SSB – Rtk splinter sessionSSB – Rtk splinter session2007-2008 perspectives2007-2008 perspectives

OSTST Hobart 2007 2

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

OSTST Hobart 2007 3

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 an agreement today on the reprocessing specifications for both TOPEX and Jason-1, so we can go ahead with the full reprocessing this year.

OSTST Hobart 2007 4

Jason-1 reprocessingJason-1 reprocessing

■MLE4 retracking ready;

■SSB ready ( or a new version shal be computed if orbit changes)

■C-band, ionosphere: ready (additional validation should be performed)

■Orbit, JMR -> see other splinters: will be ready this year

■ note that LSE applied on Jason does not differ sensibly from MLE4

■ Jason MLE4 and Topex LSE are now very consistent: no apparent wave dependency, similar SSB models.

OSTST Hobart 2007 5

Reprocessing perspectives for TOPEXReprocessing perspectives for TOPEX

■ What is ready: LSE retracking algorithm (range, SWH, Sigma0, att, skewness or not)

Note: the skewness does not eliminate all quadrant features.

PTR characterization

■ What would need some additional work SSB model (requires retracked data, orbit, corrections) 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

Alt-A SSB from agreement of 1-3 year average MAP algorithm ? C-band, ionospheric correction has to be validated

OSTST Hobart 2007 6

Other pointsOther points

■Need to investigate the leakages characteristics, as they now become the main source of error in TP/Jason consistency (~1-2cm)

■MAP algorithm

■Skewness set to 0 in LSE for SWH<1m => is there an impact?

■Sigma 0 LSE has to be in the future reprocessed TP products; more generally, are there requirements/suggestions on the content of those products

■Poseidon 1 ?

OSTST Hobart 2007 – Performance assessment TOPEX/Poseidon data- 7 -

(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

OSTST Hobart 2007 – Performance assessment TOPEX/Poseidon data- 8 -

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

OSTST Hobart 2007 – Performance assessment TOPEX/Poseidon data- 9 -

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

OSTST Hobart 2007 – Performance assessment TOPEX/Poseidon data- 10 -

Mean of Topex SSH differences at Xovers: Range impact

• Hemispheric bias increases using LSE and MAP retracking– 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 orbitMean 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

OSTST Hobart 2007 – Performance assessment TOPEX/Poseidon data- 11 -

• 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)

Status