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(link is external), 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.
“The 100 ns stability level is good enough to meet a new telecommunications standard,” said lead author Marc Weiss, a mathematical physicist at NIST. “We’ll continue trying to meet the 1 microsecond accuracy level, which is needed by critical infrastructure such as the power industry.”
The conference paper notes that if the fiber-optic network or its power source went down and had to be re-established, then GPS or some other alternative time reference would be needed to recalibrate the fiber-optic circuit. The authors suggest the fiber network could serve as a partial backup to the GPS, and the GPS could be used for calibration to correct timing delays. Or, to provide a more reliable backup for the GPS, two independent telecom network paths could be used.
In the experiment, fiber-optic cables run from NIST and USNO to their respective nearby CenturyLink offices, where the signals are multiplexed into the network on a dedicated wavelength not shared with any other customers. The experiment began in April 2014 and will run through the end of 2016.
“It appears that there is at least one commercial transport mechanism that could serve to back up GPS for time transfer at the 100 ns level,” the paper concludes. “We have some certainty that similar results will apply if this technique were used as a service across the country.”
The need for precision timing backup has grown along with the importance of GPS. According to a 2013 study by the Government Accountability Office, “GPS is essential to U.S. national security and is a key component in economic growth, safety, and national critical infrastructure sectors.” An inability to mitigate GPS disruptions could result in billions of dollars in economic losses, the study found.
The NIST research is being carried out under a Cooperative Research and Development Agreement among NIST, CenturyLink, and Microsemi, which, in addition to collaborating on the research, is providing equipment that transmits and receives timing signals. The project has been extended to January 2017, with the possibility of testing the technique in a time transfer experiment spanning the nation.
A Wayne State University researcher understands that the three most important things about real estate also apply to small ground robotic vehicles: location, location, location.
In a paper recently published in the journal IEEE Transactions on Parallel and Distributed Systems, Weisong Shi, Ph.D., associate professor of computer science in the College of Engineering, describes his development of a technique called LOBOT that provides accurate, real-time, 3-D positions in both indoor and outdoor environments. The project was supported in part by the Wayne State Career Development Chair award, which gives Shi an opportunity to explore other areas after receiving tenure at WSU.
Scientists believe small ground robotic vehicles have great potential for use in situations that are either uncomfortable or too tedious for humans. For example, a robot may become part of industrial operations, assist senior citizens or serve as a tour guide for an exhibition center. Keeping a robot as small as possible enables it to move through narrow passageways, such as tunnels.
To complete such missions, a robotic vehicle often must obtain accurate localization in real time. But because frequent calibration or management of external facilities is difficult or impossible, a completely integrated self-positioning system is ideal. In addition, that system should work indoors or outdoors without human calibration or management and cost as little as possible.
In the paper titled “LOBOT: Low-Cost, Self-Contained Localization of Small-Sized Ground Robotic Vehicles,” Shi and lead author Guoxing Zhan, one of his former graduate students, describe their technique, which combines a GPS receiver, local relative positioning based on a 3-D accelerometer, a magnetic field sensor and several motor rotation sensors.
The researchers noted that IEEE Transactions, the leading journal in the field, prominently featured their paper in its April 2013 issue. They are proud that their work was in progress before President Barack Obama’s June 2011 announcement of the National Robotics Initiative, which seeks to accelerate the development and use of robots in the United States that work beside, or cooperatively with, people.
Shi’s technique combines elements of common localization schemes for ground robotic vehicles, noting that each of those schemes has limitations. One scheme, using GPS alone, requires a lot of power. Another, radio-based positioning, requires proper calibration, a friendly environment and a set of external devices to generate or receive radio signals.
A third scheme, the use of vision techniques, relies heavily on recognition of objects or shapes and often has restricted spatial and visual requirements. Additionally, those objects and shapes must be captured and loaded into a computer which, like GPS, requires a lot of power.
A fourth scheme, inertial sensors, is part of the LOBOT design. Inertial sensors often are used to detect movement, but unlike radio- or vision-based techniques, operate independently of external environmental features and need no external reference. However, previous methods of maintaining their accuracy have resulted in high cost and calibration difficulty.
LOBOT uses a hybrid approach that localizes robotic vehicles with infrequent GPS use, a 3-D version of the accelerometer used in other inertial sensor systems and several motor rotation sensors — all installed on the robotic vehicle. All of the components are commercially available, with some costing as little as $20.