appear to a ground observer to cross the sky and "set" when they go behind the Earth beyond the visible horizon. Therefore, to provide continuous communications capability with these lower orbits requires a larger number of satellites, so that one of these satellites will always be visible in the sky for transmission of communication signals. However, due to their closer distance to the Earth, LEO or MEO satellites can communicate to ground with reduced latency and at lower power than would be required from a geosynchronous orbit. Low Earth orbit (LEO) A low Earth orbit (LEO) typically is a circular orbit about above the earth's surface and, correspondingly, a period (time to revolve around the earth) of about 90 minutes. Because of their low altitude, these satellites are only visible from within a radius of roughly from the sub-satellite point. In addition, satellites in low earth orbit change their position relative to the ground position quickly. So even for local applications, many satellites are needed if the mission requires uninterrupted connectivity. Low-Earth-orbiting satellites are less expensive to launch into orbit than geostationary satellites and, due to proximity to the ground, do not require as high signal strength (signal strength falls off as the square of the distance from the source, so the effect is considerable). Thus there is a trade off between the number of satellites and their cost. In addition, there are important differences in the onboard and ground equipment needed to support the two types of missions. Satellite constellation A group of satellites working in concert is known as a satellite constellation. Two such constellations, intended to provide satellite phone and low-speed data services, primarily to remote areas, are the Iridium and Globalstar systems. The Iridium system has 66 satellites, which orbital inclination of 86.4° and inter-satellite links provide service availability over the entire surface of Earth. Starlink is a satellite internet constellation operated by SpaceX, that aims for global satellite Internet access coverage. It is also possible to offer discontinuous coverage using a low-Earth-orbit satellite capable of storing data received while passing over one part of Earth and transmitting it later while passing over another part. This will be the case with the CASCADE system of Canada's CASSIOPE communications satellite. Another system using this store and forward method is Orbcomm. Medium Earth orbit (MEO) A medium Earth orbit is a satellite in orbit somewhere between above the