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Some Remaining Issues in Satellite Altimetry

Some Remaining Issues in Satellite Altimetry. Lee-Lueng Fu Jet Propulsion Laboratory. NASA Sea Level Workshop November 2-4, 2009, Austin, Texas. Present-Day Sea Level Change. Nerem, 2009. Global mean sea level spectrum. 60-day. Period Power 65.0745 85.1839

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Some Remaining Issues in Satellite Altimetry

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  1. Some Remaining Issues in Satellite Altimetry Lee-Lueng Fu Jet Propulsion Laboratory NASA Sea Level Workshop November 2-4, 2009, Austin, Texas

  2. Present-Day Sea Level Change Nerem, 2009

  3. Global mean sea level spectrum 60-day PeriodPower 65.0745 85.1839 64.3895 76.6947 63.7187 179.475 63.0619 115.847 62.4184 171.108 61.7879 75.2514 61.1700 412.935 60.5644 192.179 59.9706 398.861 59.3883 4128.00 58.8173 2384.98 58.2571 3428.01 57.7075 974.379 57.1682 447.638 56.6389 1183.94 56.1193 154.271 55.6091 143.652 Power density (mm^2/cyc/day) Frequency (cyc/day) 60-day Power density (mm^2/cyc/day) Period(days)

  4. Spectral peak at 58.7 days

  5. Comparison to S2 tide error estimate From R. Ray

  6. 58.7 day period

  7. Iono correction at 58.7 day period

  8. Global mean wet tropo correction Power density (mm^2/cyc/day) Period (days)

  9. Wet tropo correction at 58.7 day period

  10. Systematic errors in sea level measurement Jason-1 GRACE +ARGO (Willis et al, 2008) A slightly different analysis Leuliette and Miller (2009)

  11. Consistent with other measurements? Sea level trend estimated from an ocean model constrained by 2.1 billions observations. The altimetry global mean was not used. Global mean: 1.6 mm/yr Estimated errors of the sea level trend. (Wunsch et al, 2007)

  12. How realistic is the current uncertainty estimate ? Finally the total error budget of GMSL is : • 0.6 mm/yr in a confidence interval of 90% Ablain et al, 2009

  13. Measurement stability • OSTM/Jason-2 Level One Requirements • Maintain the stability of the global mean sea level measurement with a drift less than 1 mm/year over the life of the mission. • This requirement was not strictly propagated to the subsystems • The radiometer subsystem (AMR) took the entire 1 mm/year as its stability requirement and allocated it to the ground system.

  14. Radiometer Long Term Calibration Issues Not calibrated by on-board sources TA Radiometer Receiver Front-end path loss correction required Emission from Earth Calibration Source • mm-level long term stability is a demanding requirement for the radiometer • < 0.1 K brightness temperature stability • Radiometers on Topex, Jason-1 and Jason-2 use internal calibration technique • Disadvantage: Do not view calibration sources through same path as Earth scene • Vulnerable to calibration instability from hardware changes requiring periodic post-launch re-calibration S. Brown/JPL

  15. Observed TMR and JMR PD Instability Observed instability significant compared to sea level rise signal • JMR exhibited two jumps of about 5 mm then an additional 8 mm • 6mm/year when treated as drift JMR GDR-A TMR drifted at a rate of about 1 mm/year over the first 6 years of the mission S. Brown/JPL Brown et al. MicroRad08

  16. Recalibrated JMR JMR tuned to on-Earth brightness temperature references for GDR-B Eliminates large jumps in PD record GDR-A GDR-B S. Brown/JPL

  17. Limitations of On-orbit Recalibration On-orbit references sensitive to climate variability; require corrections; risk of aliasing geophysical signals Need to acquire sufficient data to reach mm-level 30+ days of data required to reach 2-4 mm level Validation of recalibrated product at mm/yr level against other models/sensors challenging Uncertainty near + 1mm/yr level, decreasing with record length. Observed TMR 18.0 GHz Amazon TBs TB Reference Regions in Amazon S. Brown/JPL

  18. Radiometer Long Term Calibration • Option for Jason-3: Eliminate reliance on periodic on-orbit recalibration by supplementing internal calibration system with external calibration system • On-board blackbody and cold-sky reflector calibration targets can be added to existing radiometer design • Periodic observations of on-board external calibration targets used to maintain the long term stability (e.g. once per pass or cycle over land) • Calibration is traceable to known physical quantities that are independent of the climate system and other sensors or models • External calibration approach is well established and used scanning Earth observing radiometers • MSU, AMSU, SSM/I, TMI on TRMM, WindSat, AMSR-E, SSMIS • Combination internal/external calibration approach has the potential to produce a long term calibration stability that exceeds that of each system individually • expect sub-mm/year inherent stability from such as system • 0.01 K long term TB stability estimated for MSU (Spencer et al., 1990): ~0.1mm/yr S. Brown/JPL

  19. Altimeter Bias Harvest SSH Calibration Time Series JASON-2 — JASON-1 DSSH BIAS: FROM COMMON OVERFLIGHTS: +80 ± 4 mm (N = 16, s = 16 mm, R = .76) FROM GLOBAL ANALYSIS: +77 ± 1 mm (N = 19, s = 2 mm) June 22, 2009 OSTST Meeting Harvest site, Haines et al.

  20. Assessment of Jason-1 and OSTM Global Verification Phase Sea Surface Height Collinear Residuals, Beckley et al.

  21. Tide gauge calibration has order 1 cm uncertainty? Relative biases from tide gauge calibration (Leuliette et al) • The cross-calibration between T/P and Jason-1 has been a moving target, changing from 144 mm to 100 mm depending on the SSB corrections. • Has this issue been resolved?

  22. Jason‑2 range bias per Cycle (with respect to Jason-1) during the cross-calibration phase • I was told that the Jason-2/Jason-1 cross-calibration was not a function of wave height, suggesting the original Jason-1 instrument algorithm problem had been solved. Dettmering et al.

  23. Intermission Differences Jason1 – T/P Jason2 –Jason1 Without SSB With SSB Beckley, 2009

  24. Some Remarks • 60 day error/signal • Measurement stability requirement and allocation to subsystems • Radiometer calibration • Altimeter bias • Importance of a cross-calibration between successive missions • Importance of maintaining a dedicated science team

  25. Backup Brown et al. OSTST Seattle

  26. 10 11 12 13 14 15 16 17 18 19 20 22 Saral/AltiKaFr./India HY-2B China HY-2AChina Sentinel-3A Europe CRYOSAT-2 Europe Sentinel-3B Europe SWOT France/USA Jason-2 Europe/USA Altimetry Missions Launch Date 08 09 21 Reference Missions - Higher accuracy/Medium Inclination 12/01 Jason -1 Fr./USA Jason-3 Europe/USA Jason-CS/Jason-4 Europe/USA Complementary Missions - Medium accuracy/Higher inclination 3/02 Sentinel -3C/D ENVISATEurope 2/98 GFOUSA GFO-FO USA Broad-Coverage Mission In orbit Approved Proposed Needed Wilson et al.

  27. External Calibration Hot Target Calibrate Stow Cold sky reflector

  28. AMR PD Stability Assessment Comparisons between AMR and model and other radiometers 1 mm AMR-ECMWF AMR-AMSRE AMR-TMI No conclusive evidence of long term PD instability or drift

  29. Comparison to M2 tide error estimate

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