The NAVSTAR Global Positioning System (GPS) is a space-based navigation system designed to provide users with worldwide three-dimensional position and navigation information. Almost all military aircraft, surface ships, and submarines are potential users of the system, as are some land vehicles and ground troops. Military allies and civilians could also use it. The system will consist of 24 satellites (four of which have already been launched), ground control equipment, and user equipment. The Department of Defense's (DOD) justification for the program was to consolidate navigation satellite research programs, improve weapon system effectiveness by increasing navigation accuracy and global coverage, and promote potential cost savings.