The ESA Gaia mission uses two telescopes to create the most ambitious survey of the Galaxy. The angle between them must be known with exquisite precision and accuracy. An interferometer: the Basic Angle Monitoring system measures its variations. High quality data have been retrieved and analysed for more than a year. A summary of the in-orbit performance and some early results are presented.

We outline the basic principles of scanning space astrometry, such as represented byHipparcos, Gaia, and some other astrometric satellites planned or proposed. We explain theneed for large-angle measurements, why these are essentially one-dimensional, how it ispossible to determine absolute parallaxes, and why a Hipparcos-type scanning law isfavourable. We discuss the choice of the basic angle between the two viewing directions,the principle of self-calibration, and why the resulting numerical problem must bedifficult to solve.

Gaia is an ambitious space astrometry mission of ESA with a main objective to map the sky in astrometry and photometry down to a magnitude 20 by the end of the next decade. While the mission is built and operated by ESA and an industrial consortium, the data processing is entrusted to a consortium formed by the scientific community, which was formed in 2006 and formally selected by ESA one year later. The satellite will downlink around 100 TB of raw telemetry data over a mission duration of 5 years from which a very complex iterative processing will lead to the final science output: astrometry with a final accuracy of a few tens of microarcseconds, epoch photometry in wide and narrow bands, radial velocity and spectra for the stars brighter than 17 mag. We discuss the general principles and main difficulties of this very large data processing and present the organization of the European Consortium responsible for its design and implementation.

The ESA space astrometry mission Gaia will measure the positions, parallaxes and proper motions of the 1 billion brightest stars on the sky. Expected accuracies are in the 7–25 μas range down to 15 mag and sub-mas accuracies at the faint limit (20 mag). The astrometric data are complemented by low-resolution spectrophotometric data in the 330–1000 nm wavelength range and, for the brighter stars, radial velocity measurements. The scientific case covers an extremely wide range of topics in galactic and stellar astrophysics, solar system and exoplanet science, as well as the establishment of a very accurate, dense and faint optical reference frame. With a planned launch around 2012 and an (extended) operational lifetime of 6 years, final results are expected around 2021. We give a brief overview of the science goals of Gaia, the overall project organisation, expected performance, and some key technical features and challenges.

The first astronomical photographic survey, the Carte du Ciél was initiated in 1887 by a group of French astronomers. The observational campaign was started in 1891, while the last of more than 22,000 total plates were photographed in 1950; most of observations (more than 90%) were performed prior to 1920. Detailed description of the Carte du Ciél development can be found elsewhere (Kolchinsky 1989, Eichhorn 1974, Debarbat et al. 1987).

As a follow-up to the most successful Hipparcos mission we have suggested a mini-satellite. The instrument DIVA is a pair of two small Fizeau interferometers with a fixed angular separation scanning the entire sky in a Hipparcos-type fashion. Despite its small apertures the very efficient opticaldesign and the use of CCD detectors will allow a significant increase in performance compared to the Hipparcos mission. Introducing objective prisms in front of the beam combiners will generate dispersed fringes which will allow a more accurate astrometry and much more detailed spectro-photometric information than filtered, undispered fringes. A complete optical system was designed which is suitable to achieve the desired goal and may be expanded to the size of GAIA. Likewise, several options exists to improve the capabilities and the depth of the DIVA mission.

Dispersed interferometer fringes as a new type of signal are currently under discussion for several space interferometry mission projects (GAIA, FAME, DIVA). The dispersed fringes are directly recorded by CCD mosaics whichare operated in drift-scan mode on board a continuously rotating satellite. They allow combined astrometric (perpendicular to the fringes, i.e. in the direction of the drift scan) and spectro-photometric measurements (in the direction of the dispersion of the fringes, i.e. perpendicular to the drift scan). We have simulated polychromatic dispersed fringe patterns of stars with known spectra. The specific instrument parameters used for the simulation are those envisaged for the DIVA instrument, with realistic assumptions on various noise contributions. The results can be scaled to GAIA and FAME instrument parameters. We have investigated simple algorithms which can be implemented in an on-board data reduction and used for the real-time attitude determination with bright and single stars. On the preliminary basis of 100 simulations per star of a given magnitude and spectral type, we have obtained first estimates of the achievable accuracy of the astrometric measurements and of the location of the photometric zero point λo.

The final Tycho Catalogue (ESA 1997b) has been derived from 37 months of observations with the star mapper of the astrometric satellite Hipparcos. The Hipparcos Catalogue (ESA 1997a) with about 120,000 stars is the result of the main Hipparcos mission and has, e.g., been described by Kovalevsky et al. (1995). Both catalogues will be published in 1997.

DIVA (Double Interferometer for Visual Astrometry) is a Fizeau interferometer on a small satellite. It will perform astrometric and photometric observations of at least 4 million stars. A launch in 2002 and a minimum mission length of 24 months are aimed at. A detailed description of the experiment can be obtained from the DIVA homepage at http://www.aip.de:8080/dso/diva. An overview is given by Röser et al., 1997.

The limiting magnitude of DIVA is about V = 15 for spectral types earlier than M0, but drops to about V = 17.5 for stars later than M5. Table 1 gives a short overview on DIVA’s performance. DIVA will carry out a skysurvey complete to V = 12.5. For the first time this survey will comprise precise photometry in at least 8 bands in the wavelength range from 400 to 1000 nm.

DIVA will improve parallaxes by a factor of 3 compared to Hipparcos; proper motions by at least a factor of 2 and, in combination with the Hipparcos observations, by a factor of 10 for Hipparcos stars. At least 30 times asmany stars as Hipparcos will be observed, and doing this DIVA will fill the gap in observations between Hipparcos and GAIA. DIVA’s combined astrometric and photometric measurements of high precision will have important impacts on astronomy and astrophysics in the next decade.

The HIPPARCOS Catalogue, linked to the system of extra-galactic radio sources, will soon be the reference system in optical astronomy. The Guide Star Catalog (GSC; Lasker et al. AJ 99, 2019ff, 1990), with its 18 million objects, already serves most astronomers for the planning of small field observations, optical identifications, and the reduction of CCD images. The utility of the GSC will be greatly enhanced if it is transformed to the HIPPARCOS system and its remaining systematic errors minimized. To do so we will use the forthcoming TYCHO Reference Catalogue (Röser & Høg, in Workshop on Databases for Galactic Structure, eds. Davis Philip, Hauck & Upgren, pp. 137, 1993). The reductions will use a combination of the mask and sub-plate methods (Taff, AJ 98, 1912, 1989; Taff, Lattanzi & Bucciarelli ApJ 361, 667, 1990), the co-location method (Bucciarelli, Lattanzi & Taff, ApJ Suppl. 84, 91, 1993), the new “fixed number of stars filter” of Röser, Bastian & Kuzmin (in press in the proceedings of IAU 148, 1994), and the infinitely overlapping circles (IOC) version of the latter (Taff, Bucciarelli & Lattanzi, ApJ 358, 359, 1990; Bucciarelli, Taff & Lattanzi, J. Stat. Comp. Sim. 48, 29, 1993). We present in the table below test results using the PPM as the reference catalog (typically only 4 sub-plates per plate; 25 stars for the Square Filter; 16 stars per circle for the IOC method). The first line presents the estimates for σαcosδ and σδ, both in arcsec, relative the to PPM Suppl (Röser, Bastian & Kuzmin, A&A Suppl. 105, 301, 1994); the second line contains them based on plate overlaps (all for the −30° declination zone). When comparing the numbers note that in the first line the GSC and PPM Suppl (0.″12) errors are contributing whereas in the second line the GSC errors are present twice (in the rms sense).

We are compiling a catalogue of positions and proper motions of 4 million stars, based on the Guide Star Catalogue (GSC) and the Astrographic Catalogue (AC). The GSC is re-reduced by a method described in this paper. The AC has been newly reduced recently by us for the determination of positions and proper motions of the PPM catalogue.

The expected accuracy of the proper motions of the GSC-AC catalogue ranges between 5 and 6 mas/year. Having an average star density of 100 per square degree, this catalogue will be a useful tool for the reduction of CCD frames.

The new catalogue of Positions and Proper Motions (“PPM”) was compiled to replace the AGK3 and SAO catalogues as an astrometric reference on the northern celestial hemisphere. It provides more than 180 000 reference stars, is on the system of FK5 (J2000.0) and has a higher accuracy than the two older catalogues.

Some properties of PPM and its High-Precision Subset are presented.