The Atmospheric Technology Division (ATD) at NCAR, the predecessor or the Remote Sensing Facility (RSF), developed the Scanning Aerosol Backscatter Lidar (SABL) to be a compact and reliable aerosol backscatter lidar system. There were two main goals during the development of the system. 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 can be measured with SABL.
The conceptual design for the SABL transmitter/receiver module was done at NCAR. The detailed optical/mechanical design and construction of the SABL telescope and receiver were done by Lentec Corporation of Albuquerque, NM. The telescope is of Cassegrainian design with a 14-inch diameter and a speed of f/5. The goals of the design of SABL were to maximize the signal-to-noise ratio and to build a stable instrument, one that would not require a lot of operator fine tuning to perform well. The critical components of the telescope are built of low expansion materials so that the optical performance does not change with large temperature changes. The receiver is built on an optical plate that is rigidly mounted to the base plate of the telescope to minimize the effect of vibration on system performance.
To maximize the signal-to-noise ratio several things have been done in component selection. In the infrared channel the temperature of the silicon avalanche photo diode (APD) is controlled with a thermoelectric cooler. This does two things: it reduces the dark current noise of the detector and it keeps the gain of the detector stable. The temperature of the photo multiplier tube (PMT) used
to detect the green signal, is also temperature controlled. The main benefit of cooling the PMT is not to reduced the dark current but to gain stability. To reduce the background noise in the system, narrow band interference filters are used.
Two computers are used in SABL: a VMEbus based VxWorks system is used for the real-time data acquisition and a Sun Sparc-5 is used for data display, data logging onto DLT tape and overall system control. The computers are linked together by Ethernet. The Sparc-5 can also be connected to another network so that a remote display can be sent to a scientist at a location away from the lidar.
Many other features have been built into SABL to make it possible to operate the lidar when direct contact with the transmitter/receiver module is not possible. This was necessary for SABL to be mounted under the wing of the NCAR C-130 Aircraft in a fuel pod. This pod arrangement allows complete scanning of SABL through the 180 degrees off the left side of the aircraft. The transmitter/receiver module are located in the pod while the data system remains in the main cabin of the aircraft. One of these remote functions is the ability to converge the lidar from the Sparc-5 with two piezoelectric screws that control the output beam steering mirror. Also available at the console is the ability to control the gains of the APD and PMT, the digitizing rate and the number of range gates that are recorded. Also controlled from inside is the scanning rate, or pointing direction of the turning mirror and all laser functions. Many different temperatures and pressures in the pod are also monitored.
A two-channel 12-bit, 40-MHz digitizer is used in the receiver. Using these high dynamic range digitizers allows the recording of the signals from the detectors without using logarithmic amplifiers. Removing the logamps from the receiver has made the analysis for quantitative aerosol properties easier and more accurate. The operator and scientist displays, have the option of displaying logarithmic/range corrected or raw linear data. Either the scientist or the lidar operator can save any interesting display image to a disk file by clicking on a button on the display screen. This image can be displayed or printed out at a later time to show colleagues and is also available in real time at the other NCAR supported workstations on the aircraft.
