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A light detection and ranging (LIDAR) system is often used in atmospheric studies. Some of the different LIDAR system designs are Mie and Rayleigh LIDAR, Raman and differential absorption LIDAR, Doppler and fluorescence LIDAR, and systems used as simple range finders or altimeters. The designs vary according to the subject under study, the precision of measurement required and the circumstances of their deployment. Each type of system is a product of evaluating the capabilities of the hardware and software available and how it can be used to meet the measurement objectives.
A LIDAR system usually measures laser backscatter, which is reflected laser light. It may be designed specifically to measure direct laser backscatter, wavelength-shifted backscatter, the difference in absorption rates between two wavelengths, or frequency change in backscattered light. A basic system consists of a transmitter, a receiver and a data analysis component. LIDAR system designs have either a bistatic or a monostatic configuration. In a monostatic system, the transmitter and receiver are located together, while in a bistatic design the two are separate.
Another design consideration is to employ either a biaxial or coaxial sensor arrangement. In a biaxial arrangement, the axis of the transmitter and receiver have a different orientation. Backscattered light can be detected by the receiver only when the subject is beyond a certain distance. The axis of the transmitter and receiver are the same in a coaxial arrangement.
LIDAR systems that use pulsed lasers usually have a monostatic configuration, but might have either a biaxial or a coaxial sensor arrangement. Systems using a continuous-wave laser usually have a bistatic configuration. If the range of the subject is relatively near, a coaxial arrangement of transmitter and receiver is generally preferred. If near-target capability is not an issue, a biaxial arrangement might be adopted to help avoid complications from nearby laser backscatter.
Different LIDAR system designs also employ different laser wavelengths, and various bandwidth combinations for the transmitters and receivers. Other design considerations include requirements for use as a look-up or look-down LIDAR, and whether the system will be in continuous operation or used only at night. Some designs make use of tunable lasers. These options are carefully chosen to pursue a specific measurement goal.
The data analysis component of a LIDAR system makes use of various analytic techniques. Mie, Rayliegh, Raman and fluorescence LIDARS are designed to analyze different types of laser backscatter patterns. Scatter patterns depend on wavelength. Mie analysis best describes scatter patterns when the reflecting particle is about the same size as the wavelength. Rayleigh analysis is more accurate for particles much smaller than the wavelength.
Rayliegh and Mie designs examine elastic backscatter, in which the reflected light is of the same wavelength as the light transmitted. Raman LIDAR analyzes inelastic backscatter. This results from light being shifted slightly in wavelength when reflected by a particle. The amount of shift can identify the makeup and atmospheric concentration of the reflecting particles. Fluorecence LIDAR uses a similar analysis to examine backscatter from liquids and solids.
Doppler LIDAR measures shifts in the frequency of backscattered light to determine changes in temperature and wind velocity or direction. Differential absorption transmits two wavelengths of light and measures the difference in atmospheric absorption between the two wavelengths. The relative differences in absorption can identity aerosol concentrations.
Each of the different LIDAR system designs uses a unique configuration of hardware and software to make a precise measurement of a specific quantity under a limited set of circumstances. More general-purpose systems, such as a police speed detector, return less precise results. In some systems, the analytic method to be employed in the data analysis component determines the system hardware design. In others, available hardware dictates what system designs can be employed.
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