SSB – Retrack Splinter Summary and 2007-2008 Perspectives

1. SSB – Retrack Splinter Summary and 2007-2008 Perspectives OSTST Hobart, Tasmania March 15, 2007

2. Sea State Bias and Retracking Analysis Splinter • Talks: • 0930 D. Vandemark, H. Feng, N. Tran, B. Chapron, B. Beckley Inclusion Of Wave Modeling In Sea State Bias Correction Refinement • 0950 E. Rodriguez, P. Callahan, T. Lungu Cross Calibration Of TOPEX And Jason Using MAP And LSE Retracking To Improve Global Sea Level • 1010 P. Thibaut, S. Labroue, N. Granie Evaluation Of Ground Retracking Algorithms On Jason Data • 1030 BREAK • 1100 Y. Faugere, A. Olivier, P. Thibaut, G. Dibarboure, N. Picot, J. Lambin Analysis Of The High Frequency Content Of Jason-1, Topex And Envisat Data • 1120 S. Labroue, M. Ablain, J. Dorandeu, N. Tran, P. Gaspar, O.Z. Zanife Comparison Of Topex And Jason-1 Sea State Bias Models • 1140 Discussion • Posters: • SSB-P1. TOPEX Retracked GDR – Features and Statistics, Philip S. Callahan, Ernesto Rodriguez, Ted Lungu • SSB-P2. A New Altimeter Waveform Retracking Algorithm Based On Neural Networks, Arnaud Quesney, Eric Jensou, Juliette Lambin, Nicolas Picot • SSB-P3. Unsupervised Classification Of Altimetric Waveform Over All Surface Type, Arnaud Quesney, Eric Jeansou, Christian Ruiz, Nathalie Steunou, Bruno Cugny, Nicolas Picot, Jean-Claude Souyris, Sylvie Thiria, Mustapha Lebbah • SSB-P4. Sigma0 Blooms In The Envisat Radar Altimeter Data, Pierre Thibaut, F. Ferreira, Pierre Femenias • SSB-P5. Simulator Of Interferometric Radar Altimeters: Concept And First Results, Pierre Thibaut, Olivier Germain, Fabrice Collard, Bruno Picard, Laurent Phalippou, Christopher Buck OSTST Hobart 2007

3. Outline • Brief review of presentations • Review of discussion on JPL, CNES retracking and SSB • We want to come to agreement today on the reprocessing approach for both TOPEX and Jason-1, so we can go ahead with the full reprocessing this year: • Would like endorsement of proposed approach OSTST Hobart 2007

4. Vandemark et al. APPROACH • Driving Assumption – information on wave steepness from global wave model can be integrated with altimeter Hs and U10 to improve routine sea state range corrections • TRACK 1 Nonparametric global SSB solutions using 2 input variables – SLA averaging method • Inputs are [ Hs, family of alternatives ] • Tran et al., 2006 JGR - methods and 1st results • TRACK 2 Three step clustering approach • Partition measurements using fuzzy clustering • Develop multi-class SSB solutions • Combine to give single global result OSTST Hobart 2007

5. 2000 2001 2002 Figure 7. Hard partition (max membership) class-specific direct SSB maps on U10 and Hs domain for 2000, 2001, and 2002. from TOPEX+ WW3-ecmwf (NASA-GSFC Pathfinder datasets: 1/10 of the total points) (200 samples in a cell) Vandemark et al. OSTST Hobart 2007

6. Vandemark et al. Jason-1 2004 results using clustering-based NP SSB solution (6 classes) • Systematic improvements at all latitudes and most regions in the spatial benchmark at right • Not optimized yet so results will improve OSTST Hobart 2007

7. Spectrum of oceanic signal s Noise hidden by oceanic signal Noise  Signal hidden by noise Y. Faugere High Frequency Content Method and data used • First method: spectral analysis of the SLA signal SLA(t) = s(t) + (t) , where s(t) is the geophysical signal and (t) is the noise A plateau on a power spectrum can be the signature of a white noise. Plateau  • 1Hz spectra are computed from 10 days of data • 20Hz spectraare computed from 2 days of data OSTST Hobart 2007

8. Y. Faugere 20Hz Data Cross comparison of Jason-1 and Envisat Impact of SWH selection on HF content • Legend: • High SWH EN • High SWH J1 • Small SWH EN • Small SWH J1 • 1Hz Data • 1Hz Envisat and Jason-1 are superimposed in both cases. The noise level increases with the wave height. Moreover a sort of pseudo-plateau is visible on 1Hz spectra at 1Hz only for high waves • At 20hz Envisat and Jason-1 spectra are closer for small waves (plateau almost superimposed). • The pseudo-plateau visible at 1Hz on high waves is not the signature of a instrumental white noise. It is the signature of the energy between 0.1-0.4Hz on the 20Hz spectra OSTST Hobart 2007

9. Y. Faugere First results on the cross comparison of Jason-1 and Topex RGDR HF content Variance difference of HF content: J1 MLE4 (cycle 20) - TP LSE (Cycle 360) TP MAP TP LSE J1 MLE4 -1cm² 1cm² • TP LSE and J1 MLE4 1Hz spectra are very consistent • The Geographical distribution of the difference of HF content is not as homogeneous as for Jason-1/Envisat OSTST Hobart 2007

10. Rodriguez et al. Retracking Progress • Retracked 2 yr TOPEX Alt-B and produced RGDRs with improved orbits • LSE skewness absorbs WF leakages so much reduced N/S Asc/Des (“Quadrant”) difference, but still some • MAP skewness much smaller so large variations with SWH • Need to assess waveform residuals to correction for leakages, OR rely on empirical correction • Worked issues with CNES on differences of MLE4, LSE, MAP • Processed large set of simulated data, numerous PTRs • Found no anomalies in Jason waveform residuals • However, MLE4 only agrees with LSE when solve for skewness, not fixed skewness. MAP has SWH dependence • Similar results found from simulated WF OSTST Hobart 2007

11. Rodriguez et al. TOPEX Waveform Contamination Evidence TOPEX Skewness Jason Skewness Cyc 19-21 (avg = 0.06) Asc Des OSTST Hobart 2007

12. Delta Range(TPX LSE – Jas GDR) versus (SWH and SIG0)  No remaining dependancies with SWH or SIG0 in the bulk of the data  Skewness solved Range_LSE-Range_GDR versus (SWH,SIG0) Skew solved 40 cm 50 cm Page 12

13. Delta Range(TPX LSE - Jas GDR) versus (SWH and ATT2)  No remaining dependancies with SWH or ATT2 in the bulk of the data  Skewness solved Range_LSE-Range_GDR versus (SWH,ATT) Skew solved 40 cm 50 cm Page 13

14. Delta Range(TPX LSE – Jas GDR) versus (SWH and SIG0 and ATT2)  Dependances appear when the skewness is fixedbut it was fixed to 0 (in GDR 0.1) Range_LSE-Range_GDR versus (SWH,ATT) Skew fixed Range_LSE-Range_GDR versus (SWH,SIG0) Skew fixed 40 cm 50 cm 40 cm 50 cm Page 14

15. S. Labroue The best we can do now Orbit – Range (GSFC orbits) Delta(J-TP) 1 cm 5 cm 9 cm 5 cm 9 cm 5 mm 5 cm 9 cm 5 cm 9 cm Orbit – Range - SSB (GSFC orbits) Good global results but some sea state related signals are still there when comparing the quadrants. A SSB estimated globally on Topex cannot remove all the residual sea state dependences

16. S. Labroue Jason SSB (95-131) Topex LSE (328-364) -30 cm -30 cm 0 cm 0 cm Both SSB are estimated on a full year of data. Cycles 1-21 are not enough to assess accurately the sea state variations.

17. Questions • Do retracking approaches show reduction in SSB? • What is the approach to aligning TOPEX and Jason data? • What error model should be used with the corrected data? • We want to come to agreement today on the reprocessing approach for both TOPEX and Jason-1, so we can go ahead with the full reprocessing this year: • Would like endorsement of proposed approach OSTST Hobart 2007

18. Jason-1 Reprocessing • MLE4 retracking ready • LSE applied on Jason does not differ sensibly from MLE4 • Jason MLE4 and Topex LSE are now very consistent: no apparent SWH dependence, similar SSB models • SSB processing ready, so a new version will be computed as soon as other pieces, e.g., final CNES orbit, is available • C-band, ionosphere: ready. Additional validation may be performed • Orbit, JMR: will be ready this year (-> see other splinters) OSTST Hobart 2007

19. TOPEX Reprocessing • Propose to use LSE retracking algorithm including skewness • Note: skewness does not eliminate all quadrant features. • PTR fitting program (mainly for Alt-A) exists but needs updating • Needs some additional work • SSB model (requires retracked data, orbit, corrections ) – CNES will fit on RGDR; final RGDR updated with SSB • C-band, ionospheric correction has to be validated • Correction for quadrant effects, 3 options: • Add a field with an empirical correction (a+b.SWH) by quadrant, the SSB field being the TOPEX global SSB • Add a field with an empirical correction (a+bSWH) by quadrant, the SSB field being the Jason-1 latest SSB model • Split the SSB model into quadrant • Splinter had some preference for #1. Endorsement ? • Alt-A SSB from agreement of 1-3 year average • JPL plans to complete reprocessing within approx 1 yr (may need some extension into next OSTST) OSTST Hobart 2007

20. Other Points • Should investigate the leakages characteristics, as they now become the main source of error in TP/Jason consistency (~1-2cm) • But, proposed approach will correct empirically with quadrant SSB • MAP algorithm appears not to provide good results on either Jason or TOPEX: small skewness, SWH dependence of height • Skewness set to 0 in LSE for SWH<1m => is there an impact? • Very difficult to solve; inversion tends to be unstable • Poseidon 1 ? OSTST Hobart 2007

21. Backup Material OSTST Hobart 2007

22. Y. Faugere SLA=Orbit-Range-MSS High-pass filter (20km cut-off) HF(SLA) Standard deviation σ[HF(SLA)] in 2°x2° boxes Method and data used • Second method: Filtering technique 2007 OSTST meeting - 22 - OSTST Hobart 2007

23. Use of new orbits (GRACE) Use of new orbits (GRACE) Use of new orbits (GRACE) New SSB, range, orbits Orbit : J1-CNES/TP-GSFC 4 - Impact of GSFC orbit • New orbits are provided by CNES for Jason-1(GDR ‘B’) and GSFC for TOPEX (RGDR). • Using GSFC orbits similar for Jason-1 and TOPEX, allows us to remove the East/West signal • Even if orbits are best and more homogenous between TOPEX and Jason-1, weak systematic discrepancies remain (< 1cm). -2 cm +2 cm New SSB,New ranges Orbit : J1-GSFC/TP-GSFC +2 cm -2 cm

24. 4 – SLA Consistency J1/TP (LSE) • Does new retracking methods make SLA of Jason-1 and T/P more consistent? • Using Jason-1 GDR ‘B’ cycles 1 - 21 • SLA without geophysical corrections  LSE range makes SLA of T/P more consistent with J1. SLA differences J1/TP using TP MGDR range SLA differences J1/TP using TP LSE range MGDR GSFC orbit LSE GSFC orbit -2 cm +2 cm -2 cm +2 cm

25. S. Labroue 4 – SLA Consistency J1/TP (MAP) • Using TP MAP range does not significantly decrease SLA differences between Jason-1 and T/P SLA differences J1/TP using TP MGDR range [cm] SLA differences J1/TP using TP MAP range [cm] MAP GSFC orbit MGDR GSFC orbit -2 cm -2 cm +2 cm +2 cm

26. -3 cm +3 cm Mean crossovers SSH using MGDR GSFC orbits [cm] S. Labroue Mean of Topex SSH differences at Xovers: Range impact MGDR GSFC orbit LSE & GSFC orbit • Using LSE retracking makes T/P ranges more consistent with Jason-1 ranges but residual sea state errors are still present. The errors are quadrant dependent and due to leakages in the TP waveforms. • Mean SSH differences at crossoverpoints show hemispheric bias, which increases when using retracked data instead of MGDRs (different impact of the leakages as a function of the sign of the range rate) • Since LSE retrieves 5 parameters instead of 4 (MGDR), noise on altimetric parameters is increased Mean HN : -0.89 cm HS: 1.07 cm Mean HN : -0.58 cm HS: 0.68 cm -3 cm +3 cm Mean crossovers SSH using LSE & GSFC orbit [cm] MAP & GSFC orbit Mean HN : -0.93 cm HS: 1.37 cm -3 cm +3 cm Mean crossovers SSH using MAP & GSFC orbit [cm]