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20 Years of Improvements to GEOSAT Altimetry

20 Years of Improvements to GEOSAT Altimetry. John Lillibridge 1 , Walter H.F. Smith 1 , David Sandwell 2 , Remko Scharroo 3 , Frank Lemoine 4 , and Nikita Zelensky 5 1 NOAA Laboratory for Satellite Altimetry, Silver Spring, MD 2 University of California San Diego, La Jolla, CA

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20 Years of Improvements to GEOSAT Altimetry

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  1. 20 Years of Improvements to GEOSAT Altimetry John Lillibridge1, Walter H.F. Smith1, David Sandwell2, Remko Scharroo3, Frank Lemoine4, and Nikita Zelensky5 1 NOAA Laboratory for Satellite Altimetry, Silver Spring, MD 2 University of California San Diego, La Jolla, CA 3 Altimetrics, LLC, Cornish, NH 4 NASA Space Geodesy Branch, Greenbelt, MD 5 SGT Inc., Greenbelt, MD

  2. A Brief History of Geosat • U.S. Navy mission operated by Johns Hopkins/APL • Geodetic Mission: 31-Mar-1985 to 30-Sep-1986 • Data remained classified until 28-Jun-1995 • Exact Repeat Mission: 8-Nov-1986 to 30-Dec-1989 • Onboard tape-recorders failed (and batteries not long after) • Single-ƒ Ku-band altimeter; no radiometer • Doppler Tranet tracking - declassified in 1993 • APL Ground segment created original SDR & WDR tapes • WDRs were never classified & used by NASA for ice sheet studies • SDRs and WDRs ‘separated at birth’, until now….

  3. 3 Generations of NOAA Geosat GDRs • Original GDRs: late 1980s • NSWC(GM) & NAG(ERM) orbits • FNOC model wet/dry tropo • Klobuchar iono model • Schwiderski tide model • T2 GDRs: 1991 • GEM-T2 orbits (ERM only) • TOVS/SSMI wet tropo • ECMWF model dry tropo • Same tide & iono models • JGM-3: 1997 • GM + ERM • JGM-3 orbits (TRANET) • NCEP Reanalysis model wet/dry • IRI95 iono model • CSR 3.0 tide models • SSB model from CLS • 5.0 millisec timing bias 1987: NAG / NSWC 1991: GEM-T2 1997: JGM-3

  4. 20th Anniversary Reprocessing - Motivation Global Sea Level Rise: • Extend altimetric time series from 1992 back to 1985 • Requires best corrections and improved precision orbit determination • Tie Geosat time series to modern altimetry via tide gauge network Marine Gravity & Bathymetry: • Enhance spatial resolution of geophysical fields • Reduce noise in sea surface slope by retracking ocean waveforms • Reduce correlation between SWH and range from onboard tracker • Eliminate along-track ‘phase shift’ from /tracker

  5. 20th Anniversary Data Set Improvements Reprocess original SDRs & WDRs from scratch • Reunite SDRs & WDRs using original frame-counts • 10 Hz ranges have mm resolution in SDRs vs. cm in GDRs • SDRs have 10 Hz range, SWH and AGC • GDRs have 10 Hz range but only 1 Hz SWH & AGC Improve precision time-tagging • ‘Bounce Time’ delay based on true vs. nominal range • Smoothed version of on-board-clock/UTC correlation Enhanced instrumental & geophysical corrections • Height & USO corrections for thermal effects & clock drift • Improved Doppler height correction • New TOVS wet troposphere • Hybrid SSB model based on retracked data Improved Orbits based on Grace gravity model • GGM02C Gravity Model and retracked crossovers • Doppler station coordinates from post EGM96 gravity & station coordinate solution • Improved models utilized in POD Two-Step Waveform retracking • Initial 5-parameter fit: range, SWH, attitude, power, noise • Second fit of range only after smoothing other 4 parameters

  6. SDR Tape Recovery & WDR Merge • All GM & ERM 9-track SDR tapes stored at NOAA/NODC • Original WDR data received from NASA/GSFC archive • 16 unreadable SDR tapes recovered by forensics expert: < 3 minutes of data lost from GM • 14 half-daily WDRs lost from GM • ERM 9-tracks have NOT been read

  7. Doppler correction from orbit too smooth • Extra component from MSS gradient Doppler Correction from Orbit - MSS d(Range)/dt = d(Orbit-SSH)/dt ~ d(Orbit)/dt - d(MSS)/dt Improved Doppler Correction - Mean Sea Surface • Doppler correction from range too noisy

  8. TOVSWet UTC Time AM PM TOVS Pathfinder Wet Troposphere Correction • NASA Pathfinder Dataset • 1° spatial resolution • Separate ascending/descending • Interpolate using time + wet grids

  9. total precipitable water (mm) total precipitable water (mm) Spatial Temporal Autocorrelation Functions New TOVS vs. NCEP Wet • Interpolate AM/PM TOVS data using Gaussian filtering with time & space scales of 1/2 day and 2° • NCEP Reanalysis model wet still provides better synoptic spatial resolution • TOVS provides alternative ‘measured only’ wet tropo values

  10. # Points Direct SSB Parametric SSB - BM4: a0+SWH*(a1+a2*SWH+a3*U+a4*U*U) Merged Hybrid SSB Residuals Retracked Hybrid Sea State Bias Model

  11. JGM3-GDR Orbit Improvement

  12. Orbit Determination Data Modeling Doppler Data: 45 Tranet & Opnet Doppler Tracking stations. (See Haines et al., JGR, 1990; Haines et al., JGR, 1994 for a detailed description of these data). Altimeter Data: • Retracked GEOSAT altimetry from NOAA. • Ionosphere: IRI95. • Troposphere (wet & dry) from NCEP Reanalysis. • SSB: ~4% SWH. • Crossovers edited for bathymetry (< 500 m); regions of high variability (> 20 cm); outliers (> 50 cm). • Only crossovers internal to an arc (< 6 days) are used.

  13. Avg. = 7.356 cm Geosat GM Orbit Statistics (GGM02C) • March 30, 1985 - October 1, 1986 • 110 six-day arcs • Overlap of one day between arcs • Average number of observations and RMS over all data arcs • Orbit error estimated at 5 cm • Doppler-only orbits have independent crossover fits of ~9.2 cm with the GGM02C era updated modeling (based on arcs from 850330 to 850514). • An altimeter timing bias is also estimated: Average ~0.4 msec, with periodic trends.

  14. JGM-3 GDRs GGM02C Retracked Reduction in Geographically Correlated Orbit Error

  15. Global Sea Level Rise from Altimetry

  16. Decay, k Amplitude, A Width, s Noise Level, N Arrival Time, t0 5-parameter model waveform

  17. 5 Two-step retracking strategy Monte Carlo experiments show that SSH error can be reduced if SWH error is suppressed. SWH spectrum shows noise floor around l ~= 100 km. Strategy: Assume SWH is smooth along track. Retrack each waveform, fitting all 5 parameters. Smooth s (SWH), (attitude), noise & amplitude along track. Retrack range again, constraining s to smoothed values.

  18. Before and after retracking Shown: Geosat in Equatorial Atlantic (high MSS signal, low SWH) Signal Noise Retracked noise power cut by 50%

  19. JGM-3 GDRs Retracked Sea Surface Slope Noise (18 km low-pass)

  20. Indian Ocean Triple Junction

  21. Improved SWH estimates from Retracking

  22. Retracked SDR AGC_SWH_ATT[FIB5](db) = 126.0 - 100*log10(amp) Instrumental 0 correction for Attitude/SWH • Onboard tracker’s AGC loop requires instrumental correction for waveform shape • Retracked amplitude parameter’s variations mimic AGC correction • BUT, scaling of 100 vs. 10log10 not yet understood…

  23. Conclusions • Geosat first long altimetry mission - played important role in geodesy, geophysics, oceanography & glaciology • The mission remains unique in terms of its spatial coverage/resolution & providing 1980’s era altimetry • Archiving of altimetric data sets is crucial, as future improvements cannot be fully anticipated: • Weather model reanalyses • Improved tidal and ionospheric models • Improved gravity models and precise orbit determination • New retracking methods and algorithms • Geosat 20th Anniversary Data Set • 5 cm orbits based on GRACE gravity model • 10 Hz dataset with improved corrections • Retracked SSH, SWH from 5-parameter & 1-parameter fits

  24. Geosat Waveforms - The Movie • 12-hour segment (tracking - ocean/land/ice) • Leading edge at bottom • Each frame = 1 minute • Colors represent waveform power in 63 bins

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