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SSB – Retrack Splinter Summary and Perspectives OSTST Hobart, Tasmania March 15, 2007

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OSTST Hobart /03/15 jl+psc 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

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OSTST Hobart /03/15 jl+psc 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

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OSTST Hobart /03/15 jl+psc APPROACH Driving Assumption – information on wave steepness from global wave model can be integrated with altimeter H s and U 10 to improve routine sea state range corrections TRACK 1Nonparametric global SSB solutions using 2 input variables – SLA averaging method Inputs are [ H s, family of alternatives ] Tran et al., 2006 JGR - methods and 1 st results TRACK 2Three step clustering approach Partition measurements using fuzzy clustering Develop multi-class SSB solutions Combine to give single global result Vandemark et al.

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OSTST Hobart /03/15 jl+psc Figure 7. Hard partition (max membership) class-specific direct SSB maps on U10 and Hs domain for 2000, 2001, and from TOPEX+ WW3-ecmwf (NASA-GSFC Pathfinder datasets: 1/10 of the total points) (200 samples in a cell) Vandemark et al.

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OSTST Hobart /03/15 jl+psc Jason 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 Vandemark et al.

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OSTST Hobart /03/15 jl+psc Signal hidden by noise Noise hidden by oceanic signal Noise Spectrum of oceanic signal s Plateau A plateau on a power spectrum can be the signature of a white noise. 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 1Hz spectra are computed from 10 days of data 20Hz spectra are computed from 2 days of data Y. Faugere High Frequency Content

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OSTST Hobart /03/15 jl+psc Cross comparison of Jason-1 and Envisat Impact of SWH selection on HF content 1Hz Data 20Hz Data Legend: High SWH EN High SWH J1 Small SWH EN Small SWH J1 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 Hz on the 20Hz spectra Y. Faugere

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OSTST Hobart /03/15 jl+psc First results on the cross comparison of Jason-1 and Topex RGDR HF content TP MAP TP LSE J1 MLE4 Variance difference of HF content: J1 MLE4 (cycle 20) - TP LSE (Cycle 360) -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 Y. Faugere

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OSTST Hobart /03/15 jl+psc 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 Rodriguez et al.

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OSTST Hobart /03/15 jl+psc TOPEX Waveform Contamination Evidence TOPEX SkewnessJason Skewness Cyc (avg = 0.06) Des Asc Rodriguez et al.

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Hobart OSTST Meeting Pierre THIBAUT – March 2007 Page 12 Delta Range(TPX LSE – Jas GDR) versus (SWH and SIG0) 40 cm50 cm No remaining dependancies with SWH or SIG0 in the bulk of the data Skewness solved Range_LSE-Range_GDR versus (SWH,SIG0) Skew solved

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Hobart OSTST Meeting Pierre THIBAUT – March 2007 Page 13 Delta Range(TPX LSE - Jas GDR) versus (SWH and ATT 2 ) 40 cm50 cm No remaining dependancies with SWH or ATT 2 in the bulk of the data Range_LSE-Range_GDR versus (SWH,ATT) Skew solved Skewness solved

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Hobart OSTST Meeting Pierre THIBAUT – March 2007 Page 14 Delta Range(TPX LSE – Jas GDR) versus (SWH and SIG0 and ATT2) Dependances appear when the skewness is fixed but 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 cm50 cm 40 cm50 cm

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OSTST Hobart 2007 – Performance assessment TOPEX/Poseidon data Orbit – Range (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 Orbit – Range - SSB (GSFC orbits) 5 cm9 cm 5 cm9 cm 5 cm9 cm 5 cm9 cm 1 cm 5 mm Delta(J-TP) The best we can do now S. Labroue

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OSTST Hobart 2007 – Performance assessment TOPEX/Poseidon data Jason SSB (95-131) Both SSB are estimated on a full year of data. Cycles 1-21 are not enough to assess accurately the sea state variations. Topex LSE ( ) -30 cm 0 cm -30 cm 0 cm S. Labroue

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OSTST Hobart /03/15 jl+psc 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

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OSTST Hobart /03/15 jl+psc 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)

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OSTST Hobart /03/15 jl+psc 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: 1.Add a field with an empirical correction (a+b.SWH) by quadrant, the SSB field being the TOPEX global SSB 2.Add a field with an empirical correction (a+bSWH) by quadrant, the SSB field being the Jason-1 latest SSB model 3.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)

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OSTST Hobart /03/15 jl+psc 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 is there an impact? Very difficult to solve; inversion tends to be unstable ■Poseidon 1 ?

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OSTST Hobart /03/15 jl+psc Backup Material

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OSTST Hobart /03/15 jl+psc Method and data used ■Second method: Filtering technique HF(SLA) High-pass filter (20km cut-off) SLA=Orbit-Range-MSS σ [HF(SLA)] in 2°x2° boxes Standard deviation 2007 OSTST meeting Y. Faugere

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OSTST Hobart 2007 – Performance assessment TOPEX/Poseidon data 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 Use of new orbits (GRACE) New SSB, range, orbits Orbit : J1-CNES/TP-GSFC New SSB,New ranges Orbit : J1-GSFC/TP-GSFC

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OSTST Hobart 2007 – Performance assessment TOPEX/Poseidon data – 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 SLA without geophysical corrections LSE range makes SLA of T/P more consistent with J1. SLA differences J1/TP using TP MGDR range MGDR GSFC orbitLSE GSFC orbit -2 cm +2 cm SLA differences J1/TP using TP LSE range

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OSTST Hobart 2007 – Performance assessment TOPEX/Poseidon data – SLA Consistency J1/TP (MAP) SLA differences J1/TP using TP MGDR range [cm] -2 cm +2 cm SLA differences J1/TP using TP MAP range [cm] -2 cm +2 cm MGDR GSFC orbit MAP GSFC orbit Using TP MAP range does not significantly decrease SLA differences between Jason-1 and T/P S. Labroue

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OSTST Hobart 2007 – Performance assessment TOPEX/Poseidon data Mean of Topex SSH differences at Xovers: Range impact 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 crossovers SSH using LSE & GSFC orbit [cm] -3 cm +3 cm Mean crossovers SSH using MAP & GSFC orbit [cm] -3 cm +3 cm LSE & GSFC orbit MAP & GSFC orbit Mean crossovers SSH using MGDR GSFC orbits [cm] -3 cm +3 cm MGDR GSFC orbit Mean HN : cm HS: 0.68 cm Mean HN : cm HS: 1.07 cm Mean HN : cm HS: 1.37 cm S. Labroue

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