\" \" The U.S. federal government maintains at least 24 operational GPS satellites at any given time. On the electrical grid, phase measurement units use GPS signals to make sure that sine waves of electricity match up where two grids come together. Telecommunication networks use GPS signals to keep their cellphone towers synchronized to prevent your call from dropping on that train ride to grandma's house. GPS satellites transmit weak, very high-frequency signals that travel to Earth through 12,645 miles (20,350 kilometers) of space and atmosphere. Last January 2016, the U.S. Air Force decommissioned a GPS satellite, something it's routinely doing, and in the process introduced a 13.7-microsecond error to ripple through half of the GPS satellites still in orbit.