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UNSW - A Glossary of GPS Terms Category: Meteorology and astronomy > GPS Date & country: 22/11/2012, AU Words: 85

Absolute Positioning
Mode in which a position is determined, using a single receiver, with respect to a well-defined coordinate system, typically a Geocentric system (i.e., a system whose point of origin coincides with the centre of mass of the earth). Also referred to as Point Positioning, or Single Receiver Positioning.

Antenna Splitter
An attachment which can be used to split the antenna signal into two, so that it may be fed to two GPS receivers. Such a configuration forms the basis of a Zero Baseline test.

Anywhere Fix
The ability of a receiver to start position calculations without being given an approximate location and time.

Attribute
A characteristic which describes a Feature. Attributes can be thought of as questions which are asked about the Feature. Typically associated with geospatial data gathering for inclusion within Geographic Information Systems (GIS).

Availability
The number of hours per day that a particular location has sufficient satellites (above the specified elevation angle, and perhaps less than some specified PDOP value) to make a GPS position determination possible.

Carrier-Aided Tracking
A signal processing strategy that uses the GPS carrier signal to achieve an exact lock-on the PRN code. More efficient and accurate than the standard approach.

Carrier Phase
GPS measurements made on the L1 or L2 carrier signal. May refer to the fractional part of the L1 or L2 carrier wavelength (approximately 19cm for L1, 24cm for L2), expressed in units of metres, cycles, fraction of a wavelength or angle. (One cycle of L1 is equivalent to one wavelength, and similarly for L2.) In carrier phase-based positioning, such as employed in GPS Surveying techniques, carrier phase may also refer to the accumulated or integrated measurement which consists of the fractional part plus the whole number of wavelengths (or cycles) since signal lock-on.

Carrier
A radio wave having at least one characteristic (e.g., frequency, amplitude, phase) that can be varied from a known reference value by modulation. In the case of GPS there are two transmitted carrier waves: (a) L1 at 1575.42MHz, (b) L2 at 1227.60MHz, modulated by the Navigation Message (both L1 and L2), the P-Code (both L1 and L2) and the C/A-Code (L1).

Circular Error Probable
(CEP) A statistical measure of the horizontal precision. The CEP value is defined as a circle's radius, when centred at the true position, encloses 50% of the data points in a horizontal scatter plot. Thus, half the data points are within a 2-D CEP circle and half are outside the circle.

Clock Bias
The difference between the receiver or satellite clock's indicated time and a well-defined time scale reference such as UTC (Coordinated Universal Time), TAI (International Atomic Time) or GPST (GPS Time).

Course-Made-Good
(CMG) The bearing from your starting point to your present position. Commonly used in marine or air navigation.

Control Point
Also called a Control Station or Geodetic Control Station. A monumented point to which coordinates have been assigned by the use of terrestrial or satellite surveying techniques. The coordinates may be expressed in terms of a satellite reference coordinate system (such as with respect to WGS84, or to ITRS), or a local geodetic datum.

Constellation
Refers to either the specific set of satellites used in calculating a position, or all the satellites visible to a GPS receiver at one time, or the entire ensemble of GPS satellites comprising the Space Segment.

Coarse Acquisition
(C/A) See also C/A-Code. A spread spectrum direct sequence code that is used primarily by commercial GPS receivers to determine the pseudo-range to a transmitting GPS satellite, modulated on the L1 carrier.

Code Phase
GPS measurements based on the C/A-Code. The term is sometimes restricted to the C/A- or P-Code pseudo-range measurement when expressed in units of cycles.

Crosstrack Error
(XTE) The distance you are off a desired course in either direction. Commonly used in marine or air navigation.

Cutoff Angle
The minimum acceptable satellite elevation angle (above the horizon) to avoid blockage of line-of-sight, multipath errors or too high Tropospheric or Ionospheric Delay values. May be preset in the receiver, or applied during data post-processing. For navigation receivers may be set as low as 5

Datalogger
Also known as a Data Recorder. A handheld, lightweight data entry computer. It can be used to store additional data obtained by a GPS receiver, such as Attribute information on a Feature whose coordinates are captured for a GIS project.

Data Message
Also known as the Navigation Message. A 1500 bit message modulated on the L1 and L2 GPS signal, which contains the satellite's location (or ephemeris), clock (bias) correction parameters, constellation almanac information and satellite health.

Dilution of Precision
(DOP) An indicator of satellite geometry for a unique constellation of satellites used to determine a position. Positions tagged with a higher DOP value generally constitute poorer measurement results than those tagged with lower DOP. There are a variety of DOP indicators, such as GDOP (Geometric DOP), PDOP (Position DOP), HDOP (Horizontal DOP), VDOP (Vertical DOP), etc.

Differential Positioning
Also known as Relative Positioning. Precise measurement of the relative positions of two receivers tracking the same GPS signals. Maybe considered synonymous with DGPS, or the term may be reserved for the more precise carrier phase-based baseline determination technique associated with GPS Surveying.

Differential GPS
(DGPS) A technique to improve GPS accuracy that uses pseudo-range errors measured at a known Base Station location to improve the measurements made by other GPS receivers within the same general geographic area. It may be implemented in real-time through the provision of a communication link between the GPS receivers, transmitting the correction information in the industry-standard RTCM format, or various proprietary formats. May be implemented in single Base Station mode, in the so-called Local Area DGPS (LADGPS), or using a network of Base Stations, as in the Wide Area DGPS (WADGPS) implementation.

Doppler Shift
The apparent change in the frequency of a signal caused by the relative motion of the transmitter and receiver.

Doppler-Aiding
A signal processing strategy that uses a measured Doppler Shift to help the receiver smoothly track the GPS signal. This allows for more precise velocity and position determination, especially when the receiver is moving at high speed and/or in an erratic fashion.

Dynamic Positioning
See Kinematic Positioning

Estimated-Time-of-Arrival
(ETA) The time of day of your arrival at your destination. Typically used for navigation applications.

Estimated-Time-Enroute
(ETE) The time left to your destination at your present speed. Typically used for navigation applications.

Federal Radionavigation Plan
(FRP) Congressionally mandated, joint US Department of Defense (DOD) and US Department of Transportation (DoT) effort to reduce the proliferation and overlap of federally funded radionavigation systems. The FRP is designed to delineate policies and plans for US government-provided radionavigation services. Produced annually.

Fix
A single position with latitude, longitude (or grid position), altitude (or height), time, and date.

Geodetic Survey
Global surveys for the establishment of control networks (comprised of Reference or Control Points), which are the basis for accurate land mapping. Maybe carried out using either terrestrial or satellite positioning (e.g. GPS) techniques. The outcome is a network of benchmarks which are a physical realisation of the Geodetic Datum or Reference System.

Geographic Information System
(GIs) A computer-based system that is capable of collecting, managing and analysing geospatial data. This capability includes storing and utilising maps, displaying the results of data queries and conducting spatial analysis.

Geometric Dilution of Precision
(GDOP) See Dilution of Precision. An indicator of the geometrical strength of a GPS constellation used for a position/time solution.

Global Navigation Satellite System
(GNSS) This is an umbrella term used to describe a generic satellite-based navigation/positioning system. It was coined by international agencies such as the International Civil Aviation Organisation (ICAO) to refer to both GPS and GLONASS, as well as any augmentations to these systems, and to any future civilian developed satellite system. For example, the Europeans refer to GNSS-1 as being the combination of GPS and GLONASS, but GNSS-2 is the blueprint for an entirely new system.

Global Positioning System
(GPS) A system for providing precise location which is based on data transmitted from a constellation of 24 satellites. It comprises three segments: (a) the Control Segment, (b) the Space Segment, and (c) the User Segment.

Height
(Ellipsoidal) The height coordinate determined from GPS observations is related to the surface of a Reference Ellipsoid. The coordinates are derived initially in the 3-D Cartesian system (as XYZ values), and then for display/output purposes they are transformed to Latitude, Longitude and (Ellipsoidal) Height using well known formulae to an ellipsoid such as that associated with the WGS84 Datum (semi-major axis: 6378137m; inverse flattening: 298.257223563). The surface of the ellipsoid is the zero ellipsoidal height datum. In Relative Positioning, the height component of the receiver whose coordinates are being determined relative to the Base Station can also be related to an ellipsoid by transforming the baseline vector from the 3-D form (DXDYDZ) to a change in Latitude, change in Longitude, and change in Ellipsoidal Height.

Integrity
A quality measure of GPS performance for critical applications such as civilian aviation. A high level of integrity is sought for such applications.

Ionosphere, Ionospheric Delay
The Ionosphere is that band of atmosphere extending from about 50 to 1000 kilometres above the earth's surface in which the sun's ultraviolet radiation ionises gas molecules which then lose an electron. These free electrons influence the propagation of microwave signals (speed, direction and polarisation) as they pass through the layer. The Ionospheric Delay on GPS signals is frequency-dependent and hence impacts on the L1 and L2 signals by a different amount (unlike that within the Troposphere). A linear combination of pseudo-range or carrier phase observations on the L1 and L2 carrier waves can be created to almost entirely eliminate the Ionospheric Delay. The resulting observable is known as the Ionosphere-Free carrier phase (or pseudo-range). For single-frequency receivers it is not possible to account for this signal bias in this way. A broadcast model is contained within the transmitted Navigation Message, however, it is a relatively poor model (unlikely to account for more than 50% of the effect) as the Delay is very difficult to predict. The magnitude of the Ionospheric Delay is a function of the latitude of the receiver, the season, the time of day, and the level of solar activity. The Delay in the Zenith direction can be several tens of metres, increasing as the elevation angle of the satellite signal reduces (being 3-5 times greater than in the Zenith direction). The Delay is largely eliminated in Relative or Differential Positioning, however, the residual Ionospheric Delay increases as the baseline length increases and may be a significant source of error (especially in the height component) for very high precision GPS Geodesy. Even when using dual-frequency instrumentation, the Ionospheric Delay can still ca...

Ionosphere-Free Combination
This is a particular linear combination of the observations made on the L1 and L2 carrier waves that eliminates (to the first order) the ionospheric delay on the GPS observables. The ionosphere-free L1 carrier phase combination (in units of L1 wavelengths) is: f(L1)ion-free = a1.f(L1) + a2.f(L2)

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

Kinematic Positioning
Kinematic Positioning refers to applications in which the position of a non-stationary object (vehicle, ship, aircraft) is determined.

Latitude
A north/south angular measurement of position relative to the equator, in the meridian plane which contains the earth's rotation axis.

Longitude
An east/west angular measurement of position in relation to the Prime Meridian. The angle between the two great circles, one being the Prime (or Greenwich) Meridian and the other a meridian passing through the point of interest. (A great circle that passes through the north and south poles, and hence contains the earth's rotation axis.)

Mask Angle
See Cutoff Angle

Multiplexing Channel
A channel of a GPS receiver that can be sequenced through a number of satellite signals. In contrast to a Multi-Channel Receiver in which one channel is dedicated to each satellite signal.

Multipath Error
Errors caused by the interference of a signal that has reached the receiver antenna by two or more different paths. This is usually caused by one path being bounced or reflected. The impact on a pseudo-range measurement may be up to a few metres. In the case of carrier phase, this is of the order of a few centimetres.

Multipath
Interference caused by reflected GPS signals arriving at the receiver, typically as a result of nearby structures or other reflective surfaces. May be mitigated to some extent through appropriate antenna design, antenna placement and special filtering algorithms within GPS receivers.

Multi-Channel Receiver
A GPS receiver that can simultaneously track more than one satellite signal using a dedicated signal electronics channel for each satellite. High quality receivers may have 12 channels for L1, and another 12 channels for L2 signals. Lower quality GPS navigation receivers may have only 6 or 8 channels. In contrast to a Multiplexing Channel Receiver.

Navigation Message
Also known as the Data Message, containing the satellite's broadcast ephemeris, satellite clock (bias) correction parameters, constellation almanac information and satellite health.

NAVSTAR
The name sometimes given to the GPS satellite system. NAVSTAR is an acronym for NAVigation Satellite Timing and Ranging.

On-The-Fly
(OTF) This is a form of Ambiguity Resolution (AR) which does not require that the receivers remain stationary for any length of time. Hence this AR technique is suitable for initialising carrier phase-based Kinematic Positioning. For many applications this introduces considerable flexibility. For example, aircraft do not have to be parked on the ground in order to resolve the carrier cycle ambiguities, and then require that signal lock-on be maintained throughout the kinematic survey. However, dual-frequency instrumentation capable of making both carrier phase and precise (P-Code level) pseudo-range measurements is required.

Outage
Defined as a loss of Availability, due to either there not being enough satellites visible to calculate a position, or the value of the DOP indicator is greater than some specified value (implying that the accuracy of the position is unreliable).

Post-Processed GPS
In post-processed (Differential or Relative ) GPS the base and user (or roving or mobile) receivers have no data communication link between them. Instead, each receiver records the satellite observations that will allow differential correction (in the case of pseudo-range-based positioning), or the processing of double-differenced observables (in the case of carrier phase-based positioning) at a later time. Data processing software is used to combine and process the data collected from these receivers.

Position
The 3-D coordinates of a point, usually given in the form of Latitude, Longitude, and Altitude (or Ellipsoidal Height), though it may be provided in the 3-D Cartesian form, or any other transformed map or geodetic reference system. An estimate of error is often associated with a position.

Position Dilution of Precision
(PDOP) See Dilution of Precision. Measure of the geometrical strength of the GPS satellite configuration for 3-D positioning.

Point Positioning
See Absolute Positioning

Precise Positioning Service
(PPS) The most accurate Absolute Positioning possible with GPS navigation receivers, based on the dual-frequency encrypted P-Code. Available to the military users of GPS. Typical accuracy is of the order of 10-20m.

Pseudolite
A ground-based differential GPS receiver which transmits a signal like that of an actual GPS satellite, and can be used for ranging. Originally intended as an augmentation for Local Area Augmentation Systems to aid aircraft landings. However, pseudolites may also be used where signal obstructions are such that insufficient GPS satellites can be tracked. In fact, pseudolites are feasible in circumstances where no satellite signals are observable, e.g. for indoor applications.

Range
A fixed distance between two points, such as between a starting and an ending waypoint, or a satellite and a GPS receiver. May also be referred to as Geometric Range.

Relative Positioning
The determination of relative positions between two or more receivers which are simultaneously tracking the same GPS signals. One receiver is generally referred to as the Reference or Base Station, whose coordinates are known in the satellite datum. The second receiver may be stationary or moving. However its coordinates are determined relative to the Base Station. In carrier phase-based positioning this results from the determination of the baseline vector, which when added to the Base Stations coordinates generates the User's coordinates. In pseudo-range-based GPS positioning, the coordinates are derived from the User receiver's observations after they have had the differential corrections applied (either in the real-time or post-processed mode).

Real Time Kinematic
(RTK) The Relative Positioning procedure whereby carrier phase measurements (or corrections) are transmitted in real-time from a Reference or Base Station to the user's roving receiver. Centimetre accuracy is achieved without the need to record and post-process double-differenced carrier phase observables.

RINEX
Receiver INdependent EXchange format. A set of standard definitions and formats to promote the free exchange of GPS data and facilities the use of data from any GPS receiver with any post-processing software package. The format includes definitions for three fundamental GPS observables: time, phase, and range.

Root Mean Square
(RMS) The square root of the average of the squared errors.

Rover
Any mobile GPS receiver collecting data during a field session. The receiver's position may be computed relative to another, stationary GPS receiver at a Base Station. May also be referred to as the Mobile Receiver.

Satellite Constellation
See Constellation, or Space Segment.

SINEX
Solution Independent Exchange format. A solution output format recently developed by geodesists to permit the exchange of solution information between organisations, from which the original normal equation systems for precise GPS adjustments can be reconstructed. These reconstructed equation systems can be combined with other normal equation systems to create new GPS baseline solutions.

Spherical Error Probable
(SEP) A statistical measure of the 3-D positioning precision. The SEP value is defined as a sphere's radius, when centred at the true position, encloses 50% of the data points in a 3-D scatter plot. Thus, half the data points are within a 3-D SEP sphere and half are outside the sphere.

Space Segment
The space-based component of the GPS system (i.e., the orbiting satellites and their signals). The satellites may be differentiated into various groups. e.g. the Block II, Block IIA, Block IIR, and Block IIF satellites.

Static Positioning
Location determination when the receiver's antenna is presumed to be stationary on the earth. In the case of pseudo-range-based techniques this allows the use of various averaging techniques to improve the accuracy. Static Positioning is usually associated with GPS Surveying techniques, where the two GPS receivers are static for some observation period which may range from minutes to hours (and even in the case of GPS geodesy, several days).

Troposphere, Tropospheric Delay
The Troposphere is the neutral atmosphere comprising the lower 8km of the atmosphere. The Tropospheric Delay on GPS signals is of the non-dispersive variety because it is not frequency-dependent and hence impacts on both the L1 and L2 signals by the same amount (unlike that within the Ionosphere). The wet and dry components of the Troposphere cause the Delay to the signals, with the wet component be responsible for approximately 10% of the total delay. Various Tropospheric Delay models have been developed to estimate the Delay as a function of the satellite elevation angle, receiver height, and meteorological parameters such as temperature, pressure and humidity. The Delay in the Zenith direction is approximately 2.5m, increasing as the elevation angle of the satellite signal reduces. (This behaviour is described by the so-called Mapping Function, so that the Delay near the horizon is 3-5 times higher than in the Zenith direction.) The Delay is largely eliminated in Relative or Differential Positioning, however the residual Tropospheric Delay increases as the baseline length increases and may be a significant source of error (especially in the height component) for very high precision GPS Geodesy.

Trivial Baseline
Trivial Baselines are those baselines formed when more than two GPS receivers are used simultaneously in the field to perform static GPS surveys. For example, when 3 receivers at points A, B, C are deployed only 2 baselines are independent (either A-B & A-C, AB & B-C, or AC & C-B), with the other one being trivial. This trivial baseline may be processed, but because the data used for this baseline has already been used to process the independent baselines, the baseline results should not be used for Network Adjustment or for quality control purposes unless the statistics (and variance-covariance matrix) are appropriately downweighted.

Triple-Difference
A linear combination of Double-Difference carrier phase observables by which the cycle ambiguity parameters can be eliminated and which is less affected by unrepaired cycle slips than Double-Differences. A Triple-Differenced observable is created by differencing two consecutive Double-Differences (the same pair of receivers and the same pair of satellites, but separated in time). A useful observable for obtaining approximate baseline solutions or for detecting cycle slips in the Double-Differenced observables.

Track
(TRK) The direction of movement relative to a ground position. Commonly associated with navigation applications.

Universal Transverse Mercator
(UTM) A grid coordinate system that projects global sections onto a flat surface to measure position in specific zones. These zones are 6

User Segment
That component of the GPS system that includes the user equipment, applications and operational procedures.

Velocity-Made-Good
(VMG) The speed you are closing in on a destination along a desired course. A navigation term.

Waypoint
A (usually two-dimensional) coordinate that is input into a navigation device, such as a GPS receiver, representing a position that a vessel, aircarft, vehicle or person has to navigate to, with the aid of GPS (and/or any other position fixing device).

Wide Area Augmentation System
(WAAS) WAAS is a US Federal Aviation Authority (FAA) funded system of equipment and software that augments GPS accuracy, availability and integrity. The WAAS provides a satellite signal for WAAS users to support enroute and precision approach aircraft navigation. Similar systems are under development in Europe (where it is known as EGNOS -- European Geostationary Navigation Overlay System), Japan (where it is known as MT-SAT), and Australia.

World Geodetic System 1984
(WGS84) A global Geodetic Datum defined and maintained by the US Department of Defense. As the Control Segment coordinates and the Broadcast Ephemerides are expressed in this Datum, the GPS positioning results are said to be in the WGS84 Datum. In the case of Point Positioning this is largely true, although the level of accuracy achievable under the policy of Selective Availability is so poor that the link to the WGS84 Datum is very approximate. In the case of Relative Positioning, the baseline vector may be determined to quite high accuracy (at the sub-centimetre level using precise GPS Surveying techniques), however the coordinate (and therefore the Datum) of the unknown point is almost completely defined by the Datum of the Base Station. This may not be coincident with the WGS84 Datum at better than a few tens of metres! If GPS Geodesy techniques are used, with known station coordinates expressed in the ITRS and precise ephemerides obtained from the IGS, it is more correct to state that the subsequent set of coordinates are expressed in one of the ITRS frames (e.g. ITRF92, ITRF94, etc.). The WGS84 and the ITRS are compatible at the one metre level. However, the ITRS is a more precise realisation of an earth-fixed, earth-centred terrestrial reference system.

XTE
See Crosstrack Error.

C/A-Code
The standard (Clear/Acquisition) GPS PRN code, also known as the Civilian Code or S-Code. Only modulated on the L1 carrier. Used by the GPS receiver to acquire and decode the L1 satellite signal, and from which the L1 pseudo-range measurement is made.

I/O
Abbreviation for Input/Output.

L1 Frequency
1575.42MHz GPS carrier frequency which contains the C/A-Code, the encrypted P-Code (or Y-Code) and the Navigation Message. Commercial GPS navigation receivers can track only the L1 carrier to make pseudo-range (and sometime carrier phase and Doppler frequency) measurements.

L2 Frequency
1227.60MHz GPS carrier frequency which contains only the encrypted P-Code (or Y-Code) and the Navigation Message. Military Y-Code capable receivers can, in addition to making L1 measurements, make pseudo-range measurements on the L2 carrier. The combination of the two measurements (on L1 and L2) permits the Ionospheric Delay to be corrected for. Dual-frequency GPS receivers intended for Surveying applications can make L2 measurements using proprietary signal processing techniques. Such measurements are essential if the Ionospheric Delay on carrier phase is to be corrected for (especially on baselines of length greater than about 20-30km) and/or where fast Ambiguity Resolution is needed.

L-Band
The group of radio frequencies extending from 390MHz to 1550MHz. The GPS carrier frequencies L1 and L2 are in the L-Band.

R95
A position accuracy measure. The R95 value is defined as a circle's radius, when centred at the true position, encloses 95% of the data points in a horizontal scatter plot.