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 customer’s 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 standard’s 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 x 10^-13 after 1 day of averaging.

The TMAS was introduced to meet the small but growing demand for calibration laboratories and research facilities to maintain a high accuracy time standard. This requires the laboratory to continuously generate a 1 pulse per second (pps) on-time signal, and for laboratories in the United States, to be able to state the uncertainty of that signal with respect to the Coordinated Universal Time (UTC) scale maintained at NIST, known as UTC(NIST). Once the uncertainty of the 1 pps signal is known, it can then be used as a standard for traceable measurements of time interval and/or frequency, or as a synchronization source for other timing systems.

High accuracy 1 pps signals are normally generated by either a cesium oscillator or a Global Positioning System disciplined oscillator (GPSDO). Cesium oscillators are primary laboratory standards that physically realize the base unit of time interval (the second) as defined by the International System (SI). However, they still need to be synchronized before serving as a time standard. GPSDOs are devices that usually contain a quartz or rubidium oscillator whose outputs are continuously steered to agree with signals from the GPS satellites. In contrast to a cesium oscillator, a GPSDO is inherently on-time, and can produce a 1 pps signal that is usually well within 1 µs of UTC. However, because it is not usually possible to measure the time offset of a GPSDO with respect to UTC(NIST), laboratories are often limited to using and trusting the number quoted on the manufacturer’s specification sheet as an uncertainty figure.

Laboratories that want their time standards calibrated against UTC(NIST) to accuracies better than 1 microsecond have historically had several options, all of which have shortcomings. Customers sometimes ask to send their cesium oscillator to NIST for calibration, but this is normally not a good solution, nor is it practical. If a cesium oscillator is sent to to Boulder time information is lost during the shipment to NIST and the return shipment to the customer, and the cesium would need to be resynchronized when it returns to the customer’s lab. In fact, when the device returns to the customer, even the frequency of the device might be substantially different from what it was during the calibration. A GPSDO can be sent to NIST for delay calibrations (Service ID 76120S). This works well if the antenna and cable are calibrated along with the receiver. However, due to local reception conditions, the device might perform differently at the customer’s site than it did at NIST, and the customer will be without a time reference during the interval when the unit is gone from their laboratory.

The TMAS is a remote calibration service that eliminates these problems. Unlike the traditional model, a remote calibration does not require the customer to send their device under test (DUT) to NIST. Instead, the DUT remains in place at the customer’s site, and NIST sends a measurement system to the customer. The TMAS utilizes today's technology to provide the ultimate solution to a customer’s time measurement problem. It compares the customer's time standard to UTC(NIST), 24 hours a day, 7 days a week, with the results continuously updated via the Internet so that they can easily be accessed from anywhere.

The TMAS employs the common-view GPS measurement technique to compare time standards located at remote locations from each other. Ideally, a comparison between two time standards would be made by bringing them into the same laboratory and connecting them both to some type of phase comparator, usually a time interval counter. If bringing the time standards together into the same lab is not practical or desirable, the difference between the two time standards can still be measured by simultaneously comparing both standards to a common reference signal that can be received at both sites. Both sites record their measurements and exchange their results, and the results are subtracted from each other to obtain the time difference between the two standards. The common-view signal from the GPS satellites can be thought of as a transfer standard, and its value drops out of the final measurement result.

Subscribers to the NIST service receive an already assembled and calibrated time measurement system which includes everything needed to make state-of-the-art measurements that are continuously traceable. An easy-to-read instruction manual makes installation a snap. All that is necessary is to mount a small GPS antenna in a location with a clear view of the sky, and to connect your input signals. Simply plug in a time base signal (5 or 10 MHz), the 1 pps time standard, and connect the unit to a always-on Internet connection with a dedicated IP address, and you'll be linked to UTC(NIST). Once started, the measurement unit requires no operator attention. NIST personnel will monitor your measurements from Boulder, Colorado, verify and analyze your data, and quickly troubleshoot any problems that may occur.

The measurement system blends commercially-available equipment with hardware and software developed at NIST. Measurements are made using a time interval counter with a single shot resolution of less than 30 picoseconds. The software automates the measurement process and presents data in a clear, easy-to-understand format. If any hardware component fails, NIST replaces it immediately using an overnight delivery service.

The web-based software allows you to graph the performance of your time standard over periods of up to 200 days. An array of powerful graphing features let you display the data in a variety of fashions: you can increase or decrease the scale of the x or y-axis to zoom in on sections of the graph. Results of comparisons are continously updated every 10 minutes, so you'll always know the current time difference between your primary standard and UTC(NIST). Plus, all data is stored locally and on a NIST server. At any time, you can retrieve and graph past data recorded by the system. This gives you a permanent history of the frequency and time performance of your primary standard.

The NIST Time Measurement & Analysis Service is a welcome addition to any laboratory that needs to maintain a traceable, highly accurate time standard. It allows you to continously monitor the performance of your standard with respect to UTC(NIST) in near real time.

For detailed information about the NIST Time Measurement and Analysis Service, including the theory of operation and a complete analysis of the measurement uncertainties, please see the following PDF article:

Remote Time Calibrations via the NIST Time Measurement and Analysis Service (Measure: The Journal of Measurement Science, vol. 1, no. 4, pp. 50-59, December 2006.)

For questions or more information about the NIST Time Measurement and Analysis Service, contact Michael Lombardi at email: (lombardi@nist.gov).