Showing papers by "European Space Operations Centre published in 2014"

A collinearity diagnosis of the GNSS geocenter determination

Abstract: The problem of observing geocenter motion from global navigation satellite system (GNSS) solutions through the network shift approach is addressed from the perspective of collinearity (or multicollinearity) among the parameters of a least-squares regression. A collinearity diagnosis, based on the notion of variance inflation factor, is therefore developed and allows handling several peculiarities of the GNSS geocenter determination problem. Its application reveals that the determination of all three components of geocenter motion with GNSS suffers from serious collinearity issues, with a comparable level as in the problem of determining the terrestrial scale simultaneously with the GNSS satellite phase center offsets. The inability of current GNSS, as opposed to satellite laser ranging, to properly sense geocenter motion is mostly explained by the estimation, in the GNSS case, of epoch-wise station and satellite clock offsets simultaneously with tropospheric parameters. The empirical satellite accelerations, as estimated by most Analysis Centers of the International GNSS Service, slightly amplify the collinearity of the $$Z$$ geocenter coordinate, but their role remains secondary.

Two-Line-Elements-Based Maneuver Detection Methods for Satellites in Low Earth Orbit

Abstract: Two novel methods are developed for detecting space events based on the analysis of their two-line elements history. These methods are tuned for detecting orbital maneuvers of satellites and the characterization thereof. The first method detects nonnatural anomalous events in low Earth orbit based on a consistency check between arbitrary two-line element sets of the same object. The use of background models that enable one to estimate the variable noise in the two-line element consistencies for different orbital regimes is found to further increase the detection rate. The second method uses the time series of an object containing the orbital elements, or a derived quantity thereof, and evaluates it for any type of unexpected changes by methods from robust statistics and harmonic analysis. Both methods can process the entire two-line element catalog with a minimal operator interference while yielding good results in the different orbital regimes. The generated orbital maneuvering data are used to assess th...

The pointing system of the Herschel space observatory

Abstract: We present the activities carried out to calibrate and characterise the performance of the elements of attitude control and measurement on board the Herschel spacecraft The main calibration parameters and the evolution of the indicators of the pointing performance are described, from the initial values derived from the observations carried out in the performance verification phase to those attained in the last year and half of mission, an absolute pointing error around or even below 1 arcsec, a spatial relative pointing error of some 1 arcsec and a pointing stability below 02 arsec The actions carried out at the ground segment to improve the spacecraft pointing measurements are outlined On-going and future developments towards a final refinement of the Herschel astrometry are also summarised A brief description of the different components of the attitude control and measurement system (both in the space and in the ground segments) is also given for reference We stress the importance of the cooperation between the different actors (scientists, flight dynamics and systems engineers, attitude control and measurement hardware designers, star-tracker manufacturers, etc) to attain the final level of performance

A methodology for analyzing human-automation interactions in flight operations using formal verification techniques

Abstract: When designing and developing systems in safety critical or cost intensive environments it is important to identify as much potential risks as possible prior to operating the system. This includes aspects of the interaction between human and automation systems that are prone to issues. This work-in-progress paper describes a methodology that systematically derives relevant analysis questions for complex human-automation interaction systems. It demonstrates how formal models for all components of the human-automation system can be created. These models are used by model checking algorithms to verify the safety properties associated with the selected analysis questions. While this paper includes no evaluation of the methodology, an ongoing evaluation study is outlined based on the life support system (ECLS) of the European science laboratory Columbus, which is part of the International Space Station. Each step of the formal verification methodology is illustrated with the results obtained so far on the ECLS case study.

Station-keeping for quasi-periodic orbits

Abstract: The shallow gravity gradient in the libration point regions enables manoeuvring at low ∆v expenses, but implicates a sensitivity to small perturbations. A variety of bounded orbits can be determined around each libration point and station-keeping is required to maintain them for multiple revolutions. In this paper, a station-keeping algorithm based on the orbital lifetime expectancy is proposed for so-called quasi-periodic solutions. The method introduced is based on the identification of a manoeuvre maximising the lifetime of an orbit within defined boundaries. The manoeuvre direction and magnitude is finally optimised with a differential evolution algorithm. The novelty of the method presented here is the identification of the downstream centre manifold by the lifetime analysis to preserve the orbit with its properties forward in time. The study shows that the manoeuvre direction is directly correlated to stability information that is provided by the Floquet modal theory. Finally, numerical calculations were carried out for trajectories around the far-side libration point in the Earth-Moon system to show the effectiveness of this station-keeping approach. The robustness is proven by the introduction of errors and the evaluation of their impact.