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of plane trigonometry.
Universal Time Coordinated (UTC)
UTC is the time as maintained by the U.S. Naval Observatory. Because of variations in the Earth's
rotation, UTC is sometimes adjusted by an integer second. The accumulation of these adjustments
compared to GPS time, which runs continuously, has resulted in an offset between GPS time and
UTC. After accounting for leap seconds and using adjustments contained in the navigation message,
GPS time can be related to UTC within 20 nanoseconds or better. Greenwich Mean Time (GMT) is still
the standard time zone for the prime meridian (zero longitude).
Warm Start
Start mode of the GPS receiver when current position, clock offset and approximate GPA time are
input by user or by the application software. Almanac is retained, but ephemeris data is clear.
Wide Area Augmentation System (WAAS)
Developed by the United States government, WAAS is a Satellite-Based Augmentation System
(SBAS) that calculates the errors in the GPS signal at several monitoring stations around the country,
then transmits error correction messages from geostationary satellites to GPS receivers.
World Geodetic System 1984 (WGS 84)
The primary map datum used by GPS. Secondary datums are computed as differences from the
WGS 84 standard. WGS 84 is a set of U.S. Defense Mapping Agency (DMA) parameters for
determining global geometric and physical geodetic relationships. Parameters include a geocentric
reference ellipsoid; a coordinate
system; and a gravity field model. GPS satellite orbital information
in the navigation message is referenced to WGS 84.
