PPP for TAI time links and frequency standards comparison G. Petit, A. Harmegnies, A. Kanj Bureau International des Poids et Mesures PPP Workshop June 2013 Ottawa 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 June 2013 Ottawa 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 June 2013 Ottawa GPSPPP processing for operational TAI links (TAIPPP) NRCans GPSPPP Release (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 (seeNew 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/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 ftp://tai.bipm.org/TimeLink/LkC PPP Workshop June 2013 Ottawa The TAIPPP network Geodetic estimates from PPP monthly solutions Velocity estimates based on up to 5 years ( ), quite consistent with ITRF PPP Workshop June 2013 Ottawa 5 cm/yr Post-seismic (not linear) 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 June 2013 Ottawa 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 June 2013 Ottawa Monthly computations over 5 years provide (up to) 59 overlaps => strong statistics on the monthly discontinuities. Typical (median) discontinuity: 170 ps in phase 4.5 x 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 = ns (i.e.-1 x rate) Other cases with large RMS probably due to higher code noise. Too many to look in detail PPP Workshop June 2013 Ottawa Discontinuities between monthly batches Station# disc.Disc ave /nsDisc Emq / nsSlope Emq / ns/d apl ao_ be1_ brus brux cont dmdm dt gum ip ieng impr kris liti mbro migt nict nist nm0c np nrc nrl obet opmt ptbb rjep ro_ sepb sg2p sp sydn tp twtf usn vn3p vsle wab wtza 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 June 2013 Ottawa Number of arcs / month Station# monthsAverage arc / dDisc Emq / ns apl be1_ brus cont dmdm gum kris nict nist nrc opmt ptbb sepb sp tp usn usn wab wtza impr ip nxra rjep ro_ roap vn3p 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). PPP Workshop June 2013 Ottawa cont station, month following the February 2010 earthquake Average velocity V E ~ -1.5 cm / month 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 June 2013 Ottawa Comparisons with IGS solutions Station# monthsAve diff / nsRMS diff / ns brus brux cont ieng nist opmt ptbb sydn twtf usn wab wtza Performance for TAI links u A = 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 June 2013 Ottawa 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 June 2013 Ottawa 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 January 2012 ( ), limited by change in the reference clocks: e.g. OP on 55952; PTB on gaps or other unexpected events in the fountain data quality of PPP results (no global phase reset) PPP Workshop June 2013 Ottawa Processing software and results to be compared NRCan GPSPPP Up to 40-day continuous processing NRCan GPSPPP release (2011): presently our operational solution NRCan GPSPPP release (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 June 2013 Ottawa 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 15 d Some indication that the IPPP solution is more stable for > hours PPP Workshop June 2013 Ottawa IPPP solution J. Delporte/CNES GPSPPP: 4 independent months Future of PPP for TAI The frequency accuracy (time stability) of our standard PPP processing is now again limiting PPP Workshop June 2013 Ottawa Time accuracy: Will depend on progresses in calibration (code measurements) => No direct implication Conclusions and prospects PPP has been used for TAI links for 4 years We expect the level of u A = 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 June 2013 Ottawa THANK YOU PPP Workshop June 2013 Ottawa Acknowledgements: Time laboratories participating to TAI Providers of analysis software (NRCan, CNES, ORB etc..)