Staff Sgt. Nathaniel Williams (left) briefs Pvts. Ruben Gutierrez and Patrick O'Neal on their reaction to a simulated, aerial chemical attack against their camp during Exercise Roving Sands '97 on April 21, 1997. More than 20,000 service members from all branches of the armed forces of the U.S., Canada, Germany, and the Netherlands are participating in Exercise Roving Sands '97. The exercise is designed to refine their skills in operations using an integrated air defense network of ground, missile and radar early warning systems combined with tactical fighter and bomber aircraft operating in a simulated high-threat environment. The chemical attacks are being executed by former Russian helicopters operated by members of Optec Threat Support Activity, Biggs Army Airfield, Texas.