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It is the aim of the GOCE mission to determine a model of the Earth’s gravity field with high accuracy and resolution. For this purpose, gravity gradients will be measured in combination with high–low satellite-to-satellite tracking. The gravity gradients are derived from pair-wise differenced accelerations as determined by the six three-axes accelerometers that form the GOCE gradiometer. Since the measured accelerations suffer from errors of a random and systematic nature, the gravity gradients may suffer from random and systematic errors as well. Systematic errors are, for example, a scale factor and a bias. The common accelerations of the paired accelerometers also are contaminated with such errors. The common accelerations are used in the drag-free control of the satellite and are important for the separation of the gravitational and non-gravitational forces in the gravity field determination. The checking of the gravity gradients and the common accelerations against independent data (i.e. external to the GOCE satellite) in order to free the observations as well as is possible from systematic errors is called external calibration. The possibilities and limitations of using terrestrial gravity data and global gravity models for external calibration of the gravity gradients are reviewed. It turns out that the determination of a gravity gradient scale factor and bias using just the accurate knowledge of the central term and the flattening ( J 2) of the Earth’s gravity field is not good enough. When global gravity field models are used for the calibration, higher degrees and orders should be taken into account as well. With today’s existing global models it seems to be possible to remove the greater part of the systematic errors of the GOCE gradients. A gravity gradient bias can accurately be recovered using terrestrial gravity data in a regional approach with least squares collocation. However, since regional data are used it may not be possible to determine calibration parameters valid for the whole (global) gravity gradient data set. Nevertheless, regional terrestrial gravity data could be used to validate the measured and calibrated gravity gradients. In addition, a possible use of GOCE high–low satellite-to-satellite tracking data to calibrate the common accelerations is explored; it is shown that this approach fails. If more accurate gravity field information becomes available then such a calibration may become feasible. [ABSTRACT FROM AUTHOR]

Software intensive systems (SIS) increasingly influence the ability of enterprises to be competitive in continuously changing market situations. The integration of these systems into organizations, and in particular the subsequent exploitation, maintenance and service activities, have become of utmost importance. Unfortunately the area of exploitation and operation, also known as service management, is still rather immature. Service management covers services such as performance and availability support, end-user and help desk support, education, and maintenance. One of the main concepts of service management is the Service Level Agreement (SLA). The goal of an SLA is to bridge the gap between service provider and users or customers. However, there exist many problems and unsolved questions regarding the specification and the quantification of SLAs. This paper addresses the specification of SLAs on the basis of three well-founded service management principles, respectively: ‘continuity in service management,’ the pit/shell principle of a service, and the principle of specifying the quality of both a service process and a service object. Finally, the paper addresses the validation of these principles in practice. [ABSTRACT FROM AUTHOR]