When Air France Flight 447 from Rio de Janeiro disappeared over the Atlantic in the early hours of 1 June, search and rescue teams had to look for survivors in an area almost as big as Great Britain. Not knowing exactly where the Airbus 330-200 went down, French and Brazilian authorities turned to new software developed for the U.S. Coast Guard that uses the location of debris to look back in time to estimate the most likely site of an accident.
The Coast Guard's "reverse drift" modeling program takes into consideration the types and locations of objects found floating in the water, the time of an accident, and environmental conditions--such as the speed and direction of ocean currents and winds--to calculate the location where an accident may have occurred.
The Air France recovery operation was the first real-world test of this reverse-drift capability, which was added to SAROPS, the Coast Guard's Search and Rescue Optimal Planning System, only in early May. It was developed by Metron, Applied Science Associates, and Northrop Grumman.
In the past, drift models could only look forward in time. If a person falls off a cruise ship, for example, a forward-drift computer simulation can determine the approximate area where the current and winds will carry him. But that works only if you know where the person fell off the ship.
The exact coordinates of an accident at sea are not always known, as was the case for Flight 447. That's when reverse-drift modeling comes in. Reverse drift helps provide the missing variable--the location of the accident--that rescue planners need in order to look for possible survivors and wreckage. A seat cushion and other small debris were the first clues that operators fed into the simulation program in the early days of Flight 447's search operation, officials say.
Once the reverse-drift simulation gave officials an idea of where the plane went down, they used forward-drift simulation to pinpoint where to look for debris, bodies, and possible survivors, all of which would be carried away from the crash site at different rates by wind and currents.
"The computer model was very accurate, and that's when [the rescue teams] started to find the wing debris and the bodies," says Geoff Pagels, search and rescue specialist at the Coast Guard rescue coordination center in Portsmouth, Va., who was in daily contact throughout June with officials at the rescue coordination center in Gris Nez, France. By 15 June, search teams had found the bodies of 50 of the 244 victims and more than 400 pieces of debris from the plane, which were scattered over a 200 000-square-kilometer area. "Each time they find something, they keep coming back to us and ask us to run more simulations," Pagels says.
At sea, every object is affected by winds and currents. The effect of the current is simple. "The object goes where the water goes," says Tom Kratzke, programmer at the scientific consulting firm Metron, who wrote the code for the reverse- and forward-drift models. "The effect of the wind is much more complicated," he says.
The program uses surface area and nine other parameters of typical objects to determine how the wind will drive them.
Even though the SAROPS system is equipped with a sophisticated environmental data server, which gathers ocean wind and current information from multiple sources, environmental conditions are never fully known. So the program makes 10 000 educated guesses about the different paths that objects may take and combines them to find the most likely location.
The reverse drift gives more accurate results with each new object found but produces a larger search area as time goes by, says Jack Frost, manager of development for SAROPS. "Time is the enemy," he says.