An amateur radio satellite is an artificial satellite built and used by amateur radio operators. It forms part of the Amateur-satellite service. These satellites use amateur radio frequency allocations to facilitate communication between amateur radio stations.
Many receive an OSCAR designation, which is an acronym for Orbiting Satellite Carrying Amateur Radio. 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.
These satellites can be used free of charge by licensed amateur radio operators for voice (FM, SSB) and data (AX.25, packet radio, APRS) communications. Currently, over 18 fully operational amateur radio satellites are in orbit. They may be designed to act as repeaters, as linear transponders, and as store and forward digital relays.
Amateur radio satellites have helped advance the science of satellite communications. Contributions 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 Amateur Radio Satellite community is very active in building satellites and in finding launch opportunities. Lists of functioning satellites need updating regularly, as new satellites are launched and older ones fail. Current information is published by AMSAT. AMSAT has not been actively involved in the launch and operation of most amateur satellites in the last two decades beyond allocating an OSCAR number.
The first amateur satellite, simply named OSCAR 1, was launched on December 12, 1961, barely four years after the launch of the world’s first satellite, Sputnik I. The beginning of this project was very humble. The satellite had to be built in a very specific shape and weight, so it could be used in place of one of the weights necessary for balancing the payload in the rocket stage. OSCAR 1 was the first satellite to be ejected as a secondary payload (the primary payload was Discoverer 36) and to subsequently enter a separate orbit. It carried no on-board propulsion and its orbit decayed quickly. Despite orbiting for only 22 days, OSCAR 1 was an immediate success. Over 570 amateur radio operators in 28 countries forwarded observations to Project OSCAR.