LIDAR System for Monitoring Atmospheric Turbulence Profiles

The Georgia Tech Research Institute (GTRI) has developed a new type of LIDAR system for monitoring the vertical profile of atmospheric refractive turbulence. The ground-based system makes real-time measurements by projecting a laser beam to form a laser beacon at several successive altitudes from 250 m to 15 km. The beacon is observed with a four-aperture telescope and the differential motions of pairs of the beacon images from each altitude are statistically characterized as variances. The measurement technique is similar to the astronomical instrument known as the Differential Image Motion Monitor (DIMM), which uses natural stars as sources. Whereas the DIMM only provides one number, r0, to characterize the entire atmosphere, the LIDAR uses beacons at a range of altitudes, along with an inversion algorithm that we have developed, to retrieve the turbulence profile. GTRI has developed and tested a brassboard version of the turbulence LIDAR. The brassboard system transmits 300 mJ pulses of 355 nm laser light at 50 pulses per second, (15 W) and receives backscattered light with a 40-cm telescope. Altitude ranges are selected by using an electro-optical shutter based on two Pockels cells, and image data is recorded with a specialized CCD camera manufactured by SciMeasure (this type of camera is normally used in wavefront sensors for adaptive optics systems). The LIDAR provides turbulence profiles at 10-minute intervals during both day and night, and it also has a separate receiver for a conventional aerosol LIDAR in order to characrterize aerosol and cloud layers. Tests will be conducted at the White Sands Missile Range during a two-week period in June, 2007. The tests will include truth data obtained with micro-thermal probes carried aloft by a tethered blimp. Turbulence profiles provided by the LIDAR will be compared with the truth data, and overall system performance will be discussed.