Brij N. Agrawal

Bio: Brij N. Agrawal is an academic researcher from Naval Postgraduate School. The author has contributed to research in topics: Spacecraft & Adaptive optics. The author has an hindex of 24, co-authored 134 publications receiving 2256 citations. Previous affiliations of Brij N. Agrawal include International Telecommunications Satellite Organization & United States Department of the Navy.

Light propagation through anisotropic turbulence.

Abstract: A wealth of experimental data has shown that atmospheric turbulence can be anisotropic; in this case, a Kolmogorov spectrum does not describe well the atmospheric turbulence statistics. In this paper, we show a quantitative analysis of anisotropic turbulence by using a non-Kolmogorov power spectrum with an anisotropic coefficient. The spectrum we use does not include the inner and outer scales, it is valid only inside the inertial subrange, and it has a power-law slope that can be different from a Kolmogorov one. Using this power spectrum, in the weak turbulence condition, we analyze the impact of the power-law variations α on the long-term beam spread and scintillation index for several anisotropic coefficient values ς. We consider only horizontal propagation across the turbulence cells, assuming circular symmetry is maintained on the orthogonal plane to the propagation direction. We conclude that the anisotropic coefficient influences both the long-term beam spread and the scintillation index by the factor ς2−α.

Shape control of a beam using piezoelectric actuators

Abstract: This paper presents the analytical and experimental results on optimal placement of piezoceramic actuators for shape control of beam structures. The objective is to determine the optimum piezoceramic actuator locations and voltages to minimize the error between the desired shape and the achieved shape. The analytical model for predicting beam deformation due to a piezoelectric actuator is based on the Euler-Bernoulli model. The cost function has fifth-order polynomials in the actuator locations and second-order polynomials in actuator voltages. This difference resulted in difficulty in simultaneous optimization of actuator locations and voltages. Using embedded Nader and Mead simplex algorithms to separately optimize actuator locations and voltages was found to produce reliable results, converging to the same optimum solution for a variety of initial conditions. Experimental results show that the analytical model provides a reasonable prediction of actuator performance at low input voltage, but does not account for the nonlinear behavior of the piezoceramic and effects of hysteresis.

Spacecraft Vibration Reduction Using Pulse-Width Pulse-Frequency Modulated Input Shaper*

Abstract: Minimizing vibrations of a flexible spacecraft actuated by on-off thrusters is a challenging task. This paper presents the first study of Pulse-Width PulseFrequency (PWPF) modulated thruster control using command input shaping. Input shaping is a technique which uses shaped command to ensure zero residual vibration of a flexible structure. PWPF modulation is a control method which provides pseudo-linear operation for an on-off thruster. The proposed method takes full advantage of the pseudo-linear property of a PWPF modulator and integrates it with a command shaper to minimize the vibration of a flexible spacecraft induced by on-off thruster firing. Compared to other methods, this new approach has numerous advantages: 1) effectiveness in vibration suppression, 2) dependence only on modal frequency and damping, 3) robustness to variations in modal frequency and damping, 4) easy computation and 5) simple implementation. Numerical simulations performed on an eight-mode model of the Flexible Spacecraft Simulator (FSS) in the Spacecraft Research and Design Center (SRDC) at US Naval Postgraduate School (NPS) demonstrate the efficacy and robustness of the method.

Terminal Guidance System for Satellite Rendezvous

Abstract: This paper assumes a requirement for an unmanned multiunit satellite to be assembled in orbit. The requirement to be met is to bring the satellites together so tha t they do not collide but actually rendezvous. The equations of motion of the rendezvous satellite in a relative coordinate system are derived and used to compute a final injection velocity which would effect collision after a time r. The velocity is corrected periodically by a command guidance system and just before impact retrothrust is applied. A terminal infrared homing sj^stem is required to actually accomplish physical contact and joining of the satellites. The first satellite placed in orbit is the "control satellite" and controls all the satellites to be assembled and contains the ccmputer, command guidance equipment, precision orientation equipment, and other features necessary to effect rendezvous. The succeeding satellites contain a propulsion system, a rough at t i tude control system, and a command receiver plus whatever scientific equipment they carry to perform their basic mission. This paper presents the following:

Optical Communication in Space: Challenges and Mitigation Techniques

Abstract: In recent years, free space optical (FSO) communication has gained significant importance owing to its unique features: large bandwidth, license free spectrum, high data rate, easy and quick deployability, less power, and low mass requirements. FSO communication uses optical carrier in the near infrared band to establish either terrestrial links within the Earth’s atmosphere or inter-satellite/deep space links or ground-to-satellite/satellite-to-ground links. It also finds its applications in remote sensing, radio astronomy, military, disaster recovery, last mile access, backhaul for wireless cellular networks, and many more. However, despite of great potential of FSO communication, its performance is limited by the adverse effects (viz., absorption, scattering, and turbulence) of the atmospheric channel. Out of these three effects, the atmospheric turbulence is a major challenge that may lead to serious degradation in the bit error rate performance of the system and make the communication link infeasible. This paper presents a comprehensive survey on various challenges faced by FSO communication system for ground-to-satellite/satellite-to-ground and inter-satellite links. It also provides details of various performance mitigation techniques in order to have high link availability and reliability. The first part of this paper will focus on various types of impairments that pose a serious challenge to the performance of optical communication system for ground-to-satellite/satellite-to-ground and inter-satellite links. The latter part of this paper will provide the reader with an exhaustive review of various techniques both at physical layer as well as at the other layers (link, network, or transport layer) to combat the adverse effects of the atmosphere. It also uniquely presents a recently developed technique using orbital angular momentum for utilizing the high capacity advantage of optical carrier in case of space-based and near-Earth optical communication links. This survey provides the reader with comprehensive details on the use of space-based optical backhaul links in order to provide high capacity and low cost backhaul solutions.

Liquid Sloshing Dynamics

Abstract: Preface Introduction 1. Fluid field equations and modal analysis in rigid containers 2. Linear forced sloshing 3. Viscous damping and sloshing suppression devices 4. Weakly nonlinear lateral sloshing 5. Equivalent mechanical models 6. Parametric sloshing (Faraday's waves) 7. Dynamics of liquid sloshing impact 8. Linear interaction of liquid sloshing with elastic containers 9. Nonlinear interaction under external and parametric excitations 10. Interactions with support structures and tuned sloshing absorbers 11. Dynamics of rotating fluids 12. Microgravity sloshing dynamics Bibliography Index.

Spacecraft Dynamics and Control: A Practical Engineering Approach

Abstract: 1. Introduction 2. Orbital dynamics 3. Orbital maneuvers 4. Attitude dynamics and kinematics 5. Gravity gradient stabilization 6. Single and dual spin stabilization 7. Attitude maneuvers in space 8. Momentum-biased attitude stabilization 9. Reaction jet attitude 10. Structural dynamics and liquid sloshing Appendix A. Attitude transformation in space Appendix B. Attitude determination hardware Appendix C. Orbit and attitude control hardware.