CSIR-National Physical Laboratory
सीएसआईआर-राष्ट्रीय भौतिक प्रयोगशाला
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 nano-seconds with respect to UTC. The timescale system used to generate IST consists of five caesium clocks, one 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 Cesium 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 simply a vehicle used to transfer time from one site to the other. The time signal embedded in a GNSS signal is the most commonly used source of common-view clock because of its wide 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 being emitted by a satellite and received by specific equipment installed in a laboratory. Estimating all systematic uncertainties, accurate time synchronization (~10 ns) can easily be achieved 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 allow 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 with 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 U