James R. ODonnell

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

THE MICROWAVE ANISOTROPY PROBE (MAP) MISSION

The Microwave Anisotropy Probe mission is designed to produce a map of the cosmic microwave background radiation over the entire celestial sphere by executing a fast spin and a slow precession of its spin axis about the Sun line to obtain a highly interconnected set of measurements. The spacecraft attitude is sensed and controlled using an inertial reference unit, two star trackers, a digital sun sensor, twelve coarse sun sensors, three reaction wheel assemblies, and a propulsion system. This paper presents an overview of the design of the attitude control system to carry out this mission and presents some early flight experience.

An Anomalous Force on the Map Spacecraft

The Microwave Anisotropy Probe (MAP) orbits the second Earth-Sun libration point (L 2)—about 1.5 million kilometers outside Earth’s orbit—mapping cosmic microwave background radiation. To achieve orbit near L2 on a small fuel budget, the MAP spacecraft needed to swing past the Moon for a gravity assist. Timing the lunar swing-by required MAP to travel in three high-eccentricity phasing loops with critical maneuvers at a minimum of two, but nominally all three, of the perigee passes. On the approach to the first perigee maneuver, MAP telemetry showed a considerable change in system angular momentum that threatened to cause on-board Failure Detection and Correction (FDC) to abort the critical maneuver. Fortunately, the system momentum did not reach the FDC limit; however, the MAP team did develop a contingency strategy should a stronger anomaly occur before or during subsequent perigee maneuvers. Simultaneously, members of the MAP team developed and tested various hypotheses for the cause of the anomalous force. The final hypothesis was that water was outgassing from the thermal blanketing and freezing to the cold side of the solar shield. As radiation from Earth warmed the cold side of the spacecraft, the uneven sublimation of frozen water created a torque on the spacecraft.

Attitude Control System of the Wilkinson Microwave Anisotropy Probe

The Wilkinson Microwave Anisotropy Probe mission produces a map of the cosmic microwave background radiation over the entire celestial sphere by executing a fast spin and a slow precession of its spin axis about the sun line to obtain a highly interconnected set of measurements. The attitude control system implements this spin-scan observing strategy while minimizing thermal and magnetic fluctuations, especially those synchronous with the spin period. The spacecraft attitude is sensed and controlled using an inertial reference unit, 2 star trackers, a dual-head digital sun sensor, 12 coarse sun sensors, 3 reaction wheel assemblies, and a propulsion system. Sufficient attitude knowledge is provided to yield instrument pointing to a standard deviation (1σ) of 1.3 arc-min per axis. The attitude control system also maintains the spacecraft attitude during orbit maneuvers, controls the spacecraft angular momentum, and provides for safety in the event of an anomaly. An overview of the design of the attitude control system to carry out this mission is presented, as well as some early flight experience.

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.

Solar Dynamics Observatory Guidance, Navigation, and Control System Overview

Angela M. Russo, Scott R. Starin, and Melissa F. VessNASA Goddard Space Flight Center Code 591, Greenbelt, Maryland 20771AbstractThe Solar Dynamics Observatory (SDO) was designed and built at the Goddard Space Flight Center, launchedfrom Cape Canaveral on February 11, 2010, and reached its final geosynchronous science orbit on March 16, 2010.The purpose of SDO is to observe the Sun and continuously relay data to a dedicated ground station. SDO remainsSun-pointing throughout most of its mission for the instruments to take measurements of the Sun. The SDO attitudecontrol system (ACS) is a single-fault tolerant design. Its fully redundant attitude sensor complement includessixteen coarse Sun sensors (CSSs), a digital Sun sensor (DSS), three two-axis inertial reference units (IRUs), andtwo star trackers (STs). The ACS also makes use of the four guide telescopes included as a part of one of the scienceinstruments. Attitude actuation is performed using four reaction wheels assemblies (RWAs) and eight thrusters, witha single main engine used to provide velocity-change thrust for orbit raising. The attitude control software has fivenominal control modes, three wheel-based modes and two thruster-based modes. A wheel-based Safehold running inthe attitude control electronics box improves the robustness of the system as a whole. All six modes are designed onthe same basic proportional-integral-derivative attitude error structure, with more robust modes setting their integralgains to zero. This paper details the final overall design of the SDO guidance, navigation, and control (GNCAtmospheric Imaging Assembly (AIA), led by Lockheed Martin Space and Astrophysics Laboratory; and ExtremeUltraviolet Variability Experiment (EVE), led by the University of Colorado. The basic mission is to observe theSun for a very high percentage of the 5-year mission (10-year goal) with long stretches of uninterrupted observationsand with constant, high-data-rate transmission to a dedicated ground station to be located in White Sands, NewMexico. These goals guided the design of the spacecraft bus that will carry and service the three-instrument payload.Overarching design goals for the bus are geosynchronous orbit, near-constant Sun observations with the ability to flythrough eclipses, and constant HGA contact with the dedicated ground station. A three-axis stabilized ACS isneeded both to point at the Sun accurately and to keep the roll about the Sun vector correctly positioned with respectto the solar north pole. This roll control is especially important for the magnetic field imaging of HM I.The mission requirements have several general impacts on the ACS design. Both the AIA and HMI instrumentsare very sensitive to the blurring caused by jitter. Each has an image stabilization system (ISS) with some ability tofilter out high frequency motion, but below the bandwidth of the ISS the control system must compensate fordisturbances within the ACS bandwidth or avoid exciting jitter at higher frequencies.Within the ACS bandwidth, the control requirement imposed by AIA is to place the center of the solar disk nomore than 2 arc

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.

Anomalous Force on the Wilkinson Microwave Anisotropy Probe

The Wilkinson Microwave Anisotropy Probe orbits the outer Earth‐sun libration point, about 1.5 million kilometers outside Earth’s orbit, mapping cosmic microwave background radiation. To achieve its orbit on a small fuel budget, the spacecraft needed to perform a lunar gravity assist maneuver. Timing the lunar swing-by required the spacecraft to travel in three high-eccentricity phasing loops with critical maneuvers at a minimum of two, but nominally all three, of the perigee passes. On the approach to the first perigee maneuver, spacecraft telemetry showed a considerable change in system angular momentum that threatened to cause the onboard failure detection and correction software to abort the critical maneuver. Fortunately, the system momentum did not reach the preset failure detection limit; however, the mission operations team did develop a contingency strategy should a stronger anomaly occur before or during subsequent perigee maneuvers. Simultaneously, the team developed and tested various hypotheses for the cause of the anomalous force. The final hypothesis was that water was outgassing from the thermal blanketing and freezing to the cold side of the solar shield. As radiation from Earth warmed the cold side of the spacecraft, the uneven sublimation of ice created a torque on the spacecraft.

MAP Attitude Control System Design and Flight Performance

The Microwave Anisotropy Probe (MAP) is a follow-on to the Differential Microwave Radiometer (DMR) instrument on the Cosmic Background Explorer (COBE) spacecraft. To make a full-sky map of cosmic microwave background fluctuations, a combination fast spin and slow precession motion will be used that will cover the entire celestial sphere in six months. The spin rate should be an order of magnitude higher than the precession rate, and each rate should be tightly controlled. The sunline angle should be 22.5 +/- 0.25 deg. Sufficient attitude knowledge must be provided to yield instrument pointing to a standard deviation of 1.3 arc-minutes RSS three axes. In addition, the spacecraft must be able to acquire and hold the sunline at initial acquisition, and in the event of a failure. Finally. the spacecraft must be able to slew to the proper burn orientations and to the proper off-sunline attitude to start the compound spin. The design and flight performance of the Attitude Control System on MAP that meets these requirements will be discussed.

RESTORING REDUNDANCY TO THE MAP PROPULSION SYSTEM

The Microwave Anisotropy Probe is a follow-on to the Differential Microwave Radiometer instrument on the Cosmic Background Explorer. Sixteen months before launch, it was discovered that from the time of the critical design review, configuration changes had resulted in a significant migration of the spacecrafts center of mass. As a result, the spacecraft no longer had a viable backup control mode in the event of a failure of the negative pitch axis thruster. Potential solutions to this problem were identified, such as adding thruster plume shields to redirect thruster torque, adding mass to, or removing it from, the spacecraft, adding an additional thruster, moving thrusters, bending thrusters (either nozzles or propellant tubing), or accepting the loss of redundancy for the thruster. The impacts of each solution, including effects on the mass, cost, and fuel budgets, as well as schedule, were considered, and it was decided to bend the thruster propellant tubing of the two roll control thrusters, allowing that pair to be used for back-up control in the negative pitch axis. This paper discusses the problem and the potential solutions, and documents the hardware and software changes that needed to be made to implement the chosen solution. Flight data is presented to show the propulsion system on-orbit performance.