Light Detection and Ranging (LIDAR)
It is clear that human activity has altered Earth's atmosphere. For example:
- the release of chlorofluorocarbons has caused significant thinning of the protective ozone layer, particularly in the polar regions;
- increasing levels of human-made greenhouse gases are raising temperatures in the lower atmosphere.
Modelling and observations suggests that the middle atmosphere (10–100 km altitude) is cooling in response to rising greenhouse gas concentrations and ozone loss.
It is still unclear how these changes are influencing the general structure of the atmosphere. In particular the various dynamic and chemical processes that determine its state are being influenced.
Atmospheric scientists see the polar middle atmosphere as a ‘litmus site’. Because of the region's extreme physical conditions and geographical isolation, scientists can identify climate change and test predictive models.
Physicists from the Australian Antarctic Division and the University of Adelaide have developed a complex Light Detection and Ranging (LIDAR) instrument that is being used in the detailed study of the middle atmosphere above Davis station.‘Light detection and ranging’ is a remote sensing technqiue that is the optical equivalent of radar ('radio detection and ranging'). A lidar instrument obtains information on the location and properties of distant objects by illuminating them with a beam of light (usually from a pulsed laser) and measuring the 'time-of-flight' and other characteristics of the scattered light.
The main aim of the Davis LIDAR is to investigate the long-term climate and characteristics of the Antarctic atmosphere in the study of global climate change.
There are currently only five LIDARs operating in Antarctica. The Davis lidar is one of only three Antarctic LIDAR that can probe the mesosphere (the layer between altitudes of 50 km and 95 km).
The Davis LIDAR measures atmospheric density, temperature, wind velocity and aerosol loading as a function of altitude. The altitude range is normally between 10 km and 65 km, although certain types of measurement can be made to altitudes of 95 km.
The LIDAR data are normally collected with a vertical resolution of 94 metres, although a resolution as high as 18 metres is available. The data are typically obtained with 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 532nm (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 is transmitted into the sky by a Cassegrain telescope with a 1-metre diameter primary mirror. The telescope also collects the backscattered light, and can be pointed within 45 degress of overhead. A mirror spinning at 400 revolutions per second allows the lidar to switch between transmitting and receiving modes.
The most novel aspect of the Davis LIDAR is its ability to measure atmospheric temperatures and winds using a technique known as 'incoherent Doppler LIDAR'. This involves making very high resolution measurements of the spectrum of the laser light backscattered from the sky using a scanning Fabry-Perot spectrometer, and being able to move the telescope around the sky.The accuracy of the temperature measurements depends on altitude, but is typically 1–2 degrees Celsius at an altitude of 30 km over an integration time of 30 minutes (as determined by comparison with in-situ measurements from balloons).