, we developed and tested under real conditions an operative model of a laser-optical system based on a small-size "dry" neodymium laser (peak power of 0.5 MW, pulse repetition rate up to 5 Hz). The model is able to automatically establish the position of an object at a distance of up to 0.5 km to an accuracy no worse than 20 cm in a locality blocked from direct observation.
The caused by atmosphere turbulence error of lidar measurement of power emission was estimated for a stationary impurity source. The error magnitude was determined by the first and second order statistical moments of the plume integrated concentration and by instants of sounding pulse shots as well as by coordinates of sounding beams. Statistical characteristics of plume were obtained using random force method in the turbulence theory. It was shown that the power emission measurement error was dependent on the distance bewteen the impurity source and the measurement plume cross section, pulse repetition rate, number of averaging pulses, number of sounding beams.
A problem on introduction of additional a prior assumptions to construct a closed set of lidar equations at several wavelengths and on their solutions to estimate microphysical parameters of atmospheric aerosols by multi-frequency laser sounding data is discussed. Some regression relations between spectral values of aerosol backscatter and extinction coefficients in the visible and near-IR are used as the assumptions. The regressions are constructed by model considerations. The optical atmospheric aerosol model of the World Meterological Organization is taken as a basic one. The constructed regressions enable one to evaluate the solvability of, generally, ill-conditioned lidar equations and the errors in the solutions as well as to make some estimations with respect to the determination of aerosol microstructural parameters. This work has been directed towards the design of procedures and algorithms to process laser sounding data gathered routinely by lidar setups of the Institute of Physics, Belarus National Academy of Sciences, Minsk, Republic of Belarus within the frame of a number of International and National research and development programs.
The results of comprehensive field experiment are presented. The objective of the experiment is lidar technique development for control of pollutant dispersion from pulse source. Experiment was carried out in steppe region, underlying surface was covered with sparse vegetation. Charge exploded at the altitude of 10 m stand for source of pollutant. Lidar was used to trace the cloud of explosion products. The ratio of backscatter signal from the cloud to aerosol background signal was recorded along with the time and coordinates of sounded points. Ultrasonic meteorological station and sodar 25 - 30 meters distant from explosion location were used to measure air temperature, vertical and horizontal components of wind velocity and its direction, total energy of turbulent motions, tangential turbulent friction stress and vertical turbulent heat flow. Experimental data were compared with the results of numerical modeling of pollutant spatial distribution performed on the basis of Gaussian statistical model. Numerical results were primarily in satisfactory agreement with experimental data.
A potential source of errors at remote measurements of concentration of atmospheric gases by CO2-lasers is the incomplete knowledge of atmospheric composition and temperature during measurements. This paper gives a general expression for errors owing to approximate description of the composition of the base of a statistical atmospheric model and of separate parameters measured during laser sounding. Measurements of additional atmospheric parameters are shown can reduce considerably the uncertainty in the concentration of a gas studied. A special computer code is designed to search pairs of frequencies with minimal error in mean-path horizontal measurements by using a topographic target. The performed calculations showed that a number of referenced experiments on remote measuring of atmospheric gas concentrations by CO2-lasers were conducted at frequencies that did not provide the minimal atmospheric errors.6
The simple effective method for computing the spectral thermal radiation intensity from jets of hydrocarbon fuel combustion products viewing through thick atmosphere layers is proposed. The method is based on (1) similarity of radiation characteristics of optically thin turbulent jets and (2) three-group approximation. The first approach can be used because jet radiation filtered by sufficiently thick atmospheric layer is concentrated mainly at frequencies where radiating volume is optically thin. Within three-group approximation developed for radiation transfer calculation in molecular rotational-vibrational bands the lines in each spectral interval are approximated by several (up to 3) groups of lines with the same lower energy levels. This allows one to account for the contribution of so called 'hot' lines that arise due to transitions between excited states and are weakly absorbed in low temperature atmosphere.
A possibility to use the own infrared radiation of a gas mixture to detect CO in combustion products by means of nondispersion correlation method has been investigated. Water vapor and carbon dioxide in atmosphere has a little effect on the signal. But atmosphere CO restricts registration distances to about 100 m. Presence of H2 and CO2 in a jet causes two-valued property of the calibration curve and may bring indeterminacy in detection of CO in jet. Nevertheless, registration of CO absorption mass over 0.1 atm cm in combustion products using own radiation of combustion products is possible.
Similarity of radiation characteristics of heated turbulent jets was revealed experimentally and confirmed theoretically. A universal function that describes thermal radiation of many jets with different outlet parameters was found. A method was developed for prediction of IR radiation emitted by exhaust jets in finite spectral intervals containing many spectral lines.
The results of the investigations of turbulent fluctuation influence on heat radiation of hydrocarbon fuel combustion jets are presented. These results show that temperature fluctuations are one of the basic factors forming radiation fields on non-reactive jets. The approach used is based on optically thin pulsation approximation that allow to account for temperature fluctuation contribution using local averaging of optical characteristics in radiation transfer equation. Such averaging may be completed using model probability density functions (PDF) of temperature and concentration. Several PDF models were verified on the basis of measured and calculated jet IR radiance comparison.
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