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Lighter Lidar Takes to the Sky

Photonics Spectra
Apr 2001
Richard Gaughan

Satellite pictures of the ocean surface offer insight into the levels of organics that indicate ocean health, but the volume of color imagery exceeds our ability to interpret it. Instrumentation on ships can provide surface truth measurements to help engineers produce algorithms to interpret the data, but aircraft-mounted lidar instrumentation can measure many of the same quantities and provide 12 to 20 times the coverage rate. Unfortunately, the systems have been large and heavy, requiring expensive four-engine aircraft. Now NASA's Goddard Space Flight Center has developed AOL-3, a third-generation airborne oceanographic lidar system that weighs only 200 lb and can be carried by two-engine aircraft.

AOL-3 uses a frequency-doubled and -tripled Nd:YAG laser as its transmitter. The laser's 532- and 355-nm measurement beams illuminate the ocean surface in 15- to 30-cm spots separated by about 40 cm. The scattered light and induced fluorescent radiation are collected with a 25-cm telescope and transmitted to a monochromator. Fiber optic faceplates placed at the image plane route each of eight wavelength channels to an independent photomultiplier tube, where waveforms are temporally resolved.

To validate AOL-3's performance, the system was flown with its predecessor, AOL-2, on the center's C-130Q aircraft. The test flight also took advantage of a simultaneous cruise from Bermuda to the entrance of Delaware Bay by the Cape Henlopen research vessel owned by the Newark-based University of Delaware.

The researchers compared AOL-3's measurements with AOL-2's and with the measurements from the ship at the surface. All three sets of measurements consistently reflected the change from coastal to deep waters. For example, the chlorophyll concentration varied from a high of 5 µg/l in coastal waters to less than 0.2 µg/l in pelagic waters.

Lidar in action

C. Wayne Wright, a scientist at Goddard's observational science branch and member of the research team, said that the data from AOL-3, which the team presented in the Jan. 20 issue of Applied Optics, is already being used to develop algorithms for the satellite data. He also cited the lidar system's flexibility.

"The instrument is highly configurable and is fairly easily adapted to other investigations," he said. "We are using it extensively to develop a pump-and-probe technique for measuring primary productivity [the amount of chemical energy stored by photosynthesis]." The researchers are developing techniques to monitor other indicators, such as overall carbon estimation and the ocean-floor fluorescence of coral reefs.

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