Time Transfer - Time Standard

Time Standard

Since the advent of the Global Positioning System, highly precise, yet affordable timing is available from many commercial GPS receivers. Its initial system design expected general timing precision better than 340 nanoseconds using low-grade "coarse mode" and 200 ns in precision mode. A GPS receiver functions by precisely measuring the transit time of signals received from several satellites. These distances combined geometrically with precise orbital information identify the location of the receiver. Precise timing is fundamental to an accurate GPS location. The time from an atomic clock on board each satellite is encoded into the radio signal; the receiver determines how much later it received the signal than it was sent. To do this, a local clock is corrected to the GPS atomic clock time by solving for three dimensions and time based on four or more satellite signals. Improvements in algorithms lead many modern low cost GPS receivers to achieve better than 10 meter accuracy, which implies a timing accuracy of about 30 ns. GPS-based laboratory time references routinely achieve 10 ns precision.