1. Jason-1 POD reprocessing at CNES Current status and further developments L. Cerri, S. Houry, P. Perrachon, F. Mercier. J.P. Berthias with entries from the Jason-1 POD group Ocean Topography Science Team Meeting - Hobart, Australia – March 2007

2. 2 Reprocessing: GDR-B Current Status ■GDR-B Standards in summary  Doris/SLR/GPS dynamic orbits  GPS overweighs D+L  Gravity: EIGEN-CG03c  Static with C20dot, C30dot, C40dot  C21/S21 from EIGEN-CG03c model (constants)  Solar radiation pressure : standard Box & Wing model, with 0.97 scale coefficient  Dynamic parameterization: 1/rev every 12 h, Drag coeff. every 2 revs  DPOD2000 Doris coordinates, ITRF2000 Laser coordinates  GPS constellation ephemeris and clocks from JPL (IGS00 based)  SAA correction applied before the Doris instrument change (up to cycle 90)

3. Ocean Topography Science Team Meeting - Hobart, Australia – March 2007 3 Comparison of GDR-B with GPS JPL06b ■Mean Radial RMS = 0.86 cm ■120-days signal in along and cross track components ■In phase with  ’ ■Cross track difference removed when using UCL solar radiation pressure model

4. Ocean Topography Science Team Meeting - Hobart, Australia – March 2007 4 Comparison of GDR-B with GPS JPL06b ■Order one pattern seen in mean radial difference ■Mean radial rate due to drift in Z-shift between orbits (~-1mm/yr) Mean Radial rate (mm/year) Mean Radial difference (mm)

5. Ocean Topography Science Team Meeting - Hobart, Australia – March 2007 5 Comparison of GDR-B with GSFC new D+L orbits ■GSFC D+L ITRF2000 latest release (using TVG and SAA model) ■Mean Radial RMS = 1.2 cm ■120-days signal in along and cross track components  Cross-track signal with opposite phase

6. Ocean Topography Science Team Meeting - Hobart, Australia – March 2007 6 Comparison of GDR-B with GSFC new D+L orbits ■Stronger order one pattern in mean radial difference ■Mean radial rate due to drift in Z-shift between orbits (~+1mm/yr) Mean Radial rate (mm/year) Mean Radial difference (mm)

7. Ocean Topography Science Team Meeting - Hobart, Australia – March 2007 7 Comparison of GDR-B with CNES latest D+L orbits ■CNES D+L series uses SAA model for the entire period ■The same 120-day signal is found between CNES GDR-B and CNES D+L only solution (same phase as previous plots)  No such signal when comparing GSFC and CNES D+L orbits  Likely due to some problem in our modeling of GPS measurements (under investigation)

8. Ocean Topography Science Team Meeting - Hobart, Australia – March 2007 8 Comparison of GDR-B with CNES latest D+L orbits ■Same gravity model in both orbits; mean radial difference only shows a small N/S difference ■The same pattern shown in comparison with GSFC orbits is seen in mean radial rate between GDRB and CNES D+L Mean Radial rate (mm/yr) Mean Radial difference (mm)

9. Ocean Topography Science Team Meeting - Hobart, Australia – March 2007 9 Z Bias relative to GDR-B ■Drifts in the Z bias in the order of 1 mm/yr can be identified for all sets of orbits ■120-day signal is also clearly present in GSFC orbits, while it is less visible in other orbits ■The strong variation of the Z bias of both D+L orbits corresponds to the change of Doris instrument (cycle 91)

10. Ocean Topography Science Team Meeting - Hobart, Australia – March 2007 10 Summary ■GDR-B Orbits are a clear improvement over GDR-A orbits ■GDR-B Orbits exhibit Z-biases with respect to GPS JPL06b and GSFC and CNES latest Doris/SLR orbits  Biases with respect to GSFC D+L orbits exhibit 120-day signal  Biases drift at about 1 mm/yr  Drifts have opposite signs when comparing with JPL and GSFC solutions ■Geographically correlated radial signal due to the use of a different gravity model (order 1 coefficients) can be seen between GSFC and CNES orbits

11. Ocean Topography Science Team Meeting - Hobart, Australia – March 2007 11 ITRF2005 evaluation ■Conducted using  ITRF2005 Doris coordinates when available  Used DPOD2000 coordinates for missing stations  ITRF2005 SLR rescaled solution from IERS  No missing station found over the test period ■No test run with GPS ITRF2005 constellation orbits  JPL GPS orbits are based on IGS00 which uses improved velocities with respect to ITRF2000 (closer to ITRF2005)

12. Ocean Topography Science Team Meeting - Hobart, Australia – March 2007 12 ITRF2005 evaluation ■Negligible decrease of post-fit Doris residuals (~0.001 mm/s) ■Significant decrease of post-fit SLR residuals (~2 mm)

13. Ocean Topography Science Team Meeting - Hobart, Australia – March 2007 13 ITRF2005 evaluation ■Focus on Z bias between different orbits and CNES GDR-B ■GDR-B is by construction very close to CNES GPS only solutions ■GPS JPL reduced dynamic supposed to be centered in IGS00 Increasing bias Decreasing bias GDR-B Reference

14. Ocean Topography Science Team Meeting - Hobart, Australia – March 2007 14 ITRF2005 evaluation ■Comparison of two sets of CNES and GSFC D+L orbits using ITRF2000 and ITRF2005  CNES series spans cycles 22 to 127  SAA model is applied for both instruments

15. Ocean Topography Science Team Meeting - Hobart, Australia – March 2007 15 ITRF2005 evaluation ■In ITRF2005 Vs ITRF2000 comparison a bias of ~7 mm and a drift of ~1mm/year can be seen ■Impact of ITRF2005 on CNES and GSFC (ITRF2005s_only) orbits is similar up to about cycle 90 ■More significant differences after that cycle ■ITRF2005 is handled differently by CNES - Missing ITRF2005 coordinates have been replaced by the corresponding DPOD2000/ITRF2000 coordinates - Station weights are different

16. Ocean Topography Science Team Meeting - Hobart, Australia – March 2007 16 ITRF2005 evaluation ■To evaluate the ITRF2005-based GPS constellation, a test on few Jason1 cycles over 2005 (cycles 111  127 ) was performed on GPS only orbits ■Mean bias relative to GDR-B is 4.3 mm, close to the mean value of GSFC D+L ■Not sufficient to estimate any trend

17. Ocean Topography Science Team Meeting - Hobart, Australia – March 2007 17 Centering: Z bias summary ■Moving average over 36 cycles removes periodic part of the signal (120 days and annual) ■Different behavior of D+L orbits before and after the instrument change is clearly shown cnes gps itrf2005

18. Ocean Topography Science Team Meeting - Hobart, Australia – March 2007 18 Centering: Z bias summary ■ITRF2000/IGS00 orbits exhibit relative drift of up to 2 mm/yr (between GSFC and JPL) ■CNES GDR-B Z-bias is between D+L and GPS ■ITRF2005 orbits are clearly biased and exhibit no drift relative to GDRB ■Few arcs processed with ITRF2005 constellation give results consistent with ITRF2005 D+L orbits ■Results for CNES D+L orbits are more uncertain due to the shorter time span

19. Ocean Topography Science Team Meeting - Hobart, Australia – March 2007 19 Lack of GPS measurements ■< 3000 measurements since cycle 176 ■D+L orbit is routinely produced since cycle 179 to account for the reduced GPS measurements ■Z bias of GDRB is likely to be shifted towards D+L orbits ■Along track bias shown between D+L and GDR-B (due to CNES GPS processing)  Discontinuous distribution of GPS measurements distorts the orbit along-track Cycle 186 22°<  ’<44°