The GOCE User Toolbox

Transcription

1 The GOCE User Toolbox Jérôme Benveniste - ESA Earth Observation Science and Applications Department Per Knudsen - Danish National Space Center and the GUT TEAM 37th COSPAR Scientific Assembly 2008, Montreal - A21

2 Abstract (in hidden slide - for furter reference) The Gravity and Ocean Circulation Experiment - GOCE satellite mission is a new type of Earthobservation satellite that will measure the Earth gravity and geoid with unprecedented accuracy.combining GOCE geoid models with satellite altimetric observations of the sea surface heightsubstantial improvements in the modelling of the ocean circulation and transport are foreseen.no ocean circulation products are planned to be delivered as level-2 products as part of thegoce project so that a strong need exists, for oceanographers, to further process the GOCElevel-2 geoid and merge it with Radar Altimetry. The primary requirement of oceanographersis to have access to a geoid and its error covariance at the highest spatial resolution andaccuracy possible, although required resolution depends on application. For effective use of thegeoid data, knowledge of the error covariance is mandatory. Within the ESA supported GUTSproject, the user requirements for GOCE User Toolbox associated with geodetic, oceanographicand solid earth applications are consolidated. The Toolbox functionality and results will bepresented.

3 GOCE: ESA s Gravity Mission The Gravity field and steady-state Ocean Circulation Explorer (GOCE) Its objectives are to improve understanding of: global ocean circulation and transfer of heat physics of the Earth s interior (lithosphere & mantle) topographic processes, evolution of ice sheets and sea level

4 ESA s Gravity Mission Scientific Applications Gravity field map and improved global geoid models Improved understanding of ocean circulation and energy distribution Global unification of height systems

5 ESA s Gravity Mission The satellite and its instruments Payload: Three-axis diagonal gravity gradiometer; sat-to to-sat tracking (geodetic quality multi-channel GPS receiver with Helix antenna)

6 Main Technical Challenge Highest sensitivity accelerometers in space CHAMP: 10-9 ms -2 GRACE: ms -2 GOCE: ms -2

7 GOCE User Products Final GOCE Earth gravity field model as spherical harmonic series including error estimates. Target: 1-2 cm / 1 mgal up to degree and order 200 corresponding to 100 km spatial resolution. Variance-covariance matrix of final GOCE Earth gravity field model Grids of geoid heights, gravity anomalies and geoid slopes computed from final GOCE Earth gravity field model including propagated error estimates Quality report for final GOCE gravity field model

8 Is there a need for a GOCE User Toolbox?

9 Need of a GOCE User Toolbox? Oceanographers wish to derive absolute dynamic topography (SSH-geoid) + errors A User Toolbox would help users attempt to compute dynamic topography geoid slopes error covariances anywhere A User Toolbox would help develop and validate new algorithms Absolute dynamic topography along radar altimeter tracks Gridded dynamic topography Gridded mean dynamic topography (MSS - geoid) At user required resolutions

10 Need of a User Toolbox! Clear and detailed recommendation from the to develop this toolbox See summary and recommendations in WS proceedings (SP-659)

11 User Toolbox Development Approach Two phase approach 1) Consolidate the User Requirements, write processing algorithms and input-output Specs, draft a Toolbox architecture -- by scientists 2) Detailed design, coding and testing -- by SW company + beta testing by scientists

12 GOCE User Toolbox Specification Study Objective No ocean circulation products are planned to be delivered as level-2 products as part of the GOCE project In order to facilitate the use of GOCE products for oceanographers and other communitites such as Solid Earth physicists, the development of a user toolbox was clearly recommended. The aim is to facilitate the using, viewing and postprocessing of GOCE Level 2 mission data products in conjunction with radar altimetry from ERS and ENVISAT.

13 First Phase: GOCE User Toolbox Specification 1. User Requirement Consolidation covering both Oceanography, SE and Geodesy 2. Processing Algorithm Scientific Trade Off Study 3. Toolbox System Documentation Produce a Toolbox output specification document. Produce an algorithm specification document. Produce a Toolbox architectural design document.

14 Task 1: User Requirement Consolidation Main requirements Computation of global gridded geoid heights at a given order/degree of spherical expansion (i.e. at a given spatial resolution). Computation of geoid heights at a given spatial resolution and a given point or list of points (unstructured grid, oceanographic transect) Option to replace geoid heights by geoid slopes Computation of full covariance matrix for a given order/degree of spherical expansion and/or within a given order/degree range Computation of geoid heights covariance for any couple of points on the sphere Computation of cumulated geoid height errors at a given spatial resolution (on a global regular grid or for a list of points) Option to include the omission errors

15 Task 1: User Requirement Consolidation Additional requirements regarding the computation of absolute dynamic topography Handling of external MSSH (ancillary data) Interpolation of MSSH on a given regular grid or at given points (unstructured grid) Spatial filtering of MSSH Change of reference system for the geoid and/or MSSH (reference ellipsoid, tide system) Computation of a GOCE MDT (MSSH-GOCE geoid) at a given resolution and on a given structured or unstructured grid Handling of altimetric data (altimetric heights, sea level anomalies) Computation of altimetric absolute dynamic topography Computation of altimetric absolute geostrophic velocities

16 Task 1: User Requirement Consolidation Additional requirements regarding the geoid validation Handling of ancillary data (in-situ data, external MDT (insitu/modelled), local geoids, combined CHAMP/GRACE/GOCE geoid ) Computation of differences / Root Mean Square differences / correlation coefficient / regression slopes between GOCE geoid and external geoids / GOCE MDT and external MDT / between absolute altimetric dynamic topography and in-situ absolute dynamic topography / between absolute altimetric geostrophic velocities and in situ geostrophic velocities

17 Task 2: Scientific Trade Off Study A trade off study was needed to select the best (accuracy, CPU time ) algorithms to compute the variables listed in the user requirement doc. Task: Carry out a scientific trade off study (in view of selecting the toolbox processing and viewing functions) 1) Basic processing functions (filtering functions like spherical harmonic expansion, interpolation functions) 2) Specific processing function (geoid height covariance computation, computation of the geoid height and MSSH in respect to a different reference ellipsoid / tide system, MDT - and corresponding geostrophic circulation- computation, ADT -and corresponding geostrophic circulation- computation) 3) Basic viewing functions (global/regional maps, geoid/mdt/mssh profile along a transect)

18 Task 3: Toolbox System Documentation Documentation needed to produce a toolbox:- 1) Input / output specification document, 2) an algorithm specification document 3) architectural design document mapping the required functionality and interfaces. 4) Provide a summary study report with scope of tutorial for future toolbox user These 4 documents are the input to the Toolbox implementation phase.

19 All documents available at : GUTS Task reports User Toolbox Requirements Document Using the Toolbox (leaflet) Algorithmic and trade-off study for the Generation of a GOCE User Toolbox Toolbox Functionality and Algorithm Specification Document System Specification and Architectural Design Summary and Tutorial Document Guts Final Report

20 GUT Implementation Next phase, Toolbox implementation, started in January 2008

21 GUT Primary WorkFlow

22 GUT WorkFlow Modules 1: Geoid and gravity field computation - synthesis (with errors) 2: Sea surface height and a-priori dynamic topography selection 3: Satellite Dynamic Topography computation (spatial & spectral) 4: Combined (Remove-Restore) Dynamic Topography computation (spatial & spectral filtering) 5: Dynamic Topography-derived quantities 6: Pre-viewing function

23 MDT=MSS-GOCE The GOCE User Toolbox Height difference between the TOPEX and the GRIM ellipsoids Reference frame issue Height difference between the TIDE FREE and the MEAN TIDE reference systems MDT=MSS-GOCE = Satellite-only MDT Scales down to km Centimetric accuracy Scales down to 100 km Centimetric accuracy Further filtering required Need for scales shorter than 100 km Combined MDT computation (remove restore technique) required

24 The GOCE User Toolbox Main workflow I- Handling of gravity data II- MDT computation (MSS-Geoid) III- CMDT computation (high resolution)

25 The GOCE User Toolbox I- Handling of gravity data Example: Computation of the geoid height on a grid a a given resolution Options: Output Grid : regular, ½ resolution grid Input Data: EIGEN-GL04S1 SH coefficients (reference ellipsoid=grim Tide system=free ) Output Reference ellipsoid: TP Output tide system: Mean Tide Degree/order of expansion: 40 Output m

26 The GOCE User Toolbox II- MDT computation Space domain - Workflow 1- Choice of input geoid and altimetric Mean Sea Surface 2- Computation of geoid and MSS on same grid and same reference frames (reference ellipsoid and tide system) 3- Choice of filter type and filter cut length

27 The GOCE User Toolbox II- MDT computation Example: Computation of a 400km resolution MDT using GRACE data Options: Filter Type: Gaussian Filter width=400 km Input Data: MSSCLS01 EIGEN GL04S GRACE Geoid Output cm + my_filter_matrix.fic

28 The GOCE User Toolbox II- MDT computation Spectral domain - Workflow 1- Computation of MSS coefficients 2- Computation of MDT coefficients 3- Filtering in spectral domain and back to the space domain

29 The GOCE User Toolbox II- MDT computation Spectral domain Example: Computation of a 400km resolution MDT using GRACE data GUT_WF3b Fj400 O my_filter_matrix.fic MSSCLS01 GGM02S_SH160_coef.fic MSSCLS01_EIGENGL4S_fj400_grid.fic MSSCLS01_coef.fic Input Data: - MSS CLS01 - GGM02S GRACE SH coefficients Output Options: -Filtertype: Jekeli - Filter width= 400 km cm + my_filter_matrix.fic + MSSCLS01_coef.fic

30 The GOCE User Toolbox III- CMDT computation Spatial domain - Workflow 1- Choice of a-priori MDT 2- Compute a-priori MDT and GUT MDTS on thesamegrid MDT computed previously 3- Extraction of the A-priori MDT short scales 4- Computation of high resolution MDT

31 The GOCE User Toolbox III- CMDT computation Spatial domain Example: Computation of a combined MDT in the Gulfstream area GUT_WF4a Niiler_MDT.fic R /-29.75/30.25/49.75 I 0.5/0.5 MSSCLS01_EIGENGL04S_fg400_grid.fic MDTC_niiler_grid.fic Input Data: - (Niiler et al, 2003) MDT - EIGEN-GL04S1 MDTS (filtered at 400 km) - MDTS Grid characteristics: ½ resolution LonMin= E, LonMax E LatMin=29.75 N, LatMax=49.75 N Output

32 The GOCE User Toolbox III- Geostrophic velocity computation GUT_WF6 (GUT_FA13) ADT_June_7th_2006.fic AbsGeosVel_June_7th_2006.fic Input Data: Grid of ADT on June, (SLA+MDTC) in the Gulfstream area Output

33 The GOCE User Toolbox It was recommended that the GOCE User Toolbox should be designed so that it can be used at different levels, depending on the expertise and the needs of the user. I- The workflow approach User need: Compute a MDT on a ¼ regular grid, at 100 km resolution (SH200) filtering the GUT default geoid and MSSH data in the spectral domain Workflow 3b of the GOCE User Toolbox II- The single step approach User need: Interpolate a grid of GOCE geoid heights along an altimetric track Use the Grid Interpolation routine of the GOCE User Toolbox III- The step by step approach User need: Compare the default grid of GOCE geoid heights with a user provided grid of GRACE geoid heights Four steps: 1- Compute the GOCE geoid relative to the same reference ellipsoid as the user provided geoid 2- Compute the GOCE geoid relative to the same tide system as the user provided geoid 3- Compute the GOCE geoid on the same grid as the user provided geoid 4- Compute the difference between the two geoids

34 The GOCE User Toolbox The single step approach - Example Input Data: -MSSCLS01_EIGENGL04S_fg400.fic -Characteristics of input grid (Grid.fic) LonMin=-180 E LonMax=180 E LatMin=90 S LatMax=90 N ½ resolution - List of points (Points.fic): The step by step approach - Example

35 GUT Tutorials Include: - GUT objectives - How to compute a Mean Dynamic Topography - How to use the toolbox -- «use cases» - for more technical details use the user manual

36 TOOLBOX STATUS - Technical Specification and Architectural Design --> Document accepted - Toolbox SW Development Started 1 April - First Prototype released last week - Evaluation and Testing on-going - Version 1 released before end of commissioning phase - User satisfaction survey and further requirements

37 GUT OPEN GROUP The GUTS and GUT are supported by ESA with collaborators from many European countries working in a core group with an open group of observers, reviewers and advisors. The Members of this open group are de facto the first users of the toolbox. Some have also contributed existing source code to improve the Toolbox. You can join this group and contribute to the toolbox requirements validation process.

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