TWSTFT - NGS-T20

Two-way Satellite Time and Frequency Transfer
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TWSTFT - NGS-T20

Two-way Satellite Time and Frequency Transfer

Two-way Satellite Time and Frequency Transfer (TWSTFT) is a GNSS independent technique of synchronizing multiple station clocks, irrespective of their geographic location, to a remote Master station reference clock via a dedicated satellite link.

Accord's NGS-T20, a TWTFT modem is capable of synchronizing a slave station clock to an accuracy of less than 5 ns with respect to the Master's reference clock. Moreover, in the absence of the satellite link, once synchronized, the Modem can keep the time accurate to less than one microsecond over the next 24 hours.

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Application

  • Synchronization of one station clock to a remote Master station clock at nanosecond accuracy via a satellite link
  • Comparison of a station clock reference to a remote station Clock reference for time keeping purpose

TWSTFT - NGS-T20

TWSTFT systems are used for time comparison purpose

Applications

Synchronization of one station clock to a remote Master station clock at nanosecond accuracy via a satellite link

Comparison of a station clock reference to a remote station Clock reference for time keeping purpose

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Frequently Asked Questions

1. What is time & frequency transfer?

Time and Frequency transfer consists of realization of a source system time and frequency at the client via a communication channel between them. The realized time and frequency at the client could be used for either comparison of local time and frequency or for comparison followed by local time and frequency correction. Figure shows the block diagram of a typical time and frequency transfer system. The communication channel could be either wired or wireless and either one-way or two-way.

The reader is requested to download the article ‘ The Need for Time Synchronization ’ from Whitepapers section to know more about the concept.

Accord's NGS-T20, a TWTFT modem is capable of synchronizing a slave station clock to an accuracy of less than 5 ns with respect to the Master's reference clock. Moreover, in the absence of the satellite link, once synchronized, the Modem can keep the time accurate to less than one microsecond over the next 24 hours.

2. What sort of accuracy in time & frequency is possible in a two way satellite time and frequency transfer system (TWSTFT)?

The achievable time accuracy at the slave TWSTFT system is less than 2 ns. The frequency accuracy achievable is better than 1e-12 assuming that the Master frequency source accuracy and stability is better by at least an order.

3. What sort of applications need such accurate time transfer?

Typically, the accuracy provided by TWSTFT systems are used for time comparison purpose and for keeping time that is traceable to International reference standard such as UTC. For example, National Physical Laboratory India (NPLI), Delhi is the time keeper of Indian Standard Time (IST). NPLI uses a TWSTFT link with BIPM, France, which is a keeper of UTC, to measure its time and frequency offset periodically and in-turn also contribute to the international atomic time (TAI).

Such systems also find use in defense applications where time transfer is required to be independent of GPS/GNSS systems which are easily prone to Jamming and spoofing. Since accurate time transfer is achievable, the output of such systems could also be used for synchronization of distributed radar systems to one common source of time.

4. What are the implications on reduced accuracy in time and frequency transfer?

The reduced accuracy in time or frequency transfer leads to an error in the client’s time and frequency with respect to the Server or Time-source. The implication of this depends on the application and the amount of error that is introduced. For example, in Telecom applications, a frequency error of more than 1e-11 can lead to frequent data communication errors and call-drops. In enterprise networks and Banking sectors, erroneous time could lead to inaccurate time-stamping of events rendering the administrator incapable of tracing the sequence of events and transactions. In defense, error in synchronization between radars could lead to inaccurate positioning of the target.

5. What communication protocol is used by in Accord’s two way time & frequency transfer system NGS-T20?

The communication protocol used is Accord –proprietary.

6. In what way a TWSTFT system is superior to GNSS time's accuracy between two remote systems?

Time output from GNSS Receivers is typically traceable to UTC with an accuracy of 15 to 50 ns. Deriving time from GPS/GNSS constellation via a GNSS Receiver is an example of a one-way mode of time-transfer. Here the path delay between the satellite and the receiver is perturbed by the ionospheric and tropospheric activity. This introduces time-variant delay in the communication path and leads to an inaccurate estimation of time at the receiver end, limiting the accuracy to 15 ns at best, considering a good PDOP. Therefore, the achievable time-accuracy between two such receivers placed geographically far apart can be as high as 100 ns depending on the PDOP, Ionospheric and tropospheric errors.

In contrast, the TWSTFT is a two-way time transfer system. The path delays in the communication link between the Master and the Slave via the satellite are almost symmetrical and get cancelled out at the slave. Therefore, the TWSTFT system time at Slave can be accurate to less than 2 ns with respect to the Master.

7. What are the main areas of applications for a TWSTFT system?

  • Synchronisation of multiple remote Slave clocks to a central Master clock reference and dissemination of time over the network using NTP and PTP protocols, distribution of time and frequency using 1-PPS, 10 MHz and IRIG signals.
  • Comparison of distant clocks
  • Satellite ranging for orbit determination

8. How many receiver channels are present in Accord's two way time transfer system NGS-T20?

At present 2-receive channels are present as default. The same can be extended to multiple receive channels with software upgrade alone.

9. What are the communication frequencies the NGS-T20 is capable of operating in?

The TWSTFT product has signal interface options currently available from 140-160 MHz and from 950-2050 MHz (the operating frequency can be customized to suit the user requirements). These signals are required to be up/down converted to the appropriate satellite uplink/downlink frequency. It is to be noted that the scope of Accord’s supply is currently limited to the Baseband TWSTFT modem with operation in the said frequency bands.

10. Is the satellite bandwidth required continuously?

This depends on the end application and time and frequency accuracy requirements at the slave. For more detailed information, the reader is requested to contact ibd@accord-soft.com with your requirements.

11. What happens when GNSS signals are jammed -would the two-way time transfer system still work?

The GNSS signals occupy predominantly the L-band, whereas the TWSTFT system is typically operated in C-band or Ku-band. Therefore when GNSS signals are jammed, the TWSTFT system would still work.

12. What is the maximum separation that could exist between two TWSTFT systems?

As long as the two systems involved in the time transfer are under the same satellite signal footprint on earth, direct time transfer between them is possible.