KEYWORDS: Optical communications, Sensors, Space operations, Systems engineering, Asteroids, Error analysis, Receivers, Transmitters, Ka band, Signal attenuation
Deep-Space Optical Communications is a key emerging technology that is being pursued for high data-rate
communications, which may enable rates up to ten times more than current Ka-band technology. Increasing the
frequency of communication, from Ka-band to optical, allows for a higher data rate transfers. However, as the frequency
of communication increases, the beam divergence decreases. Less beam divergence requires more accurate and precise
pointing to make contact with the receiver. This would require a three-order-of-magnitude improvement from Ka-Band
(~ 1 mrad) to optical (~ 1 urad) in the required pointing. Finding an architecture that can provide the necessary pointing
capability is driven by many factors, such as allocated signal loss due to pointing, range to Earth, spacecraft disturbance
profile, spacecraft base pointing capability, isolation scheme, and detector characteristics. We have developed a suite of
tools to 1) flow down a set of pointing requirements (Error Budget Tool), 2) determine a set of architectures capable of
meeting the requirements (Pointing Architecture Tool), and 3) assess the performance of possible architecture over the
mission trajectory (Systems Engineering Tool). This paper describes the three tools and details their use through the
case study of the Asteroid Retrieval Mission. Finally, this paper details which aspects of the pointing, acquisition, and
tracking subsystem still require technology infusion, and the future steps needed to implement these pointing
architectures.
Future space telescope programs need to assess in-space robotic assembly of large apertures at GEO and ESL2 to support
ever increasing aperture sizes. Since such large apertures will not fit within a fairing, they must rely on robotic
assembly/deployment. Proper assessment requires hardware-in-the-loop testing in a representative environment.
Developing, testing, and flight qualifying the myriad of technologies needed to perform such a test is complex and
expensive using conventional means. Therefore, the objective of the ALMOST program is to develop a methodology for
hardware-in-the-loop assessment of in-space robotic assembly of a telescope under micro-gravity conditions in a more
cost-effective and risk-tolerant manner. The approach uses SPHERES, currently operating inside ISS, to demonstrate inspace
robotic assembly of a telescope that will phase its primary mirror to optical tolerances to compensate for assembly
misalignment. Such a demonstration, exploiting the low cost and risk of SPHERES, will dramatically improve the
maturity of the guidance, navigation and control algorithms, as well as the mechanisms and concept of operations,
needed to properly assess such a capability.
The objective of the Synthetic Imaging Formation Flying Testbed (SIFFT) is to develop and demonstrate algorithms for
autonomous centimeter-level precision formation flying. Preliminary tests have been conducted on SIFFT at the Flat
Floor facility at NASA's Marshall Space Flight Center (MSFC). The goal of the testing at MSFC was to demonstrate
formation reconfiguration of three "apertures" by rotation and expansion. Results were very successful and demonstrate
the ability to position and reconfigure separate apertures. The final configuration was with three satellites floating in an
equilateral triangle. The two Follower satellites expand the formation with respect to the Master satellite, which
executes a 10° rotation. Testing was performed successfully under various initial conditions: initial Follower rotation,
initial Follower drift, and initial significant position error of each Follower. Results show roughly 10cm steady state
error and ±5cm precision. Formation capturing technique, where satellites search for each other without prior
knowledge of the position of the other satellites, were also developed and demonstrated both on the 2D flat table and in
the 3D International Space Station environment. Future work includes using a minimum set of beacons for estimation
and implementing a search algorithm so satellites can acquire each other from any initial orientation.
On-orbit servicing and assembly is a critical enabling technology for the advancement of large scale structures
in space. The goal of the SWARM project (Synchronized Wireless Autonomous Reconfigurable Modules) is
to develop and mature algorithms for autonomous docking and reconfiguration, to be used as the building
blocks for autonomous servicing and assembly. Algorithms for approach, docking, and reconfiguration have been
implemented and tested through a demonstration of the assembly of two telescope sub-apertures at Marshall
Space Flight Center (MSFC) in July 2006. The algorithms developed for reconfiguration set the mass properties
based on the configuration. Updatable parameters include the location of sensors and receivers with respect to
the geometric center, thruster locations, and control gains specific to each configuration. To test these algorithms
in a 2D environment, a ground testbed was developed to provide multiple docking ports and modular payload
attachments. Hardware components include nodes, Universal Docking Ports, posts, sub-aperture mirrors, and
a SPHERES satellite as the assembler tug. Testing at MSFC successfully demonstrated relative docking and
reconfiguration. Valuable information was gained about the performance of the docking under friction, sensitivity
to estimator initialization, thrust authority needed for different phases of the test, and control when CM changes
during the test.
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