During the preparation of the Hipparcos Input Catalogue precise positions were measured very recently by the Bordeaux automatic meridian circle (BMC) for about 5500 stars in the northern hemisphere. On average eight observations were made for each star at a mean epoch 1987. The estimated mean error in α is 0″.12 and in δ 0″. 18. With 2555 stars almost half the BMC sample lies in the declination zone -2° < δ < +32°. In this zone the (x,y) measurements of all Astrographic Catalogue plates have been newly reduced (S. Röser, priv. comm.). So, at least two independent positions are available for practically all stars down to the limiting magnitude of the AC with epochs around 1910 and with a mean positional error of 0”. 28. A combination of these two sources should provide proper motions to an accuracy of 0″. 004 yr−1.

The Hipparcos geometrical calibrations directly linked with the astrometrical precision are shortly described, and their accuracies given. An example for the basic angle shows the very high stability of the instrument.

An accurate on-ground reconstitution of the satellite three-axes attitude from the downlinked star mapper photon counts is an essential factor in the final accuracy of the Hipparcos Astrometric Catalogue. The reconstitution method developed by the FAST Consortium and currently on use to process Hipparcos data is the concern of this paper. After on outline of the main reconstitution models and algorithms, an accuracy assessment of the estimates, attitude and star coordinates, coming out from the first year data reduction, is presented; the assessment shows that attitude root mean square errors are already around the mission target of 0.1 arcseconds.

The procedures used to reduce the Hipparcos data in both FAST and NDAC consortia follow the so-called three step procedure described in 1976 by L. Lindegren (unpublished). It consists in a first place to treat data collected during a few hours and, since the sky scanning is only slowly drifting, deduce the abscissae of observed stars on a given mean great circle, called the reference great circle (RGC). The object of this presentation is to discuss the quality of the results obtained at this stage of the reduction. The results presented here are, we believe, representative of the 3000 or so great circle reductions that shall be made during the full data treatment.

Results from a test solution of the positions and parallaxes for about 10,000 stars are presented in statistical form. The solution is based on about 1300 hours of Hipparcos observations made from November 1989 to February 1991, representing about 1/5 of the scientifically useful data collected in that period. Due to the short time span of the data no attempt was made to determine proper motions. The positions, many of which are already in the 2-5 milli-arcsec accuracy range, clearly reveal systematic errors of up to 0.2 arcsec in the ground-based positions of the Input Catalogue. Perhaps the most convincing evidence of the quality of the satellite data is provided by the distribution of trigonometric parallaxes determined in this solution. By comparison with the expected distribution of true parallaxes, based on a standard model of the Galaxy, we conclude that the external errors of the best parallaxes have a standard deviation around 5 milli-arcsec. Any global offset of the parallax zero point appears to be less than 1 milli-arcsec. The accuracy obtained is consistent with pre-launch predictions, if allowance is made for the reduced data collection efficiency due to the faulty orbit, and supports the assertion that the 2 milli-arcsec goal can still be reached in the present orbit. Details of the solution are being published in Astronomy & Astrophysics.

The quasi-simultaneous measurement of the one-dimensional angular separation of stars on the sky, on both small and large angular scales, rely on the measurements made by the IDT detector, as well as on inputs from the star mapper and attitude control system. This presentation will concentrate on the results of a comparison of the results of the IDT data processing carried out by the FAST and NDAC data analysis teams. The extent to which the intensity and phase measurements agree between the two reductions, and the extent to which the differences are consistent with expected photon noise errors, will be illustrated.

The star abscissae as obtained from the reduction of Hipparcos observations have been compared with results obtained on the same circle, but on a different date. The comparison shows that the results are reproducible, taking into account the expected random and non-random effects.

For a test under real conditions the Hipparcos-measured star abscissae on 60 reference great circles have been processed covering partly the period from December 1989 to February 1991. After adapting the abscissae to a provisional instrumental system the determination of positions of some 4000 programme stars has been attempted in the course of the operational qualification of the data reduction procedures of the Consortium FAST. – Following the FAST Consortium’s decision to work exclusively in the ecliptic frame of reference the results are given in ecliptic longitude (λ) and latitude (β).

In the Tycho project astrometric and photometric data of about 1 000 000 stars to a limit of B = 12 mag will be derived. The brightest 500 000 stars will obtain magnitudes in two colours BT and VT, the fainter 500 000 usually only one broad-band magnitude T. There is confidence that an accuracy will be achieved about 0.03 arcsec for positions and 0.03 mag for magnitudes, at B = 10.5 mag. The results are obtained by appropriate treatment of the continuous data records generated by the Hipparcos satellite’s star mapper (which provide simultaneous measurements in two spectral channels), based on predicted star transits using its own ‘Tycho Input Catalogue’. The data treatment is carried out by the Tycho Data Analysis Consortium (TDAC), using calibration and satellite attitude information from the two Hipparcos data reduction consortia. An overview of the data reductions is given. Raw observation data and their numerical treatment are described. This assessment is based on a small data set, i.e. 12 weeks spread over the first 8 months of the Tycho ‘revised mission’ where the satellite is in the elliptical transfer orbit, instead of the intended geostationary orbit.

In addition to its astrometric capabilitie the Hipparcos main detector proves to be a good phototometer. The main features of the reduction are outlined altogether with a presentation of some conspicuous results which already provide good clues as to the quality of the final results when more than three years of data are analysed.

The Hipparcos satellite has a predefined observing program given by an Input Catalogue with 118 000 entries. This includes some 10 000 members of known double or multiple systems (with separations above 0.1 arcsec and magnitude-differences below 4-5), which are observably non-single and thus need a more complex reduction procedure. Also, a similar number of hitherto unknown doubles showing deviations from a single-star observation model will be included in the double-star reductions. A key feature in the NDAC reductions is the use of the main reduction results to calibrate and collect the data for individual doubles in ‘Case History Files’. This enables a global solution of both absolute and relative parameters for a double to be carried out later in a single solution. Using provisional data tapes covering small parts of a 14-month interval, tests have been made of major parts of the double star reductions. Comparisons with the main reduction results for single stars indicate that remaining calibration errors are below the 10 milliarcsec level. For known doubles, the solutions are good relative to the Input Catalogue at the expected 0.3 arcsec level, and for a few tens of systems observed with the star-mapper, the agreement is at the 0.05 arcsec level. A few hundred doubles not flagged in the Input Catalogue have also been detected and had their parameters provisionally determined. A detailed paper describing these methods and results has been accepted by Astronomy & Astrophysics.

Among the long period variable stars included in the Hipparcos observing program, 245 large-amplitude ones require brightness predictions during the mission in order to allocate the necessary observing time. We present the computation of the light ephemerides with numeric and symbolic methods using AAVSO’s 75-year data on maxima and minima dates and magnitudes and 20 years of individuai observations; the evaluation and revision of the ephemerides using ground-based observations compiled monthly by the AAVSO from observers world-wide and real-time monitoring of ephemerides stars from satellite observations performed at ESOC; the statistical results derived from one-and-a-half years of simultaneous ground-based and Hipparcos observations. We show the usefulness of all this work in understanding the physics of these pulsating variable stars.

Hipparcos observations provide a wealthy source of discovery of new double and multiple stars. We have developed adequate procedures to recognize in the signal parameters the multiplicity of the stellar source passing through the Hipparcos field of view. These procedures are outlined together with the design of the statistical tests employed. Sampling on real data indicates that about 6000 new double stars will be detected at the mission completion.

The status of our longterm joint optical- and radio observing program for the establishment of a new inertial optical- and radio reference frame is described. A special attempt is made to provide a suitable number of extragalactic sources and radiostars for an inertial HIPPARCOS reference link.

The pre-launch accuracy prediction for the Hipparcos mission is reviewed with regard to the observed performance and expected lifetime of the satellite in its present orbit. Given the excellent quality of the data it is likely that the target accuracy of 2 milli-arcsec for typical programme stars can be achieved if operations continue at least until the end of 1993. We emphasize that any extension of the operational lifetime beyond that point strongly enhances the scientific value of the results.

The homogeneous positional reference frame to be established by Hipparcos must be brought into coincidence with the new IAU frame based on extragalactic radio sources. A review is given of the methods by which this is to be achieved.

A large fraction if not all of the programmes related to the study of the galactic structure, its kinematics and chemical evolution will require the knowledge of the third component of the velocity: the stellar radial velocity. But we will also need the radial velocity to be able to determine the true distribution of masses in the solar neighborhood (corrected by the crossing time of the stars in the sampled sphere). The radial velocity will allow the determination of the statistical parallax for a sample of stars lying beyond 100 parsec. At the very beginning of the development of the Hipparcos space mission, the very need for ground-based, complementary measurements has been recognized. However, in spite of the existence of new kinds of techniques or detectors, the task to provide radial velocities for the somewhat 118’000 stars of the Hipparcos Input Catalogue is quite enormous. All presently published stellar radial velocities determined since the beginning of this century represent at the most 20 to 25% of the total number of stars to be measured!

The Hipparcos satellite generates a considerable quantity of ‘raw’ data: approximately 1Gbit per day, or some lOOOGbits over its anticipated lifetime. The Hipparcos mission follows usual practices of having elements of the observing programme selected after an invitation for proposals to the general astronomical community, as well as a strong scientific interest in the data exploitation from amongst the individuals working within the scientific consortia related to the observing programme definition and the data analysis. However, an unusual feature of this mission is the fact that the elemental observations cannot be ‘separated out’ and distributed to the proposers or the consortia ‘principal investigators’. Instead, the data analysis teams will work towards the construction of a single, agreed-upon, catalogue containing the final astrometric and photometric star parameters.

A comparison of accurate optical and radio positions is presented for 25 southern compact radio objects. There is a systematic “S” shaped trend indicating a zonal bias in the Perth 70 optical catalogue.

Besides the link of the HIPPARCOS reference frame to extragalactic objects via radio stars or by the HST, also photographic astrometry is able to calibrate the HIPPARCOS proper motions with regard to an inertial system. Numerical simulations have shown that even with a very small number of well-distributed link fields (3 to 5) the photographic method is competitive with other techniques.