Overview

Contrary to a widely held public impression, the elimination of GPS Selective Availability in 2000 did not take care of the needs that many users have for enhanced GNSS capabilities.

Indeed, various “augmentations” have been developed to meet the requirements of some applications for better accuracy, availability, or integrity (the assurance of the quality of a signal) than are available from GNSS signals in space.

These enhancements have included private solutions, such as off-the-shelf real-time kinematic positioning equipment or more robust and accurate differential corrections from commercial service providers. Additionally, several public land-based augmentation systems exist, including the U.S. Nationwide Differential GPS operated by the U.S. Coast Guard and the U.S. Army Corps of Engineers, the National Geodetic Survey’s Continuously Operating Reference Stations (CORS), and the International GNSS Service (IGS), which provides high-accuracy data on GNSS satellite orbits for use by the world’s geophysical community.

Perhaps the largest and most widespread efforts are the satellite-based augmentation systems (SBAS) being created by government entities around the world, typically associated with aviation-related agencies. These include such programs as the U.S. Wide Area Augmentation System (WAAS) operated by the Federal Aviation Administration (FAA) and the European Geostationary Navigation Overlay Service (EGNOS) currently operated by the European Satellite Services Provider on behalf on behalf of the European Union.

SBAS activities are linked by various organizations to ensure that all certified aviation receivers are compatible with all operational SBAS systems. This means that all service providers and all receiver manufacturers need to have the same understanding of the message format and contents. In the United States, an RTCA Inc. working group two meets three times a year and enables service providers, receiver manufacturers, and the user community to discuss SBAS performance, interpretation of the standards, and future planning. RTCA is also the U.S. agent for contributing to the minimum operational performance standards (MOPS) that provide the most detailed description of how to implement an SBAS.

The European Organization for Civil Aviation Equipment (EUROCAE) Working Group 62 meets twice a year to plan for inclusion of Galileo in future avionics. Finally, the International Civil Aviation Organization (ICAO) is the top-level organization in which civil aviation authorities around the world establish the standards and recommended procedures for all navigation systems.

We asked Dr. Todd Walter to bring us up to date on the status of GNSS augmentation efforts. Walter is a senior research engineer in the Department of Aeronautics and Astronautics at Stanford University and has a long history with international SBAS programs, including WAAS, for which he is helping design future architectures to provide aircraft guidance. Walter also works with the FAA and GPS Directorate on incorporating integrity concepts into GPS III. He is a fellow and current president of the Institute of Navigation.