Quality assessment of GPS reprocessed Terrestrial Reference Frame

1. Quality assessment of GPS reprocessed Terrestrial Reference Frame X Collilieux1 Z Altamimi 1 L. Métivier 1 T. van Dam 2 1 IGN/LAREG and GRGS 2 University of Luxembourg Acknowledgment : J. Ray (NGS)

2. Outline • Data • GPS Frame origin and scale. Investigation of their temporal behaviors • Station position time series • GPS TRF assessment Data Origin & Scale Station position time series TRF assessment

3. Data AC Epoch Nb of stations IG1 combined 1997.0-2009.5 570 GFZ 1998.0-2008.0 248 ESA 1997.0-2009.0 358 COD 1997.0-2009.0 224 MIT 1998.0-2008.0 748 Data Origin & Scale Station position time series TRF assessment mi1 gf1 co1 es1 IGS Nb of stations per week

4. GPS translation and scale (1/5) Comparison with ITRF2008 preliminary solution (SLR origin) mi1 Data Origin & Scale Station position time series TRF assessment

5. GPS translation and scale (1/5) Comparison with ITRF2008 preliminary solution (SLR origin) mi1 gf1 Data Origin & Scale Station position time series TRF assessment

6. GPS translation and scale (1/5) Comparison with ITRF2008 preliminary solution (SLR origin) mi1 gf1 co1 Data Origin & Scale Station position time series TRF assessment

7. GPS translation and scale (1/5) Comparison with ITRF2008 preliminary solution (SLR origin) mi1 gf1 co1 Data Origin & Scale Station position time series TRF assessment es1

8. GPS translation and scale (2/5) W.r.t. ITRF2008 prel. Data Origin & Scale Station position time series TRF assessment • GPS scale is based on ITRF2005 scale through GPS satellite antenna offsets. • This non-zero scale offset is explained by: • ITRF2008P and ITRF2005 scale difference • Adoption of absolute phase center variations

9. GPS translation and scale (3/5) Seasonal variations of the apparent geocenter motion mi1 gf1 co1 es1 Data Origin & Scale Station position time series TRF assessment SLR Good fit on Y component. Still inconsistent X and Z variations

10. GPS translation and scale (4/5) Comparison of scale variations with a loading model 6 h Loading model 12 h + + 1 month Loading effects Some examples GPS data (ITRF2008) Loading model Data Origin & Scale Station position time series TRF assessment ~ Bad fit ~Good fit

11. Station position time series – Scale issue Comparison of scale variations with a loading model Load mi1 mm gf1 co1 es1 Data Origin & Scale Station position time series TRF assessment Difference with the loading model • Cause of these variations: • Deficiency in the loading model • Draconitic period (harmonics detected in the power spectra) • Thermal effects mm As the scale behavior is stable : do not estimate scale non-linear temporal variations to avoid loosing loading signal

12. Station position time series - Spectral map (1/2) Repeatability Data Origin & Scale Station position time series TRF assessment mi1 co1 gf1 es1 ITRF2008 ITRF2005

13. Station position time series - Spectral map (2/2) • 5 latest years of IGS combined analysed for spectral content. • For the whole period, no difference between Acs, but the 6th harmonic more visible in the North Proportation of stations which detect a given frequency • Is the power of the 4th harmonic of the draconitic frequency lower in ITRF2008 data? Data Origin & Scale Station position time series TRF assessment • Comparison of the amplitude of the 4th harmonic signal between: • ITRF2005 residuals (2001-2006) • ITRF2008 residuals (2004-2009) IG1 reprocessed IGS ITRF2005

14. Station position time series - Comparison with a loading model (2/3) Amplitude of the annual signal Vertical ρ = 43% Period 1997.0-2006.0 Station considered ITRF2005 GPS (IGS) Data Origin & Scale Station position time series TRF assessment ρ = 56% GPS annual signal ampl. ITRF2008 GPS (IG1) Loading model annual signal ampl.

15. Comparison with a loading model (3/4) Amplitude of the annual signal Vertical ρ = 43% ρ = 17% ρ = -13% ITRF2005 GPS (IGS) Data Origin & Scale Station position time series TRF assessment ρ = 56% ρ = 26% ρ = -5% ITRF2008 GPS (IG1) Horizontal

16. Comparison with a loading model (4/4) Phase of the annual signal : histogramme of the difference of phase between GPS and loading model ITRF2005 GPS (IGS) Data Origin & Scale Station position time series TRF assessment ITRF2008 GPS (IG1) Significant improvement in the height only

17. Φ A .cos( ω . T–Φ) Station position time series - Annual signal and AC solutions Data Origin & Scale Station position time series TRF assessment mi1 es1 gf1 co1

18. Agreement with IVS VLBI reprocessed solution Comparison with newly reprocessed VLBI (IVS) solution Height local tie residual improvement Made on GPS-VLBI co-location sites Data Origin & Scale Station position time series TRF assessment --------------------------------------------- WRMS Positions Velocities 2D h 2D h t=2000.0 mm mm/yr ------------------------------------------------------------------------ ITRF2005 0.3 1.6 0.1 0.5 ITRF2008 P 0.1 0.8 0.1 0.4 ------------------------------------------------------------------------ If VLBI positions and velocities are fixed to their estimated values,

19. Conclusion Origin • GPS frame origins differ from SLR origin by less than 1 mm/yr for each component • The GPS translation non-linear variations are still biased compared to SLR and loading model. Scale • The scale behavior of analysis GPS reprocessed solution is stable. • 31 to 40% WRMS reduction when corrected by a loading model Station position time series • The repeatability of GPS position time series is smaller, especially in vertical, than previous non-homegenously reprocessed solution • The estimated annual variations are closer to loading model than in the past. Still larger power in horizontal for GPS. • Harmonic of the draconitic frequency still visible. Their power is comparable than previous GPS solutions. Reference positions and velocities • GPS station long term positions and velocties are closer to VLBI results