REMOTE underwater SENSING

Two hundred years after Cook, Flinders, and other explorers of the Southern Hemisphere started charting Australia`s vast coastline, one-half of the country`s continental shelf, or nearly 1 million km2, remains unsurveyed to acceptable standards. The Royal Australian Navy (RAN) estimates that it would take more than 100 years to finish surveying the Australian continental shelf using surface vessels that can cover, at best, about 4 km2/hour. The RAN, therefore, commissioned the Australian Defense

Th Lfw28798 13

REMOTE underwater SENSING

Laser system charts Australia`s coastline

Paul Mortensen

Two hundred years after Cook, Flinders, and other explorers of the Southern Hemisphere started charting Australia`s vast coastline, one-half of the country`s continental shelf, or nearly 1 million km2, remains unsurveyed to acceptable standards. The Royal Australian Navy (RAN) estimates that it would take more than 100 years to finish surveying the Australian continental shelf using surface vessels that can cover, at best, about 4 km2/hour. The RAN, therefore, commissioned the Australian Defense, Science and Technology Organization (DSTO; Adelaide) to come up with a fast hydrographic survey method to complete the coastline survey.

After 20 years and a $60 million investment, the laser airborne depth sounder (LADS) has been developed. The laser system can survey the sea floor at 50 km2/hour--equivalent to about 10 football fields per minute--and should reduce the survey time to about 15 years with a cost-per-unit-area surveyed about one-third that of a conventional shipborne survey.

LADS operation

The airborne LADS package is based on a Nd:YAG laser with 1064-nm output frequency doubled to provide visible green output at 532 nm. The laser is stabilized in a modified Fokker F27-500 twin-propeller aircraft on a platform designed to counteract pitch, roll, and sideways drift of the aircraft. The system produces 1-MW, 5-ns pulses at a pulse-repetition rate of 168 Hz; such pulses propagate well in clear ocean or coastal waters. The output beam is split by an optical coupler into infrared (IR) and green components that are eye-safe from the operating altitude of 500 m.

In operation, the IR pulses are emitted vertically downward from the aircraft and reflect from the sea surface to provide an initial sea-surface reference. A scanning mirror directs the green pulses to form a sounding pattern at 10 ¥ 10-m intervals over a width of 240 m on each survey run (see figure). The green pulses, reflected from both the sea surface and bottom, are collected by the scanning mirror and sent to a telescope with spectral, spatial, and polarizing filters. These pulses are detected by a photomultiplier tube with controlled gain and propagation characteristics and digitized at 2-nm intervals, which provides a resolution better than 0.22 m. The time differences between returning signals are computed to yield water depth. Digitized images are then combined with global-positioning-satellite data for analysis by ground equipment.

Compared to its ship-based sonar counterpart, the LADS-equipped plane travels at 145 knots, 14 times faster than a ship. It reads the sea floor to a width of 240 m and depth of 50 m, compared with sonar, which measures a strip only as wide as the water is deep, for example, an area 20 m wide in 20 m of water. Currently, the LADS system is operating from Cairns in Northern Queensland, Australia, where it is charting the Great Barrier Reef without damaging coral or marine life. It is also exposing hidden underwater dangers and finding better routes for commercial shipping, thereby saving millions of dollars in shipping costs.

Apart from hydrographic surveys, the LADS has other potential applications, including coastal and marine resource-management programs, fishery-resource assessments, oil and gas-pipeline and rig location, and military reconnaissance and tactical missions. In 1994, the US Navy (USN) evaluated the LADS together with helicopter-borne configurations from Sweden and the USA and a fixed-wing system from Canada. The USN installed the LADS in a P-3 Orion aircraft and, after extensive trials in Southern Australian and Great Barrier waters, decided to buy it; delivery was in January 1996.

Although the DSTO built the prototype system, Vision Systems Ltd. (Adelaide) subsequently designed and delivered the first LADS to the RAN Hydrographic Service. LADS Corp. (Adelaide) is contracted by the RAN to provide operational, maintenance, and logistics support to the RAN LADS Unit Survey program. LADS Corp. is also marketing the system internationally under license from the DSTO.

The company is building a $24 million system to provide a contract surveying service, which should be available worldwide by January 1998. This configuration will feature a faster-pulse solid-state diode-pumped laser (built by LADS Corp.) and compact, high-performance computing to enable closer spot spacing and greater area coverage.

Click here to enlarge image

Operating at an altitude of 500 m, the airborne, pulsed, frequency-doubled Nd:YAG laser produces a 240-m-wide sounding pattern at 10 ¥ 10-m intervals. The system can chart the sea floor to a depth of 50 m.

More in Optics