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2D operating mode - A GPS position that requires a minimum of three visible satellites and that has only horizontal coordinates.

3D operating mode - A GPS position that requires a minimum of three visible satellites and that has horizontal coordinates and elevation.

absolute positioning - The mode whereby using a single receiver a position is determined, respecting a well-defined coordinate system.

accuracy - A type of measurement used to determine how close the GPS position to the true location is.

acquisition time - see TTFF

active antenna - An amplified antenna

active leg - The segment of the route currently being traveled.

aerial map - The type of map that uses photographs taken from a plane to show what an area looks like.

agonic line - An imaginary line on the earth's surface connecting points where the magnetic declination is zero.

AGPS - Assisted Global Positioning System - A land station assists GPS in acquiring position.

alidade - Instrument used to determine direction horizontally or vertically.

almanac data - Orbital position data for each GPS satellite. Considered not to be very accurate as the data are valid for over one month.

altimeter - An instrument that measures altitude or elevation with respect to a reference level, usually mean sea level, by means of air pressure.

altitude - A distance measurement that is made in a vertical direction, using as a reference point the sea level.

analog signal - Continuous signal that is sent to the GPS receiver

anywhere fix - That ability that some GPS receivers have, to calculate a position without knowing the time and an approximate location.

antenna splitter - A piece of hardware which can be used to fed two GPS receivers, by splitting the antenna signal into two.

anti-spoofing - A method used to encrypt the P-code in order to prevent the jamming of the satellite signal.

APRS - Automatic Position Reporting System - An amateur radio based automatic position reporting system for tracking and digital communications.

arcGIS - A family of software products that form a complete GIS (Geographic Information System).

archipelago - A large group of islands.

arcInfo - Comprehensive software in the arcGIS family that has advanced geoprocessing and data conversion capabilities.

arete - A sharp, narrow mountain ridge or spur.

ARGOS - Advanced Research and Global Observation Satellite - An advanced research and development satellite launched by the United States Air Force. Carries various space and atmospheric experiments.

atlas - A collection of maps that have been traditionally been kept in a book or loose-leaf format, but that can now be found in multimedia formats.

atoll - A ringlike coral island and reef that nearly or entirely encloses a lagoon.

atomic clock - A precise clock that is used by GPS satellites and that works using elements cesium or rubidium. Such a clock has an error of only one second per million years.

automatic vehicle location - A system of providing real-time location information for emergency vehicles, delivery trucks, service vehicles, etc.

availability - The number of hours per day in which a GPS position determination is possible, by having sufficient satellites.

AVL - See automatic vehicle location.

axis - The imaginary line around which the Earth rotates.

azimuth - The horizontal angle (0 - 360 degrees) from a reference point, usually true north or true south. Also known as a bearing.

azimuth ring - The dial on a compass, marked with zero to 360 degree markings.

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