Data from ESA's GOCE mission are of fundamental importance to the oceanographic community. It is expected that in conjunction with altimetric observations, gravity data from GOCE will - for the first time in history - allow access to the absolute ocean dynamic surface topography and to compute the absolute ocean surface geostrophic currents at spatial scales down to about 100 km.

At the moment, only the variable part of the sea level, and thus the geostrophic currents, can be inferred from altimetric heights with sufficient accuracy. Despite their importance for oceanographic studies, the processing and analysis of gravity mission data has proven to be complicated to the point that the lack of proper processing software is hampering progress in the use of those data.

Success in the exploitation of GOCE data therefore seems to depend fundamentally on the proper knowledge of several steps of the detailed gravity data processing procedure in terms of spherical harmonics, their implicit consistent normalisation factors, filtering and error data, among others.

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.

Hence, to facilitate the easy use of GOCE products for oceanographers and to support the needs of specific applications, the development of a user toolbox, GUT, was identified as an urgent step at the Second International GOCE User Workshop.

Such a toolbox is required to guarantee optimal use of the existing and future gravity data acquired from GRACE and GOCE. In particular, it was recognised and accepted that software packages are required to allow the gravity field data, in conjunction and consistent with any other auxiliary data sets, to be pre-processed by users not intimately familiar with gravity field processing procedure, for oceanographic and hydrologic application, regionally and globally.

From previous work, a preliminary idea about the scope of a GUT toolbox existed already in the community, especially from experience gained through GRACE data processing.

The GOCINA project is another source of valuable information about needs and solutions, that have been consulted while defining GOCE user needs and toolboxes.

The GOCE User Toolbox project was structured in three phases:

Phase 1

In the first phase (Kick-off January 2006), ESA conducted a 12 months long algorithmic and trade-off study, coined GOCE User Toolbox Specification (GUTS), in preparation for the development of a GOCE User Toolbox in the subsequent phase.

The primary objectives of the GOCE User Toolbox Specifications study were to consolidate toolbox requirements, to develop - in close collaboration with ESA's HPF effort – algorithms, to design a draft architecture and to define input and output specifications for the subsequent generation of the GUT Toolbox.

The purpose of the study was accordingly to:

• Consolidate the User Toolbox requirements associated with geodetic, oceanographic and solid earth applications
• Carry out a scientific trade off study to select the toolbox processing and viewing functions
• Produce a toolbox output specifications document
• Produce an algorithms specifications document which details the necessary level for coding
• Produce a toolbox architectural design document mapping the required functionality and interfaces such as auxiliary data

For all applications (geodetic, oceanographic and solid earth), the absolute minimal requirement of the toolbox was identified as the computation of geoid heights from the set of spherical harmonic coefficients at a given user-specified harmonic degree and order.

For oceanographic applications, the key quantity to be computed was the mean dynamic topography, which is the height of the mean sea surface relative to the geoid.

For this application, GUT provides the tools for converting the mean sea surface and the geoid into the same geodetic system and for carrying out the needed filtering to compensate for the different resolution capabilities of the two surfaces. Furthermore, a-priori mean dynamic topography models derived from e.g. ocean circulation models, may be used.

For geodetic applications, the primary variables of interest to be provided by the toolbox were identified in the form of geoid heights, heights anomaly and deflections of the vertical. Those quantities were required to be represented in the nodes along a profile, in a grid or in discretely located points.

Finally, for Solid Earth applications, scientists showed interest in computing gravity anomalies fields and associated error information inside the toolbox.

GUTS was supported by ESA with collaborators from many European countries.

The GUT-Phase 1 consortium was composed of the following organisations:

• Alfred-Wegener-Institut (AWI), Germany
• Collecte Localisation Satellite (CLS), France
• Danish National Space Center (DNSC), Denmark
• IFREMER, France
• Nansen Environmental and Remote Sensing Center (NERSC), Norway
• National Oceanography Centre, Southampton (NOCS), UK
• Proudman Oceanographic Laboratory (POL), UK
• Technische University München (TUM), Germany
• University of Copenhagen (UCPH), Denmark
• University of Hamburg (UHH), Germany
• University of Reading (UR), UK
• University of Stuttgart (UST), Germany

The consortium was an open working group, open to any further membership and open to all donators of free source code.

Phase 2

In the second phase (Kick-off January 2008), detailed design, coding and implementation by software engineers and beta testing by scientists took place. GUT was launched by ESA as 18 months long CCN of GUTS contract.

The primary objective of the CCN was to release the first version of the user toolbox through the following steps:

• Review toolbox specification and define technical requirements
• Develop the first generation toolbox based on existing software
• Introduce visualisation functions via GUI based on BRAT
• Develop toolbox tutorial
• Evaluate and test the toolbox

Furthermore, two supporting scientific studies have been carried out; one on a new Mean Sea Surface and one on error covariance matrix handling.

The GUT-Phase 2 consortium was composed of the following organisations:

• Collecte Localisation Satellite (CLS), France
• Danish National Space Center (DNSC), Denmark + Georges Balmino, France
• Nansen Environmental and Remote Sensing Center (NERSC), Norway
• National Oceanography Centre, Southampton (NOCS), UK
• Proudman Oceanographic Laboratory (POL), UK
• Science and Technology (S&T), NL
• University of Reading (UR), UK

Phase 3

In the third phase (Kick-off March 2010) a GUT Extension/Maintenance Contract kicked-off on 25 March 2010. The main outcome from this contract was the release of GUT v2.0 but also pure research activities in the fields of the synthetic error covariance matrix evaluation and MSS error characteristics assessment was been pursued. The second public release of GUT (v2.0) was disseminated on March 2011 in concomitance of the 4th GOCE User Workshop.

GUT v2.0 had:

• More Support/Utilities for Solid Earth Community (Bouguer Anomaly, etc.)
• Anisotropic Filtering
• Spline Interpolation
• Support to ICGEM Format
• More Statistical Analysis methods
• History Attribute
• Time Attribute
• An external Tool for the analysis of Variance/Covariance Matrix
• And many others more

The GUT-Phase 3 consortium is composed of the following organisations:

• Danmarks Tekniske Universitet Space (DTU), Denmark
• Collecte Localisation Satellite (CLS), France
• National Oceanography Centre Southampton (NOCS), UK
• Science & Technology (S&T), NL
• Newcastle University (NU), UK
• Institut de Physique du Globe de Paris (IPGP), France
• Copenhagen University (CU), Denmark
• Hamburg University (HU), Germany

The GUT members work 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.

If you would like to join this group and contribute to the toolbox validation process please contact the GUT Support Team.

The second version of GUT was disseminated in concomitance of the 4th GOCE User Workshop. A user satisfaction survey is ongoing.

The GUT toolbox has been developed in a European coordinated effort supported by ESA.

The objective of the GOCE User Toolbox Specifications study (GUTS) was to develop - in close collaboration with ESA's HPF effort - algorithms and input and output specification for the subsequent generation of a user toolbox that is required by the general science community for the exploitation of GOCE level 2 and ERS-Envisat altimetry.

The GUT project dealt with the actual creation of the GOCE User Toolbox. The objective of this activity was to develop the first generation of a user toolbox that is required by the general science community for the exploitation of GOCE level 2 data. The purpose of this study is accordingly to:

• Review toolbox specification and define technical requirements,
• Develop the first generation toolbox based on existing software,
• Introduce visualisation functions via GUI based on BRAT,
• Develop toolbox tutorial,
• Evaluate and test the toolbox.

ESA Team

Consortium

Oceanography is the branch of Earth science that studies the ocean. In particular, physical oceanography studies the ocean's physical attributes including temperature-salinity structure, mixing, waves, internal waves, tides and currents.

The primary oceanography variable of interest provided by the toolbox for oceanographic applications is the mean dynamic topography resulting from the difference between mean sea surface height (MSSH) measurements from altimeter and the geoid heights.

Altimetric MSSH fields are an a priori auxiliary input data set fields from which a consistently filtered mean dynamic topography is computed by the toolbox.

This requires that a consistent reference system be chosen for geoid and the MSSH (both surfaces expressed relative to the same reference ellipsoid) as well as a consistent permanent tide system. Ideally the same tidal models or other corrections should have been used in the geoid and the altimetric fields.

Furthermore, as from the mean dynamic topography, the associated geostrophic currents can be derived.

Functionalities related to the oceanographic aspects of the toolbox include:

• Provision of apriori MSSH, MDT and Geoid data on a grid
• Computation of a GUT MDT (MSSH-GOCE geoid) at a given spatial resolution and on a given structured or unstructured grid as the difference between the apriori MSSH and the geoid. This MDT product is referred to as a "Satellite-only" MDT or MDTS.
• Spatial and spectral filtering of MSSH or MDT consistent with a specific harmonic geoid height field resolution. Several filtering types and filter scales are provided to user choice.
• Interpolation of external high resolution MDT on any regular grid or at given points with internally calculated GUT MDT, i.e. combined MDT (MDTC). The spectral content of the MDTS is limited by the spectral content of the geoid model.

  In the case of GOCE, the corresponding MDTS will thus have a centimetric accuracy at a 100 km resolution.

  In some areas of the world ocean, notably coastal areas, straits, semi-enclosed seas such as the Mediterranean Sea and close to steep bottom topography, the MDT is expected to contain signals at shorter spatial scales.

  The GOCE User Toolbox hence provides the user with more sophisticated MDT computation techniques, allowing to integrate short-scale information from other MDT sources.

  These techniques will be further referenced to as Remove-Restore techniques. Two variants of a remove-restore "combined" technique are included in GUT.

  The first (method A) utilises a high-resolution a-priori MDT, e.g. from hydrodynamic modelling or observations, to restore the small-scale structure in the 'satellite' MDTS.

  The second variant (method B) takes the a-priori MDT as the basis and restores the large-scale structure by comparing the spectral equivalents of an apriori geoid (based on the filtered difference of MSSH and a-priori MDT) and the GOCE geoid.

  This requires that we use the unfiltered version of MDTS (i.e. direct difference of MSSH – Geoid).

• Change of reference system for MSSH or MDT
• Computation of altimetric time-varying absolute dynamic topography as the difference between altimetric sea surface heights and a geoid model.
• Computation of altimetric absolute geostrophic velocities from the spatial gradients of the geoid field in north and east component and along track.
• Provision of methods to produce a global description of these gridded fields in term of spherical harmonics.

Geodesy is concerned with the measurement of the Earth's figure and the mapping of parameters related to it. Its products are used extensively in all branches of the Earth sciences. In addition, they are applied to many areas of civil engineering, exploration, mapping and cadastral work and are the basis of all geo-information systems.

The three main quantities of geodesy science are: geoid height, gravity anomaly and vertical deflections.

The geoid is that equipotential surface which would coincide exactly with the mean ocean surface of the Earth, if the oceans were in equilibrium, at rest, and extended through the continents.

The geoid height is the orthornormal height of geoid above the reference ellipsoid. The gravity anomaly in modern definition is the difference between the magnitude of the gravity vector at a point P (on the surface of the Earth), and the magnitude of the reference gravity (normal gravity) at the point Q which has the same gravity potential as at the point P and lies on the normal to the reference ellipsoid passing through P.

The deflection vertical in modern definition is the angular separation of the gravity vector at the point P and the normal gravity vector at point Q which has the same gravity potential as at the point P and lies on the normal to the reference ellipsoid passing through P.

For geodesy branches, GUT allows user for:

1. Computation of global, gridded geoid heights or gravity anomalies at a given, user-specified, degree and order of the spherical harmonic expansion (i.e. at a given spatial resolution).
2. Computation of geoid heights and gravity anomalies at a given spatial resolution and a given point or list of points (e.g. unstructured grid, transect).
3. Computation of vertical deflections at a given, user-specified, degree and order of the spherical harmonic expansion (i.e., at a given spatial resolution) over global grid, or transect.
4. Computation of height anomalies at a given, user-specified, degree and order of the spherical harmonic expansion (i.e. at a given spatial resolution) over global grid, or transect.

The gravity anomaly and vertical deflections are calculated in full (i.e. no spherical approximations used).

The tool gives users the possibility to change reference, ellipsoid, tide system, or to express the degree/order of expansion in term of scale angle or scale length.

Finally, the passage from spatial domain to spectral domain is allowed by implementation of the spherical harmonic synthesis algorithm.