Laser speckle can influence lidar measurements from a diffuse hard target. Atmospheric optical turbulence will also affect the lidar return signal. We present a numerical simulation that models the propagation of a lidar beam and accounts for both reflective speckle and atmospheric turbulence effects. Our simulation is based on implementing a Huygens-Fresnel approximation to laser propagation. A series of phase screens, with the appropriate atmospheric statistical characteristics, are used to simulate the effect of atmospheric turbulence. A single random phase screen is used to simulate scattering of the entire beam from a rough surface. We compare the output of our numerical model with separate CO 2 lidar measurements of atmospheric turbulence and reflective speckle. We also compare the output of our model with separate analytical predictions for atmospheric turbulence and reflective speckle. Good agreement was found between the model and the experimental data. Good agreement was also found with analytical predictions. Finally, we present results of a simulation of the combined effects on a finite-aperture lidar system that are qualitatively consistent with previous experimental observations of increasing rms noise with increasing turbulence level.
The ambient atmosphere between the laser transmitter and the target can affect C02 differential absorption lidar (DIAL) measurement sensitivity through a number of different processes. In this work, we will address two of the sources of atmospheric interference with C02 DIAL measurements: effects due to beam propagation through atmospheric turbulence and extinction due to absorption by atmospheric gases. Measurements of atmospheric extinction under different atmospheric conditions are presented and compared to a standard atmospheric transmission model (FASCODE). We have also investigated the effects of atmospheric turbulence on system performance. Measurements of the effective beam size after propagation are compared to model predictions using simultaneous measurements of atmospheric turbulence as input to the model. These results are also discussed in the context of the overall effect of beam propagation through atmospheric turbulence on the sensitivity of DIAL measurements. . INTRODUCTIONCO. differential absorption lidar is an important remote sensing technique for a variety of applications ranging from effluent identification and characterization to geophysical structure identification to monitoring of meteorological phenomena. Numerous ground based, airborne and satellite based systems have been deployed and utilized for various studies.'8 A number of important factors contribute to the popularity of CO. laser based LIDAR systems. One of the major advantages to using CO. laser based lidar is the relatively mature laser technology available from commercial CO, systems. Systems with a variety of output formats (CW-lOOkHz, kW average power, 100's kW peak powers) are available. Another advantage is in the CO. spectral region the atmosphere is relatively transparent allowing long range operation with modest laser energy. This reduces power and size requirements as compared to other laser technologies. A third important factor is the broad tunability (9-11 jim) available in a spectral region where many materials exhibit characteristic "fingerprint" spectral signatures. This becomes especially important in applications where identification of numerous components is important.As part of the design and characterization of a CO. LIDAR system for any application a number of issues must be considered. In this work, we not only address issues associated with atmospheric effects that influence any CO2 DIAL application, but also consider those important to applications involving multi-spectral DIAL from hard targets. In this approach, multiple wavelengths are used in the measurement rather than just two as in typical DIAL systems. Since the transmitted energy hasa broad spectral content, the spectral fingerprint of the material of interest, either the target itself in geophysical measurements or effluents in a pollution monitoring system, will be present in the spectral characteristics of the signal return. The use of a large number of wavelengths is necessary for multiple component identification and introduces additional ...
Issues related to the development of direct detection, long-range CO2 DIAL systems for chemical detection and identification are presented and discussed including : data handling and display techniques for large, multi-2 data sets, turbulence effects, slant path propagation, and speckle averaging. Data examples from various field campaigns and CO2 lidar platforms are used to illustrate the issues.
performance can be adversely affected by the ambient atmosphere between the laser transmitter and the target through a number of different processes. This work addresses two sources of atmospheric interference with multispectral C 0 2 DIAL measurements: effects due to beam propagation through atmospheric turbulence and extinction due to absorption by atmospheric gases. We compare measurements of the effective beam size after propagation to predictions from a beam propagation model that includes turbulence effects such as beam steering and beam spreading. We also compare the experimental measurements of atmospheric extinction to those predicted by both a standard atmospheric transmission model (FASCODE) and a chemometric analysis.
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