Conference and White Papers

These conference and white papers also present the latest developments in technology and regulations affecting time management. Please contact us at 1-408-428-7907 or support@symmetricom.com if we can answer any additional questions.

To help you select that appropriate document we have included abstracts for each of them and organized them into the following categories.

• Precise Frequency References & Oscillators
  • Active Hydrogen Masers
  • Cesium Beam Frequency Standards
  • Rubidium Gas Cell Frequency Standards
  • Advanced Clock Development
• Network Time Servers & NTP
• Next Generation Networks
• GPS and Time Code Instrumentation
• Advanced Timing Solutions
• Digital Broadcasting

Precise Frequency References & Oscillators

Active Hydrogen Masers

• The Long-Term Stability of the U.S. Naval Observatory's Masers (PTTI Conference 2004)
  Abstract: This paper reports on the observed long-term performance of the hydrogen masers in an operational setting. The USNO received its first five cavity-tuned hydrogen masers in 1989 and 1990, and five more masers were delivered in 1992 through 1994. These masers, coupled with the acquisition of cesium-beam standards and improved time transfer techniques, led to significant improvements in the stability of the USNO Master Clock. Download Now.

• Symmetricom Sigma Tau Standards Group - Active Hydrogen Maser MHM 2010
  Abstract: This white paper provides an overview of some of the many Active Hydrogen MASERs, delivered by the Symmetricom Sigma Tau Standards Group over the past 20 years. Read about the MHM 2010, the first commercially available Active Hydrogen Maser in the world with stand-alone Cavity Auto Tuning and how this technique, developed by the Symmetricom Sigma Tau Standards Group, enables the MHM 2010 to deliver long-term stability normally only attributed to the most stable of cesium atomic standards. Read More.

• Optically-Pumped Cesium Beam Frequency Standard for GPS-III (PTTI Conference 2001).
  Abstract: We describe an optically-pumped cesium beam frequency standard under development for deployment on the GPS-III satellite constellation. The objective of the project is to demonstrate the feasibility of a space qualified optically-pumped cesium beam frequency standard exhibiting a short-term stability of σ_y(τ)<6 x 10^-12 τ^-1/2 with lifetime and reliability comparable to that of the current GPS-IIF cesium standards. In this paper, we report on the architecture and development progress of a prototype instrument which meets these goals with a minimal introduction of unproven technology. In particular, we discuss the design choices that have been made with particular emphasis on optical pumping and interrogation techniques, laser and optical technologies, and the realization of the prototype frequency standard. Preliminary performance data is presented. Download Now.

• Rubidium 8040C
  Abstract: This white paper highlights how Symmetricom has helped communication- device manufacturers meet their challenges for several years as the market-leader in rubidium frequency standard devices. Read about how the 8040C offers higher levels of accuracy, redundancy and flexibility for manufacturers and laboratory testing-companies that need precise frequency verification. Read More.

• A Modern Militarized Rubidium Frequency Standard
  Abstract: This white paper describes the design of the Symmetricom Model 8130A Rubidium Frequency Standard (RFS), modern ruggedized RFS intended for military applications and other harsh environments. Read More.

• Rubidium Keeps the Signal Stable
  Abstract: This white paper describes the advantages of using a Rubidium Atomic frequency standard to provide a stable reference (clock) for a digital exciter. It highlights how a rubidium oscillator is not only more economical and requires less maintenance than other solutions, but that it will also provide better short-term stability and frequency accuracy. Read More.

• The Chip-Scale Atomic Clock-Prototype Evaluation (PTTI Conference 2007)
  The authors have developed a chip-scale atomic clock (CSAC) for applications requiring atomic timing accuracy in portable battery-powered applications. At PTTI/FCS 2005, we reported on the demonstration of a prototype CSAC, with an overall size of 10 cm3, power consumption »150 mW, and short-term stability sy(t)<1x10-9t-1/2. Since that report, we have completed the development of the CSAC, including provision for autonomous lock acquisition and a calibrated output at 10.0 MHz, in addition to modifications to the physics package and system architecture to improve performance and manufacturability. Read More.

• The Miniature Atomic Clock – Pre-Production Results
  The authors have developed a Miniature Atomic Clock (MAC) for applications requiring atomic timing accuracy in portable battery-powered applications. Recently, we have completed a pre-production build of 10 devices in order to evaluate unit-to-unit performance variations and to gain statistical confidence in the performance specifications, environmental sensitivity, and manufacturability. Download Now

• VCSELs for Atomic Sensors
  A new generation of small low-power atomic sensors, including clocks, magnetometers, and gyroscopes, is being developed based on recently available MEMS and VCSEL technologies. These sensors rely on spectroscopic interrogation of alkali atoms, typically rubidium or cesium, contained in small vapor cells. The relevant spectroscopic wavelengths (in vacuum) are 894.6 nm (D1) and 852.3 nm (D2) for cesium, and 795.0 nm (D1) and 780.2 nm (D2) for rubidium. The D1 wavelengths are either preferred or required, depending on the application, and vertical-cavity surface-emitting lasers (VCSELs) are preferred optical sources because of their low power consumption and circular output beam. Download Now

• The Chip-Scale Atomic Clock
  Abstract: For over 30 years, Symmetricom has been supplying modular and ruggedized atomic clock solutions that have met our military’s mobile needs. Since2002, in collaboration with the Defense Advanced Research Projects Agency (DARPA), Symmetricom has been developing a Chip-Scale Atomic Clock (CSAC) one-hundred times smaller and lower power than any existing atomic clock technology. Read More.

• The MAC - A Miniature Atomic Clock (PTTI Conference 2005)
  Abstract: The authors are developing a chip-scale atomic clock (CSAC), more than two orders of magnitude smaller and lower power than any existing technology. As an intermediate milestone, en route to the ultimate CSAC objectives, we have developed a Miniature Atomic Clock (MAC), combining the low-power CSAC physics package with a low-parts count, low power digital control and microwave system. The MAC is a complete packaged atomic clock, with overall size of 10 cm³, power consumption <200 mW, and short-term stability
  σ_y(τ) ~ 4 x 10^-10 τ^-1/2.
  
  The MAC provides a valuable testbed for the further development and refinement of the CSAC physics package as well as for the development of the CSAC control electronics prior to undertaking the costly and time-consuming size reduction effort which will be necessary to meet the ultimate CSAC objectives. The MAC itself may find applications in commercial and military timing systems which require the relatively small size and power consumption of the MAC now, rather than wait for the evolution of the 1 cm³, 30 mW CSAC. Download Now.

• The Chip-Scale Atomic Clock – Low-Power Physics Package (PTTI Conference 2004)
  Abstract: We have undertaken a development effort to produce a prototype chip-scale atomic clock (CSAC). The design goals include short-term stability, σ_y(τ) < 6 x 10^-10 τ^-1/2, with a total power consumption of less than 30 mW and overall device volume < 1 cm³. The stringent power requirement dominates the physics package architecture, necessarily dictating a small (< 1 mm³) volume gaseous atomic ensemble interrogated by a low-power semiconductor laser. At PTTI 2002 and PTTI 2003, we reported on laboratory experiments which underlie the fundamental architecture of our CSAC, based on interrogation of the cesium D1 transition by the technique of coherent population trapping (CPT).
  
  In the past year, the development effort has shifted from fundamental research and feasibility investigation to engineering and prototype development. In this paper, we report on the design of a rugged and compact physics package that is expected to exceed the ultimate performance and power requirements of the CSAC. Download Now.

• The Chip-Scale Atomic Clock – Recent Development Progress (PTTI Conference 2003)
  Abstract: We have undertaken the development of a chip-scale atomic clock (CSAC) whose design goals include short-term stability, σ_y(τ = 1 hour), of 1x10^-11 with a total power consumption of 30 mW and an overall device volume of 1 cm³. The stringent power requirement dominates the physics package architecture, dictating a small (< 5 mm³) gaseous atomic ensemble interrogated by a low-power semiconductor laser. At PTTI 2002, we reported on initial experimental investigations leading to the decision to employ the coherent population trapping (CPT) interrogation technique. This paper describes our further progress on the CSAC effort, including the development of custom vertical cavity surface emitting laser (VCSEL) sources and techniques for microfabricating miniature cesium vapor cells comprised of anodically bonded silicon and glass. Measurements of the signal contrast and linewidth of both the cesium D1 and D2 resonance transitions are compared, and frequency stability measurements of the CSAC testbed are presented. Download Now.

• Time on a Chip: The Incredible Shrinking Atomic Clock, New York Times 2004
  A media perspective on the chip scale atomic clock innovations. Download Now.

• The Chip-Scale Atomic Clock – Coherent Population Trapping vs. Conventional Interrogation
  (PTTI Conference 2002)
  Abstract: Symmetricom-TRC has undertaken a development effort to produce a prototype chip-scale atomic clock (CSAC). The overall architecture of the CSAC and, in particular, the physics package, must be defined early in the project, prior to the onset of a large-scale engineering effort. Within the constraints imposed by the performance goals of the project we have recognized two possible schemes for interrogating the ground-state hyperfine frequency of the gaseous atomic ensemble: the conventional double-resonance technique and the coherent population trapping technique. In this paper we describe a laboratory apparatus, which allows for in situ comparison of the two techniques, without the ambiguities associated with comparing data from disparate experiments. Data is presented comparing the short-term stability resultant of the two techniques, as well as environmental sensitivity to resonance cell temperature, laser intensity, and RF power. Download Now.

• VCSELs for Atomic Clocks (SPIE Photonics West 2006)
  Abstract: This paper describes the VCSEL requirements for CPT-based atomic clocks, which include single mode operation, single polarization operation, modulation bandwidth > 4 GHz, low power consumption (for the CSAC), narrow linewidth, and low relative intensity noise (RIN). A significant manufacturing challenge is to reproducibly obtain the required wavelength at the specified VCSEL operating temperature and drive current. Data are presented that show the advantage of operating at the D1 (rather than D2) resonance of the alkali atoms. Measurements of VCSEL linewidth are discussed in particular, since atomic clock performance is especially sensitive to this parameter. Download Now.

• An Ultra-Low-Power Physics Package for a Chip-Scale Atomic Clock (IEEE Transducers Conference 2005)
  Abstract: We report the design and measured thermal and mechanical performance of an ultra-low-power physics package for a Chip-Scale Atomic Clock (CSAC). This physics package will enable communications and navigation systems that require a compact, low-power atomic frequency standard. The physics package includes a unique combination of thermal isolation, mechanical stability and robustness, and small package volume. We have demonstrated temperature control at a nominal operating temperature of 75°C in a room-temperature, vacuum ambient requiring only 7mW of heating power. This represents a power reduction of over two orders of magnitude compared to the lowest power existing commercial technology and more than an order of magnitude improvement over other CSAC development efforts. Download Now.

Network Time Servers & NTP

• Cisco IP Telephony Clock Synchronization: Best Practices
  Abstract: More and more enterprises are using IP telephony solutions based on Cisco networking solutions. But many customers are overlooking an important part of the system architecture—the need to synchronize the infrastructure design with an external network time server. Proper clock synchronization is critical to ensuring good performance, improving troubleshooting, and producing accurate call detail records (CDRs) for billing. Read More.

• Network Time Synchronization: 5 Essential Elements
  Abstract: Organizations today need network time synchronization that ensures the integrity of network operations and applications yet needs little in the way of management overhead. It’s hard to imagine a company that would knowingly tolerate only some of its processes running on time or only some of its timestamps being accurate — especially if they realize how easy and inexpensive good network timekeeping can be to accomplish. To make time both pervasive and accurate, five essential elements must be present. Read More.

• 5 Dangers of Poor Network Time Keeping
  Abstract: This white paper provides a compelling argument for having accurate and synchronized time in an IT infrastructure. It discusses the technical and business consequences that can occur when clocks in a network fall out of synch. Read More.

• The Importance of Network Time Synchronization
  Abstract: As you read this, your network of workstations and servers, each with their own clock, are time stamping files, email, transactions, etc., all the while your server logs are recording every manner of transaction in the event you need that information. At some point during the day it is quite likely that automatic processes such as archiving, directory synchronization, cron jobs, etc. will execute and alter files based on time stamps. Fundamental to all of this is the belief that the time is correct. Even if the time is not absolutely correct there is often a belief that at least the time is “close enough.” This paper describes why “close enough” is no substitute for accurate network time and why network time synchronization is critically important. Read More.

• It’s All in the Timing – Synchronizing the VoIP Network for Quality, Savings and Performance
  Abstract: As we accelerate into the New World of VoIP we assume we can leave some of the trappings of wireline telecom behind, such as the need for synchronization. After all, an IP network is about as asynchronous as it gets. While this may be true in some respects, precise time and synchronization continue to permeate many areas of IP telephony operations. Customer expectations of voice quality and service reliability remain unchanged, and as a result, the need for precise time remains unchanged though it manifests itself in different ways. The bottom line is that trying to design a VoIP network without considering network synchronization is probably the shortest path to realizing that you need it. Read More.

• Stochastic Model Estimation of Network Time Variance
  Abstract: This white paper presents a study of clock drift in computers and other networked devices. The paper starts with an introduction to clock architecture, distribution of standard time, drift in oscillators, and efforts to improve clock precision. This is followed by an equation for clock drift that shows the how the calculation for modeling drift in networks was established. Finally, statistical projections are presented along with results and conclusions about the significance of network clock error, even in relatively small networks. Read More.

Next Generation Networks

• IEEE 1588 Precise Time Protocol: The New Standard in Time Synchronization
  Abstract: This white paper focuses on how the recently developed IEEE 1588 Precise Time Protocol (PTP) now promises to revolutionize time synchronization by improving accuracy and reducing cost. While certain other precise sync protocols require significant investment in hardware and cabling, PTP makes highly precise timekeeping possible using the most widely deployed medium for network connectivity – Ethernet. This is particularly important because as network computing becomes more complex and the world more interconnected, the need for more precise time synchronization has greatly increased. Read More.

GPS and Time Code Instrumentation

• Designing and Testing IEEE 1588 Timing Networks
  Abstract: IEEE 1588 Precision Time Protocol (PTP) is an emerging technology that facilitates precise time and frequency transfer over Ethernet networks. Synchronization-critical applications can see significant cost and performance improvements over existing alternatives. This white paper compares PTP with other technologies, explains how it works, describes performance criteria, and provides guidance for implementing PTP networks. In particular, this paper presents the results of synchronization tests using hardware time stamping and the IEEE 1588 protocol. It discusses the limitations, advantages, and disadvantages of using PTP for time and frequency distribution; considers PTP performance under various network topologies and traffic scenarios; and shares the test results of PTP performance over networks comprised of commercial off-the-shelf (COTS) network switches and hubs versus PTP-optimized devices. Read More.

• How Time Finally Caught Up With The Power Grid
  Abstract: Besides electricity, regional power authorities must also learn to distribute synchronized time across the gird if they want to prevent blackouts like the one on August 14, 2003. This white paper delves into the role of time, specifically synchronized time, in looking at the conditions leading up to this event. Read More.

• Why Convert to a SAASM based Global Positioning System (GPS)?
  Abstract: This application note focuses on the history and need for SAASM. This is of particular importance because as of 1 October 2006, all newly-fielded Department of Defense GPS systems will use SAASM compliant Precise Positioning System (PPS) devices. Procurement of non-SAASM GPS user-equipment will be disallowed unless wavered. Read More.

• Time & Time Code Reference
  Abstract: This application note discusses time scales of measurement, digital clock accuracy and synchronization, IRIG Time Code Formats, Other Time Code Formats and IEEE 1344 Compliance. Read More.

• The Role of Time and Frequency Systems in the Power Industry
  Abstract: This application note discusses how the massive 2003 power outage that left over 50 million people in the dark did indeed shed light on one critical issue facing power companies — the role of synchronized timekeeping. Read More.

Advanced Timing Solutions

• Platforms Using Commercial Satellite Modems
  Time based communications (TBC) involves the use of an active data channel for time transfer. TBC was demonstrated in 2000 using commercial SATCOM modems for two-way satellite time transfer between static locations. In 2002, testing was conducted with the Air Force Research Lab (AFRL) at Wright Patterson Air Force Base to demonstrate a TBC implementation from the ground to an airborne platform using standard communications channels and equipment. Algorithms to perform Dynamic Two-Way Time Transfer (DTWTT) have been developed to correct raw time transfer data for platform motion and measurement effects. Flight tests were conducted in November 2002 to demonstrate the algorithms and determine the level of performance that can be expected from dynamic two-way time transfer.

  This paper begins with a review of Time Based Communications followed by the introduction of Dynamic Two-Way Time Transfer. The flight experiment is presented with a description of the data collection hardware as well as a detailed presentation of the flight data. Conclusions on the use of DTWTT are drawn based on the results of the flight tests. Read More.

• Common-View LORAN-C as a Backup to GPS for Precise Time Recovery
  The LORAN-C network is being recapitalized to serve as a backup to GPS for navigation and timing. LORAN-C is being investigated as a precision (sub 100 ns) time recovery system for use in the continental United States. Traditionally, LORAN-C time recovery has been limited by propagation effects (both spatial and temporal) to provide time recovery in the microsecond range. By employing common view techniques long used in GPS, the potential exists to enhance time recovery performance by over an order of magnitude using LORAN-C. The LORAN-C Accuracy Panel (run by the US Coast Guard) has sponsored a research project to collect data and determine the performance level for time recovery using common-view LORAN-C. This paper presents the results of that work.

  This paper begins with a short overview of the LORAN-C recapitalization project with an emphasis on timing enhancements at the transmitting stations. A description of the commonview LORAN-C test network follows. Data collected in Boulder will be corrected via commonview using a near field station and a far field station. Results will be analyzed to determine the dependence of time recovery precision on distance. Read More.

• Ephemeris Determination Using a Connected Element Interferometer
  This paper describes a connected element interferometer that has been developed and is operating with collection elements in Tucson and Phoenix with a separation of 180 km. Coherence between the collection sites of better than 50 picoseconds is achieved through two way time transfer over a commercial fiber optics link. The description of hardware and software implementation, as well as measurement results of several operating satellites, are presented. Consider analysis is also presented to demonstrate the projected system performance against different targets, based on the characteristics of the measurement using the existing Tucson-Phoenix link. Read More.

• Continuous Satellite Two-Way Time Transfer using Commercial Modems
  Satellite two-way time transfer has traditionally been used as a periodic measurement of the offset between two-clocks. It is often used as a secondary measurement to GPS or a sole means of time recovery with performance that is superior to GPS time transfer. In this paper, we propose a continuous two-way system based on commercial modems where the two-way time transfer is performed in the background of normal data transfer. The intent of such a system is to provide a cost effective means to achieve high accuracy two-way time transfer in the background of an active data channel. The timing functionality is buried in the administrative portion of the communications channel and thus is provided as a byproduct of data transfer. This provides lower cost, lower bandwidth options for users who require sub-nanosecond time transfer between sites. It also provides a continuous record of clock differences with no impact to data transmission for users who have access to a data communications link.

  This paper begins with a review of the concept of two-way time transfer and time based communications and presents the requirements for implementing two-way over a communications channel. The requirements are then satisfied with commercial modems using two examples: an internal implementation and an external implementation. Data is presented for each case and tradeoffs are discussed. Read More.

• Dynamic Two-Way Time Transfer to Moving Platforms
  Time based communications (TBC) involves the use of an active data channel for time transfer[1]. In 2002, testing was conducted with the Air Force Research Lab (AFRL) at Wright Patterson Air Force Base to demonstrate a TBC implementation from the ground to an airborne platform using standard communications channels and equipment. Algorithms to perform Dynamic Two-Way Time Transfer (DTWTT) have been developed to correct raw time transfer data for platform motion and measurement effects. Flight tests were conducted in November 2002 to demonstrate the algorithms and determine the level of performance that can be expected from dynamic two-way time transfer. Tests were conducted using Satellite Relay links and line-of-sight (LOS) links between the ground and the aircraft. The results from the Satellite Relay case were presented in a PTTI 2002 paper entitled “Two-Way Time Transfer to Airborne Platforms Using Commercial Satellite Modems”. The LOS results are presented here.

  This paper begins with a review of Dynamic Two-Way Time Transfer for the line-of-sight communications link case. The flight experiment is presented with a description of the data collection hardware as well as a detailed presentation of the flight data. Conclusions on the use of DTWTT are drawn based on the results of the flight tests. Read More.

• Low Cost Digitally Enhanced Loran for Tactical Applications (LC DELTA)
  During the recent spate of hurricanes and tropical storms that punished the state of Florida, navigation aids of all types suffered either directly or indirectly from water and wind damage. During the months of August and September of 2004, Hurricanes Charley, Frances, Ivan, and Jeanne, and Tropical Storms Bonnie and Gaston, tore through the Atlantic and Gulf Coasts of the United States, resulting in damage estimates ranging from $11.8B to $22.8B.

  Loran-C Stations Jupiter [7980-Y], Malone [7980-M; 8970-W], and Carolina Beach [7980-Z; 9960-Y] were in the paths of Hurricanes Charley, Frances, Ivan, or Jeanne. Hurricane Charley did no significant damage to any of the stations, and Malone and Carolina Beach suffered minimal damage from Hurricanes Charley and Ivan. However, Loran-C Station Jupiter was unavailable for 60 hours as a result of Hurricane Frances, and six hours as a result of Hurricane Jeanne. Differential GPS (DGPS) sites were also affected. The Cape Canaveral, FL site was unusable for 113 hours, and the Tampa, FL site was unusable for 54 hours, because of Hurricane Jeanne-related damage. Because of Hurricane Ivan-related damage, the Mobile Point, AL site was unusable for 226 hours, and the Millers Ferry, AL site was unusable for 29 hours.

  Loran-C once again demonstrated its robustness under the harshest of environmental conditions. However, this weather “attack” pointed out the need for emergency capability in the event of other types of attacks, for critical infrastructure protection, or simply for rapid deployment of Loran for tactical use.

  This paper discusses the concept of, and the need for, a modern Tactical Loran system. We also provide a conceptual framework under which Low Cost Digitally Enhanced Loran for Tactical Applications (LC DELTA) might be developed. We show that the application of 21st century technology to an old problem results in a solution that is both physically and fiscally practical. Read More.

• Timing via the New LORAN-C System
  In 1999, the United States Federal Radionavigation Plan extended the life of the U.S. LORAN-C system while the long term benefits as a GPS backup are investigated. Since 1999, U.S. Congress has continued to provide funds via the Federal Aviation Administration (FAA) to develop and recapitalize the LORAN-C infrastructure. As a result of this recapitalization, the timing systems at the LORAN-C transmitting stations are being upgraded from its 1960’s technology. This paper re-introduces LORAN-C with an emphasis on the improvements that are being provided to the LORAN-C user community and the timing performance and applications of the new system. These improvements include new timing systems, new transmitters and new user equipment.

  The paper begins with an introduction to the LORAN-C Recapitalization Project (LRP). Next a comparison of the technology and performance of the new LORAN-C system and the existing systems is presented with an emphasis on timing performance. The new time and frequency equipment (TFE) suite is presented with details on the local timescale computation, UTC recovery, and transmitter timing adjustment loop. Performance of the new system will be presented from the factory acceptance testing and field trials. Additional details will be presented on the timing performance of the new transmitter. The paper concludes with a summary of the ongoing efforts and results from the LORAN-C Accuracy Panel. Keywords: Loran, Timing transfer Read More.

Digital Broadcasting

• Single Frequency Networks (SFNs) Require Robust Time and Frequency Synchronization
  A single frequency network (SFN) enables highly efficient distribution of digital content over a wide area. Easier, faster and less expensive to service a geographic area from a grid of transmitters than using a web of copper or fiber, SFNs address the need to bring high-quality video and audio services to mobile users – with a network that can serve all users.

  High precision and high reliability are critically important in the technology that provides timing references pervasively throughout a carrier’s single frequency network infrastructure. A user is likely to receive signals from more than one transmitter at a time and if those signals are out of synch, they will interfere with each other. Additionally, data arriving from one transmitter may be late relative to the data arriving from another, causing what the user receives as scrambled. If assigned frequencies are not maintained causing signals assigned to adjacent frequencies to overlap and interfere with each other, the signal itself may be corrupt. Such problems can be avoided if all steps – including frame encoding, modulation and transmission – are synchronized to microsecond precision among devices that are spread throughout the head end and across all transmitter stations.

  This white paper discusses pervasively distributed synchronization that is both highly precise and reliable as a key operational requirement of SFNs as well as the technique used to send data over the airwaves – coded orthogonal frequency division modulation (COFDM). The whitepaper also discusses the key attributes in the SFN’s timing reference including highly accurate timekeeping, low phase noise, redundant time sources, hot swappable technology and remote management. Read More.