The MUOS constellation is eventually expected to include four next-generation communication satellites and an in-orbit spare.

WSJ: U.S. to Trim Commercial-Satellite Use

Virgina-based Iridium is almost halfway through a multi-year plan to replace its low-earth orbit network of communication satellites with a powerful new generation.

FORBES: Iridium Preps Its NEXT Generation Of Satellites

That is, until US satellite company Iridium decided to sell real estate on a portion of its next-generation global satellites, which are set to go into orbit starting in 2015 and be fully operational by 2018.

BBC: The key to preventing lost planes

The costs of producing GPS chips is dropping constantly and a new generation of satellites that will deliver improved accuracy are due to start reaching orbit by 2008 or so.

FORBES: Magazine Article