Maps GPS Glossary – A

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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