The Time and Frequency Division (TFD) is located in Boulder, Colorado. The division consists of approximately 125 technical personnel, including federal employees and NIST associates. The majority of the staff works on the NIST Boulder campus. One TFD staff member has an appointments in both the Time and Frequency Division and the Quantum Physics Division. The TFD carries out an extensive technical program in virtually all areas of time and frequency. While the majority of division funding comes directly from the NIST appropriation, a significant amount of its support is received from the DoD and members of the national intelligence community. The breadth, depth, and impact of this program are unmatched by any similar foreign metrology organization.
ASSESSMENT OF TECHNICAL PROGRAMS
The current TFD technical program is well aligned with the strategic goals of the organization. These goals have three interrelated components: realization of the official U.S. time scale (Atomic Standards Group), dissemination of time and frequency information (Time and Frequency Services Group), and conduct of the research that underpins the quality of products today and ensures advances of these products in the future (Ion Storage, Optical Frequency Measurements, and Atomic Devices and Instrumentation groups) This is a solid strategic approach. Its results have produced state-of-the-art time and frequency products today with a firm foundation for future improvements in place. The TFD effort expended in developing and addressing a strategic perspective is in keeping with recommendations provided by the 2010 National Research Council review of the NIST Physics Laboratory.1 The recommendation calling for more thorough strategic planning has been addressed in an excellent manner by the TFD.
The quality of TFD products employed in realizing official U.S. time and frequency is outstanding. Central to this is the national frequency standard NIST-F2. This standard, based on a cesium atom hyperfine transition, is internationally recognized as the most precise and accurate national standard2 in the world. Consistent with a technical program that addresses current and future time and frequency needs, NIST-F2 is the cryogenic extension of the NIST-F1 standard. The latter standard was the first United States standard to employ cold atoms in a cesium fountain, while NIST-F2 now places the microwave interaction region in a cryogenic environment, dramatically reducing the frequency uncertainties resulting from the blackbody radiation of the standard housing. The TFD accomplishment of developing the cryogenic, cesium fountain clock as the national standard of the United States has improved national and international timekeeping.
1 National Research Council, 2010, Assessment of the National Institute of Standards and Technology Physics Laboratory: Fiscal Year 2010, The National Academies Press, Washington, D.C., Chapter 1.
2 T.P. Heavner, E.A. Donley, F. Levi, G. Costanzo, T.E. Parker, J.H. Shirley, N. Ashby, S.E. Barlow, and S.R. Jefferts, 2014, First accuracy evaluation of NIST-F2, Metrologia 51(174).
The actual U.S. timescale is maintained by an ensemble of about 10 hydrogen masers whose frequencies are periodically steered by NIST-F2 and NIST-F1. This information is a major contributor to coordinated universal time (UTC), the international time scale maintained by the BIPM,3 the French equivalent of NIST. Complementing the time scale is the exceptional progress the TFD has shown in the development of ultrastable radio-frequency (RF)/microwave signals. Employing laser comb signals, TFD laboratories have improved the stability of microwave signals from 100 to 10,000 beyond that obtained from other methods of generation. This is reflective of a technical program producing products of the highest quality.
The TFD has accomplished its goal of disseminating its time and frequency information in an exemplary manner. The TFD is a major contributor to national and international time scales. Beyond this, the TFD actively distributes its expertise domestically and internationally, providing training opportunities to guest researchers, maintaining a strong commitment to publication and conference participation, and conducting a variety of instructional events.
Underlying time scale maintenance and dissemination is the research portion of the TFD technical program. The program seeks to advance the state of the art to meet the needs of stakeholders in the future and to improve the quality of current time and frequency products. An example of applied research improving products in the near term is the effort to operate a new primary frequency standard (such as NIST-F1 or -F2) in a nearly continuous manner. This new approach to maintaining a time scale allows a single-hydrogen maser to be almost continuously steered by the primary standard, replacing the 10-maser ensemble that is only periodically adjusted by information from NIST-F2. The result will be a more stable time scale realized in a more cost-effective manner.
Looking further into the future, TFD laboratories are exploring new standards based on optical transitions. With operating frequencies approximately 100,000 greater than those of microwave transition standards, optical standards are already orders of magnitude more stable and accurate than their microwave analogs. The new generation of standards can be divided into two general categories based on atoms and ions. The atomic standard being investigated employs optical transitions in ytterbium (Yb) atoms. Currently the TFD Yb standard is the most stable in the world. Its stability is on the order of 2 × 10−18. This can be compared to the accuracy and ultimate stability of NIST-F2, whose accuracy reaches approximately 1 × 10−16. The ion standards employ either single mercury (Hg) or aluminum (Al) ions held in electromagnetic traps. Again, these devices have accuracies in the upper 10−18 range. The TFD research into trapped ion standards improved the performance of these devices and provided a foundation for significant accomplishments in the area of QI and processing. Evidence of the significance of accomplishments in this area is the Nobel prize awarded to David Wineland, a TFD staff member. A further accomplishment of the research effort is the recognition and demonstration that combining optical standards with optical frequency combs allows relating their frequencies in an extremely precise manner to lower, electronically accessible values that are more easily manipulated. The research aspects of the TFB technical program are also at the state of the art internationally.
Members of the TFD continue to work at the forefront of optical frequency combs. One area of particular accomplishment focuses on generation of microwave signals with ultralow phase noise and timing jitter through photodetection of optical frequency combs. In this so-called optical frequency division process, the already low optical phase noise associated with individual comb lines that are referenced to ultrastable resonators and atomic frequency references is divided down by the ratio of the optical to microwave frequency. Through optical frequency division TFD scientists have now demonstrated generation of 10-GHz microwave signals with the best low-frequency phase noise ever reported, substantially below what is available with the best electronic oscillators. This work is pushing technological performance and is advancing scientific understanding. One notable example involves the understanding of shot noise, a fundamental noise process in photodetection that arises because photons arrive as discrete and randomly spaced energy packets. A standard expression for shot noise has been
known for decades and is widely applied. TFD researchers showed that when short pulse laser sources such as those from optical frequency combs are used for microwave generation, the standard shot noise formula associated with continuous-wave laser excitation is modified, allowing generation of microwave signals with phase noise and timing jitter below the limit that the conventional shot noise formula would predict.
Another notable area of accomplishment within the last 5 years is in the generation of optical frequency combs through nonlinear wave mixing in optical microresonators. Such microcombs offer dramatic size reduction compared to frequency combs derived from mode-locked lasers, thereby opening the possibility of applications beyond specialized laboratory settings. TFD researchers have developed new methods for fabricating high-quality-factor optical microresonators at low cost and high speed and have used these microresonators as a platform for investigating comb generation physics and noise processes. The TFD is among about a half dozen of the most active and visible groups worldwide, and it is at or very near the top of this group in terms of application of microcombs for optical metrology and low-phase-noise signal generation. The TFD accomplishments in optical frequency division for low-phase-noise microwave generation and in microcombs have attracted a series of grants from other government agencies.
There are also interactions with companies that want to commercialize the compact Ca clock developed at the TFD. This system would serve as an ultrastable microwave source for a variety of applications, including very long baseline telescopes and radar detection of slow-moving objects. Clock-based relativistic geodesy is an intriguing possibility that challenges contemporary physics to find applications where gravitational potential is the critical quantity.
TFD activities on chip-scale atomic clocks, which originated a little more than a decade ago under funding from the Defense Advanced Research Projects Agency (DARPA), are a major success story. By employing modern microfabrication techniques and miniature lasers, the NIST group was the first to realize an atomic clock at chip scale. This effort was relatively new at the time of its inception but has now grown into a subject pursued by many groups worldwide. A commercial version of the chip-scale atomic clock was introduced in 2011 by a U.S. company, Symmetricon (now Microsemi), and is being used for applications with modest timing requirements (modest in comparison to NIST’s state-of-the-art frequency standards) such as telecommunications networks, GPS receivers, and seismic exploration. The development of the chip-scale atomic clock has impact from both a research and a dissemination perspective.
The frontier of trapped ion physics and technology is defined by Wineland’s Ion Storage Group in the TFD. Indeed, the early days of atomic cooling were pioneered by, for example, Wineland and nearly simultaneously by Dehmelt at the University of Washington. They pioneered various aspects of laser cooling of atoms, and they are theoretically and experimentally showing how the kinetic energy of a collection of resonant absorbers can be reduced by irradiating these absorbers with near-resonant electromagnetic radiation. Their process is closely related to anti-Stokes spontaneous Raman scattering. At present the use of ion traps is ubiquitous in many areas of physics, including quantum computers and atom-field matter interactions.
Over the last few years, work on chip-scale atomic sensors has been broadened to include other sensing applications. In one example, TFD researchers demonstrated the first chip-scale magnetometer. This device, which operates near room temperature, achieved performance equivalent to that of SQUID, which requires cooling to below 1 K. TFD scientists partnered with medical researchers to demonstrate sensing of weak biomagnetic fields, enabling, for example, precision measurements of fetal heart activity without the need to implant electrodes. New programs aimed at using atomic sensors for sensing of other fundamental quantities (for example. length and temperature) are now under way as part of the NIST-on-a-Chip initiative.
The dramatic progress in laboratory demonstrations of optical clocks, both in the TFD and in the Quantum Physics Division, brings them to the point where they demonstrate world-record stability of 2 × 10−18, nearly two orders of magnitude better than the NIST-F2 cesium clock (currently at ~1 × 10−16 accuracy). Several different matter systems for optical clocks are being explored (for example, various
ions in the TFD Ion Storage Group, Yb atoms in the TFD Optical Frequency Measurements Group, and Sr atoms in the Quantum Physics Division at JILA). Optical clock technology shows promise to take over as the new primary frequency standard in the years ahead. The opportunity and challenges confronting NIST researchers and researchers worldwide is to continue this progress, to develop optical clocks to the maturity needed, and to chart the evolution of optical clock technology to become a primary standard.
The performance of the best optical clocks is now at a level where even minute changes in the gravitational field, such as those associated with a few centimeters change in elevation, become observable. On the one hand, this brings new opportunities for applying such clocks to sensing. On the other hand, it raises the question whether optical clocks are nearing their accuracy limit, since with further improvements they may become dominated by the local microgravitational environment.
TFD research related to the NIST-on-a-Chip initiative appears to center on incorporation of multiple atomic sensors, such as those introduced in the previous, highly successful chip-scale atomic clock work, interconnected via on-chip waveguides with in-plane light propagation. The general idea of scaling to a larger number of sensors and exploiting waveguide photonics to enable more sophisticated device architectures is appealing. Further development of their roadmap for this work will be beneficial.
PORTFOLIO OF SCIENTIFIC EXPERTISE
The scientific and technical expertise resident in the TFD is outstanding. Developing this internationally recognized level of excellence is a significant accomplishment of the TFD. The achievements outlined in the preceding section result directly from the excellence of the scientific expertise resident in the division. This excellence is further confirmed by the numerous awards TFD staff members have received, including a 2012 Nobel prize in physics. Additionally, TFD staff members have received multiple external awards, such as the Benjamin Franklin Medal from the Franklin Institute, the Rank Foundation Prize, the IEEE Sensor Technical Achievement Award for chip-scale devices, the IEEE Rabi Awards for time-scale establishment and oscillator development, and the National Conference of Standards Laboratories International (NCSLI) Wildhack Award for remote time-measurement services. Staff members have also received many NIST and DOC awards. These include multiple Arthur S. Flemming Awards, Gold and Silver and Bronze DOC awards, and NIST’s Rabinow, Condon, and Slichter awards. The award subjects span a wide variety of TFD areas of investigation and effort. Within NIST a prestigious position is that of fellow, a rank achieved by only 2 percent of staff members. While the TFD represents 2 percent of the NIST technical staff, fellows within the TFD comprise 10 percent of the NIST population of Fellows.
The TFD scientific expertise evident during the review was primarily of an experimental nature. One consideration is whether the theoretical expertise actually resident within the TFD is sufficient to support the experimental efforts into the future. As stabilities and accuracies of frequency standards continue to improve, strong theoretical support will be required. Of course, NIST staff does collaborate closely with staff of JILA, where theoretical expertise resides. The adequacy of this expertise and ensuring continuing close interaction deserves ongoing attention.
With the current excellence of TFD scientific expertise comes the challenge of maintaining the same high level into the future. This will require ongoing effort on the part of NIST researchers and management. Not surprisingly, many current junior staff members have outstanding qualifications. Confirming this are the multiple forms of recognition already received. Examples include the International Union of Pure and Applied Physics (IUPAP), the Young Scientist Award, European Frequency and Time Forum Young Scientist Awards, and the Humboldt Foundation’s Fyodor Lynen Research Fellowship. This bodes well for ongoing excellence of the scientific expertise within the TFD.
ADEQUACY OF FACILITIES, EQUIPMENT, AND HUMAN RESOURCES
Most of the buildings on the NIST Boulder campus are about 60 years old. These older buildings do not provide the environmental controls (for example, temperature and humidity) necessary for modern precision measurement central to the TFD mission. The new precision measurement laboratory building, which was completed in 2012, represents a major improvement. The laboratories visited are well equipped. Such performance improvements in physical facility are essential to the division’s ability to maintain its scientific leadership. Challenges associated with shortage of meeting and collaboration space have also been voiced by TFD staff.
The unique facilities associated with the TFD provide remarkable synergy, enabling research that could be accomplished at few other institutions. As one example, research of optical frequency combs can draw on cold atom and ion frequency standards programs and use these highly stabilized frequency references for stabilization of the comb. Optical frequency comb research can also access maser signals associated with the U.S. primary clock for stabilization or measurement of the comb repetition rate. Conversely, the highly stabilized comb enables comparison of optical clocks based on different matter systems.
Of approximately 130 TFD personnel, 42 percent were NIST permanent employees. The remaining 58 percent were guest researchers, consisting of graduate students, postdoctoral researchers, contractors, and foreign visitors. A benefit of the large pool of guest researchers is that the division work force is being constantly refreshed. The guest researchers also provide a pool from which permanent hires can be made when openings arise. As noted above, several of the more junior TFD researchers have won awards from external bodies. Such recognitions provide evidence of the TFD’s ability to attract excellent new talent to its workforce.
Because of its unique strengths and capabilities, the TFD has enjoyed continued success in attracting funding from other government agencies. This has allowed it to broaden its research portfolio in important ways beyond its core activities. Some of this work has been sufficiently successful that it led to some of the division’s highest impact programs—for example, chip-scale atomic clocks, quantum information (QI) processing, and low phase noise signal generation from optical frequency combs.
TFD staff have voiced concerns that there are major institutional challenges associated with administrative processing in accepting funding from other agencies.
Concerns were also raised with respect to an onerous procurement process with substantial delays. Several staff spoke of a high overhead rate on capital equipment; they suggested that this makes acquisition of expensive equipment very difficult.
DISSEMINATION OF OUTPUTS
The products of the TFD program are various and are disseminated in many ways. The division distributes not only scientific expertise and findings but also technologies for measuring and maintaining precise time and frequency; it disseminates internationally as well precise time and frequency information. The dissemination efforts and their resulting impacts are outstanding, meeting the needs of both internal and external stakeholders. While there are opportunities and a limited number of areas of potential improvement, the levels of excellence in all aspects of product dissemination are at the state of the art, displaying international leadership.
The dissemination of scientific expertise and products proceeds along two primary paths. Fifty-eight percent of the division staff is composed of guest researchers, primarily graduate students, postdoctoral fellows, and visiting researchers. The ratio of permanent to guest staff is the result of a strategy that seeks to disseminate scientific expertise to scientists throughout their careers. The ongoing training of nearly 60 researchers in a field as specialized as the measurement and maintenance of extraordinarily precise time and frequency is an accomplishment worthy of note. The scale of such efforts may be compared to those of similar organizations such as BIPM. One of the BIPM’s responsibilities is
the maintenance of international time through the collection of information from timekeeping institutions around the world. Its time department lists 8 permanent staff members4 compared to the approximately 50 permanent NIST staff members in the Time and Frequency Division. By inference, the BIPM’s outreach through visiting researchers will in no way match that of the TFD.
Beyond direct training, the TFD publishes the results of its scientific investigations regularly, and its staff participates in numerous domestic and international conferences. Over the period 2010 through 2014 the TFD averaged 58 publications per year. In 2014 the BIPM Time Department produced 12 publications.5 In 2014 also, the time and frequency department of the German equivalent of the NIST, the Physikalisch-Technische Bundesanstalt, produced 5 publications.6 Similarly, in 2012, the Division of Time and Frequency of the Chinese National Institute for Metrology produced 10 publications.7 In terms of its publications, the TFD is therefore a unique institution on an international scale.
In the dissemination of time and frequency technology the TFD has excelled in a number of areas. The most accurate primary atomic frequency standard contributing to UTC is the cooled cesium fountain clock, NIST-F2. The scientific and technical excellence embodied in this device set the state of the art in 2009 when it was brought into operation, with continuing improvement through 2014, at which point it became the official primary frequency standard of the United States,8 along with the continuing NIST-F1. To distribute the unique expertise within this standard, a copy was provided to the Italian National Metrology Laboratory, Istituto Nazionale di Ricerca Metrologica, in 2009.9 Today this is the second-most-accurate standard contributing to UTC. The research that NIST conducts currently in the field of optical frequency standards will result in the next generation of primary national standards. Eventually this will significantly improve the performance of the UTC time scale. Progress in this area is disseminated through publications and participation in international conferences.
Continuing its efforts related to chip-scale devices, the TFD is applying its expertise related to small gas cell frequency standards to a new generation of extremely compact magnetometers. Such devices have the potential to make laboratory-quality magnetic field measurements available to a broad range of applications in various environments. Progress in this field is conveyed to the international community through publications.10,11,12 Of particular note is the introduction of such technology to the
5 Bureau International des Poids et Mesures, Time Department, Director’s Report on the Activity and Management of the International Bureau of Weights and Measures (January 1, 2014–December 31, 2014) Supplement: Time Department, http://www.bipm.org/en/publications/directors-report/, accessed September 14, 2015.
6 Physikalisch-Technische Bundesanstalt, Time and Frequency Department Publications, http://u99132.bs.ptb.de/cms/de/ptb/fachabteilungen/abt4/fb-44/44-literatur.html, accessed September 14, 2015.
8 T.P. Heavner, E.A. Donley, F. Levi, G. Costanzo, T.E. Parker, J.H. Shirley, N. Ashby, S.E. Barlow, and S.R. Jefferts, 2014, First accuracy evaluation of NIST-F2, Metrologia 51(174).
9 F. Levi, C. Calosso, D. Calonico, L. Lorini, E. Bertacco, A. Godone, G. Costanzo, B. Mongino, S. Jefferts, T. Heavner, and E. Donley, 2009, “The Cryogenic Fountain ITCsF2,” pp. 769-773 in Frequency Control Symposium, 2009, Joint with the 22nd European Frequency and Time Forum, IEEE International, doi:10.1109/FREQ.2009.5168289.
10 R. Jiménez-Martinez, S. Knappe, and J. Kitching, 2014, An optically-modulated zero-field atomic magnetometer with suppressed spin-exchange broadening, Review of Scientific Instruments 85:045124.
11 R. Jiménez-Martinez, D.J. Kennedy, M. Rosenbluh, E.A. Donley, S. Knappe, S.J. Seltzer, H.L. Ring, V.S. Bajaj, and J. Kitching, 2014, Optical hyperpolarization and NMR detection of 129Xe on a microfluidic chip, Nature Communications 5:3908.
12 P.J. Ganssle, H.D. Shin, S.J. Seltzer, V.S. Bajaj, M.P. Ledbetter, D. Budker, S. Knappe, J. Kitching, and A. Pines, 2014, Ultra-low-field NMR relaxation and diffusion measurements using an optical magnetometer, Angewandte Chemie Internationale Edition 53(37):9766-9770.
field of medical imaging.13 This is an example of the TFD disseminating its time and frequency expertise to fields outside its core areas of responsibility. The TFD continues to demonstrate exceptional breadth of expertise in its dissemination.
Another aspect of time and frequency technology is related to the high-precision measurement of the phase noise of electromagnetic signals. The TFD excels at an international level in this field and disseminates its state-of-the-art expertise through numerous publications. For example, a recent article identified a weakness in the accepted means of measuring phase noise. The failure of these measurement techniques under certain conditions can lead to errors in phase noise magnitudes on the order of 20 dB.14 The TFD engineers are also extending the range of frequencies over which precise phase noise measurements can be made, again disseminating information about this technique through publication.15 Finally, research in the generation of ultrastable RF signals based on the use of optical transitions and optical combs is laying out a path to the future of ultralow-phase noise microwave signals.16
Complementing its publications, the TFD presents a series of seminars and workshops addressing various aspects of precise timekeeping. The best known of these is the annual NIST time and frequency metrology seminar. Now in its fortieth year, this is the most comprehensive introductory seminar on the field of time and frequency in the world.
The final component of dissemination is the actual distribution of precise time information. For many years, the TFD has very successfully distributed time and frequency information in a number of ways. These include providing calibration services at the NIST Boulder campus, precise time distribution via various forms of radio transmissions, and, more recently, the distribution of time via the Internet. In recent years there have been several noteworthy accomplishments in the area of dissemination. The TFD contributions of ultraprecise time and frequency information to UTC and its recent improvement through NIST-F2 have already been noted. The TFD led the formation of a near real-time international time scale among members of the Sistema Interamericano de Metrologia (SIM), a regional metrology organization whose members are nations of the Organization of American States.17 The SIM Time Network is based on GPS common view intercomparisons between member state primary standards. Today the timing laboratories of 23 SIM member states participate in the SIM time network, and further expansion to more SIM members is planned. Timing information for the SIM Time Network is disseminated at the +/- 100 ns level. A unique, recent accomplishment of this time scale is its real-time nature enabled by the Internet-based exchange of intercomparison information. In contrast, UTC is a backward-looking time scale with intercomparison information delayed two to four weeks. The real-time nature of the SIM time scale is an international first and is driving UTC to a more frequent dissemination of information. The TFD SIM time-scale efforts disseminate high-quality time throughout the western hemisphere and also influence the global UTC scale.
At a lower level of precision, NIST, under TFD leadership, continues to broadcast precise timing information from high frequency and low frequency radio stations in Colorado and Hawaii to the continental United States and Pacific waters. With proper adjustment for propagation time and atmospheric conditions, this system can provide time information at the ±100 µsec level. Broadcasting in Colorado since 1965, the transmission system modified its transmission waveform in 2012, resulting in
13 O. Alem, A.M. Benison, D.S. Barth, J. Kitching, and S. Knappe, 2014, Magnetoencephalography of epilepsy with a microfabricated atomic magnetrode, Journal of Neuroscience 34(43):14324-14327.
14 C. Nelson, A. Hati, and D. Howe, 2014, A collapse of the cross-spectral function in phase noise metrology, Review of Scientific Instruments 85:024705.
15 A. Hati, C. Nelson, and D. Howe, 2014, PM noise measurement at W-band, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 61(12):1961-1966.
16 S. Diddams, J. Li, X. Li, H. Lee, and K. Vahala, 2014, Electro-optical frequency division and stable microwave synthesis, Science Magazine 345(6194):309-313.
17 J.M. López-Romero, M.A. Lombardi, N. Diaz-Muñoz, and E. de Carlos-Lopez, 2014, SIM time scale, IEEE Transactions on Instrumentation and Measurement 62(12):3343-3350.
better coverage in areas that prior to the upgrade had poor reception.18 The formal dissemination of time and frequency information has been markedly improved through this recent upgrade.
NIST TFD time dissemination efforts are also looking toward the future. The network-based dissemination of timing information is of growing importance to a variety of industries, including telecommunications and financial services. To meet growing needs the TFD is exploring the use of the precise time protocol in optical networks.19 These steps forward in the dissemination of timing information are distributed to the national and international technical communities through publications. GPS common view techniques for time transfer, along with those enabled through communication satellites, have been in operation for many years. The TFD is now investigating more precise time transfer that could be enabled through use of a hydrogen maser on the International Space Station (ISS). The ever-improving operational time dissemination services, along with research in this field, demonstrate a robust time dissemination program at the TFD. This program is at the leading edge of international activities in this field.
The emphasis on patent preparation does not appear to be stable from year to year. A careful assessment by the division of the value of patenting would be worthwhile. Once the value proposition is in hand, staff can be given more consistent direction.
The product dissemination efforts of the TFD are at the state of the art in this field and in many cases define it. The TFD has opportunities to advance the state of the art in several areas. These include improved global time transfer through use on an atomic standard on board the ISS and further improvement of network time transfer using the precision time protocol (PTP). The primary challenge to TFD progress is maintaining adequate funding for its diverse efforts in time and frequency and the ensuing dissemination of those products.
18 Y.Liang, O. Eliezer, D. Rajan, and J. Lowe, 2014, WWVB time signal broadcast: An enhanced broadcast format and multi-mode receiver, IEEE Communications Magazine 52(5):210-217.
19 M. Weiss, L. Cosart, J. Hanssen, S. Hicks, C. Chase, C. Brown, C. Allen, P. Johnson, G. Wiltsie, and D. Coleman, 2014, Ethernet time transfer through a U.S. commercial optical telecommunications network, pp. p. 214-220 in Proceedings of Annual Precise Time and Time Interval Systems and Applications Meeting, Institute of Navigation, Manassas, Va.