GPS Glossary
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2D Mode
A two-dimensional position fix that includes only horizontal coordinates. It requires a minimum of three visible satellites.
3D Mode
A three-dimensional position fix that includes horizontal coordinates plus elevation. It requires a minimum of four visible satellites.
Acquisition Time
The time it takes for a GPS receiver to acquire satellite signals and determine the initial position. Three satellites are needed for a 2D and four for a 3D-position fix.
Active Leg
The segment of a route currently being traveled.
Almanac Data
Information transmitted by each satellite on the orbits and state (health) of the entire constellation. Almanac data allows the GPS receiver to rapidly acquire satellites as soon as it is turned on.
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.
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.
Beacon
Stationary transmitter that emits signals in all directions; also called a non-directional beacon. In DGPS, the beacon transmitter 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 called an azimuth. In a GPS receiver, bearing usually refers to the direction to a waypoint.
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. Accurate to 100 meters.
Cold Start
The power-on sequence when the GPS receiver downloads almanac data before establishing a position fix. Also called initialization.
Control Segment
A worldwide chain of monitoring and control stations that control and manage the GPS satellite constellation.
Coordinate
A set of numbers that describes your location on or above the earth.
Coordinated Universal Time (UTC)
Replaced Greenwich Mean Time (GMT) as the World standard for time in 1986. It is based on atomic measurements rather than the earth's rotation. Greenwich Mean Time (GMT) is still the standard time zone for the Prime Meridian (Zero Longitude). It is the time kept by GPS satellites.
Course
The direction from the beginning landmark of a course to its destination (measured in degrees, radians, or mils).
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.
Crosstrack Error (XTE/XTK)
The distance you are off the desired course in either direction.
A math model designed to fit part of the earth's surface. Latitude and longitude lines on a paper map are referenced to a specific map datum. The map datum for a GPS receiver needs to match the datum listed on the corresponding paper map.
Desired Track (DTK)
The compass course between the "from" and "to" waypoints.
A technique used to improve the accuracy of the GPS. DGPS reduces the effect of selective availability, propagation delay, etc. and can improve position accuracy to greater than 10 meters.
Dilution of Precision (DOP)
A measure of the GPS receiver-satellite geometry. A low DOP value indicates higher accuracy. The DOP indicators are GDOP (geometric DOP), PDOP (position DOP), HDOP (horizontal DOP), VDOP (vertical DOP), and TDOP (Time clock offset).
DOD
The U.S. Department of Defense. The DOD manages and controls the Global Positioning System.
Elevation
The distance above or below average sea level.
Ephemeris
Current satellite position and timing information transmitted as part of the satellite data message. A set of ephemeris is valid for several hours.
Estimated Position Error (EPE)
A measurement of horizontal position error in feet or meters based upon a variety of factors including DOP and satellite signal quality.
Estimated Time Enroute (ETE)
The time left to your destination based upon your present speed and course.
Estimated Time of Arrival (ETA)
The time of day of your arrival at a destination.
Global Positioning System
A global navigation system based on 24 satellites orbiting the earth at an altitude of 12,000 miles and providing very precise, worldwide positioning and navigation information 24 hours a day, in any weather. Also called the NAVSTAR system.
GLONASS
The Russian Global Positioning System.
GOTO
A route consisting of one leg with your present position being the start of the route and a single defined waypoint as the destination.
Greenwich Mean Time
The mean solar time for the meridian at Greenwich, England, used as a basis for calculating time throughout most of the world. Also called universal time.
Grid
A pattern of regularly spaced horizontal and vertical lines forming square zones on a map used as a reference for establishing points.
Heading
The direction in which a ship or an aircraft is moving. This may differ from actual COG due to winds, sea conditions, etc.
I/O (Interface Option)
The one-way or two-way transfer of GPS information with another device, such as a nav plotter, autopilot, or another GPS unit.
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.
Invert Route
To display and navigate a route from end to beginning for purposes of returning back to the route's starting point.
L1 Frequency
One of the two radio frequencies transmitted by the GPS satellites. This frequency carries the Coarse Acquisition Code, P-Code, and the nav message and is transmitted on a frequency of 1575.42 MHz.
L2 Frequency
One of the two radio frequencies transmitted by the GPS satellites. This frequency carries only the P-Code, and is transmitted on a frequency of 1227.6 MHz.
Latitude
A position's distance north or south of the equator measured by degrees from 0 to 90. One minute of latitude equals one nautical mile.
Leg (route)
A portion of a route consisting of a starting (from) waypoint and a destination (to) waypoint. A route that is comprised of waypoints A, B, C, and D would contain three legs. The route legs would be from A to B, from B to C, and from C to D.
Liquid Crystal Display (LCD)
Produced by applying an electric field to liquid crystal molecules and arranging them to act as light filters.
Local Area Augmentation System (LAAS)
The implementation of DGPS to support aircraft landings in a local area (20 mile range).
Longitude
The distance east or west of the prime meridian (measured in degrees) which runs from the North to South Pole through Greenwich, England.
Long Range Radio Direction Finding System (LORAN)
A radio navigation aid operated and maintained by the U.S. Coast Guard. It is used as a supplemental system for harbor approach navigation and inland navigation. LORAN C is used in civil aviation.
Magnetic North
Represents the direction of the north magnetic pole from the observer's position. The direction a compass points.
Magnetic Variation
Errors in magnetic compass readings caused by changes in the earth's magnetic field at different locations on the planet. Navigational charts list the variation and a yearly level of increase.
Map Display
A graphic representation of a geographic area and the features in it.
Multiplexing Receiver
A GPS receiver that switches at a very rapid rate between satellites being tracked. Typically, multiplexing receivers require more time for satellite acquisition, and are not as accurate as parallel channel receivers. Multiplexing receivers are also more prone to lose a satellite fix in dense woods than parallel channel GPS receivers.
Multipath
An error caused when a satellite signal reaches the GPS receiver antenna by more than one path. Usually caused by one or more paths being bounced or reflected. The TV equivalent of multipath is "ghosting."
Nautical Mile
A unit of length used in sea and air navigation, based on the length of one minute of arc of a great circle, especially an international and U.S. unit equal to 1,852 meters (about 6,076 feet).
Navigation
The act of determining the course or heading of movement. This movement could be for a plane, ship, automobile, person on foot, or any other similar means.
Navigation Message
The message transmitted by each GPS satellite containing system time, clock correction parameters, ionospheric delay model parameters, and the satelliteÕs ephemeris and health. The information is used to process GPS signals to give the user time, position, and velocity. Also known as the data message.
NAVSTAR
The official U.S. Government name given to the GPS satellite system. NAVSTAR is an acronym for NAVigation Satellite Timing and Ranging.
NMEA (National Marine Electronics Association)
A U.S. standards committee that defines data message structure, contents, and protocols to allow the GPS receiver to communicate with other pieces of electronic equipment aboard ships.
NMEA 0183
A standard data communication protocol used by GPS receivers and other types of navigation and marine electronics.
North-Up Display
A GPS receiver's display screen that always shows North on top.
A continuous tracking receiver using multiple receiver circuits to track satellites simultaneously.
P-Code
The precise code of the GPS signal typically used only by the U.S. military. It is encrypted and reset every seven days to prevent use from unauthorized persons.
Pixel
A single display element of an LCD screen. The more pixels, the higher the resolution and definition.
Position
A geographic location on the earth commonly measured in latitude and longitude.
Position Fix
The GPS receiver's computed position coordinates.
Position Format
The way in which the GPS receiver's position will be displayed on the screen. Commonly displayed as degrees and minutes, with options for degrees, minutes, and seconds, degrees only, or one of several grid formats.
Prime Meridian
The zero meridian (0¡), used as a reference line from which longitude east and west is measured. It passes through Greenwich, England.
Pseudo-Random Code
The identifying signature signal transmitted by each GPS satellite and mirrored by the GPS receiver in order to separate and retrieve the signal from background noise.
Pseudorange
The measured distance between the GPS receiver and the GPS satellite using uncorrected time comparisons from satellite transmitted code and the local receiver's reference code.
RS-232
A serial input/output standard that allows for compatibility between data communication equipment made by various manufacturers.
Radio Technical Commission for Maritime Services (RTCM)
A commission established for the purposes of establishing standards and guidance for interfacing between radiobeacon-based data links and GPS receivers, and to provide standards for ground-based differential GPS stations.
Route
A group of waypoints entered into the GPS receiver in the sequence you desire to navigate them.
Search the Sky
A message shown when a GPS receiver is gathering data from satellites to compute a position without almanac data.
Selective Availability (SA)
The random error which the government intentionally adds into GPS signals so that their accuracy, for civilian use, is degraded. The level of SA is subject to accuracy degradation to 100m. Selective availability ended in mid-2000.
Space Segment
The satellite portion of the complete GPS system.
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.
Statue Mile
A unit of length equal to 5,280 feet or 1,760 yards (1,609 meters) used in the U.S. and other English-speaking countries.
Straight Line Navigation
The act of going from one waypoint to another in the most direct line and with no turns.
The GARMIN feature which takes your current track log and converts it into a route to guide you back to a starting position.
Track-Up Display
The direction to be followed, is always located at the top of the display.
Track (TRK)
Your current direction of travel relative to a ground position (same as COG).
Triangulation
The location of an unknown point, as in GPS navigation, found by using the laws of plane trigonometry.
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 to or subtracted from the current heading to reach the course to the intended waypoint.
Universal Time Coordinated (UTC)
A universal time standard, referencing the time at Greenwich, England. Also referred to as GMT or Zulu time.
Universal Transverse Mercator (UTM)
A worldwide coordinate projection system utilizing north and east distance measurements from reference point(s). UTM is the primary coordinate system used on U.S. Geological Survey topographic maps.
U.S.C.G.
United States Coast Guard. The Coast Guard is responsible for providing all of the navigation aids in the U.S. including DGPS.
User Interface
The way in which information is exchanged between the GPS receiver and the user. This takes place through the screen display and buttons on the unit.
User Segment
One segment of the entire GPS system that includes the GPS receiver.
Velocity Made Good (VMG)
The rate of closure to a destination, based upon your current speed and course.
Waypoint
A permanently stored and named position in the GPS receiver's memory.
Wide Area Augmentation System (WAAS)
A U.S. Federal Aviation Authority (FAA) system of equipment and software that supplements GPS accuracy, availability and integrity. The WAAS provides a satellite signal for WAAS users to support enroute and precision approach aircraft navigation.
WGS-84
World Geodetic System - 1984. The mathematical reference ellipsoid used by GPS.
Y-Code
The encrypted P-Code.