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Satellite Gravity: GRACE & GOCE

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Satellite Gravity: GRACE & GOCE Satellite Gravity: GRACE & GOCE Srinivas Bettadpur, Associate Professor, Dept of Aerospace Engineering & Engineering Mechanics and Center for Space Research University of Texas at Austin Airborne Gravimetry for Geodesy – Summer School Silver Springs, MD, USA (May 23-27, 2016) Presentation Viewpoint • Global gravity field models derived from satellite data serve as a long-wavelength reference, to support the interpretation of in situ data. – While this may not be literally exact, it serves as basis for the flow of this presentation • Therefore, I choose to classify the audience engagement with satellite gravity data into three levels: – Level-3: Start with “satellite-only” fields – resolution ≈ 300-100 km – Level-2: Start with Inter-technique data fusion • GOCE, GRACE, GRACE-FO, GNSS-tracking of low Earth orbiters, etc – Level-1: Process mission datasets at “lower” levels (metrology) Level-3 Use of Global Satellite Models Level-3: Many global models available… Level-3: Broad User Guidelines… • Many global models are available. • All models provide spherical harmonic coefficients – Nmax ranges from 180 to 280 – Data span ranges from 3 years to 12 years – Infinite variety of analyst noise • Spatio-Temporal Error Characteristics – Low Degrees – Generally very well determined – Mid Degrees – Strongly influenced by analyst choices in data fusion – High Degrees – Recognizable/unique error characteristics • A typical Level-3 User will, therefore, put in most effort in the recognition of (and accounting for) these unique error patterns. The GGM05 Model Suite • GGM05S – A “GRACE-only” model • Outcome of CSR_RL05 monthly time-variable gravity models • Unconstrained estimates to d/o 180 • GGM05G – A “GRACE+GOCE” model • Coefficients of a “smooth” (EGM08-like) field adjusted using band-pass filtered (10-50 mHz) GOCE data (XX+XZ+YY+ZZ). • Polar gap filled with synthetic gradients derived from 150x150 GGM05S at 200-km altitude • Added GGM05S after extensive experimentation with relative weights of GRACE and GOCE – trading long-wavelength benefits relative to short wavelength artifacts • GGM05C – A GRACE + GOCE + DTU13 GGM05S Ten-year combination of GRACE monthly estimates (March 2003 to April 2013) Gravity anomalies from GGM05S to degree/order 180 (100 km smoothing) GGM05G Combine GGM05S with GOCE + polar gap fill from GGM05S Gravity anomalies from GGM05G to degree/order 240 (50 km smoothing) Variations Relative to EGM08 (Pavlis et al. 2012) Image on left shows, in addition to the land gravity corrections: 1. Corrections to potential MDT built into EGM08 (evident in Southern Oceans) 2. GRACE-related artifacts (≈2-4 cm) evident over mid-Pacific 3. Near coastal artifacts (≈10 cm) arise likely from the transition between three datasets across the coasts in EGM2008 Smoothed Surface Gravity Test Statistics No solution is best everywhere, though all show improvement in areas where no gravity data was available for EGM2008 GRAV-D data comparisons show little discrimination between models Level-3 User would carry out further analysis at spatial scales of typical interest to this audience GOCO05S MDT residuals before spectral filtering (no smoothing). Color scale runs ± 35 cm GGM05G GOCO05S MDT residuals after spectral filtering (no smoothing). Color scale runs ± 35 cm GGM05G No need any longer to use GRACE-only models for this purpose Level-3: A Way Forward • Choose one (or many) candidate global field(s) – Use fields derived using both GRACE and GOCE datasets. – No reason any more to use any (current-day) GRACE-only fields • (High-resolution time-variable signals due to ice-loss are “few-mm”) • For the local region of interest, empirically build error covariance – By inspection – Upon comparison with in situ data • Use, thereafter, the global models with your own error estimates Level-2: Build your own ‘satellite-only’ field Ingredients Needed: Estimates and covariance matrices for individual datasets from each satellite gravity mission And then on to the concerns of Level-3 user… Level-2: GRACE Variations • Variable Data Quality: – 2003-2010: Flight platform was most stable; relatively constant altitude. • Exclude certain durations with very poor ground-track coverage – Post-2010: Strike a balance between poor environmental control and lower altitude (higher noise at wavelengths shorter than ≈ 500 km, compared to earlier in mission). • Formal covariance certainly does NOT reflect true errors in the mean field harmonics – For tuning GGM05G errors, a very “engineering” approach was adopted – We think we have a way to do this is a formally correct way in next Release-06 (due Spring/Summer 2017) • Stray issues – Choice of the degree-2 harmonics should be solveable Level-2: GOCE Variations • Variable Data Sensitivity – GOCE mission identifies spans with variable extent of instrument calibration and with lowering altitude • Consider treating each component of the gravity gradient independently, for its regional information contribution • Handle polar gap “carefully” – This may scare the space-geodesists more than it does the physical geodesists Level-2 Work: Covariance Tuning & Relative Weighting Level-1: To the basics… Level-1: Checklist • Global or Local Solutions? Go for global solutions • Do I need a supercomputer? Wouldn’t hurt, as it allows for rapid parameteric experiments – Each processing by itself is not too onerous computationally • Differential Corrections with Variational Equations and a spherical harmonic model will work. – All roads lead to the same place with unconstrained solutions – “striations” with GRACE, and “orange-peel” with GRACE+GOCE – Regularization or stabilization are the only meaningful game-changers in this domain, for purposes of needs of this audience. • Specialized software is needed, and is considerable effort to assemble. – Some knowledge of aerospace systems will be needed, as well. – Data screening requires a LOT of effort Oct 2004 Year:2004 DOY:304 From:65000 sec To:69000 sec Editing Orbits kbr residuals Before Extra Editing Post-fir kbr residuals Starting Early in the Space Age • The Pear-Shape of the Earth (1959) was estimated from studies of the orbits of Vanguard-1 satellites. From: O’Keefe, Eckels & Squires, Science, New Series, Vol. 129, No. 3348 (Feb 27, 1959) pp 565-566 Over the next four decades, a wide variety of techniques of observing orbital motion of near-Earth satellites were used to determine and analyze the variations in Earth’s gravity field: Optical Measurements Radar and Radio Ranging Satellite Laser Ranging Global Positioning System Radar Altimetry Status Just Before GRACE • 30+ years of analysis of terrestrial tracking data: – Hemispheric scale estimates, used as validation/constraints on climate models – Formulation: Short-arc, Precision Orbit Determination, with numerical adjustment of model parameters Figure 1, Cheng & Tapley (JGR v 109, Sep 2004) GIA + Atmosphere + Hydrology + Glaciers + … 20 years later 500 km 89° 5+2+2 years Rockot (via DLR) 3 x 1.3 x 0.9 m Otherwise “perfect” record No panel 440 kg GRM Gradio Aristoteles 0.2 µ/s (& better) < 1 cm SLR/GPS TIDES GAMES …. GRACE Measurement Concept Gravity Observations & Orbits Potential Observations of satellite motion Altimetry and analysis of perturbations (Seasat to OSTM) U (Lageos 1/2, and (TDRSS, Doris) other geodetic sats) Doppler SLR/GPS Acceleration Velocity Position Gravimetry g = ∇ U ∫ g ∫∫ g Earth’s gravity field variation spectrum ranges from sub-diurnal to millenial time-scales, and is visible at all spatial (local to global) wavelengths. Variations are caused by external (luni-solar tides) and internal (GOCE) (oceans, atmosphere, ice, elastic Earth) influences, and can be regular Gradiometry Gradients (tides), irregular (climate), or episodic (earthquakes). G = ∇ g Measurement of higher derivatives of gravity provides better determination of small spatial-scale features. GRACE: Mission Concept GPS Satellites Nominal separation Ground-based GPS Receiver Mass anomaly (fixed or moving) GRACE: Mission Concept GPS Satellites Leading satellite - approaching the anomaly - feels Ground-based a greater gravitational attraction: GPS Receiver Separation Increases Mass anomaly GRACE: Mission Concept GPS Satellites Trailing satellite - also approaching the mass Ground-based anomaly - accelerates and catches up: GPS Receiver Decreasing Separation Mass anomaly GRACE: Mission Concept GPS Satellites Leading satellite is far from the anomaly, and is Ground-based not affected by it; while the trailing satellite - having just GPS Receiver passed the anomaly - is being tugged backwards: Increasing Separation. Mass anomaly GRACE: Mission Concept GPS Satellites Trailing satellite catches back up with leading satellite but the ‘signature’ of mass ‘lump’ has been observed in K-band range data Ground-based GPS Receiver Mass anomaly KBR Signal Content Full KBR Range - Bias Cubic Spline Residual (30 second knots) Topography Along Groundtrack (glk/jpl) GRACE Mission Science Goals High resolution, mean & time variable gravity field mapping for Earth System Science applications. Mission Systems Instruments • HAIRS (JPL/SSL/APL) • SuperSTAR (ONERA) • Star Cameras (DTU) • GPS Receiver (JPL) Satellite (JPL/Astrium) Launcher (DLR/Eurockot) Operations (DLR/GSOC) Science (CSR/JPL/GFZ) Orbit Launched: March 17, 2002 Initial Altitude: 500 km Inclination: 89 deg Eccentricity: ~0.001 Separation Distance: ~220 km Lifetime: 5 years Non-Repeat Ground Track, Earth Pointed, 3-Axis Stable The Satellites Orders of Magnitude Virtually all of this signal is due to the
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