Accuracy: The degree of conformance between the measured position and its true position. GPS accuracy is usually given as a statistical measure of system error. Not to be confused with precision. Something can be measured very precisely, but still be inaccurate. Radio Navigation system accuracy can be characterized as:
Predictable Accuracy of a position solution with respect to the charted solution, where both are based on the same geodetic datum.
Repeatable The accuracy with which a user can return to a position whose coordinates have been measured at a previous time with the same system.
Relative The accuracy with which a user can measure a position relative to that of another user of the same system at the same time.
Ambiguity The uncertainty in the measurement of the number of wave cycles of the GPS carrier between the satellite and the receiver.
Argos A French-American system for positioning that uses a small transmitter which sends a signal from the surface to U.S. weather satellites, then back down to ground-based processing centers in France and the U.S. Doppler-shift distortion of the signal is used to determine position which is then transmitted back to the user
via telephone, microwave links or other conventional communication channels.
Automatic Vehicle Location (See IVHS) Any system using any sort of technology to locate or track a vehicle.
APRS (Automatic Packet Reporting System) is a real-time tactical digital communications protocol for exchanging information between a large number of stations covering a large (local) area. As a multi-user data network, it is quite different from conventional packet radio. APRS contains only four packet types, Position/objects, Status, Messages and Queries. The Position/object packets contain not only the latitude/longitude, course and speed, but also fields for power, antenna height and gain and voice operating frequency. APRS was developed since the late 80’s by Bob Bruninga, callsign WB4APR, currently a senior research engineer at the United States Naval Academy. The acronym “APRS” was derived from his callsign. In the 1990’s as GPS excitement dominated many new applications, the “P” was often referred to as “Position” instead of the original “Packet”. But this so skewed the public perception of APRS as only a GPS and Weather Position tracking system, that recently, the emphasis has returned to the broader “Packet” applications.
Base In GPS, refers to a GPS receiver stationed over a known, accurately surveyed point, used to apply differential corrections to Rover receivers.
Block I/II/IIR/IIF The four generations of GPS satellites. Block I’s were the prototypes first launched in 1978. Block II satellites are the fully operational production space vehicles. Block IIR are the Replacement, or Replenishment satellites. Block IIF will be the Follow-on satellites.
C/A-Code The coarse acquisition code (sometimes called the Civilian Acquisition Code) is a sequence of 1,023 pseudo-random binary bi-phase modulations of the GPS carrier wave at a Chipping rate
(See Chip) of 1.023 MHz, and a code repetition period of 1 millisecond (see Pseudo-Random Code).
Carrier Phase Positioning GPS measurements based on measuring the number of wavelengths of the Ll or L2 GPS carrier signal between a satellite and receiver.
Carrier Smoothing A signal processing method by which the carrier wave is used to increase the accuracy of C/A-Code positioning by smoothing the variability in individual positions in a sequence of positions.
Carrier Wave A simple radio sine wave characterized by frequency, amplitude, and wavelength, any of which can be modulated from a known reference value.
Carrier Frequency The frequency of an unmodulated output of a radio transmitter. The GPS Ll carrier frequency is 1575.42 MHz, and the GPS L2 carrier frequency is 1227.6 MHz.
CEP See Circular Error Probable.
Chip In GPS, the switch, or opportunity to switch, between a digital “0” or a “ 1” in a binary pulse code. Also more generally used to define most any type of electronic integrated circuit.
Chip Rate The number of chips per second. For example, the C/A GPS code has a chipping rate of 1.023 MHz.
Circular Error Probable (CEP) A statistical measure of GPS accuracy. In a circular normal distribution the radius of a circle containing 50% of the individual measurements being made, or the radius of a circle inside of which there is a 50% probability of being located.
Civilian Code See C/A-code
Clock Bias The difference between a clock’s indicated time and true universal time.
Clock Offset Difference in the time reading between two clocks
Coastal Confluence Zone (CCZ) Harbor entrance to 93 kilometers (50 nautical miles) offshore or the edge of the continental shelf (at the 100 fathom curve), whichever is greater.
Code-Correlation The method by which a satellite-receiver range is determined by comparing the time difference between the satellite’s and receiver’s copies of a unique code.
Code-Phase GPS GPS measurements based on measuring the time delay of the C/A-code between a group of satellites and a receiver.
Common-Use Systems Systems used by both military and civil sectors.
Control Segment The ground-based segment of the GPS triad, consisting of a worldwide network of GPS monitor stations, uplink antennas, and master control stations that ensure the accuracy of the satellites’ positions and their clocks.
Coverage The surface area or space volume covered by a radionavigation system in which the signals are adequate to permit the user to determine position to a specified level of accuracy. Such coverage is influenced by system geometry, signal power levels, receiver sensitivity, atmospheric noise, and other factors which affect signal availability.
Cycle Slip A discontinuity, or skip, in the counting of carrier sine waves between the satellites and receiver
Data Message See Nav-msg
Datum In geodesy, a fundamental measurement base fixed to the physical Earth to which a three-dimensional Cartesian coordinate system is associated which has a specified origin and orientation (usually based on an ellipsoid).
Delta-Range The difference in range between two different epochs.
DGPS Differential GPS (See Differential Correction)
Differential Correction A technique used to improve the accuracy of a radionavigation system by determining position error at a known location and applying that determined error-correction value to the data of a user of the same system at another, unknown position. This method can be applied to post-processed (see Post-Processing) and real-time data correction.
Dilution of Precision (DOP) A measure of the contribution of satellite geometry to the uncertainty in a position fix. Values for the various DOPs typically range from 1 to 10, with lower values representing
higher quality satellite geometry. Values of 1 to 3 are considered excellent; 4 to 5 good; 6 marginal; and over 6 poor to unacceptable. (See Geometric Dilution of Precision (GDOP), Position Dilution of Precision (PDOP), Horizontal Dilution of Precision (HDOP), Vertical Dilution of Precision (VDOP),
Time Dilution of Precision (TDOP).)
Distance Root Mean Square (DRMS) A statistical measure of GPS accuracy. The root mean square value of the distances from the true location of the position fixes in a collection of measurements. 1 DRMS is the radius of a circle that contains at least 68% of the fixes in a given collection at any one place. 2DRMS is the radius of a circle that contains at least 95% of the fixes in a given collection at any one place and is the most common GPS error term used. (See CEP, SEP.)
Dithering The deliberate introduction of clock timing error in the GPS signal in the implementation of selective availability.
DOP See Dilution of Precision.
Doppler Shift The predictable apparent change in frequency of a signal caused by the relative motion of the transmitter and receiver. As the transmitter and receiver move toward each other the signal is compressed and therefore the frequency becomes higher. As the transmitter and receiver move away from each other the signal is expanded, or “stretched” and the frequency becomes lower.
DRMS See Distance Root Mean Square.
Dual-Frequency Receiver A receiver capable of simultaneously receiving both the Ll and L2 frequencies transmitted by the GPS satellites. Such receivers have the advantage of being able to compensate for most ionospheric refraction.
Dynamic Positioning The determination of position while in motion, as in real-time mapping. This has come to mean by use of C/A-code, rather than with carrier-phase. (See Kinematic Positioning.)
Electromagnetic Spectrum The organization and categorization of electromagnetic waves according to their wavelength along a continuum.
Elevation Height above mean sea level. The vertical distance above the geoid. Not to be confused with ellipsoidal height.
Ellipse A two-dimensional curve where the sum of the distances from any point of the curve to two fixed points, called foci, is constant. Rotating an ellipse around its minor axis results in an ellipsoid of
revolution, or, simply, an ellipsoid.
Ellipsoid (See Geodesy) A mathematical figure formed by revolving an ellipse around its minor axis. Often used interchangeably with spheroid. Two quantities define a ellipsoid: The length of the semimajor axis (a) and the flattening (f=(a-b)/a, where (b) is the length of the semiminor axis). Frequently defined by the semi-major axis (a) and the eccentricity (e) where the eccentricity is defined by:
e = √( (a²-b²) / a² )
Ellipsoidal Height The measure of vertical distance above the ellipsoid. Not to be confused with height above sea level or the Geoid. Ellipsoidal height is the measurement used by the GPS and is based on the WGS 84. Usually given as HAE, or Height Above the Ellipsoid.
Ephemeris A list of accurate positions or locations of a celestial object as a function of time. In GPS, refers to the positions of the satellites as they move through their orbits. Available from the satellites as “broadcast ephemeris” (also called “predicted ephemeris”), or from other sources as “precise ephemeris” for post-processing. Precise ephemeris data are the actual, measured positions of thesatellites for a particular point in time, whereas the broadcast ephemeris data are predicted and are, therefore, somewhat less
accurate. Plural form: Ephemerides.
Epoch An instantaneous event of measurement that is sometimes, incorrectly, referred to as the interval between the measurements. For example, “ 10 second epochs” (incorrect), rather than “ 10 second
epoch intervals” (correct).
Fast Multiplexing In GPS, a single channel receiver that samples a number of satellite ranges in rapid sequence. Cheaper to manufacture than multi-channel receivers that are capable of receiving multiple
satellite signals simultaneously. Typical of low-end “consumer” or “sport” receivers.
Frequency In electromagnetic radiation, the number of sine waves passing a fixed point per unit of time.
Full Operational Capability (FOC) The level of GPS system capability achieved when a full complement of Block II satellites were placed in operational orbit. This occurred in July, 1995. (See Initial Operational Capability (IOC).)
Geocentric Relative to the Earth as a center, measured from the center of mass of the Earth.
Geodesy The science related to the determination of the size and shape of the Earth by such measurements as triangulation, leveling, and gravimetric observations (bahasa Indonesia Geodesi).
Geodetic Datum A mathematical model designed to best fit part or all of the geoid. Defined by an ellipsoid and the relationship between the ellipsoid and a point on the topographic surface established as the origin of a datum. (Lihay juga Geodesy.)
Geoid The equipotential (equal-gravity) surface that coincides with mean sea level that may be imagined to extend through the continents. This surface is everywhere perpendicular to the force of
gravity.
Geoidal Height The height above the geoid. Usually referred to as the elevation above mean sea level (MSL). (See Ellipsoidal Height.)
Geometric Dilution of Precision (GDOP) The collection of geometric factors that degrade the accuracy of position fixes derived from externally-referenced navigation systems. In GPS, related to the spatial orientation and distribution of the satellites above and around the user.
GPS The U.S. Department of Defense, satellite-based Global Positioning System comprising 24 satellites in 6 orbits spaced 60 degrees apart with 4 satellites each, at an altitude of 20,200 kilometers (10,900 nautical miles). GPS satellites transmit signals that permit the accurate determination of position anywhere on the Earth.
GPS Triad The three principal components of the Global Positioning System, consisting of the space segment, the control segment, and the user segment.
HAE Height Above the ellipsoid.
Hertz (Hz) Cycles per second.
Horizontal Dilution of Precision (HDOP) The Dilution of Precision in two dimensions horizontally. This value is often lower (i.e., “better”) than some other DOP values because it ignores the vertical dimension. (See Dilution of Precision.)
ILS Instrument Landing System. (See Precision Approach.)
Inclination One of the orbital parameters that specifies the elevation of an orbit relative to a reference plane. In the GPS, that plane is the Earth’s equator.
Initial Operational Capability (IOC) The level of GPS system capability achieved when a minimum of 24 GPS satellites (Block I and II) first became operational in their orbits and were available for navigation. This occurred in December, 1993. (See Full Operational Capability (FOC).)
Integrity The ability of a system to provide timely warnings to users when the system should not be used for navigation.
Intelligent Vehicle Highway Systems (IVHS) Also, IVS, or Intelligent Vehicle Systems. Any of a number of systems by which road vehicles can be accurately tracked. More and more this is coming to mean vehicle monitoring, tracking, and positioning by some method that includes the GPS as at least one of its components.
IVS, or Intelligent Vehicle System is more general in that it includes non-highway land transportation such as rail.
Ionosphere A band of electrically charged particles between 50 and 200 kilometers (25 to 120 miles) altitude (variable) which acts as a refractive medium for radio signals. (See Refraction.)
IVHS See Intelligent Vehicle Highway Systems
IVS Intelligent Vehicle Systems. (See Intelligent Vehicle Highway Systems.)
Kinematic Positioning The determination of position while in motion. This has come to mean by use of carrier-phase techniques, rather than C/A-code. (See Dynamic Positioning.)
L-Band The group of radio frequencies ranging from 1.2 GHz to 1.6 GHz. The GPS carrier frequencies (1575.42 MHz and 1227.6 MHz) are within the L-Band.
* Ll Signal (or Ll Frequency) The primary L-Band frequency used by GPS satellites centered at 1572.42 MHz. The Ll signal is modulated with the C/A-Code, P-Code, and Nav-msg data strings.
* L2 Signal (or L2 Frequency) The second L-Band frequency used by GPS satellites centered at 1227.6 MHz. The L2 signal carries the P-Code and Nav-msg data.
Loran A long-range, ground-based navigation system where position is determined by measuring the time intervals between pulsed radio signals from two or more pairs of ground stations at known locations.
Mask Angle The (usually) user-definable angle above the horizon below which satellites will not be tracked. Normally set to between 15 and 20 degrees to avoid interference caused by buildings, trees, multipath, and to minimize atmospheric refraction.
Master Control Station (MCS) One component of the ground-based control segment of the GPS triad. The main Master Control Station is located at Falcon Air Force Base in Colorado Springs, Colorado, with two back-up Master Control Stations located in Sunnyvale, California, and Rockville, Maryland. Unmanned monitor
stations passively track all GPS satellites visible to them at any given moment, collecting signal (ranging) data from each. This information is then passed on to the Master Control Station where the satellite position (ephemeris), and clock-timing data are estimated and predicted.
MHz Megahertz. Millions of cycles per second
Microsecond One-millionth of a second
Millisecond One-thousandth of a second
Monitor Stations One component of the ground-based control segment of the GPS triad used to track satellite clock and orbital parameters. Data collected at monitor stations are linked to the master control station where corrections are calculated and from which correction data are uploaded to the satellites as needed.
MSL Mean Sea Level. (See Geoidal Height.)
Multichannel Receiver A receiver capable of continuously receiving several signals from different satellites simultaneously, each on a different channel. This allows the derivation of position solutions from the simultaneous calculations of pseudo ranges.
Multipath Interference caused by reflected GPS signals arriving at a receiver as a result of nearby reflective surfaces such as buildings and bodies of water. Multipath signals have traveled longer distances and so produce erroneous pseudo-range estimates and consequent positioning errors.
Multiplexing See Fast-Multiplexing.
AND-83 North American Datum of 1983. (See Datum.)
Nanosecond (ns) One billionth of a second.
Nautical Mile (nm) 1.156 statute miles; 6,076 feet.; 1,852 meters
Navigation The process of planning, recording, and controlling the movement of a craft or vehicle from one place to another.
Nav-msg A modulation superimposed on both the C/A- and P-Codes with a data rate of 50 bits per second. The Nav-msg contains GPS system time of transmission, a hand-over word (HOW) for the transition from C/A- to P-Code tracking, ephemeris and clock data for the particular satellite being tracked, and almanac data for
all the satellite vehicles in the constellation. Additionally, it contains information such as satellite health, coefficients for ionospheric delay models for C/A-Code users, and coefficients to calculate Universal Coordinated Time (UTC).
Omega A ground-based radionavigation system that uses extremely low frequency radio waves. The system determines position by measuring phase differences between two or more transmitters. Coverage is global using only 8 transmitters. Accuracy is low; on the order of 3-6 kilometers (1.6-3.2 nautical miles).
P-Code The precise, protected, or precision code of the GPS signal, typically used without differential correction by U.S. military and other authorized users. It consists of a 267-day long sequence of pseudo-random binary biphase modulations of the GPS carrier waves with a chip rate of 10.23 MHz. Each satellite is randomly assigned a one-week segment of the code which is changed weekly. The P-Code becomes the Y-Code when encrypted, necessitating a special decryption key available only to authorized users.
PDOP See Position Dilution of Precision
Position Dilution of Precision (PDOP) The dilution of Precision in three dimensions. Sometimes called the Spherical Dilution of Precision. Probably the most commonly used DOP value. (See Dilution of Precision.)
Post-Processing A technique of differential correction where data from rover receivers are corrected for numerous sources of error by some form of computer processing with base receiver data some time after the data from all receivers have been collected.
PPS See Precise Positioning Service.
Precise Positioning Service (PPS) The highest level of military dynamic positioning provided by GPS using the dual-frequency PCode. Specified to provide 6.0 meters 1DRMS horizontal, or 16 meter SEP accuracy.
Precise Time A time requirement accurate to within 100 milliseconds.
Precision In GPS, the degree, or size of unit, to which a position can be measured. Not to-be confused with accuracy. It is possible to be very precise but inaccurate.
Precision Approach A standard aviation instrument approach procedure in which an electronic glide slope is provided, as in the Instrument Landing System (ILS). There are three main ILS categories:
* Category I (CAT I) Conditions must be such that visibility height above touchdown is 61 meters (200 feet) or greater, and runway visual range is 550 meters (1,800 feet) or greater. Instrument navigation positioning accuracy must be 17.1 meters (56 feet) horizontal, 4.1 meters (13. 5 feet) vertical.
* Category II (CAT II) Conditions must be such that visibility height above touchdown is 30.5 meters (100 feet) or greater, and runway visual range is 366 meters (1,200 feet) or greater. Instrument navigation positioning accuracy must be 5.2 meters (17 feet) horizontal, 1.7 meters (5.6 feet) vertical.
* Category III (CAT III) Conditions must be such that runway visual range is 2 13 meters (700 feet) or greater. There is no height visibility limitation. Instrument navigation positioning accuracy must be 4.1 meters (13.5 feet) horizontal, 0.6 meters (2 feet) vertical. (This is for CAT IIIA Instrument approachs. There are, in addition, CAT IIIB and CAT IIIC classifications that have runway visibility limitations of 46 meters (150 feet), and no limitations, respectively. Instrument position specifications remain the same.)
PRN Pseudo-Random Noise. (See Pseudo-Random Code.)
Pseudolite Pseudo-Satellite. A ground-based transmitter that simulates a GPS satellite that can be used for ranging. The NAV-msg portion of the signal may contain differential correction data. Because the transmitter is ground-based, it can significantly improve the DOP values for receivers located above it, such as aircraft.
Pseudo-Random Code Also called Pseudo-Random Noise (PRN). A signal so complexly modulated with l’s and O’s that it appears to be random noise, even though it is not.
Pseudo-Random Noise (PRN) See Pseudo-Random Code Pseudorange A range (distance) measurement based on the correlation of a satellite-transmitted code and a receiver’s internally generated code that has not been corrected for clock synchronization errors between the satellite and receiver’s clocks.
Radionavigation The determination of position, or the obtaining of information relating to position, for the purposes of navigation by means of the propagation properties of radio waves.
Real-Time Differential Correction A method by which a differential correction signal can be instantaneously applied in real-time to a rover receiver. (See RTCM.)
Refraction In GPS, the bending, or slight redirection, of the incoming signal satellite signal. Most commonly due to the ionosphere and the troposphere.
Relative Positioning See Differential Correction.
RINEX (Receiver Independent Exchange) A data format based on a set of standard definitions for GPS time, phase, and range observables. GPS manufacturers use their own proprietary formats for GPS data collected with their equipment. Consequently, one system cannot read the data of another. RINEX permits appropriate software to process GPS data even if such data are collected using receivers of differing manufacturers.
Rover In GPS, those receivers used to determine the positions of unknown points. Data from rover receivers are generally differentially corrected with data collected by base receivers.
RTCM (also RTCM-SC104) A common language for the real-time differential signal transmission developed by the Radio Technical Commission for Maritime Services Special Committee 104 (currently Version 2.1).
SA See Selective Availability
Satellite Constellation In GPS, the collection of orbiting satellites, located in 6 orbital planes elevated 55 degrees to the equator at an altitude of 20,200 kilometers (10,900 nautical miles), with three or four satellites per orbit for a total of 24 (21 functioning, 3 operational backups).
Selective Availability (SA) A Department of Defense method by which the accuracy of the C/A-Code is degraded to 100 meters 2DRMS by dithering the satellite clocks and ephemeris data sent on the Nav-msg.
SEP See Spherical Error Probable.
Sigma See Standard Deviation Space Segment The space segment of the GPS triad, consisting of the constellation of orbiting GPS satellites (See Satellite Constellation), and any ancillary spacecraft that provide GPS augmentation information such as differential correction.
Spherical Error Probable (SEP) A statistical measure of GPS accuracy. In a spherical normal distribution the radius of a sphere containing 50% of the individual measurements being made, or the radius of a sphere inside of which there is a 50% probability of being located. (See DRMS, CEP.)
Spheroid See Ellipsoid.
SPS See Standard Positioning Service.
Standard Deviation (Sigma) The statistical measure of the dispersion of random errors about the mean. If a large number of measurements or observations are made, the standard deviation is the square root of the sum of the squares of deviations from the mean value divided by the number of observations minus one.
Standard Positioning Service (SPS) The civilian positioning service level of accuracy achieved with a single frequency receiver using the C/A-code. With selective availability implemented, that accuracy is specified to be no worse than 100 meters 95% of the time (2DRMS).
Static Positioning In GPS, location determination using a stationary receiver, allowing the use of multiple-position averaging over time. The term has come to mean carrier phase positioning exclusively.
Survey The science and technology of making measurements to determine the relative position of points on, above, or beneath the Earth’s surface.
Surveying The branch of applied mathematics used in the art of accurately determining the area of any part of the Earth’s surface, the lengths and directions of the bounding lines, or the contour of the surface, and accurately delineating the whole on a map or chart for a specified datum. In modern time, it has come to be synonymous with legal certification, denoting that work performed by a legally certified surveyor is legally defensible in a court of law.
SV Space Vehicle, or Satellite Vehicle.
Time Dilution of Precision (TDOP) Dilution of precision with respect to time (clock offset).
Troposphere The lowest layer of the atmosphere. The zone where weather phenomena and atmospheric turbulence are most marked. Contains 75% of the total molecular or gaseous mass of the atmosphere and virtually all of the water vapor. Extends from the surface to the floor of the tropopause at between 8 and 16 kilometers (5 to 10 miles) altitude.
Universal Time Coordinated (UTC) Previously GMT. An international, highly accurate and stable uniform atomic time scale. The basis for civil time. It is occasionally adjusted by one-second increments to ensure that the difference between the uniform time scale, defined by atomic clocks, does not differ from the Earth’s rotation by more than 0.9 seconds. Maintained by the U.S. Naval Observatory.
Universal Transverse Mercator (UTM) Grid A military grid system based on the Transverse Mercator projection applied to maps of the Earth’s surface extending to 84 degrees North and South latitudes.
User Segment One segment in the GPS triad. Includes all end-user receivers that determine position and velocity with the GPS.
Vehicle Location Monitoring A service provided to maintain the orderly and safe movement of platforms or vehicles, encompassing the systematic observation of airspace, surface, and subsurface areas by electronic, visual or other means to locate, identify, and control the movement of vehicles. (See IVHS.)
Vertical Dilution of Precision (VDOP) The dilution of precision in the vertical dimension alone. Wavelength In electromagnetic radiation, the distance between two equivalent points on two successive sine waves.
World Geodetic System (WGS) A consistent set of parameters describing the size and shape of the Earth and the positions of a network of points with respect to the center of mass of the Earth. The most current is the WGS-84.
Y-Code The encrypted version of the P-Code.
From: Understanding GPS
Ambiguity The uncertainty in the measurement of the number of wave cycles of the GPS carrier between the satellite and the receiver.
Argos A French-American system for positioning that uses a small transmitter which sends a signal from the surface to U.S. weather satellites, then back down to ground-based processing centers in France and the U.S. Doppler-shift distortion of the signal is used to determine position which is then transmitted back to the user
via telephone, microwave links or other conventional communication channels.
Automatic Vehicle Location (See IVHS) Any system using any sort of technology to locate or track a vehicle.
APRS (Automatic Packet Reporting System) is a real-time tactical digital communications protocol for exchanging information between a large number of stations covering a large (local) area. As a multi-user data network, it is quite different from conventional packet radio. APRS contains only four packet types, Position/objects, Status, Messages and Queries. The Position/object packets contain not only the latitude/longitude, course and speed, but also fields for power, antenna height and gain and voice operating frequency. APRS was developed since the late 80’s by Bob Bruninga, callsign WB4APR, currently a senior research engineer at the United States Naval Academy. The acronym “APRS” was derived from his callsign. In the 1990’s as GPS excitement dominated many new applications, the “P” was often referred to as “Position” instead of the original “Packet”. But this so skewed the public perception of APRS as only a GPS and Weather Position tracking system, that recently, the emphasis has returned to the broader “Packet” applications.
Base In GPS, refers to a GPS receiver stationed over a known, accurately surveyed point, used to apply differential corrections to Rover receivers.
Block I/II/IIR/IIF The four generations of GPS satellites. Block I’s were the prototypes first launched in 1978. Block II satellites are the fully operational production space vehicles. Block IIR are the Replacement, or Replenishment satellites. Block IIF will be the Follow-on satellites.
C/A-Code The coarse acquisition code (sometimes called the Civilian Acquisition Code) is a sequence of 1,023 pseudo-random binary bi-phase modulations of the GPS carrier wave at a Chipping rate
(See Chip) of 1.023 MHz, and a code repetition period of 1 millisecond (see Pseudo-Random Code).
Carrier Phase Positioning GPS measurements based on measuring the number of wavelengths of the Ll or L2 GPS carrier signal between a satellite and receiver.
Carrier Smoothing A signal processing method by which the carrier wave is used to increase the accuracy of C/A-Code positioning by smoothing the variability in individual positions in a sequence of positions.
Carrier Wave A simple radio sine wave characterized by frequency, amplitude, and wavelength, any of which can be modulated from a known reference value.
Carrier Frequency The frequency of an unmodulated output of a radio transmitter. The GPS Ll carrier frequency is 1575.42 MHz, and the GPS L2 carrier frequency is 1227.6 MHz.
CEP See Circular Error Probable.
Chip In GPS, the switch, or opportunity to switch, between a digital “0” or a “ 1” in a binary pulse code. Also more generally used to define most any type of electronic integrated circuit.
Chip Rate The number of chips per second. For example, the C/A GPS code has a chipping rate of 1.023 MHz.
Circular Error Probable (CEP) A statistical measure of GPS accuracy. In a circular normal distribution the radius of a circle containing 50% of the individual measurements being made, or the radius of a circle inside of which there is a 50% probability of being located.
Civilian Code See C/A-code
Clock Bias The difference between a clock’s indicated time and true universal time.
Clock Offset Difference in the time reading between two clocks
Coastal Confluence Zone (CCZ) Harbor entrance to 93 kilometers (50 nautical miles) offshore or the edge of the continental shelf (at the 100 fathom curve), whichever is greater.
Code-Correlation The method by which a satellite-receiver range is determined by comparing the time difference between the satellite’s and receiver’s copies of a unique code.
Code-Phase GPS GPS measurements based on measuring the time delay of the C/A-code between a group of satellites and a receiver.
Common-Use Systems Systems used by both military and civil sectors.
Control Segment The ground-based segment of the GPS triad, consisting of a worldwide network of GPS monitor stations, uplink antennas, and master control stations that ensure the accuracy of the satellites’ positions and their clocks.
Coverage The surface area or space volume covered by a radionavigation system in which the signals are adequate to permit the user to determine position to a specified level of accuracy. Such coverage is influenced by system geometry, signal power levels, receiver sensitivity, atmospheric noise, and other factors which affect signal availability.
Cycle Slip A discontinuity, or skip, in the counting of carrier sine waves between the satellites and receiver
Data Message See Nav-msg
Datum In geodesy, a fundamental measurement base fixed to the physical Earth to which a three-dimensional Cartesian coordinate system is associated which has a specified origin and orientation (usually based on an ellipsoid).
Delta-Range The difference in range between two different epochs.
DGPS Differential GPS (See Differential Correction)
Differential Correction A technique used to improve the accuracy of a radionavigation system by determining position error at a known location and applying that determined error-correction value to the data of a user of the same system at another, unknown position. This method can be applied to post-processed (see Post-Processing) and real-time data correction.
Dilution of Precision (DOP) A measure of the contribution of satellite geometry to the uncertainty in a position fix. Values for the various DOPs typically range from 1 to 10, with lower values representing
higher quality satellite geometry. Values of 1 to 3 are considered excellent; 4 to 5 good; 6 marginal; and over 6 poor to unacceptable. (See Geometric Dilution of Precision (GDOP), Position Dilution of Precision (PDOP), Horizontal Dilution of Precision (HDOP), Vertical Dilution of Precision (VDOP),
Time Dilution of Precision (TDOP).)
Distance Root Mean Square (DRMS) A statistical measure of GPS accuracy. The root mean square value of the distances from the true location of the position fixes in a collection of measurements. 1 DRMS is the radius of a circle that contains at least 68% of the fixes in a given collection at any one place. 2DRMS is the radius of a circle that contains at least 95% of the fixes in a given collection at any one place and is the most common GPS error term used. (See CEP, SEP.)
Dithering The deliberate introduction of clock timing error in the GPS signal in the implementation of selective availability.
DOP See Dilution of Precision.
Doppler Shift The predictable apparent change in frequency of a signal caused by the relative motion of the transmitter and receiver. As the transmitter and receiver move toward each other the signal is compressed and therefore the frequency becomes higher. As the transmitter and receiver move away from each other the signal is expanded, or “stretched” and the frequency becomes lower.
DRMS See Distance Root Mean Square.
Dual-Frequency Receiver A receiver capable of simultaneously receiving both the Ll and L2 frequencies transmitted by the GPS satellites. Such receivers have the advantage of being able to compensate for most ionospheric refraction.
Dynamic Positioning The determination of position while in motion, as in real-time mapping. This has come to mean by use of C/A-code, rather than with carrier-phase. (See Kinematic Positioning.)
Electromagnetic Spectrum The organization and categorization of electromagnetic waves according to their wavelength along a continuum.
Elevation Height above mean sea level. The vertical distance above the geoid. Not to be confused with ellipsoidal height.
Ellipse A two-dimensional curve where the sum of the distances from any point of the curve to two fixed points, called foci, is constant. Rotating an ellipse around its minor axis results in an ellipsoid of
revolution, or, simply, an ellipsoid.
Ellipsoid (See Geodesy) A mathematical figure formed by revolving an ellipse around its minor axis. Often used interchangeably with spheroid. Two quantities define a ellipsoid: The length of the semimajor axis (a) and the flattening (f=(a-b)/a, where (b) is the length of the semiminor axis). Frequently defined by the semi-major axis (a) and the eccentricity (e) where the eccentricity is defined by:
e = √( (a²-b²) / a² )
Ellipsoidal Height The measure of vertical distance above the ellipsoid. Not to be confused with height above sea level or the Geoid. Ellipsoidal height is the measurement used by the GPS and is based on the WGS 84. Usually given as HAE, or Height Above the Ellipsoid.
Ephemeris A list of accurate positions or locations of a celestial object as a function of time. In GPS, refers to the positions of the satellites as they move through their orbits. Available from the satellites as “broadcast ephemeris” (also called “predicted ephemeris”), or from other sources as “precise ephemeris” for post-processing. Precise ephemeris data are the actual, measured positions of thesatellites for a particular point in time, whereas the broadcast ephemeris data are predicted and are, therefore, somewhat less
accurate. Plural form: Ephemerides.
Epoch An instantaneous event of measurement that is sometimes, incorrectly, referred to as the interval between the measurements. For example, “ 10 second epochs” (incorrect), rather than “ 10 second
epoch intervals” (correct).
Fast Multiplexing In GPS, a single channel receiver that samples a number of satellite ranges in rapid sequence. Cheaper to manufacture than multi-channel receivers that are capable of receiving multiple
satellite signals simultaneously. Typical of low-end “consumer” or “sport” receivers.
Frequency In electromagnetic radiation, the number of sine waves passing a fixed point per unit of time.
Full Operational Capability (FOC) The level of GPS system capability achieved when a full complement of Block II satellites were placed in operational orbit. This occurred in July, 1995. (See Initial Operational Capability (IOC).)
Geocentric Relative to the Earth as a center, measured from the center of mass of the Earth.
Geodesy The science related to the determination of the size and shape of the Earth by such measurements as triangulation, leveling, and gravimetric observations (bahasa Indonesia Geodesi).
Geodetic Datum A mathematical model designed to best fit part or all of the geoid. Defined by an ellipsoid and the relationship between the ellipsoid and a point on the topographic surface established as the origin of a datum. (Lihay juga Geodesy.)
Geoid The equipotential (equal-gravity) surface that coincides with mean sea level that may be imagined to extend through the continents. This surface is everywhere perpendicular to the force of
gravity.
Geoidal Height The height above the geoid. Usually referred to as the elevation above mean sea level (MSL). (See Ellipsoidal Height.)
Geometric Dilution of Precision (GDOP) The collection of geometric factors that degrade the accuracy of position fixes derived from externally-referenced navigation systems. In GPS, related to the spatial orientation and distribution of the satellites above and around the user.
GPS The U.S. Department of Defense, satellite-based Global Positioning System comprising 24 satellites in 6 orbits spaced 60 degrees apart with 4 satellites each, at an altitude of 20,200 kilometers (10,900 nautical miles). GPS satellites transmit signals that permit the accurate determination of position anywhere on the Earth.
GPS Triad The three principal components of the Global Positioning System, consisting of the space segment, the control segment, and the user segment.
HAE Height Above the ellipsoid.
Hertz (Hz) Cycles per second.
Horizontal Dilution of Precision (HDOP) The Dilution of Precision in two dimensions horizontally. This value is often lower (i.e., “better”) than some other DOP values because it ignores the vertical dimension. (See Dilution of Precision.)
ILS Instrument Landing System. (See Precision Approach.)
Inclination One of the orbital parameters that specifies the elevation of an orbit relative to a reference plane. In the GPS, that plane is the Earth’s equator.
Initial Operational Capability (IOC) The level of GPS system capability achieved when a minimum of 24 GPS satellites (Block I and II) first became operational in their orbits and were available for navigation. This occurred in December, 1993. (See Full Operational Capability (FOC).)
Integrity The ability of a system to provide timely warnings to users when the system should not be used for navigation.
Intelligent Vehicle Highway Systems (IVHS) Also, IVS, or Intelligent Vehicle Systems. Any of a number of systems by which road vehicles can be accurately tracked. More and more this is coming to mean vehicle monitoring, tracking, and positioning by some method that includes the GPS as at least one of its components.
IVS, or Intelligent Vehicle System is more general in that it includes non-highway land transportation such as rail.
Ionosphere A band of electrically charged particles between 50 and 200 kilometers (25 to 120 miles) altitude (variable) which acts as a refractive medium for radio signals. (See Refraction.)
IVHS See Intelligent Vehicle Highway Systems
IVS Intelligent Vehicle Systems. (See Intelligent Vehicle Highway Systems.)
Kinematic Positioning The determination of position while in motion. This has come to mean by use of carrier-phase techniques, rather than C/A-code. (See Dynamic Positioning.)
L-Band The group of radio frequencies ranging from 1.2 GHz to 1.6 GHz. The GPS carrier frequencies (1575.42 MHz and 1227.6 MHz) are within the L-Band.
* Ll Signal (or Ll Frequency) The primary L-Band frequency used by GPS satellites centered at 1572.42 MHz. The Ll signal is modulated with the C/A-Code, P-Code, and Nav-msg data strings.
* L2 Signal (or L2 Frequency) The second L-Band frequency used by GPS satellites centered at 1227.6 MHz. The L2 signal carries the P-Code and Nav-msg data.
Loran A long-range, ground-based navigation system where position is determined by measuring the time intervals between pulsed radio signals from two or more pairs of ground stations at known locations.
Mask Angle The (usually) user-definable angle above the horizon below which satellites will not be tracked. Normally set to between 15 and 20 degrees to avoid interference caused by buildings, trees, multipath, and to minimize atmospheric refraction.
Master Control Station (MCS) One component of the ground-based control segment of the GPS triad. The main Master Control Station is located at Falcon Air Force Base in Colorado Springs, Colorado, with two back-up Master Control Stations located in Sunnyvale, California, and Rockville, Maryland. Unmanned monitor
stations passively track all GPS satellites visible to them at any given moment, collecting signal (ranging) data from each. This information is then passed on to the Master Control Station where the satellite position (ephemeris), and clock-timing data are estimated and predicted.
MHz Megahertz. Millions of cycles per second
Microsecond One-millionth of a second
Millisecond One-thousandth of a second
Monitor Stations One component of the ground-based control segment of the GPS triad used to track satellite clock and orbital parameters. Data collected at monitor stations are linked to the master control station where corrections are calculated and from which correction data are uploaded to the satellites as needed.
MSL Mean Sea Level. (See Geoidal Height.)
Multichannel Receiver A receiver capable of continuously receiving several signals from different satellites simultaneously, each on a different channel. This allows the derivation of position solutions from the simultaneous calculations of pseudo ranges.
Multipath Interference caused by reflected GPS signals arriving at a receiver as a result of nearby reflective surfaces such as buildings and bodies of water. Multipath signals have traveled longer distances and so produce erroneous pseudo-range estimates and consequent positioning errors.
Multiplexing See Fast-Multiplexing.
AND-83 North American Datum of 1983. (See Datum.)
Nanosecond (ns) One billionth of a second.
Nautical Mile (nm) 1.156 statute miles; 6,076 feet.; 1,852 meters
Navigation The process of planning, recording, and controlling the movement of a craft or vehicle from one place to another.
Nav-msg A modulation superimposed on both the C/A- and P-Codes with a data rate of 50 bits per second. The Nav-msg contains GPS system time of transmission, a hand-over word (HOW) for the transition from C/A- to P-Code tracking, ephemeris and clock data for the particular satellite being tracked, and almanac data for
all the satellite vehicles in the constellation. Additionally, it contains information such as satellite health, coefficients for ionospheric delay models for C/A-Code users, and coefficients to calculate Universal Coordinated Time (UTC).
Omega A ground-based radionavigation system that uses extremely low frequency radio waves. The system determines position by measuring phase differences between two or more transmitters. Coverage is global using only 8 transmitters. Accuracy is low; on the order of 3-6 kilometers (1.6-3.2 nautical miles).
P-Code The precise, protected, or precision code of the GPS signal, typically used without differential correction by U.S. military and other authorized users. It consists of a 267-day long sequence of pseudo-random binary biphase modulations of the GPS carrier waves with a chip rate of 10.23 MHz. Each satellite is randomly assigned a one-week segment of the code which is changed weekly. The P-Code becomes the Y-Code when encrypted, necessitating a special decryption key available only to authorized users.
PDOP See Position Dilution of Precision
Position Dilution of Precision (PDOP) The dilution of Precision in three dimensions. Sometimes called the Spherical Dilution of Precision. Probably the most commonly used DOP value. (See Dilution of Precision.)
Post-Processing A technique of differential correction where data from rover receivers are corrected for numerous sources of error by some form of computer processing with base receiver data some time after the data from all receivers have been collected.
PPS See Precise Positioning Service.
Precise Positioning Service (PPS) The highest level of military dynamic positioning provided by GPS using the dual-frequency PCode. Specified to provide 6.0 meters 1DRMS horizontal, or 16 meter SEP accuracy.
Precise Time A time requirement accurate to within 100 milliseconds.
Precision In GPS, the degree, or size of unit, to which a position can be measured. Not to-be confused with accuracy. It is possible to be very precise but inaccurate.
Precision Approach A standard aviation instrument approach procedure in which an electronic glide slope is provided, as in the Instrument Landing System (ILS). There are three main ILS categories:
* Category I (CAT I) Conditions must be such that visibility height above touchdown is 61 meters (200 feet) or greater, and runway visual range is 550 meters (1,800 feet) or greater. Instrument navigation positioning accuracy must be 17.1 meters (56 feet) horizontal, 4.1 meters (13. 5 feet) vertical.
* Category II (CAT II) Conditions must be such that visibility height above touchdown is 30.5 meters (100 feet) or greater, and runway visual range is 366 meters (1,200 feet) or greater. Instrument navigation positioning accuracy must be 5.2 meters (17 feet) horizontal, 1.7 meters (5.6 feet) vertical.
* Category III (CAT III) Conditions must be such that runway visual range is 2 13 meters (700 feet) or greater. There is no height visibility limitation. Instrument navigation positioning accuracy must be 4.1 meters (13.5 feet) horizontal, 0.6 meters (2 feet) vertical. (This is for CAT IIIA Instrument approachs. There are, in addition, CAT IIIB and CAT IIIC classifications that have runway visibility limitations of 46 meters (150 feet), and no limitations, respectively. Instrument position specifications remain the same.)
PRN Pseudo-Random Noise. (See Pseudo-Random Code.)
Pseudolite Pseudo-Satellite. A ground-based transmitter that simulates a GPS satellite that can be used for ranging. The NAV-msg portion of the signal may contain differential correction data. Because the transmitter is ground-based, it can significantly improve the DOP values for receivers located above it, such as aircraft.
Pseudo-Random Code Also called Pseudo-Random Noise (PRN). A signal so complexly modulated with l’s and O’s that it appears to be random noise, even though it is not.
Pseudo-Random Noise (PRN) See Pseudo-Random Code Pseudorange A range (distance) measurement based on the correlation of a satellite-transmitted code and a receiver’s internally generated code that has not been corrected for clock synchronization errors between the satellite and receiver’s clocks.
Radionavigation The determination of position, or the obtaining of information relating to position, for the purposes of navigation by means of the propagation properties of radio waves.
Real-Time Differential Correction A method by which a differential correction signal can be instantaneously applied in real-time to a rover receiver. (See RTCM.)
Refraction In GPS, the bending, or slight redirection, of the incoming signal satellite signal. Most commonly due to the ionosphere and the troposphere.
Relative Positioning See Differential Correction.
RINEX (Receiver Independent Exchange) A data format based on a set of standard definitions for GPS time, phase, and range observables. GPS manufacturers use their own proprietary formats for GPS data collected with their equipment. Consequently, one system cannot read the data of another. RINEX permits appropriate software to process GPS data even if such data are collected using receivers of differing manufacturers.
Rover In GPS, those receivers used to determine the positions of unknown points. Data from rover receivers are generally differentially corrected with data collected by base receivers.
RTCM (also RTCM-SC104) A common language for the real-time differential signal transmission developed by the Radio Technical Commission for Maritime Services Special Committee 104 (currently Version 2.1).
SA See Selective Availability
Satellite Constellation In GPS, the collection of orbiting satellites, located in 6 orbital planes elevated 55 degrees to the equator at an altitude of 20,200 kilometers (10,900 nautical miles), with three or four satellites per orbit for a total of 24 (21 functioning, 3 operational backups).
Selective Availability (SA) A Department of Defense method by which the accuracy of the C/A-Code is degraded to 100 meters 2DRMS by dithering the satellite clocks and ephemeris data sent on the Nav-msg.
SEP See Spherical Error Probable.
Sigma See Standard Deviation Space Segment The space segment of the GPS triad, consisting of the constellation of orbiting GPS satellites (See Satellite Constellation), and any ancillary spacecraft that provide GPS augmentation information such as differential correction.
Spherical Error Probable (SEP) A statistical measure of GPS accuracy. In a spherical normal distribution the radius of a sphere containing 50% of the individual measurements being made, or the radius of a sphere inside of which there is a 50% probability of being located. (See DRMS, CEP.)
Spheroid See Ellipsoid.
SPS See Standard Positioning Service.
Standard Deviation (Sigma) The statistical measure of the dispersion of random errors about the mean. If a large number of measurements or observations are made, the standard deviation is the square root of the sum of the squares of deviations from the mean value divided by the number of observations minus one.
Standard Positioning Service (SPS) The civilian positioning service level of accuracy achieved with a single frequency receiver using the C/A-code. With selective availability implemented, that accuracy is specified to be no worse than 100 meters 95% of the time (2DRMS).
Static Positioning In GPS, location determination using a stationary receiver, allowing the use of multiple-position averaging over time. The term has come to mean carrier phase positioning exclusively.
Survey The science and technology of making measurements to determine the relative position of points on, above, or beneath the Earth’s surface.
Surveying The branch of applied mathematics used in the art of accurately determining the area of any part of the Earth’s surface, the lengths and directions of the bounding lines, or the contour of the surface, and accurately delineating the whole on a map or chart for a specified datum. In modern time, it has come to be synonymous with legal certification, denoting that work performed by a legally certified surveyor is legally defensible in a court of law.
SV Space Vehicle, or Satellite Vehicle.
Time Dilution of Precision (TDOP) Dilution of precision with respect to time (clock offset).
Troposphere The lowest layer of the atmosphere. The zone where weather phenomena and atmospheric turbulence are most marked. Contains 75% of the total molecular or gaseous mass of the atmosphere and virtually all of the water vapor. Extends from the surface to the floor of the tropopause at between 8 and 16 kilometers (5 to 10 miles) altitude.
Universal Time Coordinated (UTC) Previously GMT. An international, highly accurate and stable uniform atomic time scale. The basis for civil time. It is occasionally adjusted by one-second increments to ensure that the difference between the uniform time scale, defined by atomic clocks, does not differ from the Earth’s rotation by more than 0.9 seconds. Maintained by the U.S. Naval Observatory.
Universal Transverse Mercator (UTM) Grid A military grid system based on the Transverse Mercator projection applied to maps of the Earth’s surface extending to 84 degrees North and South latitudes.
User Segment One segment in the GPS triad. Includes all end-user receivers that determine position and velocity with the GPS.
Vehicle Location Monitoring A service provided to maintain the orderly and safe movement of platforms or vehicles, encompassing the systematic observation of airspace, surface, and subsurface areas by electronic, visual or other means to locate, identify, and control the movement of vehicles. (See IVHS.)
Vertical Dilution of Precision (VDOP) The dilution of precision in the vertical dimension alone. Wavelength In electromagnetic radiation, the distance between two equivalent points on two successive sine waves.
World Geodetic System (WGS) A consistent set of parameters describing the size and shape of the Earth and the positions of a network of points with respect to the center of mass of the Earth. The most current is the WGS-84.
Y-Code The encrypted version of the P-Code.
From: Understanding GPS
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