ssb – retrack splinter summary and 2007-2008 perspectives

SSB – Retrack Splinter Summary and SSB – Retrack Splinter Summary and 2007-2008 Perspectives2007-2008 Perspectives

OSTST Hobart, Tasmania

March 15, 2007

OSTST Hobart 2007 22007/03/15 jl+psc

Sea State Bias and Retracking Analysis SplinterSea 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


OutlineOutline

■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


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

Vandemark et al.


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)

2000 2001 2002

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

Vandemark et al.


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 usedMethod and data used

■ First method: spectral analysis of the SLA signalSLA(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. FaugereHigh Frequency Content


Cross comparison of Jason-1 and EnvisatCross 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 0.1-0.4Hz on the 20Hz spectra

Y. Faugere


First results on the cross comparison of Jason-1 and First results on the cross comparison of Jason-1 and Topex RGDR HF contentTopex RGDR HF content

TP MAPTP LSEJ1 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


Retracking Progress 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.


TOPEX Waveform Contamination EvidenceTOPEX Waveform Contamination Evidence

TOPEX Skewness Jason Skewness Cyc 19-21 (avg = 0.06)

Des

Asc

Rodriguez et al.

Hobart OSTST MeetingPierre THIBAUT – March 2007

Delta Range(TPX LSE – Jas GDR) versus (SWH and SIG0)

40 cm 50 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


Delta Range(TPX LSE - Jas GDR) versus (SWH and ATT2)

40 cm 50 cm

No remaining dependancies with SWH or ATT2 in the bulk of the data

Range_LSE-Range_GDR versus (SWH,ATT) Skew solved

Skewness solved


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 cm 50 cm 40 cm 50 cm

OSTST Hobart 2007 – Performance assessment TOPEX/Poseidon data- 15 -

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 cm 9 cm5 cm 9 cm

5 cm 9 cm 5 cm 9 cm

1 cm

5 mm

Delta(J-TP)The best we can do now S. Labroue


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 (328-364)

-30 cm 0 cm -30 cm 0 cm

S. Labroue


QuestionsQuestions

■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


Jason-1 ReprocessingJason-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)


TOPEX Reprocessing 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)


Other PointsOther 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 ?


Backup Material Backup Material


Method and data usedMethod 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- 22 -

Y. Faugere


(TP- J 1)=- 6.9 cm

(TP- J 1)=- 7.1 cm

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

+2 cm

+2 cmUse of new orbits (GRACE)Use of new orbits (GRACE)Use of new orbits (GRACE)

New SSB, range, orbitsOrbit : J1-CNES/TP-GSFC

New SSB,New rangesOrbit : J1-GSFC/TP-GSFC


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

MGDR GSFC orbit LSE GSFC orbit

-2 cm -2 cm+2 cm +2 cm

SLA differences J1/TP using TP LSE range


4 – 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


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 : -0.58 cm HS: 0.68 cm Mean HN : -0.89 cm HS: 1.07 cm

Mean HN : -0.93 cm HS: 1.37 cm

S. Labroue

ssb – retrack splinter summary and 2007-2008 perspectives

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