acquisition time - see TTFF

active antenna - An amplified antenna

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

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.

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.

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.

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

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

AVL - See automatic vehicle location.

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