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SPSx51 Modular GPS Receivers User Guide 183
Glossary
SNR See signal-to-noise ratio.
triple frequency GPS A type of receiver that uses three carrier phase measurements (L1, L2, and L5).
UTC Universal Time Coordinated. A time standard based on local solar mean time at the
Greenwich meridian.
VRS Virtual Reference Station. A VRS system consists of GPS hardware, software, and
communication links. It uses data from a network of base stations to provide
corrections to each rover that are more accurate than corrections from a single base
station.
To start using VRS corrections, the rover sends its position to the VRS server. The VRS
server uses the base station data to model systematic errors (such as ionospheric
noise) at the rover position. It then sends RTCM correction messages back to the rover.
WAAS Wide Area Augmentation System. WAAS was established by the Federal Aviation
Administration (FAA) for flight and approach navigation for civil aviation. WAAS
improves the accuracy and availability of the basic GPS signals over its coverage area,
which includes the continental United States and outlying parts of Canada and
Mexico.
The WAAS system provides correction data for visible satellites. Corrections are
computed from ground station observations and then uploaded to two geostationary
satellites. This data is then broadcast on the L1 frequency, and is tracked using a
channel on the GPS receiver, exactly like a GPS satellite.
Use WAAS when other correction sources are unavailable, to obtain greater accuracy
than autonomous positions. For more information on WAAS, refer to the FAA website
at http://gps.faa.gov.
The EGNOS service is the European equivalent and MSAS is the Japanese equivalent of
WAAS.
WGS-84 World Geodetic System 1984. Since January 1987, WGS-84 has superseded WGS-72 as
the datum used by GPS.
The WGS-84 datum is based on the ellipsoid of the same name.