Satellites provide a platform for a wide variation of applications.  The equipment placed on the satellite to accomplish the intended mission is what pays for the satellite and is designated as the Payload. Although communications is one of the most common uses, payloads can include imaging of the Earth or objects in space, global positioning systems (GPS), ranging and altimeters used in mapping, spectral analysis devices to determine the composition of the atmosphere.  In the past the payloads have been selective in three primary categories, Communications, Imaging and Scientific. I would consider GPS satellites as a forth category unto itself. Some satellites are now being built with shared payloads to provide a more cost effective means to collect scientific data for long term studies.

Communications satellite payloads range in complexity from a few channels in one frequency band to hybrid systems that have multiple channels using a number of frequency bands including frequency conversion and switching systems to translate and rough the traffic to selective areas. These satellites can receive signals from the ground and retransmit them to the ground or another satellite. Applications are also in place to communicate with ships, trains, trucks and airplanes. The basic types of communications are voice, video and data.

Imaging satellite payloads are governed by the resolution of the ground image they can provide. The images can be produced in the visible light spectrum, normal color or black and white (for higher resolution), inferred spectrum for tracking temperature differentials or other ranges based on the design of the camera used. Space imaging also uses Ultraviolet and Gama-ray spectrum among other types. These satellite may also include mapping instruments that produce topographical mapping information utilizing for example, radar or laser mapping techniques.

Scientific payloads are just that, and include a diverse range of equipment. These satellites are designed to collect very specific information. Some of the applications have been to collect data and measure the irradiation from the Sun, Sun spot activity, solar flair and wind measurements, measure the ozone layer, map carbon dioxide concentrations, search the stars and the list goes on. Scientists are developing new missions every day to gain more knowledge.