2D Operating Mode
A two-dimensional GPS position fix that includes only horizontal coordinates (no GPS elevation). It requires a minimum of three visible satellites.
3D Operating Mode
A three-dimensional GPS position fix that includes horizontal coordinates, plus elevation. It requires a minimum of four visible satellites.
Accuracy
A measure of how close an estimate of a GPS position is to the true location.
Acquisition Time
The time it takes a GPS receiver to acquire satellite signals and determine the initial position.
Active Antenna
An antenna that amplifies the GPS signal before sending it to the receiver.
Active Leg
The segment of a route currently being traveled. A "segment" is that portion of a route between any two waypoints in the route.
Almanac Data
Information transmitted by each satellite on the orbits and state (health) of every satellite in the GPS constellation. Almanac data allows the GPS receiver to rapidly acquire satellites shortly after it is turned on.
Altimeter
An instrument for determining elevation, especially an aneroid barometer used in aircraft that senses pressure changes accompanying changes in altitude. The Garmin® eTrex® Vista and Summit models contain a basic GPS with a built-in barometric altimeter.
Analog Signal
The principal feature of analog signals is that they are continuous. In contrast, digital signals consist of values measured at discrete intervals.
Anti-Spoofing
Encryption of the P-code to protect the P-signals from being "spoofed" through the transmission of false GPS signals by an adversary.
Atomic Clock
A very precise clock that operates using the elements cesium or rubidium. A cesium clock has an error of one second per million years. GPS satellites contain multiple cesium and rubidium clocks.
AutoLocate®
This is a proprietary feature of Garmin GPS receivers. A Garmin unit displays the "AutoLocate" status when it is looking for and collecting data from satellites that were visible at its last known or initialized position (almanac data), but it has not collected enough data to calculate a position fix.
Azimuth
The horizontal direction from one point on the earth to another, measured clockwise in degrees (0-360) from a north or south reference line. An azimuth is also called a bearing.
Basemap
Garmin mapping units come with permanently built-in basemaps, which typically include coverage of oceans, rivers, and lakes; principal cities, smaller cities, and towns; interstates, highways, and local thoroughfares; and railroads, airports, and political boundaries. Basemaps are available in a variety of global coverage areas, depending on the user’s needs.
Beacon
Stationary transmitter that emits signals in all directions (also called a non-directional beacon). In DGPS, the beacon transmitter also broadcasts pseudorange correction data to nearby GPS receivers for greater accuracy.
Bearing
The compass direction from a position to a destination, measured to the nearest degree (also call an azimuth). In a GPS receiver, bearing usually refers to the direction to a waypoint.
C/A Code
See Coarse/Acquisition Code.
Carrier Frequency
The frequency of an unmodulated output of a radio transmitter. The GPS L1 carrier frequency is 1575.42 MHz.
Cartography
The art or technique of making maps or charts. Many GPS receivers have detailed mapping—or cartography—capabilities.
CDI
See Course Deviation Indicator.
CDMA
See Code Division Multiple Access.
Code Division Multiple Access (CDMA)
A method whereby many radios use the same frequency, but each one has a unique code. GPS uses CDMA techniques with codes for their unique cross-correlation properties.
Clock Bias
The difference between the indicated clock time in the GPS receiver and true universal time (or GPS satellite time).
Clock Offset
A constant difference in the time reading between two clocks, normally used to indicate a difference between two time zones.
CMG
See Course Made Good.
Coarse/Acquisition Code (C/A Code)
The standard positioning signal the GPS satellite transmits to the civilian user. It contains the information the GPS receiver uses to fix its position and time, and is accurate to 100 meters or better.
COG
See Course Over Ground.
Cold Start
The power-on sequence where the GPS receiver downloads almanac data before establishing a position fix.
Control Segment
A worldwide chain of monitoring and control stations that control and manage the GPS satellite constellation.
Coordinates
A set of numbers that describes your location on or above the earth. Coordinates are typically based on latitude/longitude lines of reference or a global/regional grid projection (e.g., UTM, MGRS, Maidenhead).
Coordinated Universal Time (UTC)
Replaced Greenwich Mean Time (GMT) as the world standard for time in 1986. UTC uses atomic clock measurements to add or omit leap seconds each year to compensate for changes in the rotation of the earth.
Course
The direction from the beginning landmark of a course to its destination (measured in degrees, radians, or mils), or the direction from a route waypoint to the next waypoint in the route segment.
Course Deviation Indicator (CDI)
A technique for displaying the amount and direction of crosstrack error (XTE).
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.
Course Up Orientation
Fixes the GPS receiver's map display so the direction of navigation is always "up."
Crosstrack Error (XTE/XTK)
The distance you are off the desired course in either direction.
