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Fieldwork

NASA EAARL Lidar Test at Wallops Flight Facility

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photograph of EARRL laser points
Figure 1. EARRL laser points scanned across Wayne Wright's hand.
John Brock, Damaris Torres-Pulliza, and Georgia De Stoppelaire traveled to the NASA Wallops Flight Facility in Virginia to participate in performance tests of the NASA Experimental Airborne Advanced Research Lidar (EAARL) (Figure 1). Wayne Wright, the NASA Principal Investigator for EAARL, provided us with an update on the instrument design and capabilities, and described recent progress to prepare a twin engine Cessna 310 to serve as the EAARL aircraft platform (Figure 2).

EAARL will provide new capabilities to survey sandy beaches, coastal vegetation, shallow bathymetry, and nearshore benthic habitats. The design sensor combines a hyperspectral scanner with a lidar that can precisely record the full reflected laser pulse (Figure 3). In contrast, the NASA Airborne Topographic Mapper (ATM) lidar currently in use by the CMG Program measures only the range to the leading edge of the first laser reflection. Aircraft costs are reduced for EAARL relative to the ATM, due to its compact design, low power requirement, and the substitution of an array of GPS antennas in the place of an Inertia Navigation System. All of these innovations minimize the airborne payload, and enable the use of light aircraft that are commonly available at low cost.

EAARL also has the capability to sense the vertical complexity of the surface target "on the fly" during a given survey. This greatly reduces data volume over bare terrain, while simultaneously enabling the capture of detailed reflected pulse waveforms over forests and shallow water. The instrument's ability to adjust itself to the terrain on a pulse-by-pulse basis means that a single survey can provide a data set that is keyed to multiple applications. The possible uses for EAARL include coral community mapping, studies of changes along sandy coasts, the three-dimensional assessment of plant communities, and shallow bathymetric sounding.

Cessna 310 aircraft in the hanger
Figure 2. Cessna 310 aircraft in the hanger at Wallops Flight Facility undergoing modification for EAARL installation.

An equipment rack mounted over a port in the Cessna 310 recently passed the NASA engineering safety analysis, and the aircraft itself has passed the NASA safety review for flights over coastal waters. For the initial test flights this spring over Chincoteague Bay, Assateague Island, and coral reefs in Biscayne National Park, the survey crew will consist of a pilot and an instrument operator, and a vacant third seat will carry an observer. A future development goal for EAARL is to enable the pre-programming of flight waypoints and instrument operation. Following this upgrade, it may become possible to reduce the required onboard survey personnel to just a pilot.

Display of the EAARL transmitted (left) and received (right) pulse waveforms
Figure 3. Display of the EAARL transmitted (left) and received (right) pulse waveforms during the ground test to simulate bathymetric surveying. Note the separate peaks in the reflected waveform that correspond to the window screen target and the primary white target.

photograph of EARRL targets
Figure 4. Georgia De Stoppelaire (foreground) and Damaris Torres-Pulliza (background) beside the forward window screen target and the rearward primary white target. These targets were positioned about 150 m feet away from the EAARL lidar during these tests.
While at NASA WFF, USGS staff members assisted in tests designed to determine the shallowest subaqueous surfaces that EAARL will be able to survey. The EAARL laser was aimed horizontally at a target about 150 m away that represented the sea bottom. A window screen placed in front of the target to simulate the water surface was moved progressively closer to the primary target (Figure 4). EAARL was determined to be capable of separately ranging the distances to both the screen and the primary target down to a minimum separation of about 0.5 m. Based on proof-of-concept tests performed over Carysfort Reef off the upper Florida Keys, under ideal clear water conditions the maximum EAARL survey depth should slightly exceed 25 m.


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