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. |
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3D Operating Mode
A three-dimensional GPS position fix that includes horizontal coordinates, plus elevation. It requires a minimum of four visible satellites. |
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Accuracy
A measure of how close an estimate of a GPS position is to the true location. |
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Acquisition Time
The time it takes a GPS receiver to acquire satellite signals and determine the initial position. |
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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. |
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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.
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Cold Start
The power-on sequence where the GPS receiver downloads almanac data before establishing a position fix. |
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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). |
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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. |
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Course Deviation Indicator (CDI)
A technique for displaying the amount and direction of crosstrack error (XTE). |
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Datum
A math model which depicts a part of the surface of the earth. Latitude and longitude lines on a paper map are referenced to a specific map datum. The map datum selected on a GPS receiver needs to match the datum listed on the corresponding paper map in order for position readings to match. |
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Distance
The length (in feet, meters, miles, etc.) between two waypoints or from your current position to a destination waypoint. This length can be measured in straight-line (rhumb line) or great-circle (over the earth) terms. GPS normally uses great circle calculations for distance and desired track.
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Elevation
The distance above or below mean sea level. |
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Ellipsoid
A geometric surface, all of whose plane sections are either ellipses or circles. |
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Ephemeris Data
Current satellite position and timing information transmitted as part of the satellite data message. A set of ephemeris data is valid for several hours. |
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Geocaching
A high-tech version of hide-and-seek. Geocachers seek out hidden treasures utilizing GPS coordinates posted on the Internet by those hiding the cache. |
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Geodetic Datum
A math model representing the size and shape of the earth (or a portion of it). |
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Geographic Information System (GIS)
A computer system or software capable of assembling, storing, manipulating, and displaying geographically referenced information (i.e., data identified according to their location). In practical use, GIS often refers to the computer system, software, and the data collection equipment, personnel, and actual data.
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Geosynchronous Orbit
A specific orbit around where a satellite rotates around the earth at the same rotational speed as the earth. A satellite rotating in geosynchronous orbit appears to remain stationary when viewed from a point on or near the equator. It is also referred to as a geostationary orbit. |
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GIS
See Geographic Information System. |
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Global Positioning System (GPS)
A global navigation system based on 24 or more satellites orbiting the earth at an altitude of 12,000 statue miles and providing very precise, worldwide positioning and navigation information 24 hours a day, in any weather. Also called the NAVSTAR system. |
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GMT
See Greenwich Mean Time. |
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GPS
See Global Positioning System.
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Greenwich Mean Time (GMT)
The mean solar time for Greenwich, England, which is located on the Prime Meridian (zero longitude). Based on the rotation of the earth, GMT is used as the basis for calculating standard time throughout most of the world.
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Grid
A pattern of regularly spaced horizontal and vertical lines forming square zones on a map used as a reference for establishing points. Grid examples are UTM, MGRS, and Maidenhead. |
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Heading
The direction in which a vehicle is moving. For air and sea operations, this may differ from actual Course Over Ground (COG) due to winds, currents, etc. |
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Healthy
A term used when an orbiting GPS satellite is suitable for use. "State" is also used to refer to satellite health.
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Input/Output (I/O)
The two-way transfer of GPS information with another device, such as a nav plotter, autopilot, or another GPS unit. |
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Initialization
The first time a GPS receiver orients itself to its current location and collects almanac data. After initialization has occurred, the receiver remembers its location and acquires a position more quickly because it knows which satellites to look for. |
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Ionosphere
A region of the earth's atmosphere where ionization caused by incoming solar radiation affects the transmission of GPS radio waves. It extends from a height of 50 kilometers (30 miles) to 400 kilometers (250 miles) above the surface. |
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LAAS
See Local Area Augmentation System.
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Latitude
A position's distance north or south of the equator, measured by degrees from zero to 90. One minute of latitude equals one nautical mile. |
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LCD
See Liquid Crystal Display. |
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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. |
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Lithium Battery
A soft, silvery, highly reactive metallic element that is used in batteries where weight and cold weather conditions are concerns. |
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Liquid Crystal Display (LCD)
A display circuit characterized by a liquid crystal element sandwiched between two glass panels. Characters are produced by applying an electric field to liquid crystal molecules and arranging them to act as light filters. |
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Local Area Augmentation System (LAAS)
The implementation of ground-based DGPS to support aircraft landings in a local area (20-mile range). |
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Longitude
The distance east or west of the prime meridian (measured in degrees). The prime meridian runs from the north to south pole through Greenwich, England. |
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Magnetic North
Represents the direction of the north magnetic pole from the observer's position. The direction a compass points. |
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Map Display
A graphic representation of a geographic area and its features. |
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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. |
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NAVSTAR
The official U.S. Government name given to the GPS satellite system. NAVSTAR is an acronym for NAVigation Satellite Timing and Ranging. |
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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. |
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NMEA Standard
A NMEA standard defines an electrical interface and data protocol for communications between marine instrumentation.
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Parallel Channel Receiver
A continuous tracking receiver using multiple receiver circuits to track more than one satellite simultaneously. |
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Pixel
A single display element on an LCD screen. The more pixels, the higher the resolution and definition. |
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Position
An exact, unique location based on a geographic coordinate system. |
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Position Fix
The GPS receiver's computed position coordinates. |
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Position Format
The way in which the GPS receiver's position will be displayed on the screen. Commonly displayed as latitude/longitude in degrees and minutes, with options for degrees, minutes and seconds, degrees only, or one of several grid formats. |
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RAIM
Receiver Autonomous Integrity Monitoring; A GPS receiver system that would allow the receiver to detect incorrect signals being transmitted by the satellites by comparing solutions with different sets of satellites. |
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Route
A group of waypoints entered into the GPS receiver in the sequence you desire to navigate them. |
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Time To First Fix (TTFF)
If you have not used your GPS unit for several months, the almanac data for the satellites may be out of date. The unit is capable of recollecting this information on its own, but the process can take several minutes. Time to First Fix (TTFF) is the time it takes a GPS receiver to find satellites after the user first turns it on (when the GPS receiver has lost memory or has been moved over 300 miles from its last location). |
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Troposphere
The lowest region of the atmosphere between the surface of the earth and the tropopause, characterized by decreasing temperature with increasing altitude. GPS signals travel through the troposphere (and other atmospheric layers). |
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TTFF
See Time To First Fix. |
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Universal Transverse Mercator (UTM)
A nearly worldwide coordinate projection system using north and east distance measurements from reference point(s). UTM is the primary coordinate system used on U.S. Geological Survey topographic maps. |
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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. |
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UTC
See Coordinated Universal Time. |
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UTM
See Universal Transverse Mercator. |
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WAAS
See Wide Area Augmentation System. |
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Waypoints
Waypoints are locations or landmarks worth recording and storing in your GPS. These are locations you may later want to return to. They may be check points on a route or significant ground features. (e.g., camp, the truck, a fork in a trail, or a favorite fishing spot). Waypoints may be defined and stored in the unit manually by taking coordinates for the waypoint from a map or other reference. This can be done before ever leaving home. Or more usually, waypoints may be entered directly by taking a reading with the unit at the location itself, giving it a name, and then saving the point. Waypoints may also be put into the unit by referencing another waypoint already stored, giving the reference waypoint, and entering the distance and compass bearing to the new waypoint. |
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Wide Area Augmentation System (WAAS)
A system of satellites and ground stations that provide GPS signal corrections for better position accuracy. A WAAS-capable receiver can give you a position accuracy of better than three meters, 95 percent of the time. (At this time, the system is still in the development stage and is not fully operational.) WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. |
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WGS-84
World Geodetic System, 1984. The primary map datum used by GPS. Secondary datums are computed as differences from the WGS 84 Standard |
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