Lidar Measurements Supporting the Ocular Hazard Distance Calculation Using Atmospheric Attenuation

Abstract

A series of lidar measurements has been performed at the Vidsel Test Range, Vidsel, situated in the inland of the very northern part of Sweden, as a part of an assessment of reducing the laser hazard distance using atmospheric attenuation within the calculations of nominal ocular hazard distance (NOHD). The question was “How low is the atmospheric attenuation as function of height in this area, using a wavelength of 1064 nm?” The work included building a ground based backscatter lidar, performing a series of measurements and analyzing the results. The measurements were performed during June to November, 2014, with the objective to measure at clear air and good weather situations. The lidar measurements at 1064 nm showed a very low atmospheric attenuation as a function of height to altitudes of at least 10 km at several occasions. The lowest limit of backscatter coefficient possible to measure with this instrument is 0.3·10-7 m-1 sr-1. Assuming a lidar ratio varying between 30 – 100 sr, this was leading to an extinction coefficient of about 0.9 - 3·10-6 m-1. The atmospheric attenuation reduces the laser hazard distance with about 50 – 56 % depending on the lidar ratio. A recommendation is to monitor the atmospheric attenuation at the occasions when the method to the reduced laser hazard distance using atmospheric attenuation is used