U.S. Air Force satellite specialists recently figured out how to extend the life of most of the GPS satellites in service. By tweaking the battery charging it became possible to keep these satellites in orbit 12-24 additional months (depending on how long a satellite had already been up there). The main limiting factor on GPS satellites is battery endurance so this tweak was a big deal.
Small but continuous improvements to the GPS satellites has been a common feature of the GPS program. The first ten GPS Block 1 (GPS I) satellites went up in the 1980s to aid in development of the system. In 1989 and 1990 nine GPS II satellites went up these provided partial coverage of the globe. These are the ones that U.S. forces used during the 1991 Gulf War and the success of GPS in that conflict proved that the concept worked and publicized that fact because of the war coverage. Between 1990 and 1997 nineteen of the GPS IIA satellites went up. Like the GPS II birds these were designed to last 7.5 years but some lasted twice as long.
The first of the IIR models went up in 1997. Currently 19 of the 28 GPS satellites in use are IIRs. After the first 12 of the one ton IIRs were sent up eight more improved GPS IIR-14 (M) satellites went up between 2005 and 2009. The IIR-14 (M) offered several new features, like a more powerful GPS signal, making it easier, and faster, for receivers to pick up a signal, and report where the user is. There are also two new encrypted signals for military users, which increased the accuracy of smart bombs. Along those lines, there was new encryption and anti-jamming capabilities for military users.
The 28 active GPS satellites in orbit provide coverage for the entire planet, as well as spare satellites to take over if one breaks down. By the end of the decade, military users will have access to the better accuracy and anti-jamming features over most of the planet. Currently the IIF models are being launched. Two of 12 are in orbit with the first of them launched in 2010. These 1.6 ton satellites are designed to last 12 years. The GPS IIIA is in development and these will last 15 years in orbit.