Satellite Formation Flying

I have done work with the AF Research Lab to assesses the frequency and average size of maneuvers necessary to stationkeep various satellite formations. Effects from three sources of error are considered: modeling (guidance algorithms are based on simple dynamics), knowledge (navigation errors in position and velocity) and control (execution errors in the burn). A particular focus of my current research is the effect of the Earth’s oblateness on stationkeeping. Even without knowledge or control errors, this perturbation will cause the formation to drift and, therefore, will require maneuvers to maintain position relative to the reference satellite. The effect of this perturbation can be reduced by redefining the nominal trajectory so the target state will lead to an orbit that does not drift with respect to the reference satellite - the J2-invariant orbit described by Schaub and Alfriend.

Tethered Formations

One potential means of reducing the stationkeeping fuel for a satellite formation is to tether the platforms together. The tethers keep the objects from drifting apart, but unfortunately do not keep them from colliding. To mitiigate this possibility, the entire system is spun up. To keep the system oriented toward Earth, the Likins-Pringle rigid body equilibria are used as a baseline design. A flexible lumped-mass model is used to assess the stability of the tethered system. Three parameters related to the formation size, masses and spin rate are varied in order to find designs that demonstrate desirable dynamic behavior. While none of the current designs are Lyupanov stable, formations with a spin axis near the local vertical are well-behaved over time spans of several orbits.

Biomechanics

This research aims at developing physics-based models of human motion (e.g. walking, running, raquet/club swing) and using computer optimization to a) improve performance in elite athletes and b) develop new designs for sport equipment and prosthetics. With regard to performance, data collected from sensors on athletes can be compared to computer models to suggest possible corrections in technique. With regard to equipment design, the computer models can be used to test the effectiveness of new or altered equipment designs, or prosthetics for Special Olympians. The current research of this new initiative entails a simple model of multiple rigid links with torque actuators at the joints. An optimal control algorithm is used to achieve desired behavior (e.g. maximum height on a jump) with minimal control effort.

Other Areas:

Satellite Maneuvering via Electrodynamic Tethers

Space Transportation with a Tether Sling (See Space Grant)