In a lidar system an intense laser pulse emitted into the atmosphere undergoes both absorption and scattering, and backscattered light can be received at the telescope of the lidar. The magnitude of the received signal is determined by ths backscattering properties of the atmosphere. Atmospheric backscattering depends on the wavelength of the laser lines and the concentration, size, shape, and refractive properties of the particles or molecules in the atmosphere. The precise analysis of the lidar return signals allows the remote sensing of the absorption and the scattering properties of the atmosphere, and indirectly the determination of many important parameters including air pollution. The experimental setup for this consists of a telescope and a biaxially mounted pulse laser. In most cases, however, not all the light actually hits the detector, which is usually placed in the focal plane of the receiving optics for optimum collecting efficiency from infinity. In practice, the majority of lidar systems employ reflecting (Newtonian or Cassegrainian) telescopes. These always require some kind of mirror supporting structure which partly blocks the returned radiation. In general, interpretation of the lidar return signal is more complicated from geometrical considerations which include the degree of overlap between the laser beam and the field of view of the receiving optics as well as the details of the telescope.
Numerical and experimental investigations were performed on the peak position for lidar return signals with the variation of lidar geometry, and showed that the laser beam divergence angle(Θ) and the inclination angle between the telescope and laser axes(δ) are very important parameters in designing or aligning a lidar system to receive a good lidar signal. The peak position for lidar return signals changes from 405 m to 285 m with the variation of Θ from 0 to 1200 μrad, but it has a peak value of 510 m in the near 200 μrad, from 600 m to 285 m with the variation of δ from -400 μrad to 1200 μrad. The results of numerical and experimental data are in good agreement, and clearly indicate that to receive good return signals in field measurement the lidar geometry must be carefully adjusted with consideration of the characteristics of each angular parameter.
The slope method of determining the geometrical form factor(G-Factor) is presented. We assumed that the appropriately averaged aerosol distribution is homogeneous, and so assumed the constant to be independent of distance R of the volume backscattering coefficient(β) and of total aerosol scattering coefficient(σ). The determination of G-Factor is achieved by an experimental technique which uses the slope method for the lidar signal received in a clean atmosphere along (near) the horizontal path. We obtained a good set of results by this method. For an inhomogeneous atmosphere, the G-Factor is experimentally determined by using the polynomial regression method in the lidar equation.
The DIAL technique for remote sensing of ethylene was evaluated for the lidar dual-pulse system. Mismatch of the G-Factor in lidar dual-pulse operation was identified as strong error source of DIAL measurement. A new method was described to correct the error caused by mismatch of the G-Factors in lidar dual-pulse operation.
$C_2H_4$ concentration was monitored over a heavily trafficked roadway. Using the new method and applying the G-Factor determined by slope method to the calculation of concentration, the range averaged concentration of $C_2H_4$ was corrected from 55ppb to 63ppb. The technique thus shows a good correction of concentration error due to lidar dual-pulse operation, especially in the short range.
