Andrew N. Novick

Characterizing the performance of GPS disciplined oscillators with respect to UTC(NIT)

Global Positioning System Disciplined Oscillators (GPSDOs) are now the primary standard of time and frequency at many laboratories and calibration facilities. They are typically accepted as self-calibrating standards, and their users generally assume that they meet the manufacturers specifications. To gain a better understanding of the actual performance of GPSDOs, this paper presents a method of characterizing both their long and short-term performance that uses the UTC(NIST) time scale as a reference. It then describes how this method is used to characterize four GPSDOs, including two that use an oven controlled quartz oscillator (OCXO) as their time base, and two equipped with a rubidium oscillator. All four devices were simultaneously tested using the same antenna over two 60 d measurement intervals. During the first 60 d measurement, a previously surveyed antenna position was used and the same coordinates were applied to all four devices. During the second 60 d measurement, each GPSDO performed an independent survey of the antennas position and applied its own coordinates. Both the timing output (1 pulse per second) and the frequency output (10 MHz) of each GPSDO was measured during both 60 d intervals. A low-noise dual mixer time difference system was used to characterize the short-term frequency stability of each devices 10 MHz output, and all measurement results are presented and summarized.

The inter-American metrology system (SIM) common-view GPS comparison network

The inter-American metrology system (SIM) consists of national metrology institutes in 34 member nations located throughout North, Central, and South America, and the Caribbean region. Currently, at least ten SIM laboratories pursue time and frequency metrology, and are responsible for maintaining the national time and frequency standards for their respective countries. To benefit these laboratories and advance metrology in the SIM region, we have developed a measurement network to continuously intercompare these standards and to make the data readily accessible to all SIM members. This network utilizes the multi-channel common-view GPS technique and the near real-time exchange of data via the Internet. This paper discusses the challenges of implementing the SIM comparison network, and provides a technical description of the measurement system supplied to participating laboratories. It describes the method used to calibrate the SIM measurement systems, and describes how the measurement results are processed and reported to the laboratories. It presents data collected from comparisons between the national frequency and time standards located in Canada, Mexico, and the United States. It validates these data by comparing them to data collected from previously established time links. It concludes with a discussion of the measurement uncertainties.

Remote Time Calibrations via the NIST Time Measurement and Analysis Service

Abstract: The National Institute of Standards and Technology (NIST) now offers a new remote calibration service designed to assist laboratories that maintain an accurate local time standard. The service monitors the local time standard by continuously comparing it to the national time standard and reports the comparison results to the customer in near real-time. This new service, called the NIST Time Measurement and Analysis Service, or TMAS, works by making simultaneous common-view measurements at NIST and at the customers laboratory with up to eight Global Positioning System (GPS) satellites. Each customer receives a time measurement system that performs the measurements and sends the results to NIST via the Internet for instant processing. Customers can then view their standards performance with respect to NIST in near real-time, using an ordinary web browser. Time is measured with a combined standard uncertainty of less than 15 nanoseconds, and frequency is measured with an uncertainty of less than 1 × 10−13 after 1 day of averaging. This paper describes the multi-channel GPS common-view technique used by the service and the measurement system sent to each customer. It also explains how NIST calibrates each measurement system prior to shipment, how measurement results are reported to the customer, and how the measurement uncertainties are estimated.

WWVB Radio Controlled Clocks: Recommended Practices for Manufacturers and Consumers

Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the entities, materials, or equipment are necessarily the best available for the purpose. iii FOREWORD Radio controlled clocks represent a true revolution in timekeeping. Clocks that synchronize to NIST radio station WWVB now number in the millions in the United States, and new sales records are being established every year. As a result, many of us are now accustomed to having clocks in our homes, offices, and on our wrists that always display the correct time and that never require adjustment. This NIST Recommended Practice Guide was written to provide guidance to both manufacturers and consumers of radio controlled clocks. Through voluntary compliance with the recommended practices listed here, manufacturers can benefit by continuing to develop more reliable and usable radio controlled products, increasing both consumer confidence and sales. Consumers can benefit by using this guide to help them select and purchase radio controlled clock products, to learn how the products work, and to help troubleshoot reception problems.

Reducing local oscillator phase noise limitations on the frequency stability of passive frequency standards: tests of a new concept

We report on the experimental test of a new concept for reducing the limitation on short-term frequency stability of passive frequency standards due to local oscillator phase noise. This concept is general and can be applied to many passive frequency standards. Systems that use sinewave modulation to interrogate a stable resonance are limited in short-term frequency stability by phase noise at the second harmonic of the modulation, f/sub m/. This effect limits the fractional frequency stability to approximately /spl sigma//sub v/(/spl tau/7)=0.9(f/sub m///spl nu//sub 0/) (S/sub /spl phi//(2f/sub m/))/sup 1/2//spl tau//sup -1/2/, where /spl nu//sub 0/ is the carrier frequency and S/sub /spl phi//(2f/sub m/) is the phase noise at twice the modulation frequency. (Contributions from higher even harmonics of the modulation generally can be neglected). This new concept uses notch filters at /spl plusmn/2f/sub m/ from the carrier to reduce this effect. Tests on a modified passive rubidium standard demonstrate an improvement of approximately 18 in /spl sigma//sub y/(/spl tau/). The dual notch filters proved to be feasible and were obtained commercially. Measurements suggest that ultimate performances of less than 2/spl times/10/sup -14//spl tau//sup -1/2/ are possible if the atomic resonance has sufficient quality.<<ETX>>

Reducing the effect of local oscillator phase noise on the frequency stability of passive frequency standards

The authors report on the experimental test of a new concept for reducing the limitation on the short-term frequency stability of passive frequency standards due to local oscillator phase noise. Systems that use sine-wave modulation to interrogate a stable resonance are limited in short-term frequency stability by phase noise at the second harmonic of the modulation, f/sub m/. The new concept uses notch filters at /spl plusmn/2f/sub m/ from the carrier to reduce this effect. Tests on a modified passive rubidium standard demonstrate an improvement of approximately 18 in /spl sigma//sub y/(/spl tau/). The dual notch filters proved to be feasible and were obtained commercially. Measurements suggest that ultimate performances of approximately 2 /spl times/ 10/sup -14/ /spl tau//sup -1/2/ are possible if the atomic resonance has sufficient quality.<<ETX>>

Examination of time and frequency control across wide area networks using IEEE-1588v2 unicast transmissions

The IEEE 1588–2008 Precision Time Protocol (PTP) version 2 (IEEE 1588v2) can be used to synchronize a slave clock to a grandmaster clock over a wide area network (WAN). However, many of the algorithms the slaves use to steer to the master are optimized for a scenario where both devices are on the same subnet or local area network (LAN). This paper is a study of existing PTP hardware from a number of different manufacturers in unicast mode. We characterize the performance of the equipment, beginning with the timing outputs of the masters that are locked to their built-in Global Positioning System (GPS) receivers. Next, we compare the results of steering unicast clients to their masters through a LAN versus several wider-area network configurations such as virtual-LANs and the public Internet. Analysis of the results will show how clients of different manufacture handle the various network paths. It is our hope that these comparisons will instigate changes to clock steering and synchronization algorithms, which may help improve the overall capabilities of PTP for telecom and other networking environments. As network synchronization techniques improve, the quality of the PTP masters will become more significant. Therefore, the performance and calibration of PTP masters with respect to UTC(NIST) is also discussed.

Practical limitations of NTP time transfer

The Network Time Protocol (NTP) is commonly utilized to synchronize computer clocks in packet-switched, wide area networks (WANs) such as the public Internet. The delay asymmetry in WANs, often due to inconsistent routing and/or bandwidth saturation, is usually the dominant source of error. It typically limits NTP time transfer uncertainty to about one millisecond. This paper discusses the uncertainty of NTP time transfer when network asymmetry is largely eliminated. We performed NTP measurements over a local area network (LAN) when both the server and client are referenced to a common clock. Three variations of a LAN are tested, including a direct connection between the server and client with an Ethernet crossover cable. The elimination of network asymmetry reveals other uncertainty sources that serve as practical limitations for NTP time transfer, including client instability, asymmetry in network interface cards, and server instability.

A Low-Cost Time Transfer Receiver for Contributions to Coordinated Universal Time

This paper describes a low-cost time transfer receiver that allows timing laboratories, including national metrology institutes and other designated institutions, to contribute data to the computation of Coordinated Universal Time (UTC). The time transfer receiver compares a laboratory’s local realization of UTC, to signals broadcast by Global Positioning System (GPS) satellites. It stores the measurement results in a format compatible with international standards, and sends data via the Internet to the Bureau International des Poids et Mesures (BIPM) for inclusion in the UTC computation. In addition to being inexpensive, the receiver was designed to be easy to use, allowing recently established timing laboratories to begin contributing to UTC with a minimal investment in training.