GPS constellation of 32 satellites orbiting the Earth in MEO that provide coordinates to our smartphones (have we become too reliant on it ?), and is the mainstay of many of sensor networks on earth by providing them location and a universal time reference. One other use of this constellation is to image the ionosphere, i.e. perform an inverse problem from the measurements of the delay of message transmission between the satellites and some known antenna on the ground (here is a layman's description of the issue) to evaluate the concentration of electron in the ionosphere.
Here is an animated gif version of the solving of this inverse problem. The sensor network is made of 32 satellites plus any number of antennas on the ground.and its size scales directly with the size of the earth.