TPF-I SWG Report - Exoplanet Exploration Program - NASA

T E C H N O L O G Y R OADMAP FOR TPF-I
There are four operating modes that have been defined: safe stand-off, reconfiguration, hand-off, and
observation. The precise boundary between the different operating modes is to some extent arbitrary,
since it depends on the capability of the sensors that are chosen. The acquisition sensor has the poorest
resolution but the widest coverage in range and bearing angle. The acquisition sensor will be used
primarily to establish the array configuration, reconfigure it, and recover from faults that would cause
elements of the array to lose their station. The fine sensor has the most restricted coverage in bearing
angle and is used to maintain the formation during science observations. The medium sensor has a
capability allowing hand-off between the acquisition sensor and the fine sensor. It is the delay and delayrate
limitations of the interferometer during the science observations that drive the formation-flying
requirements.
The requirements on formation flying are decoupled as much as possible from the requirements on
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 for 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 optomechanical 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.
The optical path-difference is required to be controlled to ±1 cm along the beam path, for both in-plane
and out-of-plane directions. This ±1-cm control requirement is shown in Table 6-3, row 10 under the
‘Observation’ operating mode. 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.
Specific requirements are also levied on the ground technology development during the current pre-Phase
A efforts. Since the flight conditions can be simulated in a high-fidelity simulation, the Formation
Algorithms and Simulation Testbed is identical to the flight requirements listed in Table 6-3. The
requirements for the multi-robot based Formation Control Testbed are somewhat relaxed, though scalable
to flight requirements, and are given in Table 6-4.
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