Nederlands Instituut van Navigatie Workshop Time is of the Essence The relevance of Time and Timing. Timing

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1 Nederlands Instituut van Navigatie Workshop Time is of the Essence The relevance of Time and Timing Timing Basisprincipes van tijdbepaling en tijdoverdracht (Basic Principles of Time Determination and Time Transfer) P.Waller (ESA/ESTEC) VSL, 16 October 2015

2 Outline Introduction Who needs time and for what? Basic principles of Time Transfer Few Examples Trends and perspectives Conclusions

3 Introduction Timing? The technologies, techniques, systems and associated services to generate, maintain and distribute time information with a known level of accuracy Two pillars of Timing: Time Generation Time Transfer / Dissemination

4 Introduction: few definitions Accuracy, Stability, Precision Source: John R. Vig tutorial March 2008

5 Introduction: few figures 1 day is ~1e5 sec 1sec/day ~ 1e-5 1 year is ~3e7 sec 1sec/year ~ 3e-8 Homo sapiens: ~6e12 sec 1sec/THSapiens ~ 1e-13 Earth: ~1e17 sec 1sec/Tearth ~ 1e-17 Universe: ~4e17 sec 1sec/Tuniverse ~ 2e-18

6 Who needs accurate time? Timing Displays (rails, airports ) Power grid, smart grid Data centers, big data Banking, High Frequency Trading (100 s millions transactions/sec!) Digital Video Broadcasting (DVB-T, DVB-H, DVB-S ) Digital Cellular Networks (CDMA, TD-CDMA, WIMAX, LTE ) PNT (Loran, GNSS, ) Spacecraft Navigation Very Long Baseline Interferometry Relativistic Geodesy, Fundamental Physics experiments

7 Who needs accurate time? Generic Requirements: Everywhere, at any time Seamless integration, scalable, extendable Backward compatibility Easy and Cheap access Specific Requirements: Accuracy Availability Traceability Authentication.

8 How to provide accurate time? The two pillars: 1) Time generation = a clock = a timescale 2) Time transfer, allows for: Time comparison Time Dissemination Clock synchronization

9 Simple case: time transfer between two clocks A? B What is the time difference between clocks A and B: TA TB?

10 Method 1: Clock Transportation A c B ma = TA - Tc A c B mb = TB - Tc ma - mb = TA - TB

11 Method 1: Clock Transportation Advantages: Simple method Independent of external infrastructure Drawbacks No simultaneous comparison Accuracy is affected by Stability of the transportable clock Stability of clocks A and B during transport Number of measurements and clocks limited by transport duration

12 Method 2: Common View c tca tcb A B ma = Tc + tca - TA mb = TC + tcb - TB mb - ma = TA TB + (tcb tca)

13 Method 2: Common View Advantages: Simultaneous and continuous comparison Multiple clocks simultaneous comparison (Almost) independent of clock C Drawbacks Use of external infrastructure Travel time between clock C and clocks A/B shall be known

14 Common View, a practical example: GNSS Time Transfer ρa ρb A RXA RXB B

15 Common View, a practical example: GNSS Time Transfer fixed and known Nav. Message or IGS Nav. Message or IGS P = RA Rsat c (TA Tref) + c (Tsat Tref) + Iono + Tropo + HWdelays + noise Nav. Message or Iono combination Modelled To be measured!

16 From Common-View to All-in-View A RXA RXB B

17 Common View, a practical example: GNSS Time Transfer

18 Common View, a practical example: GNSS Time Transfer

19 Common View, a practical example: GNSS Time Transfer

20 Method 3: Bi-directional links / two-way A ma = TB + tba - TA tba tab B mb = TA + tab - TB mb - ma = 2 (TA TB) + (tab tba) TA TB = ½ (mb - ma) ½ (tab tba)

21 Method 3: Bi-directional links / two-way Advantages: No need for external clock No need for travel time measurement Delays compensation for symmetric link Drawbacks Use of dedicated infrastructure Requires coordinated scheduling for multiple clocks comparison No continuous measurements

22 Method 3, a practical example: Two-Way Satellite Time and Frequency Transfer A B

23 Method 3, a practical example: Two-Way Satellite Time and Frequency Transfer ma = TB TXB + SPUB + SPTB +SPDA +RXA TA mb = TA TXA + SPUA + SPTA +SPDB +RXB TB diff.delay in A diff.delay in B mb - ma = 2 (TA TB) + (TXA RXA) - (TXB RXB) + (SPUA SPDA) + (SPUB SPDB) + (SPTA SPTB) up-down in A up-down in B diff.delay in GEO transponder

26 Typical Performances Technique Precision Accuracy Frequency Comparison GNSS (code) 1~2 ns few ns 1day GNSS (phase) ~100ps few 5ns 1day TWSTFT (code) ~100ps 1ns 1day

27 Trends and Perspectives GNSS Technique Use of Carrier Phase (PPP Time Transfer) Hardware and bias calibrations Increased number of systems and signals! Vulnerability, use back-up systems (e.g. LORAN/eLORAN) Two-Way Technique Use of carrier phase TWTFT (10ps precision?) TW technique successfully implemented on optical fiber links

28 Trends and Perspectives Bi-directional packet-based protocols Network Time Protocol, Precise Time Protocol Network Time Security White Rabbit Optical fiber 300km time-transfer link in Poland 2 x 1000km optical carrier link in Germany 2 x 137km White Rabbit link demonstrated in NL Growing network in Europe Very promising alternative to GNSS, TW.. Transcontinental scale?

29 Trends and Perspectives

30 Conclusion The techniques and technologies for time transfer are being continuously improved to address the needs and challenges from an increased number of interconnected users. Classical techniques (GNSS, TWSTFT) have significantly improved their performance and ease of usage, and are expected to pursue their developments, in particular thanks to the growing number of available GNSS signals. Other very promising time transfer techniques are being developed, in particular using optical fiber techniques and technologies.

31 THANK YOU!

Time and frequency transfer methods based on GNSS. LIANG Kun, National Institute of Metrology(NIM), China

Time and frequency transfer methods based on GNSS LIANG Kun, National Institute of Metrology(NIM), China Outline Remote time and frequency transfer GNSS time and frequency transfer methods Data and results