OSTST Hobart 2007 – Performance assessment Jason-1 data M.Ablain, S.Philipps, J.Dorandeu, - CLS N.Picot - CNES Jason-1 GDR data Performance assessment

OSTST Hobart 2007 – Performance assessment Jason-1 data 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

OSTST Hobart 2007 – Performance assessment Jason-1 data 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 from cycle 136 onwards [cm] -3 cm Mean SSH crossover differences [cm] -3 cm Jason-1 TOPEX

OSTST Hobart 2007 – Performance assessment Jason-1 data 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] Jason-1 TOPEX Jason-1 TOPEX GDR ‘B’GDR ‘A’

OSTST Hobart 2007 – Performance assessment Jason-1 data 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 –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 SSH crossovers [cm] GDR ‘A’ - Jason-1 GDR ‘B’ – Jason-1 STD of SSH crossovers [cm] GDR ‘A’ - Jason-1 GDR ‘B’ – Jason-1

OSTST Hobart 2007 – Performance assessment Jason-1 data 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 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. Mean SSH crossovers using GDR A orbits [cm] Mean SSH crossovers using GDR B orbits [cm] -3 cm +3 cm -3 cm +3 cm -3 cm

OSTST Hobart 2007 – Performance assessment Jason-1 data JMR correction 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. Daily differences between JMR and ECMWF model and in comparison to other missions Jump, Cycle 69 Drift, cycles 27 to 32

OSTST Hobart 2007 – Performance assessment Jason-1 data – JMR Correction Slope (JMR – Model) is 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. -4 mm/year+4 mm/year Global and local slopes of differences between JMR and ECMWF model

OSTST Hobart 2007 – Performance assessment Jason-1 data – 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 +/ (LSR) - IB correction applied - Seasonal signals removed - GIA not applied T/P J1+T/P J1

OSTST Hobart 2007 – Performance assessment Jason-1 data Conclusion Good performances of Jason-1 GDR data : 5.1 cm RMS at crossover, and stable Data performances better with GDR ‘B’: SSH variance at crossovers using GDR “A” [cm²] SSH variance at crossovers using GDR “B” [cm²] 0 cm²100 cm² 0 cm² 100 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) Variance gain = 21 cm²  35 %