• 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.

• Time transfer using TWSTFT (Two Way Satellite Time and Frequency Transfer) TWSTFT is based on exchange of timing signals through a Geostationary (Geo-sat) telecommunication satellite. TWSTFT is potentially one of the most accurate methods for comparing the timescales located geographically under the footprint of that Geo-sat used. With this method, frequencies can be compared with an uncertainty of 10^-15 @ 1day averaging time, and time scale differences can be compared at the ns level. The high accuracy is obtained by users simultaneously exchanging signals via a Geo-Sat, which will cancel out the delays as the path between the time scales is symmetric. The conventional setup of TWSTFT requires a modem to generate pseudo random noise (PRN) as the time transfer signal, a VSAT (Very Small Aperture Terminal) which generally consists of dish antenna of 1.8 m to 3.6 m, power amplifier, a low noise amplifier and 2.5 MHz band width of satellite transponder for Ku band link. NPL presently has two TWSTFT station links:1. TWSTFT link with ISTRAC- ISRO: To provide UTC time traceability to IRNSS (Indian Regional Navigation Satellite System) CSIR-NPL has a TWSTFT station link with ISTRAC-Bangalore and ISTRAC-Lucknow. The link is operational since 2018 using the GSAT-8 satellite.2. TWSTFT link with UTC: To have a TWSTFT link with PTB Germany which is the pivot of the UTC, an International TWSTFT station is also set up at NPL. This setup is upgraded recently and is in the process of establishing the link with 7 International NMIs, including PTB, via Express-80 Geo-satThe above figure shows the outdoor portion of two VSAT’s of 2.4m diameter dish antenna each at NPLI. Left one is being used for international link and the right one is used for domestic link

• Time transfer over the Internet (NTP Service) NTP is an internet standard protocol that uses a reliable time source, i.e., UTC(NPLI), as a 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 signal directly from an authoritative clock source, e.g., an atomic clock or a 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 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.Considering the crucial role of time synchronization in cyber security of the nation, the Indian Computer Emergency response team (CERT-In) has issued a directive to all service providers, intermediaries, data centers, body corporates, and government organizations to synchronize their ICT infrastructure to any of the NTP servers of the National Informatics Centre (NIC), National Physical Laboratory (NPL), or accurate and standard time sources other than NPL and NIC. Multiple stacks of NTP servers are available at NPL for time dissemination in the public domain with the domain name “time.nplindia.org”. Many customers get benefitted from the NTP services of CSIR – NPL.

  To check time of your device with IST maintained by CSIR-NPL Click Here

For any query related to Time generation and dissemination, please contact Dr. Ashish Agarwal at ashish@nplindia.org

a. Time Dissemination:

• R&D on Time Dissemination using Network Time Protocol (NTP)
  NTP is used widely for achieving time synchronization in computer networks. CSIR-NPL is working on the NTP hierarchical structure and evaluating the performance of the NTP servers available at different stratum levels. CSIR-NPL is working on many other aspects related to NTP like, Time & frequency aspects of NTP servers; NTP security; NTP performance analysis at end client/user; NTP for Internet of Things applications, etc.
  For any query related to NTP time dissemination, please contact Ms. Divya Singh Yadav at divya.yadav@nplindia.org or Dr. Deepak Sharma at deepak.sharma@nplindia.org.

• Precise Time Dissemination via PTP using dedicated optical fibres
  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 the 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) however it works in controlled networks (usually local area networks) where latencies/residence time in each network element 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 the absence of GPS signals, GPS signal spoofing, etc. It also eliminates the need to fix GPS antennae on rooftops physically. A proof of principle experiment for PTP based time dissemination both inside and locally outside the NPL campus using dark telecom fibres has been carried out recently.
  For any query related to PTP, please contact Dr. Manoj Das at manoj.das@nplindia.org
  
  
• Telephone Time Dissemination via Fonoclock
  The research activity aims to establish a robust telephone time dissemination service with a time synchronization accuracy of ±10 ms over the 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 the industry for commercial production. Thereafter, the service will be opened for public use. Due to millisecond accuracy, this service will appeal to defense-related activities where cyber security is the prime concern over NTP services. Even the banking and telecommunication sector will benefit from new improved telephone time service. Once IST becomes the legal time of the country, this technology will become the most appropriate means to synchronize with IST. An ordinary person in a rural village can access IST via a telephone line. It is also better than GPS-based timing receivers, which are prone to jamming and 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 are one of the easiest means to get time, even in rural areas. All developed nations, e.g., the USA, UK, Germany, and Japan, still offer telephone time dissemination services, which will hold relevance for at least the next decade. The conceptual design of the technology is presented in the figure.
  

  Conceptual design of FonOclock service over the 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 a master slave clock.
  For any query related to FonOclock, please contact Dr. Poonam Arora at arorap@nplindia.org
  
  

• NPLI Disciplined Oscillator (NPLI- DO)
  A device with a stable oscillator like Rubidium (Rb) providing required outputs to the users, which will be remotely steered in near real-time with respect to UTC(NPLI), shall be defined as an “NPLI-Disciplined Oscillator (NPLI-DO)”. Instead of using GNSS signals (like GPS, Glonass etc.) for which there is no authenticity provided to the client if the user has an NPLI-DO the timing output shall be synchronised with UTC(NPLI) and monitored by NPL, a monthly traceability report shall be provided for the same. The expected frequency stability of the NPLI-DO is 5*10-12/s, 2*10-15/day, and the synchronization accuracy is within ± 20ns with respect to IST.
  For any query related to NPLI Disciplined Oscillator, please contact Dr. V. Bharath at bharath.v@nplindia.org
  
  
• Time dissemination through White Rabbit Precision Time Protocol (WR-PTP) based optical fibre linkAt CSIR-National Physical Laboratory, work in progress towards establishing phase stabilized optical fibre link utilizing White Rabbit Precision Time Protocol (WR-PTP) technique for precise and accurate transfer of time and frequency signal over long distances. WR-PTP is one of the most advance techniques, developed at CERN, for transferring time and frequency signals through optical fibre. Optical fibre-based time transfer links are quite suitable for precise and secure transfer of time and frequency signal as it provides very low attenuation and high reliability of signals during its transmission. However, the optical path length of the fibres may vary due to mechanical stress or temperature variations around the optical fibres and the ambient variation also influence the stability as well as accuracy of the optical fibre link. Conventional WR technique is not capable of compensating dynamic time delay or phase variation arises due to ambient temperature variation. We have developed a technique and introduced a feedback loop for estimating and compensating dynamic time delay or phase variation, arises mainly due to varying ambient condition. Introduction of active phase compensation improves the stability as well as accuracy of the WR network-based time transfer link and makes it enable to transfer time with very low uncertainty (within 200ps) over a very long optical fibre link.

For any query related to White Rabbit Precision Time Protocol (WR-PTP) based optical fibre link, please contact Dr. Subhasis Panja at panjas@nplindia.org.

Experimental setup for phase stabilized optical fibre link over 22 km long underground fibre and time offset variation with time

b. Microwave Frequency Standards:

• Cs Fountain Primary Frequency Standard
  The current 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 the highest accuracy is called the primary standard of time and frequency. A Cesium fountain frequency standard provides the most precise and accurate measurements of SI unit of time and hence it 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) in the USA, UK, France, Germany, Italy, and Japan. Russia, China, and India recently developed their Cs fountain primary standards. The International Atomic Timescale (TAI) is a weighted average of time kept by atomic clocks worldwide. 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 a better and more 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 enables to carefully investigate the systematic errors 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 Dr. Poonam Arora at arorap@nplindia.org

c. R&D on Optical Frequency Standards:

• In the 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 can provide much better accuracy than microwave atomic clocks. The realization of optical clocks is 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 is in progress towards devolving an optical clock based on an ultra-narrow transition of a 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 ²F_7/2, which can be populated from the ²S^1/2 ground state via an electric octupole transition at ~ 467 nm. The long-lived ²F_7/2 state has a 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 ²S_1/2 and ²P_1/2 states at ~ 369.5 nm will be utilized for reducing the temperature of the ion to a few hundred 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 ²D_3/2 or ²F_7/2. 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 towards the clock transition ²S_1/2 (F =0) – ²F_7/2(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.

For any query related to optical frequency standards, please contact Dr. Subhasis Panja at panjas@nplindia.org

d. Lasers for T&F related Applications: Lasers play a critical role in metrology and related applications. In Time & frequency (T&F) division, they are being used in Cs fountains (the primary standard of SI second) for cooling the Cs atoms. In optical atomic clocks (candidate for re-definition of SI second) experiment, they are being used to generate ions from neutral atoms and then cooling the ions. In optical clocks, narrow linewidth lasers are used for performing precision spectroscopy of the clock transition. In pressure metrology, the next generation of quantum Pascal standards will be based on laser interferometer setup.

The laser laboratory in T&F division has embarked on the indigenous design and development of external cavity diode lasers (ECDL) based on semiconductor diodes. Such lasers are promising in terms of single frequency and single transverse mode operation. They offer wide frequency tunability with high-bandwidth frequency modulation based on current and Piezo transducer feedback.

Ultra-stable High-finesse Optical Cavities (OC): The laser lab is also working on indigenous design and development of ultra-stable high-finesse optical cavities. Such cavities offer excellent long-term frequency stability with predictable linear drifts. The frequency linewidth of ECDL’s locked to such cavities can be brought down to Hz level, a pre-requisite for the oscillators in optical clock. The work is comprehensive and includes cavity calculations, design of optical elements professional machining level 3D CAD mechanical designs, electrical and electronics.

The lab is working on applications based on such ECDL and cavity development for Quantum technologies at CSIR-NPL:

• Atomic clocks: Microwave & Optical (Cs fountain and Yb+ ion trap clock)
• Quantum pressure standard based on cavity refractometers

e. Ultra-stable Direct Optical Frequency Transfer through Optical Fibers for Metrology Applications: Such transfer techniques based on active fiber noise cancellation offers stability at 10^-17 at 1s for short distances. They find applications in comparing two optical clocks for same atomic species as well as frequency ratio measurements for different atomic species. They can also be used to establish microwave to optical link using frequency combs.

Recently, Earthquake detection based on stable lasers, using widely used Internet optical fibre network has been demonstrated. The laser lab has started initial study on developing such stable lasers and optical fiber as sensors.

The lab is working on the feasibility study regarding development of quantum nodes based on optical nanofibers for technologies like quantum communication.

For queries related to direct optical frequency transfer techniques, please contact Dr. Manoj Das at manoj.das@nplindia.org

f. Cyber Secured Critical Information Infrastructure of IST: Demilitarized Isolation of Public Servers for Internet Time Dissemination Service (NTP Service)

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

• Calibration: Services are available for calibration of Rubidium & Cesium Atomic Clocks, Time Interval Counters, Hours Meters, Teleclocks, GPS timing receivers and Stopwatches.
• Training: Training is available in the area of Time and Frequency metrology and related areas. Theoretical course with hands on training (organized on request).
• Consultancy: We provide consultancy on time and frequency synchronization and capability building. Consultancy services are available for setting up timing laboratory, time synchronization systems and time dissemination systems.
• Traceability: Time traceability via satellite links is available for selected strategic sectors who need ns accuracy with respect to IST. a). Time traceability for the precision users. b). MoU signed with Dept. of Telecommunication (DoT), Ministry of Consumer Affairs (MoCA), IFR Information Dissemination Services Pvt. Ltd for providing IST time traceability. c). CSIR-NPL provides time traceability to Indian Space Research Organization (ISRO) via CVGNSS and TWSTFT.

• FonOclock
• Pulse Distribution Amplifier

• 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.
• PhD positions.

Dr. Ashish Agarwal
Senior Principal Scientist & Head
Email: ashish@nplindia.org