The paper presents control design and results of a leading experiment in the distance stabilization of large bodies, emulating optical mirrors, with picometer repeatability. The experiment, called COSI (Control Optics Structure Interaction), was funded by the European Space Agency (ESA) in view of future space telescopes needing picoradian precision over time scales >1 s. Distance stabilization is achieved by actively controlling the optical length of Fabry-Perot cavities in the vacuum. The first experiments stabilized three 0.5 m distances between two 7 kg plates with a residual control error better than 3 pm (1σ), in presence of severe environment noise and artificial micrometer distance variations, thus fully demonstrating feasibility of COSI concept and technology

The paper presents an original formulation of discrete-event dynamic systems (DEDS) strictly consistent with the Kalman definition of dynamic systems. The paper starts with a clear definition of event as a pair (occurrence time, fact), where the time is a real number and the fact is an element of a set with algebraic properties. The introduction of the concept of event sequences and of suitable operations over their set allows to formulate DEDS as causal operators transforming input e\'ent sequences into output event sequences. The definition of a state for such operator allows to give a state representation of the input-output relation. The state representation is a state equation as in the standard continuous or discrete-time systems, and allows to compute the free and the forced responses of the system. The paper terminates by providing the elementary stability defmitions and the state equations of linear and time-invariant DEDS.

The paper presents a further original development of the Manufacturing Algebra aiming at describing the dynamics of the production procesess taking place in factories. Starting from the definition of events and event sequences, a theory of discrete-event dynamic systems is developed which is appropriate for modelling the time and space evolution of discrete (or by part) production processes, at any level of detail. The result is a set of state equations modellling the state evolution of the storage and production units of a factory under the action of a real-time production control.

The paper presents the major features of the ESPRIT Basic Research HIMAC, proposed to the EC after a thorough investigation of the state of the art of manufacturing systems control. HIMAC has followed a new approach, by developing a specific mathematIcs, the Manufactunng Algebra (MA), for modelling and controlling the production processes of discrete manufacturing systems. As a key feature, the MA models can be bottom-up aggregated starting from the very detailed levels used in simulation, thus making poSSIble a coherent model hierarchy at the base of the design and realization of Hierarchical control strategies and architectures. The formulation of the original mathematical approach and its validation criteria are outlined in the paper. For a deeper understanding, the mterested reader can take advantage of the list of references given at the end.

We outline the principles by which absolute trigonometric parallaxes have been derived from the first 30 months of data acquired by the Hipparcos astrometry satellite. Distributions of the parallaxes and formal errors of more than one hundred thousand stars are presented, indicating median standard errors of about 1.5 milliarcsec (mas). Tests which provide confidence in the quality of the results are described, together suggesting that the corresponding 'external' errors are unlikely to be underestimated by more than about 10-20 per cent. Parallaxes are absolute, with present estimates of the limit on any global zero-point offset, i.e. common to all stars, of less than 0.1 mas. The Hertzsprung-Russell diagram derived from the satellite data provides powerful confirmation of the astrometric data quality, with a well-defined broad main sequence extending to M = -5 mag, a well-defined giant branch, and a distinct degenerate sequence. Solar neighbourhood 'clump giants', stars in a post-helium flash stage of evolution, are located in the HR diagram on the basis of trigonometric parallax distance estimates for the first time, and the first direct luminosity calibration of M giants is reported. A comparison with stars previously considered to lie within 25 pc demonstrates significant discrepancies for those distances less reliably determined from ground-based observations, and indicates that a corresponding re-evaluation of stellar masses and luminosities within the solar neighbourhood is implied.

The data analysis methods for the determination of the astrometric parameters from observations by the Hipparcos satellite as used in FAST and NDAC consortia for the construction of intermediate 18-month and 30-month solutions are presented. The quality of the results depends upon calibrations of the instrumental parameters, examples of which are given. Results for the 18 and 30 month reductions are presented: these include histograms of the rms errors of each astrometric parameters, the FAST-NDAC differences as a function of coordinate, magnitude or colour index and results of a first attempt at combining the two solutions. The actual and pre-launch accuracy predictions are compared and discussed.