TPF-I SWG Report - Exoplanet Exploration Program - NASA

A PPENDIX C
Figure C-3. Example of controller tracking error for both attitude and translation control, the TPF-I
requirements can be met with the TSC.
pressure of 0.5 mN in the inertial z-axis, (vi) mass and inertia uncertainties of 3%, and (vii) kinematic
decoupling errors, which result from realizing inertial force commands with body-fixed thrusters and
imperfect attitude knowledge. To ensure performance with margin, the solar-induced disturbances are
larger than will be expected in practice.
The steady-state simulation results are given in Fig. C-3, which show, respectively, the body x-axis angular
tracking error and the inertial x-axis relative translation tracking error. There is a considerable transient (not
shown) due to the low bandwidth of the controllers required by the TSC. However, the transient will be
reduced when the controllers are integrated with the formation guidance, which provides feed-forward
accelerations.
Mode Commander (Formation/S/C)
The Mode Commander (MDC) coordinates the algorithms with the current mission phase and hardware
capabilities. For example, it lets the FACS algorithms know when the star tracker can provide
measurements or when all spacecraft are ready for a synchronized rotation for covering the u-v plane. All
FACS modes are contained within the two main modes (namely “standalone” and “formation”) and these
two nodes enable the end-to-end operations of the FACS across all mission phases, including the various
anticipated system hardware configurations.
In standalone mode, each of the spacecraft in TPF-I formation can perform basic system checkout at a safe
separation. Upon successful spacecraft system checkout, the spacecraft will be ground commanded to a
closer range, within the formation sensor acquisition range (nominally at 10 km), while still in standalone
operational mode. At this closer range, the ISC and formation sensors signals are to be acquired. Once
formation knowledge and inter-s/c communication is established, the flight system can be commanded to
enter the formation operational mode.
While in the formation mode, the spacecraft within the TPF-I formation can be ground commanded to reconfigure
to any desired safe formation separation. While in formation mode, a loss of ISC or range and
bearing (spacecraft-to-spacecraft position) knowledge will result in automatic transition back to the
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