The requirements on formation-flying are decoupled as much as possible from the requirements of nulling. The formation-flying system is envisioned as the 'coarse stage' of a multi-layer control system that maintains the optical pathlengths. Centimeter-level variations in the relative positions of the spacecraft are sensed by the instrument's fringe tracking system and compensated by the optical delay lines in each beamtrain, each of which is required to provide a control range of ±10 cm of optical delay. The small changes in the relative bearing angles between the spacecraft are compensated by the articulation of steering mirrors on the collector and combiner spacecraft. The thrusters and reaction wheels will be important disturbance sources for the interferometer, but opto-mechanical modeling will be needed to establish the appropriate requirements.

The range and bearing control requirements during science observations are imposed by the limitations of the fringe sensor and delay line of the interferometer. If the fringes are allowed to move beyond the throw of the delay line, they will be lost. Similarly, if the fringes move too quickly for the fringe tracker to sense them, they will also be lost, even if they are within range of the delay line. The limitations of the delay line therefore impose requirements on range and bearing angle, and limitations of the fringe tracker impose requirements on range rate and bearing rate. A bearing requirement of ±0.33 arcmin is equivalent to ±1 cm spacecraft position control at the shortest baseline with the collector spacecraft center-to-center separation of 100 m.