Lidar signal-to-noise ratio improvements: Considerations and techniques

The primary objective of this study is to improve lidar signal-to-noise ratio (SNR) and hence extend attainable lidar ranges through reduction of the sky background noise (BGP), which dominates other sources of noise in daytime operations. This is particularly important for Raman lidar techniques where the Raman backscattered signal of interest is relatively weak compared with the elastic backscatter lidars.;Two approaches for reduction of sky background noise are considered: (1) Improvements in lidar SNR by optimization of the design of the lidar receiver were examined by a series of simulations. This part of the research concentrated on biaxial lidar systems, where overlap between laser beam and receiver field of view (FOV) is an important aspect of noise considerations. The first optimized design evolved is a wedge shaped aperture. While this design has the virtue of greatly reducing background light, it is difficult to implement practically, requiring both changes in area and position with lidar range. A second more practical approach, which preserves some of the advantages of the wedge design, was also evolved. This uses a smaller area circular aperture optimally located in the image plane for desired ranges. Simulated numerical results for a biaxial lidar have shown that the best receiver parameters selection is one using a small circular aperture (field stop) with a small telescope focal length f, to ensure the minimum FOV that accepts all return signals over the entire lidar range while at the same time minimizing detected BGP and hence maximizing lidar SNR and attainable lidar ranges. The improvement in lidar SNR was up to 18%. (2) A polarization selection technique was implemented to reduce sky background signal for linearly polarized monostatic elastic backscatter lidar measurements. The technique takes advantage of naturally occurring polarization properties in scattered sky light, and then ensures that both the lidar transmitter and receiver track and minimize detected sky background noise while maintaining maximum lidar signal throughput. Measurements, carried at 532 nm, show as much as a factor of 10 improvement in SNR and the attainable lidar range up to 34% over conventional un-polarized schemes. For vertically pointing lidars, the largest improvements are limited to the early morning and late afternoon hours, while for lidars scanning azimuthally and in elevation at angles other than vertical, significant improvements are achievable over more extended time periods. Observed changes in SNR improvements were also related to relative humidity and modification of underlying aerosol microphysics.;A second, distinct objective of this research was to utilize multiwavelength lidar techniques to separate plume and cloud particles. Choice of the study location and time for this work was driven mainly by the availability of satellite data collected by NASA INTEX-NA and NOAA NEAQS experiment over New York City on July 21, 2004 in support of MODIS imagery. The lidar results identify smoke plumes over New York City and validate the plume source origin location using NOAA-HYSPLIT back trajectory analysis. Surface measurements, at the time, from in-situ particle counters are presented and show no enhanced PM2.5 loading. This result is supported by lidar measurements, which confirm that nearly all of the aerosol plumes are located above the normal aerosol boundary layer showing that satellite measurements are often incomplete and are not sufficient for assessing surface air quality.