With heritage from Environment Canada's ground-based and airborne lidar designs, UMBC's atmospheric lidar group constructed the Elastic Lidar Facilty (ELF) in 2002 (Comer, 2003). The ELF, being a common example of all elastic lidars, shares the same characteristics of all lidar systems. The transmission stage of the ELF consists of a laser and a prism. The laser is a flash lamp pumped, Q-switched Continuum Surelite II Nd:YAG, which is operating at 1064 nm, the fundamental frequency of Nd:YAG, and 532 nm, the first harmonic. The repetition rate for the laser is 10 Hz. After the beam leaves the laser, the beam encounters a steering prism directing the beam skyward.
The next figure shows the path of the beam as it exits the prism. The prism sits upon a stage assembly that has 2 degrees of freedom, enabling the beam to be aligned and aimed.
The next part of the ELF system is the receiving stage. A 14” Celestron Schmidt-Cassegrain telescope collects the backscattered light.
On the focal plane the telescope, there is an optics package that is comprised of removable neutral density filters, a dichroic filter, and two narrow band pass filters. Since Mie scattering has such a large backscattering cross section, the signal is often large enough to saturate the detectors. Neutral density filters in the optical path are used to alleviate this, and can be replaced with higher or lower values in order to obtain the best signal under the changing atmospheric conditions. The spectral transmission factor of the receiver x(l) is a function of the filters, mirrors, and detectors used in this stage.
This is a schematic showing the ray tracing of light inside the telescope and optics package. Light, after being collected by the telescope’s main mirror, reflects off the secondary mirror, and is then focused to the back of the telescope. The light then passes though the first stage of neutral density and to a dichroic filter, which is used to split the light into its 532 and 1064 nm components. From this filter, the 1064 nm wavelengths are allowed to pass straight through, going through another neutral density filter, through a 1064 narrow band pass filter, and finally into the APD. The 532 nm beam reflects 90 degrees off the dichroic, through another neutral density filter, and then finally through a 532 narrow band pass filter before it reaches the PMT. The ELF system uses a Hamamatsu H6780 PMT for the 532 nm channel, and an Analog Modules 710-427 APD for the 1064 nm channel. From the detectors, the signals are then routed via coaxial cables to a Gage 12100 series digitizer card installed in a PC computer.
ELF Processing Algorithm
The time series of backscatter is initially processed with a fixed extinction to backscatter ratio (Sa) and system constant (K, derived from Rayleigh returns on very clear days). The extinction profile is integrated to give an optical depth which is compared to the AERONET column AOD. By adjusting the Sa value and iterating, we close the lidar derived AOD on the AERONET column and fix the Sa value throughout the day. In the following figure, the AOD time series of lidar derived AOD and AERONET AOD are shown.
The choice of an Sa of 77 sr-1 gives agreement between ELF and AERONET. The algorithm is described on the next figure:Image prepared by Meloë Kacenelenbogen