Robert G. Melton

Time-Explicit Representation of Relative Motion Between Elliptical Orbits

Theclassictreatmentofrendezvousmechanicsandotherproblemsinvolvingtherelativemotionoftwospacecraft assumesa circularreferenceorbit, allowing a simpleclosed-form description ofthemotion. Asolution isdeveloped using an elliptical reference orbit, expanding the state transition matrix in powers of eccentricity, while retaining the explicit time dependence of the three-dimensional motion. The solution includes separate matrix elements for e rst-and second-order terms in eccentricity and for both Cartesian and cylindrical coordinates. Assessment of the maximum errors in position and velocity components over one complete revolution of the reference satellite shows that the solution is accurate for practical purposes with eccentricities in the range 0 ‐0.3. An example application is given for the proposed laser interferometer space antenna gravity wave experiment.

Constrained Station Relocation in Geostationary Equatorial Orbit Using a Legendre Pseudospectral Method

An algorithm for numerical approximation is formulated using a Legendre pseudospectral method to solve the problem of constrained station relocation, changing current position to another longitude, in geostationaryequatorial orbit. The problem was stated as a Bolza form in optimal control theory to calculate the control. A collision avoidance term, which is included in the objective function as an integral form and serves as a soft constraint, is considered during transfer by specifying a maximum or minimum radius, depending on whether the transfer is in the east or west direction. Several east-and-west-direction station relocation maneuvers are simulated; the states and control behavior are obtained and all the variables are found to be within feasible ranges. This method would be possible to implement using very-low-thrust engines, in particular, by employing attitude control thrusters.

Numerical Analysis of Constrained, Time-Optimal Satellite Reorientation

Previous work on time-optimal satellite slewing maneuvers, with one satellite axis (sensor axis) required to obey multiple path constraints (exclusion from keep-out cones centered on high-intensity astronomical sources) reveals complex motions with no part of the trajectory touching the constraint boundaries (boundary points) or lying along a finite arc of the constraint boundary (boundary arcs). This paper examines four cases in which the sensor axis is either forced to follow a boundary arc, or has initial and final directions that lie on the constraint boundary. Numerical solutions, generated via a Legendre pseudospectral method, show that the forced boundary arcs are suboptimal. Precession created by the control torques, moving the sensor axis away from the constraint boundary, results in faster slewing maneuvers. A two-stage process is proposed for generating optimal solutions in less time, an important consideration for eventual onboard implementation.

Performance of optical sensors in the control of flexible structures

This paper demonstrates the use of embedded single-mode optical fibers in the active control of smart structures. The experimental prototype structure, a composite lattice configuration fashioned from graphite/bismaleimide laminate, exhibits complex bending and torsional modes, and forms a sub-scale model of a similar structure being analyzed at the Air Force Astronautics Laboratory. Simulations that include effects of Gaussian observation and process noise on a linear quadratic regulator/ loop transfer recovery (LQG/LTR) controller for active vibration damping are presented. The fiber-optic sensor demonstrates advantages over conventional strain gage sensors in terms of reduced sensitivity to sensor placement, reduced problems with observation and control spillovers, and improved control stability.

Multiple Rigid-Body Reorientation Using Relative Motion with Constrained Final System Configuration

Moveable appendages in multibody spacecraft can augment or replace the attitude control actuators. In this work, motionsof the movable bodies relative to themain body areused to adjustthesystem’ s inertial attitude so as to approach or attain a desired target attitude. A control algorithm designed to generate the maneuver commands that cause the necessary relative motions is tested with several cases representing a variety of dynamic conditions. The control can accommodate many different system cone gurations and dynamic conditions such as nonzero system momentum, a problem that historically has proved dife cult to solve in a generalized, three-dimensional mode. Additionally, thecontrolcan return the system’ s geometric cone guration to itsinitial stateby the conclusion of thereorientation. Theresults indicatethat the control can accomplish nearly completereorientationsin all cases tested while meeting the system constraints.

Coast-Arc Orbit Stability During Spiral-Down Trajectories About Irregularly Shaped Bodies

Theorbit stability for a spacecraftwhile in aminimumpropellant, optimal low-thrust transfer fromahigh-altitude orbit to low-altitude orbit around an irregularly shaped body is addressed. To ensure the spacecraft’s safety, it is necessary to know that if the spacecraft’s main engines safe during the period of orbit transfer, then the resulting coast orbit is stable or unstable with low-probability of the spacecraft colliding with the body or escaping from orbit. To answer this question, a Monte Carlo simulation, developed in FORTRAN 90, was developed to analyze a sufficiently large set of coast orbits under the influence of a high-fidelity gravitational model. The perturbations arising from the nonspherical harmonics were derived using Hotine’s partially nonsingular geopotential formulation. Thismethodwas chosen because of the higher efficiency of Hotine’smethodwhen comparedwith using a spherical harmonic analysis. The simulation examines the orbital radius to determine the danger of spacecraft crash or escape.

An overview of space science and engineering education at Penn State

This provides an overview of space science and space systems engineering education at Penn State University. Students at the graduate and undergraduate levels participate in an educational program consisting of three interdependent components: space systems-related courses, space systems project work, and research. Courses range from Introduction to Space Physics to Spacecraft Design. Student project involvement is realized through a number of student-driven space systems projects completed throughout the past two decades. Students are provided opportunities to do independent study projects, honors theses, M.S. theses, and Ph.D. theses on a number of space science and space systems-related topics. Our educational goal is to prepare students at the undergraduate and graduate levels for productive careers in technical and nontechnical fields relating to space systems. Due to student interest, we are developing a certificate in Space Systems Engineering for undergraduates in the College of Engineering. Strengthening Penn States position in space engineering are its Center for Space Research Programs, membership in the USAFs Space Education Consortium, and its role in serving as the lead institution within the Commonwealth for the Pennsylvania Space Grant Consortium

The fallacy of using NII in analyzing aircraft operations

Three measures of noise annoyance (Noise Impact Index, Level-Weighted Population, and Annoyed Population Number) are compared regarding their utility in assessing noise reduction schemes for aircraft operations. While Nil is intended to measure the average annoyance per person in a community, it is found that the method of averaging can lead to erroneous conclusions, particularly if the population does not have uniform spatial distribution. Level-Weighted Population Annoyed Population Number are shown to be better indicators of noise annoyance when rating different strategies for noise reduction in a given community.