Scanning Underwater with Smaller Lidars and UAVs

Figure 1: The GTRI Lightweight Lidar Prototype Gantry-Mounted Over a Laboratory Water Tank.

Figure 1: The GTRI Lightweight Lidar Prototype Gantry-Mounted Over a Laboratory Water Tank.

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Bathymetric lidars, which are devices that employ powerful lasers to scan beneath the water’s surface, are used today primarily to map coastal waters.  Unfortunately, at nearly 600 lbs, the systems are large and heavy, requiring costly piloted aircraft to carry them.  But that situation may be changing.  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—called Active Electro-Optical Intelligence, Surveillance, and Reconnaissance (AEO-ISR)—would allow modest-sized unmanned aerial vehicles (UAVs) to carry bathymetric lidars, which could lower costs substantially. 

Furthermore, unlike current bathymetric systems, AEO-ISR technology is designed to gather and transmit data in real time, allowing systems to produce high-resolution three-dimensional (3-D) undersea imagery with greater speed, accuracy, and usability.  Together, these advanced capabilities could support a wide range of military uses, such as anti-mine and anti-submarine intelligence and nautical charting, as well as civilian mapping tasks.  In addition, the new technique could enable bathymetric lidar to probe not only maritime zones but forested areas as well.

“Lidar has completely revolutionized the way that ISR is done in the military, and also the way that precision mapping is done in the commercial world,” said principal research scientist Grady Tuell.  “GTRI has extensive experience in atmospheric lidar going back 30 years, and we’re now bringing that knowledge to bear on a growing need for small, real-time bathymetric lidar systems.”

Tuell and his team have developed the Pathfinder Lidar, a prototype that has successfully demonstrated AEO-ISR techniques in the laboratory.  The team has also completed a preliminary design for a deployable mid-size bathymetric device that is less than half the size and weight of current systems and requires half the electric power.

Measuring Laser Light

To simulate the movement of an actual aircraft, the Pathfinder prototype must be “flown” over a laboratory pool.  To do this, the researchers have installed the lidar onto a gantry above a large water tank in Georgia Tech’s Mechanical Engineering Department and operate it in a manner that simulates flight (see Figure 1).

The Pathfinder uses a special green laser (shown in Figure 2) that can penetrate water to considerable depths.  Firing a laser beam every 10,000th of a second, the proxy aircraft allows the team to study the best methods for producing accurate images of objects on the floor of the pool.  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 Pathfinder must make a series of adjustments that let it measure reflected laser beams as if there were no water present.

One challenge is that when a tightly focused light beam, such as a laser, hits water, it loses speed and bends, which is a familiar underwater effect called refraction.  Due to changes in the water surface, the angle of refraction varies constantly, and these changes in the refracted angle must be accounted for when computing the path of the light.

Another challenge is that the photons in the laser beam scatter in the water, like light from a car headlight hitting fog.  The amount of this scattering depends on the water’s turbidity, which refers to the number of particles suspended in it.  In addition, the water absorbs some of the light. 

Figure 2: The Special Green Laser of GTRI Lightweight Lidar Prototype System Used to Penetrate Water and Help Researchers Study the Best Methods forProducing Accurate Images of Objects on the Pool Floor.

Figure 2: The Special Green Laser of GTRI Lightweight Lidar Prototype System Used to Penetrate Water and Help Researchers Study the Best Methods forProducing Accurate Images of Objects on the Pool Floor.

Because of these two effects, a lidar system receives back only a tiny signal when its laser beam bounces off an underwater surface, such as the sea floor.  The Pathfinder’s signal-conditioning and sensor-processing capabilities must be sophisticated enough to detect that small returning signal in an overall sea and air environment that is extremely “noisy” (i.e., the environment is filled with extraneous signals that interfere with the desired data).

Improving Critical Techniques

The ultimate product of a bathymetric lidar is a 3-D 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).  Using statistics, calculus, and linear algebra, the TPU technique propagates errors from the individual measurements—navigation, distance, refraction angle—to estimate the accuracy of sea-floor measurements.

In a major milestone, 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.  The team has also produced the first hybrid lidar combining a waveform-resolved linear mode green lidar with an infrared geiger mode lidar.  This hybrid lidar enables precise beam steering through the water surface to improve the accuracy and fidelity of 3-D images of the seafloor.

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 Pathfinder’s high-speed computer needs only an additional nanosecond to digitize the returned beam and convert the analog light signal containing floor-reflectance points into digital location coordinates, from which distance and other information can be computed. 

“In our laboratory tests,” Tuell said, “we’re computing about 37 million points per second, which is exceptionally fast for a lidar system and gives us a great deal of information about the sea floor in a very short period of time.  The key is we’re using FPGAs to do the necessary signal conditioning and signal processing, and we’re doing it at exactly the time that we convert from an analog signal to a digital signal.”

A Deployable Design

In addition to developing the proof-of-concept Pathfinder prototype, the GTRI team has produced a computer-aided design (CAD) 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.

The longer-term aim is to use AEO-ISR technology to develop bathymetric lidars that could fly on small UAVs with payloads of 30 lbs or less.  To help these lidars deliver maritime surveillance and mapping data in real time, most of the necessary signal processing would be performed on the aircraft and only essential data would be transmitted to ground stations.

“We’ve provided a prototype that demonstrates the key technology,” Tuell said, “and we’ve completed a design for a mid-size design.  In the future, we believe small bathymetric lidars will perform military tasks, and also civilian geographic tasks such as county-level mapping, with increased convenience and at greatly reduced cost.”