Space-to-space Radio Interferometry System from Medium Earth Orbits

This paper presents a new concept to perform space-to-space Very Long Baseline Interferometry which enables the imaging of cosmic sources at high-resolution and high-sensitivity with small antennas. Several individual apertures are embarked on separate identical satellites staggered in height into Polar or Equatorial Circular Medium Earth Orbits (PECMEO orbits). These orbits are stable and allow GNSS-based on-the-fly centimeter-level relative positioning. Coherent operation is possible by exchanging local oscillator components and measured signals through Inter-Satellite Links (ISL). On-board cross correlation is performed at each satellite over a delay window compatible with the accuracy of the on-the-fly relative positioning and the result sent to the ground. Image reconstruction is completed on the ground thanks to sub-millimeter baseline retrieval from accurate GNSS orbits, ISL ranging and spacecraft attitude information. The application of this concept to image the Super Massive Black Hole Sgr A^* is hinted.