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The mode of operation in which a GPS receiver uses an assumed altitude (either last known good or supplied externally) plus three Pseudorange measurements to compute a position. Contrast with 3D Operating Mode
The mode of operation in which a GPS receiver computes a position using at least four Pseudorange measurements. Contrast with 2D Operating Mode
A measure indicating how closely the reported GPS position matches the true position.
The minimum signal power level at which a GPS receiver can autonomously acquire satellites and calculate a navigation solution. The limiting factor is generally the receiver's ability to decode the Navigation Message from the satellite signal. Even if the receiver can track a signal, it cannot form a position without ephemeris data from the Navigation Message. Aided (or Assisted) GPS techniques have been developed to supply the receiver with ephemerides to work around this limitation. See Navigation Sensitivity and Tracking Sensitivity
Period required by a GPS receiver to determine initial position based on acquisition of satellite signals.
A type of antenna that pre-amplifies the low-level RF signals from GPS satellites before the signal arrives at the receiver. See Low Noise Amplifier LNA . Contrast with Passive Antenna
The leg of a route currently being travelled. See Leg
A data set of approximate orbit information, clock corrections, and atmospheric delay parameters for all GPS satellites. It is transmitted by each GPS satellite over a 12.5 minute period to facilitate acquisition of satellites by GPS receivers. The data may be useful for several weeks and is normally refreshed once per week. See Cold Start Also see Ephemeris
An instrument used to determine elevation. It is typically an aneroid barometer that senses pressure changes accompanying changes in altitude, typically used in aircraft.
A continuously varying signal directly representing a desired value. Contrast with Digital Signal
The process of encrypting P-Code information so that it cannot be replicated by an adversary. The resulting encrypted code is called the Y-code. The encryption data can only be decoded by GPS receivers with special decryption circuitry, guarding against fake (spoofed) transmissions of satellite data.
A precision clock that depends for its operation on an electrical oscillator regulated by the natural vibration frequencies of a beam of cesium or rubidium atoms. A cesium clock has an error of one second per million years. GPS satellites contain multiple cesium and rubidium clocks.
The number of hours per day that a particular location has sufficient satellites (above the specified elevation mask angle) with less than the specified PDOP value to make a GPS position fix.
In GPS the horizontal angle between True North and a satelllite's position.
A receiver set up at a known location for specifically collecting data to differentially correct rover files. A base station calculates the error for each satellite, improving the accuracy of GPS positions collected at unknown locations by a roving GPS receiver through differential correction. Sometimes called a reference station.
A pair of base stations for which simultaneous GPS data has been collected.
A map database containing only major features (e.g. Interstate and State highways) but not detailed features (e.g. County roads).
A radio station (Operated by the Coast Guard in the US) that broadcasts Differential Corrections for GPS receivers to improve accuracy. See DGPS
The direction from a position to a destination, In GPS bearing usually refers to the direction to a waypoint.
The proposed Chinese counterpart to GPS
See Coarse/Acquisition Code
An unmodulated radio wave. It can be modulated in amplitude, frequency, or phase to convey information.
The frequency of the unmodulated fundamental output of a radio transmitter. For GPS the L1 carrier frequency is 1575.42 MHz.
A measurement of the phase angle of the GPS carrier signal.
A signal processing strategy that uses the GPS carrier phase to augment the measured code phase for better accuracy.
The art or technique of making maps or charts. Many GPS receivers have detailed mappingor cartographycapabilities.
See Course Deviation Indicator
See Code Division Multiple Access
See Circular Error Probable
A statistical measure of horizontal precision accuracy. CEP is defined as the radius of a circle (centered on the true position) that contains 50% of the reported positions.
The difference between the clock time maintained in the GPS receiver and GPS satellite time.
See Clock Bias
See Course Made Good
The GPS spread spectrum code modulated on the L1 Frequency that is used by commercial GPS receivers to determine the Pseudorange from the transmitting GPS satellite. See Standard Positioning Service SPS .
A method whereby multiple carriers share a frequency, but each one is modulated by a unique code. GPS uses a set of PRN codes called Gold codes chosen for their low cross-correlation properties.
A measurement of the difference in phase between the received GPS PRN code and the PRN code generated internally in the receiver. Used to calculate pseudorange.
See Course Over Ground
The start mode in which a GPS receiver has no previous information available except (perhaps) almanac data. Specifically, its position, the time, and ephemeris data is unknown. Contrast with Hot Start and Warm Start
The set of GPS satellites in orbit.
In surveying, a monumented point to which coordinates have been, or are being assigned. The National Geodetic Survey maintains a nationwide set of control points. Sometimes called a control station.
A worldwide chain of monitoring and control stations that control and manage the GPS satellite constellation. See Space Segment and User Segment
Officially known as UTC or Universal Time Coordinated, this is a time standard based on a worldwide group of atomic clocks. UTC is corrected by adding (or subtracting) leap seconds as required to match solar time (which is based on the Earth's rotation). UTC replaced Greenwich Mean Time GMT as the world standard for time in 1986.
A set of numbers that uniquely identifies a location on surfaces of two dimensions or in space of three dimensions. Coordinates are typically based on latitude/longitude lines of reference or a global/regional grid projection (e.g., UTM MGRS Maidenhead .
The direction from the current position to the destination.
An instrument that dispays the magnitude and direction of Crosstrack Error
The bearing from the active from waypoint to the current position, independent of the path taken to arrive at the current position.
The direction you are currently travelling in. See Speed Over Ground SOG .
The heading which is needed to be maintained in order to reach a destination. See Heading
Displaying a map with the current course up. Contrast with North Up Orientation
Incorrect recognition of one PRN code for another.
The distance (right or left) you are from the desired course.
A discontinuity of an interger number of cycles in the measured carrier phase due to a temporary loss-of-lock in the carrier tracking loop of a GPS receiver.
See Navigation Message
A GPS receiver that is capable of recording its position at specified times for later playback. It may also permit data entry, e.g. from a keyboard.
A reference model which represents the surface of the earth. Latitude and longitude lines on a paper map are referenced to a specified datum. The datum selected on a GPS receiver must match the datum listed on the corresponding paper map or errors of hundreds of meters can result. The GPS system normally uses WGS-84
The United States Department of Defense DOD . The DOD manages and controls the Global Positioning System
The course between the from and to waypoints.
See Differential GPS
A technique to improve position accuracy by supplying pseudorange corrections transmitted by a Beacon close to the GPS receiver. In the US, DGPS beacons are operated by the Coast Guard.
Accurate measurement of the relative positions of two receivers tracking the same GPS signals.
A signal having only two states (On/Off, 0/1, High/Low, Active/Inactive, etc.). It must be decoded to recover the represented value.
A measure of the quality of satellite geometry as seen by the GPS receiver. A lower DOP value indicates better geometry and correspondingly higher accuracy. Useful DOPs are GDOP (geometric DOP), HDOP (horizontal DOP), PDOP (position DOP), TDOP (time DOP), and VDOP (vertical DOP). Also see User Equivalent Range Error UERE and Estimated Position Error EPE .
The length between two waypoints or from your current position to a destination waypoint. This length can be measured in constant-course (rhumb line) or great-circle (over the earth) terms. GPS normally uses great circle calculations for distance and desired track.
The introduction of digital noise into a value. See Selective Availability
See Department of Defense
See Dilution Of Precision
A signal processing strategy that uses a measured doppler shift to help the receiver smoothly track the GPS signal. Allows more precise velocity and position measurement.
A change in the observed frequency of a wave, occurring when the receiver and transmitter are in motion relative to each other, with the frequency increasing when the source and observer approach each other and decreasing when they move apart. The motion of the source causes a real shift in frequency of the wave, while the motion of the observer produces only an apparent shift in frequency. Also called Doppler effect.
A transmission path for the communication of signals and data from a communications satellite or other space vehicle to the earth. See Uplink
See Desired Track
See European Geostationary Navigation Overlay Service
The vertical distance above or below ground level.
A simplified model of the Earth's surface generated by rotating an ellipse about the north-south (minor) axis.
See Estimated Position Error.
A data set of precise orbit information for a GPS satellite. It is transmitted by each GPS satellite every 30 seconds to allow calculation of the satellite's position in space. The data may be used for several hours and is normally refreshed every two hours. See Almanac
Errors in the ephemeris data transmitted by a GPS satellite. Ephemeris errors can be removed by differential correction.
An estimate of horizontal position error. EPE = Horizontal Dilution of Precision HDOP * User Equivalent Ranging Error UERE .
Estimated time it will take to reach your destination based upon your current distance and speed.
Estimated time upon arrival at your destination based upon your current distance and speed.
See Estimated Time Of Arrival.
See Estimated Time Enroute
The European Satellite Based Augmentation System SBAS system similar to the Wide Area Augmentation System WAAS .
Congressionally managed document of policies and plans for U.S. government-provided radionavigation services. Developed by joint Department of Defense DOD and Department of Transportation (DOT) effort to reduce the proliferation and overlap of federally funded radionavigation systems.
A navigation solution report usually with Position (e. g. horizontal position + Altitude), Velocity, and Time.
The ratio of valid fixes to total (valid plus invalid) fixes for a set of position measurements. A fix density of 100% implies that a valid navigation solution was computed for each measurement epoch and 0% implies that no position fixes were found. To make the density more meaningful, it is useful to exclude epochs for which a valid fix would be impossible, such as time spent in a tunnel, etc. A fix density of 100% is expected for a receiver with an antenna exposed to open sky.
See Federal Radionavigation Plan
The proposed European counterpart to GPS
Abbreviation for Geometric Dilution of Precision See Dilution of Precision DOP .
An outdoor treasure-hunting game in which the participants use a GPS receiver or other navigational techniques to hide and seek containers (called geocaches or caches) anywhere in the world.
See Datum
Global or regional surveys done to establish control networks (comprised of reference or control points) as a basis for accurate land mapping.
A computer system capable of assembling, storing, manipulating, and displaying geographically referenced information (i.e., data identified according to location). In practical use, GIS often refers to the computer system, software, and the data collection equipment, personnel, and actual data.
A model of the Earth's surface which approximates Mean Sea Level Since the Earth's mass is not evenly distributed, it is somewhat irregular.
The difference in elevation between the Geoid and Ellipsoid (which varies from -105 to +85 m over the Earth's surface). The value is positive when the geoid is above the ellipsoid. Also called Geoidal Undulation and Geoid Height. Altitude(MSL) = Ellipsoidal Altitude - Geoidal Separation
See Dilution of Precision DOP .
A geosynchronous orbit with a latitude of zero. A satellite in a geosynchronous orbit appears to remain stationary over a fixed point on the equator. This is very useful for communications satellites.
A specific orbit where a satellite rotates around the earth at the same rotational speed as the earth. A satellite in geosynchronous orbit appears to remain at a fixed longitude (but its latitude may vary)
See Geographic Information System
The Russian counterpart to GPS GLONASS is designed to provide worldwide coverage, however, its performance is optimized for the northern latitudes.
The system of user, space, and control segments providing position, velocity, and time service which is operated by the US Department of Defense Its full name is NAVSTAR GPS
See Global Navigation Satellite System
See Greenwich Mean Time
A subset of Pseudo-Random Numbers with low cross-correlation properties (used for GPS PRNs).
A route consisting of one leg, with the current position being the start of the route and a single defined waypoint as the destination.
See Global Positioning System
The mean solar time at Greenwich, England, which is located on the Prime Meridian (zero longitude). Based on the rotation of the earth, GMT has been replaced by UTC as the standard for time throughout the world.
A pattern of regularly spaced horizontal and vertical lines forming square zones on a map used as a reference for establishing points.
See Speed Over Ground SOG .
The plot of positions reported over time by a navigation device such as a GPS receiver. Latitude longitude positions may be shown with a background of roads, buildings, etc. as provided by a mapping program. Since a ground track usually includes substantial straight portions, Crosstrack Error is readily visible. Also, Fix Density is apparent from the plotted positions while the receiver is moving.
Abbreviation for Ground Speed See Speed Over Ground SOG .
Abbreviation for Horizontal Dilution of Precision. See Dilution of Precision DOP .
The direction in which a vehicle is pointing. For air and sea operations, this may differ from Course due to winds and currents.
A term used when an orbiting GPS satellite is suitable for use. State is also used to refer to satellite health.
See Quadrifilar Helix Antenna
See Dilution of Precision DOP .
The start mode in which a GPS receiver has its position, time, and ephemeris data available. Contrast with Cold Start and Warm Start
See Input/Output
The startup process for a GPS receiver. It may involve acquiring satellite signals, receiving almanac and ephemeris, calculating pseudoranges, and developing a position estimate. This information may be stored to speed up subsequent starts.
The two-way transfer of GPS information with another device, such as a nav plotter, autopilot, or another GPS unit.
To display and navigate a route from end to beginning for purposes of returning to the route's starting point.
That part of the atmosphere, extending from about 30 to 250 miles (50 to 400 kilometers), where ionization due to solar radiation can reflect and/or refract GPS electromagnetic waves.
The radio frequencies that extend from 390 MHz to 1550 MHz. The GPS carrier frequencies are in this band. See L1, L2, L5.
One of the carrier frequencies transmitted by the GPS satellites. This signal carries the Coarse Acquisition Code (C/A code), P(Y) Code (Precise code), and the navigation message, and is transmitted on a frequency of 1575.42 MHz.
One of the carrier frequencies transmitted by the GPS satellites. This signal carries the P(Y) Code (Precise code) and is transmitted on a frequency of 1227.6 MHz.
Proposed carrier frequency transmitted by Block 2F GPS satellites. This signal will carry the L5 Civil (L5C) code and is transmitted on a frequency of 1176.45 MHz.
See Local Area Augmentation System
The angular distance north or south from the equator of a position. Typically measured in Degrees, Minutes, Seconds or Degrees & Minutes. One minute of latitude is approximately one nautical mile.
A portion of a route consisting of a starting (from) waypoint and a destination (to) waypoint. A route that is comprised of waypoints A, B, C, and D would contain three legs. The route legs would be from A to B, from B to C, and from C to D. Also referred to as a segment.
Of an electromagnetic wave, propagation in which the transmission path is a direct straight line from the transmitter to the receiver. An unobstructed LOS is required for GPS signals.
See Low Noise Amplifier
The FAA system of short range ground-based Differential GPS to support precision instrument approaches of aircraft.
The angular distance east or west from the prime meridian (near Greenwich, England). Typically measured in Degrees, Minutes, Seconds or Degrees & Minutes. One minute of longitude is approximately one nautical mile at the equator, decreasing to zero at the poles.
A radio frequency (RF) amplifier specially designed to add very little noise to the amplified signal.
The direction from the current position to the North Magnetic Pole. The direction a compass points in the absence of deviations and anomalies.
The angular difference (east or west) between true north and magnetic north. Variation changes with location and (slowly) over time.
The Maidenhead Locator System is a scheme used by amateur radio operators for identifying positions on the Earth. Maidenhead locators are also commonly referred to as grid locators or grid squares, despite having a non-square shape on almost any cartographic projection.
The elevation angle below which a satellite should not be used in the nav solution. Typically 5 or 10 degrees.
The average level of the ocean's surface, as measured by the level halfway between mean high and low tide. Used as a reference in determining altitude, land elevation or sea depths. See Geoid
See Military Grid Reference System
The geographic standard used by the United States Armed Forces and NATO for locating any point on the earth with a 2 to 10 character geocode. A two digit code implies a precision of 10km and a ten digit code corresponds to a 1m precision with intermediate steps of 1km, 100m, and 10m. It is always displayed in an even number of characters preceded by an alpha-numeric code describing the larger Earth area to which it belongs.
See MTSAT Satellite-based Augmentation System
The proposed Japanese Satellite Based Augmentation System SBAS , covers parts of Asia and the Pacific.
A GPS receiver which can simultaneously track more than one satellite signal.
Errors caused by the multipath interference. In television, displays as a ghost image.
A phenomenon in the physics of waves whereby a wave from a source travels to a detector via two or more paths and, under the right condition, the two (or more) components of the wave interfere. In GPS signal tracking, this typically occurs due to reflection of the signal off of buildings in urban areas.
A U.S. standards committee. The NMEA 0183 specification defines the electrical interface and message protocol for communication among navigation equipment onboard a ship. It is widely used by GPS receiver manufacturers to report GPS data.
A unit of length used in sea and air navigation, approximately one minute of arc of a great circle. About 1852 meters or 6076 feet.
The message transmitted by each GPS satellite. It includes system time, clock correction parameters, ionospheric delay model parameters, the satellite's ephemeris data and health plus almanac data for all satellites. Also known as the Data Message. The message is defined in the Navstar GPS Interface Specification IS-GPS-200.
The minimum signal power level at which a GPS receiver can continue navigation once it has already calculated a solution. Since ephemerides are valid for several hours, a receiver can maintain a navigation solution without decoding the Navigation Message However, the solution will be lost as ephemerides age out and satellites set below the mask angle. See Acquisition Sensitivity and Tracking Sensitivity
The official U.S. Government name given to the GPS satellite system. NAVSTAR is an acronym for NAVigation Satellite Timing and Ranging.
See National Marine Electronics Association
Displaying a map with North up. Contrast with Course Up Orientation
Abbreviation for Original Equipment Manufacturer.
See P-Code
The precise code of the GPS signal typically used only by the U.S. military. It is encrypted (becoming Y code) and reset every seven days to prevent use by unauthorized persons.
An antenna that does not amplify the GPS signal before sending it to the receiver. See LNA Contrast with Active Antenna
A type of GPS antenna in which a flat area forms the receiving surface of the antenna. Contrast with Quadrifilar Helix Antenna
Abbreviation for Position Dilution of Precision See Dilution of Precision DOP .
A location described by coordinates (typically Latitude Longitude and Altitude).
See Dilution of Precision DOP .
The GPS receiver's reported position coordinates.
See Precise Positioning Service
The GPS positioning service available only to authorized users with a key to decrypt the P(Y) code. Can also use dual-frequency (L1 and L2) to nearly eliminate ionospheric erors. See Standard Positioning Service SPS
The line traced out by longitude zero and passing through Greenwich, England. The prime meridian forms the origin for the longitude part of the geographic coordinates and divides the eastern and western hemispheres.
See Pseudo-Random Noise
A ground-based transmitter transmits a signal like that of an actual GPS satellite which can be used for ranging.
A signal with properties like random noise, however it is a defined sequence of 1's and 0's.
The unique CDMA code identifying each GPS satellite.
A measure of the apparent propagation time from the satellite to the receiver antenna, expressed as a distance. The apparent propagation time is determined from the time shift required to align a replica of the GPS code generated in the receiver with the received GPS code. The time shift is the difference between the time of signal reception (measured in the receiver time frame) and the time of emission measured in the satellite time frame). Pseudorange is obtained by multiplying the apparent signal-propagation time by the speed of light. Pseudorange differs from the actual range by the amount that the satellite and receiver clocks are offset, by propagation delays, and other errors including those introduced by selective availability.
A type of GPS antenna in which four spiraling elements form the receiving surface of the antenna. For GPS use, quadrifilar antennas are typically half-wavelength or quarter-wavelength size and encased in a plastic cylinder for durability. Contrast with Patch Antenna
The Japanese Quasi-Zenith Satellite System augmentation system. See Satellite Based Augmentation System SBAS .
Abbreviation for Quasi-Zenith Satellite System augmentation system. See Satellite Based Augmentation System SBAS .
A committee created for the purposes of establishing standards and guidance for interfacing between radio beacon-based data links and GPS receivers, and to provide standards for ground-based differential GPS stations. See SC-104
See Receiver Autonomous Integrity Monitoring
The Differential GPS procedure whereby carrier phase corrections are transmitted in real time from a reference station to the user's roving receiver.
A feature of some GPS receivers to detect incorrect signals being transmitted from a satellite by comparing solutions with different sets of satellites.
The process of determining the relative difference in position between two locations by placing a receiver over each site and making simultaneous measurements observing the same set of satellites at the same time. This technique allows the receiver to cancel errors that are common to both receivers, such as satellite clock and ephemeris errors, propagation delays, etc.
Receiver INdependent EXchange format. A set of standard definitions and formats to promote the free exchange of GPS data which facilitates the use of data from any GPS receiver with any software package. The format includes definitions for three fundamental GPS observables: time, phase, and range.
A group of waypoints in the sequence you desire to navigate them. See Leg
The recommended standard from the IEEE defining serial input/output between Data Terminal Equipment (DTE) and Data Communications Equipment (DCE). Widely used by various electronics equipment.
See Radio Technical Commission For Maritime Services
See Real Time Kinematic
See Selective Availability
A system designed to improve the accuracy of GPS using ground stations to monitor the GPS signals, calculate corrections, and then transmit them from satellites to the GPS receiver. See Wide Area Augmentation System WAAS , European Geostationary Navigation Overlay Service EGNOS .
See Constellation
See Satellite Based Augmentation System
See Special Committee 104
The intentional degradation of the performance capabilities of the NAVSTAR satellite system for civilian use, accomplished by transmitting dithered clock and/or ephemeris data. SA has been turned off since 2000 by presidential order.
The process of sending data one bit at one time, sequentially, over a single path, rather than simultaneously over two or more lines, as in parallel transmission.
The minimum signal power level at which a specified function will operate. For GPS receivers, there are three common sensitivity measurements. Please refer to Acquisition Sensitivity Navigation Sensitivity and Tracking Sensitivity
See Speed Over Ground
The space-based component of the GPS system (i.e. the satellites). See Control Segment and User Segment
The RTCM standard defining communication from a Differential GPS beacon receiver and the Differential GPS receiver.
The speed with respect to the ground. This may differ from airspeed or nautical speed due to winds or currents.
In GPS the L-band signal is spread over a frequency band much wider than the minimum bandwidth needed to transmit the information being sent. This is done by modulating with a pseudo-random code, and provides the ability to receive all satellites unambiguously and to give some resistance to noise and multipath.
See Standard Positioning Service
The GPS positioning service available to all users. Typically, commercial receivers use the single frequency L1 C/A code only.
Location determination when the receiver's antenna is presumed to be stationary on the earth. This allows the use of various averaging techiques that can greatly improve accuracy.
A unit of length equal to 5,280 feet or 1,760 yards (1,609 meters) used in the U.S. and some other English-speaking countries.
Abbreviation for Satellite Vehicle. Also known as Space Vehicle.
Abbreviation for Time Dilution of Precision. See Dilution of Precision DOP .
See Dilution of Precision DOP .
The time it takes a GPS receiver to report a valid fix from startup. See Cold Start
Same as Course Up Orientation
The minimum signal power level at which a GPS receiver will keep the tracking loops closed. This level is not directly visible through standard NMEA data and is the least important measurement for most pedestrian, vehicle, marine, or aviation applications. See Acquisition Sensitivity and Navigation Sensitivity
The lowest layer of the atmosphere and contains about 95% of the mass of air in the Earth's atmosphere. The troposphere extends from the Earth's surface up to about 10 to 15 kilometers. All weather processes take place in the troposphere. See Ionosphere
The direction of the north pole from your current position. Magnetic compasses indicate differently due to variation (between true north and magnetic north), deviation, and anomalies.
See Time To First Fix
See User Equivalent Range Error.
A system of plane coordinates based upon 60 north-south trending zones, each 6 degrees of longitude wide, that circle the globe. Used to derive geographic coordinates, normally in meters, east and north of an origin that are defined uniquely for each zone. UTM is the primary coordinate system used on U.S. Geological Survey topographic maps.
A transmission path by which radio or other signals are sent from the ground to an aircraft or a communications satellite. See Downlink
The total expected magnitude of position errors due to measurement uncertainties. Multiply by Horizontal Dilution of Precision HDOP to get the Estimated Position Error EPE .
The segment of the complete GPS system that includes the GPS receiver and user. See Control Segment and Space Segment
See Coordinated Universal Time (Universal Time Coordinated).
See Universal Transverse Mercator
Abbreviation for Vertical Dilution of Precision. See Dilution of Precision DOP .
The rate of closure to a destination based upon your current speed and course.
See Dilution of Precision DOP .
See Velocity Made Good
See Wide Area Augmentation System
The start mode in which a GPS receiver has its position and approximate time available but not ephemeris data. Contrast with Cold Start and Hot Start
The distance between successive points of equal amplitude and phase on a wave (for example, crest to crest or trough to trough). It is the inverse of Frequency.
A location designated by a set of coordinates and stored in a GPS receiver to be later used as a GOTO destination or as part of a route.
World Geodetic System, 1984. The World Geodetic System defines a fixed global reference frame for the Earth, for use in geodesy and navigation. The latest revision dates from 1984, and will be valid up to about 2010. It is the primary map datum used by GPS Other datums are computed as differences from the WGS 84 standard.
A system of satellites and ground stations that provide GPS signal corrections for better position accuracy. WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellites. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message which is transmitted by geostationary satellites to the WAAS-capable receiver. WAAS is a type of Satellite Based Augmentation System SBAS .
See Crosstrack Error
The P-code after encryption has been applied.
The number of 1.5 second intervals elapsed since the beginning of the GPS week (just before midnight on Saturday).