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PPP for TAI time links and frequency standards comparison

PPP for TAI time links and frequency standards comparison. G. Petit, A. Harmegnies, A. Kanj Bureau International des Poids et Mesures. Outline. Background on PPP in TAI Study of the stability of operational TAI PPP links

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PPP for TAI time links and frequency standards comparison

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  1. PPP for TAI time links and frequency standards comparison G. Petit, A. Harmegnies, A. Kanj Bureau International des Poids et Mesures PPP Workshop 12-14 June 2013 Ottawa

  2. Outline • Background on PPP in TAI • Study of the stability of operational TAI PPP links • A test-bed for comparing processing techniques/software for frequency comparisons • Conclusions PPP Workshop 12-14 June 2013 Ottawa

  3. Background: PPP for TAI links • Precise Point Positioning makes sense for TAI time links • Each station processed independently (no need for a network), result is [UTC(k) – IGS time]. The time link is then obtained by simple difference. • The CCTF endorsed the introduction of PPP links in TAI • First official use in September 2009, more than 30 links now concerned • Time accuracy: about unchanged with respect to P3 code links • Frequency accuracy expected to be much improved • Better use of Primary frequency standards: The link to TAI is no more the dominant part of the uncertainty. PPP Workshop 12-14 June 2013 Ottawa

  4. GPSPPP processing for operational TAI links (TAIPPP) • NRCan’s GPSPPP Release 05211 (2011) • IGS Rapid SP3 orbits and 5-min SV clocks (RINEX format) – fixed • SV antenna offset values and station antenna phase center variations taken from most recent IGS file igs08.atx, completed when antenna info is not available. • Ocean loading applied (see http://holt.oso.chalmers.se/~loading/) • New yaw/attitude models; adaptative cycle slip detection and repair • Use iono-free L3/P3 measurements (cc2noncc for C1/P2) with a priori weights = 1 cm phase, 1 m pseudorange, elevation cut-off: 10° • Batches of 35 (or 40) days, continuous computation. • No a priori model of clock stability • Tropospheric delay estimated as 3mm/√hr random walk, without gradient. • Station coordinates estimated on each 1-month batch (or when important change) • ~35 stations present in our most recent computation • TAIPPP results are made available in ftp://tai.bipm.org/TimeLink/TAIPPP/ • PPP links results are also available in the data base of TAI link comparisons at ftp://tai.bipm.org/TimeLink/LkC PPP Workshop 12-14 June 2013 Ottawa

  5. The TAIPPP network PPP Workshop 12-14 June 2013 Ottawa

  6. Geodetic estimates from PPP monthly solutions Velocity estimates based on up to 5 years (0804-1304), quite consistent with ITRF Post-seismic (not linear) 5 cm/yr Post-seismic (not linear) PPP Workshop 12-14 June 2013 Ottawa

  7. Outline • Background on PPP in TAI • Study of the stability of operational TAI PPP links • A test-bed for comparing processing techniques/software for frequency comparisons • Conclusions PPP Workshop 12-14 June 2013 Ottawa

  8. Influence of doing a monthly computation • The PPP solution is continuous and follows the phase as long as there is no global reset of ambiguities (=> new ‘arc’). • The number of arcs/month is an indicator of the quality • Some 50% of stations have quite long arcs (typically 1 or 2 arcs/35-day) • Over each month, the code measurement provide time reference for the phase => the time reference is nearly independent from month to month. • The month-to-month discontinuity of the PPP solution provides an indicator of its 1-month stability • Solving one position for a 35 day interval has some implications: If the position is not actually constant, the PPP (clock) solution is affected PPP Workshop 12-14 June 2013 Ottawa

  9. Discontinuities between monthly batches • Monthly computations over 5 years provide (up to) 59 overlaps => strong statistics on the monthly discontinuities. • Typical (median) discontinuity: • 170 ps in phase • 4.5x10-16 in rate • Study allows to detect a few outliers (out of 1500): monthly discontinuities of several ns due to events where phase and code are not consistent. • nist discontinuities seem significantly biased: average = -0.329 ns (i.e.-1x10-16 rate) • Other cases with large RMS probably due to higher code noise. Too many to look in detail… PPP Workshop 12-14 June 2013 Ottawa

  10. Number of arcs / month • A majority of stations have less than two arcs per months on average, i.e. typically 1, 2 (or 3) arcs. • For stations with long arcs, monthly discontinuities should match the technique performance.. • For stations with short arcs, monthly discontinuities may be difficult to interpret. • Study to be pursued to correlate with • Receiver type / condition • Monthly discontinuity • …….. PPP Workshop 12-14 June 2013 Ottawa

  11. Effect of station velocity • Using a wrong station position (mostly east-west) is associated with obtaining a fake clock rate. => Using a fixed position while the station actually moves causes a clock drift. • This was mostly evidenced in the post-seismic relaxation after the M8.8 Chile earthquake (February 2010) and the M9.0 Japan earthquake (March 2011). • Amplitude of order 100 ps per mm/month of East velocity • Typical plate motion in ITRF is 20 mm/yr (Western Europe), -15 mm/yr (North America) => this should not be completely neglected (as we have done in practice). cont station, month following the February 2010 earthquake Average velocity VE ~ -1.5 cm / month PPP Workshop 12-14 June 2013 Ottawa

  12. Comparisons with IGS solutions • Some 10 TAI stations also are present in IGS clock solutions. • Differences between TAIPPP and IGS rapid solutions typically at the 150 ps level. • However several stations show significant (close to 100 ps) average bias, most notably: • twtf (Taiwan, Z12-T) • Some systematic differences have been associated with the different handling of C1-P1 biases (before we used cc2noncc); should affect only nist. PPP Workshop 12-14 June 2013 Ottawa

  13. Performance for TAI links • uA = 0.3 ns (1 sigma) for a link • expected to represent time instability over one month • should be smaller over shorter averaging time • Room for improvement (without changing much our procedures) • How to treat the breaks in clock solutions (ambiguity resets)? Using clock stability may help • Account for known station velocity • Encourage stations to follow “IGS standards” PPP Workshop 12-14 June 2013 Ottawa

  14. Outline • Background on PPP in TAI • Study of the stability of operational TAI PPP links • A test-bed for comparing processing techniques/software for frequency comparisons • Conclusions PPP Workshop 12-14 June 2013 Ottawa

  15. Comparison studies: a baseline data set • Requirements: • Rinex data continuously available, without phase resets • Availability of comparison techniques and solutions • TWTT • IGS clock solutions • Availability of local ultra-stable clocks • 96-day period October 2011- January 2012 (55855-55951), limited by • change in the reference clocks: e.g. OP on 55952; PTB on 55852 • gaps or other unexpected events in the fountain data • quality of PPP results (no global phase reset) PPP Workshop 12-14 June 2013 Ottawa

  16. Processing software and results to be compared • NRCan GPSPPP • Up to 40-day continuous processing • NRCan GPSPPP release 05211 (2011): presently our operational solution • NRCan GPSPPP release 01413 (2013) tested, not operationally implemented - no significant difference in results found for our standard processing • CNES IPPP / GINS • Integer ambiguity resolution => indefinitely continuous processing (in principle) • ORB Atomium • 1-day processing (float AR, so far) • IGS clock solutions • 1-day processing PPP Workshop 12-14 June 2013 Ottawa

  17. Comparisons of IPPP with GPSPPP • OP-PTB 96-day comparison GPSPPP = concatenation of 4 monthly results • IPPP = No obvious problem in linking each day • RMS difference = 114 ps ~1.5x10-16 @ 15 d • Some indication that the IPPP solution is more stable for t > hours IPPP solution J. Delporte/CNES GPSPPP: 4 independent months PPP Workshop 12-14 June 2013 Ottawa

  18. Future of PPP for TAI • Time accuracy: Will depend on progresses in calibration (code measurements) => No direct implication • The frequency accuracy (time stability) of our standard PPP processing is now again limiting PPP Workshop 12-14 June 2013 Ottawa

  19. Conclusions and prospects • PPP has been used for TAI links for 4 years • We expect the level of uA = 0.3 ns (time stability over one month) • Nevertheless some improvements are possible (and necessary) and we would like to more precisely assess the performance between 5 and 30 days. • Major goal: Improve frequency accuracy for frequency standards comparison PPP Workshop 12-14 June 2013 Ottawa

  20. THANK YOU Acknowledgements:Time laboratories participating to TAIProviders of analysis software (NRCan, CNES, ORB etc..) PPP Workshop 12-14 June 2013 Ottawa

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