Comment GPS vs inertial navigation (Score 1) 298
BTW, I recently learned that the GPS system began as a clever hack by JHU APL engineers, who determined the orbit of the original Russian Sputnik satellite by listening to its Doppler shift (this technique was recently used to locate flight MH370 by measuring the Doppler shift of its telemetry signal). After doing that, they realized that this calculation can be reversed: geographic coordinates could be obtained by lmeasuring signals from a constellation of satellites in known orbits.