One of the problems that arises when restoring the reflectivity of the target surface from the echo pulse of a navigation lidar is the effect of the tilt of the reflecting target surface on the broadening of the echo pulse. In this paper, a model is proposed that describes the broadening of the reflected echo pulse of a short-range lidar at different angles of incidence of the laser beam on the target. This model also takes into account the main parameters of the lidar, the atmosphere, the reflecting surface of the target and its inclination, which allow calculations to be made in a wide range of conditions.
Possibility of application of orbital lidars for laser detection of gas leaks from orbital vehicles has been considered. The optical characteristics of the main light-scattering components – molecules and atoms of gases, free electrons, aerosols, and space garbage are estimated at altitudes of 100–500 km from the Earth surface. Based on equations of laser sensing and laser detection and ranging, the lidar return signal powers are calculated. It has been shown that an orbital lidar with modern technical parameters can reliably detect signals from dispersed components in space at distances of several hundred meters from an orbital vehicle. Nowadays this problem becomes relevant in view of the Soyuz spacecraft sprung a leak on December 15, 2022 sending streams of coolant into space. The results of calculations have also shown that that space garbage can be detected with a space lidar at distances from several units to tens of kilometers.
KEYWORDS: LIDAR, Signal attenuation, Mass attenuation coefficient, Particles, Ocean optics, Airborne laser technology, Water, Monte Carlo methods, Scattering, Polarization
Results of experimental determination of the laser radiation extinction coefficient by sea water from the depth profiles of the airborne lidar return signal power by the gradient method for pure and coastal waters at depths from 5 to 50 m are presented. The extinction coefficients have been calculated for the “Makrel” polarization lidar with a wavelength of 532 nm. The proposed method expands the possibilities of remote hydrooptical sensing of the optical characteristics of the subsurface inhomogeneous water layers.
The special features of hydrooptical signals of airborne polarization lidar sensing of non-uniform water depths are considered. Depth profiles of lidar return signal intensity and depolarization calculated by the Monte Carlo method in the presence of underwater hydrosol layers are compared with the data of their airborne polarization lidar measurements. The results obtained can be used to expand the possibilities of lidar return signal interpretation aimed at profiling of the optical characteristics of water, especially non-uniform water in ambiguous situations.
A system of imaging of backscattered lidar pulses based on a SPAD camera developed at the IAO SB RAS is described. The main technical characteristics of the system are presented. Examples of 3 images of a single laser pulse with wavelength of 532 nm recorded by the SPAD camera with imaging array of 64 × 32 pixels from distances of 75, 90, and 285 m are given.
Results of statistical simulation of laser sensing of water depth in an experimental water pool by the Monte Carlo method are presented in the report. As a rule, experimental pools have limited sizes. The influence of pool walls on the lidar signal characteristics is analyzed. The power of laser radiation with a wavelength of 0.53 m is estimated depending on sizes of the experimental water pool, water extinction index, laser beam divergence angle, and field of view angle of the receiver. The results obtained demonstrate not too great influence of radiation reflection from the pool walls when the pool size exceed 20 m. Hence, this effect can be disregarded for pools of larger sizes.
A concept of hyperspectral LIF lidar based on a time-of-flight SPAD camera with high spatial resolution is suggested. Hyperspectral registration of the FIF spectrum of an oil film with the time-of-flight SPAD camera is numerically modeled. A quantitative dependence of the oil concentration on the distance and technical characteristics of the transceiving lidar system is refined.
The paper describes a developed sample of a mobile (helicopter) laser spectroscopic system, provides diagrams, and presents discussion of the results of experimental studies of fluorescence and Raman spectra of model objects, simulating the presence of heavy hydrocarbons in various concentrations.
Results of laser sensing of fluctuation of the upper cloud boundary altitude are presented in the report. A nonlinear relationship between the vertical and horizontal sizes of cloud inhomogeneities is established.
For statistical estimation of the potential possibilities of the lidar with matrix photodetector placed on board an aircraft, the nonstationary equation of laser sensing of a complex multicomponent sea water medium is solved by the Monte Carlo method. The lidar return power is estimated for various optical sea water characteristics in the presence of solar background radiation. For clear waters and brightness of external background illumination of 50, 1, and 10–3 W/(m2⋅μm⋅sr), the signal/noise ratio (SNR) exceeds 10 to water depths h = 45–50 m. For coastal waters, SNR ≥ 10 for h = 17–24 m, whereas for turbid sea waters, SNR ≥ 10 only to depths h = 8–12 m. Results of statistical simulation have shown that the lidar system with optimal parameters can be used for water sensing to depths of 50 m.
General features of forming lidar returns for sensing of depth profiles of the optical characteristics of sea water comprising pelagic fish schools are considered. Based on the data of statistical simulation by the Monte Carlo a method, an algorithm of analysis of two polarization components of lidar returns has been developed in which the threshold value is the signal from the surrounding water mass with the known light scattering properties.
The possibility of using of lidar return for detecting of low-contrast irregularities in above-water atmospheric is discussed in this paper. Numerical calculation of lidar signals is carried out considering in the double-scattering approximation.
The possibility of reconstructing the cloudiness parameters from measurements with a lidar immersed at depths of 10– 25 m is analyzed. Theoretical analysis is performed by the Monte Carlo method for lidars with realistic technical parameters, two optical types of ocean waters, different optical cloud models, and marine atmosphere in the presence of a fog. It is demonstrated that under typical conditions, cloudiness can reliably be detected up to altitudes of several hundred meters above the sea level. A change of the laser divergence angle and receiver field-of-view angle has a stronger effect on the character of lidar signal reflected from sea water than from cloudiness. A large contribution of multiply scattered radiation to lidar return signals calls for the development of mathematical methods that take it into account to increase the measurement accuracy.
Physical and mathematical description of laser sensing of ocean and sea waters is much more complicated than of atmospheric meteorological formations. The main problems here are higher optical density (turbidity) of water and much more complicated and variable scattering matrices of such waters compared to atmospheric ones due to the presence of biological components having very diverse sizes and shapes. Description of the lidar return signal from such media is complicated by the need to account for multiple scattering contribution. Approximation of double light scattering can be a certain compromise for not too turbid water. Results of numerical modeling of lidar returns from sea water with stratified inhomogeneity are discussed in the present paper.
The capability of lidar placed onboard an orbital platform to measure the atmospheric density profiles at altitudes from the Earth surface 20–100 km is investigated. The International Space Station whose Russian Segment will be equipped with one more measuring module is planned to be used as such an orbital platform. The distinguishing feature of the lidar caused by the problem being solved is the application of a laser with two working wavelengths from the UV range.
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