Reconnaisance or Spy Satellites

Reconnaissance satellites are used to spy on other countries. They provide intelligence information on the military activities of foreign countries. These satellites can even detect missile launches or nuclear explosions in space. Reconnaissance satellites can pick up and record radio and radar transmissions while passing over a country. They can also be used as an orbiting weapon by placing warheads on a low orbit satellite to be launched at a ground target.

There are basically four types of reconnaisance satellites:

Optical-imaging satellites have light sensors that detect missile launches and "see" enemy weapons on the ground.

Radar-imaging satellites are able to observe the Earth using radar technology through cloud cover.

Signals-intelligence or ferret satellites are essentially super-sophisticated radio receivers that capture the radio and microwave transmissions emitted from any country on Earth.

Relay satellites make military satellite communications around the globe much faster by transmitting data from spy satellites to stations on Earth.

Spy satellites usually circle the Earth in polar orbits, taking pictures.