The Global Positioning System [GPS], established beginning in 1973 by the US Department of Defense, has evolved from its secretive beginnings to something close to a household name. The system is still used for military applications, of course, but it has a much wider customer base. It is used in mobile phones, so they can be located by emergency services; in navigation devices for vehicles; and for many applications that rely on accurate time measurements. (Part of the GPS signal is a time stamp from the atomic clock on board the satellite.)
Now, according to an article at the PhysOrg.com site, the GPS is to receive a significant upgrade, at an estimated cost of $8 billion. The existing system uses a “constellation” of 24 satellites in orbit around the Earth at an altitude of about 20,200 km; the orbits are arranged so that there are always six satellites above the horizon. The GPS receiver calculates its position based on the signals from several satellites that it can “see”, and from the time information in those signals. (In order for the calculation to be accurate enough, adjustments must be included for the effects of General Relativity.) Current civilian GPS receivers can give horizontal position information accurate within about a five meter radius.
The new system upgrade involves the construction of 30 new satellites, to allow for six spares, by Boeing and Lockheed-Martin. These satellites will have more available commercial signals, and more accurate atomic clocks, which should have the effect of reducing the error radius to less than two meters. The upgrade
… will also make the system faster, and there will be provision to prevent disruptions such as accidental jamming of GPS, which in the recent past have caused disruption to emergency services and mobile phone services, as well as causing power outages.
The upgrade is expected to take about ten years to complete.