product user manual for sea level sla products...

PRODUCT USER MANUAL For Sea Level SLA products

SEALEVEL_GLO_SLA_L3_OBSERVATIONS_008_001 SEALEVEL_MED_SLA_L3_OBSERVATIONS_008_002

SEALEVEL_BS_SLA_L3_OBSERVATIONS_008_003 SEALEVEL_EUR_SLA_L3_OBSERVATIONS_008_004 SEALEVEL_ARC_SLA_L3_OBSERVATIONS_008_005

WP leader: CLS, Gilles Larnicol, France Issue: 3.4

Contributors: Françoise Mertz, Rosmorduc Vinca, Anne Delamarche, Gilles Larnicol

MyOcean version scope : Version 2.2

Approval Date : 5 February 2013

Project N°: FP7-SPACE-2007-1

Work programme topic: SPA.2007.1.1.01 - development of upgraded capabilities for existing GMES fast-track services and related (pre)operational services. Duration: 39 Months

PUM for Sea Level SLA Products Ref : MYO2-SL-PUM-008-001-005

SEALEVEL_*_SLA_L3_OBSERVATIONS_008_00* Date : 5 February 2013Issue : 3.4

CHANGE RECORD

Issue Date § Description of change Author Validated by

1.0 2011/01/01 all First version of document Françoise Mertz,Vinca Rosmorduc,Anne Delamarche

Gilles Larnicol

1.1 2011/02/23 all Correction of anomalies inthe format of the file

Françoise Mertz,Vinca Rosmorduc,Anne Delamarche

Gilles Larnicol,Gérald Dibar-boure

2.0 2011/09/01 all Version V2 of MyOceanproducts



2.1 2011/10/31 all New template Françoise Mertz,Vinca Rosmorduc,Anne Delamarche


2.2 2012/02/24 all Version V2.1 Françoise Mertz,Vinca Rosmorduc,Anne Delamarche


3.0 2012/05/25 all End of Envisat mission andaddition of Jason-1 on itsgeodetic orbit (j1g)



3.1 2012/07/20 all New version D of Jason-2IGDR



3.2 2012/10/20 all Integration of Cryosat-2 inReprocessed products



3.3 2012/12/04 all NRT: Improvement in theprocessing of Cryosat-2, Re-processed: integration ofJason-1 geodetic mission andnew version D of Jason-2

Françoise Mertz,Vinca Rosmorduc,Caroline Maheu


3.4 2013/02/05 all Global NRT: Integration of"OGDR on the fly" data

Françoise Mertz,Vinca Rosmorduc,Caroline Maheu


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CONTENTS

I. Introduction 11

II. Updates in 2012 and 2013 13II.1. Integration of most recent data as soon as available: "OGDR on the fly" (February 2013) . . 13II.2. All the Sea Level SLA files are compressed (February 2013) . . . . . . . . . . . . . . . . . 13II.3. Version "D" of input IGDR/OGDR Jason-2 data (July 30, 2012 for NRT products , Decem-

ber 2012 for Reprocessed products) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13II.4. Integration of Jason-1 on its geodetic orbit (May 2012 for NRT products, December 2012

for Reprocessed products) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14II.5. Envisat has stopped sending data (April 2012) . . . . . . . . . . . . . . . . . . . . . . . . . 14II.6. Integration of the Cryosat2 Mission (February 2012 in NRT and in Reprocessed products) . 15

II.6.1. Generalities about the mission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15II.6.2. Integration in DUACS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

II.7. Addition of Europe and Arctic products (January 2012) in NRT processing . . . . . . . . . . 17

III.SSALTO/DUACS system 19III.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19III.2. Near Real Time processing steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

III.2.1. Input data, models and corrections applied . . . . . . . . . . . . . . . . . . . . . . . 21III.2.2. Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24III.2.3. Homogenization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25III.2.4. Input data quality control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25III.2.5. Multi-mission cross-calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26III.2.6. Product generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

III.2.6.1. Computation of raw SLA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27III.2.6.2. Cross validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29III.2.6.3. Filtering and sub-sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

III.2.7. Quality control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30III.2.7.1. Final quality Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30III.2.7.2. Performance indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

III.3. Delayed Time processing steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32III.3.1. Input data, models and corrections applied . . . . . . . . . . . . . . . . . . . . . . . 32III.3.2. Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35III.3.3. Homogenization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35III.3.4. Input data quality control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35III.3.5. Multi-mission cross-calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36III.3.6. Product generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

III.3.6.1. Computation of raw SLA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37III.3.6.2. Cross validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39III.3.6.3. Filtering and sub-sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

III.3.7. Quality control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40III.3.7.1. Final quality Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

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IV. MyOcean Products 41IV.1. Near Real Time Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

IV.1.1. Delay of the products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41IV.1.2. Temporal availibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

IV.2. Delayed Time Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41IV.2.1. Delay of the products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42IV.2.2. Temporal availibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

V. Description of the product specification 43V.1. General information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

VI. Nomenclature of files 48VI.1. Nomenclature of files downloaded through the MyOcean Web Portal DirectgetfileService . . 48

VI.1.1. Nomenclature of the product line . . . . . . . . . . . . . . . . . . . . . . . . . . . 48VI.1.2. Nomenclature of the datasets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49VI.1.3. Nomenclature of the NetCdf files . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

VII.Data format 51VII.1.NetCdf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51VII.2.Structure and semantic of NetCDF along-track files . . . . . . . . . . . . . . . . . . . . . . 52

VIII.How to download a product 56VIII.1.Download a product through the MyOcean Web Portal Directgetfile Service . . . . . . . . . 56

IX. News and Updates 57IX.1. [Duacs] Operational news . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57IX.2. Updates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

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LIST OF TABLES

1 Characteristics of the geodetic orbit for Jason-1 since May 7th, 2012 . . . . . . . . . . . . . 142 SSALTO/DUACS Near-Real Time Input data overview . . . . . . . . . . . . . . . . . . . . 213 Corrections and models applied in SSALTO/DUACS NRT products produced from IGDRs. . 224 Corrections and models applied in SSALTO/DUACS NRT products produced from OGDRs. 235 SSALTO/DUACS Delayed Time Input data overview . . . . . . . . . . . . . . . . . . . . . 326 Corrections and models applied in SSALTO/DUACS DT products for TOPEX/Poseidon,

Jason-1, Jason-2 and GFO. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337 Corrections and models applied in SSALTO/DUACS DT products for ERS-1, ERS-2, En-

visat and Cryosat-2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

LIST OF FIGURES

1 DUACS and AVISO, a user-driven altimetry service . . . . . . . . . . . . . . . . . . . . . . 112 SSH-MSS for Cryosat-2 with LRM mode only (left) or LRM+pseudo-LRM modes (right) . . 153 Coverage of the Europe product for Jason-1, Jason-2 and Envisat from August, 18th to

September, 9th 2011 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 Coverage of the Arctic region with Envisat from October 28th 2011 to January 3rd 2012 . . . 185 SSALTO/DUACS processing sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 Overview of the near real time system data flow management . . . . . . . . . . . . . . . . . 247 Merging pertinent information from IGDR and OGDR processing . . . . . . . . . . . . . . 258 Example with the key performance indicator on 2009/06/27 . . . . . . . . . . . . . . . . . . 31

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LIST OF ACRONYMS

ATP Along-Track ProductADT Absolute Dynamic TopographyAVISO Archiving, Validation and Interpretation of Satellite Oceanographic dataBGLO Biais Grande Longueur d’OndeCal/Val Calibration - ValidationCERSAT Centre ERS d’Archivage et de TraitementCMA Centre Multimission Altimetry centerCORSSH CORrected Sea Surface HeightC2 Cryosat-2DAC Dynamic Atmospheric CorrectionDT Delayed TimeDTU Mean Sea Surface computed by Technical University of DanemarkDUACS Data Unification and Altimeter Combination SystemE1 ERS-1E2 ERS-2EN EnvisatENN Envisat on its non repetitive orbit (since cycle 94)ECMWF European Centre for Medium-range Weather ForecastingENACT ENhanced ocean data Assimilation and Climate predictionG2 Geosat Follow OnGIM Global Ionospheric MapsGDR Geophysical Data Record(s)IERS International Earth Rotation ServiceIGDR Interim Geophysical Data Record(s)J1 Jason-1J1N Jason-1 on its new orbit (since cycle 262)J1G Jason-1 on its geodetic orbit (since May 2012)J2 Jason-2JPL Jet Propulsion LaboratoryLAS Live Access ServerLWE Long Wavelength ErrorsMADT Map of Absolute Dynamic TopgraphyMDT Mean Dynamic TopographyMOE Medium Orbit EphemerisMP Mean ProfileMSLA Map of Sea Level AnomalyMSS Mean Sea SurfaceNRT Near-Real TimeOE Orbit ErrorOER Orbit Error ReductionOpendap Open-source Project for a Network Data Access ProtocolPF Polynom FitPO.DAAC Physical Oceanography Distributed Active Archive CentrePOE Precise Orbit EphemerisRD Reference DocumentSAD Static Auxiliary Data

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SI Signed IntegerSLA Sea Level AnomalySL TAC Sea Level Thematic Assembly CentreSSALTO Ssalto multimission ground segmentSSH Sea Surface HeightTAC Thematic Assembly CentreT/P Topex/PoseidonTPN Topex/Poseidon on its new orbit (since cycle 369)

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REFERENCES

[1] Andersen, O. B., The DTU10 Gravity filed and Mean sea surface (2010), Second international sympo-sium of the gravity field of the Earth (IGFS2), Fairbanks, Alaska.

[2] Boy, F. et al (2011): "Cryosat LRM, TRK and SAR processing". Presented at the 2011 Ocean Sur-face Topography Science Team meeting. http://www.aviso.oceanobs.com/fileadmin/documents/OSTST/2011/oral/01_Wednesday

[3] Carrere, L., F. Lyard, 2003, Modeling the barotropic response of the global ocean to atmo-spheric wind and pressure forcing- comparisons with observations. J. Geophys. Res., 30(6), 1275,doi:10.1029/2002GL016473.

[4] Carrere L., 2003, Etude et modélisation de la réponse HF de l’océan global aux forçagesmétéorologiques. PhD thesis, Université Paul Sabatier (Toulouse III, France), 318 pp.

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[6] Cartwright, D. E., A. C. Edden, 1973, Corrected tables of tidal harmonics, Geophys. J. R. Astr. Soc.,33, 253-264.

[7] Following the scientific recommendations from the OSTST meeting (San Diego, October 2011), theESA Cryosat Project and the CNES SALP Project have been collaborating to generate these Cryosat-derived L3 and L4 products. Level 1B and Level 2 products derived from CNES processors are notdistributed by AVISO as per the CNES / ESA agreement.

[8] Dibarboure G., C. Renaudie, M.-I. Pujol, S. Labroue, N. Picot, 2011, "A demonstrationof the potential of Cryosat-2 to contribute to mesoscale observation", J. Adv. Space Res.,doi:10.1016/j.asr.2011.07.002. http://dx.doi.org/10.1016/j.asr.2011.07.002

[9] Dibarboure G., P. Schaeffer, P. Escudier, M-I.Pujol, J.F. Legeais, Y. Faugère, R. Morrow, J.K. Willis,J. Lambin, J.P. Berthias, N. Picot, 2010: Finding desirable orbit options for the "Extension of Life"phase of Jason-1. Submitted to Marine Geodesy.

[10] Dibarboure G., M-I.Pujol, F.Briol, PY.Le Traon, G.Larnicol, N.Picot, F.Mertz, M.Ablain, 2011: Jason-2 in DUACS: first tandem results and impact on processing and products. Marine Geodesy, 34,(3-4),214-241.

[11] Dibarboure G., 2009: Using short scale content of OGDR data improve the Near Real Time productsof Ssalto/Duacs, oral presentation at Seattle OSTST meeting (pdf).

[12] Dorandeu, J., M. Ablain, Y. Faugère, F. Mertz, B. Soussi, and P. Vincent, 2004: Jason-1 global statisti-cal evaluation and performance assessment. Calibration and cross-calibration results. Marine Geodesy,27,(3-4), 345-372

[13] Dorandeu, J., M. Ablain, P.-Y. Le Traon, 2003: Reducing Cross-Track Geoid Gradient Errors aroundTOPEX/Poseidon and Jason-1 Nominal Tracks: Application to Calculation of Sea Level Anomalies.J. of Atmosph. and Ocean. Techn.,20, 1826-1838.

[14] Dorandeu, J. and P.-Y. Le Traon, 1999: Effects of global mean atmospheric pressure variations onmean sea level changes from TOPEX/Poseidon. J. Atmos. Oceanic Technol., 16, 1279-1283.

[15] Ducet, N., P.-Y. Le Traon, and G. Reverdin, 2000: Global high resolution mapping of ocean circulationfrom TOPEX/Poseidon and ERS-1 and -2. J. Geophys. Res., 105, 19477-19498.

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[16] Egbert, Gary D., Svetlana Y. Erofeeva, 2002: Efficient Inverse Modeling of Barotropic Ocean Tides. J.Atmos. Oceanic Technol., 19, 183-204. doi: 10.1175/1520-0426(2002)019<0183:EIMOBO>2.0.CO;2

[17] Gaspar, P., and F. Ogor, Estimation and analysis of the Sea State Bias of the ERS-1 altimeter. Reportof task B1-B2 of IFREMER Contract n˚ 94/2.426 016/C., 1994.

[18] Gaspar, P., F. Ogor and C. Escoubes, 1996, Nouvelles calibration et analyse du biais d’état de mer desaltimètres TOPEX et POSEIDON. Technical note 96/018 of CNES Contract 95/1523, 1996.

[19] Gaspar, P., and F. Ogor, Estimation and analysis of the Sea State Bias of the new ERS-1 and ERS-2altimetric data (OPR version 6). Report of task 2 of IFREMER Contract n˚ 96/2.246 002/C, 1996.

[20] Gaspar, P., S. Labroue and F. Ogor. 2002, Improving nonparametric estimates of the sea state bias inradar altimeter measurements of seal level, J. Atmos. Oceanic Technology, 19, 1690-1707.

[21] Hernandez, F., P.-Y. Le Traon, and R. Morrow, 1995: Mapping mesoscale variability of the AzoresCurrent using TOPEX/POSEIDON and ERS-1 altimetry, together with hydrographic and Lagrangianmeasurements. Journal of Geophysical Research, 100, 24995-25006.

[22] Hernandez, F. and P. Schaeffer, 2000: Altimetric Mean Sea Surfaces and Gravity Anomaly maps inter-comparisons AVI-NT-011-5242-CLS, 48 pp. CLS Ramonville St Agne.

[23] Hernandez, F., M.-H. Calvez, J. Dorandeu, Y. Faugère, F. Mertz, and P. Schaeffer, 2000: Sur-face Moyenne Océanique: Support scientifique à la mission altimétrique Jason-1, et à une mis-sion micro-satellite altimétrique. Contrat SSALTO 2945 - Lot 2 - A.1. Rapport d’avancement.CLS/DOS/NT/00.313, 40 pp. CLS Ramonville St Agne.

[24] Iijima, B.A., I.L. Harris, C.M. Ho, U.J. Lindqwiste, A.J. Mannucci, X. Pi, M.J. Reyes, L.C. Sparks,B.D. Wilson, 1999: Automated daily process for global ionospheric total electron content maps andsatellite ocean altimeter ionospheric calibration based on Global Positioning System data, J. Atmos.Solar-Terrestrial Physics, 61, 16, 1205-1218

[25] Labroue S., A. Ollivier, M. Guibbaud, F. Boy, N. Picot, P. Féménias, "Quality as-sessment of Cryosat-2 altimetric system over ocean", 2012, OSTST in Venice, avail-able at http://www.aviso.oceanobs.com/fileadmin/documents/OSTST/2012/posters/Labroue_Ollivier_Guibbaud_Final.pdf

[26] Labroue S., F. Boy, N. Picot, M. Urvoy, M. Ablain, "First quality assessment of the Cryosat-2 altimetricsystem over ocean", J. Adv. Space Res., 2011, doi:10.1016/j.asr.2011.11.018. http://dx.doi.org/10.1016/j.asr.2011.11.018

[27] Labroue, S., 2007: RA2 ocean and MWR measurement long term monitoring, 2007 report for WP3,Task 2 - SSB estimation for RA2 altimeter. Contract 17293/03/I-OL. CLS-DOS-NT-07-198, 53pp.CLS Ramonville St. Agne

[28] Labroue, S., P. Gaspar, J. Dorandeu, O.Z. Zanifé, F. Mertz, P. Vincent, and D. Choquet, 2004: Nonparametric estimates of the sea state bias for Jason-1 radar altimeter. Marine Geodesy, 27, 453-481.

[29] Lagerloef, G.S.E., G.Mitchum, R.Lukas and P.Niiler, 1999: Tropical Pacific near-surface currentsestimated from altimeter, wind and drifter data, J. Geophys. Res., 104, 23,313-23,326

[30] Le Traon, P.-Y. and F. Hernandez, 1992: Mapping the oceanic mesoscale circulation: validation ofsatellite altimetry using surface drifters. J. Atmos. Oceanic Technol., 9, 687-698.

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[31] Le Traon, P.-Y., P. Gaspar, F. Bouyssel, and H. Makhmara, 1995: Using Topex/Poseidon data to en-hance ERS-1 data. J. Atmos. Oceanic Technol., 12, 161-170.

[32] Le Traon, P.-Y., F. Nadal, and N. Ducet, 1998: An improved mapping method of multisatellite altimeterdata. J. Atmos. Oceanic Technol., 15, 522-534.

[33] Le Traon, P.-Y. and F. Ogor, 1998: ERS-1/2 orbit improvement using TOPEX/POSEIDON: the 2 cmchallenge. J. Geophys. Res., 103, 8045-8057.

[34] Le Traon, P.-Y. and G. Dibarboure, 1999: Mesoscale mapping capabilities of multi-satellite altimetermissions. J. Atmos. Oceanic Technol., 16, 1208-1223.

[35] Le Traon, P.-Y., G. Dibarboure, and N. Ducet, 2001: Use of a High-Resolution Model to Analyze theMapping Capabilities of Multiple-Altimeter Missions. J. Atmos. Oceanic Technol., 18, 1277-1288.

[36] Le Traon, P.Y. and G. Dibarboure, 2002 Velocity mapping capabilities of present and future altimetermissions: the role of high frequency signals. J. Atmos. Oceanic Technol., 19, 2077-2088.

[37] Le Traon, P.Y., Faugère Y., Hernandez F., Dorandeu J., Mertz F. and M. Ablain, 2002: Can we mergeGEOSAT Follow-On with TOPEX/POSEIDON and ERS-2 for an improved description of the oceancirculation, J. Atmos. Oceanic Technol., 20, 889-895.

[38] Le Traon, P.Y. and G. Dibarboure, 2004: An Illustration of the Contribution of the TOPEX/Poseidon-Jason-1 Tandem Mission to Mesoscale Variability Studies. Marine Geodesy, 27 (1-2).

[39] Mertz F., F. Mercier, S. Labroue, N. Tran, J. Dorandeu, 2005: ERS-2 OPR data quality assessment ;Long-term monitoring - particular investigation. CLS.DOS.NT-06.001 (pdf)

[40] MSS_CNES_CLS11 was produced by CLS Space Oceanography Division and distributed by Aviso,with support from Cnes (urlhttp://www.aviso.oceanobs.com/)".

[41] Pascual, A., Y. Faugère, G. Larnicol, P-Y Le Traon, 2006: Improved description of the oceanmesoscale variability by combining four satellite altimeters. Geophys. Res. Lett., 33

[42] Pascual A., C. Boone, G. Larnicol and P-Y. Le Traon, 2009. On the quality of Real-Time altimetergridded fields: comparison with in-situ data. Journ. of Atm. and Ocean. Techn. Vol. 26(3) pp. 556-569,DOI: 10.1175/2008JTECHO556.1

[43] Prandi P., M. Ablain, A. Cazenave, N. Picot, 2011, A new estimation of mean sea level in the ArcticOcean from satellite altimetry. Submitted to Marine Geodesy.

[44] Pujol M-I. et al., 2009. Three-satellite quality level restored in NRT, poster at OSTST meeting (pdf)

[45] Ray, R., 1999: A Global Ocean Tide model from TOPEX/Poseidon Altimetry, GOT99.2. NASA Tech.Memo. NASA/TM-1999-209478, 58 pp. Goddard Space Flight Center, NASA Greenbelt, MD, USA.

[46] Rio, M.-H. and F. Hernandez, 2003: A Mean Dynamic Topography computed over the worldocean from altimetry, in-situ measurements and a geoid model. J. Geophys. Res., 109, C12032,doi:10.1029/2003JC002226.

[47] Rio, M.-H. and F. Hernandez, 2003: High frequency response of wind-driven currents measured bydrifting buoys and altimetry over the world ocean. J. Geophys. Res., 108, 39-1.

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[48] Rio, M.-H., 2003: Combinaison de données in situ, altimétriques et gravimétriques pour l’estimationd’une topographie dynamique moyenne globale. Ed. CLS. PhD Thesis, University Paul Sabatier(Toulouse III, France), 260pp.

[49] Scharroo, R. and P. Visser, 1998: Precise orbit determination and gravity field improvement for theERS satellites. J. Geophys. Res., 103, 8113-8127

[50] Scharroo, R., J. Lillibridge, and W.H.F. Smith, 2004: Cross-calibration and long-term monitoring ofthe Microwave Radiometers of ERS, Topex, GFO, Jason-1 and Envisat. Marine Geodesy, 97.

[51] Tran N. and E. Obligis, December 2003, "Validation of the use of ENVISAT neural algorithms onERS-2", CLS.DOS/NT/03.901.

[52] Tran, N., S. Labroue, S. Philipps, E. Bronner, and N. Picot, 2010 : Overview and Update of the SeaState Bias Corrections for the Jason-2, Jason-1 and TOPEX Missions. Marine Geodesy, accepted.

[53] Vincent, P., Desai S.D.,Dorandeu J., Ablain M., Soussi B., Callahan P.S. and B.J. Haines, 2003: Jason-1 Geophysical Performance Evaluation. Marine Geodesy, 26, 167-186.

[54] Wahr, J. W., 1985, Deformation of the Earth induced by polar motion,J. of Geophys. Res. (Solid Earth),90, 9363-9368.

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I. INTRODUCTION

MyOcean is a European Network project aiming at monitoring, analyzing and forecasting the Ocean. Ituses satellite and in-situ data to describe the ocean in 3 dimensions and real time. More information can befound on http://www.myocean.eu.org.The aim of this document is to describe the products delivered by the Sea Level TAC (Thematic AssemblyCentre) which is one of the five TAC of the MyOcean project.The data produced in the frame of this TAC are generated by the processing system named DUACS (DataUnification and Altimeter Combination System).DUACS is part of the CNES multi-mission ground segment (SSALTO). It processes data from all altimetermissions: Cryosat-2, OSTM/Jason-2, Jason-1, Topex/Poseidon, Envisat, GFO, ERS-1&2 and even Geosat.At this time (February 2013) DUACS is using three different altimeters in near real time .Developed and operated by CLS, it started as an European Commission Project (Developing Use Of Altime-try for Climate Studies), funded under the European Commission and the Midi-Pyrénées regional council.It has been integrated to the CNES multi-mission ground segment SSALTO in 2001, and it is maintained,upgraded and operated with funding from CNES with shared costs from EU projects.At the beginning of 2004, DUACS was redefined as the Data Unification Altimeter Combination System.

Figure 1: DUACS and AVISO, a user-driven altimetry service

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The products lines described in this user manual are the following:SEALEVEL_GLO_SLA_L3_OBSERVATIONS_008_001SEALEVEL_MED_SLA_L3_OBSERVATIONS_008_002SEALEVEL_BS_SLA_L3_OBSERVATIONS_008_003SEALEVEL_EUR_SLA_L3_OBSERVATIONS_008_004SEALEVEL_ARC_SLA_L3_OBSERVATIONS_008_005

The data provided to users have a global coverage (PL SEALEVEL_GLO_SLA_L3_OBSERVATIONS_008_001)and regional products are also computed over specific areas:Mediterranean Sea (PL SEALEVEL_MED_SLA_L3_OBSERVATIONS_008_002)and Black Sea (PL SEALEVEL_BS_SLA_L3_OBSERVATIONS_008_003).Thanks to updates in 2012 (see section II.), new regional products are now available:Europe (PL SEALEVEL_EUR_SLA_L3_OBSERVATIONS_008_004)and Arctic (PL SEALEVEL_ARC_SLA_L3_OBSERVATIONS_008_005).

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II. UPDATES IN 2012 AND 2013

This section describes the updates of the SSALTO/DUACS system that occurred during 2012 and 2013,allowing to add new products (Europe and Arctic), the integration of the Cryosat-2 data in the system, thestopping delivery of Envisat data, the addition of Jason-1 data on geodetic orbit (j1g), the new version ofJason-2 GDRs.To have the information of the DUACS changes, improvements and updates of the system, please refer to:http://www.aviso.oceanobs.com/en/data/product-information/duacs/presentation/updates/index.html.

II.1. Integration of most recent data as soon as available: "OGDR on the fly"(February 2013)

Since February 5th, 2013, the most recent OGDR data are taken into account in the GLOBAL NRT productas soon as available: indeed, the two latest files delivered each day can be updated during all the day in orderto take into account the latest available data. For example, if you download files at 15:00 for the day D:

• Before February 5, the last date of data that you could get in the last file delivered was 20:00 of theday D-1.

• As from February 5, the same file is updated and you get data until 13:00 of the day D at the best (ifOGDR is valid).

This concerns all the datasets of the following product: SEALEVEL_GLO_SLA_L3_NRT_OBSERVATIONS_008_001_a

II.2. All the Sea Level SLA files are compressed (February 2013)

All the Sea Level products are now compressed:

• Before February 5, you got a zip file containing *.nc files

• As from February 5, you get a zip file containing *.nc.gz files

This concerns all the datasets of the following products:SEALEVEL_GLO_SLA_L3_NRT_OBSERVATIONS_008_001_aSEALEVEL_BS_SLA_L3_NRT_OBSERVATIONS_008_001_aSEALEVEL_MED_SLA_L3_NRT_OBSERVATIONS_008_001_aSEALEVEL_EUR_SLA_L3_NRT_OBSERVATIONS_008_001_aSEALEVEL_ARC_SLA_L3_NRT_OBSERVATIONS_008_001_aSEALEVEL_GLO_SLA_L3_RAN_OBSERVATIONS_008_001_bSEALEVEL_MED_SLA_L3_RAN_OBSERVATIONS_008_002_bSEALEVEL_BS_SLA_L3_RAN_OBSERVATIONS_008_003_b

II.3. Version "D" of input IGDR/OGDR Jason-2 data (July 30, 2012 for NRTproducts , December 2012 for Reprocessed products)

On July 30, 2012, the version of Jason-2 IGDR/OGDR has changed and is now the "D" version. Some

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corrections have thus been updated as detailed in tables 3 and 4 for Near Real Time processing and in tables6 and 7 for Delayed Time processing.

II.4. Integration of Jason-1 on its geodetic orbit (May 2012 for NRT products,December 2012 for Reprocessed products)

An anomaly occured on March 3rd, 2012 on the Jason-1 mission. On April 23rd, CNES began to commandthe satellite into a nadir orientation and after some propulsion anomalies, CNES and NASA management,through the Joint Steering Group, have directed the Jason-1 Project to then begin a series of maneuvers toplace Jason-1 into a new orbit as defined in table 1. After checking the current orbit carefully, the operationalteam determined that a geodetic mission was still possible. It was also decided to preserve all remaining fuelfor future station keeping maneuvers which is mandatory in a geodetic orbit. Core payloads were switchedON on May 4th and after some POSEIDON2 radar (PRF) adjustments the mission was resumed on May 7that 15:12:48 UTC.Below are the characteristics of the new orbit which will be maintained, as before, within +/- 1km controlbox at the Equator:

Semi major axis: 7702.437 kmEccentricity: 1.3 to 2.8 10-4Altitude equator: 1324.0 kmOrbital period: 6730s (1h52’10”)Inclination: 66.042˚Cycle: 406 daysSub cycles: 3.9 - 10.9 - 47.5 - 179.5

days

Table 1: Characteristics of the geodetic orbit for Jason-1 since May 7th, 2012

For this new phase of the Jason-1 mission (called j1g), the cycle numbering will restart at 500. Off-lineproducts will be produced once a day for the IGDR, and every 11 days for the GDR’s. The integration ofJason-1 data in the DUACS system is available since May 25th, 2012.

II.5. Envisat has stopped sending data (April 2012)

Since April 8th 2012, Envisat has stopped sending data to Earth. Esa’s mission control has worked tore-establish contact with the satellite but there has been no reaction from the satellite. Thus Esa has de-clared officially the end of mission for Envisat (see http://www.esa.int/esaCP/SEM1SXSWT1H_index_0.html). Therefore, no SL TAC NRT data from Envisat can be released since this date.

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http://www.esa.int/esaCP/SEM1SXSWT1H_index_0.html



II.6. Integration of the Cryosat2 Mission (February 2012 in NRT and in Re-processed products)

Since February 6th, 2012, Cryosat-2 mission has been integrated in the sytem. This mission is dedicated tothe observation of the floating sea-ice as well as the continental ice sheets, but all data acquired over oceanare valuable for the observation of oceanic circulation and mesoscale variations. This major change is theresult of the long-standing and fruitful partnership between ESA and CNES and a response to the requestfrom scientific and operational oceanography users [7]. The integration of Cryosat-2 impacts the deliveringof Near real time and Delayed time Sea Level Anomalies (SLA) .

II.6.1. Generalities about the mission

A Cryosat-2 Processing Prototype (C2P) (described in Boy et al, 2011, [2]) has been developed on CNESside to lay the ground for various SAR processing studies. The processing chains ingest Level-0 telemetryfiles distributed by ESA, and perform the following steps to generate Sea Level Anomalies (SLA) values foreach altimeter measurements:

• Level-1: Decommutation, time-tagging and localization of measurements

• Level-1b: Calculation of instrumental corrections and geophysical/meteorological corrections

• Level-2: MLE4 waveforms Retracking and calculation of SLA

The prototype processes data almost continuously over ocean, either in Low Resolution Mode (LRM) or inthe Doppler/SAR mode processed as pseudo-LRM mode allowing to increase the coverage (figure 2).

Figure 2: SSH-MSS for Cryosat-2 with LRM mode only (left) or LRM+pseudo-LRM modes (right)

Although Cryosat-2 raw SSH can be only corrected with GPS-derived ionospheric correction and ECMWFwet troposphere correction, validation activities showed (Labroue et al, 2011, [26]) that C2P outputs have anaccuracy roughly equivalent to Envisat’s Level 2 products if the latter are processed with the same standards.

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II.6.2. Integration in DUACS

The Cryosat-2 data are included in the Near Real time and Delayed time process. The chain deliversnow (for global coverage, Mediterranean, Black Seas, Europe and Arctic) along track data from Cryosat-2(SLAs) . Note that the products distributed are of Level-3 and don’t replace Level-2 products distributed byagencies.The details of the processing are given in Dibarboure et al, 2011 [8].Here, we just give an overview: the data are computed with the corrections given in section III.2.1.. Thefiltering and sub-sampling of SLAs (section III.2.6.3.) are detailed in [8].

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II.7. Addition of Europe and Arctic products (January 2012) in NRT process-ing

Since the MyOcean V2, two new along-track products are computed in Near Real Time, intended foraddressing the needs of data assmilation and validation in regional models:

• Following the TAPAS (Tailored Altimeter Product for Assimilation Systems) initiative launched bySL TAC with all the Modeling and Forecasting Centers (MFCs), the Europe regional products (PLSEALEVEL_EUR_SLA_L3_OBSERVATIONS_008_004) have been proposed (figure 3). They areavailable in Near Real Time only.They are studied to partly fulfil the needs of the Centers with adapted filtering and resolution. Thoseparameters have been tuned in order to provide boundary conditions for the Atlantic assimilationmodels. They are different from the ones used for Mediterranean and Black Seas products. If you needto study the Mediterranean Sea or the Black Sea, you’d better use the dedicated products describedabove since the processing parameters have been fitted to be adapted to the dynamics of the ocean inthose regions.

Figure 3: Coverage of the Europe product for Jason-1, Jason-2 and Envisat from August, 18th to September,9th 2011

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• The Arctic along-track products (PL SEALEVEL_ARC_SLA_L3_OBSERVATIONS_008_005) coverlatitudes between 45 and 82 degrees (figure 4). They are available in Near Real Time only.

Figure 4: Coverage of the Arctic region with Envisat from October 28th 2011 to January 3rd 2012

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III. SSALTO/DUACS SYSTEM

III.1. Introduction

This chapter presents the input data used by SSALTO/DUACS system and an overview of the different pro-cessing steps necessary to produce the output data.SSALTO/DUACS system is made of two components: a Near Real Time one (NRT) and a Delayed Time(DT) one.In NRT, the system’s primary objective is to provide operational applications with directly usable high qual-ity altimeter data from all missions available.In DT, it is to maintain a consistent and user-friendly altimeter database using the state-of-the-art recom-mendations from the altimetry community.Following figure gives an overview of DUACS system, where processing sequences can be divided into 7main steps:

• acquisition

• homogenization

• input data quality control

• multi-mission cross-calibration

• product generation

• merging

• final quality control.

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Figure 5: SSALTO/DUACS processing sequences

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III.2. Near Real Time processing steps

III.2.1. Input data, models and corrections applied

To produce SLA in near-real time, the DUACS system uses two flows, based on the same instrumental mea-surements but with a different quality:

• The IGDR that are the latest high-quality altimeter data produced in near-real-time.

• The OGDR that includes real time data (OSTM/Jason-2 and Jason-1 OSDR) to complete IGDR. Thesefast delivery products do not always benefit from precise orbit determination, nor from some exter-nal model-based corrections (Dynamic Atmospheric Correction (DAC), Global Ionospheric Maps(GIM)).

Integration of OGDR data and the introduction of Jason-2/Jason-1 tandem increased the resilience and pre-cision of the system. A better restitution of ocean variability is observed, especially in high energetic areas.

Altimetric product Source Availability Type of orbit

Jason-2 IGDR CNES ~24h CNES MOE GDR-Cuntil 2012/07/30

CNES MOE GDR-Dsince 2012/07/31

OGDR NOAA/EUMETSAT ~3 to 5 h

Jason-1 IGDR CNES/NASA ~48 h CNES MOE GDR-Dsince 2012/05/07

OGDR CNES/NASA ~3 to 12 h

Cryosat-2 IGDR-like ESA/CNES ~48 h CNES MOE GDR-D

not delivered

Table 2: SSALTO/DUACS Near-Real Time Input data overview

See Figure 6: Overview of the near real time system data flow management.

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DUACS NRT product from IGDR1

j2 j1n;j1g c2Productstandard ref

version C until 2012/07/30and D after

version C CPP [2]

Orbit CNES MOE GDR-C until2012/07/30 GDR-D after

CNES MOE GDR-C for j1nand CNES MOE GDR-D

for j1g

CNES MOE GDR-D

Ionopheric bi-frequency altimeter range measurements GIM model for c>64(Iijima et al,1998[24])

Dry tropo Model computed from ECMWF Gaussian grids (new S1 and S2 atm tidesapplied)

Wet tropo-sphere

AMR radiometer(enhancement in coastal

regions since 2012/07/30)

JMR radiometer Model computed fromECMWF Gaussian grids

DAC MOG2D High Resolution forced with ECMWF pressure and wind fields (S1and S2 were excluded) (Carrere and Lyard, 2003[3])+ inverse barometer.

Filtering temporal window is un-centered (no data in the future)Ocean tide GOT4v8 (S1 and S2 are included) and TPX 07.2 [16] for Arctic productsPole tide [Wahr, 1985 [54]]Solid earthtide

Elastic response to tidal potential [Cartwright and Tayler, 1971[5]],[Cartwright and Edden, 1973[6]]

Loading tide GOT4v8 (S1 and S2 are included)Sea statebias

Non parametric SSB[Labroue, 2004[28]] (with

cycles J1 1 to 111 usingGDR-B standards until2012/07/30 and derived

from Jason-2 after)

Non parametric SSB[Labroue, 2004[28]](with cycles 1 to 121

using GDR-B standards)

Same as Jason-1 withcorrection of a drift onSigma0 (Labroue et al.,

2012[25]

Orbit error Global multi-mission crossover minimization (LeTraon and Ogor, 1998[33])

Long wave-lengh errors

Optimal Interpolation [Le Traon et al., 1998[32]]

Intercalibration Reference from cycle 20Mean pro-file (seeIII.2.6.1.)

Computed with cycles 11-353 TP data and with cy-cles 11-250 J1 data; refer-enced [1993,1999]

TPN/J1N: computedwith cycles 369-479TPN data; referenced[1993,1999]. J1G: NoMP can be used, instead,the MSS_CNES_CLS11[40] is used

No Mean Profile canbe used for Cryosat-2. Instead, theMSS_CNES_CLS11[40] is used

(1) A flag included in the along-track files indicates the source of the production (OGDR/FDGDR or IGDR). If flag=0, the

processed data comes from OGDRs/FDGDRs; if flag=1, the processed data comes from IGDRs.

Table 3: Corrections and models applied in SSALTO/DUACS NRT products produced from IGDRs.

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DUACS NRT product from OGDR 2

j2 j1n;j1gProduct stan-dard ref

version C until 2012/07/30 and D after version C

Orbit NavigatorIonopheric From-dual frequency altimeter range measurementsDry troposphere Model computed from ECMWF Gaussian grids (new S1 and S2 atmospheric

tides are applied)Wet tropo-sphere

AMR radiometer (enhancement incoastal regions since 2012/07/30)

JMR radiometer

DAC Inverse Barometer forecastedOcean tide GOT4v8 (S1 and S2 are included) and TPX 07.2 [16] for Arctic productsPole tide [Wahr, 1985 [54]]Solid earth tide Elastic response to tidal potential [Cartwright and Tayler, 1971[5]],

[Cartwright and Edden, 1973[6]]Loading tide GOT4v8 (S1 and S2 are included)Sea state bias Non parametric SSB [Labroue,

2004[28]] (with cycles J1 1 to 11 usingGDR-B standards until 2012/07/30 andderived from Jason-2 after)

Non parametric SSB [Labroue,2004[28]] (with cycles 1 to 21 usingGDR-B standards)

Orbit error Specific filtering of long-wavelengthsignal3

Long wave-lengh errors

Optimal Interpolation [Le Traon et al., 1998[32]]

Intercalibration Reference from cycle 20Mean profile(see III.2.6.1.)

Computed with cycles 11-353 TP dataand with cycles 11-250 J1 data; refer-enced [1993,1999]

TPN/J1N: computed with cycles 369-479 TPN data; referenced [1993,1999].J1G: No MP can be used, instead, theMSS_CNES_CLS11 [40] is used

(2) A flag included in the along-track files indicates the source of the production (OGDR or IGDR). If flag=0, the processed data

comes from OGDRs; if flag=1, the processed data comes from IGDRs.

(3) Specific data processing was applied on long wave-length signal (§III.2.3. of the user manual)

Table 4: Corrections and models applied in SSALTO/DUACS NRT products produced from OGDRs.

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III.2.2. Acquisition

The acquisition process is twofold:

• straightforward retrieval and reformatting of altimeter data and dynamic auxiliary data (pressure andwet troposphere correction grids from ECMWF are provided by Meteo France, TEC grids from JPL,NRT MOG2D corrections,...) from external repositories.

• synchronisation process.

To be homogenized properly, altimeter data sets require various auxiliary data. The acquisition softwaredetects, downloads and processes incoming data as soon as they are available on remote sites (externaldatabase, FTP site). Data are split into passes if necessary. If data flows are missing or late, the synchroni-sation engine put unusable data in waiting queues and automatically unfreezes them upon reception of themissing auxiliary data. This processing step delivers "raw" data, that is to say data that have been dividedinto cycles and passes, and ordered chronologically.The acquisition step uses two different data flows in near-real time: the OGDR flow (within a few hours),and the IGDR flow (within a few days).For each OGDR input, the system checks that no equivalent IGDR entry is available in the data base beforeacquisition; for each IGDR input, the system checks and delete the equivalent OGDR entry in the data base.These operations aim to avoid duplicates in SSALTO/DUACS system.

Figure 6: Overview of the near real time system data flow management

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III.2.3. Homogenization

The Homogenization process consists in applying the most recent corrections, models and references rec-ommended for altimeter products. Each mission is processed separately as its needs depend on the baseinput data. The list of corrections and models currently applied is provided in tables 3 and 4 for NRT data.The system includes SLA filtering to process OGDR data. DUACS extract from these data sets the shortscales (space and time) which are useful to better describe the ocean variability in real time, and merge thisinformation with a fair description of large scale signals provided by the multi-satellite observation in nearreal time (read: IGDR-based DUACS data). Finally an "hybrid" SLA is computed.

Figure 7: Merging pertinent information from IGDR and OGDR processing

III.2.4. Input data quality control

The Input Data Quality Control is a critical process applied to guarantee that DUACS uses only the mostaccurate altimeter data. Thanks to the high quality of current missions, this process rejects a small percent-age of altimeter measurements, but these erroneous data could be the cause of a significant quality loss.The quality control relies on standard raw data editing with quality flags or parameter thresholds, but alsoon complex data editing algorithms based on the detection of erroneous artefacts, mono and multi-missioncrossover validation, and macroscopic statistics to edit out large data flows that do not meet the system’srequirements.

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III.2.5. Multi-mission cross-calibration

The Multi-mission Cross-calibration process ensures that all flows from all satellites provide a consistentand accurate information. It removes any residual orbit error (OE, Le Traon and Ogor, 1998[33]), or longwavelength error (LWE, Le Traon et al., 1998[32]), as well as large scale biases and discrepancies betweenvarious data flows.

This process is based on two very different algorithms: a global multi-mission crossover minimization fororbit error reduction (OER), and Optimal Interpolation (OI) for LWE.

Multi-satellite crossover determination is performed on a daily basis. All altimeter fields (measurement,corrections and other fields such as bathymetry, MSS,...) are interpolated at crossover locations and dates.Crossovers are then appended to the existing crossover database as more altimeter data become available.This crossover data set is the input of the Orbit Error Reduction (OER) method. Using the precision of thereference mission orbit, a very accurate orbit error can be estimated. This processing step does not concernOGDR data.LWE is mostly due to residual tidal or inverse barometer errors and high frequency ocean signals. The OIused for LWE reduction uses precise parameters derived from:

• accurate statistical description of sea level variability

• localized correlation scales

• mission-specific noise and precise assumptions on the long wavelength errors to be removed (from arecent analysis of one year of data from each mission).

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III.2.6. Product generation

The product generation process is composed of four steps: computation of raw SLA, cross-validation,filtering&sub-sampling, and generation of by-products.

III.2.6.1. Computation of raw SLA

Since the geoid is not well known yet, the SSH cannot be used directly, the SSH anomalies are used instead.They are computed from the difference of the instantaneous SSH - a temporal reference. This temporalreference can be a Mean Profile (MP) in the case of repeat track analysis or a gridded MSS when the repeattrack analysis cannot be used. The errors affecting the SLAs, MPs and MSS have different magnitudes andwavelengths. The computation of the SLAs and their errors associated are detailed in Dibarboure et al, 2010[9].

Use of a Mean ProfileIn the repeat track analysis (when the satellites flies over a repetitive orbit), measurements are re-sampledalong a theoretical ground track (or mean track) associated to each mission. Then a Mean Profile (MP) issubtracted from the re-sampled data to obtain SLA. The MP is a time average of similarly re-sampled dataover a long period.

• The Mean Profile used for Jason-2 is computed with 10 years of T/P (cycles 11 to 353) and 6 years ofJason-1 (cycles 11 to 250).

• The Mean Profile used from Jason-1 cycle 262 to 374 (where satellites are on interleaved ground-tracks) is computed with 3 years of T/P (cycles 369 to 479).

• No Mean Profile can be used for Envisat on its new orbit (enn), Jason-1 on its geodetic orbit (j1g) norfor Cryosat-2 mission (c2) because there is no repetitive track. The MSS must be used instead (seebelow).

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Computation of a Mean ProfileThe computation of a Mean Profile is not a simple average of similarly co-located SSH data from the sameground track on the maximum period of time as possible .

• Indeed, as the satellite ground track is not perfectly controlled and is often kept in a band of about1km wide, precise cross-track projection and/or interpolation schemes are required to avoid errors.

• The mesoscale variability error (which is <3.5 cm for MP between 3 to 5 years and <1cm for WLof 100-200km for MP between 7 and 15 years) is eliminated with an iterative process using a prioriknowledge from Sea Level maps derived from previous iterations or from other missions.

• Moreover, the inter-annual variabilty error (<5cm for WL>5000km and <5-8cm for WL of 200-500km) is accounted for by using the MSS computed over 1993-1999 (e.g. the GFO MP is computedon 2000-2006 but referenced onto 1993-1999 for the sake of coherency with other missions).

• Finally, for these Mean Profiles, the latest standards and a maximum of data were used in order toincrease as much as possible the quality of their estimation . Note that a particular care was broughtto the processing near coasts.

Use of a MSSThe repeat track analysis is impossible for Envisat since november 2010, for Jason-1 on its geodetic orbit(j1g) and for Cryosat-2 mission (c2) because the satellite is not in a repetitive orbit phase. The alternative isto use the MSS instead. The gridded MSS is derived from along track MPs and data from geodetic phases.Thus any error on the MP is also contained in the MSS. There are essentially 4 types of additional errors ongridded MSS which are hard to quantify separately:

• To ensure a global MSS coherency between all data sets, the gridding process averages all sensor-specific errors and especially geographically correlated ones.

• The gridding process has to perform some smoothing to make up for signals which cannot be resolvedaway from known track, degrading along-track content.

• There are also errors related to the lack of spatial and temporal data (omission errors).

• The error stemming from the geodetic data: the variability not properly removed before the absorptionin the MSS and the impossibility to compute mean sea surface height content.

Note that for the Arctic regional products, the DTU10 MSS [1] is taken into account instead of the CLS01MSS. Indeed, the accuracy of DTU10 compared to CLS01 is further demonstrated by the significant sealevel anomaly variance reduction found over the whole Arctic Ocean as shown in Prandi et al., [43]..

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III.2.6.2. Cross validation

After the repeat track analysis, the cross-validation technique is used as the ultimate screening process ofisolated and slightly erroneous measurements. Small SLA flows are compared to previous and independentSLA data sets using a- 12 year climatology and a 3 sigma criteria for outlier removal.

III.2.6.3. Filtering and sub-sampling

Residual noise and small scale signals are then removed by filtering the data using a Lanczos filter. As dataare filtered from small scales, a sub-sampling is finally applied. Along-track SLA are then produced.Note that the filtering and sub-sampling is adapted to each region and product as a function of the char-acteristics of the area and of the assimilation needs.

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III.2.7. Quality control

The production of homogeneous products in a high quality data with a short delay is the key feature of theDUACS processing system. But some events (failure on payload or on instruments, delay, maintenance onservers), can impact the quality of measurements or the data flows. A strict quality control on each process-ing step is indispensable to appreciate the overall quality of the system and to provide the best user services.

III.2.7.1. Final quality Control

The Quality Control is the final process used by DUACS before product delivery. In addition to dailyautomated controls and warnings to the operators, each production delivers a large QC Report composed ofdetailed logs, figures and statistics of each processing step. Altimetry experts analyse these reports twice aweek.

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III.2.7.2. Performance indicators

To appreciate the quality situation of the DUACS system, new performance indicators are computed daily.They aim at evaluate the status of the main processing steps of the system: the input data availability, theinput data coverage, the input data quality and the output product quality. These indicators are computed foreach and every currently working satellite, and combined to obtain the overall status.

Figure 8: Example with the key performance indicator on 2009/06/27

See the description, the latest and previous indicators on Aviso website:http://www.aviso.oceanobs.com/index.php?id=1516

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III.3. Delayed Time processing steps

III.3.1. Input data, models and corrections applied

Delayed Time MyOcean products are generated:

• from Aviso GDR products for T/P, Jason-1, Jason-2 and Envisat (GDR-A: cycles 1 to 22 / GDR-B:cycles 23 to 85 / GDR-C: from cycle 86),

• from NOAA GDR for GFO and from CERSAT (IFREMER) OPR for ERS-1 and ERS-2 (phases C(1st 35-day repeat orbit period), phase E and F (geodetic phases), phase G for ERS-1 (last 35-dayrepeat orbit period, tandem phase with ERS-2 ; phase A for ERS-2 (1st 35-day repeat orbit period,tandem phase with ERS-1)).

All GDR products are computed with a Precise Orbit Ephemeris (POE) and are delivered within 2 to 3months depending on the mission. For several missions, an updated orbit is used:

• For Jason-2, GDR-C orbit is used until 2012/06/18 and GDR-D after

• For Jason-1 geodetic orbit, GDR-D orbit is used (GDR-C before)

• For ERS-1&-2, the orbit used is DGME-04 provided by Delft Institute (http://www.deos.tudelft.nl/)until June 2003,

• For Topex/Poseidon, the orbit used is GSFC (std0809) for the whole mission,

• For Envisat, CNES POE of GDR-C standard is used for the whole mission,

• For the whole GFO mission, the orbit used is GSFC (std0809) and when not available, NASA POE isused.

Altimetric product Source Availability Type of orbit

Topex/Poseidon GDR NASA/CNES - GSFC POE

Jason-1 GDR (GDR-C) CNES/NASA ~40 days CNES POE

Jason-2 GDR (GDR-C) CNES/NASA ~60 days CNES POE

GFO GDR NOAA - GSFC/NASA POE

ERS-1&2 IFREMER/ESA - DGME-04

Envisat (GDR-A, GDR-B andGDR-C from cycle 86)

ESA - CNES POE

Cryosat-2 ESA/CNES ~40 days CNES POE

Table 5: SSALTO/DUACS Delayed Time Input data overview

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DUACS DT product (>V3.0.0)j2 j1;j1n;j1g tp;tpn g2

Productstandard ref

GDR-C until 2012/06/18and GDR-D after

GDR-C GDR (NOAA)

Orbit Cnes POE GDR-C until2012/06/18 and GDR-D

after

Cnes POE GDR-C forj1,j1n and GDR-D for

j1g

GSFC (ITRF2005,GRACE last

standards)

GSFC (ITRF 2005,GRACE last standards

Ionopheric Dual frequency altimeter range measurements Dual frequencyaltimeter range

measurements (Topex),DORIS (POSEIDON)

From GIM model(Iijima et al, 1998 [24])

Dry tropo-sphere

Model computed fromECMWF Gaussian grids

(new S1 and S2atmospheric tides are

applied)

Model computed from ECMWF rectangular grids and gaussian grids fromNovember 20th, 2010 (new S1 and S2 atmospheric tides are included)

Wet tropo-sphere

JMR/AMR radiometer further than 50 km fromthe coasts, From ECMWF model for distances

between 10 and 50 km

TMR radiometer(Scharoo et al. 2004

[50])

GFO radiometer

DAC MOG2D High Resolution forced with ECMWF pressure and wing fields (S1 and S2 wereexcluded) + inverse barometer computed from rectangular grids

Ocean tide GOT4v7 (S1 and S2 are included)Pole tide [Wahr, 1985[54]]Solid earthtide

Elastic response to tidal potential Cartwright and Tayler, 1971[5],Cartwright and Edden, 1973[6]

Loading tide GOT4v7 (S1 parameter is included)Sea statebias

NP SSB [Labroue,2004[28]] (with cycles

J1 1 to 111 usingGDR-B standards until2012/06/18 and derived

from Jason-2 after)

NP SSB [Labroue,2004[28]] (with cycles 1

to 121 using GDR-Bstandards)

Non parametric SSB [N.Tran and al. 2010[52]]

Non parametric SSB [N.Tran et al., 2010[52]]

Orbit error Global multi-mission crossover minimization (Le Traon and Ogor, 1998[33])Long wave-lengh errors

Optimal Interpolation (Le Traon et al., 1998[32])

Intercalibration Reference from cycle 11 Reference from cycle 1to 354

Correction of regionalbias J2/J1 deduced from

intercalibration phase( cycle 11 J2)

Correction of globalbiais J1/TP déducedfrom intercalibrationphase ( cycles 11 J1)

Mean pro-file (seeIII.3.6.1.)

Computed with cycles11-353 TP data and withcycles 11-250 J1 data;referenced [1993,1999]

TP/J1: computed with cycles 11-353 TP data andwith cycles 11-250 J1 data; referenced

[1993,1999]TPN/J1N: computed with cycles 369-479 TPN

data; referenced [1993,1999]

Computed with cycles37-187 G2 data ;

referenced [1993,1999]

Period of use from cycle 9 from cycle 9 cycles 1 to 481 cycles 37 to 222

Table 6: Corrections and models applied in SSALTO/DUACS DT products for TOPEX/Poseidon, Jason-1,Jason-2 and GFO.

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DUACS DT product (>V3.0.0)e1 e2 en;enn c2

Productstandard ref

OPR GDR-A (c≤22)/GDR-B(23≤c≤85)/GDR-C (c≥93)

CPP [2]

Orbit DGME-04 (Scharroo and Visser, 1998[49] CNES POE (GDR-Bstandards for c≤22; GDR-C

standards c≥23)

CNES POE GDR-D

Ionopheric Bent model Bent model (c≤36), GIMmodem c≥37 ((Iijima et al,

1998 [24])

Dual-frequency altimeterrange measurement (c 9-64)

and GIM model c≥65(Iijima et al., 1999[24])

corrected from 8 mm bias

From GIM model(Iijima et al, 1998

[24])

Dry tropo-sphere

Model computed from ECMWF rectangular grids and gaussian grids from November 20th, 2010(new S1 and S2 atmospheric tides are included)

Wet tropo-sphere

MWR radiometer MWR corrected of23.6GHz TB drift (Scharroo

et al., 2004[50])+NeuralNetwork algorithm (Tran

and Obligis, 2003[51]

MWR (dist≥50km from thecoasts) and corrected from

side lobes (c≥41), fromECMWF model

(50≥dist≥10km).

ECMWF Model

DAC MOG2D High Resolution forced with ECMWF pressure and wing fields (S1 and S2 wereexcluded) + inverse barometer computed from rectangular grids

Ocean tide GOT4v7 (S1 and S2 are included)Pole tide [Wahr, 1985[54]]Solid earthtide

Elastic response to tidal potential [Cartwright and Tayler, 1971[5]], [Cartwright and Edden,1973[6]]

Loading tide GOT4v7 (S1 parameter is included)Sea statebias

BM3 [Gaspar andOgor 1994[17]]

Non parametric SSB [Mertzet al., 2005[39]]

Non parametric SSB[Gaspar et al, 2002] with

GDR-B standards c≤85 andwith GDR-C standards

c≥86

Same as Jason-1with correction of a

drift on Sigma0(Labroue et al.,

2012[25]Orbit error Global multi-mission crossover minimization (Le Traon and Ogor, 1998[33])Mean pro-file (seeIII.3.6.1.)

Computed with cycles 1-85 E2 data and withcycles 10-72 EN data; referenced [1993,1999]

EN: Computed with cycles1-85 E2 data and withcycles 10-72 EN data;

referenced [1993,1999]ENN: use of

MSS_CNES_CLS11 [40]

Use ofMSS_CNES_CLS11

[40]

Period of use cycles 15 to 43Including E-F

geodetic phases

cycles 1 to 83 from cycle 9 cycle 29

Table 7: Corrections and models applied in SSALTO/DUACS DT products for ERS-1, ERS-2, Envisat andCryosat-2

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III.3.2. Acquisition

The acqusition process in Delayed time is much simpler than in Near Real time: it consists in a synchronisa-tion process of all the auxiliary data required to homogenize propely the altimeter data sets. The acquisitionstep uses the GDRs or the OPRs provided by the agencies.

III.3.3. Homogenization

The Homogenization process consists in applying the most recent corrections, models and references rec-ommended for altimeter products. Each mission is processed separately as its needs depend on the baseinput data. The list of corrections and models currently applied is provided in tables 6 and 7 for DT data.

III.3.4. Input data quality control

The Input Data Quality Control is a critical process applied to guarantee that DUACS uses only the mostaccurate altimeter data. Thanks to the high quality of current missions, this process rejects a small percent-age of altimeter measurements, but these erroneous data could be the cause of a significant quality loss.The quality control relies on standard raw data editing with quality flags or parameter thresholds, but alsoon complex data editing algorithms based on the detection of erroneous artefacts, mono and multi-missioncrossover validation, and macroscopic statistics to edit out large data flows that do not meet the system’srequirements.

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III.3.5. Multi-mission cross-calibration

The Multi-mission Cross-calibration process ensures that all flows from all satellites provide a consistentand accurate information. It removes any residual orbit error (OE, Le Traon and Ogor, 1998[33]), or longwavelength error (LWE, Le Traon et al., 1998[32]), as well as large scale biases and discrepancies betweenvarious data flows.

This process is based on two very different algorithms: a global multi-mission crossover minimization fororbit error reduction (OER), and Optimal Interpolation (OI) for LWE.

Multi-satellite crossover determination is performed on a daily basis. All altimeter fields (measurement,corrections and other fields such as bathymetry, MSS,...) are interpolated at crossover locations and dates.Crossovers are then appended to the existing crossover database as more altimeter data become available.This crossover data set is the input of the Orbit Error Reduction (OER) method. Using the precision of thereference mission orbit, a very accurate orbit error can be estimated. LWE is mostly due to residual tidalor inverse barometer errors and high frequency ocean signals. The OI used for LWE reduction uses preciseparameters derived from:

• accurate statistical description of sea level variability

• localized correlation scales

• mission-specific noise and precise assumptions on the long wavelength errors to be removed (from arecent analysis of one year of data from each mission).

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III.3.6. Product generation

The product generation process is composed of four steps: computation of raw SLA, cross-validation,filtering&sub-sampling, and generation of by-products.

III.3.6.1. Computation of raw SLA

Since the geoid is not well known yet, the SSH cannot be used directly, the SSH anomalies are used instead.They are computed from the difference of the instantaneous SSH - a temporal reference. This temporalreference can be a Mean Profile (MP) in the case of repeat track analysis or a gridded MSS when the repeattrack analysis cannot be used. The errors affecting the SLAs, MPs and MSS have different magnitudes andwavelengths. The computation of the SLAs and their errors associated are detailed in Dibarboure et al, 2010[9].

Use of a Mean ProfileIn the repeat track analysis (when the satellites flies over a repetitive orbit), measurements are re-sampledalong a theoretical ground track (or mean track) associated to each mission. Then a Mean Profile (MP) issubtracted from the re-sampled data to obtain SLA. The MP is a time average of similarly re-sampled dataover a long period.

• The Mean Profile used for T/P (cycles 1 to 364), Jason-1 (cycles 1 to 259) and Jason-2 is computedwith 10 years of T/P (cycles 11 to 353) and 6 years of Jason-1 (cycles 11 to 250).

• The Mean Profile used for T/P (cycles 368 to 481) and from Jason-1 cycle 262 to 374 (where satellitesare on interleaved ground-tracks) is computed with 3 years of T/P (cycles 369 to 479).

• The Mean Profile used for ERS-1 in its 35 days repetitive orbit mission, ERS-2, and Envisat (only forthe first orbit, before November 2010) is computed with 8 years of ERS-2 (cycles 1 to 85) and 6 yearsof Envisat (cycles 10 to 72).

• No Mean Profile can be used for Envisat on its new orbit (enn), Jason-1 on its geodetic orbit (j1g) norfor Cryosat-2 mission (c2) because there is no repetitive track. The MSS must be used instead (seebelow).

• The Mean Profile used for GFO is computed with 7 years of GFO cycles 37 to 187.

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Computation of a Mean ProfileThe computation of a Mean Profile is not a simple average of similarly co-located SSH data from the sameground track on the maximum period of time as possible .

• Indeed, as the satellite ground track is not perfectly controlled and is often kept in a band of about1km wide, precise cross-track projection and/or interpolation schemes are required to avoid errors.

• The mesoscale variability error (which is <3.5 cm for MP between 3 to 5 years and <1cm for WLof 100-200km for MP between 7 and 15 years) is eliminated with an iterative process using a prioriknowledge from Sea Level maps derived from previous iterations or from other missions.

• Moreover, the inter-annual variabilty error (<5cm for WL>5000km and <5-8cm for WL of 200-500km) is accounted for by using the MSS computed over 1993-1999 (e.g. the GFO MP is computedon 2000-2006 but referenced onto 1993-1999 for the sake of coherency with other missions).

• Finally, for these Mean Profiles, the latest standards and a maximum of data were used in order to in-crease as much as possible the quality of their estimation (see tables 6 and 7: Corrections and modelsapplied in SSALTO/DUACS Delayed-Time products). Note that a particular care was brought to theprocessing near coasts.

Use of a MSSThe repeat track analysis is impossible for ERS-1 for its 168 days geodetic mission (phases E-F from April1994 to March 1995) or for Envisat since november 2010, and for Cryosat-2 mission (c2) because thesatellite is not in a repetitive orbit phase. The alternative is to use the MSS instead. The gridded MSS isderived from along track MPs and data from geodetic phases. Thus any error on the MP is also containedin the MSS. There are essentially 4 types of additional errors on gridded MSS which are hard to quantifyseparately:

• To ensure a global MSS coherency between all data sets, the gridding process averages all sensor-specific errors and especially geographically correlated ones.

• The gridding process has to perform some smoothing to make up for signals which cannot be resolvedaway from known track, degrading along-track content.

• There are also errors related to the lack of spatial and temporal data (omission errors).

• The error stemming from the geodetic data: the variability not properly removed before the absorptionin the MSS and the impossibility to compute mean sea surface height content.

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III.3.6.2. Cross validation

After the repeat track analysis, the cross-validation technique is used as the ultimate screening process ofisolated and slightly erroneous measurements. Small SLA flows are compared to previous and independentSLA data sets using a- 12 year climatology and a 3 sigma criteria for outlier removal.

III.3.6.3. Filtering and sub-sampling

Residual noise and small scale signals are then removed by filtering the data using a Lanczos filter. As dataare filtered from small scales, a sub-sampling is finally applied. Along-track SLA are then produced.Note that the filtering and sub-sampling is adapted to each region and product as a function of the char-acteristics of the area and of the assimilation needs.

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III.3.7. Quality control

The production of homogeneous products in a high quality data with a short delay is the key feature of theDUACS processing system. But some events (failure on payload or on instruments, delay, maintenance onservers), can impact the quality of measurements or the data flows. A strict quality control on each process-ing step is indispensable to appreciate the overall quality of the system and to provide the best user services.

III.3.7.1. Final quality Control

The Quality Control is the final process used by DUACS before product delivery. In addition to dailyautomated controls and warnings to the operators, each production delivers a large QC Report composed ofdetailed logs, figures and statistics of each processing step. Altimetry experts analyse these reports twice aweek.

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IV. MYOCEAN PRODUCTS

IV.1. Near Real Time Products

The purpose of the NRT DUACS component is the acquisition of altimeter data from various altimeter mis-sions in near-real time (i.e. within a few days at most), the validation and correction of these altimeter datasets (i.e edition and selection, update of corrections and homogenization, orbit error reduction) in order toproduce each day along-track productsProducts are as follows:

Along-track products, global and regional (Mediterranean and Black Seas , Arctic Ocean and Europeareas):

• Sea Level Anomaly (NRT-SLA) for each mission, with NRT-SLA ephemeris

IV.1.1. Delay of the products

The availability of the products in near real time is three to twelve hours after the measurement for along-track products . Those products are delivered every day.

IV.1.2. Temporal availibility

The following table presents the available products by mission and by data period:Near real time products:

NRT Jason-2 Jason-1 and Jason-1geodetic

Envisat new Cryosat-2

Temporal Time availability y/m∗

ongoingy/m∗-2012/03/03 for

repeat track and2012/05/07-ongoing for

geodetic track

y/m∗

2012/04/082012/01/01

ongoing

NRT-SLA X X X X

∗y/m: the temporal coverage for NRT products is fluctuating and depends on updates of Delayed Time prod-ucts. When DT products are updated (3 to 4 times per year), the oldest NRT products are replaced by thebetter quality DT products so that the 2 series are overlapping on few weeks or few months.

IV.2. Delayed Time Products

The Delayed Time component of SSALTO/DUACS system is responsible for the production of processedCryosat-2, Jason-1, Jason-2, T/P, Envisat, GFO, ERS1/2 and even Geosat data in order to provide a homo-geneous, inter-calibrated and highly accurate long time series of SLA .DT products are more precise than NRT products. Two reasons explain this quality difference. The firstone is the better intrinsic quality of the POE orbit used in the GDR processing. The second reason is that

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in the DT DUACS processing, the products can be computed optimally with a centred computation timewindow for OER, LWE . On the contrary in NRT case, "future" data are not available so the computationtime window is not centred and therefore not optimal.As for NRT products, improved altimeter corrections and processing algorithms are used: ocean tide modelto correct altimeter data, improved methods for orbit error reduction and mapping.

Along-Track products, global and regional (Mediterranean and Black Seas):

• Sea Level Anomaly (DT-SLA) for each mission,

IV.2.1. Delay of the products

Weekly products are delivered. The availability of the products in delayed time is at the best two monthsafter the date of the measurement. The product generation needs all the GDR data of all the missions to takeinto account the best corrections as possible. The time delay can be longer in the case of a missing mission.

IV.2.2. Temporal availibility

The following table presents the available products by mission and by data period:Delayed time SSALTO/DUACS products:

Temporal availibility DT-SLA

begin date end date

Cryosat-2 2012/04 y/m∗ X

Jason-2 2008/10 y/m∗ X

Jason-1 geodetic∗∗ ?? y/m∗ X

Jason-1 new∗∗ 2009/02 2012/03 X

Jason-1 2002/04 2008/10 X

GFO 2000/01 2008/09 X

Envisat new∗∗ 2010/10 2012/08 X

Envisat 2002/10 2010/10 X

ERS-1/ERS-2∗∗ 1992/10 2003/04 X

Topex new∗∗ 2002/09 2005/10 X

Topex 1992/09 2002/04 X

∗ y/m: those dates are updated regularly (3 to 4 times per year) and are avalaible when you download thedata at http://www.myocean.eu/web/24-catalogue.php∗∗ Jason-1 geodetic orbit: starting ???, Jason-1 new orbit : starting 2009/02, Envisat new orbit : starting 2010/10, T/Pnew orbit : starting 2002/09, ERS-1: Geodetic phases (E-F) are included. No ERS-1 data between December 23, 1993and April 10, 1994 (ERS-1 phase D - 2nd ice phase).

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V. DESCRIPTION OF THE PRODUCT SPECIFICATION

V.1. General information

Product Specification SEALEVEL_GLO_SLA_L3_OBSERVATIONS_008_001

Geographical coverage global

Variables latitude

longitude

SLA

track

time

mean_profile_point_index

flag

cycle

Near Real time Yes

Reanalysis Yes

Available time series see sections IV.2.2. for RAN and IV.1.2. for NRT

Temporal resolution Daily for NRT and weekly for RAN

Target delivery time 3-4 months for RAN and up to 10 times a day for NRT

Delivery mechanism MyOcean Information System

Horizontal resolution 20-50km for filtered, 7km for unfiltered

Number of vertical levels 1

Format Netcdf CF1.0

SEALEVEL_GLO_SLA_L3_OBSERVATIONS_008_001 Product Specification

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Product Specification SEALEVEL_MED_SLA_L3_OBSERVATIONS_008_002

Geographical coverage 5.5˚W-35˚E ; 30˚N-46˚N

Variables latitude

longitude

SLA

track

time


flag

cycle

Near Real time Yes

Reanalysis Yes



Target delivery time 3-4 months for RAN and daily for NRT


Horizontal resolution 14km for filtered, 7km for unfiltered


Format Netcdf CF1.0

SEALEVEL_MED_SLA_L3_OBSERVATIONS_008_002 Product Specification

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Product Specification SEALEVEL_BS_SLA_L3_OBSERVATIONS_008_003

Geographical coverage 27˚E-42˚E ; 40˚N-47˚N

Variables latitude

longitude

SLA

track

time


flag

cycle

Near Real time Yes

Reanalysis Yes



Target delivery time 3-4 months for RAN and daily for NRT


Horizontal resolution 7km


Format Netcdf CF1.0

SEALEVEL_BS_SLA_L3_OBSERVATIONS_008_003 Product Specification

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Product Specification SEALEVEL_EUR_SLA_L3_OBSERVATIONS_008_004

Geographical coverage 25˚W-42˚E ; 21˚N-66˚N

Variables latitude

longitude

SLA

track

time


flag

cycle

Near Real time Yes

Reanalysis Yes


Temporal resolution Daily for NRT

Target delivery time daily for NRT




Format Netcdf CF1.0

SEALEVEL_EUR_SLA_L3_OBSERVATIONS_008_004 Product Specification

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Product Specification SEALEVEL_ARC_SLA_L3_OBSERVATIONS_008_005

Geographical coverage 0˚W-360˚E ; 50˚N-82˚N

Variables latitude

longitude

SLA

track

time


flag

cycle

Near Real time Yes

Reanalysis Yes


Temporal resolution Daily for NRT

Target delivery time daily for NRT




Format Netcdf CF1.0

SEALEVEL_ARC_SLA_L3_OBSERVATIONS_008_005 Product Specification

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VI. NOMENCLATURE OF FILES

VI.1. Nomenclature of files downloaded through the MyOcean Web PortalDirectgetfile Service

VI.1.1. Nomenclature of the product line

The description of the syntax for the SL TAC SLA products line is given by:

SEALEVEL_<REGION>_SLA_L3_<DELAY>_OBSERVATIONS_008_<ZONE>_<LETTER>where:

REGION GLO global geographic coverage product

BS Black Sea products

MED Mediterranean products

ARC Arctic products (for NRT products)

EUROPE Europe products (for NRT products)

DELAY NRT near real time products

RAN delayed time or reanalysis products

ZONE 001 number on 3 digits

002

003

004

LETTER a letter on 1 digit

b

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VI.1.2. Nomenclature of the datasets

The nomenclature used is:

dataset-duacs-<delay>-<zone>-<mission>-<type of sla>-l3where the fileds in "<>" are described below:

delay nrt near-real time productsran delayed time products

zone global global geographic coverage productmedsea Mediterranean productsblacksea Black Sea productsarctic Arctic products (only for nrt)europe Europe products (only for nrt)

mission e1 ERS-1 (only for ran)e2 ERS-2 (only for ran)tp TOPEX/Poseidon (only for ran)tpn TOPEX/Poseidon on its new orbit (only for

ran)g2 GFO (only for ran)j1 Jason-1 (only for ran)j1n Jason-1 on its new orbitj1g Jason-1 on its geodetic orbitj2 Jason-2en Envisat (only for ran)enn Envisat on its new orbitc2 Cryosat-2

type of sla sla filtered slasla_unfiltered non filtered sla (only for ran products)

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VI.1.3. Nomenclature of the NetCdf files

The nomenclature used is:

<delay>_<zone>_<mission>_sla_<variable>_<datebegin>_<dateend>_<dateprod>.<format> where the fileds in "<>"are described below:

delay nrt near-real time productsdt delayed time products

zone global global geographic coverage productmed Mediterranean products (for dt)mfstep Mediterranean products (for nrt)blacksea Black Sea productsarctic Arctic products (only for nrt)europe Europe products (only for nrt)

mission e1 ERS-1e2 ERS-2tp TOPEX/Poseidontpn TOPEX/Poseidon on its new orbitg2 GFOj1 Jason-1j1n Jason-1 on its new orbitj1g Jason-1 on its geodetic orbitj2 Jason-2en Envisatenn Envisat on its new orbitc2 Cryosat-2

variable vfec filtered and sub-sampled slavxxc non filtered and non sub-sampled sla (only

for dt)datebegin YYYYMMDD(*) beginning date of the datasetdateend YYYYMMDD(*) end date of the datasetdateprod YYYYMMDD production date of the datasetformat .nc.gz compressed NetCdf CF1.0

(*) For dt data: as the dates in the file names are rounded, the user wanting data for date T has to read the filescontaining dates T+1 and T-1 in their name and check inside the files if there are some data for date T.

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VII. DATA FORMAT

This chapter presents the data storage format used for MyOcean products.

VII.1. NetCdf

The products are stored using the NetCDF format. NetCDF (network Common Data Form) is an interface for array-oriented data access and a library that provides an implementation of the interface. The netCDF library also definesa machine-independent format for representing scientific data. Together, the interface, library, and format support thecreation, access, and sharing of scientific data. The netCDF software was developed at the Unidata Program Center inBoulder, Colorado. The netCDF libraries define a machine-independent format for representing scientific data. Pleasesee Unidata NetCDF pages for more information, and to retreive NetCDF software package on:http://www.unidata.ucar.edu/packages/netcdf/index.html.

NetCDF data is:

• Self-Describing. A netCDF file includes information about the data it contains.

• Architecture-independent. A netCDF file is represented in a form that can be accessed by computers with dif-ferent ways of storing integers, characters, and floating-point numbers.

• Direct-access. A small subset of a large dataset may be accessed efficiently, without first reading through allthe preceding data.

• Appendable. Data can be appended to a netCDF dataset along one dimension without copying the dataset orredefining its structure. The structure of a netCDF dataset can be changed, though this sometimes causes thedataset to be copied.

• Sharable. One writer and multiple readers may simultaneously access the same netCDF file.

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VII.2. Structure and semantic of NetCDF along-track files

The NetCDF Sea Level TAC files are based on the attribute data tags defined by the Cooperative Ocean/AtmosphereResearch Data Service (COARDS) and Climate and Forecast (CF) metadata conventions. The CF convention gener-alises and extends the COARDS convention but relaxes the COARDS constraints on dimension and order and specifiesmethods for reducing the size of datasets.A wide range of software are available to write or read NetCDF/CF files. API are made available by UNIDATA(http://www.unidata.ucar.edu/software/netcdf):

• C/C++/Fortran

• Java

• MATLAB, Objective-C, Perl, Python, R, Ruby, Tcl/Tk

In addition to these conventions, the files are using a common structure and semantic:

• 1 dimension is defined:

– time: it is used to check NetCDF variables depending on time.

• 8 variables are defined:

– short SLA : contains the Sea Level Anomaly values for each time given,

– int longitude : contains the longitude for each measurement,

– short track : contains the track number for each measurement,

– double time : contains the time in days since 1950-01-01 00:00:00 UTC for each measurement,

– short mean_profile_point_index: index of the corrsponding point in the track of the mean profile

– byte flag : flag=0, the processed data comes from OGDR; if flag=1, the processed data comes from theIGDR

– int latitude : contains the latitude for each measurement,

– short cycle : contains the cycle number for each measurement.

• global attributes:

– the global attributes gives information about the creation of the file.

Example of a NetCDF sla file:

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netcdf nrt_global_j1_sla_vfec_20101230_20101230_20101230 {dimensions:

time = 10385 ;variables:

short SLA(time) ;SLA:coordinates = "longitude latitude" ;SLA:long_name = "Sea Level Anomaly" ;SLA:_FillValue = 32767s ;SLA:add_offset = 0. ;SLA:scale_factor = 0.001 ;SLA:units = "m" ;SLA:standard_name = "sea_surface_height_above_sea_level" ;

int longitude(time) ;longitude:long_name = "Longitude of measurement" ;longitude:_FillValue = 2147483647 ;longitude:add_offset = 0. ;longitude:standard_name = "longitude" ;longitude:scale_factor = 1.e-06 ;longitude:units = "degrees_east" ;

short track(time) ;track:long_name = "Track in cycle the measurement belongs to" ;track:_FillValue = 32767s ;track:units = "1" ;track:coordinates = "longitude latitude" ;

double time(time) ;time:long_name = "Time of measurement" ;time:_FillValue = 1.84467440737096e+19 ;time:axis = "T" ;time:standard_name = "time" ;time:units = "days since 1950-01-01 00:00:00 UTC" ;

int latitude(time) ;latitude:long_name = "Latitude of measurement" ;latitude:_FillValue = 2147483647 ;latitude:add_offset = 0. ;latitude:standard_name = "latitude" ;latitude:scale_factor = 1.e-06 ;latitude:units = "degrees_north" ;

short flag(time) ;flag:coordinates = "longitude latitude" ;flag:_FillValue = 3267s ;flag:long_name = "data origin" ;flag:flag_values = 1s, 0s ;flag:flag_meanings = "igdr_like non_igdr_like" ;flag:comment = "The origin of the data is determined by the types of geophysical datarecords (GDR) used in computation of the SLA: 1 for the Interim GDR (IGDR) and0 for Operational GDR (OGDR)." ;

short cycle(time) ;cycle:long_name = "Cycle the measurement belongs to" ;cycle:_FillValue = 32767s ;cycle:units = "1" ;cycle:coordinates = "longitude latitude" ;

// global attributes::history = "22-Feb-2011 15:52:43 CET : File created by the SSLATO/DUACS system version 10.0.1" ;:comment = "Produce from ENVISAT Extension Phase satellite altimetry data" ;:institution = "CLS/CNES" ;

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:references = "http://www.aviso.oceanobs.com/fileadmin/documents/data/tools/hdbk_duacs.pdf" ;:source = "satellite altimetry" ;:title = "along track Sea Level Anomaly" ;:Conventions = "CF-1.4" ;

}data:SLA =

7, 7, 11, 10, -2, -22, -39, -41, -30, -13, -3, -5, -16, -26, -32, -35,-35, -35, -37, -35, -31, -29, -31, -35, -32, -21, -10, -10, -19, -20, -1,...-69, -57, -49, -45, -39, -30, -28, -31, -27, -13, -2, -5, -15, -3, 11,12, 3, -7, -10, -14, -15, -22, -31, -37, -32, -17, -10 ;

longitude =74743328, 74868400, 74993968, 75120040, 75246616, 75373728,75501368, 75629568, 75758320, 75887656, 76017568, 76148080, 76279208,76388952, 76477056, 76565456, 76654144, 76743128, 76832400, 76921984,...329429568, 329535424, 329641792, 329748672, 329856096, 329964032,330072512, 330181536, 330291072, 330401184 ;

track =92, 92, 92, 92, 92, 92, 92, 92, 92, 92, 92, 92, 92, 92, 92, 92, 92,92, 92, 92, 92, 92, 92, 92, 92, 92, 92, 92, 92, 92, 92, 92, 92, 92, 92,...113, 113, 113, 113, 113, 113, 113, 113, 113, 113, 113, 113, 113, 113,113, 113, 113, 113, 113, 113, 113, 113, 113 ;

time =22278.0000565705, 22278.0001314719, 22278.0002063733,22278.0002812747, 22278.0003561761, 22278.0004310775, 22278.0005059788,22278.0005808802, 22278.0006557816, 22278.000730683, 22278.0008055844,22278.0008804858, 22278.0009553872, 22278.001017805, 22278.0010677393,...22278.8257112198, 22278.8257486705, 22278.8257861212, 22278.8258235719,22278.8258610226, 22278.8258984733, 22278.8259359239, 22278.8259733746,22278.8260108253, 22278.826048276 ;

mean_profile_point_index =1872, 1878, 1884, 1890, 1896, 1902, 1908, 1914,1920, 1926, 1932, 1938, 1944, 1949, 1953, 1957, 1961, 1965, 1969, 1973,...2306, 2309, 2312, 2315, 2318, 2321, 2327, 2330, 2333, 2336, 2339, 2342,2345, 2348, 2351, 2354, 2357, 2360, 2363, 2366 ;

flag =0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,...0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ;

latitude =-16225506, -16539731, -16853833, -17167810, -17481660, -17795380,-18108966, -18422418, -18735729, -19048899, -19361924, -19674801,-19987527, -20248015, -20456329, -20664569, -20872739, -21080837,...39481297, 39628277, 39775133, 39921866, 40068472, 40214953, 40361308,40507533, 40653629, 40799592, 40945423, 41091120, 41236684, 41382111 ;

cycle =331, 331, 331, 331, 331, 331, 331, 331, 331, 331, 331, 331, 331,

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...331, 331, 331, 331, 331, 331, 331, 331, 331, 331, 331, 331, 331, 331,331, 331, 331, 331, 331, 331, 331, 331, 331, 331, 331, 331 ;

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VIII. HOW TO DOWNLOAD A PRODUCT

VIII.1. Download a product through the MyOcean Web Portal DirectgetfileService

You first need to register. Please find below the registration steps:http://www.myocean.eu/web/34-products-and-services-faq.php#1Once registered, the MyOcean FAQ http://www.myocean.eu/web/34-products-and-services-faqwill guide you on How to download a product through the MyOcean Web Portal Directgetfile Service.

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http://www.myocean.eu/web/34-products-and-services-faq



IX. NEWS AND UPDATES

IX.1. [Duacs] Operational news

To be kept informed on events occurring on the satellites and on the eventual interruption of the services of the DUACSprocessing system, see the [Duacs] operational news on the Aviso website:http://www.aviso.oceanobs.com/en/data/operational-news/index.html.

IX.2. Updates

To have the information of the DUACS changes, improvements and updates of the system, please refer to:http://www.aviso.oceanobs.com/en/data/product-information/duacs/presentation/updates/index.html.Since 2010, a complete reprocessing of all altimetry data (cumulated total of about 55 years of data) is available. Themain changes introduced in the Duacs DT v3.0.0 reprocessed data set in "SSALTO/DUACS reprocessed DT data set"are listed here:http://www.aviso.oceanobs.com/fileadmin/documents/data/duacs/duacs_DT_2010_reprocessing_impact.pdf

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http://www.aviso.oceanobs.com/fileadmin/documents/data/duacs/duacs_DT_2010_reprocessing_impact.pdf

http://www.aviso.oceanobs.com/fileadmin/documents/data/duacs/duacs_DT_2010_reprocessing_impact.pdf

product user manual for sea level sla products...

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