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GPS Terminology & Glossary
Atomic Clock
A very precise clock that operates using the elements cesium or rubidium. A
cesium clock has an error of one second per million years. GPS satellites
contain multiple cesium and rubidium clocks.
Azimuth
The horizontal direction from one point on the earth to another, measured
clockwise in degrees (0-360) from a north or south reference line. An
azimuth is also called a bearing.
Cartography
The art or technique of making maps or charts. Many GPS receivers have
detailed mapping—or cartography—capabilities.
Coordinates
A set of numbers that describes your location on or above the earth.
Coordinates are typically based on latitude/longitude lines of reference or
a global/regional grid projection (e.g., UTM, MGRS, Maidenhead).
Differential
GPS (DGPS)
An extension of the GPS system that uses land-based radio beacons to
transmit position corrections to GPS receivers. DGPS reduces the effect of
selective availability, propagation delay, etc. and can improve position
accuracy to better than 10 meters.
Waypoint
Waypoints are locations or landmarks worth recording and storing in your
GPS. These are locations you may later want to return to. They may be check
points on a route or significant ground features. (e.g., camp, the truck, a
fork in a trail, where Charlie buried his treasure,).
Waypoints may be defined and stored in the unit manually, by taking
coordinates for the waypoint from a map or other reference. This can be done
before ever leaving home. Or more usually, waypoints may be entered directly
by taking a reading with the unit at the location itself, giving it a name,
and then saving the point. Waypoints may also be put into the unit by
referencing another waypoint already stored, giving the reference waypoint,
and entering the distance and compass bearing to the new waypoint.
Difference between bearing and heading
'Bearing' is the direction you are aiming at while 'heading' is the
direction you are actually going. Sometimes they are the same but sometimes
you can't head directly where you want to go because of fences, wind, road,
and other reasons. Heading is sometimes called 'track' so there are three
words in use to describe these things. When flying you can actually point
the plane in one direction while flying in another (due to wind) and neither
may actually be toward the final destination (due to mountains).
Route
A route is a series of waypoints entered in the order that you want to
navigate them.
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. Time to
First Fix (TTFF) is the time it takes a GPS receiver to find satellites
after the user first turns it on (when the GPS receiver has lost memory or
has been moved over 300 miles from its last location).
8 or 12 Channel Receivers
The number of channels used by your GPS receiver is directly related to the
number of satellite transmissions it can interpret at once. For example, if
you have an 8 Channel receiver, then you can access eight different
satellites at once. A 12 Channel receiver can interpret signals from twelve
satellites. This is important if accuracy and consistency is a prime
concern.
WAAS
WAAS stands for Wide Area Augmentation System. WAAS is a system of
satellites and ground stations that provide GPS signal corrections, giving
you even better position accuracy. A WAAS-capable receiver can give you a
position accuracy of better than three meters, 95 percent of the time. And
you don't have to purchase additional receiving equipment or pay service
fees to utilize WAAS. WAAS consists of approximately 25 ground reference
stations positioned across the United States that monitor GPS satellite
data. Two master stations, located on either coast, collect data from the
reference stations and create a GPS correction message. This correction
accounts for GPS satellite orbit and clock drift plus signal delays caused
by the atmosphere and ionosphere. The corrected differential message is then
broadcast through one of two geostationary satellites, or satellites with a
fixed position over the equator. The information is compatible with the
basic GPS signal structure, which means any WAAS-enabled GPS receiver can
read the signal.
Carrier frequency
Frequency of the
unmodulated output of a radio transmitter. L1 carrier signal
broadcasts at 1575.42 MHz (19 cm wavelength). L2 carrier signal
broadcasts at 1227.60 MHz (24 cm wavelength).
Epoch
A specific
instant in time. GPS carrier phase measurements are made at a given
frequency (e.g. every 30 seconds) or epoch rate.
Epoch
Date
The date, usually
expressed in decimal years, for which published coordinates and data are
valid.
Estimated
Position Error (EPE)
A measurement of horizontal position error in feet or meters based upon a
variety of factors including DOP and satellite signal quality.
Estimated Time Enroute (ETE)
The time it will take to reach your destination (in hours/minutes or
minutes/seconds) based upon your present position, speed, and course.
Estimated Time Of Arrival (ETA)
The estimated time you will arrive at a destination.
Geocaching
A high-tech version of hide-and-seek. Geocachers seek out hidden treasures
utilizing GPS coordinates posted on the Internet by those hiding the cache.
Geographic Information System
(GIS)
A computer system or software capable of assembling, storing, manipulating,
and displaying geographically referenced information (i.e., data identified
according to their location). In practical use, GIS often refers to the
computer system, software, and the data collection equipment, personnel, and
actual data.
GPS week
Incremental
number of weeks, starting at 0 hour UTC on the date January 6, 1980. April
6, 1997 is the first day of GPS week 900.
Greenwich Mean
Time (GMT)
The mean solar time for Greenwich, England, which is located on the Prime
Meridian (zero longitude). Based on the rotation of the earth, GMT is used
as the basis for calculating standard time throughout most of the world.
Latitude
A position's distance north or south of the equator, measured by degrees
from zero to 90. One minute of latitude equals one nautical mile.
Ionosphere
A region of the earth's atmosphere where ionization caused by incoming solar
radiation affects the transmission of GPS radio waves. It extends from a
height of 50 kilometers (30 miles) to 400 kilometers (250 miles) above the
surface.
Ionospheric delay
Signal delay or
acceleration as a wave propagates through the ionosphere. Phase delay
depends upon the electron content and affects the carrier signal. Group
delay depends upon the dispersion in the ionosphere as well, and affects the
code signal.
Kinematic
GPS
Observations
while a receiver is in motion. In surveying applications, kinematic refers
to uninterrupted carrier-phase measurements following successful solution of
the integer ambiguities. This can be accomplished in a continuous mode where
the receiver remains in motion for precise positioning of a vehicle, or in
an intermittent mode where data is recorded only after a receiver is brought
to a stationary point, and the observations while in motion are tracked as a
way to maintain the integer ambiguities.
Nautical Mile
A unit of length used in sea and air navigation, based on the length of one
minute of arc of a great circle, especially an international and U.S. unit
equal to 1,852 meters (about 6,076 feet).
Navigation
The act of determining the course or heading of movement. This movement
could be for a plane, ship, automobile, person on foot, or any other similar
means.
Pseudo-static GPS
Also known as
pseudo-kinematic and repeat occupation, this relative positioning technique
relies upon two or more simultaneous observations at a point pair, separated
by some time interval (typically 60 minutes or more), in order to solve the
integer bias terms from the change in satellite geometry occurring between
the repeat observations.
Static
GPS
Carrier phase
differencing technique where the integer ambiguities are resolved from an
extended observation period through a change in satellite geometry.
TracBack
The proprietary Garmin feature which takes your current track log and
converts it into a route to guide you back to a starting position.
Track Up Orientation
Fixes the GPS receiver’s map display so the current track heading is at the
top of the screen.
Track (TRK)
Your current direction of travel relative to a ground position (same as
Course Over Ground).
Universal
time
Local solar mean
time at Greenwich Meridian. Some commonly used version of Universal Time
are:
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