Introduction

Motivation and Scientific Aim

The very high angular resolution of Earth-based cm-wave VLBI can be increased further in two ways, either by using longer baselines or by observing at shorter wavelengths. The first approach leads to "space-VLBI" (VLBI with one or more orbiting antennas), the second approach to "millimeter-VLBI" (mm-VLBI). In the more distant future, both techniques may be combined. This would lead to space-VLBI at millimeter wavelenghts ("mm-space-VLBI"). To date space-VLBI observations (e.g. with the RadioAstron mission) have been shown to be technical feasible in the cm-bands, down to 1.3cm wavelength. Millimeter VLBI offers a much higher angular resolution than ground or space based VLBI at centimeter wavelengths and has another very significant advantage: in the spectral mm-bands it is possible to study emission regions which appear self-absorbed (and are therefore invisible) at longer wavelengths. This has important consequences for a better understanding of the physical processes acting in Active Galactic Nuclei (AGN) and in the vicinity of supermassive Black Holes. After years of continuous development and technical improvement, mm-VLBI is now able to provide good quality images in the mm-bands, with an angular resolution of typically 50-70 micro-arcseconds at 3.5 mm wavelength. At shorter wavelengths the VLBI technique is being developed further but at present is limited to fewer VLBI antennas. The Event Horizon Telescope (EHT) observes at 230 GHz and future observations at 260 GHz and 345 GHz are planned. Global mm-VLBI allows the imaging of compact galactic and extragalactic radio sources with an angular resolution which is unsurpassed by any other present astronomical imaging method. The "Global mm-VLBI Array" (GMVA) has been set up by a group of radio observatories interested in performing regular astronomical VLBI observations at millimeter wavelengths and with open access for the scientific community. The GMVA offers to the User Community a large and constantly growing VLBI network combining the most sensitive radio telescopes in the mm-band, and hence an improved sensitivity and image fidelity in the resulting micro-arcsecond scale resolution VLBI images. As the success of the GMVA depends on the scientific results it will produce, we invite all interested scientists to make use of it. Please see the other documents and links on this web page for further and more detailed information.