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 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 CVGNSS method. Additionally, ISRO is provided with traceability to IST using CVGNSS method as well.
- Time transfer using TWSTFT (Two Way Satellite Time and Frequency Transfer)
The TWSTFT technique utilizes a telecommunications geostationary satellite to compare clocks located in two different positions, i.e., at receiving and emitting stations. Two-way observations are scheduled between pairs of laboratories so that their clocks are simultaneously compared at both ends of the baseline using the satellite’s transponder. With the installation of automated stations in most laboratories, the TWSTFT link observations among the stations can be made at regular intervals, with the consequence of achieving a statistical uncertainty below 1 ns. Due to unavailability of footprints on any international satellite, the international TWSTFT link is currently not operational. However, ISRO timescale is provided with traceability to IST using domestic TWSTFT system.
For any query related to Time generation and dissemination, please contact Dr. Ashish Agarwal at firstname.lastname@example.org
- Time transfer over internet (NTP Service)
NTP is an internet standard protocol which uses a reliable time source, i.e., UTC(k) as reference for precise synchronization of servers and network devices. NTP servers follow a hierarchy with Stratum 0 as the “Primary Reference Clock” located at the NMIs of the county and can go up to Stratum 15. A primary server (referred to as a stratum 1) is a server that receives a UTC time signal directly from an authoritative clock source, e.g. atomic clock or GPS signal source. A stratum 2 server receives its time signal from a stratum 1 server, a stratum 3 server from stratum 2 servers, and so on. Clients peer with servers in order to synchronize their internal clocks to the NTP time signal.
The latest version of NTP (NTPv4.0) can maintain time with an uncertainty of less than 50 ms on WAN. However, the absolute level of uncertainty in NTP depends on network conditions. NTP is a highly scalable and fault-tolerant protocol that automatically selects the best of several available time servers. The latest 4th version of NTP comes with several security features, including protection from Kiss of death attacks, supports symmetric and asymmetric cryptographic authentication, panic is disallowed after first clock update upon synchronization, clock discipline algorithm that improves uncertainty, handling of network jitter, and polling intervals, support for the nano kernel implementation that provides nanosecond precision, fast synchronization at startup and after network failures, automatic server discovery etc.
Since its inception in 2009 Internet Time Dis