Algorithmic and tradeoff study for the Generation of a GOCE User Toolbox

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Algorithmic and trade-off study for the
Generation of a GOCE User Toolbox

• Per Knudsen
• Danish National Space Center
• Jérôme Benveniste
• ESA
• Earth Observation Science and Applications
  Department

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Abstract(in hidden slide - for furter reference)

• A strong impact is expected from GOCE Mission, in
  conjunction with altimetric missions, to access
  for the first time to the absolute ocean dynamic
  topography and consequently to the absolute ocean
  circulation at spatial scales down to 100 km.
  Currently, only the variable part of the sea
  level can be deduced from altimetric heights at
  an accuracy compatible with ocean circulation
  studies. The GOCE mission is therefore of the
  greatest interest for the oceanographic
  community. However, no ocean circulation products
  are planned to be delivered as level-2 products
  as part of the GOCE project so that a strong need
  exists, for oceanographers, to further process
  the GOCE level-2 geoid and merge it with Radar
  Altimetry. This need, and subsequent
  recommendations for an optimal use of GOCE data
  by oceanographers, have been explicitly expressed
  during the second International GOCE Workshop
  which was held in ESRIN from 8-10 March 2004.
• In order to facilitate the use of GOCE products
  for oceanographers and other commnuitites such as
  Solid Earth physicists and answer to particular
  needs of specific applications, the development
  of a user toolbox was clearly recommended. The
  aim is to facilitate the using, viewing and
  post-processing of GOCE Level 2 mission data
  products in conjunction with radar altimetry from
  ERS and ENVISAT, for an optimal use by
  oceanographers and solid Earth physicists at
  higher levels and the handling of necessary
  auxiliary data. For oceanographers, such a tool,
  and its tutorial alongside, would be a basis for
  the computation and validation of an ocean
  absolute dynamic topography and, indirectly, for
  the validation of Level-2 products.
• ESA has conducted an algorithmic and trade-off
  study, coined GOCE User Toolbox Specification
  (GUTS), in preparation for the development of an
  open source GOCE User Toolbox in a subsequent
  phase. The Toolbox will, among other
  functionality, produce an ocean absolute dynamic
  topography using GOCE level 2 product, a geoid,
  and radar altimetry from ERS and ENVISAT. The
  study has produced an algorithms specification
  and an architecture design to be used for the
  phase 2, the subsequent development of the GOCE
  User Toolbox. This paper will report the
  achievement of this study and show what the group
  of scientists involved in this study has
  recommended for a first version of the GOCE user
  Toolbox.

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GOCE ESAs 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 Earths interior (lithosphere
  mantle)
• topographic processes, evolution of ice sheets
  and sea level change

www.esa.int/livingplanet/goce
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ESAs Gravity Mission
GOCE
Examples of Scientific Applications
Gravity field map and improved global geoid models
Improved understanding of ocean circulation and
energy distribution
Global unification of height systems
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ESAs Gravity Mission
GOCE
The satellite and its instruments
Payload Three-axis diagonal gravity gradiometer
sat-to-sat tracking (geodetic quality
multi-channel GPS receiver with Helix antenna)
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Main Technical Challenges

• Highest sensitivity accelerometers in space
• CHAMP 10-9 ms-2
• GRACE 10-10 ms-2
• GOCE 10-12 ms-2

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get a feeling for the numbers
2E-03 N
Downforce
Super-tanker acceleration
0.2 gram
1 000 000 tonne
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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
  post-processing of GOCE Level 2 mission data
  products in conjunction with radar altimetry from
  ERS and ENVISAT.

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GOCE User Toolbox Specification Study Objective

• The objective of GUTS is to develop algorithms
  and input/output specification for a user toolbox
  that is required for the exploitation of GOCE
  level 2 and ERS-ENVISAT altimetry.
• The purpose of GUTS is accordingly to
• Consolidate the User Toolbox requirements.
• Carry out a scientific trade off study to select
  the toolbox processing and viewing functions.
• Produce a Toolbox output specification document.
• Produce an algorithm specification document.
• Produce a Toolbox architectural design document.

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Key Activities

• Prioritise functionality and products from the
  User Requirements Document
• Trade-off study into filtering methods for
  Dynamic Topography calculation
• Determine Toolbox WorkFlows
• Define Algorithms required for WorkFlows, given
  results of trade-off study

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Basic Requirement

• Computation of global, gridded geoid heights at a
  given, user-specified, degree and order of the
  spherical harmonic expansion (i.e., at a given
  spatial resolution)
• Computation of geoid heights at a given spatial
  resolution (i.e. specified degree and order of
  the spherical harmonic expansion) at a given
  point or list of points (e.g. unstructured grid,
  transect)

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Generation of Dynamic Topography

• Provision of a priori MSSH, MDT and Geoid data on
  a grid
• Computation of a GOCE MDT (MSSH-GOCE geoid) at
  a given spatial resolution, on a given structured
  or unstructured grid
• Filtering of MSSH consistent with a specific
  harmonic degree geoid height field expansion.

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Trade-Off Study

• Generation of a reliable Dynamic Topography is
  heavily reliant on appropriate filtering of input
  Sea Surface Height and Geoid data
• Trade-off study examined relative merits of a
  range of filters
• Also examined direct vs remove restore methods

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Filter Grading Summary
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Space vs Spectral
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Direct vs Remove-Restore
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Global cumulative error
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Primary WorkFlow
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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

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The workflow approach
Workflow 1a
GUT_WF1a Fg R 89.75/-89.75/0.25/359.75 I
0.5/0.5 EIGENGL4S_SH150_coef.fic
EIGENGL4S_SH40_Etp_MT_grid.fic
Input Data EIGEN-GRACEGL4S SH coefficients
(reference ellipsoidGRIM Tide systemFREE
) Output Reference ellipsoid TP Output tide
system Mean Tide Degree/order of expansion 40
Options Output Grid regular, ½ resolution grid
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The workflow approach
Workflow 1b
GUT_WF1b Fg R 89.75/-89.75/0.25/359.75 I
0.5/0.5 EIGENGL4S_SH150_cov_mat.fic
EIGENGL4S_SH50_Etp_MT_com_error_grid.fic
Input Data EIGEN-GRACEGL4S error covariance
matrix of SH coefficients (reference
ellipsoidGRIM Tide systemFREE ) Output
Reference ellipsoid TP Output tide system Mean
Tide Degree/order of expansion 50
Options Output Grid regular, ½ resolution grid
mm
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The workflow approach
Workflow 3a
GUT_WF3a Fg400 O my_filter_matrix.fic
MSSCLS01_grid.fic EIGENGL04S_SH150_grid.fic MSS
CLS01_EIGENGL4S_fg400_grid.fic
Input Data MSSCLS01 EIGEN GL04S GRACE Geoid
Options Filter Type Gaussian Filter width400 km
cm
my_filter_matrix.fic
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The  step by step  approach
Example 3
1- Compute the MDTS
1-3 GUT_FA09 apply a Gaussian filter with a 400
km width
1-2 GUT_FA14 Substract the EIGENGL04S geoid from
the CLS01 MSS
1-1 GUT_FA15 and GUT_FA16 Compute the EIGENGL04S
geoid relative to the TP ellipsoid and in the
mean tide system
MDTS
MSS CLS01-EIGENGL04S
m
cm
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GUTS Tutorials

• Toward the GUT tutorial rather than GUTS
  prototype tutorial
• I.e. what GUT will look like when built

• Include
• GUT objectives
• How to compute a Mean Dynamic Topography
• How to use the toolbox
• user cases - for more technical details see the
  user manual

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Outlook

• The Building of the GOCE User Toolbox (GUT) will
  start now, based on the effort done during the
  GUTS project
• First version disseminated before the end of the
  GOCE commissioning phase