Michael A. Lombardi

The Use of GPS Disciplined Oscillators as Primary Frequency Standards for Calibration and Metrology Laboratories

Abstract: An increasing number of calibration and metrology laboratories now employ a Global Positioning System disciplined oscillator (GPSDO) as their primary standard for frequency. GPSDOs have the advantage of costing much less than cesium standards, and they serve as “self-calibrating” standards that should not require adjustment or calibration. These attributes make them an attractive choice for many laboratories. However, a few of their characteristics can make a GPSDO less suitable than a cesium standard for some applications. This paper explores the use of GPSDOs in calibration laboratories. It discusses how GPSDOs work, how measurement traceability can be established with a GPSDO, and how their performance can vary significantly from model to model. It also discusses possible GPSDO failure modes, and why a calibration laboratory must be able to verify whether or not a GPSDO is working properly.

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.

Microsecond accuracy at multiple sites: is it possible without GPS?

Critical infrastructure timing systems rely upon GPS clocks simply because microsecond accuracy is easy to achieve with GPS and difficult to achieve without it. This paper has discussed how GPS clocks meet critical infrastructure timing requirements, why they are vulnerable, and some potential backup strategies. It seems clear that achieving microsecond accuracy at thousands of geographically dispersed sites without relying on GPS clocks is a very challenging problem. It seems equally clear that solving this problem is in the best interest of the United States.

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.

The Development of a Unified Time and Frequency Program in the SIM Region

Abstract: The Sistema Interamericano de Metrologia (SIM) is one of five major regional metrology organizations (RMOs) recognized by the Bureau International des Poids et Mesures (BIPM). SIM is composed of the national metrology institutes (NMIs) located in the 34 member nations of the Organization of American States (OAS). Its goal is to create a unified measurement network that extends to the entire SIM region, ensuring the uniformity of measurements and strengthening traceability throughout North, Central, and South America back to the International System (SI) of units. To help reach this goal, SIM sponsors working groups in ten different metrological fields, including time and frequency. Developing a unified time and frequency measurement network in the SIM region has been a challenging task, but much progress has been made in recent years and many obstacles have been overcome. This paper summarizes work done by the SIM Time and Frequency Metrology Working Group from 2004 to 2010. It discusses the challenges faced by the working group, the progress made by individual laboratories, and the important role played by metrology education. It also provides an overview of two major achievements of the working group: the SIM Time Network (SIMTN) and the SIM Time (SIMT) scale.

The GOES Time Code Service, 1974-2004: A Retrospective

NIST ended its Geostationary Operational Environmental Satellites (GOES) time code service at 0 hours, 0 minutes Coordinated Universal Time (UTC) on January 1, 2005. To commemorate the end of this historically significant service, this article provides a retrospective look at the GOES service and the important role it played in the history of satellite timekeeping.

Frequency Uncertainty Analysis for Josephson Voltage Standard

The uncertainty in voltage measurement in a Josephson voltage standard (JVS) is proportional to the uncertainty in frequency measurement. The 10 MHz time-base from various commonly used frequency standards for JVS are analyzed using Allan variance to determine the contribution to the uncertainty budget for voltage measurement. The 75 GHz microwave source along with phase-lock electronics and frequency counter for a Josephson junction array is also analyzed for its stability and contribution to JVS uncertainty budget. The results provide realistic estimation for uncertainty contributions of time base and frequency measurements in JVS

SIM Time Scale

Time measurement is of great importance for science, technology, and commerce. Among the seven base units of the International System of Units, the second can be realized with the smallest uncertainty, currently reaching parts in 1016. Keeping track of the continuous accumulation of seconds allows the formation of time scales that serve as references for applications that require synchronization to national and international standards. This paper presents and discusses a multinational time scale developed for the Sistema Interamericano de Metrologia (SIM). This time scale, known as the SIM time scale, or SIMT, was developed to complement the official world time scale, Coordinated Universal Time, by providing real-time support to the operational timing systems within the SIM region. SIMT is generated from automated comparisons of time standards in North, Central, and South America, and is believed to be the first operational multinational time scale whose results are continuously published in real time via the Internet.