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Smaller lidars might enable UAVs to conduct underwater scans

05 December 2014

Bathymetric lidars – devices that employ powerful lasers to scan beneath the water's surface – are used today primarily to map coastal waters.

GTRI's test pool (photo: Rob Felt/Georgia Tech)

At nearly 275kg, the systems are large and heavy, and they require costly, piloted aircraft to carry them. now, a team at the Georgia Tech Research Institute (GTRI) has designed a new approach that could lead to bathymetric lidars that are much smaller and more efficient than the current full-size systems.

The new technology, would enable modest-sized unmanned aerial vehicles (UAVs) carry bathymetric lidars, lowering costs substantially. And, unlike currently available systems, the technology is designed to gather and transmit data in real time, allowing it to produce high-resolution 3-D undersea imagery with greater speed, accuracy, and usability.

The lidar uses a high-power green laser that can penetrate water to considerable depths. Firing a laser beam every 10,000th of a second, a proxy aircraft allows the research team to study the best methods for producing accurate images of objects on the floor of a test pool (see illustration).

The ultimate goal is to obtain accurate reflectance from the sea floor, but the presence of water makes that difficult. To capture good images, the GTRI lightweight lidar must make a series of adjustments that let it measure reflected laser beams as if there were no water present.

Because of the effects of refraction, scattering due to turbidity and absorption, a lidar system receives back only a tiny signal when its laser beam bounces off an underwater surface such as the sea floor. The signal-conditioning and sensor-processing capabilities of the lightweight lidar must be sophisticated enough to detect that small returning signal in an overall sea and air environment that is very noisy.

The ultimate product of a bathymetric lidar is a three-dimensional point cloud that describes the seafloor at high spatial resolution. Users of these data need to know the accuracy of each point.

GTRI’s researchers have devised a new approach for accuracy assessment called total propagated uncertainty (TPU).
Based on a mathematical approach, the TPU technique propagates errors from the individual measurements – navigation, distance, and refraction angle – to estimate the accuracy of sea-floor measurements.

In an important development, the GTRI team was the first to demonstrate bathymetric lidar coordinate computation and TPU estimates in real time. To achieve the necessary processing speed, the team employs a mixed-mode computing environment composed of field programmable gate arrays (FPGAs), along with central-processing and graphics-processing units.

Each time a laser is fired, it takes only a few nanoseconds for the beam to reach the bottom of the pool and bounce back. Once the beam returns, the lidar's high-speed computer digitises the returned beam and computes ranges, coordinates, and TPU before the next shot of the laser.

In laboratory tests, the team computes about 37 million points per second – which is exceptionally fast for a lidar system and provides a great deal of information about the sea floor in a very short period of time.

In addition to developing the proof-of-concept lidar prototype, the GTRI team has produced a CAD design for a deployable bathymetric device that is half the size and weight of current devices and has lower power needs. The immediate goal is to field such a mid-size device on a larger UAV such as an autonomous helicopter.

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