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PRECISION NAVIGATION
Archaeology is the study of physical cultural remains, and the contexts in which those remains are found convey critical information. DeepArch is committed to the idea that archaeology in deep water must conform to the same professional standards as followed on land and in shallow water. Consequently, site mapping often requires an accuracy of at least 1cm resolution, sufficient to reconstruct the position and orientation of objects, including very small artifacts. The EXACT precision acoustic navigation system (200 kHz) achieves this high standard of accuracy. In simplest terms, the system is an acoustic tape measure, providing position information by trilateration. First, accurate measurements of the local speed of sound through the water are collected by the research team. Then, two or three battery-powered acoustic transponders are placed around the archaeological feature, establishing baselines. One "host" transponder is fitted to the underwater vehicle working on the site. Several times per second the host transponder interrogates the other transponders around the site by sending an acoustic signal, and the time lag between send and receive is recorded. This time lag is a range measurement: with two or more EXACT ranges, the vehicle's position can be determined within a cubic centimeter. When fed back into the vehicle's control system, these data allow the ROV to be put into "closed loop control." This computer control makes it possible for the vehicle to hover over the site and to run precise, repeatable survey lines. The EXACT system
designed by David Mindell and his research team can be considered a
new generation wireless SHARPS (the cabled navigation net used in shallow
waters). The EXACT system was successfully field-tested at the MONITOR
2003 expedition. During the upcoming field seasons at the Black Sea
and the eastern Mediterranean (Summer
2003), the EXACT system will provide essential navigational accuracy
for the narrow-beam sub-bottom profiler
to render accurate topographical maps of several deep-water shipwreck
sites. For more information, see: D.A. Mindell, H. Singh, D. Yoerger, L.Whitcomb, J.Howland, "Precision mapping and imaging of underwater sites at Skerki Bank using robotic vehicles," in A.M. McCann and J.P. Oleson, eds., Deep-water Shipwrecks off Skerki Bank: the 1997 Survey, Journal of Roman Archaeology, Suppl. Series. Forthcoming, 2004. B.P. Foley and D.A. Mindell, "Precision Survey and Archaeological Methodology in Deep Water," ENALIA The Journal of the Hellenic Institute of Marine Archaeology, Vol. VI, 49-56, 2002. D.A. Mindell
and B. Bingham, "New
Archaeological Uses of Autonomous Undersea Vehicles" IEEE/MTS
Oceans Conference, November 2001.
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