Light detection and ranging is a remote sensing technique that is the optical equivalent of radar (radio detection and ranging). A LIDAR instrument gets information on the location and properties of distant objects by illuminating them with a beam of light (usually from a pulsed laser). It measures the ‘time-of-flight’ and other characteristics of the scattered light.
2 generations of LIDAR have been used at Davis to investigate the long-term climate and characteristics of the Antarctic atmosphere. They have been used to study global climate change and particular phenomena such as polar stratospheric clouds, polar mesospheric clouds, meteoritic dust, and smoke from Australian bushfires. They have also been used to find out how and why tropospheric clouds over the southern ocean are different to the rest of the world.
The first LIDAR installation at Davis was of high enough power to probe much of the middle atmosphere. This required an intense laser beam, highly specialised filters (to separate the laser light from the sunlight) and specially trained staff. The science goals of our second generation LIDAR allowed for simplified and remote operation. Davis station also played host to a LIDAR operated by the Institute for Atmospheric Physics in Kuhlunsgborn, Germany during 2011 and 2012.
How a LIDAR works
The LIDAR installed at Davis in 2001 provides measurements of atmospheric density and temperature. These measurements are a function of height and various properties of clouds and aerosols along the path of the laser beam. The maximum altitude range is typically between 5 km and 65 km during daytime, and up to 100 km at night. The measurements are collected with a vertical resolution of 18 m and an integration time of 60 seconds.
The light beam transmitted by the LIDAR comes from a pulsed solid state (Nd:YAG) laser at a wavelength of 532 nanometres. This is in the green part of the optical spectrum. The average power of the laser is 30 Watts. Each laser pulse comprises about 3 billion times more photons than the output from a standard electric light globe. About 25% of the laser light transmitted into the sky is scattered or absorbed within the atmosphere — the rest continues out into space.
The laser light backscattered from the atmosphere is collected by two telescopes with apertures of 280 mm. The received light can be analysed with a scannable Fabry-Perot Spectrometer. This is a crucial component for daytime measurements that isolate the laser light from the bright background skylight. Two shutters, spinning at 400 revolutions per second, are used to accept light scattered from certain altitude ranges. This process avoids the overwhelmingly strong signals returned from clouds. The scattering properties of light can be used to infer the concentration of molecular nitrogen through the atmosphere and the presence of water droplets and ice crystals in clouds.