TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 6
Two-Spacecraft Distributed Interferometer
The purpose of this simulation was to demonstrate nominal, ground-based operation of a distributedinterferometer
formation with high-level commands and autonomous formation reconfigurations with
collision avoidance. The spacecraft are in deep space and have several flexible modes due to a large
sunshield with a fundamental mode at 0.5 Hz. An additional requirement for this FACS design was that
all thruster firings for both attitude and relative position control across all spacecraft occur in the same 4-s
window every 60 s. This requirement allows nanometer-level interferometer control loops to reacquire a
fringe and make a science observation between impulsive disturbances due to thrusters. Subsequently, the
selection of throttle-able ion thrusters that do not impart impulsive disturbances obviated this thruster
quiescence requirement.
The spacecraft go through individual modes to become power positive (such as detumble, Sun-search, and
Sun-point) and then begin to acquire the formation. In this scenario, the two spacecraft are assumed to be
in a stack and to separate via springs. Propagation using accelerometers is used to determine an
approximate position of the other spacecraft. With this knowledge, the spacecraft point their formation
sensors at one another. The autonomous-formation flying (AFF) sensor was used in this simulation,
which is RF-based with a Global Positioning System-like (GPS-like) signal structure. The field-of-view
of the sensors is 70 deg. After acquiring relative sensor lock and establishing inter-spacecraft
communication (ISC), a coarse control loop is closed to maintain a constant distance, and the formation
rotates toward the Sun on the formation solar panels. This formation is held until a command is received.
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Figure 6-7. Visualization of FAST demonstration of two-robot FCT.