TPF-I SWG Report - Exoplanet Exploration Program - NASA

T E C H N O L O G Y R OADMAP FOR TPF-I
Moreover, the residual center-of-gravity (CG) mass imbalance torque of 5 mN-m of the top attitude
platform of the FCT robot is about 33 times worse than the expected solar pressure induced torque of
0.15 mN-m on the TPF-I spacecraft sunshade due CG to center of solar-pressure offset of ~0.25 m.
The integration of the FACS software was successfully completed in February 2006, enabling the two
FCT robots to operate in a formation mode for the first time. There are a number of calibration activities
underway (inertia, thrusters, etc.); however, initial test results indicate that the closed-loop formation
performance is very close to the required level (within factor of 2). Further formation tests will be
performed once the ongoing calibration activities are completed.
Figure 6-9 shows the two-robot baseline-hold performance while pointing to a specified (non-zenith)
position. The plot shows the performance during the baseline/target hold, after two robots have
maneuvered to achieve the desired baseline (2 m) and target star pointing (20 deg. off zenith). The
required performance bounds are also indicated on the plot.
The FCT currently does not have formation sensors to directly measure the inter-robot/spacecraft range
and bearing. The range and bearing values for the FCT robots are currently derived from the position and
attitude measurements from the star tracker on each robot — using parallax from the near-field FCT
pseudo-star beacons mounted on the dome ceiling. An optical range and bearing sensor is currently under
final hardware integration and test (I&T) phase to provide direct relative sensing capabilities in the near
future. The difference of a factor of 60 in bearing control between the flight design and FCT requirement
(1 vs. 60 arcmin) arises from the greatly reduced spacing between the 'spacecraft' in the testbed. The size
of the control volume within which each spacecraft is constrained at a given time is comparable for the
two cases. The technology gap between testbed and flight is really the precision of the bearing sensor that
is needed to maintain the formation. Bearing sensing in the flight design will be achieved with a
combination of inertial star trackers on each spacecraft and encoders on the steering mirrors that direct the
science and metrology beams between the spacecraft, both of which are well within the current state-ofthe-art.
In FY2006, using the two deployed FCT robots, FCT has functionally validated the FAST algorithms and
the end-to-end formation-flying architecture, and the FCT team is currently in the process of validating
the FAST predicted performance.
6.4 Future Hardware for General Astrophysics
The Terrestrial Planet Finder Interferometer (TPF-I) has immense potential to broadly transform
astrophysics as well as to detect Earth-like planets. This potential relies on exploiting the high angular
resolution and high sensitivity of TPF-I simultaneously in an imaging mode. Both hardware and software
for aperture synthesis imaging capabilities must be developed to work well with a high-dynamic range
over a small field-of-view (from one to a few diffraction-limited ‘primary beams’) using faint off-axis
guide stars. This may require substantial modifications to the TPF system architecture beyond the
minimum hardware architecture necessary for the baseline requirement of nulling interferometry over a
narrow field (within one primary beam) around bright targets. However, the discovery potential for TPF-I
will be so radically expanded by incorporating these additional imaging capabilities that they should be
seriously considered as part of the overall TPF-I design. In this section we explore the potential hardware
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