We review the development of the space geodetic techniques of Very-Long Baseline Interferometry (VLBI) and the Global Positioning System (GPS) at the Onsala Space Observatory. This facility has been participating already in the first transatlantic VLBI measurement. Throughout the period from 1980 to date Onsala has been involved in more than 1000 individual baseline determinations. If we include the first technical developent phase we can look back at 25 years of history during which the observatory has contributed to refining our understanding of plate tectonics.

The Galactic Center radio source Sgr A * was detected for the first time with VLBI at 43GHz. Observations made with four antennas of the Very Long Baseline Array suggest a size of the source exceeding that of a scatter-broadened unresolved object by a factor of 2-3. The data indicate that the structure is elongated along P.A. = −25 ± 10°. A first image was made, with an angular resolution of ∼ 1 mas (corresponding to 8.5 AU at r 0 = 8.5 kpc)

Recent improvements in data analysis and receiver techniques have allowed us to produce maps of the 100 GHz emission from the compact cores of active galactic nuclei with the unsurpassed resolution of 50-mu-as (microarcseconds). We present here hybrid maps of a set of compact radio sources observed at two epochs with a global VLBI array. The high resolution enables us to show details of active galactic nuclei on size scales of 10(16)-10(17) cm. Jets are shown to be more curved in these inner parts than further out in the areas mapped with VLBI at lower frequencies. Our maps of the quasar 3C345 show that the curvature seen with lower resolution instruments continues very close to the core. New components are seen separating from the cores of 3C84 and BL Lac. We observe a component in 3C84 separating from the core with an apparent speed approximately 21000 km sec-1. The radio source OJ287 is still unresolved with our array, having a core size of less-than-or-similar-to 10(17) cm. There is no indication of any compact component in 3C279 which would be associated with the outburst in integrated flux density which happened some months before our observation. The flux density of the most compact component we observe in 3C279 agrees well with that of the quiet core as extrapolated from its radio spectrum at lower frequencies. The inner part of the radio jet of the giant elliptical galaxy M87 also shows a continuation of the structure on a larger size scale, with a structure we interpret as parts of a helical pattern. No fringes were found for 4C39.25 or Sgr A.

Recent improvements in data analysis and receiver techniques have allowed us to produce a map of the 100GHz emission from the compact radio source 3C273 with the unsurpassed resolution of 50-mu-as (microarcseconds). Our map shows that the structure within 300-mu-as (approximately 1.5.10(18).h-1 cm) has a position angle significantly different from the position angle of the jet observed at lower frequencies. There are also indications in our map that the inner structure has a more pronounced wiggling structure than has been observed on larger scales. The observations were made about 60 days from the start of the outburst of 1988. Most of the flux from the outburst is concentrated in a component which is elongated approximately (56 x 5).10(16).h-1 cm perpendicular to the overall jet-axis. The distance between this component and the core is approximately 128-mu-as, which corresponds to the distance expected from an apparent velocity of approximately 800-mu-as year-1.

The positions of circumstellar molecular masers relative to each other, to the central star, and to the compact extragalactic sources are of astrophysical and astrometrical interest. The SiO masers at λ = 7 mm are especially interesting as the maser spots are located very close to the photosphere of the star. For astrometry we would like to see a proper distribution of the maser spots and several epochs of observations in order to estimate the position of the central star and eventual motions of the spots.The optical position of this star can be determined with ground-based astrometric facilities, and many of the stars showing SiO emission are in theHipparcoscatalogue. A link between the Optical and Radio Reference Frames is achieved in a second step relating the maser spot positions to the Extragalactic Reference Frame (EGRF) established with VLBI (Baudryet al., 1984).

A series of 3.6 cm VLBI maps of 3C345 (1980.79 to 1988.38) is presented. The evolution of the jet components in this quasar is briefly discussed. Recent behavior of component C5 is described and the presence of a new component, tentatively labeled C6, is suggested.

Eight VLBI maps of 3C 345 were obtained at 8.3 GHz between 1980 and 1986. During this period, the two jet components C4 and C5 were shown to emerge from the core in succession and to move superlumi ...