The GOCE High Level Processing Facility (HPF) is an assignment by ESA for the scientific processing (Level 1B to Level 2) of the dedicated gravity field satellite mission GOCE, including the hardware and software development and the operation, performed by the European GOCE Gravity Consortium EGG-C.

In the framework of the GOCE HPF, the Graz GOCE team is task leader and has full responsibility for the computation of an optimum gravity field model using the time-wise approach (WP 6000).

The operational software system is composed of two main components:

Quick-Look Gravity Field Analysis: Computation of fast approximate gravity field solutions based on SGG and hl-SST data, for the purpose to derive a fast diagnosis of the GOCE system performance and of the level 1b input data in parallel to the mission.

Core Solver: Rigorous ultimate-precision solution of the very large normal equation systems applying parallel processing strategies. The Core Solver is composed of the Final Solver, taking the full normal equation matrix into account, and the Tuning Machine (based on the method of preconditioned conjugate gradients), which will verify and tune the involved software components of the Final Solver. Concerning the hl-SST processing, the energy integral approach is applied.

The developed, implemented and integrated software system is conceived in a highly modular manner that allows the investigation of specific aspects of gravity modeling such as filtering, numerical stability and optimum regularization, complementary of SST and SGG and their optimum weighting. The complete software system required to fulfill the specified tasks of WP 6000 shall finally be operated at the Sub-Processing Facility SPF 6000, located at TU Graz.

The Core Solver is a component of the Off-line Sub-Processing Facilities. The objective is to compute a high-accuracy, high-resolution spherical harmonic model including a quality description of the static Earth's gravity field from GOCE SGG and SST observations. The parameterization of the model will be complete at least up to degree and order 200, and a resolution up to degree and order 250 is envisaged, depending on the actual accuracy of the SGG observations. Additionally, a quality description in terms of a full variance-covariance matrix will be provided.

The software system is a tailored product dedicated to the processing of GOCE observables. The Core Solver is based on an epoch-wise (time-wise) processing of SST and SGG observations following the least-squares principle, applying parallel processing strategies implemented on a Beowulf cluster and facilitating the Scientific Supercomputing resources at TU Graz.

The Space Research Institute contributes to the following main modules:

• SGG assembling: Given the precise GOCE orbit, the calibrated gravity gradients defined in the Gradiometer Reference Frame are directly related to the unknown potential coefficients resulting in the linear observation model for all relevant tensor components, allowing to exploit the high degree of precision and resolution of the data. The complications arising from the colored noise of the gradiometer are managed by a recursive filter procedure in time domain.

• Solution: The mathematical models for SGG and SST data are combined to the overall mathematical model by means of superposition of the normal equations, applying an optimum weighting of the individual data types. The solution will be processed applying a parallel Cholesky reduction. The ill-posedness of the normal equations due to the polar gaps and the downward continuation are managed by optimized regularization techniques.