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The NIST Time Measurement and Analysis Service (TMAS) was designed to assist laboratories that maintain an accurate local time standard. The service monitors the customer's local time standard by continuously comparing it to the national time standard, and reports the comparison results to the customer in near real time. The 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 5 x 10-14 after 1 day of averaging.
The TMAS meets the requirements of calibration and research laboratories that need to maintain a high accuracy time standard. These laboratories must continuously generate a 1 pulse per second (pps) on-time signal and be able to state the uncertainty of that signal with respect to Coordinated Universal Time (UTC) so traceability to the International System (SI) of units can be established.
TMAS subscribers receive an already assembled and calibrated time measurement system which includes everything needed to make state-of-the-art measurements that are continuously traceable to the SI. 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 an Internet connection 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 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 continuously 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.
Optional NIST disciplined oscillator (NISTDO)
The TMAS offers a NIST disciplined oscillator option for laboratories that lack a time and frequency standard or who prefer to have NIST supply their standard. The NISTDO is a rubidium or cesium oscillator that is continuously disciplined to UTC(NIST) with a time uncertainty of less than 10 nanoseconds and a frequency uncertainty of less than 1 x 10-14 at an averaging time of one day. The NISTDO outperforms the best GPS disciplined oscillators (see graph below) and the time and frequency signals that it produces are referenced directly to UTC(NIST), and not to GPS time.
The rubidium NISTDO provides five output signals that can be configured to provide either 5 MHz, 10 MHz, or 1 pps. The cesium NISTDO includes three 1 pps outputs, two 10 MHz outputs, and a 1 MHz output.
Either version of the NISTDO can be used to calibrate other devices and can serve as the ultimate laboratory frequency and time measurement reference. It's like having a miniature NIST time scale in your laboratory!
Why does my laboratory need the TMAS?
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 cannot recover time by themselves, and 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 microsecond 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 all of these these problems. Customers don't send their time standard to NIST, instead NIST sends a measurement system to the customer. The customer's time standard is then compared 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 NIST Time Measurement and Analysis Service is a welcome addition to any laboratory that needs to maintain a traceable, highly accurate time standard. It allows you to continuously monitor the performance of your standard with respect to UTC(NIST) in near real time.
Lead Organizational Unit:pml
NIST Time and Frequency Division
For more information about the NIST Time Measurement and Analysis Service and the NIST disciplined oscillator option, please download the following technical articles: