Design, alignment and characterization of the scanning aerosol backscatter lidar

Craig Walther

National center for Atmospheric Research

P.O. Box 3000

Boulder, Colorado 80307-3000

Ph: (303) 497-2054 Fax: (303) 497-2044

Email: craigw@ucar.edu

Design, Alignment, and Characterization of the

Scanning Aerosol Backscatter Lidar

at the

National Center for Atmospheric Research

Craig Walther, Tim Rucker

The Scanning Aerosol Backscatter Lidar (SABL) is a compact and reliable aerosol backscatter lidar system. It was developed by the Atmospheric Technology Division (ATD) of the National Center for Atmospheric Research (NCAR). SABL consists of five main components: the laser transmitter, the telescope, the receiver optics, the data system and the aircraft wing pod mounted scanning hardware. The laser transmitter consists of a commercially available NdYag laser that emits energy in the IR at 1064 nm and in the visible (green) at 532 nm. Therefore, SABL has two channels, an IR channel and a green channel. The telescope is of Cassegrainian design with a 35 cm diameter. The receiver optics direct the received signal to an Avalanche Photodiode (APD) to detect the IR signal and a Photo Multiplier Tube (PMT) to detect the green signal. The SABL data system consists of two computers, a VMEbus based system that is used for the real-time data acquisition and a Sun Sparc-5 that is used for data display, data logging and overall system control. The signal from each detector is digitized to allow for up to 2000 gates to be sampled down to 3.75 meter gate spacing. The SABL scanner is mounted under the wing of the NCAR C-130 in a modified fuel pod. This pod arrangement allows complete scanning of SABL through the 180 degrees off the left side of the aircraft.

There were two main goals during the development of the SABL. The first was to produce an instrument that scientists could use to see the aerosol structure of the atmosphere in real-time. This qualitative information could then be used, for example, to direct an aircraft to a layer of unusually high or low aerosol content. The second objective was to be able to make quantitative aerosol measurements. Extinction coefficients, scattering ratios and backscatter coefficients are some of the quantitative properties of aerosols that could be measured with SABL. During it first years of operation SABL has been used in its real time role with much success. However, a range of obstacles have prevented serious quantitative measurements from being achieved. One of these obstacles is being able to consistently field the lidar in a known aligned state. Also, power sample measurements have been hard to quantify, alignment processes have had to be developed and the field of view and overlap function have had to be measured. An alignment technique using a columnated beam directed at the telescope and then verifying this alignment by using light from a star has been developed. Other techniques have been developed to quantify the power sample measurements, to place the dynamic range of the detectors in the optimal span of the entire received signal, and to measure the field of view and the beam width. These techniques will be discussed in the paper.