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Altimeter sea surface height bias calibration with in situ network

Altimeter sea surface height bias calibration with in situ network. G. Jan 1 , P. Bonnefond 2 , Y. Ménard 3 , O. Laurain 2 1 NOVELTIS, Toulouse, France 2 OCA, GEMINI, Grasse, France 3 CNES, Toulouse, France. Method. 1. 2. 3. 3 Mean sea surface catamaran GPS technique.

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Altimeter sea surface height bias calibration with in situ network

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  1. Altimeter sea surface height bias calibration with in situ network G. Jan1, P. Bonnefond2, Y. Ménard3, O. Laurain2 1 NOVELTIS, Toulouse, France 2 OCA, GEMINI, Grasse, France 3 CNES, Toulouse, France

  2. Method 1 2 3 3 Mean sea surface catamaran GPS technique From P. Bonnefond, OCA • Objective: Multi Satellites, passes and sites in situ calibration • With tide gauges and the local satellite pass over the calval site • AND with offshore passes using several mean sea level slope along satellite passes paths. • Several sites, geodetically referenced. Network: Senetosa (1998), Macinaggio (2004), Ibiza, Ajaccio (dec-2005), Capraia is schedulded, Wusi (Near the New Zealand, since 1999)

  3. Method • How the merged method (regional and then local) can improve the sea surface height bias estimation and reinforce it statistically? With 1 CalVal site : • n passes for 1 satellite calibration • several calibrations of N satellites (Jason-1, Topex - Poseidon, GFO, ENVISAT) • Tests on formation flight period Jason-1 & Topex-Poseidon (8 months) • GDR-b, GDR-a : impact of the new GDR on the sea surface height bias • Impact of the regional calval method, rms, number of cycles, passes used.

  4. Sea surface height bias for JASON-1 • Bias results on Jason-1 pass 085 : 96.7+-2.1 mm ; std=21.1 mm • GDR-b, GDR-a difference: variance ssh decreases • Ssh bias difference with Harvest pass 043 (B. Haines) =18 mm UNITS mm

  5. Sea surface height bias for JASON-1 • Calibration with offshore passes? UNITS mm • TO calibration several satellites with one site with the background idea that is continuity in altimetric time series

  6. 2002 2003 2004 2001 2005 From OSTST 2006: form regional CalVal method Absolute SSH bias (SSH) continuity between altimetry missions Envisat B= 45.4cm , rms=4cm (merged method) B= 43.3cm, rms=7.3cm Ajaccio cycles 12 to 31 + -0.4 cm (rms=1.9cm) 7.2 cm (rms=4.6cm) 45.4 cm (rms=4cm) 9.5 cm (rms=1.5cm)

  7. Impact of a gap ino altimetric data between 2 missions • Discontinuity in altimeters time series sets the problem of : • the precise knowledge of the bias drift • its extrapolation • the restart with one satellite • Interest from In Situ calval => maintain exactitude of instruments (geodetically referenced) to insure a fix stable reference level. • It means: homogeneous products in referenced to the same reference. • Precision of the match = F (knowledge of the instruments drifts , time period without SSH bias estimation) • Time period of the gap impacts the error on estimation. • If there is a 1 year gap and no drift: • Rms will increase compared to a complete configuration, stable bias.

  8. Impact de trous dans la série Altimétrie • But, in case of measurements drift: • In situ CalVal in situ to extrapolate data between 2 time series with a gap between. • Interest of inter comparison between altimetry missions • We could think to a system with: studies on satellites systems drifts / knowledge of these drifts (if existing) / put in the in situ absolute ssh bias. P. Bonnefond et al. P. Bonnefond et al. • In situ calval technique can be • An advantage to restart in a one operational satellite configuration • One major problem comes from errors specific to each site. Development of other sites could be useful.

  9. Suggestion of a common framework for in situ CalVal and data consistencyG.Jan (1), Y. Ménard (2), P. Bonnefond (3), O. Laurain (3), L. Roblou (1) (1) NOVELTIS (2) CNES (3) OCA Jas-1 ascending passes n°9, 85,187 Ja-1descending passes n° 44, 70, 222 Ja-1 passes N° 19, 109 Ja-1 passes n°43, 206 Source figure: Xscan software (F. Lyard et al.) : global bathymetry from POC-LEGOS database • Proposal to compare and synthesize sea surface height (ssh) bias results from in situ CalVal methods. • To analyse the results from CalVal sites on a computing basis as comparable as possible • Nothing new. But, the exercise could give a common framework for ssh bias computing with conditions as homogeneous as possible. This exchange plan would be made with the participantCalVal teams. It would be proposed to be applied before OSTST future meetings. Jas-1 ascending passes n°47, 149, 225 Ja-1 descending passes n° 12,88,164

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