The purpose of the paper is to discuss the use of the theory of boundary value problems for partial differential equations in satellite gradiometry. An approximation of an energetic level of the satellite orbit by a geo-centric sphere is treated in connection with the problem of a boundary and boundary data definition. The choice of basic observables and their reduction to the sphere of approximation as well as possibilities to substitute the knowledge of the instrument frame orientation in space by means of invariants of the gravitational tensor are discussed. Within the framework of a linear theory the separation of field and orbit perturbation is investigated.
In the introductory part of the paper the importance of the topic for gravity field studies is outlined. Some concepts and tools often used for the representation of the solution of the respective boundary-value problems are mentioned. Subsequently a weak formulation of Neumann's problem is considered with emphasis on a particular choice of function basis generated by the reproducing kernel of the respective Hilbert space of functions. The paper then focuses on the construction of the reproducing kernel for the solution domain given by the exterior of an oblate ellipsoid of revolution. First its exact structure is derived by means of the apparatus of ellipsoidal harmonics. In this case the structure of the kernel, similarly as of the entries of Galerkin's matrix, becomes rather complex. Therefore, an approximation of ellipsoidal harmonics (limit layer approach), based on an approximation version of Legendre's ordinary differential equation, resulting from the method of separation of variables in solving Laplace's equation, is used. The kernel thus obtained shows some similar features, which the reproducing kernel has in the spherical case, i.e. for the solution domain represented by the exterior of a sphere. A numerical implementation of the exact structure of the reproducing kernel is mentioned as a driving impulse of running investigations.
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