ATD has 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 make quantitative aerosol measurements. Extinction coefficients, scattering ratios and backscatter coefficients are quantitative properties that can be measured with SABL.

The technical characteristics for SABL are summarized in the table. 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 a 14-inch Cassegrainian having a speed of f/5. The design goals were to maximize the signal-to-noise ratio and to build a stable instrument, one that would require little operator fine tuning. The critical components of the telescope are built of low expansion materials so that the optical performance is temperature insensitive. The receiver is built on an optical plate rigidly mounted to the telescope base to minimize the effect of vibration.

Careful component selection maximizes the signal-to-noise ratio. The temperature of the IR channel silicon avalanche photodiode (APD) is controlled with a thermoelectric cooler to reduce the dark current noise of the detector and stabilize its gain. The temperature of the green channel photomultiplier tube (PMT) is also temperature controlled. The main benefit of cooling the PMT is to gain stability. To reduce the background noise in the system, narrow band interference filters are used. Currently, the filters are less than 1.0 nm wide; plans are in place to obtain filters of less than 0.2 nm in the future. The system will then be dark current noise limited except under high background light level conditions. An example of a high background condition is viewing the top of a sunlit cumulus cloud.

Two computers are used in SABL: 1) a VMEbus based VxWorks system is used for real-time data acquisition and data logging on Exabyte tapes and 2) a Sun Sparc-5 is used for data display and overall system control. The computers are linked with Ethernet. The Sparc-5 can also be connected to another network so that a display can be sent to remote locations. During ACE-1 the remote display was used by the mission scientist on the flight deck to monitor the depth of the marine boundary layer and aerosol layers.

Features have been built into SABL to allow remote operation of the lidar. This will be the case when the scanner and optical head of SABL are mounted in the C-130 wing pod while the data system remains in the main cabin. One remote function is the ability to converge the lidar with two piezoelectric screws that control the output beam steering mirror. Also available at the Sparc-5 console is the ability to control the gains of the APD and PMT, the digitizing rate and the number of recorded range gates.

A two-channel 12-bit, 40-MHz digitizer allows the recording of the signals from the detectors without logarithmic amplifiers. The operator and scientist displays have the option of displaying logarithmic/range corrected or raw linear data. Either the scientist or the operator can save display images to a disk file by clicking on a button on the display screen. This image can later be viewed or printed.

SABL Characteristics

Wavelength/energy	1064 nm/75 mJ (IR) 532 nm/50 mJ (Green)
Pulse length	15 nsec (2.5 m)
Pulse rate	< 60/second
Transmit beam divergence	1 - 4 mrad
Telescope diameter/speed	14 inch f/5.
Telescope field-of-view	1 - 5 mrad
Background filter bandwidths	0.92 nm @ 1064 nm 0.16 nm @ 532 nm
Detectors	Avalanche photo diode @ 1064 nm Photomultiplier tube @ 532 nm
Digitizer - 2 channel	12 bits@40 MHz (4.0m)
Number of range gates per channel	< 2500
Recording media	Exabyte tape (8 mm)
Data System	VME Bus/VxWorks Sun Sparc-5/Unix
Transmitter/receiver size/weight	15x15x32 in./100 lbs.
Rack Space/Weight	48 in. of rack plus monitor/150 lbs.