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The concept for space interferometry from Polar or Equatorial Circular Medium Earth Orbits (the PECMEO concept) is a promising way to acquire the image of the “shadow” of the event horizon of Sagittarius A* with an angular resolution of circa 5 microarcseconds.The concept is intended to decrease the size of the main reflector of the instrument to about 3 m using a precise orbit reconstruction based on Global Navigation Satellite System (GNSS) navigation,inter-satellite range and range-rate measurements,and data from the Attitude and Orbit Determination System (AODS).The paper provides the current progress on the definition of the subsystems required for the concept on the basis of simulations,radio regulations,and available technology.The paper proposes the requirement for the localization of the phase centre of the main reflector.The paper provides information about the visibility of GNSS satellites and the needed accuracies of the AODS.The paper proposes the frequency plan for the instrument and its Inter-Satellite Links (ISLs).The concepts for measurement of range and range rate using ISLs (as well as for the data exchange at these ISLs) are presented.The block diagram of the interferometer is described and its sensitivity is estimated.The link budget for both ISLs is given as well as their critical components.The calculated measurement quality factors are given.The paper shows the expected performance of the sub-systems of the interferometer.The requirements for the localization of the main reflectors and the information about the availability of the GNSS satellites are based on the simulations results.The frequency plan is obtained according to the PECMEO concept and taking into account the radio regulations.The existing technology defines the accuracies of the AODS as well as the link budgets and the measurement accuracies for both ISLs and the sensitivity of the instrument.The paper provides input information for the development of the orbit reconstruction filter and the whole PECMEO system.