FUTURE DEVELOPMENTS IN VERY LONG BASELINE INTERFEROMETRY R. T. SCHILIZZI Joint Institute for VLBI in Europe P.O. Box 2, 7990 AA Dwingeloo, The Netherlands Email: schilizzi@jive.nl 1 Introduction Radio astronomy will undergo fundamental changes in the next 10 to 15 years to enable the momentum of the past decades to be maintained in the future. The key issue to be tackled is sensitivity across the spectrum from meter to sub-millimeter wavelengths. At the long wavelength end of the spectrum (meters to centimeters), the Square Kilometer Array (SKA) will provide a massive increase in sensitivity. At millimeter and sub-millimeter wavelengths, the Atacama Large Millimeter Array (ALMA, see R. S. Booth - these Proceedings) will also provide a large increase in sensitivity. These two instruments are expected to be fully operational 10 (ALMA) to 15 (SKA) years from now. In the coming decade, developments in VLBI will also focus on achieving higher sensitivity with existing arrays, and on the exploration of the potential of ALMA and SKA for even higher sensitivity VLBI. These three elements - VLBI, ALMA and SKA - form the basis of the radio astronomy programme in Europe for the coming decades. 2 Current VLBI arrays At meter and centimeter wavelengths, three major regional arrays are in operation: the European VLBI Network in Europe and Asia, the VLBA in the USA, and the Asia-Pacific Telescope centred on Australia (see Figure 1). These networks are used primarily for astronomical and astrometric observations in stand-alone mode, in combination (EVN+VLBA), and in combination with the Japanese HALCA orbiting radio telescope (Figure 2). The VLBA is a dedicated array, the EVN and APT are part-time arrays observing about 30% and 10% of the year respectively. The Coordinated Millimeter VLBI Array (CMVA, Figure 1) makes astronomical measurements at a wavelength of 3 mm several weeks a year. Preliminary experiments are also being carried out at wavelengths as short as 1 mm. The northern hemisphere arrays use compatible MkIV or VLBA data acquisition systems based on multi-track linear tape recorders which allow recording rates up to 1024 Mbit/sec (MkIV) or 512 Mbit/sec (VLBA with 2 acquisition drives). The APT uses the Canadian S-2 recording system based on helical scan video cassette recorders and allowing recording rates up to 128 Mbit/sec. The third major recording system has been developed in Japan based on the ID-1 standard for the communications industry, and is in use in the VSOP space VLBI mission. The VLBI technique can also be applied to geodetic measurements. The International VLBI Service has been set up to define and maintain the celestial reference frame monitor universal time and the length of day monitor the coordinates of the celestial pole measure Earth orientation parameters at regular intervals, and contribute to the maintenance of the terrestrial reference frame Some 30 telescopes participate in regional or global networks on a regular basis to make the required measurements. A.B. Smolders and M.P. van Haarlem (eds.) Perspectives on Radio Astronomy Technologies for Large Antenna Arrays Netherlands Foundation for Research in Astronomy - 1999
Figure 1. The world-wide distribution of radio telescopes involved in astronomical VLBI Correlation of astronomical data currently takes place at a number of locations around the world: NRAO Array Operations Center in Socorro, New Mexico USA (20 stations) for VLBA, VLBA+EVN, and ground array+halca observations using the VLBA and MkIV systems Max-Planck-Institute for Radioastronomy in Bonn, Germany (6 stations) for MkIII observations on the EVN and CMVA MIT Haystack Observatory, Westford, Massachusetts USA (6 stations) for MkIII observations on the CMVA National Astronomical Observatory at Mitaka, Japan (10 stations) for ground array+halca observations using the VSOP system Australia Telescope National Facility, Sydney Australia (6 stations) for APT observations using the S2 system Herzberg Institute for Astrophysics, Penticton Canada (6 stations) for ground array+halca observations using the S2 system A series of MkIV correlators are being brought on-line at the present time. The design and construction was carried out under the auspices of an international collaboration involving European radio observatories and the Haystack Observatory in the USA. They are located at: JIVE Dwingeloo The Netherlands (16 stations) for EVN observations (and EVN+VLBA in early 2000) MPIfR Bonn (9 stations, replacing the MkIII correlator) for geodetic and CMVA observations US Naval Observatory, Washington (8 stations, replacing a 6 station MkIII correlator) for astrometric and geodetic observations Haystack Observatory (4 stations replacing the MkIII correlator) for geodetic and CMVA observations In addition there are geodetic VLBI correlators at the Institute for Applied Astronomy in St Petersburg Russia, and at the Communications Research Laboratory and the Geographical Survey Institute both in Japan. 12
The main goals for high sensitivity VLBI observations using these current facilities are (sub)- milli-arcsec astronomical imaging of sub-mjy sources at centimeter wavelengths 10 s of micro-arcsec imaging of 10 s mjy sources at millimeter wavelengths astrometric precision of better than 100 micro-arcsec geodetic precision of 1 part in 10 10 3 Future developments in VLBI The main technical challenges in the next decade will encompass 1) data transport at even higher data rates using magnetic recording or optical fibres, 2) second generation space VLBI missions, 3) VLBI as an integral part of the ALMA and SKA arrays, and 4) next generation correlators. Realisation of points 1, 2, and 3 will have the effect of further increasing the sensitivity of VLBI observations perhaps to sub-microjy levels, while point 4 will be a necessary part of exploiting the new observational capabilities. Data transport Developments in data recording technology are directed at low wear-rate, low cost headstacks, higher areal density, and higher data rates (Whitney, 1999). Thin film (TF) head technology developed for the computer disk industry holds the promise of VLBI headstacks whose lifetime may be double that of conventional designs and whose cost is kept low due to the commercial element. These same headstacks should in principle provide ~ 4 times the areal density of the MkIV system using conventional headstacks by allowing twice the lateral track density and twice the longitudinal bit density. With TF heads it may also be possible to increase the number of tracks recorded simultaneously in order to increase the data rate up to 4 or 8 Gbit/sec. However, VLBI applications are not high on the priority list of the TF head manufacturers, and this possibility may never eventuate. Another route forward is expected to involve off-the-shelf commercial components from the computer industry in a similar development to that of the S2 system which uses commercial VHS recorders, but with individual recorder rates near 1 Gbit/sec rather than 16 Mbit/sec. Industry expectations are that this should be possible in 5 years; a bank of 8 such recorders at each telescope would provide a data rate of 8 Gbit/sec per station (Whitney, Haystack Observatory Note, 30 Sept 1999). This direction is already being pursued for VLBI applications by W. Cannon and his group at Crestech in Canada, where the development phases are called S3 (1 Gbit/sec) and S4 (multi-gbit/sec). Optical fibre links between telescope and correlator are another obvious means of transporting data at the high rates required, although at the present time the commercial costs of operating such an enterprise at rates of tens of Gbit/sec are not within reach of the scientific community. The potential advantages of electronic VLBI (Whitney 1999) are its higher reliability tape recorders are the source of most current reliability problems, lower operational costs through full automation, and faster data turn-around. Fibre links have been installed on a modest scale in radio astronomy projects in Japan (OLIVE) and in the USA connecting the VLBA telescope at Pie Town to the VLA. A feasibility study of linking the Cambridge MERLIN Telescope to Jodrell Bank by fibre has just been completed, and an extension of the study to the EVN will begin shortly. A VLBI Standard Interface is currently being defined by a study group formed by the URSI Global VLBI Working Group and the International VLBI Service - representing the astronomical and geodetic communities - in order that subsequent generations of tape recorder based acquisition systems or fibre systems developed in different regions of the world will have the same data formats and protocols. 13
Figure 2. The HALCA satellite in the VLBI Space Observatory Program (VSOP) Second generation space VLBI Following the magnificent achievement of launching the first ever space VLBI satellite, HALCA, in 1997, the Institute for Space and Aeronautical Science in Japan is contemplating a second mission, VSOP-2, that will build on the experience already gained. In prospect is a satellite with perhaps 15 m diameter HALCA has 8m, higher frequencies of observation - up to 43 GHz, lower system temperature receivers, and higher bandwidths of data transmission down to the ground several Gbit/sec. The sensitivity will be at least 20 times higher than achieved by HALCA. Other mission concepts under study are 1) ARISE in the USA that envisages a 30 m diameter inflatable antenna operating perhaps to 90 GHz and sensitivities almost 2 orders of magnitude higher than HALCA, 2) a European study of the potential of using the International Space Station as a base for constructing a 30m class solid panel radio telescope that would be later boosted into its operational orbit, and 3) Millimetron, a Russian concept for a space radio telescope operating at millimeter wavelengths which would follow Radioastron, a first generation mission that hopefully will be launched within the next few years. ALMA, SKA, and VLBI Plans for the ALMA correlator include the possibility of phasing up the entire ALMA array so that it can function as the most sensitive element in the worldwide millimeter VLBI array. In the companion volume to this, Garrett (1999) argues persuasively that the SKA configuration should allow for approximately 50% of the collecting area to be spread around the globe so that high angular resolution sub-microjy VLBI can carried out. Particular targets are the distant starburst galaxies that can be expected to dominate the source counts at these flux density levels. Next generation correlator By 2010, new generations of VLBI correlators will be necessary to take account of the higher data rates and possibly different data transport methods. Undoubtedly, such correlator designs will build on the new ideas being developed for the ALMA and SKA projects. 14
4 References [1] M. A. Garrett, Limitations of ad hoc SKA+VLBI configurations and the need to extend SKA to trans-continental dimensions in Perspectives in Radio Astronomy: Scientific Imperatives at cm and m Wavelengths (Dwingeloo:NFRA), Edited by: M. P. Haarlem & J. M. van der Hulst, (1999) [2] A. R. Whitney, Directions in VLBI technology in New Astronomy Reviews (1999), in press. 15
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