Before GPS became the big dog of satellite navigation systems, there was a satellite system run by the U.S. Navy called Transit. Voyagers knew the system by its generic name SatNav. One of the fascinating aspects of Transit was the impressive longevity of its satellites. Spacecraft that were expected to last 18 months sometimes continued to operate for many years longer than expected. Many of the GPS satellites have also proved to have long operational lives. Six of the Block IIA satellites, launched between 1990/1997, are still plugging away in their 12,550-mile high orbits, helping us avoid running aground and guiding us to the nearest marine store to buy new gear for our boats.
The current GPS constellation, which was officially expanded by the Air Force in 2011 from 24 satellites to 27 satellites, contains Block IIA, Block IIR, Block IIR(M) and Block IIF satellites. The next generation of spacecraft, the GPS III, is slated to be launched starting in 2016. Below is a illustration showing the various types of satellites that currently make up the space segment of GPS.