<P> 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 (Recall that signal strength falls off as the square of the distance from the source, so the effect is dramatic). Thus there is a trade off between the number of satellites and their cost . </P> <P> In addition, there are important differences in the onboard and ground equipment needed to support the two types of missions . </P> <P> A group of satellites working in concert is known as a satellite constellation . Two such constellations, intended to provide satellite phone services, primarily to remote areas, are the Iridium and Globalstar systems . The Iridium system has 66 satellites . </P> <P> 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 . </P>

Different categories of satellite based on mass and application