The spare can be then be moved into position in a matter of days, plugging the gap in the network. Radio signals sent from an earth station are received by the satellite, amplified and processed, and transmitted back to earth. The electrical power for the transponders that carry out this task is derived from massive solar cell 'wings'. The introduction of larger and more powerful rocket launcher systems has enabled heavier satellites to be placed in orbit. Hence satellites can now be equipped with a large number of transponders, typically twenty or more, operating in different frequency bands and dedicated to different applications.
Transponders vary both in terms of power and operating channels.
Differing amounts of power are required depending on the application. For example, direct TV broadcasting uses higher power transmitters as the residential receiver equipment is relatively simple. Often a dish as small as a 45 cm is used to receive the satellite signal. At the other end of the spectrum KPN for instance uses 34 metre dishes to receive international traffic, which are more efficient in capturing even weak signals. The footprint of a satellite is the area on earth that is covered by the signal of the satellite.