Concepts for Automated, Precise Low Earth Orbiter Navigation With the Global Positioning System

The Global Positioning System (GPS) is widely used for satellite positioning and navigation and for numerous geolocation activities. Real-time, onboard positioning accuracies for low Earth orbiters (LEOs) currently vary from 50 to 100 m for standalone conventional GPS tracking to somewhat better than 10 m with sophisticated onboard data flltering. Wide-area difierential techniques, such as those supported by the Wide Area Augmentation System (WAAS) under development by the U.S. Federal Aviation Administration, ofier real-time, kinematic positioning accuracies ranging from a few meters to better than a meter over well-deflned local regions. This article describes a concept for extending the wide-area difierential GPS techniques to achieve global, real-time positioning of LEOs at submeter accuracies. GPS design and operation policy issues that currently limit real-time, onboard precision positioning are discussed. The article then examines a number of proposed system design enhancements under consideration by the U.S. Department of Defense for the next-generation GPS, termed GPS III. These potential enhancements, if implemented, would enable global real-time, stand-alone position accuracies of a few decimeters for kinematic users and better than 10 cm for LEOs. Such capabilities could dramatically impact NASA missions by greatly lowering ground operations costs, as well as navigation and orbit determination costs in general.