Showing papers by "European Space Operations Centre published in 1986"

Limits of X-ray variability in active galactic nuclei

Abstract: A significant correlation is demonstrataed here between the X-ray luminosity and the timescale of X-ray variability for Seyfert galaxies and quasars. This is interpreted as evidence that the emitting plasma is near the limit of being dominated by electron-positron pairs. BL Lac objects do not follow this pattern; this may be due either to relativistic beaming or to the differing importance of the pair production process.

On the optimal station keeping control of halo orbits

Abstract: Techniques for computing and controlling a halo orbit are considered in this paper. It presents a semi-analytical theory for the halo orbits in the Restricted Three Bodies Problem (RTBP), that is valid and amenable to computation to any order. Results are presented up to order 11. The Floquet modes of the monodromy matrix are used to define a local optimal control procedure through the concepts of projection and gain functions.

Estimation of daytime downward longwave radiation at the surface from satellite and grid point data

Abstract: A hybrid method is developed for the estimation of the daytime downward longwave radiation flux (DLF) at the surface. The method makes use of the grid point thermodynamic fields at the surface and at the 1000 and 850 hPa levels. The cloud parameters are derived from the infrared and visible image data of the satellite METEOSAT-2. The calculation of the DLF is split into a clear-sky contribution, which is calculated from empirical formulae, and a cloud contribution which depends on cloud amount, cloud base height, and temperature.

Long-term evolution of near-geostationary orbits

Abstract: A model for the long-term evolution of free-drifting near-geostationary satellite orbits is presented. A firstorder analytical averaging transformation is applied to the perturbation equations in order to eliminate the short-term (with period of order of one day) variations of the orbital elements. The model includes lunisolar gravitational forces up to the second parallactic term of the moon, zonal and tesseral harmonics of the Earth's potential field up to the fourth degree, as well as the solar radiation force. The algebraic computations have been carried out by an automated Poisson series manipulation. Extremely compact expressions could be established after manually recombining the computer-generated results in terms of a few well-selected parameters. The results obtained are of particular interest for predicting the motion of geostationary spacecraft after their useful lifetime has expired and stationkeeping maneuvers are no longer executed. The validity of the model presented has been evaluated by a comparison with numerical results obtained for the European Space Agency's GEOS-2 satellite, which is at present orbiting about 260 km above geostationary altitude.

Attitude determination covariance analysis for geostationary transfer orbits

Abstract: Fundamental biases in V-slit sun and pencil-beam infrared Earth sensor measurements are identified and modeled in terms of representative covariances. Their effects on attitude determination accuracy are assessed by means of covariance transformations following the calculations involved in a realistic attitude estimation process. The results are useful in identifying the best attitude determination intervals and for providing quantitative estimates of the expected error for selected launch configurations. On this basis the most favorable sensor settings can be selected for a given launch window. This is of particular interest for present-day communication satellites equipped with variable sensor mountings which can be adjusted before launch.

Relative effects of solar and infrared radiative forcing in a mesoscale model

Abstract: A new efficient parameterization scheme for solar short-wave radiative heating, as a component of the net radiative effects in the atmosphere, is tested in a three-dimensional mesoscale model. This model is designed with moist convective processes in mind, so that the radiative parameterization (solar plus thermal infrared) are interactive with the cloud field. Previous work by the authors with only an infrared scheme has demonstrated that cloud-radiation interactions are characterized by strong cloud-top cooling, leading to upper cloud-layer destabilization. The effects of including solar heating are to modulate the strength of the strong infrared cooling, thereby leading to weaker interactions between clouds, radiation, and mesoscale fields. The present study shows that even on the mesoscale and for relatively short time-spans, radiative processes in the presence of clouds are not negligible. As a further step, a simple fractional cloud cover parameterization is introduced and the model response is compared with results omitting this parameterization.

Pathfinder - Accuracy improvement of comet Halley trajectory for Giotto navigation

Abstract: The 'Pathfinder' project uses the resources of NASA, ESA and Intercosmos to support the Giotto space probe flyby of comet Halley. The required 500-km approach to comet Halley on the sunward side was so uncertain as to preclude the trajectory's realization with sufficient accuracy on the basis of ground-based astronomical observations alone. Spaceborne observations by the two Vega satellites were accordingly conducted; these, in combination with a very accurate determination of the Vega trajectories using VLBI, allowed a flyby distance of 600 km to be achieved with only + or - 40 km uncertainty.

Networking ground‐based images of comet Halley during the Giotto encounter

Abstract: During the period immediately before and after the European, Russian, and Japanese spacecraft encounters with Comet Halley in early March 1986, sequences of ground-based electronic images of the comet, obtained at Table Mountain Observatory (TMO), CA, were transmitted via the Space Physics Analysis Network (SPAN) to the European Space Operations Centre (ESOC), and to University College London (UCL). During the 48-h period when the European Space Agency spacecraft Giotto was within the extended coma of Comet Halley, the ground-based images revealed that the comet displayed several spectacular near-nuclear and large-scale features. The TMO images provided a format for the interpretation of the unique in situ results obtained during the closest of the five spacecraft encounters with Comet Halley.