These spacecraft are of several different types. Some are used for reconnaissance, some for navigation, some for military communications. Those that keep an eye on, or eavesdrop on potential enemies are often called spy satellites. Some photo- reconnaissance satellites (such as Big Bird) carry cameras and take high-resolution photographs of the surface. Then they eject the exposed film in canisters so that it can be recovered. Other photo-reconnaissance satellites (such as KH-11 or Keyhole) send back images electronically. Another type of military satellite is designed to give early warning of missile attacks. They carry sensitive infrared detectors that can spot the heat emitted from missile exhausts. The current US type is called IMEWS (integrated missile early warning system).
