Edmund M. Kong

Electromagnetic Formation Flight for Multisatellite Arrays

The use of propellant to maintain the relative orientation of multiple spacecraft in a sparse aperture telescope such as NASA’s Terrestrial Planet Finder (TPF) poses several issues. These include fuel depletion, optical contamination, plume impingement, thermal emission, and vibration excitation. An alternative is to eliminate the need for propellant, except for orbit transfer, and replace it with electromagnetic control. Relative separation, relative attitude, and inertial rotation of the array can all be controlled by creating electromagnetic dipoles on each spacecraft, in concert with reaction wheels, and varying their strengths and orientations. Whereas this does not require the existence of any naturally occurring magnetic fields, such as the Earth’s, such fields can be exploited. Optimized designs are discussed for a generic system and a feasible design is shown to exist for a five-spacecraft, 75-m baseline TPF interferometer.

Electromagnetic formation flight for sparse aperture telescopes

The use of propellant to maintain the relative orientation of multiple spacecraft in a sparse aperture telescope such as NASAs Terrestrial Planet Finder (TPF) poses several issues. These include fuel depletion, optical contamination, plume impingement, thermal emission, and vibration excitation. An alternative is to eliminate the need for propellant, except for orbit transfer, and replace it with electromagnetic control. Relative separation, relative attitude, and inertial rotation of the array can all be controlled by creating electromagnetic dipoles on each spacecraft and varying their strengths and orientations. This paper not only discusses optimized designs for a generic system but also shows that feasible designs exist for a five spacecraft, seventy-five meter baseline architecture for TPF.

Optimal spacecraft reorientation for earth orbiting clusters: applications to Techsat 21

Abstract Aperture synthesis using distributed satellite systems opens up new challenges for designing propulsive maneuvering algorithms as well as orbital configurations. The ability to place these individual satellites in their desired locations is directly related to the overall performance of the system. This paper discusses such issues and determines the optimal cluster trajectories for problems ranging from cluster initialization to cluster reorientation for minimum energy problems. The determination of these optimal trajectories is based upon the calculus of variation approach. As in most optimal control problems, determination of the costates is not an easy task. Assuming time-invariant first-order dynamics (Hills Equations) leads to a simple linear quadratic tracking problem, thus allowing the co-state trajectories to be estimated. These co-state estimates are then used to determine the optimal trajectories for the clusters through a second-order multiple shooting algorithm. Results specific to cluster initialization and reorientation for the Techsat 21 flight experiment system are presented. It was found that the flight experiment can be initialized rather quickly but a maneuvering time of at least three orbital periods for the geo-location problem is required.

Dynamics and Control of Tethered Formation Flight Spacecraft Using the SPHERES Testbed

This paper elaborates on the theory and experiment of controlling tethered spacecraft formation without depending on thrusters. In dealing with such underactuated systems, much emphasis is placed on complete decentralization of the control and estimation algorithms in order to reduce the dimensionality and complication. The nonlinear equations of motions of multi-vehicle tethered spacecraft are derived by Lagrange’s equations. Decentralization is then realized by the diagonalization technique and its stability is proven by contraction theory. The preliminary analysis predicts unstable dynamics depending on the direction of the tether motor. The controllability analysis indicates that both array resizing and spin-up are fully controllable only by the reaction wheels and the tether motor, thereby eliminating the need for thrusters. Based upon this analysis, gain-scheduling LQR controllers and nonlinear controllers by feedback linearization have been successfully implemented into the tethered SPHERES testbed, and tested at the NASA MSFCs flat floor facility using two and three SPHERES configurations. The relative sensing mechanism employing the ultrasound ranging system and the inertial gyro is also described.

Development and verification of algorithms for spacecraft formation flight using the SPHERES testbed: application to TPF

The MIT Space Systems Laboratory and Payload Systems Inc. has developed the SPHERES testbed for NASA and DARPA as a risk-tolerant medium for the development and maturation of spacecraft formation flight and docking algorithms. The testbed, which is designed to operate both onboard the International Space Station and on the ground, provides researchers with a unique long-term, replenishable, and upgradeable platform for the validation of high-risk control and autonomy technologies critical to the operation of distributed spacecraft missions such as the proposed formation flying interferometer version of Terrestrial Planet Finder (TPF). In November 2003, a subset of the key TPF-like maneuvers has been performed onboard NASAs KC-135 microgravity facility, followed by 2-D demonstrations of two and three spacecraft maneuvers at the Marshall Space Flight Center (MSFC) in June 2004. Due to the short experiment duration, only elements of a TPF lost in space maneuver were implemented and validated. The longer experiment time at the MSFC flat-floor facility allows more elaborate maneuvers such as array spin-up/down, array resizing and array rotation be tested but in a less representative environment. The results obtained from these experiments are presented together with the basic estimator and control building blocks used in these experiments.

Architecting the search for terrestrial planets and related origins (ASTRO)

As a cornerstone in NASAs Origins program, the primary goal of the Terrestrial Planet Finder (TPF) mission is to directly detect the existence of Earth-like planets around nearby stars. This paper presents a process and a software tool, based on a quantitative systems engineering methodology, to conduct architectural trade studies during the TPF mission conceptual design phase.

SPHERES tethered formation flight testbed: advancements in enabling NASA's SPECS mission

This paper reports on efforts to control a tethered formation flight spacecraft array for NASAs SPECS mission using the SPHERES test-bed developed by the MIT Space Systems Laboratory. Specifically, advances in methodology and experimental results realized since the 2005 SPIE paper are emphasized. These include a new test-bed setup with a reaction wheel assembly, a novel relative attitude measurement system using force torque sensors, and modeling of non-ideal tethers to account for tether vibration modes. The nonlinear equations of motion of multi-vehicle tethered spacecraft with elastic flexible tethers are derived from Lagranges equations. The controllability analysis indicates that both array resizing and spin-up are fully controllable by the reaction wheels and the tether motor, thereby saving thruster fuel consumption. Based upon this analysis, linear and nonlinear controllers have been successfully implemented on the tethered SPHERES testbed, and tested at the NASA MSFCs flat floor facility using two and three SPHERES configurations.

Design of an algorithm for autonomous docking with a freely tumbling target

For complex unmanned docking missions, limited communication bandwidth and delays do not allow ground operators to have immediate access to all real-time state information and hence prevent them from playing an active role in the control loop. Advanced control algorithms are needed to make mission critical decisions to ensure safety of both spacecraft during close proximity maneuvers. This is especially true when unexpected contingencies occur. These algorithms will enable multiple space missions, including servicing of damaged spacecraft and missions to Mars. A key characteristic of spacecraft servicing missions is that the target spacecraft is likely to be freely tumbling due to various mechanical failures or fuel depletion. Very few technical references in the literature can be found on autonomous docking with a freely tumbling target and very few such maneuvers have been attempted. The MIT Space Systems Laboratory (SSL) is currently performing research on the subject. The objective of this research is to develop a control architecture that will enable safe and fuel-efficient docking of a thruster based spacecraft with a freely tumbling target in presence of obstacles and contingencies. The approach is to identify, select and implement state estimation, fault detection, isolation and recovery, optimal path planning and thruster management algorithms that, once properly integrated, can accomplish such a maneuver autonomously. Simulations and demonstrations on the SPHERES testbed developed by the MIT SSL will be executed to assess the performance of different combinations of algorithms. To date, experiments have been carried out at the MIT SSL 2-D Laboratory and at the NASA Marshall Space Flight Center (MSFC) flat floor.

An investigation of electromagnetic control for formation flight applications

The use of propellant to maintain the relative orientation of multiple spacecraft in a sparse aperture telescope such as NASAs Terrestrial Planet Finder (TPF) poses several issues. These include fuel depletion, optical contamination, plume impingement, thermal emission, and vibration excitation. An alternative is to eliminate the need for propellant, except for orbit transfer, and replace it with electromagnetic control. Relative separation, relative attitude, and inertial rotation of the array can all be controlled by creating electromagnetic dipoles on each spacecraft and varying their strengths and orientations. This paper addresses some of the control issues that arise when using electromagnets to control formation geometry.

Exploiting orbital dynamics for interstellar separated spacecraft interferometry

Due to the much improved angular resolution achievable with an interferometer, a number of future NASA missions have resorted to using such an imaging system in an attempt to answer fundamental questions posed in the Origins Program. In this paper, the orbit designs of a Sun-synchronous separated spacecraft interferometer operating in the visible spectrum for interstellar imaging are presented. Particularly, the concept of using delay lines to coherently interfered the reflected science light is emphasized. Comparison between the same interferometer placed outside the gravity-well indicates that the use of delay lines becomes viable only when a delay length-mass conversion of 0.006 kg/m is achieved, assuming that a 500 sec specific impulse propulsive system is used.