Time and Frequency Metrology Section (DU 6.0, DP 6.01)

CSIR-NPL is the Indian Time Authority

Time and Frequency (T&F) section plays leading role as the Indian Time Authority (ITA) who is responsible for measuring“time” and “frequency”with highest level of accuracy in India and keep them traceable to the International Bureau of Weights and Measures (BIPM) using ultraprecise satellite links to realize “second”, i.e., the unit of time in International System (SI) of Units.

In simple words, T&F section at CSIR-NPL is authorized (through an act of Parliament) to realize and maintain the Indian Standard Time (IST)(http://www.nplindia.in/clockcode/html/index.php)and disseminate it to the users in pan India. IST is realized using a timescale system, which consists of bank of five Cesium (Cs) atomic clocks and two active Hydrogen masers. These ultra-stable clocks are accurate to one second over a period of about three lakh years. IST is kept traceable to Coordinated Universal Time (UTC) maintained by BIPM using satellite links. At present, IST is traceable to UTC with a systematic link uncertainty of ±2.8 nanoseconds (ns) . Dissemination of IST is done through network time service and satellite links. Calibration of precision clocks, oscillators, GNSS receivers, stop watches etc. is also provided. In addition, the section carries out research to develop state-of-the-art ultramodern atomic clocks and dissemination techniques for improving time keeping and dissemination capabilities.

Seeing the criticality of IST infrastructure, the whole critical information Infrastructure is secured physical via Bio- Metric based Exit and Entry to all labs and also is monitored 24X7 over CCTV surveillance. All the public servers are secured behind Firewalls in De-Militarized Zone.


Dr. Anurag Gupta
Senior Principal Scientist and Head,
India Standard Time Division
Phone: +91-11-4560 8343
Fax: +91-11-4560 9310
Email: anurag@nplindia.org Ph: +91 4560 8224

Group photograph, April 2019.(Top row - left to right): Mahavir P Olaniya, Trilok Bhardwaj, Subhasis Panja, Ashish Agarwal, Anurag Gupta, Manoj Das, and V. BHARAT(Bottom row - left to right): Amutha Arunachalam, Suchi Yadav, Poonam Arora, Pranalee Thorat, Preeti Kandpal, and Subhadeep De.

Group members

Dr. Anurag Gupta, Senior Principal Scientist
Dr. Ashish Agarwal, Principal Scientist
Dr. Poonam Arora, Senior Scientist
Dr. Subhasis Panja, Senior Scientist
Mrs. Pranalee Thorat, Senior Scientist
Shri. Trilok Bhardwaj, Senior Scientist
Dr. Manoj Das, Senior Scientist
Dr. Subhadeep De, Scientist
Dr. V. Bharat, Scientist
Mrs. Suchi Yadav, Technical Officer
Mrs. Preeti Kandpal, Technical Officer
Shri Mahavir P Planiya, Technical Officer
Mrs. Amutha Arunachalam's

Major Activities

1. Time Generation and Dissemination

  • 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 ashish@nplindia.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 Dissemination via NTP at CSIR-NPL, within a short span of time has become very popular. This service is highly in use. Currently a total stack of more than 20 NTP servers are ready to provide this service. NPL has gained many customers like State bank of India, Stock Holding Corporation of India, UADAI AADHAR etc.
    Currently user can access this service via: time.nplindia.in

    To check time of your device with IST maintained by CSIR-NPL Click Here (http://www.nplindia.in/clockcode/html/index.php)

    For any quarries related to NTP service please contact- Mrs. Pranalee Thorat at thoratpp.npl@gov.in

2. R&D Activities

  1. Time Generation and Dissemination:
    • Internet Time Dissemination via PTP (under development)
    • With the goal of resilient, traceable and certified time distribution service for sectors like telecommunications, power-grid and finances that completely eliminates reliance on GPS, CSIR-NPL is working on time dissemination using internet packets based on IEEE 1588 protocol also known as precision time protocol (PTP).

      The PTP based time transfer is similar to the well-known Network time protocol (NTP), but works in controlled networks (usually local area networks) where latencies/residence time in each network elements is taken into account to deliver accuracies on the order of 1 µs, which is three-orders of magnitude better than that possible using NTP. It is immune to absence of GPS signals, GPS signal spoofing etc. It also eliminates the need to fix physically GPS antennae on rooftops.

      A proof of principle experiment is now going on connecting the primary time scale laboratory and the back-up time scale laboratory of the NPL, both located inside the NPL campus. Also, disseminating time using PTP outside the NPL campus is currently under progress.

      For any query related to PTP, please contact Dr. Manoj Das at manoj.das@nplindia.org

      The aim of this research activity is to establish a robust telephone time dissemination service with time synchronization accuracy of ±10 ms over landline telephone line. Compact and affordable receivers have been developed to access this service. The service is being tested at CSIR-NPL. The know-how of the receivers will be transferred to industry for commercial production. Thereafter, the service will be opened for public use. Due to millisecond accuracy, it will appeal to defense sectors that are afraid to use NTP service which exposes their network to hacking. Even banking and telecommunication sector will benefit with new improved telephone time service. Once IST becomes legal time of the country, this technology will become the most appropriate means to synchronize to IST. Even a common man sitting in a rural village will be able to access IST using is telephone line connection which is really available. It is better than GPS based timing receivers as well which are prone to jamming and they do not work in the basements and inside the building with no clear view for receiving the satellite signals. Telephone lines are available all over India and one of the easiest means to get time even in rural areas. All developed nations, e.g. USA, UK, Germany and Japan still offer telephone time dissemination services and this will hold relevance at least for next one decade. The conceptual design of the technology is presented in the figure.

      Figure: Conceptual design of FonOclock service over public switched telephone network (PSTN).

      The key-features of the service are:

      Complete time information (Day, Date, Hour, Minute, Second) is transmitted.
      Each time during synchronization the transmission link delay is calibrated.
      Overall synchronization accuracy is ± 10 ms.
      External 1 PPS output available for device synchronization.
      The receiver module can perform as master slave clock.
      For any query related to FonOclock, please contact Dr. Poonam Arora at arorap@nplindia.org

    • Study on necessity of two time zones: IST-I (UTC+5:30 h) and IST-II (UTC+6:30 h) in India

    • Figure: Proposed map of India showing two-time zones: IST-I (UTC+5:30 h) and IST-II IST (UTC+6:30h). A new centre for generating IST-II though “Primary Time Ensemble-I” is proposed in the north-eastern state, which will be synchronized to the “Primary Time Ensemble-II” generating IST-I at CSIR-NPL, New Delhi as well as to UTC at BIPM.

      A strong demand of a separate time zone by Northeast populace has been a matter of great debate for a very long period. However, no implementable solution to this genuine problem has yet been proposed. CSIR-National Physical Laboratory, CSIR-NPL (the National Measurement Institute, NMI, of India and custodian of Indian Standard Time, IST), proposes an implementable solution that puts the country in two-time zones: (i) IST-I (UTC+5:30h, represented by longitude passing through 82º33’ E) covering the regions falling between longitudes 68º7’ E to 89º 52’ E; and (ii) IST-II (UTC+6:30h, represented by longitude passing through 97º30’ E) encompassing the regions between 89º 52’ E to 97º 25’ E. The proposed demarcation line between IST-I and IST-II, falling at longitude 89º 52’ E, is derived from analyses of synchronizing the circadian clocks to normal office hours (9:00 am to 5:30 pm). This demarcation line passes through the border of West Bengal and Assam and has a narrow spatial extension, which makes it easier to get implemented from the railways point of view. Once approved, the implementation would requireestablishment of a lab for “Primary Time Ensemble - II” generating IST-II in any of the North-eastern states, which would be equivalent to the existing “Primary Time Ensemble -I” at CSIR-NPL, New Delhi.

      For any query related to CSIR-NPL’s proposal of two time zones in India, please contact

  2. Microwave Frequency Standards:
    • Cs Fountain Primary Frequency Standard
    • The present SI definition of a second is “The duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the Cesium 133 atom”. The device that realizes this definition with highest accuracy is called the primary standard of time and frequency. A Cesium fountain frequency standard provides most precise and accurate measurements of SI unit of time and hence is a primary standard. In a Cs fountain clock, Cs atoms are cooled and launched up, passed through a microwave cavity on the way up and down and are probed for their state in the detection region.

      Worldwide only 12 such fountains are operational at leading National Metrology Institutes (NMI) at USA, UK, France, Germany, Italy and Japan. Russia, China and India recently developed their Cs fountain primary standards. International atomic timescale (TAI) is a weighted average of time kept by atomic clocks all over the world. The increasing number of operational fountain frequency standards has contributed to the maintenance of TAI at an unprecedented level of accuracy, with fractional uncertainty of the time interval below 10−15.

      At CSIR-NPL (NPLI), India’s first ever Cesium fountain primary frequency standard, NPLI-CsF1 was indigenously designed and developed. It became operational in 2012. It was evaluated several times before participation in successful international inter-comparison with fountains from Germany, Russia and China in 2013. It was approved as primary frequency standard (PFS) by CCTF (consultative committee on time and frequency) constituted by BIPM. It contributed to the calibration of international atomic time (TAI) for few months. However, due to some inherent design issues, the fractional frequency uncertainty of the fountain could not be improved beyond 2.5 x 10-15. With the motivation to have better and stable fountain with improvised design features, a second-generation Cs fountain is currently under development at CSIR-NPL.Some new design features have been added to the second fountain which enable to carefully investigate the systematic errors in order to enhance the accuracy of NPLI-CsF2 to a few parts in 1016.

      For any query related to Cs fountain primary frequency standards, please contact Poonam Arora at arorap@nplindia.org

  3. R&D on Optical Frequency Standards:
  4. In near future the reference frequency for realizing the SI unit of time, i.e., Second will no longer be in the microwave region, rather it will be in the optical (visible light) region of the spectrum. Because of its higher operational frequency, an optical atomic clock or optical clock is capable of providing much better accuracy than the microwave atomic clocks. Realization of optical clocks are based on the spectroscopic interrogation of a narrow linewidth electronic transition of laser cooled neutral atoms within optical traps or a single atomic ion confined and laser cooled within a specially designed ion trap.

    At CSIR-NPL, work in progress towards devolving an optical clock based on an ultra-narrow transition of single Ytterbium ion (171Yb+) confined and laser cooled within a specially designed Radio Frequency (RF) ion Trap. For 171Yb+ the lowest lying excited state is 2F7/2 which can be populated from 2S1/2 ground state via an electric octupole transition at ~ 467 nm. The long lived 2F7/2 state has natural decay time of several years, which leads to an extremely narrow natural line width (sub Hz) of that transition. A radio frequency ion trap with special geometry (end-cap type) has been constructed for trapping single 171Yb+ and a cyclic electronic transition between 2S1/2 and 2P1/2 states at ~ 369.5 nm will be utilized for reducing the temperature of the ion to few micro Kelvin by laser cooling technique. There are certain possibilities that during laser cooling, the ion may be populated to one of its metastable states, namely at 2D3/2 or2F7/2. Two repump lasers at ~ 935 nm and ~ 760 nm will be utilized for depopulating those metastable states. The ion will be monitored by imaging its fluorescence at ~ 369.5 nm and finally the ions will be interrogated for the clock transition towards the clock transition 2S1/2 (F =0) – 2 F7/2(F =3) at~ 467 nm utilizing an ultra-stable narrow linewidth laser generated through an Ultra-Low expansion (ULE) cavity and the state of the ion will be determined by electron shelving technique.

    Figure: (a) Optical frequency standard lab at Metrology building, CSIR-NPL. (b) Vacuum housing for the ion trapping, and (c) pictorial representation of the single trapped ion experiment.

    For any query related to Cs fountain primary frequency standards, please contact Dr. Subhadeep De at subhadeep@nplindia.org and Dr. Subhasis Panja at panjas@nplindia.org

  5. Cyber Secured Critical Information Infrastructure of IST:
    • Demilitarized Isolation of Public Servers for Internet Time Dissemination Service (NTP Service)
    • Complete Physical Secured IST Infrastructure ( Bio-Metric and CCTV Monitoring)
    • Separate Cyber Secured Isolation of IST Infrastructure over dedicated Optical Fibre Link through ISP NIC-NKN ( Under Process)

    For any query related to Cyber Secured Critical Information Infrastructure of IST please contact Shri. Trilok Bhardwaj at trilok@nplindia.org

3. Services

In case of availing any of the services and related queries, kindly contact Ashish Agarwal at ashish@nplindia.org.

  1. Calibration Services:
  2. Atomic Clocks (Rb & Cs Frequency Standards), Time Interval Counter, Hour Meter, Teleclocks, GPS timing receivers and Stopwatch.

  3. Traceability Services:
    • time traceability for the precision users.
    • MoU signed with Dept. of Telecommunication (DoT), Ministry of Consumer Affairs (MoCA), Indian Space Research Organization (ISRO), IFR Information Dissemination Services Pvt. Ltd for providing IST time traceability.
    • CSIR-NPL provides time traceability to ISRO via CVGNSS and TWSTFT.
  4. Consultancy Service:
  5. We provide consultancy on time and frequency synchronization and capability building.

  6. Training Course on Time and Frequency:
  7. Theoretical course with hands on training (Organizing on request)
    To avail any of the above mentioned services, please contact Dr. Anurag Gupta/ Ashish Agarwal at anurag@nplindia.org / ashish@nplindia.org

4. Technology development

5. Outreach and Human Resource Development

  • Demonstration of atomic clock to school students (Jigyasa program).
  • Time and frequency exhibit for visitors on CSIR open day.
  • Projects for B.Tech/M.Tech/ M.sc. students.
  • Ph.D. positions.

For specific queries related to outreach and HRD, please contact HRD group at hrd@nplindia.org