Laser-Armed Planes Map Uncharted Seas at Top Speed
By Afshin Molavi
for National Geographic News
|February 10, 2004|
Criss-crossing the sky in a coastline-hugging pattern, the blinking
aircraft fires a flurry of green laser light into shallow waters
below. Over the next seven hours, more than six million laser pulses
will be transmitted into the water as the low-flying plane completes
A futuristic invasion? A military war game? Neither. Actually, the fixed wing aircraft is engaged in old-fashioned nautical sea chartingwith a high-tech, high-flying twist.
The twist is airborne lidar bathymetry. Bathymetry is the measurement of water depths. Airborne lidar bathymetry (ALB) is a way of measuring water depth from the air, using lidar. Lidar works like radar but substitutes laser pulses for radar's microwaves.
Faster, more accurate, more environmentally friendly, and cheaper than traditional, boat-based methods, ALB could go a long way toward mapping the literally millions of uncharted coastline around the world. Used by governments, private companies, and military planners to chart coastal waters as deep as 230 feet (70 meters), it has emerged as an increasingly important tool in recent years.
How It Works
Here's how ALB works: The lidar fires a laser pulse from the aircraft toward the sea. The laser is reflected at both the water's surface and the seafloor. ALB equipment registers those two reflections.
The amount of time that passes between the two reflections determines the water depthtaking into account the speed of light in water and variables such as ocean waves and tides. This process repeats millions of times during a survey.
The result? A map of the waters below that can be used for navigation, oil and gas exploration, national defense, economic-zone demarcation, and beach and reef management.
"It may be hard to believe that you can get a good map of the waters from the sky," says Mark Sinclair, a pilot with Australia's Tenix LADS Corporation, which is a leader in ALB. "People don't believe until they see it in their backyard," he says.
Veteran hydrographer (or water mapper) Jeff Andrews, originally a skeptic, proclaims himself "a believer." Andrews says he was "amazed by the results from the air survey" conducted for his firm, Coastal Planning and Engineering, Inc. The firm used the results for beach-restoration projects and environmental studies.
Thus far, ALB mapping has proved comparable to ship-based mapping in accuracy. ALB still cannot, however, match the most precise form of ship survey used for heavily trafficked coastal waters. But ALB can produce surveys up to 20 times faster than ship-based systemsand at roughly half the cost, advocates say.
Says John Longneck, an ALB expert at the U.S. National Oceanic and Atmospheric Administration (NOAA): "As the need to chart large areas of coastline grows, ALB has become an essential tool. ALB is fast, cost-effective, and most importantly, safe. Airborne hydrographers, unlike ship-based hydrographers, are operating free from the rocky, treacherous coastline and harsh ocean environment."
Following Captain Cook
Recently, NOAA contracted a U.S. Tenix LADS (Laser Airborne Depth Sounder) subsidiary to survey vast swaths of Alaska coastline.
In Cold Bay, Alaska (population: approximately 60), a 15-member Tenix LADS crew set up shop in April 2003 to begin a three-month survey.
"We are dealing with very complex, rugged coastline here," project manager Darren Stephenson said from Cold Bay. "It would be very difficult for a ship to do this survey. We are looking mainly to spot the hazards, such as the rocks, in the shallow water," he said.
Hazard spotting has always been an important component of nautical charting. Prior to one Australian survey by Tenix LADS, pilot Neville Balding was surprised to find that the only usable charts dated back to the early 1800s, when British navigator Mathew Flinders famously charted much of the continent's coast.
"In one sense, the LADS pilots and the onboard surveyors are picking up where [legendary 18th-century British ocean explorer and cartographer James Cook] left off," said Philip Smart of the National Air Support company, which flies and maintains aircraft on behalf of Tenix LADS in Australia.
"They are the new pioneers, because they are charting unknown territory," Smart added.
Origins and Innovations
The origins of ALB date back to the late 1960s, when the U.S. Navy began to use airborne laser-based systems for submarine detection. Prototype ALB systems emerged in the late 1970s and early 1980s.
Today there are two leading ALB systems: SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey), which is administered by the U.S. Army Corps of Engineers, and LADS Mk II, which is owned by Tenix LADS.
Over the past ten years SHOALS has conducted a wide range of surveys, from post-hurricane assessment in Florida to a survey of uncharted waters off Mexico's Yucatán Peninsula.
So far, ALB laser transmissions into water appear to have no ill effects on the environment. In fact, the laser beams are carefully calibrated to be "eye safe." That means that if you were standing on a boat and looking directly at the laser pulse coming from an ALB aircraft, you'd be safe.
Plus, NOAA's Longenecker argues, the environmental benefits of greater coastal mapping is immense. The data obtained can be used to better develop storm models as well as to evaluate the health of U.S. coastline. ALB data can also help prevent shipwrecks that can pollute pristine waters and cause human tragedies.
The work of ALB crews in Alaska, Australia, and around the world could mean the difference between a shipwreck and a safe journey.
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