As we find new terms and develop new features, this page shall be updated. Please note some terms are used for certain types of GPS devices, and GSM communication. The terms are used on this site and may differ from other systems and can also differ from country to country.
Terms are generally related to GPS, electronic device on GPS/GSM applications, wireless communication, internet access and tracking software applications.

- A -
acquisition time - see TTFF
active antenna - An amplified antenna
active leg - The segment of the route currently being traveled.
ajax - With Ajax, web applications can retrieve data from the server asynchronously in the background without interfering with the display and behavior of the existing page. It is commonly used in moving map applications.
AGPS - Assisted Global Positioning System - A land station assists GPS in acquiring position.
almanac data - Orbital position data for each GPS satellite. Considered not to be very accurate as the data are valid for over one month.
altimeter - An instrument that measures altitude or elevation with respect to a reference level, usually mean sea level, by means of air pressure.
arcInfo - Comprehensive software in the arcGIS family that has advanced geo-processing and data conversion capabilities.
ARGOS - Advanced Research and Global Observation Satellite - An advanced research and development satellite launched by the United States Air Force. Carries various space and atmospheric experiments.
attribute - A characteristic which describes a Feature. Attributes can be thought of as questions which are asked about the Feature. Typically associated with geospatial data gathering for inclusion within Geographic Information Systems (GIS).
automatic vehicle location (AVL) - A system of providing real-time location information for emergency vehicles, delivery trucks, service vehicles, etc.
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.
GPS availability: The number of hours per day that a particular location has sufficient satellites (above the specified elevation angle, and perhaps less than some specified PDOP value) to make a GPS position determination possible.
azimuth - The horizontal angle (0 - 360 degrees) from a reference point, usually true north or true south. Also known as a bearing.
azimuth ring - The dial on a compass, marked with zero to 360 degree markings.

- B -
Base station – In GPS navigation, this is a receiver that is set up on a known location specifically to collect data for differentially correcting data files of another receiver (which may be referred to as the "mobile" or "rover" receiver).
In GSM wireless mobile network, it is the localized readiating site interacting with the mobile phones connected to this datation. The information is relayed to the hierarchical centre for further processing. Some network operators trasnmt the local suburb names in the palce of a based station code reference.
bearing - An azimuth; a horizontal direction of a line or direction of travel with true north or true south as a reference.
bench mark - A material object, natural or man-made, with a known elevation or horizontal location. Bench marks can be used as reference points when traveling a route or in determining the elevation of nearby land features.
bias - All GPS measurements are affected by biases and errors. Their combined magnitudes will affect the accuracy of the positioning results (they will bias the position or baseline solution). Biases may be defined as being those systematic errors that cause the true measurements to be different from observed measurements by a "constant, predictable or systematic amount", such as, for example, all distances being measured too short, or too long.
bitmap image - An image with 1 bit of color information per pixel, also known as a bitmapped image.
Bluetooth - A standard of radio technology that makes it possible to transmit signals over short distances between telephones, computers and other devices

- C -
C/A code - The standard (Clear/Acquisition) GPS PRN code, also known as the Civilian Code or S-Code. Only modulated on the L1 carrier. Used by the GPS receiver to acquire and decode the L1 satellite signal, and from which the L1 pseudo-range measurement is made.
cadastre - A public record, survey, or map of the value , extent, and ownership of land as a basis of taxation.
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 1575.42MHz, (b) L2 at 1227.60MHz, modulated by the Navigation Message (both L1 and L2), the P-Code (both L1 and L2) and the C/A-Code (L1).
cartography - The art or technique of making maps or charts.
CF (Compact Flash) - A standard that PDAs, laptops, and other computer-related devices use to accept removable accessories such as additional memory or GPS receivers.
channel - Circuitry necessary to receive the signal from one GPS satellite.
code phase – GPS measurements based on the C/A-Code. The term is sometimes restricted to the C/A- or P-Code pseudo-range measurement when expressed in units of cycles.
COG - see course over ground
cold start - The ability of a GPS receiver to start giving position data without any almanac data stored in its memory.
collar - The area surrounding the actual map itself containing necessary information such as scale, legend, latitude and longitude, etc.
constellation - 1) All GPS satellites. 2) The satellites visible to a GPS receiver at one time.
continental margin - The zone of transition from a continent to the adjacent ocean basin. It usually includes a continental shelf, continental slope, and continental rise.
continental rise - The gently sloping surface located at the base of a continental slope.
continental shelf - The submerged margin of a continental mass extending from the shore to the first prominent break in slope.
constellation - Refers to either the specific set of satellites used in calculating a position, or all the satellites visible to a GPS receiver at one time, or the entire ensemble of GPS satellites comprising the Space Segment.
contour interval - The difference, in feet or meters, in elevation between two adjacent contour lines.
contour line - A line on a topographic map that represents a specific elevation.
control point - Also called a control station. Locations on the earth's surface used for mapping references. Can be horizontal or vertical or both. Often these are bench marks. The coordinates may be expressed in terms of a satellite reference coordinate system (such as with respect to WGS84, or to ITRS), or a local geodetic datum.
coordinates - A set of numbers that represents a precise location anywhere on Earth. Usually stated as latitude and longitude.
course - The direction from the starting waypoint or location to the destination waypoint, measured in degrees.
course made good - The bearing from the "active from" waypoint to the current position, independent of the path taken to arrive at the current position.
course over ground - The direction of movement relative to the Earth.

- D -
data message - A message included in the GPS signal which reports the satellite's location, clock corrections and health. Included is rough information about the other satellites in the constellation. In the context of location based tracking application, this is the message send by the respective devices to the receiving control station, generally containing identification reference, location readings, UTC time and date, speed and course ,etc… The format may vary from one manufacturer from another.
data logger - Also known as a Data Recorder. Generally, a handheld lightweight data entry computer. It can be used to store additional data obtained by a GPS receiver, such as Attribute information on a Feature whose coordinates are captured for a GIS project.
datum - A model of the earth's surface that is comprised of a network of physical points that serves as a reference for maps, for both horizontal and vertical data. In the U.S. early topographic maps were based on the North American Datum 1927 (NAD27). More recent maps are based on a North American Datum 1983 (NAD83). GPS uses a datum called World Geodetic System 1984 (WGS84). There are over 100 datums in use around the world.
dead reckoning - The technique of determining position by computing distance traveled on a given course. Distance traveled is determined by multiplying speed by elapsed time.
degree - A unit of measurement; 360 degrees equal one complete circle, as around the earth on a line of latitude; measured from 0 to 90 degrees from the equator to each pole along a line of longitude.
Differential positioning – Also known as Relative Positioning. Precise measurement of the relative positions of two receivers tracking the same GPS signals. Maybe considered synonymous with DGPS, or the term may be reserved for the more precise carrier phase-based baseline determination technique associated with GPS Surveying.
Dilution of precision (DOP) - An indicator of satellite geometry for a unique constellation of satellites used to determine a position. Positions tagged with a higher DOP value generally constitute poorer measurement results than those tagged with lower DOP. There are a variety of DOP indicators, such as GDOP (Geometric DOP), PDOP (Position DOP), HDOP (Horizontal DOP), VDOP (Vertical DOP), etc.
DGN - microstation design file - A file format/extension.
DGPS - Differential GPS - A method of improving GPS accuracy using measurements at a known location to improve the measurements made by other GPS receivers within the same general area.
Doppler shift - The apparent change in the frequency of a signal caused by the relative motion of the transmitter and receiver.
draw - A small, natural depression or valley in the side of a hill; a gully.
DRG - Digital Raster Graphics - See "Raster Maps."
DTM - Digital Terrain Model - A digital model of the terrain surface - similar to a DEM but often enhanced by plan metric information.
dual band – in GSM mobile phone network, there are 4 commonly used frequency bands deployed in the world, generally as 850, 900, 1800 and 1900 MHz A dual band refers to a combination of 2 of the 4 bands. The pairing of the 4 bands are 900/1800MHz and 850/1900MHz (north American and a few other countries in Latin America).
dual frequency – Refers to the instrumentation that can make measurements on both L-Band frequencies, or to the measurements themselves (e.g., L1 and L2 pseudo-range or carrier phase measurements). Dual-frequency measurements are useful for high precision (pseudo-range-based) navigation because the Ionospheric Delay bias can be determined, and the data corrected for it. In the case of Double-Differenced carrier phase, dual-frequency observations can account for the residual ionospheric bias (for case of long baselines), or aid Ambiguity Resolution for "rapid static" or "kinematic" baseline determination. All "top-of-the-line" GPS receivers are of the dual-frequency variety, and are comparatively expensive because of the special signal processing techniques that must be implemented to make measurements on the L2 carrier under the policy of Anti-Spoofing.
DXF - Drawing eXchange Format - Used for transferring CAD (Computer Aided Drafting) files.

- E -
EGNOS - European Geostationary Navigation Overlay Service - A European system of satellite navigation. It will augment the two satellite navigation systems now operating, the U.S. GPS and GNSS systems. It consists of three geostationary satellites and a network of ground stations. EGNOS will achieve its aim by transmitting a signal containing information on the reliability and accuracy of the positioning signals sent out by GPS and GNSS.
elevation - The vertical distance of a point above or below a reference surface, usually sea level.
ellipsoid - The mathematical function used to describe the shape of the earth for geodetic computations.
EPE - Estimated Position Error - The distance the GPS receiver may be off target.
ephemeris data - Precise orbital position and clock data for each GPS satellite. Valid for about 30 minutes.
equator - An imaginary line around the Earth at 0 degrees latitude.
ETA - Estimated Time of day of your arrival at your destination. Typically used for navigation applications
ETE - Estimated Time Enroute - The time left to your destination at your present speed. Typically used for navigation applications.

- F -
FIFO – Stands for First in First out, a process in which the oldest point or counting entity shall be pushed out (or erased) when a new arrival point causes an over-flow condition in the already full storage area.
Fix - A single position with latitude, longitude (or grid position), altitude (or height), time, and date.

- G -
Galileo - Europe's satellite navigation system.
GANS - Global Access, Navigation, and Safety - A United States Air Force program that is a potential vehicle for collaboration. GANS is an umbrella avionics program that integrates GPS, navigation and safety equipment, Navigation Warfare (NAVWAR), avionics modernization, military ground-based infrastructure, Global Air Traffic Management (GATM), and the Joint Precision Approach and Landing System (JPALS).
GDOP - See Geometric Dilution of Precision
geocoding - See geotagging. In tracking, it is a term that describes the conversion of a textual address from latitude and longitude values. The reverse process is called “reverse geocoding”.
geodetic datum - A mathematic model designed to best fit part or all of the geoid.
geodetic height - The distance of a point above an ellipsoidal surface. The difference between a point's geodetic height and its orthometric height equals the geoidal height.
geodetic survey – Global surveys for the establishment of control networks (comprised of Reference or Control Points), which are the basis for accurate land mapping. Maybe carried out using either terrestrial or satellite positioning (e.g. GPS) techniques. The outcome is a network of benchmarks which are a physical realisation of the Geodetic Datum or Reference System.
Geographic Information System – A computer-based system that is capable of collecting, managing and analysing geospatial data. This capability includes storing and utilising maps, displaying the results of data queries and conducting spatial analysis.
geofence - An imaginary boundary set which contains a GPS device - if that GPS device exceeds the boundary, an alarm is activated.
Geometric Dilution of Precision - Since a GPS receiver determines position by triangulation, when the GPS satellites that are being received are clustered too close together, the positional accuracy determined by the receiver is diluted. The wider the angle between satellites, the better the measurement.
geostationary - A satellite in a position always fixed to a relative point on the earth.
geotagging – (see Geocodoing). The process of adding geographical identification metadata to various media such as websites, RSS feeds or images. This data usually consists of latitude and longitude coordinates, though it can also include altitude and place names.
geosynchronous - see geostationary
GIS - Geographic Information System - A system of hardware and software used for storage, retrieval, mapping, and analysis of geographic data.
Globalstar - A handheld satellite phone service.
GLONASS – (Global Orbiting Navigation Satellite System)This is the Russian counterpart to GPS. It consists of a constellation of 24 satellites (though the number may vary due to difficulties in funding for the system) transmitting on a variety of frequencies in the ranges from 1597-1617MHz and 1240-1260MHz (each satellite transmits on two different L1 and L2 frequencies).
GNSS - Global Navigation Satellite System - A Soviet space-based navigation system comparable to the American GPS system.
GPRS - General Packet Radio Service - technology that enables high-speed wireless Internet and other data communications for uses such as mobile phones and computers.
GPS (Global Positioning System) - A system of 24 satellites used to locate any point on the earth by triangulation and distance measuring.
GPS III - A new initiative by the U. S. government to upgrade the current GPS system, which includes replacing all current satellites.
GPX - GPS eXchange format is an XML schema designed for transferring GPS data between software applications. It can be used to describe waypoints, tracks, and routes.
great circle - A circle described by the intersection of the surface of the Earth with a plane passing through the center of the Earth. The shortest distance between two points on the surface of the Earth is a segment of a great circle. All longitudes are great circles; the only latitude that is a great circle is the equator.
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 - The velocity you are travelling relative to a ground position. Typically measured in "knots" (nautical miles per hour), but may be expressed in km/hr or m/s.
GSM - Global System for Mobile communications - A world standard for digital cellular communications

- H -
heading - The direction one is traveling.
HTDP - Horizontal Time Dependent Positioning - A computer database and interpolation program to predict horizontal displacements between coordinate points over time.
index contour - A contour line on a topographic map that is labeled as to elevation; used as a reference.
initialization - The first time a GPS receiver orients itself to its current location.
INMARSAT - International Mobile Satellite Organization - Established in 1979 to serve the maritime industry by developing satellite communications for ship management and distress and safety applications.
ionosphere - A region of the Earth's atmosphere where ionization caused by incoming solar radiation affects the transmission of radio waves. It extends from a height of 43 miles (70 kilometers) to 250 miles (450 kilometers) above the surface.

- I -
IMEI – An identification number developed to track down stolen mobile phone. It is a 15-digit number with information on its registereing body, manufacturer’s code and its serial number.
index contour - A contour line on a topographic map that is labeled as to elevation; used as a reference.
initialization - The first time a GPS receiver orients itself to its current location.
INMARSAT - International Mobile Satellite Organization - Established in 1979 to serve the maritime industry by developing satellite communications for ship management and distress and safety applications.
ionosphere - A region of the Earth's atmosphere where ionization caused by incoming solar radiation affects the transmission of radio waves. It extends from a height of 43 miles (70 kilometers) to 250 miles (450 kilometers) above the surface.

- J -
JPO (Joint Program Office)
That part of the U.S. Department of Defense responsible for managing the GPS development, deployment and operation of the GPS system (in particular the Control Segment and the Space Segment, as well as the military User Segment).

- K -
kinematic GPS - Observations while a receiver is in motion. It refers to applications in which the position of a non-stationary object (vehicle, ship, and aircraft) is determined.
knot - A unit of speed of one nautical mile per hour, equal to 1.15 statute mile per hour (1.85 kilometers per hour).

- L -
L band - The group of radio frequencies extending from 390MHz to 1550MHz. The GPS carrier frequencies L1 and L2 are in the L-Band.
latitude - The distance north or south of the equator of a point on the earth's surface.
leg - The portion of a route between waypoints.
Live Tracking viewing – It offers an automatic update of a tracking point on a viewing page as each new point arrives. No manual intervention is needed to call up the latest point.
Local Area Augmentation System - A specialized type of DGPS to support aircraft landings in a local area (32KM). The first system is scheduled to be operational by late 2006.
location-based services (LBS) - Location-Based Services - A service which uses knowledge provided about where a user is located in order to deliver information relevant to the user's position.
longitude - The distance east or west of the prime meridian of a point on the earth's surface.
L1 frequency - 1575.42MHz GPS carrier frequency which contains the C/A-Code, the encrypted P-Code (or Y-Code) and the Navigation Message. Commercial GPS navigation receivers can track only the L1 carrier to make pseudo-range (and sometime carrier phase and Doppler frequency) measurements.
L2 frequency - 1227.60MHz GPS carrier frequency which contains only the encrypted P-Code (or Y-Code) and the Navigation Message. Military Y-Code capable receivers can, in addition to making L1 measurements, make pseudo-range measurements on the L2 carrier. The combination of the two measurements (on L1 and L2) permits the Ionospheric Delay to be corrected for. Dual-frequency GPS receivers intended for Surveying applications can make L2 measurements using proprietary signal processing techniques. Such measurements are essential if the Ionospheric Delay on carrier phase is to be corrected for (especially on baselines of length greater than about 20-30km) and/or where fast Ambiguity Resolution is needed.

- M -
magnetic north - A point on the earth about 500 miles from the north pole toward Hudson Bay in Canada; this is where a compass points. In the U.S., magnetic north can vary up to approximately 22 degrees from true north.
magnetic variation – (magnetic declination) The difference, in degrees, between magnetic north and true north. Same as magnetic declination.
map datum - What reference map is used in determining the Fixes.
map projection - The method used to convert the spherical shape of the earth onto the plane of a map. While we commonly use flat sheets of maps for viewing, it is required to spread a curved earth surface onto a flat form. To do this, a curved surface has to be "projected" onto a flat surface (the map paper). Mercator, Orthographic and Conic are common types of projections.
map projection deformation - The error or distortion that results when the spherical shape of the earth is projected onto the plane of a map.
map offset – Some street and road map data vary slightly from the satellite image data either due to intentional adjustments or by use of some older conversion techniques. These variations are off-sets and need to be used to bring the map data into conformity with more conventional map presentations. China data in the public domain is one such example.
map sources (public domain) – A number of organizations like Google, Microsoft, Yahoo, etc have published quality map data in the public domain as part of their business models. The data is available for general consumption and their deployment are governed by their own set of terms of use.
mean sea level - The average height of the sea surface, based upon hourly observation of the tide height on the open coast or in adjacent waters that have free access to the sea. In the United States, it is defined as the average height of the sea surface for all stages of the tide over a nineteen year period. Mean sea level, commonly abbreviated as MSL and referred to simply as 'sea level,' serves as the reference surface for all altitudes in upper atmospheric studies.
Mercator projection – It is a cylindrical map projection. Its parallels and meridians are straight lines, and the unavoidable east-west stretching away from the equator is accompanied by a corresponding north-south stretching, so that at each location the east-west scale is the same as the north-south-scale.
meridian - An imaginary line circling the earth, passing through the north and south poles; a line of longitude.
minute - A unit of measurement; one degree contains 60 minutes.
moving map - A program that displays your position in real time on the top of either a Vector or Raster map using GPS input.
Motion sensor – To improve battery life, many device manufacturers have introduced motion sensor to turn off its functions like GPS and GSM,..etc. The motion sensor, when moved, shall turn on the functions, hence conserving energy if the device is not required to perform functions when stationary.
multi-channel receiver - A GPS receiver that can simultaneously track more than one satellite signal using a dedicated signal electronics channel for each satellite. High quality receivers may have 12 channels for L1, and another 12 channels for L2 signals. Lower quality GPS navigation receivers may have only 6 or 8 channels. In contrast to a Multiplexing Channel Receiver.
Multiplexing channel - A channel of a GPS receiver that can be sequenced through a number of satellite signals. In contrast to a Multi-Channel Receiver in which one channel is dedicated to each satellite signal.

- N -
NAD 27 - North American Datum 1927
NAD 83 - North American Datum 1983
National Marine Electronics Association (NMEA) - A US standards committee that defines the data message structure, contents, and protocols that allow GPS receivers to communicate with other electronic equipment.
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 - equal to appr. 1852 meters (6076 feet).
NAVD 88 - Vertical reference system (elevation) for U.S., Canada, and Mexico; orthometric heights are derived from a particular point on the St. Lawrence Seaway.
NAVSTAR (Navigation Satellite Timing and Ranging) - The name given to the system of GPS satellites.
Near Real Time tracking – A term that is commonly used synonymously as real time tracking in location based services. (see Real Time tracking).
NMEA - see National Marine Electronics Association
NEMA 0183: A communication protocol used by GPS units and other types of navigation and marine electronics.

- O -
O E M - Original Equipment Manufacturer. Typically GPS receiver "board-sets" or "engines" that a product developer can embed within some application or hardware package.
orthographic projection - The two-dimensional graphic representation of an object formed by the perpendicular intersections of lines drawn from points on the object to a plane of projection.
Outage - Defined as a loss of Availability, due to either there not being enough satellites visible to calculate a position, or the value of the DOP indicator is greater than some specified value (implying that the accuracy of the position is unreliable).

- P -
P.O.I. – It stands for points of interest, and is commonly called “landmark” points.
packet - A unit of data that is routed between an origin and a destination on the Internet or any other packet-switched network.
parallel of latitude - An imaginary line circling the surface of the earth that is parallel to the equator; a line of latitude.
patch antenna - An antenna based on a plated section of metal on a circuit board and its associated electronics used for receiving GPS signals.
PCMCIA - Personal Computer Memory Card International Association - A standard for a credit card-size memory or input/output device that fits into a notebook, laptop or personal computer.
PDA - Personal Digital Assistant - A small mobile hand-held device that provides computing and information storage for personal or business use, often for keeping calendars and address information.
photogrammetry - The process of making maps from photographs, usually aerial photographs.
position - The 3-D coordinates of a point, usually given in the form of Latitude, Longitude, and Altitude (or Ellipsoidal Height), though it may be provided in the 3-D Cartesian form, or any other transformed map or geodetic reference system. An estimate of error is often associated with a position.
position error - The distance that the displayed position is off by.
position fixing - Using a coordinate system to determine a point on a map's location.
Projection - Representing the two-dimensional curved surface of the earth on a plane; the method used to transpose a map of the earth onto a flat surface.
power saving – in handheld tracking devices, power consumption is a critical factor as its operating life cycle depends on its batty capacity. To extend the operating life cycle, various power saving techniques are used to conserve battery. A common method is the inclusion of motion sensor.

- Q -
Quad band – (see dual band) It describes a receiver capable of receiving GPS signals within all the GPS channels in all the 4 frequency bands that are used in the world.
quadrangle - A four-sided area of the surface of the earth used as a unit in mapping.
quadrifilar helix antenna - A type of GPS receiver antenna in which four spiraling elements form the receiving surface of the antenna.

- R -
range – A fixed distance between two points, such as between a starting and an ending waypoint, or a satellite and a GPS receiver. May also be referred to as Geometric Range.
raster maps - Raster maps are commonly referred to as "bit mapped" and are in a GIF or JPEG format. Raster images are well-suited for web sites where editing or resizing are not required. They can be made smaller, but are not meant to be enlarged or customized in the ways that vector maps can be manipulated.
real time – The instance as it happens. In real life, this is theoretical impossible as it takes time for some information to pass from point A to B. This term used in the world of location based services (like navigation and tracking) is generally in the context against data logger where data is collected and stored on site, and is taken away to workbench or laboratory for further analysis. Hence, when a tracking device activated to report, it sends out a message via the vast global communication networks to reach a receiving processor and presents information to a viewer in seconds, it is generally accepted as real time with the intent of its speed (as against data logging). This term is also commonly used as “near real time”.
RFID - Radio Frequency Identification - A method of identification using radio frequency. For objects, animals, or people and does not require direct contact or line-of-sight scanning.
RKP - Relative Kinematic Positioning - Improves GPS accuracy by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests—possibly with processing in real-time (real-time kinematic positioning, RTK).
route - A set of waypoints that define the intended path of travel.

- S -
scale - The distance on a map that corresponds to the same points on the earth.
SD card – Stored digital card, a light weight portable storage medium in portable devices.
second - A unit of measurement; one minute contains 60 seconds; one degree contains 3600 seconds.
secular variation - Complex fluid motion in the outer core of the Earth that causes the magnetic field to change slowly with time.
secure communication - A type of technology for improving the security of the information being passed over the communication media – wired and wireless forms. Various forms of encryption are commonly used to confuse any one intercepting and retrieving the information along its route.
selective availability - S/A - The intentional degradation of GPS accuracy for civilian use (S/A has been deactivated indefinitely as of May 2, 2000). S/A limited the accuracy of civilian GPS systems to 300 feet or less 95% of the time.
SIM card- (Subscriber Identity Module) A SIM card is used in GSM mobile phones to identify the user for billing and other services.
SMS - Short Message Service (also called text messaging) - A function available on digital GSM networks allowing text messages to be sent and received via the network operator to a mobile phone, or from the Internet, using an "SMS gateway" website.
Speed Over Ground - The speed at which the GPS unit and the person operating it are moving with respect to the earth's surface (ignoring tidal and current activity).
statute mile - A unit of length equal to 5280 feet (1609 meters).
SVG - Scalable Vector Graphics - An XML markup language for describing two-dimensional vector graphics, both static and animated. It is an open standard created by the World Wide Web Consortium, which is also responsible for standards like HTML and XHTML.

- T -
TIGER - Topologically Integrated Geographic Encoding & Referencing - A U.S. Census Bureau mapping system.
time to first fix - See TTFF.
topography - The graphic representation of the landform on a map by the use of contour lines.
track - A collection of points that show the path travelled.
track point - A single point on the path travelled.
Track viewing – Generally referring to extracting stored data and present it on a view page, either as a textual report or against the background of a map image. This traditional approach does not show the immediate action happening in the field but needs to call up the data at that instance. A more advance method is the live-tracking viewing (see Live Tracking”)
true north - The geographical north pole.
TTFF - Time To First Fix - The time it takes for a GPS receiver to lock onto the satellite signals and determine the initial position

- U -
UMTS - Universal Mobile Telecommunications System - A transmission system for text, digitized voice, video, and multimedia for mobile computer and phone users no matter where they are located in the world.
Universal Transverse Mercator (UTM) - A coordinate system that uses a perpendicular grid with constant linear surface distance values between each of its grid lines in all directions. Coordinates are in meters as opposed to degrees, minutes, and seconds.
UPS - Universal Polar Stereographic - An azimuthal projection that is used with Universal Transverse Mercator (UTM) projection systems to represent polar regions.
USGS - United States Geological Survey
UTC - (Coordinated Universal Time) Formerly referred to as GMT or Greenwich Mean Time. This is the basis of "civilian time".
UTM - See Universal Transverse Mercator.

- V -
Vector map - Maps that use lines and polygons as opposed to pixels to display an image. Image resolution is sharp even when the image is displayed or printed at an enlarged perspective.
Velocity made good - The speed you are closing in on a destination along a desired course – a common navigation term.

- W -
WAAS - Wide Area Augmentation System - A GPS-based navigation and landing system that provides precision guidance to aircraft.
waypoint - A checkpoint; a point of reference. A (usually two-dimensional) coordinate that is input into a navigation device, such as a GPS receiver, representing a position that a vessel, aircraft, vehicle or person has to navigate to, with the aid of GPS (and/or any other position fixing device).
WGS84 – A global Geodetic Datum defined and maintained by the US Department of Defense. As the Control Segment coordinates and the Broadcast Ephemerides are expressed in this Datum, the GPS positioning results are said to be in the WGS84 Datum. In the case of Point Positioning this is largely true, although the level of accuracy achievable under the policy of Selective Availability is so poor that the link to the WGS84 Datum is very approximate. In the case of Relative Positioning, the baseline vector may be determined to quite high accuracy (at the sub-centimetre level using precise GPS Surveying techniques), however the coordinate (and therefore the Datum) of the unknown point is almost completely defined by the Datum of the Base Station. This may not be coincident with the WGS84 Datum at better than a few tens of metres! If GPS Geodesy techniques are used, with known station coordinates expressed in the ITRS and precise ephemerides obtained from the IGS, it is more correct to state that the subsequent set of coordinates are expressed in one of the ITRS frames (e.g. ITRF92, ITRF94, etc.). The WGS84 and the ITRS are compatible at the one metre level. However, the ITRS is a more precise realisation of an earth-fixed, earth-centred terrestrial reference system.

- X -
XTE/XTK – Crosstrack Error - The lateral distance in either direction you are off the desired course

- Y -
Y-code - The term used to refer to the encrypted P-Code, generated within the satellites and transmitted on both the L1 and L2 carrier signals under the policy of "Anti-Spoofing". Civilian GPS receivers use proprietary signal processing techniques to make measurements of pseudo-range and carrier phase on both L-Band frequencies.

- Z -
zenith - The point on the celestial sphere that is directly above the observer.
Zero baseline - A Zero Baseline test can be used to study the precision of receiver measurements (and hence its correct operation), as well as the data processing software. The experimental setup, as the name implies, involves connecting two GPS receivers to the same antenna. When two receivers share the same antenna, biases such as those which are satellite (clock and ephemeris) and atmospheric path (troposphere and ionosphere) dependent, as well as errors such as multipath CANCEL during data processing. The quality of the resulting "zero baseline" is therefore a function of random observation error (or noise), and the propagation of any receiver biases that do not cancel in double-differencing.

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