GPS Definitions |
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Here are some terms that you may come across when dealing with GPS receivers. |
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12-Channel | A GPS that can track a maximum of 12 GPS satellites at any one time. |
| 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. |
| A-GPS (Assisted GPS) |
A technique of providing a GPS receiver with data (or equivalent information) that it would ordinarily have to download from the GPS satellites. This technique speeds the satellite acquisition time, and the time-to-fix of the GPS receiver. |
| Absolute Positioning | Mode in which a position is identified by a single receiver, with respect to a well-defined coordinate system, commonly a Geocentric system (i.e. a system whose point of origin coincides with the center of mass of the earth). Also referred to as Point Positioning or Single Receiver Positioning. |
| 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 route. |
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| Almanac | 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. |
| Altitude | The vertical distance of a point either on or above the surface of the earth from mean sea level. The term can also be used to measure the angle of a celestial body above the visible horizon. |
| Analog Signal | The principal feature of analog signals is that they are continuous. In contrast, digital signals consist of values measured at discrete intervals. |
| Aneroid Barometer | A barometer that measures pressure without using fluids. |
| Antenna | That part of the GPS receiver hardware which receives (and sometimes amplifies) the incoming L-Band signal. |
| Anti-Spoofing (AS) | Encryption of the P-code to protect the P-signals from being "spoofed" through the transmission of false GPS signals by an adversary. |
| Anywhere Fix | The ability of a receiver to start position calculations without being give an approximate location and time. |
| Application Programming Interface (API) | A set of definitions of the ways in which one piece of computer software communicates with another. Typically used to provide a well-defined functional interface that is used by other software element(s). |
| Arrival Alarm | In marine navigation, an alarm signal issued by a GPS/chart plotter that indicates arrival at or at a predetermined distance from a waypoint (see arrival circle). |
| Arrival Circle | In Marine navigation, an artificial boundary placed around the destination waypoint of the present navigation leg, the entering of which will signal an arrival alarm. |
| 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. |
| Attribute | A characteristic which describes a Feature. Attributes can be thought of as questions which are asked about the Feature. In Geographic Information Systems (GIS), information or data that applies to a specific geographic object, area or location. |
| Auto Routing | Auto routing is a feature that calculates a driving route to a destination. It is a basic feature on vehicle navigation systems but is often not found on the less expensive handheld GPS models. |
| Automatic Vehicle Location (AVL) | A type of system using any sort of technology to track or locate a vehicle. |
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| Automatic Vehicle Monitoring (AVM) | The technique of using a navigation system to determine a vehicle's position, which is then transmitted to a central unit that monitors and tracks the vehicle's position and movement. |
| Availability | The percentage of time that the services of a navigation system can be used within a particular coverage area. Signal availability is the percentage of time that navigational signals transmitted from external sources are available for use. Availability is a function of both the physical characteristics of the operational environment and the technical capabilities of the transmitter facilities. |
| Azimuth | The direction of travel or the direction between two points in reference to true or magnetic north. When expressed in degrees, its value ranges from 0 to 360°. A compass heading is an azimuth. |
| Accuracy | A measure of how close an estimate of a GPS position is to the true location. |
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| Bandwidth | The range of frequencies in a signal. |
| Base Station | Also called a Reference Station. In GPS navigation, this is a receiver that is set up on a known location specifically to collect data from differentially correcting data files of another receiver (which may be referred to as the "mobile" or "rover" receiver). Also called a cellular transmitter. |
| Base Transceiver Station (BTS) | BTS is a cellular transmitter. Also referred to simply as base station. |
| Baseline | A Baseline consists of a pair of stations for which simultaneous GPS data have been collected. Mathematically expressed as a vector of coordinate differences between the two stations, or an expression of the coordinates of one station with respect to the other (whose coordinates are assumed known, and is typically referred to as a "Base" or "Reference" Station). |
| Basemap | A basemap is a basic set of map data that is built into a GPS device that cannot be removed. Basemaps range from those including only major highways and towns, to large and detailed databases that are very useful and comprehensive right out of the box. |
| Baud | Symbols per seconds. Measure of the speed of a communications channel. Different than bps (one symbol may represent several bits). |
| Beacon | Stationary transmitter that emits signals in all directions (also called a non-directional beacon). In DGPS, the beacon transmitter also broadcasts pseudo-range correction data to nearby GPS receivers for greater accuracy. |
| Bearing (BRG) | 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. |
| Bits Per Second (BPS) | Measure of speed of a communications channel. |
| Bluetooth | Bluetooth is the name of a short-range wireless communications technology that is low power, low cost, robust and moderately secure. It allows for a wide variety of devices to communicate with each other. |
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| C-Code | The less accurate Civilian GPS signal. |
| Cache | A small container hidden at a particular location by a geocacher. |
| Carrier | A radio wave having at least one characteristic (e.g. frequency, amplitude, phase) that can be varied from a known reference value by modulation. In the case of GPS, there are two transmitted carrier waves: (a) L1 at 1575.42 MHz, (b) L2 at 1227.60 MHz, modulated by the Navigation Message (both L1 and L2), the P-Code (both L1 and L2) and the C/A-Code (L1). |
| Carrier Frequency | The frequency of an unmodulated output of a radio transmitter. |
| Carrier Phase | GPS measurements made on the L1 or L2 carrier signal. May refer to the fractional part of the L1 or L2 carrier wavelength (approximately 19cm for L1, 24cm for L2), expressed in units of meters, cycles, fraction of a wavelength or angle. One cycle of L1 is equivalent to one wavelength, and similarly for L2. |
| Carrier-Aided Tracking | A signal processing strategy that uses the GPS carrier signal to achieve an exact lock-on the PRN code. More efficient and accurate than the standard approach. |
| Cartography | The art or technique of making maps or charts. Many GPS receivers have detailed mapping (cartography) capabilities. |
| Celestial Navigation | Navigating by use of traditional methods having to do with observations of stars, the sun, and moon. |
| CF Memory | Compact flash memory is removable memory used by some navigation systems for storing map data. CF memory has lost market share with the introduction of SD (Secure Digital) memory. Either is a good choice. |
| Channel | The number of signals that a GPS receiver can receive at one time. The more the better when trying to get a speedy and accurate position. |
| Chartplotters | Marine GPS receivers that feature detailed mapping capabilities. |
| Chronometer | An instrument that keeps very accurate time and is particularly used in navigation. |
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| Clear/Acquisition Code (C/A Code) | The standard GPS PRN code, also known as the Civilian Code or S-Code. Only modulated on the L1 carrier. Used by GPS receivers to acquire and decode the L1 satellite signal, and from which the L1 pseudo-range measurement is made. |
| 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. |
| Coarse/Acquisition (C/A Code) | Coarse/Acquisition is a spread spectrum direct sequence code that is used primarily by commercial GPS receivers to determine the pseudo-range to a transmitting GPS satellite, modulated on the L1 carrier. |
| Code Division Multiple Access (CDMA) | This is a method whereby many radios use the same frequency, but each one has a unique code. GPS uses CDMA techniques with codes for the unique cross-correlation properties. |
| Code Phase GPS | GPS measurements based on pseudo-random code (C/A or P) as opposed to carrier of that code. |
| Cold/Warm Start | A cold start is the power-on sequence where the GPS receiver downloads almanac data before establishing a position fix. A warm start is when the receiver powers on with current almanac data. |
| Constellation | Refers to either the specific set of satellites used in calculating positions or all the satellites visible to a GPS receiver at one time. |
| Control Segment | A worldwide chain of monitoring and control stations that control and manage the GPS satellite constellation. |
| Coordinate System | A coordinate system is a reference system based on mathematical rules for specifying positions (locations) on the surface of the earth. The coordinate values can be spherical (latitude and longitude) or planar (such as Universal Transverse Mercator). |
| 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. |
| Correlator | The GPS receiver "software" or electronic means, implemented in some fashion (either analog or digital) within a Tracking Channel, used to shift or compare the incoming signal with an internally generated signal. This operation is performed on the PRN codes, but may be used for more "exotic" mixed signals in the case of L2 measurements, where under the policy of Anti-Spoofing (AS) the L2 PRN code is not known. Correlator design may be influenced such that it is optimized for accuracy, mitigation of multipath, acquisition of signal under foliage, etc. |
| Course | The direction of travel with respect to true north when navigating to a waypoint. |
| Course Deviation Indicator (CDI) | CDI is a technique for displaying the amount and direction of crosstrack error (XTE). |
| Course Made Good (CMG) | CMG is the bearing from the 'active from' position (your starting point) to your present position. |
| Course Over Ground (COG) | COG is the direction of movement relative to a ground position. |
| Course to Steer (CTS) | 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) | A Crosstrack Error is the distance you are off the desired course in either direction. |
| Custom POI | A custom POI is a location you enter or load into your GPS. On many GPS models, they are managed differently than waypoints and usually have special features associated with them. |
| Cycle Slip | A discontinuity in the measured carrier beat phase resulting from a temporary loss of lock in the carrier tracking loop of the GPS receiver. |
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| Data Dictionary | Defines the fields you will fill during data collection using the GPS equipment. Very similar to describing the fields in a record for a database program. |
| Data Links | Data links are Differential Global Positioning System (DGPS) radio signals transmitted from a reference station or relay antenna at a known location to one or more rover receivers, to provide the positioning correction data that gives GPS systems maximum accuracy. |
| Data Logger | Also called a data recorder. A handheld, lightweight data entry computer. It can be used to store additional data obtained by a GPS receiver. |
| Data Message | A message included in the GPS signal that reports the satellite's location, clock corrections, and health. Included is approximate information about the other satellites in the system as well. |
| Datum | A mathematical model of the Earth established by a collection of points based on their latitude and longitude. The datum used by GPS is WGS-84. |
| Dead Reckoning | Dead Reckoning is the process of estimating one's current position based upon a previously determined position, or fix, and advancing that position based upon known or estimated speeds over elapsed time, and course. |
| Default | The value or setting automatically chosen by the unit unless otherwise directed. |
| Degree | A degree is the most common unit used for defining a position on the surface of the earth. The polar and equatorial circumferences are divided into 360 degrees. Latitude is measured in degrees north or south of the equator with the North and South Pole being 90° north and south, respectively. Longitude is measured east and west from the prime meridian to 180° at the International Date Line. |
| Desired Track (DTK) | DTK is the compass course between the "from" and "to" waypoints. |
| Detail Map | A detail map is a set of map data that can be loaded into a GPS navigation system. Detail maps usually include a lot more information than a basemap. They commonly include streets of all sizes, rivers, lakes, parks, and even individual street addresses. |
| Deviation | Errors from your course. Either built in or unintentional. One type of deviation is magnetic which is the difference between a true course and what a compass will tell you. |
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| Differential Correction | The technique of comparing GPS data collected in the field to GPS data collected at a known point. By collecting GPS data at a known point, a correction factor can be determined and applied to the field GPS data. |
| Differential GPS (DGPS) | DGPS is an extension of the GPS system that uses land-based radio beacons to transmit position corrections to GPS receivers. DGPS reduces the effect of selective availability, propagation delay, etc. and can improve position accuracy to better than 10 meters (32.81 feet). |
| Differential Positioning | Accurate measurement of the relative positions of two receivers tracking the same GPS signals. |
| Dilution of Precision (DOP) | Dilution of Precision is a measure of the quality of the GPS data being received from the satellites. DOP is a mathematical representation for the quality of the GPS position solution. The main factors affecting DOP are the number of satellites being tracked and where these satellites are positioned in the sky. The effect of DOP can be resolved into Horizontal DOP, Vertical DOP, Position DOP, Time DOP and Relative DOP. |
| Distance (DST) | 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. |
| Distance Root Mean Square (DRMS) | A measurement used to describe the accuracy of a fix. It is twice the square root of the sum of the squares of all radial errors surrounding a true point divided by the total number of measurements. |
| Distance to Go (DTG) | Reading displayed only when traveling to a waypoint. It simply indicates the remaining distance from your present position to the next waypoint. |
| Dithering | The introduction of errors into the satellite clock readout on GPS signals for the purpose of degrading GPS position accuracy for civil users. |
| DOD | The United States Department of Defense. The DOD manages and controls the Global Positioning System. |
| Doppler-Aiding | A signal processing strategy that uses a measured Doppler shift to help the receiver smoothly track the GPS signal. This allows more precise velocity and position measurement. |
| Doppler-Shift | The apparent change in the frequency of a signal caused by the relative motion of the transmitter and receiver. |
| Downlink | A transmission path for the communication of signals and data from a communications satellite or other space vehicle to the earth. |
| Download | To transfer information from a remote unit, such as a GPS receiver, to a computer. |
| Dynamic Positioning | The process of collecting GPS data while the GPS antenna is in motion. Often associated with Line or Area Features. |
| EGNOS | The European WAAS (Wide Area Augmentation System). |
| Electronic Compass | A compass that outputs a digital heading. Used in GPS receivers to calculate a heading without satellites or when not moving. |
| Elevation | Often used interchangeably with altitude, elevation refers to the vertical distance above or below mean sea level, of a point on the surface of the earth. |
| Ellipsoid | A geometric surface, all of whose plane sections are either ellipses or circles. |
| Ephemeris Data | Current satellite position and timing information transmitted as part of the satellite date message. A set of ephemeris data is valid for several hours. |
| Ephemeris Errors | Errors which originate in the ephemeris data transmitted by a GPS satellite. Ephemeris errors are removed by differential correction. |
| Estimated Position Error (EPE) | Also called Horizontal Dilution of Precision. EPE is a measurement of horizontal position error in feet or meters based upon a variety of factors including DOP and satellite signal quality. See Dilution of Precision (DOP). |
| Estimated Time Enroute (ETE) | ETE is the time it will take to reach your destination (in hours/minutes or minutes/seconds) based upon your present position, speed, and course. |
| Estimated Time of Arrival (ETA) | ETA is the estimated time you will arrive at a destination. |
| Event Marker | A graphic symbol that can be placed on the plotter display (and saved in memory) to represent some special event or area of interest to the GPS user. Also known as Icon. |
| External Antenna | An external GPS antenna lets you install your navigation system in an area of your vehicle where GPS reception is poor. Most navigation systems have a built-in antenna, but many can use an external antenna instead. |
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| Fake GPS | Fake GPS is a general term describing the use of signals from cell phone towers or Wi-Fi hotspots to triangulate and display the position of a user of a mobile electronic device. However, there are limitations. Fake GPS does not work outside of cellular tower range, or if Wi-Fi signals are used, out of Wi-Fi range. Also, in order for Wi-Fi hotspots to serve as fixed location points, they must be accurately identified and their coordinates accurately recorded in a database, and not all Wi-Fi hotspots are databased. Users will still receive the most accurate (within 3 meters with WAAS (Wide Area Augmentation System) and reliable location data from true GPS, but fake GPS is a viable option and a good backup, especially in major metro areas. |
| Fast Switching Channel | A single channel which rapidly samples a number of satellite ranges. "Fast" means that the switching time is sufficiently fast (2 to 5 milliseconds) to recover the data message. |
| Feature | Any item about which you want position information. Your data dictionary will describe the features you wish to collect. A feature may be a single point, a line, or an area. |
| Firmware | Firmware refers to the program code that a GPS receiver uses to do its work. All the instructions on how to properly provide navigational information and execute other features is stored in the firmware. Most GPS receivers allow for firmware updates, which are provided by the manufacturer and can be loaded onto the receiver via a USB connection or on a memory card. These updates will improve existing features, fix bugs or glitches, patch potential problems, or provide entirely new features. |
| Fishfinder | A sounder that uses sonar technology to locate fish. |
| Fix | Defining position, where two lines such as latitude and longitude cross, as determined by one or more navigation aids or techniques. |
| Frequency | The number of repetitions per unit time of a complete waveform, as of a radio wave (see L1 and L2 frequencies). |
| Frequency Band | A particular range of frequencies. |
| Frequency Modulation | A method of encoding information about a carrier signal by altering the frequency while amplitude remains constant. |
| Frequency Spectrum | The distribution of signal amplitudes as a function of frequency. |
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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. |
| Geocode | The process of determining the precise geographic location or a set of coordinates for an object derived from non-geographic information such as an address or zip code. |
| Geocoin | A geocoin is a special coin created by individuals or groups of geocachers as a kind of signature item or calling card. Each geocoin is assigned a unique tracking ID which allows them to travel from geocache to geocache or to be passed among friends, picking up stories along the way. |
| Geodetic Datum | A math model representing the size and shape of the earth (or a portion of it). |
| Geographic Information System (GIS) | GIS is 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. |
| Geographical Operations System (GOS) | A GOS integrates software applications and consolidates data while providing geographic visual representation of database information. |
| Geometric Dilution of Precision (GDOP) | GDOP is a measure of the overall uncertainty in a GPS position solution. See Dilution of Precision (DOP). |
| Geostationary Orbit | See Geosynchronous Orbit. |
| Geosynchronous Orbit | Also known as geostationary orbit. It is a specific orbit 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. |
| Geotag | An attachment of geographic coordinates to media and multimedia such as photos, news feeds, images, etc. to facilitate a geography-based search engine. |
| Getting, Ivan A. | Getting (1912-2003) is considered the Father of the GPS. As president of Aerospace Corp., founded by the US Air Force, physicist Getting helped develop what became the Global Positioning System (GPS), a multi-satellite military targeting program which has been adapted to certain civilian applications. |
| Global Positioning System (GPS) | GPS is a global navigation system based on 24 or more satellites orbiting the earth at an altitude of 12,000 statute 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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| GLONASS | The Global Orbiting Navigational Satellite System; the Russian counterpart to the United States' GPS system. |
| GNSS | Global Navigation Satellite System is a satellite system that is used to pinpoint the geographic location of a user's receiver anywhere in the world. Two GNSS systems are currently in operation: the U.S. Global Positioning System (GPS) and the Russian Federation's Global Orbiting Navigation Satellite System (GLONASS). The newest is Europe's GPS system called Galileo which was supposed to be operational by 2008, but is still on the ground. |
| 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. |
| Graphical User Interface (GUI) | Visual elements such as buttons or Windows® that the user interacts with to control the device. |
| Great Circle Route | A great circle is defined as a circle on the earth's surface the plane of which passes through the center of the earth. For navigation purposes: The great circle track is the shortest distance between two places on the earth's surface. The great circle track appears as a straight line on the Gnomonic (great circle) charts. The vertices of the great circle are the two points nearest the poles which have a course on the great circle track due East/West. |
| Greenwich Mean Time (GMT) | GMT is 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. Also referred to as Zulu Time. Was obsoleted in 1986 by UTC (Coordinated 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. |
| Ground Speed (GS) | Your actual speed; your speed relative to the ground being traversed in a given amount of time. |
| Hardover Word | The word in the GPS message that contains synchronization information for the transfer of tracking from the C/A to P code. |
| Heading (HDG) | 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. |
| Healthy | A term used when an orbiting GPS satellite is suitable for use. "State" is also used to refer to satellite health. |
| Horizontal Dilution of Precision (HDOP) | HDOP is a measure of how well the positions of the satellites, used to generate the Latitude and Longitude solutions, are arranged. PDOP less than 4 gives the best accuracy, between 4 and 8 gives acceptable accuracy and greater than 8 gives unacceptable poor accuracy. Higher HDOP values can be caused if the satellites are at high elevations. See Dilution of Precision (DOP). |
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| Icon | A graphic symbol that can be placed on the plotter screen (and saved in memory) to represent some special event or area of interest to the GPS user. Also known as Event Marker. |
| Illumination | The signal coming from the GPS satellites. |
| 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. |
| Input/Output (I/O) | The two-way transfer of GPS information with another device, such as a navigation plotter, autopilot or another GPS unit. |
| Integrity | The ability of a system to supply timely warnings in the event of a loss of navigation solution, excessive noise, or other factors affecting measured position. |
| Invert Route | To display and navigate a route from end to beginning for purposes of returning to the route's starting point. |
| 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 443 kilometers (275 miles) above the surface. |
| Ionospheric Refraction | The change in the propagation speed of a signal as it passes through the ionosphere. |
| IPX7 Waterproof Standard | A standard indicating that a device can be submerged 1 meter for up to 30 minutes. |
| Kinematic Positioning | Positioning a continuously moving platform by using GPS carrier-phase data while operating in a differential mode. |
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| L-Band | The radio frequencies that extend from 390 MHz to 1550 Mhz. The GPS carrier frequencies are in the L band (1227.6 MHz and 1575.42 MHz). |
| L1 Frequency | One of the two radio frequencies transmitted by the GPS satellites. This frequency carries the Coarse Acquisition Code (C/A code), P-Code, and the navigation 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 zero to 90. One minute of latitude equals one nautical mile. |
| Leg (Route) | A part 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. |
| Line of Sight (LOS) Propagation | Of an electromagnetic wave, propagation in which the direct transmission path from the transmitter to the receiver is unobstructed. An unobstructed LOS is required for GPS signals. |
| Liquid Crystal Display (LCD) | A type of screen commonly found in small electronic devices. |
| Local Area Augmentation System (LAAS) | LAAS is the implementation of ground-based 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). The prime meridian runs from the north to sound pole through Greenwich, England. |
| LOng RAnge Navigation (LORAN) | LORAN is a grid of radio waves in many areas of the globe that allows accurate position plotting. LORAN transmitting stations around the globe continually transmit 100kHz radio signals. Special shipboard LORAN receivers interpret these signals and provide readings that correspond to a grid overprinted on nautical charts. By comparing signals from two different stations, the mariner uses the grid to determine the position of the vessel. |
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| Magnetic North | Represents the direction of the north magnetic pole from the observer's position. The direction a compass points. |
| Magnetic Variation | In navigation, at a given place and time, the horizontal angle (or difference) between true north and magnetic north. Magnetic variation is measured east or west of true north. |
| Map Data | Map data is a set of digital information that a GPS unit uses to represent things such as streets, railroads, airports, parks, rivers, and lakes. Map data in most automotive systems shows the location of roads, addresses, and other points of interest (POIs). It can also include details such as speed limit, time limitations (closed to traffic during certain hours of the day), and mid-road barriers (which only allow you to turn one direction from a side street). |
| Map Display | A graphic representation of a geographic area and its features. |
| Mean Sea Level | The average level of the ocean's surface, as measured by the level halfway between mean high and low tide. Used as a standard in determining land elevation or sea depths. |
| MicroSD Memory | Formerly known as TransFlash. It is removable memory that is very common in mobile phones. It is also used by some navigation systems for storing map data. This format is much newer than SD or CF. |
| Moving Map | A digital map in which the center of the screen represents your location while the digital map underneath your position moves as you move. |
| Multi-Channel Receiver | A GPS receiver that can simultaneously track more than one satellite signal. |
| Multi-Destination Routing | Multi-destination routing lets you specify several locations you want to visit and then creates a single route that passes through all of them. Better GPS systems should be able to adjust the order of destinations to create a more efficient route. This feature is found in the more sophisticated navigation systems. |
| Multipath | Interference caused by reflected GPS signals arriving at the receiver, typically as a result of nearby structures or other reflective surfaces. |
| Multipath Error | 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". |
| 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. |
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| Nautical Charts | A series of detailed maps for the purpose of aiding safe marine navigation. |
| 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 Data Bits | The data that is broadcast by the GPS satellites. This data contains information that tells GPS receivers the satellite locations, velocities, and clock offsets. The broadcast data rate is 50 bits per seconds. |
| Navigation Message | The message transmitted by each GPS satellite containing system time, clock correction parameters, ionospheric delay mode l parameters, and the satellite's ephemeris data and health. The information is used to process GS 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 | Stands for National Marine Electronics Association that is 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 Standard | NMEA Standard 0183 is used on virtually every GPS receiver as a standard data transmission protocol between GPS receivers and other electronic instruments. |
| North Up Orientation | Fixes the GPS receiver's map display so north is always fixed at the top of the screen. |
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| OEM | Original equipment manufacturer. |
| Observable | Any feature of a physical object that can be measured or observed. The basic GPS observables are code-pseudo ranges and carrier phases as well as Doppler measurements. |
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| 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. |
| Parallel Channel Receiver | A continuous tracking receiver using multiple receiver circuits to track more than one satellite simultaneously. |
| Pedestrian or Walking Mode | Most in-car portable GPS receivers have a pedestrian or walking mode. The pedestrian mode optimizes the route for walking, and may also adjust arrival times to match walking, rather than driving speeds. |
| Pixel | A single display element on an LCD screen. The more pixels, the higher the resolution and definition. |
| PND | PND stands for Portable/Personal Navigation Device. A GPS receiver that is most commonly optimized for vehicle navigation. Such devices include a detailed street-map database, Points of Interest (POIs) and software that provides the user turn-by-turn directions to a specific destination. |
| Point of Interest (POI) | POI stands for Point of Interest. A POI can be any interesting place like a restaurant, movie theater, hospital, store, etc. |
| Point Positioning | See Absolute Positioning. |
| Position | An exact, unique location based on a geographic coordinate system. |
| 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 latitude/longitude in degrees and minutes, with options for degrees, minutes and seconds, degrees only, or one of several grid formats. |
| Positional Dilution of Precision (PDOP) | PDOP is a measure of overall uncertainty in a GPS position solution with TDOP not included in the estimated uncertainty. The best PDOP (lowest value) would occur with one satellite directly overhead and three others evenly spaced about the horizon. See Dilution of Precision (DOP). |
| Post-Processed Differential GPS | In Post-Processed Differential GPS, the base and roving receivers have no active data link between them. Instead, each records the satellite observations that will allow differential correction at a later time. Differential correction software is used to combine and process the data collected from these receivers. |
| Propagation Delay | The time it takes for a signal to travel to the satellite and back to the earth. Satellite signals travel at the speed of light and complete the trip in 270 milliseconds. |
| Propagation Speed | The speed that a signal travels at--the speed of light. |
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| Precise Positioning Service (PPS) | The most accurate dynamic positioning possible with GPS, based on the dual frequency P-Code. |
| Prime Meridian | The zero longitude location from where east and west is measured. It passes through Greenwich, England. |
| Pseudo-Lite | A ground-based transmitter that mimics a GPS signal. These devices can be used around airports for such tasks as precision instrument landings. |
| Pseudo-Random Noise Code | A binary code (sequence of "1"s and "0"s) that exhibits noise-like properties in its power spectral density. Used in the GPS system to provide a unique binary code for each satellite's encoded signal. |
| Pseudo-Random Number (PRN) | A unique number that identifies a specific GPS satellite your GPS receiver is tracking. Each GPS satellite broadcasts its own "ID code" commonly referred to as a PRN. |
| Pseudo-Range | 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. |
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| Quadrifilar Helix Antenna | A type of GPS antenna in which four spiraling elements form the receiving surface of the antenna. For GPS use, quadrifilar antennas are typically half-wavelength or quarter-wavelength size and encased in a plastic cylinder for durability. |
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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. |
| Range | A fixed distance between two points, such as between a starting and an ending waypoint or a satellite and a GPS receiver. |
| Real Time Kinematic (RTK) | The DGPS procedure where the carrier phase corrections are transmitted in real time from a reference station to the user's roving receiver. |
| Real-Time Differential GPS | A base station which computes, formats, and transmits corrections usually through some sort of data link (ex. VHF radio or cellular telephone) with each new GPS observation. The roving unit requires some sort of data link receiving equipment to receive the transmitted GPS corrections and get them into the GPS receiver so they can be applied to its current observations. |
| Real-time traffic | Information that is sent, in real-time or near real-time to your GPS navigation system to alert you of traffic delays along your path of travel. |
| Receiver | Hardware device that receives data, such as from GPS satellites. |
| Relative Positioning | The determination of relative positions between two or more receivers which are simultaneously tracking the same GPS signals. |
| Relative Dilution of Precision (RDOP) | It is usually in units of meters/cycle. Multiplying RDOP by the uncertainty of a double-difference measurement yields the spherical relative-position error. RDOP is used as a guide to the adequacy of receiver observation during real-time surveying measurements in static mode. See Dilution of Precision (DOP). |
| Residual | A quality indicator for a GPS position that is determined during the differential correction process. Indicates uncorrectable error. High residuals are not desirable. |
| Rhumb Line | A line that passes through all meridians at the same angle. When drawn on a Mercator chart, the rhumb line is a straight line, because the Mercator chart is a distortion of a spherical globe on a flat surface. The rhumb line results in a longer course than a great circle route. |
| RINEX | Receiver INdependent EXchange format. A set of standard definitions and formats to promote the free exchange of GPS data and facilitates the use of data from any GPS receiver with any software package. The format includes definitions for three fundamental GOS observables: time, phase, and range. |
| Route | A route is a series of two or more waypoints. |
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| Route Leg | Route leg is the straight line between two adjacent waypoints in a route. |
| Rover | Any mobile GPS receiver collecting data during a field session. The receiver's position can be computed relative to another stationary GPS receiver. |
| RS-232 | A serial input/output standard that allows for compatibility between data communication equipment made by various manufacturers. |
| Satellite | An object in orbit around the earth. In the case of GPS, it is a device that broadcasts time-coded signals to help determine position. |
| Satellite Constellation | The arrangement in space of a set of satellites. |
| Satellite Geometry | The geometric quality of a constellation of satellites is measured by Position Dilution of Precision (PDOP). It is also measured by Geometric Dilution of Precision (GDOP). The difference between PDOP and GDOP is that GDOP considers time, where PDOP only considers geometry. |
| SD Memory | Secure digital (SD) memory is removable memory used by some navigation systems for storing map data. SD cards are physically about half the size of CF memory cards but otherwise have the same capabilities. |
| Search The Sky | A message shown when a GPS receiver is gathering satellite almanac data. This data tells the GPS receiver where to look for each GPS satellite. |
| Selective Availability (SA) | The random error, which the government can intentionally add to GPS signals, so that their accuracy for civilian use is degraded. SA is not currently in use. |
| Sensitivity | A measurement of the ability of a GPS receiver to acquire and track GPS signals. A highly sensitive GPS receiver would be able to acquire and track GPS signals in weak signal environments. |
| Serial Communication | The sequential transmission of the signal elements of a group representing a character or other entity of data. The characters are transmitted in a sequence over a single line, rather than simultaneously over two or more lines, as in parallel transmission. The sequential elements may be transmitted with or without interruption. |
| Single Receiver Positioning | See Absolute Positioning. |
| Slow Switching Channel | A sequencing GPS receiver channel that switches too slowly to allow the continuous recovery of the data message. |
| SONAR | A system using transmitted and reflected underwater sound waves to detect and locate submerged objects or measure the distance to the floor of a body of water. |
| Sound Pulse | A short burst of sound energy. Sound pulses are used in sonar devices, which can interpret the pulse's echo. |
| Sounder | A device that determines water depth by measuring sound echoes through water. |
| Space Segment | The satellite portion of the complete GPS system. |
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| Speed Over Ground (SOG) | SOG is the actual speed the GPS unit is moving over the ground. This may differ from airspeed or nautical speed due to such things as head winds or sea conditions. For example, a plane that is going 120 knots into a 10-knot head wind will have a SOG of 110 knots. |
| Spoofing | Spoofing, a not-quite-technical term first coined in the radar community, is the transmission of fake GPS signals that receivers accept as authentic ones. |
| Spread Spectrum | The received GPS signal has wide bandwidth and low power. The L-band signal is spread over a frequency band much wider than the minimum bandwidth needed to transmit the information being sent. This is done by modulating with a pseudo-random code, and provides the ability to receive all satellites and to give some resistance to noise and multipath. |
| Standard Positioning Service (SPS) | The normal civilian positioning accuracy obtained by using the single frequency C/A code. |
| State | State is used to refer to a satellite that is in orbit and suitable for use. |
| Static Positioning | Location determination when the receiver's antenna is presumed to be stationary in the earth. This allows the use of various averaging techniques that improve the accuracy by factors of over 1000. |
| Statute Mile | A unit of length equal to 5,280 feet or 1,760 yards (1,609 meters) used in the U.S. and some 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. |
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| Text-to-speech | Text-to-speech is a feature found on many GPS receivers that provides vocalized pronunciations of local street names, making turn-by-turn directions more helpful. Instead of just saying "Turn Left", you will hear "Turn Left at Maple Street". The receiver will have read the name of the street from the map and articulated it using software. This is an extra bit of navigational information that will help you to know what's coming up so you can keep your eyes on the road, instead of on the GPS screen in search of a street name. |
| Time Dilution of Precision (TDOP) | Time Dilution of Precision is a measure of how the satellite geometry is affecting the ability of the GPS receiver to determine time. See Dilution of Precision (DOP). |
| Time to First Fix (TTFF) | Time to First Fix (TTFF) is the amount of time it takes a GPS receiver to compute its position from a warm start (current almanac, known position, approximate time). |
| Time to Go (TTG) | Reading showing the time remaining from your current position to the next waypoint. This function takes into account your Distance to Go (DTG), and your Velocity Made Good (VMG) to give you as closely as possible the amount of time left to reach your waypoint. |
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| Touch Screen | A computer display that enables the user to interact with the computer by touching areas on the screen. |
| Track (TRK) | Your current direction of travel relative to a ground position. |
| Track Up Orientation | Fixes the GPS receiver's map display so the current track heading is at the top of the screen. |
| Tracklog | Tracklog is the electronic equivalent of the famous bread crumb trail. If you turned (automatic) tracklog on, your receiver will, at fixed intervals or at special occasions, save the position, together with the time, to its memory. This can be invaluable if at any moment during your trip you have to go back exactly along the route that brought you to your actual position. |
| Transducer | A device, much like a microphone, that converts input energy of one form into output energy of another. Fish finders separate and enhance the information received from a transducer to show underwater objects. |
| Travel Bug | Travel bugs are numbered tags that are placed in Geocaches as part of the GPS game of geocaching. |
| Triangulation | A mathematical technique to determine a GPS user's position, speed, and elevation. |
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| Tropopause | The thin layer that divides the troposphere from the stratosphere is called the "tropopause", located at an altitude of around 5 miles in the winter, to around 8 miles high in the summer, and as high as 11 or 12 miles in the deep tropics. |
| Troposphere | The troposphere is the lowest major atmospheric layer, and is located from the Earth's surface up to the bottom of the stratosphere. It has decreasing temperature with height (at an average rate of 3.5 degrees F per thousand feet (6.5 degrees C per kilometer). GPS signals travel through the troposphere (and other atmospheric layers). |
| True North | The direction of the north pole from your current position. Magnetic compasses indicate north differently due to the variation between true north and magnetic north. A GPS receiver can display headings referenced to true north or magnetic north. |
| Turn (TRN) | The degrees which must be added to or subtracted from the current heading to reach the course to the intended waypoint. |
| Universal Serial Bus (USB) | A Universal Serial Bus (USB) is the most common connector for computer peripherals and accessories. Most GPS receivers use a USB cable to connect to your personal computer for updates like downloading additional maps or software. |
| Universal Time Coordinated (UTC) | 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. |
| Universal Transverse Mercator (UTM) | Universal Transverse Mercator (UTM) is 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. |
| Update rate | The frequency in which a GPS receiver updates its position. |
| Uplink | A transmission path by which radio or other signals are sent from the ground to an aircraft or a communications satellite. |
| 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 | The segment of the complete GPS system that includes the GPS receiver and operator. |
| Value | Descriptive information about a Feature. Values can be thought of as the answers to the questions posed by Attributes. |
| Velocity Made Good (VMG) | VMG is a true indication of the speed being made to the selected waypoint. |
| Vertical Dilution of Precision (VDOP) | Vertical Dilution of Precision is a measure of how well the positions of the satellites, used to generate the vertical component of a solution, are arranged. Higher VDOP values mean less certainty in the solutions and can be caused if the satellites are at low elevations. See Dilution of Precision (DOP). |
| Via Points | Via points let you create a route that passes through a particular place on your car navigation system. Via points are especially useful for longer trips where you want to pass through a particular city or use a favorite road or highway. |
| Voice-Activated/Speech-Recognition | GPS receivers that feature "Voice Activated" commands or "Speech Recognition" technology let drivers verbalize their desires, be it a destination, route change, or other instruction that would normally be accessed with the touch screen. The GPS receiver can hear and interpret vocal commands and execute them effectively, so you never have to take your hands off the wheel to modify settings or directions. |
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| Wavelength | The distance between points of corresponding phase of two consecutive cycles of a wave. |
| Waymark | A waymark is a waypoint with specific information about the location with respect to its waymark category such as a recreation area, services, cost, hours of operation, etc. |
| Waypoint (WPT) | A waypoint is a location on your route that is not the destination or the originating point. These can be tourist attractions, gas stations, hotels and rest areas, as well as other locations, depending on the type of GPS device you are using. Waypoints allow you to plan a full route with little trouble. |
| Wi-Fi Hotspot | A Wi-Fi hotspot is an area where wireless internet access is available to anyone with a wireless-enabled laptop. |
| Wi-Fi Positioning System (WPS) | WPS is a term pioneered by Skyhook Wireless to describe its Wi-Fi based location system. Skyhook has built and maintains a large global database of Wi-Fi access points and their precise locations. This data may then be used by a mobile electronic device to triangulate a user's position. WPS may be combined with cell phone tower triangulation and GPS to provide reliable and accurate position data under a wide range of conditions, including among tall buildings and indoors, when GPS signals may be weak or intermittent. However, WPS does not work when out of range of Wi-Fi signals, and the Wi-Fi hotspots database must be constantly updated to keep up with Wi-Fi hotspot changes. |
| Wide Area Augmentation System (WAAS) | Wide Area Augmentation System is a satellite-based navigation system designed to improve the accuracy, availability and integrity of signals from GPS satellites for aviation. |
| World Geodetic System 1984 (WGS-84) | The primary map datum used by GPS. Secondary datums are computed as differences from the WGS 84 standard. |
| Worldwide Reference Network (WWRN) | A wide area network of GPS reference stations that provides assistance data to location-enabled devices anywhere in the world. |
| XTE/XTK (Crosstrack Error) | The distance you are off the desired course in either direction. |
| Y-Code | Y-Code is the encrypted P-Code. |
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