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

Power density (mm^2/cyc/day) Global mean sea level spectrum Frequency (cyc/day) Period(days) Power density (mm^2/cyc/day) 60-day Period Power day

Spectral peak at 58.7 days

Comparison to S2 tide error estimate From R. Ray

58.7 day period

Iono correction at 58.7 day period

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

Wet tropo correction at 58.7 day period

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

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) Consistent with other measurements?

Source of error for the MSL calculation MSL trend uncertainties from 1993 to 2009 MinimaMaxima Orbit : Cnes POE (GDR B) for Jason-1 and GSFC (ITRF2000) for T/P mm/yr0.15 mm/yr Radiometer Wet troposphere correction: JMR (GDR B) & TMR (with drift correction) mm/yr0.30 mm/yr Dynamical atmospheric and dry troposphere corrections using ECMWF pressure fields mm/yr0.10 mm/yr Sigma0 drift impacting altimeter wind speed and sea state bias correction 0.05 mm/yr0.10 mm/yr Bias uncertainty to link TP A / TP B, TOPEX and Jason-1, Jason-1 and Jason mm/yr0.25 mm/yr Finally the total error budget of GMSL is : 0.6 mm/yr in a confidence interval of 90% Ablain et al, 2009 How realistic is the current uncertainty estimate ?

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.

Radiometer Long Term Calibration Issues 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 Radiometer Receiver Calibration Source Emission from Earth Front-end path loss correction required TATA Not calibrated by on-board sources S. Brown/JPL

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

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

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

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

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

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

Relative biases from tide gauge calibration (Leuliette et al) Tide gauge calibration has order 1 cm uncertainty? 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?

Jason ‑ 2 range bias per Cycle (with respect to Jason-1) during the cross-calibration phase Dettmering et al. 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.

Intermission Differences Jason1 – T/PJason2 –Jason1 Without SSB With SSB Beckley, 2009

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

Backup Brown et al. OSTST Seattle

CRYOSAT-2 Europe Complementary Missions - Medium accuracy/Higher inclination Broad-Coverage Mission SWOT France/USA Saral/AltiKa Fr./India HY-2B China HY-2A China GFO-FO USA Reference Missions - Higher accuracy/Medium Inclination Jason-2 Europe/USA Jason-CS/Jason-4 Europe/USA Launch Date ProposedIn orbitNeededApproved 12/01 2/98 GFO USA Sentinel-3B Europe Sentinel-3A Europe 3/02 Jason -1 Fr./USA ENVISAT Europe Sentinel -3C/D Jason-3 Europe/USA Altimetry Missions Wilson et al.

External Calibration Stow Calibrate Hot Target Cold sky reflector

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

Comparison to M2 tide error estimate