Date of Award

Summer 1-1-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Aerospace Engineering Sciences

First Advisor

Penina Axelrad

Second Advisor

George Born

Third Advisor

Hanspeter Schaub

Abstract

Angles-only orbit estimation of geosynchronous objects is a unique challenge due to the dense population of clustered geosynchronous objects, the singularities of and perturbations to geosynchronous motion, and the error inherent to experimental observations of geosynchronous objects. Passive optical tracking of geosynchronous space objects has traditionally been performed by ground-based sensors, and the capability has advanced significantly through the introduction of space-based angles-only tracking. This research addresses three key facets of geosynchronous orbit estimation accuracy: improvement to the accuracy via appropriate coordinate modeling, empirical characterization of achievable ground-based angles-only estimation accuracy, and analytic modeling of the space-based angles-only estimated uncertainty. This research develops and analyzes improvements to geosynchronous orbit estimation based on high-fidelity dynamic modeling with a specialized set of coordinates designed specifically to address the geosynchronous orbit conditions. The use of an appropriate representation, the GEO elements, enhances the orbit estimation accuracy compared to the more traditional inertial Cartesian state space representation of geosynchronous motion. Simulation and experimental studies demonstrate that GEO element estimation better recovers the in-track motion than inertial position and velocity state estimation. The short-term estimation accuracy given ground-based tracking is characterized empirically using the Wide Area Augmentation System satellite reference ephemerides. The results show that 10 meter accuracy is possible given short sampling intervals (10 to 30 seconds) and long nightly track lengths (3 or more hours). Several tracking scenarios are found to meet accuracy requirements on the order of 100 meters. The observability of relative states using space-based angles-only tracking of geosynchronous objects by a geosynchronous sensor is analyzed, and first-order analytic expressions for the predicted uncertainty of the along-track separation and intersatellite range are developed assuming space-based passive tracking. The uncertainty models are validated via Monte Carlo analysis. The results demonstrate that 1 hour of continuous space-based passive tracking can estimate the range to the order of tens of meters, and 12 hours produces range uncertainty on the order of meters. The outcome of this research is a set of methods to improve the performance of geosynchronous orbit estimation, and an enhanced understanding of the accuracy possibilities of angles-only ground-based and space-based geosynchronous orbit estimation.

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