The tanker spacecraft
We have developed a proprietary design for a pasive tanker spacecraft (no power required), which would contain 3 to 6 tones of storable propellants. It will have an “easy to catch” attitude behavior (spinner with passive nutation dumping) and a passive docking interface. It could be produced in 3-4 months and it could survive for a long time unattended in a GEO-Transfer orbit.
It could be the target of a chaser spacecraft, the Utility Agent (UA), to demonstrate passive docking capability.
It could provide fuel to the UA. (fuel sufficient to reach GEO with surplus for further service demonstrations). In broad terms it is a primitive early substitute of the UB.