Players control a Strategic Defense Initiative satellite orbiting the Earth and must destroy enemy missiles and satellites with its weapons, when activated the satellite's weapon systems fire at the crosshair present onscreen during play. The arcade version features a joystick to control the satellite and a trackball to control the crosshair. Home computer versions use different control schemes, such as depressing the fire button to control the crosshair, alternating between controlling the satellite and its weapons. It is also possible to use a joystick and mouse in combination to control the satellite and crosshair at the same time, emulating the arcade game's controls.
Each stage is split into two sections; offensive mode and defensive mode. During offensive mode the player engages a number of enemies, with the aim of destroying them all without the satellite being destroyed by the enemy. Should the player destroy all enemies during offensive mode, they are awarded 20,000 bonus points and begin the next stage on the offensive. If any enemies evade destruction during offensive mode, the player must complete defensive mode, where they are tasked with protecting the homeland from incoming warheads. Completion of defensive mode advances the player to the next stage, where they go on the offensive again.
This so-called "Star Wars" program was introduced by U.S. President Reagan to promote defensive weaponry that could shoot ICBM's out of space.
Source: Wikipedia, "Global Defense", available under the CC-BY-SA License.