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and multipath.
TIME TO FIRST FIX (TTFF)
The time it takes to find the satellites is called the Time to First Fix (TTFF). If you
have not used your GPS unit for several months, the almanac data for the satellites
may be out of date. The unit is capable of recollecting this information on its own,
but the process can take several minutes. The time it takes after the user first turns
on the GPS receiver, when a GPS receiver has lost memory, or has been moved
over 300 miles from its last location.
TRIANGULATION
A method of determining the location of an unknown point, as in GPS navigation, by
using the laws 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.
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