电信网络可作为GPS时间信号的候补网

Precision time signals sent through the Global Positioning System (GPS) synchronize cellphone calls, time-stamp financial transactions, and support safe travel by aircraft, ship, train and car. What if GPS goes down? The National Institute of Standards and Technology (NIST) and the U.S. Naval Observatory (USNO), which operate U.S. civilian and military time standards, respectively, have worked with two companies--Monroe, Louisiana-based CenturyLink, and Aliso Viejo, California-based Microsemi--to identify a practical backup possibility: Commercial fiber-optic telecommunications networks.

In GPS systems, transmissions can be disrupted unintentionally by radio interference or the weather in space, for instance. Various types of intentional interference are possible also. Federal agencies have long recognized the need to back up GPS, a collection of several dozen satellites that has provided users with time and position information since the 1970s.

To explore the possibility of using commercial telecom networks as a backup for time services, an ongoing experiment connects the NIST time scales in Boulder, Colorado, with the USNO alternate time scale at Schriever Air Force Base in Colorado Springs by means of CenturyLink's fiber-optic cables. The two federal time scales, 150 kilometers apart, are ensembles of clocks that generate versions of the international standard for time, Coordinated Universal Time (known as UTC), in real time.

In this experiment, time signals were sent at regular intervals in both directions between the two locations. Researchers measured the differences between the remote (transmitted) and local time. 

The results, just presented at a conference, showed UTC could be transferred with a stability of under 100 nanoseconds (ns, or billionths of a second)--thus meeting the project's original goal for this metric--as long as the connection remained unbroken. Stability refers to how well the remote and local clocks remain synchronized. Because the signals were forwarded by various pieces of equipment along each path, they experienced significant unequal delays in the two different directions. This reduced overall performance, resulting in an accuracy that did not meet the stated goal of 1 microsecond (millionths of a second).* With the GPS available to calibrate (and thus correct for) the unequal delays, time transfer could be accomplished maintaining that calibration within 100 ns if GPS were to "disappear," the study suggests.