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Glossary of Terms

2D Lock/Location - the GPS receiver has fixed the current latitude and longitude - no elevation. A minimum of three visible satellites are required.

3D Lock/Location - the GPS receiver has fixed the current latitude, longitude and elevation. A minimum of four visible satellites are required.

Almanac - all GPS satellites transmit almanac data, which describes their orbits and states. GPS receivers store this data to speed up initialization time.

Azimuth - an azimuth is also referred to as a bearing. It is the horizontal direction from one point to another, measured in degrees clockwise.

Beacon - differential GPS uses stationary beacons to transmit differential correction data to nearby beacon receivers.

Bearing - see azimuth

Cold Start - when a GPS receiver downloads almanac data from satellites and then uses it to establish a fix.

Datum - a mathematical model which describes a portion of the Earth's surface.

Geosynchronous Orbit - an orbit where a satellite rotates around the Earth at the same rotational speed as the Earth.

Ionosphere - a region of the Earth's atmosphere between 40 and 500 kilometers high that interferes with the transmission of GPS signals.

Latitude - the angular distance of a location north or south of the equator,

Longitude - the angular distance of a position east or west of the prime meridian (which runs through Greenwich, England between the two poles)

Multiplexing Receiver - a receiver which has to switch between each satellite signal it is tracking. Now essentially obsolete.

NAVSTAR - NAVigation Satellite Timing and Ranging - official name for the global positioning system.


NMEA Standard - A National Marine Electronics Association standard that defines the communication protocols between marine instrumentation.

Parallel Channel Receiver - a receiver which is able to simultaneously track multiple satellites, instead of switching between signals.

Pseudo-random Code - the identifying signal unique to each GPS satellite.

Selective Availability - the US Government program which intentionally degraded GPS accuracy for civilian receivers. Currently off.