E2A — Amateur radio in space: amateur satellites; orbital mechanics; frequencies and modes; satellite hardware; satellite operations; experimental telemetry applications. Working the satellites is a very popular amateur radio activity. Perhaps the most important thing you need to know when trying to communicate via satellite is where the satellites are. Amateur radio satellites are not in a geostationary orbit. That is to say they are constantly changing position in relationship to a point on the Earth. When determining where a satellite is, you might want to know its orbital period.
An amateur radio satellite is an artificial satellite built and used by amateur radio operators for use in the Amateur-satellite service. The designation is assigned by AMSAT , an organization which promotes the development and launch of amateur radio satellites. Because of the prevalence of this designation, amateur radio satellites are often referred to as OSCARs. Currently, over 18 fully operational amateur radio satellites in orbit  act as repeaters , linear transponders , or store and forward digital relays. Throughout the years, amateur radio satellites have helped make breakthroughs in the science of satellite communications. A few advancements include the launch of the first satellite voice transponder OSCAR 3 and the development of highly advanced digital "store-and-forward" messaging transponder techniques. The information presented regarding functional satellites is outdated quickly as the Amateur Radio Satellite community has become very active in building and being provided educational secondary cargo launch opportunities.