Active and Passive Satellite:
Active and Passive Satellite – Possible communication satellites may be active or passive and the major difference between the two is that whether the communication relay involves passive reflection or an active electronic system.
An active satellite is one which has transmitting equipment abroad, such as a transponder. It is a device which receives a signal from earth, amplifies it, and retransmits the same signal back to the earth (either immediately or after a delay).
A passive satellite merely reflects or scatters incident radiations from the earth, a portion of the radiation being reflected or scattered back in the direction of the earth.
Passive satellite relays would require surface transmitters of much greater power than would active relays (unless the passive reflectors are extremely large); however active satellite relays must carry aboard receiving and transmitting equipment and the necessary power sources. Thus decreasing reliability and life. One may provide all active satellite with omnidirectional transmitting antennas (radiating equally in all directions) or directive antennas (radiating most of the energy toward the earths).
Directive antennas would require much less transmitted power but would require antenna stabilization so as to direct the radiated energy towards the earth. A passive satellite relay could consist of an omnidirectional scatterer such as spherical body, like a balloon satellite, or an directive scatterer such as corner reflector.
Advantages and Disadvantages:
A corner reflector has the advantage that it tends to reflect radiation in the approximate direction from which the radiations come. Passive satellites may have significant advantages for military systems because of the relative invulnerability of the satellites.