How does the Coast Guard find people lost at sea?

Arthur A. Allen, a physical oceanographer with the U.S. Coast Guard Office of Search and Rescue in Washington, D.C., answers (as told to Adam Hadhazy):

We begin by interviewing the people who reported the problem. We try to find out where and when the boaters got in trouble, when they left port, where they intended to go, and where else they may have headed—what their plan B was. We also want to know what boat they were in and what survival gear they had. We basically determine all the possible scenarios about the incident and establish what it is we are looking for.

Then, based on that information, we build a strategy with the help of search-planning software called the Search and Rescue Optimal Planning System (SAROPS), which simulates the trajectory of various kinds of objects as they drift. SAROPS is a Monte Carlo–based system that simulates units called particles. Some particles will represent people in the water; others, the boat. They can all start drifting at different times and locales. With SAROPS, we can make more than 10,000 guesses about where boaters got in trouble and when and where they might end up. The program then assesses which scenario is most probable. There is always uncertainty, of course.

To begin devising the search in SAROPS, we pick from a list of objects whose rates of drifting under various conditions have already been modeled mathematically. We have information on the drift characteristics of many different items, from people to 55-gallon oil drums to various kinds of vessels, such as life rafts, sea kayaks, sailboats, skiffs and refugee rafts. In a recent case, for instance, we knew that the lost individuals had taken off in a sports boat with a center console, so we fed that option into the model.

SAROPS also considers the effects of wind on various currents in the ocean. Say I’m sitting at my desk at 10:30 A.M. and planning a 12 to 3 P.M. helicopter flight. I need to know the wind patterns from when the accident happened all the way through this afternoon to predict where survivors may have drifted in the intervening time. For that information, we have developed a powerful tool called the Environmental Data Server, which draws on a great variety of National Oceanic and Atmospheric Administration, U.S. Navy and academic sources of wind and current data that are updated several times a day. The server translates all these data into a common format, so that we can plug the information into SAROPS.

With our best projections in hand as to where the victims might be, we generally deploy helicopters, C-130 planes, boats called cutters and motor lifeboats to try to find them. For each kind of aircraft and boat, we know the probability of detection if we take a given path. We account for such effects as white caps on waves in these predictions, because whitecaps decrease visibility. The ocean surface is a very tough place to find someone. Although we are searching many, many square miles, the ocean is very, very large, and you are very small. It is like looking for a soccer ball—a person’s head above water—in an area the size of the state of Connecticut.

If search and rescuers do locate someone, then we interview that person, if feasible, and go all the way back to the beginning of the scenarios and readjust them accordingly. In any case, we continuously update our models and optimize search patterns to account for time passing and conditions changing.

Another aspect of our search-and-rescue procedures involves survival models. We have models, for instance, that calculate the net temperature a person in cold water is likely to have when heat loss to the water and heat generated by shivering are considered. This is a situation where being big and fat or muscular is helpful. People can also become dehydrated, which exacerbates hypothermia. Besides losing heat, a victim also loses water through metabolism, respiration and sweating, which comes into play in warmer waters. Other threats to life include predation and running out of food; we do not have models for those yet.

Even if weather conditions would allow a search to continue, we may call it off if our models tell us the victims have virtually no chance of still being alive. Unfortunately, despite our technology and best efforts, not everyone who is lost at sea is found.

Have a question?...Send it to experts@SciAm.com or go to www.ScientificAmerican.com/asktheexperts