15 years of improvements in altimetry performance
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15 years of improvements in altimetry performance. J. Dorandeu and all the CLS CalVal team. Outline. Introduction To what extent altimeter performances have improved? Which are the main fields of improvement? Why and how it could be possible? Present and future conclusions.

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15 years of improvements in altimetry performance

15 years of improvements in altimetry performance

J. Dorandeu

and all the CLS CalVal team


Outline

Outline

  • Introduction

  • To what extent altimeter performances have improved?

  • Which are the main fields of improvement?

  • Why and how it could be possible?

  • Present and future

  • conclusions


Introduction 1 2

Sensor measurement

Orbit precision

System performance

Users, Scientists

Geophysical corrections

Reference surfaces

CalVal Loop

Introduction (1/2)

  • Which performance do we deal with?

    • Altimetry performance on ocean surfaces

    • Consider the altimeter system as a whole

      • Because users are interested in the overallerror budget for their particular application

      • Because each component is interacting with each other through the overall system performance


Introduction 2 2

Menard and Haines, 2001

Introduction (2/2)

  • Errors impacting the altimeter system performance

    • Random errors

    • Systematic errors (or low frequency signals)

  • Metrics:

    • Sea Surface Height variance (crossovers, collinear differences)

    • Comparison to external data (e.g. tide gauges)

  • Data:

    • From ERS-1 – TOPEX/Poseidon to Envisat – Jason-1

    • Most examples derived from T/P: from post-launch assessment to the more recent improvements

    • A “historical” perspective: 1992 – 2006: T/P provides the longest series of continuous altimeter measurement

    • Tremendous number of studies for data quality and improvement (SWT-OSTST)

    • Other missions (ERS, Envisat, GFO or Jason-1) also provide significant examples of impressive improvements


Main improvement steps in t p data performance

Main improvement steps in T/P data performance

Jason-1 launch:

Orbit: JGM-3 ITRF2000

Tide: GOT99

SSB: NP Labroue

IB (varying Pref)

MGDR-C:

Orbit: JGM-3

Tide: CSR3.0

SSB: BM4 Gaspar

IB

MGDR-B: after the in-flight calibration:

Orbit: JGM-2

Tide: Cartwright & Ray

SSB: BM3: Walsh

IB

  • 4 major improvement steps identified along the T/P mission lifetime

    • 1rst step: after the in-flight verification: M-GDR-B

    • 2nd step: 1rst reprocessing of T/P data: M-GDR-C

    • 3rd step: Jason-1 launch. New algorithms and corrections

    • 4th step: 2006: ready to reprocess Jason-1, ENVISAT and T/P?

- 30%

- 27%

Variance reduction

- 29%

Last updates:

Orbit: JGM-3 ITRF2000

Tide: GOT00

SSB: NP Labroue

MOG2D HF. Corr.


Main improvement steps in t p data performance1

Main improvement steps in T/P data performance

1992-1993

1995-1996

2001-2002

2005-2006


Significant examples of improvements 1 2

Mitchum,1998

MSL differences with T/P M-GDR CNES and NASA orbits

Significant examples of improvements (1/2)

  • Improvements in orbit calculation:

    • reference frames and new standards

    • gravity fields: JGM-2, JGM-3, GRACE models

  • Improvements in sensor processing:

    • Altimeter

      • Precise monitoring of altimeter measurement:

        • T/P USO drift detection

        • Wallops calibrations

        • TOPEX/Poseidon relative bias, TOPEX-A / TOPEX-B relative bias

      • Sea State Bias models: Walsh et al., Fu & Glazman, BM4 Gaspar et al., Non Parametric Labroue et al.

    • Radiometer:

      • Drift corrections: Ruf et al., Scharoo et al.

      • Detection of yaw effects


Significant examples of improvements 2 2

TMR instead of ECMWF wet troposhere correction

Global improvement ~10% of variance

Significant examples of improvements (2/2)

  • Improvements in geophysical corrections

    • Ocean tides: Cartwright & Ray, CSR3.0/FES95, GOT99/FES2000, GOT00/FES2004

    • Atmospheric corrections:

      • Improvements in ECMWF model: IB and dry troposhere correction, wet troposphere correction

      • Time varying reference pressure for IB correction

      • Using MOG2D barotropic model for resolving High Frequency aliasing

    • Handling S1/S2 non gravitational tides

  • Improvements in reference surfaces

    • Mean Sea Surfaces: OSU91, OSU95, GSFC, CLS01

    • Geoids: not yet directly used for altimeter SSH, but large improvements for orbit accuracy and Mean Dynamic Topography


Present recent changes new gdrs 2006

Improvement provided by new Jason-1 retracking (MLE4)

Global variance reduction: 36%

Present, recent changes: new GDRs (2006)

  • Important changes in Jason-1 and ENVISAT new GDRs:

    • GRACE gravity field used in orbit calculation

    • High Frequency correction from MOG2D

    • New retracking algorithm (MLE4 – Jason-1 only)

  • Impact in terms of altimeter data performances:

    • Large improvement in geographically correlated errors (orbit)

    • Large improvement : 36% in terms of variance reduction

      • 50% from (orbit + altimeter measurement)

      • 50% from geophysical corrections

  • Improvements are still possible in 2006!

  • The geophysical domain deserves further attention


The virtuous circle of improvements

The virtuous circle of improvements

  • Typical example: aliasing of High Frequency signals in altimetry:

    • Problem raised by, among others, Stammer et al. (1999), Tierney at al. (2000), Hirose et al. (2001), Carrere and Lyard (2003)

    • SWT recommendation: test barotropic models: PPHA, MOG2D

    • CalVal tests for model corrections

    • SWT recommendation: MOG2DHF + IBBF

    • MOG2DHF + IBBF implementation in Jason/ENVISAT ground segment

    • Performance assessment

    • Progress in ocean circulation studies

    • New user needs

Quality forum

T/P Jason OSTST

ENVISAT QWG


Present and future of altimetry multi mission altimetry

Pascual et al.

Present and future of altimetry: multi-mission altimetry

  • From TOPEX/Poseidon and ERS-1, combined datasets have proved to be necessary

    • complementary time / space sampling for:

      • Low frequency signals and orbit accuracy: at least one precise mission needed (T/P, Jason, Envisat)

      • Mesoscale: ERS-1/2, Envisat provide space sampling

    • Need precise cross-calibration and combination

      • Example of Jason verification phase:

        • Detection of geographically correlated signals: orbit errors, SSB

        • Detection of yaw effects on both TMR and JMR

      • Efficient combination procedures are needed:

        • Developed for instance in the SSALTO/DUACS system: NRT and off-line

        • Long wave length error reduction and mapping techniques

        • Operational since 1998

        • Continuous developments and improvements

    • Year 2003: 5 altimeters flying together! But what will happen in the next 4 years?

    • Any chance to get ERS-2 in the system again?


Conclusion

Conclusion

  • Through the past 15-year period, altimetry has evolved “from a small academic discipline into a large community dealing with numerous issues of societal interest in addition to the scientific problems” (Carl Wunsh, this morning)

    • Many changes and improvements made it possible

    • All system actors and components have played a key role

      • Scientists, users, space agencies, engineers, …

      • Instrument, orbit, geophysical corrections, CalVal, …

    • Lessons learned:

      • To gather all people, all skills, all needs in a same quality forum is still the key of success

      • To keep scientists and users with us: even if things have become more operational and trustworthy, we need them to push

    • There are still so many fields of improvement: new applications (regional, coastal, …) will require higher resolution, higher frequency

    • We are at the beginning!


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