Engineering topics

Flatley, Thomas W.

Publications and source records attributed to Flatley, Thomas W..

9 records

NASA NTRS1999

Backup Attitude Control Algorithms for the MAP Spacecraft

The Microwave Anisotropy Probe (MAP) is a follow-on to the Differential Microwave Radiometer (DMR) instrument on the Cosmic Background Explorer (COBE) spacecraft. The MAP spacecraft will perform its mission, studying the early origins of the universe, in a Lissajous orbit around the Earth-Sun L(sub 2) Lagrange point. Due to limited mass, power, and financial resources, a traditional reliability concept involving fully redundant components was not feasible. This paper will discuss the redundancy philosophy used on MAP, describe the hardware redundancy selected (and why), and present backup modes and algorithms that were designed in lieu of additional attitude control hardware redundancy to improve the odds of mission success. Three of these modes have been implemented in the spacecraft flight software. The first onboard mode allows the MAP Kalman filter to be used with digital sun sensor (DSS) derived rates, in case of the failure of one of MAP's two two-axis inertial reference units. Similarly, the second onboard mode allows a star tracker only mode, using attitude and derived rate from one or both of MAP's star trackers for onboard attitude determination and control. The last backup mode onboard allows a sun-line angle offset to be commanded that will allow solar radiation pressure to be used for momentum management and orbit stationkeeping. In addition to the backup modes implemented on the spacecraft, two backup algorithms have been developed in the event of less likely contingencies. One of these is an algorithm for implementing an alternative scan pattern to MAP's nominal dual-spin science mode using only one or two reaction wheels and thrusters. Finally, an algorithm has been developed that uses thruster one shots while in science mode for momentum management. This algorithm has been developed in case system momentum builds up faster than anticipated, to allow adequate momentum management while minimizing interruptions to science. In this paper, each mode and algorithm will be discussed, and simulation results presented.

ODonnell, James R., Jr.↗

NASA NTRS1997

Polar Spin Axis Anomaly

The Global Geospace Science (GGS) Polar Plasma Laboratory (POLAR) spacecraft was launched on February 24, 1996, by a Delta 2. The spacecraft, a major axis spinner, appeared to function nominally throughout the early mission phase, which included several deployments, and orbit and attitude maneuvers. Of particular interest is the fact that the spacecraft was launched with a deliberate dynamic imbalance. During a segment of early orbit operations, a pair of Lanyard Deployed Booms (LDB) were extended. These booms were not identical; the intent was that the spacecraft would be nearly dynamically balanced after they were deployed. The spacecraft contained two dynamic balance mechanisms intended to fine tune the balance on orbit. However, subsequent images taken by the science instruments on the Despun Platform during the dynamic balancing segment indicated an offset of the principal spin axis from the geometric axis. This offset produced a sinusoidal blurring of the science images sufficiently large to degrade science data below mission requirement specifications. In the end, the imbalance encountered in flight was significantly outside the correction capability of the balances. The purpose of this paper is to examine the flight data during the various deployment and maneuver stages of the early orbit operations coupled with analytical simulations to discuss some of the potential causes of the resultant imbalance.

Crouse, Patrick L.↗

NASA NTRS1995

SAMPEX special pointing mode

A new pointing mode has been developed for the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) spacecraft. This pointing mode orients the instrument boresights perpendicular to the field lines of the Earth's magnetic field in regions of low field strength and parallel to the field lines in regions of high field strength, to allow better characterization of heavy ions trapped by the field. The new mode uses magnetometer signals and is algorithmically simpler than the previous control mode, but it requires increased momentum wheel activity. It was conceived, designed, tested, coded, uplinked to the spacecraft, and activated in less than seven months.

Markley, F. Landis↗

NASA NTRS1995

A magnetic hysteresis model

The Passive Aerodynamically Stabilized Magnetically Damped Satellite (PAMS) will be deployed from the Space Shuttle and used as a target for a Shuttle-mounted laser. It will be a cylindrical satellite with several corner cube reflectors on the ends. The center of mass of the cylinder will be near one end, and aerodynamic torques will tend to align the axis of the cylinder with the spacecraft velocity vector. Magnetic hysteresis rods will be used to provide passive despin and oscillation-damping torques on the cylinder. The behavior of the hysteresis rods depends critically on the 'B/H' curves for the combination of materials and rod length-to-diameter ratio ('l-over-d'). These curves are qualitatively described in most Physics textbooks in terms of major and minor 'hysteresis loops'. Mathematical modeling of the functional relationship between B and H is very difficult. In this paper, the physics involved is not addressed, but an algorithm is developed which provides a close approximation to empirically determined data with a few simple equations suitable for use in computer simulations.

Flatley, Thomas W.↗

NASA NTRS1994

An Earth Albedo Model: A Mathematical Model for the Radiant Energy Input to an Orbiting Spacecraft Due to the Diffuse Reflectance of Solar Radiation from the Earth Below

Past missions have shown that the earth's albedo can have a significant effect on the sun sensors used for spacecraft attitude control information. In response to this concern, an algorithm was developed to simulate this phenomenon, consisting of two parts, the physical model of albedo and its effect on the sun sensors. This paper contains the theoretical development of this model, practical operational notes, and its implementation in a FORTRAN subroutine.

Flatley, Thomas W.↗

NASA NTRS1993

Testing of the on-board attitude determination and control algorithms for SAMPEX

Algorithms for on-board attitude determination and control of the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) have been expanded to include a constant gain Kalman filter for the spacecraft angular momentum, pulse width modulation for the reaction wheel command, an algorithm to avoid pointing the Heavy Ion Large Telescope (HILT) instrument boresight along the spacecraft velocity vector, and the addition of digital sun sensor (DSS) failure detection logic. These improved algorithms were tested in a closed-loop environment for three orbit geometries, one with the sun perpendicular to the orbit plane, and two with the sun near the orbit plane - at Autumnal Equinox and at Winter Solstice. The closed-loop simulator was enhanced and used as a truth model for the control systems' performance evaluation and sensor/actuator contingency analysis. The simulations were performed on a VAX 8830 using a prototype version of the on-board software.

Mccullough, Jon D.↗

NASA NTRS1992

SAMPEX science pointing with velocity avoidance

The Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) science pointing mode is presented with the additional constraint of velocity avoidance. This constraint has been added in light of the orbital debris and micrometeoroid fluxes that have been revealed by the Long Duration Exposure Facility (LDEF) recovered in January 1990. These fluxes are 50-100 times higher than the flux tables that were used in the September 1988 proposal to NASA for the SAMPEX mission. The SAMPEX Heavey Ion Large Telescope (HILT) sensor includes a flow-through isobutane proportional counter that is susceptible to penetration by orbital debris and micrometeoroids. Thus, keeping the HILT sensor pointed away from the velocity vector, the direction of maximum flux, will compensate for the higher than expected fluxes. Using an orbital debris model and a micrometeoroid model developed at the Johnson Space Center (JSC), and a SAMPEX dynamic simulator developed by the Guidance and Control Branch at the Goddard Space Flight Center (GSFC), an 'optimal' minimum ram angle (the angle between the HILT boresight and the velocity vector) of 90 degrees has been determined. It is optimal in the sense of minimizing the science pointing performance degradation while providing approximately an 89 percent chance of survival for the HILT sensor over a three year period.

Frakes, Joseph P.↗

NASA NTRS1990

On-board attitude determination and control algorithms for SAMPEX

Algorithms for onboard attitude determination and control of the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) were developed. The algorithms include spacecraft ephemeris and geomagnetic field models, attitude determination with 2 degree accuracy, control of pitch axis pointing to the sun and yaw axis pointing away from the Earth to achieve control of pitch axis within 5 degrees of sunline, momentum unloading, and nutation damping. The closed loop simulations were performed on a VAX 8830 using a prototype version of the on-board software.

Flatley, Thomas W.↗

NASA NTRS1986

Cosmic Background Explorer (COBE) transfer orbit attitude control system

The Cosmic Background Explorer (COBE) spacecraft will be launched by the Shuttle from Vandenberg AFB into a 300 km altitude, 99 deg inclination, 6 a.m. or 6 p.m. ascending node orbit. After release from the Remote Manipulator System (RMS) arm, an on-board monopropellant hydrazine propulsion system will raise the orbit altitude to 900 km. The spacecraft continuously spins during transfer orbit operations with the spin axis nominally horizontal and in or near the orbit plane. The blowdown propulsion system consists of twelve 5 lb thrusters (3 'spin', 3 'despin', and 6 'axial') with the latter providing initially 30 lb of force parallel to the spin axis for orbit raising. The spin/despin jets provide a constant roll rate during the transfer orbit phase of the mission and the axials control pitch and yaw. The axial thrusters are pulsed on for attitude control during coast periods and are normally on- and off-modulated for control during orbit raising. Attitude sensors employed in the control loops include an array of two-axis digital sun sensors and three planar earth scanners for position measurements, as well as six gyroscopes for rate information. System redundancy is achieved by means of unique three-axes-in-a-plane geometry. This triaxial concept results in a fail-safe operational system with no performance degradation for many different component failure modes.

Placanica, Samuel J.↗