Jason-1 GDR data Performance assessment

1. Jason-1 GDR dataPerformance assessment M.Ablain, S.Philipps, J.Dorandeu, - CLS N.Picot - CNES

2. Introduction • CalVal activities – under CNES contract (SALP) • Work routinely performed at CLS • Jason-1 data have been analyzed and monitored since the beginning of the mission • Monitoring of all altimeter and radiometer parameters • SSH performances at crossovers and along-track SLA • Specific studies : Impact of new retracking algorithms, new orbits, new geophysical corrections, … • Objectives • 1 – SSH Performances since the beginning of the mission • 2 – GDR version ‘B’ reprocessing assessment • 3 – Impact of new JMR correction • 4 – Mean sea level

3. Jason-1 TOPEX 1 - SSH performances • Data used : GDR ‘A’ from cycle 1 to 135 and in version ‘B’ from cycle 136 onwards. • The cyclic mean of SSH crossover differences is stable and similar with T/P one • The map of mean differences shows weak signal (< 2 cm) only with GDR ‘B’ => Small hemispheric bias (1 cm) is detected thanks to the very good quality of new orbits Mean SSH crossover differences [cm] -3 cm -3 cm Mean SSH crossover differencesfrom cycle 136 onwards [cm]

4. Jason-1 TOPEX Jason-1 TOPEX GDR ‘A’ GDR ‘B’ 1 - SSH performances • Cyclic standard deviation of SSH differences is very good : • 5.2 cm RMS at crossovers for GDR‘B’ • Standard deviation decreases from cycle 136 onwards : GDR ‘A’ => GDR ‘B’ • Similar results for the SLA along track : • 10.3 cm RMS along-track • TOPEX performances slightly lower than for Jason-1: • TOPEX SSH high frequency content is lower than Jason-1 due to different ground processing • After applying similar retracking algorithms (MLE4 & LSE), both missions have the same high frequency content • see dedicated Faugere’s talk STD of SSH crossovers [cm] STD of along-track SLA [cm]

5. GDR ‘A’ - Jason-1 GDR ‘B’ – Jason-1 GDR ‘A’ - Jason-1 GDR ‘B’ – Jason-1 2 - GDR reprocessing in version ‘B’ • Reprocessing of the GDR in version B : • JPL (cycles 1 to 21) and CNES (cycles 128 to 135), in early 2006. • Reprocessing of the GDR in version B by JPL is on going from cycle 22 to 127 • Half of the cycles have already been reprocessed (just before OSTST). • The main evolutions in the GDR ‘B’ : • new retracking : order 2 MLE-4, • new precise orbit based on a GRACE gravity model • new geophysical corrections (tidal models, DAC, Sea State Bias). • Cyclic mean crossovers slightly more stable • Standard deviation improved from 6.1 cm to 5.2 cm with GDR ‘B’ data. mean SSHcrossovers [cm] STD of SSHcrossovers [cm]

6. 2 - GDR reprocessing in version ‘B’ • SSH differences at crossovers are very homogenous with GDR ‘B’ : • Large structures of high positive or negative differences are observed using GDR ‘A’. • These patches are removed using GDR ‘B’’ • New orbits using EIGEN-CG03C gravity field explain this improvement. • Thanks to this new orbit quality, small signals can now be detected such as an hemispheric bias : +/- 1 cm : • This signal corresponds to a pseudo time-tag bias of about 0.3 ms • A time shift of 0.173 ms has been added in the GDR ‘B’ L1-B processing (seems wrong sign), but the sign of this correction has been assessed • The origin of this weak signal remains unknown. -3 cm -3 cm +3 cm Mean SSH crossovers using GDR A orbits [cm] -3 cm +3 cm Mean SSH crossovers using GDR B orbits [cm]

7. 3 - JMR correction Drift, cycles 27 to 32 • GDR ‘A’ JMR corrections exhibits several anomalies : • a jump of 9 mm at cycle 69, • a drift of 5 mm between cycles 27 to 32, • 60 day signals of almost 5 mm amplitude due to yaw mode transitions. • New JMR correction provided by PODAAC (in GDR ‘B’) : • The jump at cycle 69 is very well corrected, • The drift is not completely removed • yaw mode transitions are reduced but still visible • A stronger signal is detected at the end (6mm) • Daily wet tropophere mean values are noisier for Jason-1 than for the other missions => This apparent noise is linked to signals generated by yaw mode transitions. Jump, Cycle 69 Daily differences between JMR and ECMWF model and in comparison to other missions

8. 4 – JMR Correction • Slope (JMR – Model) is -0.26 mm/year and local slopes can reach 5 mm/year. • Until now, the ECWMF model was used to calculate the Jason-1 MSL (for stability) : • Use of JMR leads to an increase of 0.26 mm/year of the MSL • Considering that the JMR correction is not completely well tuned, the use of the ECMWF model is probably preferable for MSL estimations (model stability has to be monitored) • Finally this analysis shows that the wet troposphere correction is a major item in the error budget of the Jason-1 MSL. Global and local slopes of differences between JMR and ECMWF model -4 mm/year +4 mm/year

9. 4 – Mean Sea Level • T/P and Jason-1 MSL have to be fitted together since T/P mission is finished, applying : • homogeneous corrections • Jason-1 / T/P SSH bias • MSL slope of 2.9 mm/year • Some discrepancies exist between the two missions: • TMR correction is used for T/P instead of the ECMWF model for Jason-1, • T/P and Jason-1 orbits are not homogeneous over the whole period. • Other corrections could impact the MSL as for example the ECMWF pressure fields in the T/P data. • Studies are shared between GOHS/LEGOS and CLS to estimate a more realistic error budget MSL slope (TP+J1) = 2.9 mm/year +/- 0.03 (LSR) J1+T/P J1 T/P • IB correction applied • Seasonal signals removed • - GIA not applied 1995 1997 2001 2003 1993 1999 2005

10. Good performances of Jason-1 GDR data : 5.1 cm RMS at crossover, and stable Data performances better with GDR ‘B’: Variance gain = 21 cm²  35 % Conclusion 0 cm² 100 cm² SSH variance at crossovers using GDR “A” [cm²] • Some improvements will be provided in next GDR release : • New SSB (Labroue, Venice 2007) • JMR corrections • New Orbits • New geophysical corrections :DAC (MOG2D HR) 0 cm² 100 cm² SSH variance at crossovers using GDR “B” [cm²]