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GPS GlossaryAlmanacSet of orbit parameters sent by GPS satellites that allows calculation of approximate GPS satellite positions and velocities. It is used by GPS receivers to determine satellite visibility during acquisition of signals. Most receivers provide a facility for battery backup or non-volatile storage of the almanac to make power-on acquisition faster. The almanac is less accurate then ephemeris data, but remains valid for a longer period (1-2) years.AttenuationA reduction in signal strength.Ascending NodeThe point at which an orbiting object passes the reference plane from south to north.Automatic Vehicle Location (AVL)The general term used to describe systems that locate or track vehicle locations.AzimuthHorizontal direction of a celestial point from a terrestrial point, expressed as the angular distance from 0 degrees?(reference) clockwise through 360 degrees. The reference point is generally True North, but may be Magnetic North, or relative to a fixed point on a vehicle.BandwidthThe width of the spectrum of a signal expressed in Hz.BaselineThe 3D vector distance between a pair of stations for which simultaneous GPS data has been collected and processed with differential techniques.Beat FrequencyEither of two additional frequencies generated when two frequencies are mixed. One beat frequency is equal to the sum of the frequencies, the other the difference of the frequencies.Binary Biphase ModulationData represented by phase changes of either 0 (binary zero) or 180 degrees (binary one) on a constant frequency carrier. GPS signals use this modulation technique.C/A (Coarse Acquisition) CodeSpread spectrum direct sequence code that is used primarily by commercial GPS receivers to determine the range to GPS satellite. The code is a sequence of 1023 pseudorandom binary biphase modulations at a chipping rate of 1.023 MHz.CarrierSteadily transmitted RF signal whose amplitude, frequency or phase (or a combination) may be modulated to carry information.Carrier Beat PhaseThe phase of the signal that remains after the incoming satellite carrier signal is mixed with the constant reference frequency in the receiver. This is used to determine Doppler shift of the satellite's signal.Carrier FrequencyThe frequency of the unmodulated fundamental output of an RF transmitter.ChipThe length of time required to transmit a binary pulse code. For the GPS C/A code this is approximately 977ns.Chip RateThe number of chips sent per second.Circular Error Probable (CEP)The radius of a circle, centered at the user's true location, that contains 50 percent of the individual position measurements made using a navigation system.Code Division Multiple Access (CDMA)A method of frequency sharing where many transmitters use the same frequency but each one has a unique code to generate pseudorandom sequences. GPS uses CDMA with Gold codes.Cold StartUsed to describe state where a GPS receiver is powered on and does not have valid almanac data available. This lengthens the time required to locate satellites and arrive at the first position solution.Control SegmentUsed to describe the Master Control Station and the globally dispersed Monitor Stations used to manage GPS satellites and synchronize their clocks.Cycle SlipAn error in the continuous reception of carrier phase cycles.Dead Reckoning (DR)Process of determining a vehicle's approximate position by calculating from its last known GPS position a vector representing the movement since that point. Typically this is calculated using a combination of bearing and speed measurements from devices such as compasses and odometers.Differential GPS (DGPS)A technique to improve GPS accuracy that uses errors measured at a known location to improve the measurements made by other GPS receivers within the same general geographic area. The calculated errors are normally transmitted to the GPS receiver in RTCM format.Dilution of Precision (DOP)A value expressing the confidence factor in the accuracy of the position solution based on current satellite geometry. The lower the value the greater the confidence in the solution. DOP can be expressed in the following forms:GDOP - All parameters are uncertain (latitude, longitude, height, clock offset). HDOP - 2D parameters are uncertain (latitude, longitude). HTDOP - 2D parameters and time are uncertain (latitude, longitude, time). PDOP - 3D parameters are uncertain (latitude, longitude, height). TDOP - Clock offset is uncertain. VDOP - Height is uncertain. DoDUnited States Department of Defense. The government agency that controlled the development and deployment of the GPS system.Doppler ShiftThe change in frequency of a wave (such as RF emissions) caused by movement of the source relative to the observer.Earth-Centered-Earth-Fixed (ECEF)Usually refers to a coordinate system centered on the WGS-84 reference ellipsoid, having the Z-axis aligned with the Earth's spin axis, the X-axis through the intersection of the Prime Meridian (Greenwich) with the Equator and the Y axis rotated 90 degrees East of the X axis about the Z-axis.EccentricityRatio of the distance from the center of an ellipse to its focus to the semi-major axis. e = (1 - sqr(b) / sqr(a)) - 0.5 where a and b and the semi-major and semi-minor axes.ElevationHeight above mean sea level or vertical distance above the reference geoid.Elevation Mask AngleThe elevation angle below which satellites are ignored in the position calculation. Typically set to around 5-10 degrees to avoid interference problems from obstructions and atmospheric errors.Ellipsoid HeightMeasure of vertical distance above the ellipsoid. GPS outputs a height (altitude) referenced to the WGS-84 datum.EphemerisSet of satellite orbital parameters used by a GPS receiver to calculate precise GPS satellite positions and velocities. The ephemeris is used in the determination of the navigation solution and is updated periodically by the satellite to maintain the accuracy of GPS receivers. Available as a broadcast ephemeris or as a post-processed precise ephemeris.EpochMeasurement interval or frequency at which data is measured. For example a GPS receiver reporting a position once every two seconds as an epoch of two seconds.FlatteningA parameter used to define the shape of an ellipsoid. The WGS-84 ellipsoid has a flattening of 1 / 298.257223563.Fundamental FrequencyThe fundamental frequency of GPS is 10.23MHz. The carrier L1 is 154F = 1575.42MHz and the L2 carrier is 120F = 1227.60MHz.GDOPSee Dilution of Precision (DOP).GeocenterThe center of the earth's mass.Geodetic DatumThe reference ellipsoid surface that defines a coordinate system. Picked to try and best fit part or all of the geoid.GeoidRepresentation of the earth's surface considered as a sea level surface extended continuously through the continents. The actual geoid is an equipotential surface coincident with mean sea level to which at every point the plumb line (direction of gravitational pull) is perpendicular.Geoid HeightHeight above the geoid, often referred to as elevation above mean sea level.GeostationarySatellite in orbit along the equator that results in a constant fixed position over a particular reference point on the earth's surface. GPS satellites are not geostationary.Global Positioning System (GPS)The NAVSTAR Global Positioning System consists of a space segment (24 orbiting satellites, four equally spaced around each of six different orbital planes), the control segment (five monitor stations, one master control station and three uplinks). The GPS receiver is referred to as the user segment. NAVSTAR satellites carry atomic clocks and broadcast coherent signals so that the position to each satellite may be determined by calculating Doppler shift. See also GLONASS.Global Navigation Satellite System (GLONASS)This is the Russian counterpart to the NAVSTAR system run by the United States. It also offers global positioning capability although performance is optimized for use in the northern hemisphere, where performance is comparable to that of NAVSTAR. Some GPS receivers use a combination of both NAVSTAR and GLONASS to provide enhanced capabilities.GPS ICD-200Specification that contains full technical details on how a GPS receiver must interface with the system.HDOPSee Dilution of Precision (DOP).HTDOPSee Dilution of Precision (DOP).InclinationThe angle between the orbital plane of a body and a reference plane.JPOJoint Program Office for GPS located at the United States Air Force Space Division in California. Contains members of a number of government agencies with an involvement in controlling the GPS system.Kinematic SurveyingMethod of surveying that involves starting from a known baseline and tracking a minimum of four satellites. One receiver remains static at a control point while other receivers are moved around to obtain measurements.L1 frequencyThe 1575.42MHz GPS carrier frequency that contains the coarse acquisition (C/A) code and the encrypted P-code for the precise positioning service. Also carries the NAVDATA message used by commercial GPS receivers.L2 frequencyThe 1227.60MHz GPS carrier frequency carries the encrypted P-code and the NAVDATA message.L BandRange of radio frequencies covering the band from 390MHz to 1550MHz.Mask AngleSee Elevation Mask Angle.Monitor StationStation to monitor and control operation of the GPS system.Multichannel ReceiverA receiver containing many independent channels that offers the best level of performance because each channels tracks one satellite continuously.Multipath ErrorsA positioning error caused by the interaction of the GPS signal and their reflections. Typically sources of reflections include trees, buildings and mountains.NAVDATAA 1500-bit navigation message broadcast at 50 bits per second on the L1 and L2 channels. The message contains time, clock correction, ephemeris, satellite health and ionospheric delay model parameters.NAVSTARName given to GPS satellites. The acronym stands for NAVigation Satellite Timing and Ranging.Original Equipment Manufacturer (OEM)Manufacturers of equipment designed for use by end consumers. OEM GPS modules are named as such because they require electronics knowledge to interface and therefore are used most commonly by OEMs.P-CodeThe protected or precise code sent on both the L1 and L2 channels. The code is very long pseudorandom sequence that does not repeat itself for about 38 weeks. Each satellite as a unique one-week segment of the code which is changed each week. This sequence forms part of the encryption scheme used to ensure only authorized users can gain access to the precise positioning service (PPS).PDOPSee Dilution of Precision (DOP).PerigeeThe closest approach of an orbiting body.Point PositioningPositions calculated by a single GPS receiver without reference to other receivers or known points. Position accuracy is typically 15 to 25 meters now that selective availability has been turned off.Precise Positioning Service (PPS)The highest level of GPS positioning accuracy provided only available to authorized users such as the military. It is based on the dual-frequency P-Code and has resistance to jamming and spoofing.PRNThe Pseudorandom Noise number is unique to each satellite and allows them to operate on the same frequency using CDMA. The number is often used to identify the particular satellites used to arrive at a position solution.PseudoliteAn Earth-based transmitter that sends signals similar to that of a GPS satellite. They are designed to improve accuracy of GPS over localized areas such as airports.PseudorangeThe calculated range from the GPS receiver to the satellite determined by taking the difference between the measured satellite transmit time and the time of reception and multiplying by the speed of light.RTCMRadio Technical Commission for Maritime Services. Body that defined the RTCM SC-104 standard for transmission of DGPS correction information to a GPS receiver.Selective Availability (SA)Errors introduced into the GPS signal by the Dod to limit the accuracy of GPS receivers available to civilians. Selective Availability has since been turned off increasing the accuracy of typically receivers from around 100 meters to around 15-25 meters.Semi-major AxisOne half of the major axis of an ellipse.Sequential ReceiverGPS receiver in which the number of satellite signals to be tracked exceeds the number of available receiver channels. This type of receiver must sequentially step between channels to track satellites.Sidereal DayTime between two upper transits of the vernal equinox. A sidereal day is approximately four minutes shorter than a solar day.Solay DayTime between two upper transits of the sun.Spherical Error Probable (SEP)Statistical measure defined as the 50th percentile of 3D position error statistics. This means half of the results are within the 3D SEP.Standard Positioning Service (SPS)Standard accuracy GPS service made available to civilian users.TDOPSee Dilution of Precision (DOP).TOWTime on week in seconds from midnight Saturday in UTC.Three-dimensional (3D) navigationMode in which at least four satellites are available so that both position and altitude may be determined.Time-To-First-Fix (TTFF)Time that a GPS receiver takes to acquire its first position after being powered on. This time will be shorter for a warm start where valid almanac data is still available.Two-dimensional (2D) navigationMode where not enough satellites are available for a 3D position calculation so the altitude is taken as being based on a fixed value.Universal Time Coordinated (UTC)Solar time in the Greenwich Mean Time (GMT) time zone but only ever adjusted in one-second increments. Because the GPS time is different to UTC a correction factor is applied to the received time that is reported by GPS receivers in UTC.VDOPSee Dilution of Precision (DOP).Velocity Made Good (VMG)The speed a mobile unit is closing in on a destination when travelling along the desired course.Warm StartUsed to describe state where a GPS receiver is powered on and still has valid almanac data available. This shortens the time required to locate satellites and arrive at the first position solution.Wide Area Augmentation System (WAAS)This system provides a satellite signal to augment GPS for precision aircraft flight and landing navigation.WGS-84World Geodetic System 1984 is an ellipsoid designed to fit the shape of the entire Earth as well as possible with a single ellipsoid. GPS uses this ellipsoid that is defined with a flattening of 1 / 298.257223563 and equatorial radius of 6378137.0. |