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Absolute Positioning

A mode in which a position is identified with respect to a well-defined coordinate system, commonly a geocentric system (i.e., a system whose point of origin coincides with the center of mass of the earth)

Accuracy

The degree of closeness of a measurement result to its true value. The degree of conformance between the estimated or measured position, time, and/or velocity of a GPS receiver and its true time, position, and/or velocity as compared with a constant standard. Radionavigation system accuracy is usually presented as a statistical measure of system error

Accuracy relative

The accuracy with which a user can measure position relative to that of another user of the same navigation system at the same time

Accuracy statistic

The industry standard definition of stataic accuracy is the standard deviation of error if a point set by a receiver was tested in a fix position for 24 hours.

Acquisition

The ability to find and lock on to satellite signals for ranging

Additional Secondary Factor (ASF)

Error propagation time caused by radiowaves slowing down as they pass over different terrain. This is in addition to slowing down as radiowaves pass over sea water

ADRG

ARC-digitized raster graphics. A United States military map format for providing maps on CD

AFE

Atlas Florae Europaeae. The AFE grid is modified from the Military Grid Reference System (MGRS)

Algorithm

A special method used to solve a certain type of mathematical problem

Almanac

A data file that contains orbit information on all satellites, clock corrections, and atmospheric delay parameters. It is transmitted by a GPS satellite to a GPS receiver, where it facilitates rapid satellite vehicle acquisition within GPS receivers. Almanac data must be acquired before GPS navigation can begin

Altimeter

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

Ambiguity

The initial bias in a carrier-phase observation of an arbitrary number of cycles. The initial phase measurement made when a GPS receiver first locks onto a GPS signal is ambiguous by an integer number of cycles because the receiver has no way of knowing the exact number of carrier wave cycles between the satellite and the receiver. This ambiguity, which remains constant as long as the receiver remains locked on the signal, is established when the carrier-phase data are processed

Amplitude

Height of a radiowave as measured from an imaginary center line to the wave peak

Amplitude Modulation (AM)

A method of encoding a message on the carrier signal by altering the height of the signal while keeping its frequency constant

AMPS

Analog Mobile Phone System. Non-digital cellular mobile phones

Analog Mobile Phone System (AMPS)

Non-digital cellular mobile phones

Antarctic Circle

An imaginary circle on the surface of the earth at 66.5°S lat.or 23.5° north of the South Pole. It marks the southernmost point at which the sun can be seen at the winter solstice (about June 22) and the northernmost point of the southern polar regions at which the midnight sun is visible. .

Anti Spoofing (AS)

Encryption of the P code signal transforming it to Y code which is unavailable to civilian users. AS prevents an encryption-keyed GPS receiver from being "spoofed" by a bogus, enemy-generated GPS P-code signal

Anywhere fix

The ability of a receiver to start position calculations without being given an approximate location and approximate time

ARC-digitized raster graphics (ADRG)

A United States military map format for providing maps on CD

Arctic Circle

An imaginary circle on the surface of the earth at 66.5°N latitude, or, 23.5° south of the North Pole. It marks the northernmost point at which the sun can be seen at the winter solstice (about Dec. 22) and the southernmost point of the northern polar regions at which the midnight sun is visible

AROF

Ambiguity Resolution On the Fly Fast ambiguity resolution GPS surveying where the solution is determined while the receiver is in motion

ASF

Additional Secondary Factor. Error propagation time caused by radiowaves slowing down as they pass over different terrain. This is in addition to slowing down as radiowaves pass over sea water

Atlas Florae Europaeae (AFE)

A modified from the Military Grid Reference System (MGRS)

Automatic Packet Reporting System (APRS)Terminal Information Service (ATIS)

A protocol that uses packet radio to transmit data that depicts the position of a station with other

Automatic Terminal Information Service (ATIS)

Is is a continuous broadcast of recorded information at selected high activity airports

Automatic Vehicle Location (AVL)

The technique of using a navigation system to determine a vehicle's position, which is then transmitted to a central unit that monitors and tracks the vehicle's position and movement

Automatic Vehicle Monitoring (AVM)

The technique of using a navigation system to determine a vehicle's position, which is then transmitted to a central unit that monitors and tracks the vehicle's position and movement

Availability

The number of hours per day that a particular location has sufficient satellites to make a GPS position fix

Azimuth

This is the direction of a celestial object, measured clockwise around the observer's horizon from north. So an object due north has an azimuth of 0°, one due east 90°, south 180° and west 270°. Azimuth and altitude are usually used together to give the direction of an object in the topocentric coordinate system