Physical model of snow precipitation interaction with a 3D lidar scanner.

Snow precipitation interaction with a generic 3D lidar is modeled. The randomness and the intensity of the signal as a function of the visibility and snowflake size and density distribution are reproduced. To do so, a representative snow density distribution is modeled as a function of visibility. Taking into account the laser beam and pulse characteristics, the probability to have one or many snowflakes of a given size in the lidar sampling cell is calculated. Knowing the number and the size of the snowflakes, the magnitude of the lidar signal is calculated. Finally, a filtering algorithm based on the relative intensity of the snowflakes is discussed.

Experimental validation of D parameter model for droplet sizing using off-axis lidar measurements.

Information about the size distribution of liquid droplets in a fog can be retrieved by measuring the backscattering lidar depolarization parameter D in circular polarization. Using a polarimetric off-axis lidar, measurements at different backscattering angles are performed on fogs made of water droplets and of mineral oil. Estimation of the effective droplet size is obtained using constrained linear inversion. Mie theory is used to calculate the variation in depolarization parameters for different effective droplet sizes. The calculation is performed for various scattering angles. These calculations provide a kernel for the constrained linear inversion scheme. It is shown that the refractive index has an effect on the retrieved droplet sizes as well as the choice of scattering angles. These measurements confirm that the circular depolarization parameter measured near the backscattering angle can be modeled as a function of the forward-scattering diffraction peak. The results of the constrained linear inversion of measurements are consistent with in situ measurement of the droplet size distribution.

Enhanced scanning agility using a double pair of Risley prisms.

Scanners with one pair of Risley prisms are robust and precise and they can be operated continuously. In this paper, we present a new scanner based on the use of two pairs of Risley prisms. The concept was driven by the need to add flexibility to Risley prism scanners used for lidar 3D mapping applications, while maintaining compactness and robustness. The first pair covers a FOV narrower than the second pair. The second pair is used to position the first Risley pair scan pattern anywhere within its own, larger, FOV. Doing so, it becomes possible, without additional scanner components, to increase the sampling point density at a specific location, to increase the sampling uniformity of the scanned area, and, while in motion, to maintain the sampling of a specific area of interest.

Estimation of normalized point-source sensitivity of segment surface specifications for extremely large telescopes.

We present a method which estimates the normalized point-source sensitivity (PSSN) of a segmented telescope when only information from a single segment surface is known. The estimation principle is based on a statistical approach with an assumption that all segment surfaces have the same power spectral density (PSD) as the given segment surface. As presented in this paper, the PSSN based on this statistical approach represents a worst-case scenario among statistical random realizations of telescopes when all segment surfaces have the same PSD. Therefore, this method, which we call the vendor table, is expected to be useful for individual segment specification such as the segment polishing specification. The specification based on the vendor table can be directly related to a science metric such as PSSN and provides the mirror vendors significant flexibility by specifying a single overall PSSN value for them to meet. We build a vendor table for the Thirty Meter Telescope (TMT) and test it using multiple mirror samples from various mirror vendors to prove its practical utility. Accordingly, TMT has a plan to adopt this vendor table for its M1 segment final mirror polishing requirement.

Comparison of the relationships between lidar integrated backscattered light and accumulated depolarization ratios for linear and circular polarization for water droplets, fog oil, and dust.

Recently, an empirical relationship between the layer integrated backscattered light and the layer accumulated depolarization ratio has been established for linear polarization for the case of water droplet clouds. This is a powerful relation, allowing calibration of space lidar and correction of the lidar signal for multiple scattering effects. The relationship is strongly based on Monte Carlo simulations with some experimental evidence. We support the empirical relationship with strong experimental data and then show experimentally and via second order scattering theoretical calculations that a modified relationship can be obtained for circular polarization. Also, we demonstrate that other empirical relationships exist between the layer accumulated linear and circular depolarization ratios and the layer integrated backscattered light for submicrometer particles and nonspherical particles.