GPS is getting an $8-billion upgrade

[S]cientists 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 story mentions that a predecessor of GPS was Transit, to support Polaris submarines. I went to a talk recently which mentioned other programs as well: there was SECOR (SEquential COllation of Range), 621B and TIMATION. I found a brief history of these programs. The military was testing various strategies for geolocation, and each had its strengths and weaknesses. You could have the satellites be autonomous or rely on ground stations; autonomous satellites need good space-qualified clocks, which were tough to come by in the 60s, but if ground station was lost, the whole system would go down. Orbital altitude was another variable — geostationary satellites had poor coverage at high latitudes, but you required more satellites as you got into lower orbits, with progressively shorter observation windows. (A low-earth orbit (LEO), like the ISS, would require of order 100 satellites for good coverage) And various communication strategies could be employed.

They were able to draw on the experiences of each program and come up with a system that seems to have worked out pretty well.