A few years back, I had an idea for an ionospheric sounding-like experiment. We could install a software routine that would scan AM & FM radio frequencies inside of every car (provided that the radio was not in use) and then transmit the data whilst passing by a data collection station. We'd need the GPS data as well, collected by the car during the process of logging signal integrity as it scans wavelengths. Then, you cross correlate the signal origins (aka AM & FM radio towers) via a database, correlate it to the car's data, and BAM, science just happened.

We could employ a somewhat similar distributed network amongst smartphones for GPS. This makes some sense, as you would have the best spatial resolution within cities, with a higher population density. It's problematic when you're out in the middle of nowhere with no line of sight to another GPS-networked device, on your own, requiring GPS to survive. That reason alone will be why we'll always have at least a few GPS satellites. The network of distributed smartphone GPS would free up some bandwidth on the satellites, as we add a couple billion more GPS-capable devices to Earth. For all we know, these capabilities may already exist through NSA backdoor routines.

Obviously, the problem with both of these ideas is privacy. No one wants the battery drain of being a node, and they sure as hell don't want to feed any government entity, or any network of people information about their whereabouts at all times. Despite the fact that we're basically already doing this every time we use GPS, or even carry a cellphone on us.

To address this need, DARPA is investing in radically new technologies that have the potential to deliver GPS-quality position, navigation and timing information for military systems, novel inertial measurement devices that use cold-atom interferometry; chip-scale self-calibrating gyroscopes, accelerometers and clocks; and pulsed-laser-enabled atomic clocks and microwave sources.

Thanks, now I have to clean the drool out of my keyboard. I admire the (apparent?) transparency of DARPA. It seems to be the only acronym containing "defense" that doesn't typically piss me off. Even DoE gets my goat sometimes.

Relativity accounts for these differences, which amount to about 38 microseconds per day — a tiny amount, but just enough to misread your position by miles.

I have done this calculation myself as homework and the contribution from general relativity (i.e. space-time curvature due to Earth's mass, on the surface vs. at the GPS satellites' altitude) outweighs the contribution from special relativity (the difference in relative velocities, on the surface vs. at the GPS satellites' speed). In case you were curious.