Peiman Maghami

Reaction Wheel Disturbance Modeling, Jitter Analysis, and Validation Tests for Solar Dynamics Observatory

The Solar Dynamics Observatory (SDO) aims to study the Suns influence on the Earth by understanding the source, storage, and release of the solar energy, and the interior structure of the Sun. During science observations, the jitter stability at the instrument focal plane must be maintained to less than a fraction of an arcsecond for two of the SDO instruments. To meet these stringent requirements, significant amount of analysis and test effort have been devoted to predicting the jitter induced from various disturbance sources. One of the largest disturbances onboard is the reaction wheel. This paper presents the SDO approach on reaction wheel disturbance modeling and jitter analysis. It describes the verification and calibration of the disturbance model, and ground tests performed for validating the reaction wheel jitter analysis. To mitigate the reaction wheel disturbance effects, the wheels will be limited to operate at low wheel speeds based on the current analysis. An on-orbit jitter test algorithm is also presented in the paper which will identify the true wheel speed limits in order to ensure that the wheel jitter requirements are met.

A μNewton thrust-stand for LISA

The success of the LISA project depends on the ability of the disturbance reduction system to shield the proof masses from all external forces and to maintain tight pointing requirements relative to the other two spacecrafts. μN-thrusters are required to compensate for the solar radiation pressure acting on the spacecraft. The force noise from these thrusters must be low enough not to disturb the freely floating proof masses. To date, these noise requirements have not been demonstrated, mostly because no thrust-stand exists with sufficient sensitivity. We present the status of our μNewton thrust-stand that will verify that the thrusters proposed for LISA will meet the noise requirements.

Solar Dynamics Observatory On-Orbit Jitter Testing, Analysis, and Mitigation Plans

The recently launched Solar Dynamics Observatory (SDO) has two science instruments onboard that required sub-arcsecond pointing stability. Significant effort has been spent pre-launch to characterize the disturbances sources and validating jitter level at the component, sub-assembly, and spacecraft levels. However, an end-to-end jitter test emulating the flight condition was not performed on the ground due to cost and risk concerns. As a result, the true jitter level experienced on orbit remained uncertain prior to launch. Based on the pre-launch analysis, several operational constraints were placed on the observatory aimed to minimize the instrument jitter levels. If the actual jitter is below the analysis predictions, these operational constraints can be relaxed to reduce the burden of the flight operations team. The SDO team designed a three-day jitter test, utilizing the instrument sensors to measure pointing jitter up to 256 Hz. The test results were compared to pre-launch analysis predictions, used to determine which operational constraints can be relaxed, and analyzed for setting the jitter mitigation strategies for future SDO operations.

Space Technology 7 disturbance reduction system - precision control flight validation

The NASA New Millennium Program Space Technology 7 (ST7) project validates technology for precision spacecraft control. The disturbance reduction system (DRS) is part of the European Space Agencys LISA Pathfinder project. The DRS controls the position of the spacecraft relative to a reference to an accuracy of one nanometer over time scales of several thousand seconds. To perform the control, the spacecraft use a new colloid thruster technology. The thrusters operates over the range of 5 to 30 micro-Newtons with precision of 0.1 micro-Newton. The thrust is generated by using a high electric field to extract charged droplets of a conducting colloid fluid and accelerating them with a precisely adjustable voltage. The control reference is provided by the European LISA Technology Package, which includes two nearly free-floating test masses. The test mass positions and orientations are measured using a capacitance bridge. The test mass position and attitude is adjustable using electrostatically applied forces and torques. The DRS controls the spacecraft position with respect to one test mass while minimizing disturbances on the second test mass. The dynamic control system covers eighteen degrees of freedom: six for each of the test masses and six for the spacecraft. After launch in late 2009 to a low Earth orbit, the LISA Pathfinder spacecraft is maneuvered to a halo orbit about the Earth-Sun LI Lagrange point for operations

Control Modes of the ST7 Disturbance Reduction System Flight Validation Experiment

The Space Technology 7 (ST7) experiment will perform an on-orbit system-level validation of two specific Disturbance Reduction System technologies: a gravitational reference sensor employing a free-floating test mass and a set of micronewton colloidal thrusters. The ST7 Disturbance Reduction System (DRS) is designed to maintain the spacecrafts position with respect to a free-floating test mass to less than 10 nm/√Hz over the frequency range of 1 to 30 mHz. This paper presents the overall design and analysis of the spacecraft drag-free and attitude controllers. These controllers close the loop between the gravitational sensors and the micronewton colloidal thrusters. There are five control modes in the operation of the ST7 DRS, starting with the attitude-only mode and leading to the science mode. The design and analysis of each of the control modes as well as the mode transition strategy are presented.

An acquisition control for the laser interferometer space antenna

The Laser Interferometer Space Antenna mission is a planned gravitational wave detector consisting of three spacecraft in heliocentric orbit. Laser interferometry is used to measure distance fluctuations between test masses aboard each spacecraft to the picometre level over a 5 million km separation. The disturbance reduction system comprises the pointing and positioning control of the spacecraft, electrostatic suspension control of the test masses and point-ahead and acquisition control. This paper presents an approach for the acquisition control of the LISA formation. The approach establishes one link at a time. For each link, it defocuses the incoming beams to make its light detectable by the receiving spacecraft. Simulations are performed to demonstrate the feasibility of the proposed approach.

Mode Transitions for the ST7 Disturbance Reduction System Experiment

The Space Technology 7 Disturbance Reduction System experiment will perform an on-orbit system-level validation of two technologies: a gravitational reference sensor employing a free-floating test mass and a set of colloidal micronewton thrusters. The Disturbance Reduction System is designed to maintain the spacecraft s position with respect to a free floating test mass to less than 10 nm/& over the frequency range of 1 to 30 m= mi paper presents the modes that compose the Disturbance Reduction System spacecraft control as well as the strategy used to transition between modes. A high-fidelity model of the system, which incorporates rigid-body models of the spacecraft and two test masses (18 degrees of freedom), is developed and used to evaluate the performance of each mode and the efficacy of the transition strategy.

Pointing acquisition and performance for the laser interferometry space antenna mission

The laser interferometer space antenna (LISA) mission, a space-based gravitational wave detector, uses laser metrology to measure distance fluctuations between proof masses aboard three spacecraft. Each spacecraft has two incoming and two outgoing laser beams for a total of six laser links. These links are established sequentially at the start of the mission, and the spacecraft control systems must aim their lasers at each other with pointing motions less than 8 nrad Hz−1/2 in the frequency band 1–100 mHz. The process for acquiring the laser links as well as the simulated performance is described.

Disturbance reduction system: testing technology for precision formation control

The Disturbance Reduction System (DRS) is a space technology demonstration within NASAs New Millennium Program. DRS is designed to validate system-level technology required for future gravity missions, including the planned LISA gravitational-wave observatory, and for formation-flying interferometers. DRS is based on a freely-floating test mass contained within a spacecraft that shields the test mass from external forces. The spacecraft position will be continuously adjusted to stay centered about the test mass, essentially flying in formation with the test mass. Colloidal microthrusters will be used to control the spacecraft position within a few nanometers, over time scales of tens to thousands of seconds. For testing the level of acceleration noise on the test mass, a second test mass will be used as a reference. The second test mass will also be used as a reference for spacecraft attitude. The spacecraft attitude will be controlled to an accuracy of a few milliarcseconds using the colloidal microthrusters. DRS will consist of an instrument package and a set of microthrusters, which will be attached to the European Space Agencys SMART2 spacecraft with launch scheduled for August 2006.

Systems engineering on the James Webb Space Telescope

The James Web Space Telescope (JWST) is a large, infrared-optimized space telescope scheduled for launch in 2014. System-level verification of critical performance requirements will rely on integrated observatory models that predict the wavefront error accurately enough to verify that allocated top-level wavefront error of 150 nm root-mean-squared (rms) through to the wave-front sensor focal plane is met. This paper describes the systems engineering approach used on the JWST through the detailed design phase.