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INTRODUCTION

Time transfer (T2L2) between SLR stations: Applications to space geodesy A. Belli, P. Exertier (OCA/ Geoazur , Astrogeo group). INTRODUCTION. T2L2/Jason-2: launched in June 2008 (for 2 yrs ): Instrument: built in OCA; continued for 6 yrs

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INTRODUCTION

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  1. Time transfer (T2L2) between SLR stations:Applications to spacegeodesyA. Belli, P. Exertier(OCA/Geoazur, Astrogeo group)

  2. INTRODUCTION • T2L2/Jason-2: launched in June 2008 (for 2 yrs): • Instrument: built in OCA; continued for 6 yrs • Organisation: ILRS network // CNES <-J2 mission-> OCA, dailyoperations • Objectives: • Metrological (performance, comparisonwithother techniques) • Scientific (spacegeodesy and fundamentalphysics) • Modeling of the DORIS oscillator (Jason-2 ... Jason-3) • Time transferbetweenSLRs • Consequences on spacegeodesy (LAGEOS and Jason) • Interest for the international service (ILRS -> ITRF)

  3. Ground-to-Space Time Transfer Jason-2 / T2L2 • : « triplets » • : groundtime at the emisson (cross axis) • : twowaydelay • : on-board time (recorded by T2L2) • : one waydelaywhichiscomputedfrom&Sagnac correction • . • gsTT : ground to space time transfer (relative to the pass) MeO Time calibration : CalL H-maser Short-termstability ground

  4. Time Transfer in non COMMON VIEW Grasse Greenbelt

  5. Non Common View: method < Jason-2 / T2L2 • gsTT(A) = t(A)B– (t(A)Ge + dt_A1wa ) +correct • gsTT(M) = t(M)B– (t(M)Ge + dt_M1wa ) +correct • t(i)B proper time on-boardcomingfrom the station i • t(i)Ge time at the emissionfrom the station i MeO GPS H-maser SLR master station SLR A station & clock ground Time calibration : CalL (100-200 ns) Ground link -> time server (GPS time) CalG+ [distribution timing signal] Integration of the frequency of the oscillator (modelised)

  6. MASTER station -> GPS time (~UTC) MeO GPS (GTR-50) H-maser LTB(t) = STC(t) + CalG + Zph(t) t(x)Ge = LTB(t) + CalL ground Time calibration : CalL (100-200 ns) LTB : Local Time Base STC : station time, regarding UTC CalG,L: calibration, delays in cableetc… Zph(t) : phase delay, jump, shift… t(x)Ge:emission time (cross axis) for station X Ground link -> time server (GPS time) CalG+ [distribution timing signal]

  7. Long-term SLR clocks nanosec = visibility of T2L2, for laser stations equipedwithH-maser or Cesium T2L2 as a time linkbetweenSLR’s

  8. Long-term SLR clocks microsec. nanosec * 1-year process: 2013 For the dates, XXXXX-XXX meansthat TB canbeconsidered constant on thatperiod *

  9. TB: impact on POD TB effect on the RMS POD for Lageos Satellite Effects in Meter 1-year process: 2013

  10. TB: impact on Positionning TB, in nanosec.

  11. CONCLUSIONS • T2L2 waslaunch in June 2008 (as a two-way+one-wayoptical TT technique) • It has been tracked by 22-24 SLR stations of the ILRS network, thatprovided the FR data to CDDIS/EDC data centers; rencently, 9 stations (1868 to, 1891) provided CDDIS withtheir FR data from 2016. • Time transferfrom Grasse master SLR station (accuratelytied to UTC) to the otherSLRs (TT in non CV) has been developed to establish a « on-board time » available on a continuous basis; • The synchronization of the station clocks to the same time reference, revealed time offsets (TB) os 0 to a few microseconds. • The technologicaldiagnosisis: • lack of time calibration of laser systems(delays in cables and devices), access to the time server (GPS), etc.

  12. TB as a systematicerror • TB have small but non negligeableeffects in station coordinates, depending on the station and the period of time • AC’s analyses are able to detect range errors in the measurements (mm level) whereas time errors are very difficult to estimate although recent improvements made : • In monitoring the engineering feedback : “data_handling_file” • In the precise orbit determination process • TB cannotbedetermined by PODactually, and itseffectisentirelytransfered on station coordinates (east-west component); • The impact on ITRF-like solutions (LAGEOS) shouldbeaccuratelyestimated and TB multi-yr time seriesincluded in the « standard » • The SLR data analysismethods (QC daily, multi-yr, etc.) shouldconsider TB in addition to rangeB.

  13. Thank You

  14. QC dailyAnalysis Grasse(56974-982: +61.1 ms); T. Otsubo, Japan Combrinck, 2010 TB.Vsat<-> a few mm Along-T / msec. Current LAGEOS A-Tracksensitivity : 20mm // 5msec

  15. DORIS oscillator USO model & T2L2 freq. data Long-termhistory (drift: aging + …) Freq drift Temperature Radiations Model: precision of few 10-13 (df/f), and accuracy 10-12 Belli at al., 2015; Jayles et al. 2016

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