In a guarded hangar on Long Island, N.Y., experts are piecing together the Boeing 747 that was once TWA flight 800. Investigators from the National Transportation Safety Board (NTSB) and the FBI are reassembling the aircraft in an effort to learn the cause of the explosion that blew the Paris-bound jet out of the sky shortly after taking off from JFK International on the night of July 17, 1996, killing all 230 aboard and scattering wreckage over five miles of the Atlantic.

From wreckage recovered to date, investigators have determined that the jetliner's center fuel tank exploded, but though most of the victims and parts of the plane -- including all four engines -- have been found, neither forensic examination nor analysis of the assembled sections has yet revealed the cause of the explosion. As of this writing, the search for the tank and remaining parts of the aircraft continues. If these prove to be too small for electronic detection, scallop trawlers may be used to comb the sea floor. Otherwise, the remaining pieces will probably be found with advanced technologies similar to those used by the National Oceanic and Atmospheric Administration (NOAA) ship, the Rude (pronounced "Rudy"), and its support group in the first few weeks following the crash.

LOCATING THE WRECKAGE

One of 15 ships in the NOAA fleet, the Rude routinely conducts hydrographic survey operations off the Atlantic and Gulf Coasts, using sophisticated side-scan sonar and multibeam echo sounder systems to locate and chart submerged wrecks and obstructions to navigation. Data provided by the vessel are used to update U.S. nautical charts.

At the time of the crash, the Rude was en route to Rhode Island after completing a hydrographic survey of the approaches to New York. She was one of the first ships to reach the crash site, following the Coast Guard Cutter Juniper. After assisting in recovering floating wreckage, the Rude's commanding officer, NOAA Corps Cmdr. Sam De Bow, was requested to begin side-scan sonar sweeps. Within two hours, the Rude located the wreckage, 110 feet down on the ocean floor.

Originally designed for military applications, side-scan sonar has become the primary search tool for locating sunken ships, downed aircraft and other objects on the sea bottom. In side-scan operations, the sensor, or "towfish," is towed behind the ship at a predetermined depth. As it moves through the water, the sensor radiates sound waves in a fan-shaped pattern, directly down and out to each side of the towfish. Echoes, or contacts, reflecting from objects protruding from the ocean floor are detected by the sensor and sent to the side-scan recorder aboard the ship, where they are processed, recorded and translated into imagery on a thermal printer. As each sonar contact is made, it is time stamped and geocoded with differentially corrected GPS coordinates. Since the Rude uses corrections from the Coast Guard Differential GPS (DGPS) Navigation Service, the positional accuracy of each contact was in the range of 3 meters to 5 meters.

The effectiveness of side-scan imagery is largely determined by the depth at which the sensor is towed. The closer to the bottom, the more defined the shadow created by the sound waves as they pass over the object (comparable to shadows cast by lamplight). Shadows not only highlight the presence of objects, they are used to estimate the height of the object off the bottom. "When we wanted to get higher-resolution imagery," explained NOAA Hydrographic Survey chief, Capt. Andy Armstrong, "we dropped the towfish closer to the bottom and used a smaller range scale."

Once contact was made -- using an AS600 side-scan sonar from EdgeTech -- the Rude began a series of parallel runs (track lines) that produced overlapping sonar images. As the survey progressed, the sonar data, track and position of the ship were processed, combined, recorded and displayed by a data acquisition system (