Local Oscillator Concept for the Event Horizon Imager

The Event Horizon Imager (EHI) is a mission concept conceived to provide a radio image of the surroundings of the event horizons of SgrA* and M87* supermassive black holes with an angular resolution at least an order of magnitude better than that achievable by radio telescopes on ground. The concept is based on performing space-to-space very long baseline interferometry (VLBI) using two satellites in medium Earth orbit at slightly different altitudes. Projected baselines up to about 26,000 km and down to a few tens of kilometers can be realized. Free of atmospheric perturbations, EHI is observed in three channels: 557 GHz, the main frequency to achieve the finest angular resolution by minimizing the scattering of the inter-stellar medium towards our galactic center; 230 GHz, inherited from the event horizon telescope (EHT); finally, 43 GHz in support of the relative delay determination. The whole EHI concept relies on very precise relative positioning between the two satellites including wavelength bootstrapping and a novel local oscillator concept that is the topic of this paper. It is shown that atomic clocks cannot provide sufficient coherence over the needed integration time. Instead, a system consisting of conventional crystal oscillators and an inter-satellite link (ISL) that can work coherently for extended periods of time, longer than required, and is therefore better suited for EHI is demonstrated.