1OSTST, Hobart 2007

Comparison of Topex and Jason-1 sea state bias models

S. Labroue, M. Ablain, J. Dorandeu, N. Tran, P. Gaspar and O.Z. Zanife

2OSTST, Hobart 2007

Introduction and Objective

Question 1 : What is the impact of the LSE retracking on TOPEX waveforms regarding SSB issue?

– Analysis of RGDR products for Topex (LSE retracking which seems to be more mature than MAP)– Results presented last year showed that LSE retracking changes Topex SSB for waves greater than 4 m

and that the new Topex SSB is closer to the Jason one.=> Confirmation of these results are presented

Question 2 : Can TOPEX leakages induce a sea state related signal and therefore pollute empirical SSB estimation from Topex data if these leakages are not well accounted for in the retracking procedure?

3OSTST, Hobart 2007

Topex SSB, MGDR Topex LSE 328-364

-30 cm 0 cm -30 cm 0 cm

SWH filtering (/20 cm), SSH filtering 3cmGIM ionospheric correctionGourrion/Vandemark Wind Speed + Sigma0 from MDR not consistent with Range and SWHMGDR+RGDR editing

More physical variations

Same behavior

Except for the minimum at 12

m/s

4OSTST, Hobart 2007

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

5OSTST, Hobart 2007

Jason SSB - Topex SSB

+1 cm-1 cm

-5 cm +5 cm

Mean SSB difference, Cycles 1-21

Mean value 3.5 mm

The SSB difference noticed for very strong sea states is not present on the geographic analysis.

6OSTST, Hobart 2007

Consistency between Jason and Topex JA(GDRB) - TP(RGDR_LSE), cycles 1-21

Orbit-Range, 2007 dataGSFC ITRF 2000 orbit for Jason

Orbit-Range, Venice 2006

Mean value 7.6 cm Mean value 7.8 cm

+1 cm-1 cm +1 cm-1 cm

Thanks to the good quality of orbits and with a consistent orbit applied to Topex and Jason, geographically correlated signals disappear and the remaining patterns can be more easily correlated with sea state or not…

Comparisons of range measurements alone show a small signal of 5 mm that can be related to sea state.

7OSTST, Hobart 2007

Consistency between Jason and Topex, JA(GDRB) - TP(RGDR_LSE), cycles 1-21

Mean value 7.4 cm Mean value 7.8 cm

+1 cm-1 cm +1 cm-1 cmOrbit-Range-SSB Orbit-Range

SWH

Very good global consistency.

The new SSB models slightly improve the

consistency.

SSB dif

+1 cm-1 cm

8OSTST, Hobart 2007

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

9OSTST, Hobart 2007

The 4 quadrants show the same differences between retracked Range and MGDR Range regarding SWH variations, except for SWH > 7 m.

=> LSE retracking LSE does not change the quadrant differences of the Topex range (except for a constant value that may explain the greater magnitude of the North/South signal observed on the crossover mean difference).

The 4 plots exhibit 2 cm on the range between 3 m et 6 m of SWH.

2 cm2 cm

2 cm 2 cm

LSE Range-MGDR Range = f(SWH)

North Desc

South Desc

South Asc

North Asc

10OSTST, Hobart 2007

South Asc SSB – South Desc SSB Jason

South Asc SSB – South Desc SSB TPB LSE

No quadrant effects on Jason SSB but there is a difference on Topex

We focus on South hemisphere since there are more (SWH,U) situations compared to North hemisphere

-5 cm +5 cm -5 cm +5 cm

11OSTST, Hobart 2007

South Asc SSB – South Desc SSB TPB LSE

South Asc SSB – South Desc SSB TPB MGDR

Ascending and descending south have differents Range/SWH correlations on Topex, whatever the processing applied (retracking or MGDR corrections)

-5 cm +5 cm-5 cm +5 cm

12OSTST, Hobart 2007

South Asc SSB – South Desc SSB TPA MGDR

South Asc SSB – South Desc SSB TPB MGDR

Topex side A seems more consistent than side B between quadrants of different range rate sign.

-5 cm +5 cm -5 cm +5 cm

13OSTST, Hobart 2007

South Desc South Asc

Topex side A and side B have different sea state behaviour globally and by quadrants.

=> Different leakages between side A and side B

=> Which one gives the most accurate SSB?

=> Need of Jason SSB to make comparisons

North Desc North Asc

-5 cm +5 cm -5 cm +5 cm

TPB SSB – TPA SSB

MGDR data

14OSTST, Hobart 2007

Jason SSB – TP LSE SSB South Asc

Topex is closer to Jason on the ascending South. This is in agreement with geographic analysis.

Jason SSB – TP LSE SSB South Desc

-5 cm +5 cm -5 cm +5 cm

15OSTST, Hobart 2007

Conclusions

• LSE retracking on Topex has changed the SSB for Topex=> Retracking performed on TOPEX side A should be done in order to confirm this result on side A.

• Jason SSB (MLE4) et Topex SSB (LSE) are in very good agreement. All the empirical SSB models on altimetric missions are now very close to each other (Topex, Jason, EnviSat and GFO).

• The MAP algorithm exhibits other sea state related signals which appear dubious since all the skewness coefficients meant to absorb leakages errors remain at zero values.

• Topex retracking makes Jason and Topex SSB more consistent, but it did not change the leakage effects.

• Leakage effects induce a constant range bias by quadrant + sea state errors (Cf side A and side B SSB differences + SSB differences between quadrants)=> It is difficult to estimate an empirical SSB model on Topex without including leakage errors in the SSB.

• Are the ground segment MGDR corrections that replace retracking in MGDR (Acceleration correction and SWH/Att correction) affected by leakage errors?

• Further studies on Topex waveforms are needed in order to improve the processing on the measurements affected by the leakages. It is worth working on this subject since Topex side B appears to be more impacted by the leakages than side A and we are now looking at millimeter residual signals.

16OSTST, Hobart 2007

Consistency between Jason and Topex JA(GDRB) - TP(RGDR), cycles 1-21

H-R-SSB Venise 2006

H-R-SSB 2007Mean value 7.4 cm Mean value 8.2 cm

+1 cm-1 cm+1 cm-1 cm

17OSTST, Hobart 2007

Topex - Ascending/Descending/North/South features

-2 cm +2 cm

Orbit – Range 2007 (GSFC orbits)

LSE Skewness

18OSTST, Hobart 2007

Topex - Ascending/Descending/North/South featuresOrbit – Range 2006

Orbit – Range 2007 (GSFC orbits) => Different signals related to the

sea state depending on the quadrant (on the sign of the range rate)

5 cm 9 cm 5 cm 9 cm

19OSTST, Hobart 2007

Jason SSB – Topex SSB, 2007 Jason SSB – Topex SSB, 2006

The SSB difference is more and more difficult to assess accurately since the residual differences betweenTopex and Jason SSB are very small and therefore more impacted by several changes in the estimation process (period, data editing, smoothing…).

-5 cm +5 cm -5 cm +5 cm

20OSTST, Hobart 2007

Jason 2007 – Jason 2006 Topex 2007 – Topex 2006

Estimation period

Smoothing

Smoothing+ Editing

Wind speed Change

Smoothing

-5 cm +5 cm -5 cm +5 cm