The National Satellite Testbed Receiver at Mauna Loa
Aeronautics and Astronautics, Stanford University, California 94305-4035
In time, the Global Positioning System (GPS) will be used for a wide variety of aircraft operations. These will include flight over oceanic routes and en route through our domestic airspace. GPS will also be used in the so-called terminal areas where flights converge in crowded metropolitan airspaces. Examples of heavily used terminal areas include the New York-Newark and the San Francisco Bay areas with their multiplicity of airports. GPS will be used on the final approach to airports; these operations demand the greatest safety and reliability. Airport approach applications include: nonprecision approach where GPS will be used solely for horizontal positioning and precision approach where GPS will be used for both horizontal and vertical position fixing. To serve all of these applications, GPS must be augmented to meet stringent requirements with respect to accuracy, integrity, continuity, and time availability. The Wide Area Augmentation System (WAAS) is currently being developed by the Federal Aviation Administration (FAA), because it will allow satellite navigation to
be used as the primary means of navigation for aircraft over the United States.
The WAAS will use a ground network to develop differential corrections for the errors that limit the accuracy of SPS. The ground network includes a geographically distributed set of GPS receivers at precisely known locations. To enable the Category I precision approach of aircraft, this network must have receivers throughout the coverage area with spacings of approximately 600 km. These reference receivers send raw GPS measurements back to master control stations.
The master stations use the reference observations to develop two corrections for each monitored satellite. One correction is for the satellite clock and the other is a three-dimensional correction for the satellite location. The master station also estimates a set of corrections for the ionospheric delays. All of these corrections will be included on the WAAS broadcast and will improve GPS position accuracy from 100 m to approximately 4 m. With this vertical accuracy, WAAS will provide vertical guidance down to decision heights of 60 to 106 m.
Other countries are deploying similar wide-area augmentation systems. The Europeans are installing the European Geostationary Navigation Overlay System (EGNOS) and the Japanese are launching the Multitransport Satellite Augmentation System (MSAS) for their airspaces. This international network will provide the primary means of navigation worldwide.
The FAA has deployed a prototype of the WAAS. This system is known as the National Satellite Testbed (NSTB) and is currently being used to explore issues associated with the international use of WAAS. These issues include ionospheric effects on low and high latitude users of satellite navigation. They also include data exchange between neighboring wide-area augmentation systems such as the WAAS and the Japanese system. The receiver at the Mauna Loa Observatory, Hawaii, is an important part of this research effort.
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