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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"> <HTML> <HEAD> <TITLE>Abstract AAS 97-629</TITLE> </HEAD> <BODY BGCOLOR="ffffff"> <h2>AAS 97-629</h2> <h2>AUTONOMOUS NAVIGATION WITH LIDAR ALTIMETRY </h2> <h4> Dr. C.J. Weeks, A. Girardi and J. Lynch - Loyola Marymount University </h4> <h2> Abstract </h2> NASA has planned current and future missions to rendezvous with comets and asteroids, and to execute close orbits around them. Comets and asteroids are small, irregularly shaped bodies with weak gravitational fields. In addition, comet outgassing may produce a turbulent atmospheric environment. The time delays inherent in Earth-based navigation will be undesirable. <br><br> This project investigates the feasibility of autonomous navigation for a spacecraft in a 10 kilometer orbit about a comet 3 kilometers in diameter, by means of a laser altimeter. The project is in three stages. First the data and spacecraft trajectory are simulated. The shape of the comet is then estimated for both eighth and sixteenth degree harmonic models. Computer images of the resulting estimates are displayed. Finally, orbit determination accuracy is tested for both polar and equatorial orbits. Results indicate that autonomous navigation is feasible for a polar orbit, but not an equatorial orbit. The convergence regions are very narrow, indicating that autonomous navigation must be performed continuously.