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GPS is something so ubiquitous that it almost feels like magic – but behind every Foursquare check-in there’s a fleet of satellites orbiting the earth that helps devices like smartphones know where they are.

Yesterday a new satellite was launched to join the fleet, replacing a one that has been in service since 1998. That older satellite, which is still functioning, will be moved into a reserve position in case any other GPS satellites break down. The new one launched yesterday has a 12 year service life.

There are currently 31 GPS satellites in an orbiting “constellation” operated by the United States Air Force. The satellites operating in the constellation are launched in “blocks” and are replaced incrementally over time to prevent equipment failures that could cripple the accuracy of geolocation on the ground. Once this current block is placed into orbit a new generation of more powerful satellites will be launched that will accuracy for military and civilian uses.