The National Satellite Testbed Receiver at Mauna Loa
Per Eng
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.