Other
Scientific paper
Jun 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991phdt........23a&link_type=abstract
Ph.D. Thesis Stanford Univ., CA.
Other
Control Systems Design, Feedback Control, Global Positioning System, Gravity Probe B, Line Of Sight, Navigation, Onboard Equipment, Orbital Maneuvers, Satellite Orbits, Errors, Geopotential, Gyroscopes, High Frequencies, Microrocket Engines, Receivers, Relativistic Effects, Spacecraft Propulsion, Time Optimal Control
Scientific paper
An onboard closed loop navigation and control system capable of executing extremely precise orbit maneuvers is described. In particular, a system to adjust the orbit of the Gravity Probe B (GP-B) spacecraft is presented. This onboard system relies on the Global Positioning System (GPS) to provide navigation information directly to the vehicle, thus alleviating the need for ground support. GP-B is a NASA project to measure two relativistic effects on orbiting gyroscopes to an unprecendented accuracy. The ideal orbit is circular, polar, and contains the line of sight to a guide star which serves as a distant inertial reference. The target injection orbit is specified based on orbit modeling over the entire 18 month mission, in order to best satisfy the ideal conditions on average. The perturbing effects of the geopotential, the Sun and the Moon, and other forces on the orbit are considered. Simulations show that the target inclination must be achieved to an accuracy of 2 x 10(exp -4) deg (25 m) to satisfy the experiment goals. The design of a GPS based navigation system and onboard orbit trim system is presented. The role of the navigation system is to provide an estimate of the satellite position and velocity state to the control system in a form that can be used to determine the necessary orbit adjustments. Two types of GPS receivers are considered: a high quality, dual frequency receiver capable of eliminating the effects of Selective Availability (SA), and a lower accuracy variety which is subjected to SA level errors. The objective of the trim system is to achieve the target state in minimum time.
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