Glossary

12-channel A GPS receiver that can keep a lock on 12 satellites simultaneously.

14-channel A GPS receiver that can keep a lock on 14 satellites simultaneously.

Accuracy A measure of how close the position given by the GPS is to the true position.

Acquisition time The time it takes for the GPS receiver to determine its initial position.

Active antenna An antenna that amplifies the GPS signal before feeding it to the GPS receiver.

Active leg The segment of a route that is currently being traveled.

Almanac A data file that contains orbit information on the GPS satellites, clock corrections, and atmospheric delay information. The almanac is transmitted by the GPS satellites to a GPS receiver to enable it to keep a lock and reacquire a lost signal quickly.

Atomic clock A very accurate clock that uses the decay of elements such as cesium or rubidium to measure the passage of time. A typical cesium clock has an error of about one second per million years. GPS satellites make use of multiple atomic clocks.

Availability The number of hours that a particular location has sufficient satellites to allow a GPS receiver to work.

Breadcrumb trail A visual representation of a path taken, shown on a map. Also known as a track.

C/A-code Short for “Coarse/Acquisition” or “S-code,” a name given to the civilian GPS signal.

COG See Course over ground.

Cold start The power-on sequence in which the GPS receiver downloads almanac data before establishing a fix.

Constellation The set of GPS satellites in orbit around the Earth.

Control segment The Earth-based component of the GPS network (the satellite control system).

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Coordinates A unique numeric or alphanumeric scheme used to describe a precise location.

Course over ground The direction in which the GPS receiver is traveling.

Course to steer The direction to take in order to return on course.

Crosstrack Error The distance you are off a desired course in either direction.

CTS See Course to steer.

Cutoff angle See Mask angle.

Datum A position for which an accurate measurement of latitude and longitude is known. This is used in map-making and in GPS for determining coordinates across the whole map. It is used in mapping and surveying to ensure map accuracy.

DOP Dilution of Precision. An estimate made by the GPS of how much error exists in the reading.

Ephemeris The predictions of current satellite positions that are transmitted to the GPS receiver.

External antenna An antenna designed to be attached and detached from a GPS receiver.

Fix A single position that includes latitude, longitude, altitude, time, and date.

GLONASS This is the Russian counterpart to GPS.

GPS Global Positioning System.

GPSr Another term for a GPS receiver.

Healthy A term used to describe an orbiting GPS satellite suitable for use. “State” can also be used to refer to satellite health.

Latitude A north-south measurement of position perpendicular to the Earth’s polar axis.

Longitude An east-west measurement of position. This is measured in relation to the prime meridian (the line corresponding to 0o longitude), an imaginary circle that passes through both the north and south poles.

Magnetic north The direction in which the north end of the compass needle points.

Mask angle The minimum elevation of a satellite above the horizon to ensure a good signal. This can be between 5 degrees and 15 degrees. GPS receivers usually ignore satellites below this elevation.

Multipath error Error caused by the interference of a GPS signal. It occurs when a signal from one satellite has reached the receiver antenna more than once because it has taken two or more different paths. It is generally caused when one signal has been bounced or reflected.

P-code Short for “Precision,” a name given to the encrypted military GPS signal.

Position The latitude, longitude, and altitude of a particular point. Often, an estimate of error is associated with a position.

SA See Selective availability.

Satellite constellation The arrangement of the GPS satellites at any particular time.

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Selective availability An intentional random error in the clock time of the civilian GPS signal that was intended to degrade the signal and thereby only allow the military/government to have access to precise location information. This was removed in May 2000.

Space segment The satellite component of the GPS network.

SPS Another name given to the civilian C/A code.

SV Space vehicles. Another name for the GPS satellites.

Track The plot of a route taken or a route to be followed, stored on a GPS.

True north The direction to the north pole.

User segment The receivers using the GPS signals.

UTC Universal Coordinated Time. The time system formerly known as Greenwich Mean Time (GMT).

WAAS Wide Area Augmentation System. A system of ground-based beacons or additional satellites that improve the baseline GPS system. To make use of this, you need a WAAS-enabled GPS receiver.

Warm start The power-on sequence in which the GPS receiver does not need to download the almanac data before establishing a fix.

Waypoint A position stored in the GPS.

XTE See Crosstrack error.

Y-code Another name given to the encrypted P-code.