Glossary of Terms

Acquisition Time

The time it takes for your GPS unit to acquire a lock onto enough satellites (3 for a 2 Dimension and 4 for a 3 Dimension) position fix.

Almanac Data

Constellation information transmitted by the satellites and collected by the GPS unit. Almanac data allows the unit to quickly acquire satellites as soon as it's turned on.

Azimuth

The angle of measurement between the horizon and a satellite or other object.

Baseline

The measured distance between two receivers or two antennae.

Bearing (BRG)

The compass direction from your position to a destination, measured to the nearest degree.

C/A Code (Course Acquisition Code)

The standard civilian GPS code.

Course

The direction from the beginning landmark of a course to its destination (measured in degrees, radians, or mils).

Course Made Good (CMG)

The bearing from the 'active from' position (your starting point) to your present position.

Course Over Ground (COG)

Your direction of movement relative to a ground position.

Course to Steer

The heading you need to maintain in order to reach a destination.

Data Message

An electronic message giving the satellite's location, clock corrections and health included in the GPS signal received from each satellite.

Datum

A geographic model obtained by referencing the earth's sea level surface area and applying theoretical mathematical calculations. Map data may vary from chart to chart. The map datum for your GPS unit should match the datum listed on the corresponding chart. Refer to the map legend to find the specific datum for your chart.

Differential GPS (DGPS)

A system of beacons which broadcasts signals to help increase the accuracy of GPS positioning.

Dilution of Precision (DOP)

A measure of the receiver-satellite(s) geometry. DOP is a geometric calculation of the uncertainty in a position fix. Also referred to as GOP or GDOP.

Distance to Waypoint

A measurement in statute, nautical or metric increments from your current position to your current destination waypoint.

Ellipsoid

A mathematical figure formed by revolving an ellipse about its minor axis. It is often used interchangeably with spheroid.

Ephemeris (pronounced: ee-fem-er-is)

Current satellite position and timing information transmitted as part of the satellite data message.

Fast-Switching Channel

A channel, which samples a number of satellites, ranges in rapid order. "Fast" is categorized as quick enough (2 to 5 milliseconds) to recover the data message.

Geodetic Datum

An ellipsoid typically defined by eight parameters which establish its dimensions, define its center with respect to the Earth's center of mass and specify its orientation in relation to the Earth's average spin axis and Greenwich reference meridian.

Geometric Quality

The probable accuracy of a position fix found by taking into consideration the relative position of satellites.

Initialization

The first time a GPS receiver orients itself to its current location. After initialization has occurred, the receiver remembers its location and acquires a position more quickly because it doesn't need a large amount of satellite information.

Magnetic Variation

Errors in magnetic compass readings caused by variance in the Earth's magnetic field at different locations on the planet. Navigational charts list the variation and a yearly level of increase.

Multipath Error

An error caused when a signal reaches the receiver antenna by more than one path. Usually caused by one or more paths being bounced or reflected.

Multiplexing Channel

A GPS receiver channel that can track a variety of satellite signals in sequential order.

Navigation Satellite for Timing And Ranging (NAVSTAR)

The name given to GPS satellites built by Rockwell International.

National Oceanic and Atmospheric Administration (NOAA)

Responsible for the production of navigational charts and for monitoring and informing mariners of weather conditions.

Parallel Channel Receiver

A receiver employing multiple receiver circuits, each dedicated to receiving and processing the signal from a single satellite.

Pseudo-Random Code

The identifying signature transmitted by each GPS satellite and mirrored by the GPS receiver, in order to separate and retrieve the signal from background noise.

Pseudorange

A distance measurement using uncorrected time comparisons from satellite transmitted code and the local receiver's reference code.

Route

A planned course of travel that is defined by a sequence of waypoints.

Speed Over Ground (SOG)

The actual speed the GPS unit is moving over the ground. This may differ from airspeed or nautical speed due to such things as sea conditions or head winds. For example a plane that is going 120 knots into a 10 knot head wind may have a SOG of 110 knots.

TracBack

The GARMIN feature which takes your current track log and converts it into a route of up to 30 waypoints to guide you back to a starting position.

Track (TRK)

Your current direction of travel relative to a ground position (same as COG).

True North

The direction of the North Pole from your current position. Magnetic compasses are slightly incorrect due to effects of the Earth's magnetic field. GPS units correct for magnetic influences.

Turn (TRN)

The degrees which must be added or subtracted from the current heading to reach the course to the intended waypoints.

Universal Time Coordinator (UTC)

A universal time standard, referencing the time at Greenwich, England. Also referred to as GMT or Zulu time.

Velocity Made Good (VMG)

The rate, at which you are approaching a destination, based upon your current speed and course.

Waypoint

A permanently stored and named position in the GPS receiver's memory.