A global array with baseline lengths up to 10 000 km gives an angular resolution of 4 mas @ 21cm and less than 0.1 mas @ 3.5mm.
In 2015, Tuomas Savolainen of Aalto University gave a presentation on Very long baseline interferometry. This is the technique of using a large array of radio telescopes. These are the radio telescopes that were recently used to make the first image of a black hole.
Earth’s size limits the baseline lengths below 12000km. Higher resolutions can be obtained by sending an antenna to space.
The Japanese HALCA satellite of VSOP project (1997-2003) had an orbital height of 21,400km and observed the radio bands: 1.6, 5 GHz with a maximum angular resolution of 0.6 mas.
The Russian RadioAstron mission (2011-) has an apogee height: 360 000km
Observing bands: 0.3, 1.6, 5, 22 GHz
Max nominal angular res.: 7 μas
The brightness temperature of what can be observed by the radio telescopes array is a limitation.
These limit the observation targets to:
• Supernova shock waves
• Magnetically active stars
• Jets from accreting black holes
The Event Horizon Telescope Project used VLBI capability at 1.3mm. They imaged black hole at the center of a Galaxy.
China Has Proposed Launching Two Space-Based Radio Telescopes
The Chinese VLBI (Very Long Baseline Interferometry) Network – CVN consists of five radio telescopes and one data processing center. The earth-based CVN will plan to extend into space in the future and there are several proposals.
The first one is the mm Space VLBI Array, which includes two earth satellites with 10m antennas and the highest band of 46GHz. The research purpose of the mm Space VLBI Array is Super Massive Black Hole, Disk Structure and dynamics, mass of SMBH and Jets in Active Galactic Nuclei (AGN).
Another proposal is the Radio Observatory in Space for Exploring the universe (ROSE), which is proposed to image the fine structure of compact celestial objects such as black hole, pulsar and so on. The mission plans to launch a 30 meter-diameter radio telescope into a 2000km x 70000km elliptical orbit, and works together with the earth-based VLBI network, Square Kilometre Array (SKA) and Five-hundred-meter Aperture Spherical radio Telescope (FAST) to get very high resolution and very high sensitivity. ROSE is operated at decimeter and long centimeter.
In the subsequent China’s Lunar Exploration Project, there will be an Earth-moon VLBI experiment, using the antenna of the lunar orbit Tracking and Data Relay Satellite (TDRS) to construct the first Earth moon space VLBI experimental system. They hope using this earth-moon VLBI system to verify the key space VLBI technology. Because the Earth moon VLBI baseline is over 300km, the experiments of astrometry, astrophysics and the deep-space tracking will be carried. Later a 10 meter antenna is planned to be constructed on the moon surface in the future.