Employees of ManTech International of Fairfax begin work this month on a five-year, $49m Navy contract to research and test underwater sonar systems for identifying and tracking submarines and ships. ManTech expects to hire about 40 to 50 engineers, technicians, scientists and other workers globally under the contract with the Naval Surface Warfare Center, Carderock Division, in Bethesda, said Kenneth J. Farquhar, president of ManTech Systems Engineering. Some of those workers will be based in Bethesda while others will be located in the District, in Florida and at Navy locations worldwide. ManTech had been doing similar work for the Navy under a previous contract that expired. The company has more than 6,000 employees and had 2005 revenue of $980.3 million.