A satellite constellation, also called a satellite swarm, is a system of satellites that work together to achieve a single purpose. For some purposes, a single satellite is fine. However, where constant contact with a point on the globe or instant global communication is required, a network of satellites may be needed to avoid latency and breaks in communication. Depending upon how high above Earth the satellite orbits, breaks can happen if a single communication point as it goes over the horizon and the planet blocks the signal. To ensure that a satellite for a given purpose is in contact at all times requires, more satellites must be deployed in orbit.
Most communications satellite constellations are geosynchronous, which means they are parked in an orbit above the equator at an altitude synchronized with the rotation of the earth below. Although the satellites appear to reside in the same location in the sky at all times, there is really a round-trip delay of a bit more than a half-second, and this latency can cause frustration for those trying to communicate in real time. Deploying multiple satellites to overlap coverage can prevent latency.
In December of 2016, NASA launched a constellation of micro-satellites from an airliner. The constellation, which is called the Cyclone Global Navigation Satellite System, measures surface winds at the center of tropical cyclones, hurricanes and typhoons. Other uses for satellite constellations include telecommunications, tracking and location awareness systems, government and military monitoring and espionage. Examples include direct broadcast satellite (DBS) systems, Global Positioning Systems (GPS'), the Russian Federation's Global Navigation Satellite System (GLONASS) and satellite-based Lidar.
Satellite constellations are also being used for broadband internet and satellite phone and cellular phone networks. Although the performance doesn't yet match that of contemporary landlines, in situations where satellite-based networking may be the only option for those without access to terrestrial wired or wireless services, the compromises involved can be more than tolerable.