scientists and engineers — including those at a sprawling satellite-making factory in El Segundo — are developing an $8-billion GPS upgrade that will make the system more reliable, more widespread and much more accurate.

The new system is designed to pinpoint someone's location within an arm's length, compared with a margin of error of 20 feet or more today. With that kind of precision, a GPS-enabled mobile phone could guide you right to the front steps of Starbucks, rather than somewhere on the block.

The 24 satellites that make up the GPS constellation — many of them built at the former Rockwell plant in Seal Beach — will be replaced one by one. The first replacement was scheduled to be launched from Cape Canaveral this weekend. The overhaul will take a decade......