It took 20 years and A$60M to build the laser airborne depth sounder (LADS). The laser component is now stabilized in a modified Fokker F27-500, a twin-propeller aircraft, on a platform designed to counteract pitch, roll, and sideways drift of the aircraft. 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 240m, and depth of 50m, compared with sonar which measures a strip as wide as the water is deep, for example, an area 20m wide in 20m of water.
The RAN estimated that it would take more than 100 years to finish surveying the Australian Continental Shelf using surface vessels that cover, at best, 4 square kilometers per hour. But LADS, which can survey the sea floor at 50 square kilometers per hour can reduce this to 15 years, and the cost per-unit area surveyed will be about one-third of that of a conventional shipborne survey.
The airborne package consists of a 168-Hz Nd:YAG infra-red laser whose output is frequency doubled to produce visible green light (532nm) in one megawatt, five-nanosecond pulses that propagate well in clear ocean or coastal waters. An optical coupler splits the output into infra-red and green components which are eye-safe from the operating altitude of 500m. Infra-red pulses are emitted vertically 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 of 10 by 10-m intervals over a width of 240m on each survey run.
The green pulses, reflect from both the sea surface and the bottom, are collected by the scanning mirror and sent to a telescope with spectral, spatial and polarizing filters. The pulses are detected by a photomultiplier with controlled gain and propagation characteristics and digitized at 2-nm intervals, providing a resolution better than 0.22m. The time differences between returning signals are computed to yield water depth. The digitized images are then combined with satellite global-positioning data for analysis by ground equipment.
At present, LADS 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, 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 LADS together with helicopter-borne systems from Sweden and the US as well as a fixed-wing system from Canada. The USN installed the system in a P-3 Orion aircraft and, after extensive trials in Southern Australian and Great Barrier waters, decided to buy the system; they took delivery in January 1996.
Although the DSTO built the prototype system, Vision Systems Limited (Adelaide) subsequently designed and delivered the first LADS to the RAN Hydrographic Service. LADS Corporation (Adelaide) is contracted by the RAN to provide operational, maintenance and logistics support to the RAN's LADS Unit's Survey program. LADS Corporation is also marketing the system internationally under license from DSTO. The company is building a $24-million LADS system to provide a contract surveying service which has been available worldwide from January 1998. This system will feature a faster pulse solid-state diode laser (built by LADS Corp.) and compact, high-performance computing to enable closer spot spacing and greater area coverage.
