सीएसआईआर-राष्ट्रीय भौतिक प्रयोगशाला
CSIR-National Physical Laboratory
Time and Frequency Metrology
- Realization and Maintenance of IST CSIR-NPL has a “Primary Timescale” generating UTC(NPLI), which is traceable to the Coordinated Universal Time (UTC) provided by International Bureau of Weights and Measurers (BIPM) located in Sevres, France. UTC(NPLI) is the realization of UTC at NPLI. The IST (i.e., UTC(NPLI) plus 5:30 hours), generated using a bank of caesium clocks and hydrogen masers, has current systematic uncertainty of ±2.8 nanoseconds with respect to UTC. The timescale system generating IST consists of five caesium clocks, one passive hydrogen maser, two active hydrogen maser, measurement system and an international satellite links for clock comparison and traceability link. The caesium clocks provide absolute atomic reference of the time which has exceptional long-term stability, whereas the hydrogen maser has ultimate short-term stability. UTC(NPLI) is realized as the steered output of an Active Hydrogen Maser (AHM). However, the timescale ensemble has five high performance Caesium clocks as well. All Cs clock output frequencies and steered output from microphase stepper are connected to an automatic switching unit which enables time-based switching of measurement of a pair of clocks through a frequency/phase comparator or a time interval counter.
- Time transfer using GNSS (Global Navigation Satellite System) The common-view clock signal is a vehicle used to transfer time from one site to another. The time signal embedded in a GNSS signal is the most commonly used source of the common-view clock because of its comprehensive visibility, ease of reception with good signal-to-noise ratio, and insensitivity to propagation effects. CVGNSS time transfer is a one-way method, the signal emitted by a satellite and received by specific equipment installed in a laboratory. Accurate time synchronization (~10 ns) can easily be achieved after estimating all associated systematic uncertainties by the CVGNSS method. Dual-frequency receivers remove the ionospheric delay and improve the time transfer accuracy. Such data is known as GPS P3, which allows clock comparisons with less than a nanosecond statistical uncertainty. CSIR-NPL has multiple dual-frequency GNSS receivers. Recently, two new GNSS timing receivers have been installed and have been calibrated using the travelling GNSS calibrator from Group-1 laboratory, i.e., NICT, Japan, and the internal delays were calculated with respect to NICT G1. With these efforts, the traceability link to UTC was calibrated, and the associated systematic uncertainty improved to ±2.8 ns with effect from October 2018. The traceability of IST to UTC is maintained using the CVGNSS method. Additionally, ISRO is provided with traceability to IST using the CVGNSS method as well.