CAPE CANAVERAL, Fla. -

The first of three new Air Force Global Positioning System satellites launched from Cape Canaveral.

The launch, which was delayed about 20 minutes after being scheduled for 8:40 p.m. Thursday, was part of an effort to replace aging satellites already in orbit, some of which are 21 years old, according to Local 6 news partner Florida Today.

The 3,900 pound United Launch Alliance Delta IV rocket launched from Launch Complex 37.

The new-generation satellite, known as IIF, is the fifth of 12 of its kind. The spacecraft it will replace has reached 16 years of age, which is nearly double its expected lifespan.

Of the 36 GPS satellites, 31 are part of an active constellation that provides precision position, navigation and timing information used for everything from guiding weapons to driving directions. The satellites orbit at about 11,000 miles in the air.

The five remaining satellites are in “residual” status, and can be called into service if necessary.

Eight members of the active group are part of the oldest “IIA” batch. Launched between 1990 and 1997, the oldest IIA satellite is 23.

Thursday's launch was originally scheduled for October, but was delayed while the Air Force and ULA investigated a fuel leak. The leak previously resulted in low thrust by a Delta IV upper stage engine during the successful launch of another GPS satellite in October of 2012.

The 206-foot, liquid-fueled Delta IV about to launch is in its "Medium-plus" configuration, with two ATK solid rocket motors assisting the first-stage booster.

The rocket was scheduled to deploy the GPS satellite more than three-and-a-half hours after launch.