Global Positioning System (GPS) Definitions

Below is a comprehensive of definitions that you may come across while evaluating GPS technologies and solutions.If you have need further assistance in locating a GPS solution, tell us about the application you are considering.

AAAAccuracy. How close a fix comes to the actual position.Acquisition. The ability to find and lock on to satellite signals for ranging.Additional Secondary Factor (ASF). Error propagation time caused by radiowaves slowing down as they pass over different terrain. This is in addition to slowing down as radiowaves pass over sea water.Algorithm. A special method used to solve a certain type of mathematical problem.Almanac. A set of parameters included in the GPS satellite navigation message that a receiver uses to predict the approximate location of a satellite. The almanac contains information about all of the satellites in the constellation.Ambiguity. The initial bias in a carrier-phase observation of an arbitrary number of cycles. The initial phase measurement made when a GPS receiver first locks onto a GPS signal is ambiguous by an integer number of cycles because the receiver has no way of knowing the exact number of carrier wave cycles between the satellite and the receiver. This ambiguity, which remains constant as long as the receiver remains locked on the signal, is established when the carrier-phase data are processed.Amplitude. Height of a radiowave as measured from an imaginary center line to the wave peak.Amplitude Modulation (AM). A method of encoding a message on the carrier signal by altering the height of the signal while keeping its frequency constant.Antispoofing (AS). The process of encrypting the P-code by modulo-2 addition of the P-code and a secret encryption W-code. The resulting code is called the Y-code. AS prevents an encryption-keyed GPS receiver from being "spoofed" by a bogus, enemy-generated GPS P-code signal.Anywhere Fix. The ability of a receiver to start position calculations without being given an approximate location and time.Automatic Vehicle Location (AVL). The technique of using a navigation system, such as GPS, to determine a vehicle's position.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.

BBBBandwidth. That range of frequencies that compose a signal.Bearing. The angle between two different points on the earth's surface with respect to true north.Binary Biphase Modulation. The phase modulation technique used to transmit a GPS signal. The phase of a carrier generated by a satellite is shifted by 180 degrees when there is a code or message binary signal level transition, either from 0 to 1 (normal to mirror image) or from 1 to 0 (mirror image to normal).Bit. A unit of information in an electronic system expressed as a choice between two possible values, for example, 0 or 1.

CCCCarrier. A radio wave that conveys or carries some kind of modulation.Carrier-Aided Tracking. A technique to improve accuracy by using the GPS carrier signal to get a more exact lock on the pseudorandom code.Carrier Phase. The accumulated phase of either the L1 or L2 carrier of a GPS signal, measured by a GPS receiver since locking onto the signal. Also called integrated Doppler.Carrier-to-Noise Power Density (C/N0). The ratio of the power level of a signal carrier to the noise power in a 1-Hz bandwidth. This is a key parameter in the analysis of GPS receiver performance. Nominal GPS receiver C/N0 values often are in the 40 to 50-dB-Hz range.Carrier-Tracking Loop. A module in a GPS receiver that demodulates, or extracts, the satellite message by aligning the phase of the receiver's local oscillator signal with the phase of the frequency-shifted, received carrier. Once the local oscillator signal is locked to the carrier, its phase can be measured to provide the carrier-phase observation.Channel. In a receiver, a path for an electronic signal that is reserved for a specific satellite and used for various functions.Chip. Binary elements or digits that, unlike bits, convey no information. A PRN code consists of a sequence of chips.Circular Error Probable (CEP). A measure of accuracy in navigation. CEP is the radius of the circle, inside of which the true horizontal coordinates of a position have a 50-percent probability of being located.Clock Aiding. An accuracy enhancement technique in which an additional atomic clock (rubidium) provides accurate time to the receiver for calculating satellite clock frequency, phase bias, and clock drift.Clock Bias. The difference between a clock's indicated time and true universal time.Coarse Acquisition (C/A)-Code. A family of PRN codes transmitted by GPS satellites. Each satellite is assigned one of 32 unique codes in the family. Each code consists of 1,023 chips and is sent at a rate of 1.023 megabits per second. The code sequence repeats every millisecond. The C/A-codes are Gold codes -- PRN codes that are distinguished by a very low cross correlation between any two codes (that is, they are nearly orthogonal). C/A-codes currently are transmitted only on the L1 frequency.Coastal Confluence Zone. That area extending from a harbor's entrance 50 nautical miles seaward or to the edge of the continental shelf (100 fathom curve) whichever is greater.Code-Tracking Loop. A module in a GPS receiver used to align a PRN code sequence present in a signal coming from a satellite with an identical PRN code sequence generated within the receiver. Alignment is achieved by appropriately shifting the receiver-generated code chips in time so that a particular chip in the sequence is generated at the same instant its twin arrives.Cold Start. The ability of a GPS receiver to start providing position updates without the assistance of any almanac information stored in its memory.Control Segment. That portion of the Global Positioning System that consists of a master control station, monitoring stations, and ground antennas, which monitor and update satellite signals and upload correction data to the satellites. Coordinated Universal Time (UTC). See UTC.Course over the Ground (COG). A vessel's track over the bottom as measured in degrees.Cross-Track Error. The difference between a vessel's actual position and its desired position on a given heading. This is usually measured as a range error in nautical miles but may also be expressed graphically using symbols.Cycle Slip. A discontinuity in GPS carrier-phase observations, usually of an integer number of cycles, caused by temporary signal loss. If a GPS receiver loses a signal temporarily, due to obstructions for example, when the signal is reacquired there may be a jump in the integer part of the carrier-phase measurement due to the receiver incorrectly predicting the elapsed number of cycles between signal loss and reacquisition.

DDDData 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. The coordinate system used to define position on the earth's surface.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.Delay-Lock Loop. Another term for a code-tracking loop.Demodulation. Separating coded data from the carrier signal.Differential GPS (DGPS). A technique for reducing the error in GPS-derived positions by using additional data from a reference GPS receiver at a known position. The most common form of DGPS involves determining the combined effects of navigation message ephemeris and satellite clock errors (including the effects of SA) at a reference station and transmitting pseudorange corrections, in real time, to a user's receiver, which applies the corrections in the process of determining its position. Dilution of Precision (DOP). A dimensionless number that accounts for the contribution of relative satellite geometry to errors in position determination. DOP has a multiplicative effect on the UERE. Generally, the wider the spacing between the satellites being tracked by a GPS receiver, the smaller the position error. The most common quantification of DOP is through the position dilution of precision (PDOP) parameter. PDOP is the number that, when multiplied by the root mean square (rms) UERE, gives the rms position error. Other DOPs include the geometric dilution of precision (GDOP), horizontal dilution of precision (HDOP), and vertical dilution of precision (VDOP).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.Dithering. Introducing errors into the satellite clock readout on GPS signals for the purpose of degrading GPS position accuracy for civil users.Doppler Aiding. A signal processing strategy that uses a measured doppler shift to help the receiver smoothly track the GPS signal, thereby enabling a more precise velocity and position measurement.Doppler Effect. The shift in the frequency of a received radio signal due to the relative motion of the transmitter and receiver.Doppler Shift. The apparent change in the frequency of a signal caused by the relative motion of the signal source (transmitter) and receiver.Double Difference. A GPS observable formed by arithmetically differencing carrier phases simultaneously measured by a pair of receivers tracking the same pair of satellites. First, the phases obtained by each receiver from the first satellite are differenced. Second, the phases obtained by each receiver from the second satellite are differenced. And third, those differences are differenced. This procedure removes essentially all of the satellite and receiver clock errors. Although primarily used with carrier phases, the procedure can also be applied to pseudoranges.Drift. The rate of a vessel? departure from a given course as measured in knots.

EEEElectromagnetic Spectrum. The continuous distribution of energy in the form of electromagnetic waves, which are arranged in order of their frequencies or wavelengths.En Route. Navigation between the point of departure and point of arrival.Ephemeris. A description of the path of a celestial body indexed by time (from the Latin word, ephemeris, meaning diary). The navigation message from each GPS satellite includes a predicted ephemeris for the orbit of that satellite valid for the current hour. The ephemeris is repeated every 30 seconds and is in the form of a set of 16 Keplerian-like parameters with corrections that account for the perturbations to the orbit caused by the earth? gravitational field and other forces.Epoch. A particular instant of time or a date for which values of data are given, or a given period of time during which a series of events take place.Error Budget. A set of individual error sources with statements of the percentage of the total system error contributed by each source.

FFFFederal Radionavigation Plan (FRP). An official U.S. government document, published bienially, that outlines radionavigation planning and policy.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 waves passing a specific point within a unit period of time, expressed in Hertz (cycles per second).Frequency Modulation. A method of encoding information about a carrier signal by altering the frequency while amplitude remains constant.Full Operational Capability (FOC). For NAVSTAR, FOC occurred in 1994, when 24 operational GPS satellites (Block II/IIA) began functioning in their assigned orbits and were available for navigation use.

GGGGeodesy. The branch of applied mathematics that deals with the measurement, curvature, and shape of the earth.Geodetic. Geographic, or pertaining to geodesy. Latitude and longitude readings are geodetic coordinates.Geodetic Datum. A specifically oriented reference ellipsoid. Typically, eight parameters are required to define a geodetic datum: two to specify the dimensions of the ellipsoid, three to specify the location of its center with respect to the earth? center of mass, and three to specify the orientation of the ellipsoid with respect to the average spin axis of the earth and the Greenwich reference meridian. Also called a horizontal datum.Geodetic Height. The height of a point above an ellipsoidal reference surface. Also known as ellipsoidal height. The difference between the geodetic height of a point and its orthometric height is equal to the geoidal height.Geoid. The undulating, but smooth, equipotential surface of the earth? gravity field, which coincides most closely with mean sea level. The geoid is the primary reference surface for heights.Geometric Dilution of Precision (GDOP). The effects of the combined errors of four variables (latitude, longitude, altitude, and time) on the accuracy of a three-dimensional fix.Geostationary Satellites. Those satellites situated in a constant orbit position relative to a given area of the globe with the purpose of maintaining constant coverage of that area.GLONASS. Russia Global Navigation Satellite System (Globalaya Navigatsionnaya Sputnikovaya Sistema). The operation of this system is similar to GPS.GPS Systems Time. The time scale to which GPS signals are referenced. GPS Time derives from a composite or ?aper?clock consisting of all operational monitor station and satellite atomic clocks. It is steered over the long run to keep it within about 1 micro-second of UTC, as maintained by the Master Clock at the U.S. Naval Observatory, ignoring the UTC leap seconds. At the integer second level, GPS Time equalled UTC in 1980, but currently, due to the leap seconds that have been inserted into UTC, it is ahead of UTC by 10 seconds.GPS Week. The number of elapsed weeks (modulo 1024) since the week beginning January 6, 1980. The week number sequentially increments at Saturday/Sunday midnight in GPS (system) Time.Ground Wave. A radiowave that travels along the earth? surface.

HHHHand-Over Word (HOW). The second word in each subframe of the navigation message. It contains the Z-count at the leading edge of the next subframe and is used by a GPS receiver to determine where in its generated P-code to start the correlation search process.Height Aiding. An accuracy enhancement technique in which the known height of the receiver is entered into the navigation solution, which in effect provides another ?atellite?range.Hertz. A unit used to measure a wave? frequency, one cycle per second.Horizontal Dilution of Precision (HDOP). The effects on accuracy of the combined errors in a two-dimensional fix obtained from crossing two lines of position.

I,JInclination. One of the orbital parameters that describes the orientation of an orbit. It is the angle between the orbital plane and a reference plane, the plane of the celestial equator for geocentric orbits and the ecliptic for heliocentric orbits.Initialization. Entering such data as time, time off-set, approximate position, and antenna height into a receiver (cold start with no almanac) to help the unit find and track satellites.Inmarsat. An international consortium chartered in the mid-1970s to provide improved maritime public correspondence and radio determination capabilities.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.Interference. Any distortion of the transmitted signal that impedes the reception of the signal at the receiver (same as noise).Ion. An electrically charged particle formed by either adding or taking away electrons from neutral particles.Ionization. The process by which atoms form electrically charged particles called ions.Ionosphere. That layer of the atmosphere approximately 30-300 miles above the earth's surface that contains electrically charged particles (ions). These charged particles interfere with or distort transmissions of electromagnetic signals through the layer.Ionospheric Refraction. The change in the propagation speed of a signal as it passes through the ionosphere.Interferometric Differencing. A variation of phase differencing in which two different antennas sample the GPS signal wavefront at two locations and then feed the information into a single amplifier and mixer (interferometer) in which phase difference observations are made.

KKKKalman Filter. An optimum mathematical procedure for recursively estimating dynamically changing parameters, such as the position and velocity of a vessel, from noise-contaminated observations.Keplerian Elements. A set of six parameters that describe the position and velocity of a satellite in a purely elliptical (Keplerian) orbit. These parameters are the semimajor axis and eccentricity of the ellipse, the inclination of the orbit plane to the celestial equator, the right ascension of the ascending node of the orbit, the argument of perigee, and the time the satellite passes through the perigee.Kilohertz. A radio signal that has 1,000 cycles per second.Kinematic. Pertaining to motion or moving objects.Kinematic Positioning. Positioning a continuously moving platform by using GPS carrier-phase data while operating in a differential mode.Kinematic Surveying. A precision differential GPS surveying technique in which the roving user does not need to stop to collect precision information. Meter to centimeter-level accuracy is available using mode, dual-frequency, carrier-phase measurement techniques.

LLLL-Band. The segment of the microwave portion of the radio spectrum nominally between 1 and 2 GHz.L1 Frequency Band. One of the two ultra high radio frequencies transmitted by NAVSTAR GPS satellites. This carrier frequency of 1575.42 MHz transmits both C/A (Standard Positioning Service) and P (Precise Positioning Service) code.L2 Frequency Band. One of the two radio frequencies transmitted by NAVSTAR GPS satellites. This carrier frequency of 1227.6 MHz carries the P (Precise Positioning Service) code only.Latitude. The distance north or south of the equator as measured in degrees, with 1 degree of latitude equaling 60 nautical miles and 1 minute of latitude being 1 nautical mile. Latitude is constant on a parallel. One must distinguish between astronomical and geodetic (for example, World Geodetic System of 1984 [WGS 84]) latitude.Local Area DGPS (LADGPS). A form of DGPS in which the user's GPS receiver receives real-time pseudorange and, possibly, carrier- phase corrections from a reference receiver generally located within line of sight. The corrections account for the combined effects of navigation message ephemeris and satellite clock errors (including the effects of SA), and usually, atmospheric propagation delay errors at the reference station. With the assumption that these errors are also common to the measurements made by the user? receiver, the application of the corrections will result in more accurate coordinates.Line of Position (LOP). Locus of points have a constant measurement (such as range, range difference). A fix is determined by crossing two lines of position.Longitude. The distance east or west from the prime meridianas measured in degrees, with each degree becoming smaller the closer it comes to either of the poles. At the equator, 1 degree of longitude equals 60 nautical miles. Along a meridian, the longitude is constant. One must distinguish between astronomical and geodetic (for example, World Geodetic System of 1984 [WGS 84]) longitude.Loran-C. Long range navigation system that determines position by comparing the arrival times of radio signals with two or more master/secondary station pairs.

MMMMagnetic Deviation. The error in a magnetic compass reading caused by the difference between the true north pole and the magnetic north pole, also referred to as magnetic variation.Megahertz (MHz). A radio signal of one-million cycles.Microstrip Antenna. A type of antenna commonly used with GPS receivers. It is usually constructed of one or more (typically rectangular) elements that are photoetched on one side of double-coated, printed-circuit board. It is often called a patch antenna.Modulation. A method of encoding a message signal on top of a carrier, which can be decoded at a later time.Multipath. Signal arrival at a receiver? antenna by way of two or more different paths such as a direct, line-of-sight path and one that includes reflections off nearby objects. The difference in path lengths causes the signals to interfere at the antenna and can corrupt the receiver? pseudorange and carrier-phase measurements.Multiplexing. The technique used in some GPS receivers of rapidly sequencing the signals of two or more satellites through a tracking channel. This ensures navigation messages from the satellites tracked by the channel are essentially acquired simultaneously.

NNNNarrow Correlator. A correlator in a code-tracking loop in which the spacing between the early and late versions of the receiver-generated reference code is less than one chip. The use of narrow correlators results in pseudorange observations with lower noise.Narrow Lane. The GPS observable obtained by summing the carrier-phase observations simultaneously measured, in cycles, on the L1 and L2 frequencies. The effective wavelength of the narrow-lane observable is 10.7 centimeters. The narrow-lane observable can help resolve carrier-phase ambiguities.Navigation Message. A 37,500-bit data message included in the GPS signal. The message, sent at a rate of 50 bits per second, includes the satellite ephemeris, clock data, almanac, and other information about the satellites and their signals.Nautical Mile. The unit of distance used in most maritime navigation. It is 1.15 times longer than a statute mile, or 6076.1 feet.NMEA 0183. The committee of the National Marine Electronics Association that developed a standard for interfacing marine electronic devices. The standard is widely used for interfacing GPS receivers.Noise. An interfering signal that tends to mask the desired signalat the receiver output and which can be caused by space and atmospheric phenomena, can be human made, or can be caused by receiver circuitry.Notice Advisory to GLONASS Users (NAGU). A periodic bulletin alerting GLONASS users to changes in system performance.Notice Advisory to NAVSTAR Users (NANU). A periodic bulletin alerting GPS users to changes in system performance.

OOOOn-the-Fly (OTF). The term used to identify a technique that resolves differential carrier-phase integer ambiguities without requiring a GPS receiver to be stationary at any time.Orthometric Height. The height of a point above the geoid.Oscillator. A device that generates a signal of a given frequency.

PPPParity. An extra bit at the end of a string of bits. Used in error detection, this reveals whether the number of 1s is odd or even. Precision (P)-Code. A PRN code transmitted by GPS satellites. The code consists of about 2.35 3 1014 chips and is sent at a rate of 10.23 megabits per second. At this rate, it would take 266 days to transmit the complete code. Each satellite is assigned a unique one-week segment of the code that is reset at Saturday/Sunday midnight. The P-code is currently transmitted on both the L1 and L2 frequencies.Phase Angle. The time difference between the same point on two different waves, usually measured in fractions of a cycle (radians or degrees).Phase Differencing. The technique of using different GPS receivers at different locations to measure the phase angles of the carrier signal from the same satellite. These angles are compared by a communications link between the two locations if real-time operations are required.Phase-Lock Loop. Another term for carrier-tracking loop.Phase Modulation. Encoding information on a carrier signal by changing the phase so that some segments of the carrier are out of phase while others are in phase. With GPS, only two phase angles are used, 0 and 180, representing the two values, I or O.Precise Positioning Service (PPS). The full-accuracy, single-receiver GPS positioning service provided to the United States and its allied military organizations and other selected agencies. It includes access to the unencrypted P-code and the removal of SA effects.Pseudolite. A ground-based transmitter that mimics a GPS signal. These devices can be used around airports for such tasks as precision instrument landings.Pseudorandom Noise (PRN) Code. Deterministic binary sequences with noise-like properties. Also called pseudonoise codes. These codes are used in spread-spectrum communications systems and in ranging systems such as GPS. Two PRN codes are transmitted by GPS satellites: the C/A-code and P-code.Pseudorange. The range between the antenna phase centers of a GPS satellite and a receiver, measured by the receiver's delay-lock loop using either the C/A- or P-code. The range is biased by the offset of the clock in the receiver from that in the satellite and by atmospheric propagation delays.

Q,RQuadrifilar Helix. A type of circularly polarized antenna used with some GPS receivers. The antenna consists of two orthogonal, fractional-turn bifilar wire or metal strip helices fed in phase quadrature. It is also known as a volute antenna.Radio Direction Finder. A radio receiver that features a directional antenna and a visual null indicator for use in determining lines of position from radiobeacons at known positions.Ranging. A technique used to determine a line of position by calculating the distance between a receiver and a known reference point.Receiver Autonomous Integrity Monitoring (RAIM). A technique by which a GPS receiver system detects incorrect satellite signals by comparing solutions with different sets of satellites.Real-Time Kinematic (RTK). The DGPS procedure whereby carrier-phase corrections are transmitted in real time from a reference receiver to the user? receiver. RTK is often used for the carrier-phase integer ambiguity resolution approach.Reference Station. A ground station at a known location used to derive differential corrections. The reference station receiver tracks all satellites in view, computes their pseudoranges, corrects these for errors, and then transmits the corrections to users.Relative Accuracy. The accuracy with which a user can measure position relative to that of another user on the same navigation system at the same time.Reliability. The ability to perform a specific function without failure under specified conditions for a given length of time.RINEX. This is the Receiver-Independent Exchange format for GPS observations. Provision is made for pseudorange, carrier-phase, and Doppler observations.RTCM SC-104. The special committee of the Radio Technical Commission for Maritime Services that developed recommended standards for DGPS.

SSSSelective Availability (SA). The policy and procedure of denying to most nonmilitary GPS users the full accuracy of the system. SA is achieved by dithering the satellite clock (delta-process) and degrading the navigation message ephemeris (epsilon-process). Currently SA primarily uses the delta-process. The effects can be removed with encryption keys or through DGPS techniques.Signal to Noise Ratio (SNR). The ratio of incoming signal strength to the amount of interfering noise as measured in decibels on a logarithmic scale.Single Difference. A GPS observable formed by arithmetically differencing carrier phases that are simultaneously measured by a pair of receivers tracking the same satellite (between-receiver single difference), or by a single receiver tracking a pair of satellites (between-satellite single difference). The between-receiver?ingle-difference procedure essentially removes all satellite clock errors. The between-satellite?ingle-difference procedure essentially removes all receiver clock errors. Although primarily used with carrier phases, the procedure can also be applied to pseudoranges.Space Segment. That portion of the Global Positioning System that is in space: the GPS satellites.Spatial Decorrelation. The distance between the user and the reference station. When calculating differential corrections, the greater the distance between the two, the greater the error of corrections.Spherical Error Probable (SEP). A measure of accuracy in navigation. SEP is the radius of the sphere inside of which the true three-dimensional coordinates of a position have a 50-percent probability of being located.Spoofing. The deliberate transmission of fake signals to skew the position calculations of a GPS receiver. The spoofer mimics a GPS satellite, rather like a pseudolite, but with disruptive intent. Spread-Spectrum. A signal normally requiring a narrow transmission bandwidth but spread over a much larger bandwidth. For example, the 50-bits-per-second GPS navigation message, which could be transmitted in a bandwidth of 50 Hz or so, is spread over a bandwidth of about 1 MHz by the C/A-code. Standard Positioning Service (SPS). The GPS single receiver (stand-alone) positioning service available to any user on a continuous, worldwide basis. It is intended to provide access only to the C/A-code and the L1 carrier. The horizontal-position accuracy, as degraded by SA, currently is advertised as 100 meters, the vertical-position accuracy as 156 meters, and time accuracy as 334 nanoseconds ?all at the 95-percent probability level. SPS also guarantees the user-specified levels of coverage, availability, and reliability.Static Positioning. Location determination when the receiver's antenna is presumed to be stationary on the earth. This allows the use of various averaging techniques to greatly improve accuracy.Stop-and-Go Surveying. A precision DGPS surveying technique, also referred to as semikinematic, by which the rover is used to collect data a point for a few seconds or minutes before moving on.

TTTTemporal Decorrelation. The age or time lapse in corrections used in differential GPS. The longer the time lapse between the corrections, the less accurate they become.Transit. The satellite-based system that measured successive Doppler (frequency) shifts of signals transmitted from satellites in polar orbits to determine position.Triple Difference. The arithmetic difference of sequential, doubly differenced carrier-phase observations. The triple-difference observable is free of integer ambiguities. It is a useful observable for determining initial, ap- proximate coordinates of a site in relative GPS positioning and for detecting cycle slips in carrier-phase data.Tropospheric Delay. Retardation of GPS signals caused by elements in the troposphere such as temperature, air pressure, and water vapor.

U,VUTC (Coordinated Universal Time). The time scale based on the atomic second but occasionally corrected, by the insertion of leap seconds, to keep it approximately synchronized with Earth? rotation. The leap second adjustments keep UTC within 0.9 seconds of UT1.User Equivalent Range Error (UERE). Any error contributing to the error budget of stand-alone GPS receiver positioning, expressed as an equivalent error in the range between a user?s receiver and a satellite. Also known as user range error (URE). UERE errors originate from different sources and thus are independent of each other. The total UERE is the square root of the sum of the squares of the individual errors. A prediction of maximum anticipated total UERE (minus ionospheric error) is provided in each satellite?s navigation message as the user range accuracy (URA).User Segment. The part of the whole Global Positioning System that includes the receivers of GPS signals.UT1. A time scale based on Earth?s axial spin. UT1 is a measureof the true angular orientation of Earth in space. Because Earth does not spin at exactly a constant rate, UT1 is not a uniform time scale.

W,X,Y,ZWarm Start. The ability of a GPS receiver to begin navigating using almanac information stored in its memory from previous use.Waypoint. A destination. The coordinates of locations along the desired path as measured in geographic coordinates of longitude and latitude.Wide Area Augmentation System (WAAS). A system that enhances the GPS SPS and is available over a wide geographical area. The WAAS being developed by the Federal Aviation Administration, in concert with other agencies, will provide WADGPS corrections, additional ranging signals from geostationary (GEO) satellites, and integrity data on the GPS and GEO satellites.Wide Area DGPS (WADGPS). A form of DGPS in which the user?s GPS receiver receives corrections determined from a network of reference stations distributed over a wide geographical area. Separate corrections are usually determined for specific error sources ? such as satellite clock, ionospheric propagation delay, and ephemeris ? and are applied in the user?s receiver or attached computer in computing the receiver?s coordinates. The corrections are typically supplied in real time by way of a geostationary communications satellite or through a network of ground-based transmitters. Corrections may also be provided at a later date for postprocessing collected data.Wide-Lane Observable. The GPS observable obtained by differencing the carrier-phase observations simultaneously measured, in cycles, on the L1 and L2 frequencies. The effective wavelength is 86.2 centimeters. It can be useful in resolving carrier-phase ambiguities.World Geodetic System 1984 (WGS 84). A set of parameters, established by the U.S. Defense Mapping Agency, for determining geometric and physical geodetic relationships on a global scale. The system includes a geocentric reference ellipsoid; a coordinate system; and a gravity field model. The ellipsoid is essentially that of the International Union of Geodesy and Geophysics Geodetic Reference System 1980. The coordinate system is a realization of the conventional terrestrial system, as established by the International Earth Rotation Service. The descriptions of the GPS satellite orbits in the navigation message are referenced to WGS 84.Y-Code. The encrypted P-code.Z-Count. The fundamental GPS time unit. It is a 29-bit binary number, of which the 10 most significant bits give the binary representation of the GPS week number, and the 19 least significant bits give the time-of-week (TOW) count in units of 1.5 seconds. A truncated version of the TOW ? with 6-second epochs ? is included in the navigation message hand-over word.

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