2drms

A position accuracy measure defined as twice the RMS of the horizontal error. This approximately corresponds to the 95% confidence interval, or "two sigma" value. For example, under the policy of "Selective Availability" GPS Absolute Positioning accuracy is claimed to be 100m 2drms, which means that approximately 95% of the horizontal position solutions will be within 100m of the correct value.

3D Position

A 3-dimensional position fix that only includes the X (longitude), Y(latitude) and Z(altitude) coordinates. To calculate a 3D fix you need at least 4 visible satellites.

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.

AGPS

AGPS is short for Assisted GPS. Assisted GPS is used to speed up start-up times of GPS equipment. GPS may have problems getting a lock when the signal is weak and in such a case Assisted GPS would assist in getting a lock. An example of Assisted GPS is downloading almanac data using internet to shorten the time to first fix.

Almanac data

GPS receivers use almanac data to predict which satellites are nearby when they're looking for GPS signals. Almanac data includes a set of parameters for each GPS satellite that can be used to calculate its approximate location in orbit. Using almanac data saves time by letting the receiver skip looking for satellites that are below the horizon. GPS satellites include almanac data in the signals they transmit to GPS receivers. Although variations in satellite orbits can accumulate with time, almanac data doesn't need to be highly accurate to be useful. Data collected before your receiver was last switched off may remain usable for weeks or months.

Ambiguity

Carrier phase measurements can only be made in relation to a cycle or wavelength of the L1 or L2 carrier waves because it is impossible to discriminate different carrier cycles (they are all "sine waves" if one ignores the modulated messages and PRN codes). Integrated carrier phase measurements may be made by those receivers intended for carrier phase-based positioning.

In this case the change in receiver-satellite distance can be measured by counting the number of whole wavelengths since initial signal lock-on and adding the instantaneous fractional phase measurement. However, such a measurement is a biased range or distance measurement because the initial number of whole (integer) wavelengths in the receiver-satellite distance is unknown. This unknown value is referred to as the "ambiguity".

It is different for the different satellites, and different for the L1 and L2 measurements. It is, however, a constant if signal tracking continues uninterrupted through an observation session. If there is signal blockage, then a "cycle slip" occurs, causing the new ambiguity after the cycle slip to be different from the value before.

Cycle slip repair therefore restores the continuity of carrier cycle counts and ensures that there is only one ambiguity for each satellite-receiver pair.

Ambiguity Resolution

If the initial integer ambiguity value for each satellite-receiver pair could be determined, then the ambiguous integrated carrier phase measurement can be corrected to create an unambiguous, but very precise (millimetre observation accuracy), receiver-satellite distance measurement. A solution using the corrected carrier phase observations is known as an "ambiguity-fixed" or "bias-fixed" solution.

The mathematical process or algorithm for determining the value for the ambiguities is Ambiguity Resolution. Tremendous progress has been made in AR techniques, making today's carrier phase-based GPS systems very efficient by cutting down the length of observation data needed (resulting in so-called "rapid static surveying" techniques) and even allowing this process to occur while the receiver is itself in motion (in so-called "on-the-fly" AR techniques).

(In practice, the AR process and the ambiguity-fixed solutions are carried out on the double-differenced carrier phase observables, not on the one-way satellite-receiver measurements.)

Antenna

That part of the GPS receiver hardware which receives (and sometimes amplifies) the incoming L-Band signal. Antennas come in all shapes and sizes, but most these days use so-called "microstrip" or "patch" antenna elements. The geodetic antennas, on the other hand, may use a "choke-ring" to mitigate any multipath signals.

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.

Anti-Spoofing (AS)

Is a policy of the U.S. Department of Defense by which the P-Code is encrypted (by the additional modulation of a so-called W-Code to generate a new "Y-Code"), to protect the militarily important P-Code signals from being "spoofed" through the transmission of false GPS signals by an adversary during times of war.

Hence civilian GPS receivers are unable to make direct P-Code pseudo-range measurements and must use proprietary (indirect) signal tracking techniques to make measurements on the L2 carrier wave (for both pseudo-range and carrier phase).

All dual-frequency instrumentation must therefore overcome AS using these special signal tracking and measurement techniques.

Anywhere Fix

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

Attribute

A characteristic of a map feature described as a text or number. Attributes are often stored in a tabular format. An example of an attribute, is a value indicating the number of residents in a city or area.

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.

Baseline

A Baseline consists of a pair of stations for which simultaneous GPS data have been collected. Mathematically expressed as a vector of coordinate differences between the two stations, or an expression of the coordinates of one station with respect to the other (whose coordinates are assumed known, and is typically referred to as a "Base" or "Reference" Station).

Bearing

Also referred to as the Azimuth. The compass direction from a position to a destination. The "north" direction is "zero bearing", and the angle is measured clockwise through 360┬░. May be referred to a number of "north" directions, including magnetic north, (projection) grid north, or geographic north.

Bias

All GPS measurements are affected by biases and errors. Their combined magnitudes will affect the accuracy of the positioning results (they will bias the position or baseline solution). Biases may be defined as being those systematic errors that cause the true measurements to be different from observed measurements by a "constant, predictable or systematic amount", such as, for example, all distances being measured too short, or too long. Biases must somehow be accounted for in the measurement model used for data processing if high accuracy is sought. There are several sources of biases with varying characteristics, such as magnitude, periodicity, satellite or receiver dependency, etc. Biases may have physical bases, such as the atmosphere effects on signal propagation or ambiguities in the carrier phase measurements, but may also enter at the data processing stage through imperfect knowledge of constants, for example any "fixed" parameters such as the satellite ephemeris information, station coordinates, velocity of light, antenna height errors, etc.

Random errors will not bias a solution. However, outlier measurements, or measurements significantly affected by multipath disturbance (which may be considered a transient, unmodelled bias), will bias a solution if the proportion of affected measurements is relatively high compared to the number of unaffected measurements. For this reason, long period static GPS Surveying is more accurate (less likely to be biased) than "rapid static surveying" or kinematic (single-epoch) positioning.

Binary Shift-Key (BSK) Modulation

BSK is a modulation technique by which a binary message, such the Navigation Message or the PRN codes (consisting of 0's and 1's), is imprinted on the carrier wave.

Unlike Amplitude Modulation (AM) and Frequency Modulation (FM), BSK Modulation does not alter the signal level (the "amplitude") or the carrier wavelength (the "frequency").

At a change in value of the message from 0 or 1, or from 1 to 0, the carrier wave is reversed (the phase is "flipped" by 180┬░). All reversals take place at the zero-crossings of the carrier (sine) wave (i.e., where the phase is zero).

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.

CAD

CAD stands for Computer Aided Design. An automated system for the design, drafting and display of graphically oriented information.

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).

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-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.

Centroid

A centroid is the x,y coordinate of the center of a one or two dimensional shape.

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.

Class of Survey

Class of Survey is a means of categorising the internal quality, or precision of a survey. The number of categories, the notation applied, and the accuracy tolerances defining the transition from one class to another are defined by individual nations.

Typically they are based on traditional geodetic surveying categories, supplemented by several extra categories of higher precision applicable to GPS Surveying and GPS Geodesy techniques, and may be different for horizontal surveys and vertical surveys.

The attachment of a particular Class "label" (e.g. A, B, etc.) to a survey, comprising a few or many points within a "network", carried out using GPS or any other technique, is performed as part of the process of "network adjustment" in which the relative error ellipses (in the horizontal case) between coordinated stations are computed and compared with the accuracy standards that must be met for various categories of Class. See Minimally Constrained.

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).

CMG

Course Made Good

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.

COG

Course Over Ground

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.

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.

Control Segment

A world-wide network of GPS monitoring and upload telemetry stations operated by, or on behalf of, the US Department of Defense. The tracking data is used by the Master Control Station at Colorado Springs to calculate the satellites' positions (or "broadcast ephemerides") and their clock biases.

These are formatted into the Navigation Message which is uploaded on a daily (perhaps more frequently) basis by the Control Segment stations.

Correlator

The GPS receiver "software" or electronic means, implemented in some fashion (either analogue or digital) within a Tracking Channel, used to shift or compare the incoming signal with an internally generated signal. This operation is performed on the PRN codes, but may be used for more "exotic" mixed signals in the case of L2 measurements, where under the policy of Anti-Spoofing (AS) the L2 PRN code is not known.

Correlator design may be influenced such that it is optimised for accuracy, mitigation of multipath, acquisition of signal under foliage, etc.

Course

The course is defined as the direction between two points. The course is expressed in degrees or radians.

Course-Made-Good (CMG)

The bearing from your starting point to your present position. Commonly used in marine or air navigation.

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┬░, while for GPS Surveying typically a cutoff angle of 15┬░ is used.

Cycle Slip

A discontinuity of an integer number of cycles in the measured (integrated) carrier phase resulting from a temporary loss-of-lock in the carrier tracking loop of a GPS receiver.

This corrupts the carrier phase measurement, causing the unknown Ambiguity value to be different after the cycle slip compared with its value before the slip. It must be "repaired" (the unknown number of "missing" cycles determined and the carrier observation subsequent to the cycle slip all corrected by this amount) before the phase data is processed in double-differenced observables for GPS Surveying techniques.

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.

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.

Datum

A Datum is a means by which coordinates determined by any means may be related to a well-defined Reference Frame. The Reference Frame may be visualised as a 3-D Cartesian coordinate system consisting, as a minimum, of information concerning the origin of the axes, and the directions of two principal axes fixed to the earth.

The Reference Frame may be globally applicable, such as WGS84 or ITRF, in which case it is "geocentric" (having its origin at the earth's centre of mass), or be locally applicable as in the case of traditional national geodetic frames such as the Australian Geodetic Datum. In any case, the Datum may be considered synonymous to the Reference Frame, or be restricted to refer to the set of coordinates of geodetic stations or benchmarks which provide the physical realisation of the Reference Frame. A satellite-defined Datum such as WGS84 may, in addition, be realised by the time-varying coordinates of the satellites themselves (the Ephemerides). Finally, the Datum may be defined only in the horizontal sense or for the vertical component.

An example of a Horizontal Datum is a Reference Ellipsoid (located and oriented in such a way as to be compatible to the Reference Frame to which it is attached), upon which coordinate information is expressed in terms of Latitude and Longitude.

(WGS84 has a Reference Ellipsoid associated with it.) A Vertical Datum may be defined by a local realisation of Mean Sea Level, or as height above the Reference Ellipsoid.

DGPS

DGPS is short for Differential GPS. Differential GPS is an extension of the GPS system that uses land-based radio beacons (Long Wave, UHF or R.D.S.) to transmit position corrections to GPS receivers.

DGPS reduces the effect of selective availability, propagation delay, etc. and can improve position accuracy to better than 10 meters.

Another example of DGPS is WAAS/EGNOS, which is using GPS satellites to broadcast position corrections. WAAS/EGNOS is less accurate the land based radio beacons, because a correction for a larger area has to be sent.

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.

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.

Dithering

The introduction of digital noise into the system. "Clock dithering" is the process by which the U.S. Department of Defense (DoD) degrades the accuracy of the Standard Positioning Service (i.e. absolute positioning of a C/A-Code capable receiver).

"Clock dithering" is the additional satellite clock "bias" induced by the DoD's "Selective Availability" policy that cannot be corrected for by the broadcast Navigation Message clock correction parameters.

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.

Double-Difference

A data processing procedure by which the pseudo-range or carrier phase measurements made simultaneously by two GPS receivers are combined so that, for any measurement epoch, the observations from one receiver to two satellites are subtracted from each other (in a so-called "between-satellite single-difference") to remove that receiver's clock error (or bias).

(Similarly for the other receiver's observations to the same two satellites.) Then the two single-differences are subtracted so as to eliminate the satellite clock errors as well as to reduce significantly the effect of unmodelled atmospheric biases and orbit errors. (The order may be reversed, i.e., take "between-receiver single-differences" to each satellite in turn, and then difference between the single-differences.) The resulting set of Double-Differenced observables (for all independent combinations of two-satellite-two-receiver combinations) can be processed to solve for the baseline (linking the two receivers) components and, in the case of ambiguous carrier phase measurements, the integer ambiguity parameters.

All high precision positioning techniques use some form of Double-Difference processing: pseudo-range, unambiguous carrier phase within a "bias-fixed" solution (i.e., after the double-differenced ambiguity values have been estimated and applied to the original carrier measurements), or ambiguous carrier phase data within a "bias-free" solution.

Dual-Frequency

Refers to the instrumentation that can make measurements on both L-Band frequencies, or to the measurements themselves (e.g., L1 and L2 pseudo-range or carrier phase measurements).

Dual-frequency measurements are useful for high precision (pseudo-range-based) navigation because the Ionospheric Delay bias can be determined, and the data corrected for it.

In the case of Double-Differenced carrier phase, dual-frequency observations can account for the residual ionospheric bias (for case of long baselines), or aid Ambiguity Resolution for "rapid static" or "kinematic" baseline determination. All "top-of-the-line" GPS receivers are of the dual-frequency variety, and are comparatively expensive because of the special signal processing techniques that must be implemented to make measurements on the L2 carrier under the policy of Anti-Spoofing.

Dynamic Positioning

See Kinematic Positioning

EGNOS

EGNOS stands for European Geostationary Navigation Overlay Service. Like WAAS in North America, EGNOS is an enhancement to GPS in Europe, that can allow specially equipped GPS receivers to more accurately calculate their position.

EGNOS uses a network of ground-based stations that compare their precisely known location with locations calculated from GPS satellite signals. Any differences found can be used to create correction data that's broadcast from EGNOS satellites.

EGNOS can help correct for error caused by distortion of GPS signals as they pass through the ionosphere as well as clock and orbital variations associated with individual GPS satellites.

EPE

Estimated Position Error. The estimated position error in meters calculated by the GPS receiver based on the GPS satellite positions.

Ephemeris

The file of values from which a satellite's position and velocity (the so-called "satellite state vector") at any instant in time can be obtained.

The "Broadcast Ephemeris (or Ephemerides)" for a satellite are the predictions of the current satellite position and velocity determined by the Master Control Station, uploaded by the Control Segment to the GPS satellites, and transmitted to the user receiver in the Data Message. "Precise Ephemeris (or Ephemerides)" are post-processed values derived by, for example, the International GPS Service (IGS), and available to users post-mission via the Internet.

Ephemeris Errors

Errors (or "biases") which are present in the (Broadcast or Precise) Ephemeris data. Broadcast Ephemeris errors are typically at the few metre level, while Precise Ephemeris errors are at the decimetre-level.

Ephemeris errors are largely mitigated by differential correction (in DGPS Positioning) or in double-differenced observables (formed from carrier phase measurements) when the receivers are not up to a few tens of kilometres apart.

In very high precision applications and/or where the baseline lengths are hundreds or thousands of kilometres, residual Ephemeris Errors may limit the accuracy of the baseline solution.

EPN

EUREF Permanent Network. European network of 140 permament GPS reference stations in more then 30 European countries.

Estimated-Time-Enroute (ETE)

The time left to your destination at your present speed. Typically used for navigation applications.

Estimated-Time-of-Arrival (ETA)

The time of day of your arrival at your destination. Typically used for navigation applications.

ETRS

European Terrestrial Reference System. European Reference System based on the ITRS system. Also called ETRS89.

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.

Galileo

Europe's own satellite navigation system, also known as GNSS (Global Navigation Satellite System). The system is expected to work in 2012.

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.

Geofence

A geofence is a virtual boundary around a geographical space.

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.

Geoid

The fundamental surface in Geodesy. It is defined as the equipotential surface of the gravity field that most closely approximates the Mean Sea Level.

(The MSL deviates from the Geoid surface by 1-2 metres due to the Sea Surface Topography caused by wind-driven or geostrophic currents.) The Geoid is the Vertical Datum surface both from a mathematical viewpoint (i.e., the sum of the Orthometric Height and the Geoid Height equals the Ellipsoidal Height of a point), as well as in practice by making the land height system synonymous with "height above MSL".

Models of the Geoid Height have been determined from the combined processing of satellite-derived potential models, surface gravity observations and the ocean gravity anomalies derived from Satellite Altimetry. Their accuracy may range from a few metres in the open ocean areas, down to the few decimetre level in land areas where there is a good coverage of surface gravity.

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.

GIS

Geographic Information System

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 Orbiting Navigation Satellite System (GLONASS)

This is the Russian counterpart to GPS. It consists of a constellation of 24 satellites (though the number may vary due to difficulties in funding for the system) transmitting on a variety of frequencies in the ranges from 1597-1617MHz and 1240-1260MHz (each satellite transmits on two different L1 and L2 frequencies).

GLONASS provides worldwide coverage, however, its accuracy performance is optimised for northern latitudes, where it is better than GPS's SPS (there being no "Selective Availability" on GLONASS satellites). GLONASS positions are referred to a different Datum to those of GPS, i.e. PZ90 rather than WGS84.

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.

GLONASS

Global Navigation Satellite System. GLONASS is the Russian global positioning system that's similar to the United States' Navstar GPS system. The first satellites for GLONASS were launched in the early 1980s. Today, GLONASS has fewer satellites and is not as widely used as the United States' GPS system.

GPS

Global Positioning System. A global navigation system based on 24 or more satellites orbiting the earth at an altitude of 12,000 statue miles and providing very precise, worldwide positioning and navigation information 24 hours a day, in any weather. Also called the NAVSTAR system.

GPS Surveying

Conventional static GPS surveying has the following characteristics:

• (1) The points being coordinated are not moving, i.e. they are "static".
• (2) GPS data are collected over some "observation session", typically ranging in length from an hour to several hours (or perhaps days for very precise GPS Geodesy applications).
• (3) The results are not required immediately, for in-the-field use.
• (4) The relative positioning mode of operation is the only mode employed, requiring the use of a minimum of two GPS receivers for all survey work.
• (5) The measurements used for data reduction are those made on the transmitted L-Band carrier wave, requiring specialised hardware and software.
• (6) A variety of processing algorithms can be employed, including "bias-free" and "bias-fixed" solutions.
• (7) Mostly associated with the traditional surveying and mapping functions.

GPS Time (GPST)

GPST is a form of Atomic Time, as is, for example, Coordinated Universal Time (UTC). GPST is "steered" over the long run to keep within one microsecond of UTC.

The major difference is that while "leap seconds" are inserted into the UTC time scale every 18 months or so to keep UTC approximately synchronised with the earth's rotational period (with respect to the sun), GPST has no leap seconds. At the integer second level, GPST matched UTC in 1980, but because of the leap seconds inserted since then, GPST is now (end 1998) ahead of UTC by 12 seconds (plus a fraction of a microsecond that varies from day to day).

The relationship between GPST and UTC is transmitted within the Navigation Message.

GPX

GPS Exchange Format. XML format used to exchange GPS data (track, waypoints and routes) between GPS mapping software programs.

Grid

A map coordinate system that projects the surface of the earth onto a flat surface such as a "map", using square zones for position measurements.

Common map grids include that defined by the UTM (Universal Transverse Mercator) projection.

Ground Speed

The velocity you are travelling relative to a ground position.

Typically measured in "knots" (nautical miles per hour), but may be expressed in km/hr or m/s.

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.

Height (Orthometric)

The Orthometric Height is the height of a station on the earth's surface, measured along the local plumbline direction through that station, above the Geoid surface. It is approximated by the "Height Above Mean Sea Level", where the MSL Datum is assumed to be defined by the mean tide gauge observations over several years.

The relationship between Orthometric Height (H) and Ellipsoidal Height (h) is : h = H + N, where N is the Geoid Height or Geoid Undulation with respect to the Reference Ellipsoid.

Orthometric Height is traditionally derived from geodetic levelling (using such techniques as optical levelling, trigonometrical levelling, barometric levelling).

Independent Baseline

These are baselines observed using GPS Relative Positioning techniques which are the minimum necessary to transfer the Datum from one Base Station to all other stations within a ground network. For example, if there are M stations, there will be M-1 independent baselines linking all the stations.

Any extra baselines that are measured are "redundant" baselines which may improve the quality and reliability of the station coordinates after Network Adjustment.

International GPS Service (IGS)

An initiative of the International Association of Geodesy, as well as several other scientific organisations, that was established as a service at the beginning of 1994. The IGS comprises of many component civilian agencies working cooperatively to operate a permanent global GPS tracking network, to analyse the recorded data and to disseminate the results to users via the Internet. The range of "products" of the IGS include precise post-mission GPS satellite ephemerides, tracking station coordinates, earth orientation parameters, satellite clock corrections, tropospheric and ionospheric models.

Although these were originally intended for the geodetic community as an aid to carrying out precise surveys for monitoring crustal motion, the range of users has since expanded dramatically, and the utility of the IGS is such that it is vital to the definition and maintenance of the International Terrestrial Reference System (and its various "frame realisations" ITRF92, ITRF94, ITRF96, etc.).

International Terrestrial Reference System (ITRS)

The most precise, geocentric, globally-defined coordinate system or datum on the earth's surface. It is a more accurate and more convenient a Satellite-Based Datum than the WGS84 Datum.

The various "frames" (such as ITRF96, etc.) are realisations of the ITRS for a particular epoch in time, consisting of a set of 3-D coordinates and velocities for hundreds of geodetic stations around the world (all coordinates of fixed stations on the earth change with time due to "continental drift").

Although some of the stations are Satellite Laser Ranging (SLR) stations, or Very Long Baseline Interferometry (VLBI) stations, the vast majority are GPS tracking stations of the IGS network.

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 cause problems during the process of rapid Ambiguity Resolution when phase and range combinations other than the Ionosphere-Free one are used.

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)

with a1 = f12f12 - f22 and a2 = - f1f2f12 - f22 , f1 and f2 are the frequencies of the L1 and L2 carrier waves respectively. (A similar expression can be developed for the ionosphere-free L2 carrier phase.) The ionosphere-free pseudo-range combination (in metric units) is: Pion-free = b1.P(L1) + b2.P(L2)

with b1 = f12f12 - f22 and b2 = - f22f12 - f22.

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.

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.

Latitude

A position's distance north or south of the equator, measured by degrees from zero to 90. One minute of latitude equals one nautical mile.

Layer

In mapping, a layer is a collection of features with the same feature type. An example of a layer, is a collection of line features (ways, streams, contours), a collection of area features (lakes, towns, land coverage) or a collection of points (cities, sightings, heights).

L-Band

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

Local Area Augmentation System (LAAS)

Plan by which Local Area Differential GPS (LADGPS), which generates and transmits differential corrections to appropriately equipped aircraft users, is augmented with integrity messages transmitted from the ground and additional ranging signals.

LAAS is set up near a major airport, and consists of the DGPS reference station, the integrity monitoring receiver and a pseudolite which transmits "satellite-like" PRN-coded signals to incoming aircraft.

Longitude

The distance east or west of the prime meridian (measured in degrees). The prime meridian runs from the north to south pole through Greenwich, England.

Map Feature

A map feature is a representation of a real world object on a map. Features can be represented as points, polygons, lines, texts, or a cell or pixel in a raster format. A feature can contain coordinates and attributes (optional).

Mask Angle

See Cutoff Angle.

Minimally Constrained

A form of least squares solution in which the observed baseline vectors are treated as "observations" in a secondary network adjustment (see Network Adjustment), and only one coordinate must be held fixed to its known value while all others are allowed to adjust. Typically GPS surveys measure more baselines than the minimum needed to coordinate all the points in the network.

These extra "observations" are redundant information that a minimally constrained network adjustment uses to derive optimum estimates of the coordinate parameters, as well as valuable quality information in the form of parameter standard deviations and error ellipses (or ellipsoids).

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.

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.

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.

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.

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.

Network Adjustment

A form of least squares solution in which the observed baseline vectors are treated as "observations" in a secondary adjustment (see Minimally Constrained).

It may be a minimally constrained network adjustment with only one station coordinate held fixed, or it may be constrained by more than one fixed (known) coordinates. The latter is typical of a GPS survey carried out to densify or connect some newly coordinated points to a previously established control or geodetic framework (see Datum).

NMEA

National Marine Electronics Association. An U.S. standards committee that defines data message structure, contents, and protocols to allow the GPS receiver to communicate with other pieces of electronic equipment aboard ships. Also called NMEA-0183. The latest version of NMEA is called NMEA2000 which uses a bus system instead of serial communications.

NTRIP

Networked Transport of RTCM via Internet Protocol. Industry standard on how to transmit RTCM data over an IP based network.

OEM

Original Equipment Manufacturer. Typically GPS receiver "boardsets" or "engines" that a product developer can embed within some application or hardware package.

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.

Order of Survey

In an analogous manner to "Class of Survey", Order of Survey is a means of categorising the quality, or precision, of a static survey. However, it relates to the external quality, and is influenced by the quality of the "external" network information. The number of categories, the notation applied, and the accuracy tolerances defining the transition from one order to another are defined by individual nations. Typically they mirror the categories of Class of Survey, hence an A Class survey may correspond to a 1st Order survey. The labeling of a particular Order (e.g. 1st, 2nd, etc.) to a survey points within a "network" (whether it is carried out using GPS or any other technique) is performed as part of the process of Network Adjustment in which the relative error ellipses (in the horizontal case) between coordinated stations are computed and compared with the accuracy standards that must be met for various categories of Order.

However, unlike the Minimally Constrained Network Adjustment that is a prerequisite to establishing the Class of Survey, the Network Adjustment must be constrained to the surrounding geodetic control. Hence a very high quality GPS network (therefore a high Class survey) may be distorted to "fit" the existing control which may have been determined using a lower Class survey.

The resulting Order of the Survey would have to match the lower of either the Class of the GPS survey or the Class of the existing geodetic control. If the existing geodetic control is of a lower quality to what can be achieved using modern GPS Surveying techniques, then the geodetic control network must be upgraded or "renovated" using more precise GPS Geodesy techniques.

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).

P-Code

The Precise or Protected code. A very long sequence of PRN binary biphase modulations on the GPS L1 and L2 carrier at a chip rate of 10.23MHz, which repeats about every 267 days.

Each one week segment of this code is unique to a GPS satellite and is reset each week. Under the policy of "Anti-Spoofing" the US Dept. of Defense has encrypted the P-Code (replacing it with a so-called Y-Code). Only US military and other authorised users are able to overcome AS using special receivers.

Phase-Smoothed Pseudo-Range

The pseudo-range measurement which has had its "noise" level (random errors) reduced by being combined with the high precision carrier phase. It is still an unambiguous "range" measurement which can be processed using the standard algorithms of Point Positioning or Relative Positioning.

Point Positioning

See Absolute Positioning.

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.

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.

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.

Pseudo-Random Noise (PRN)

A binary signal with random noise-like properties. It is generated by mathematical algorithm or "code", and consists of repeated pattern of 1's and 0's. This binary code can be modulated on the GPS carrier waves using Binary Shift-Key (BSK) modulation. The C/A-Code and the P-Code are examples of PRN codes.

Each satellite transmits a unique C/A-Code and P-Code sequence (on the same L1 and L2 frequencies), and hence a satellite may be identified according to its "PRN number", e.g. PRN2 or PRN14 are particular GPS satellites.

Pseudo-Range

A distance measurement based on the correlation of a satellite's transmitted code (may be the C/A-Code or the encrypted P-Code) and the local receiver's reference code (for that PRN satellite number), that has not been corrected for errors in synchronisation between the transmitter's clock and the receiver's clock. Hence a pseudo-range measurement is a time-error biased distance measurement.

The precision of the measurement is a function of the resolution of the code, hence C/A-Code pseudo-range measurements may have a "noise" at the few metre level for standard GPS receivers (and at the sub-metre precision level in the case of so-called "narrow correlator" GPS receivers).

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.

Radio Technical Committee for Maritime Applications (RTCM)

RTCM Special Committee 104 has developed standard message types for use by differential GPS transmitting stations.

The message content has been defined and hence when the RTCM-104 standard (version 2.2 is the latest) is implemented within a user receiver, it is able to decode and apply the DGPS corrections to its raw data in order to generate a DGPS-corrected coordinate.

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.

Real-Time DGPS

A Base Station computes, formats, and transmits pseudo-range corrections via some sort of data communication link (e.g., VHF or UHF radio, cellular telephone, FM radio sub-carrier or satellite com link).

The roving receiver requires some sort of data link receiving equipment to receive the transmitted DGPS corrections so that they can be applied to its current observations. Most GPS receivers are so-called "RTCM-capable", which means that they can accept industry standard DGPS correction messages if the real-time data link is provided.

Receiver Autonomous Integrity Monitoring (RAIM)

A form of receiver self-checking in which redundant pseudo-range observations are used to detect if there is a problem or "failure" with any of the measurements -- only four measurements are needed to derive 3-D coordinates and the receiver clock error, hence any extra measurements can be used for checking. Once the failed measurements have been identified they may be eliminated from the navigation fix.

RAIM is a concept that has been introduced by aviation users who are concerned that GPS does not have the level of Integrity necessary for non-precision airport approaches or GPS-aided landing.

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).

RINEX

Receiver Independend Exchange format. Raw data used to post process data received from a GPS receiver. All satellite information (timings, positions etc) is stored in this file.

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.

RS-232

A serial communications standard to transfer data between various communication equipment. This protocol is used by PC serial ports.

RTCM

Radio Technical Commision for Maritime Service. Standard used to exchange data between GPS devices and correction signal receivers or datalinks.

RTK

Real Time Kinematic. GPS method where a base station or datalink is used which sends corrections to the mobile GPS receiver. Using this method it is possible to measure positions realtime and with very high accuracy.

SA

Selective Availability. Selective Availability Selective Availability is a feature of the U.S. GPS system that enables the Department of Defense (DOD) to purposely degrade the accuracy of signals that are used for civilian purposes. This system is no longer active after it has been turned off in the year 2000.

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.

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.

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.

Standard Positioning Service (SPS)

The civilian Absolute Positioning accuracy obtained by using the pseudo-range data obtained with the aid of a standard single-frequency C/A-Code GPS receiver. Under "Selective Availability" the horizontal accuracy is stated to be 100m 2drms (or 95% of the time).

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).

Stop-and-Go Positioning

This is a GPS Surveying "high productivity" technique which is used to determine centimetre accuracy baselines to static points, using site observation times of the order of 1 minute.

Only carrier phase that has been converted into unambiguous "carrier pseudo-range" is used, necessitating that the ambiguities be resolved BEFORE the survey starts (and again at any time the satellite tracking is cut, e.g. due to signal obstructions).

It is known as the "stop & go" technique because the coordinates of the receiver are only of interest when it is stationary (the "stop" part), but the receiver continues to function while it is being moved (the "go" part) from one stationary setup to the next. As the receiver must track the satellite signals at all times, hence the transport of the receiver from one static point to another must be done carefully.

strItem

strDescription

Topology

Topology is the mathematics of relationships. In the world of GIS, it refers to such relationships as adjacency (e.g. two polygons next to each other), connectivity (i.e. line joins) and spatial overlay. We can overlay a polygon layer (e.g. park) with a distribution of animal sightings (e.g. points) and determine how many and which sightings occur within the park.

Track (TRK)

The direction of movement relative to a ground position. Commonly associated with navigation applications.

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.

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.

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)./br>
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.

TTFF

Time to First Fix (TTFF) describes the time and process required for a GPS device to acquire enough usable satellite signals and data to provide accurate navigation.

User Segment

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

UTC (Coordinated Universal Time)

Formerly referred to as GMT or Greenwich Mean Time. This is the basis of "civilian time".

UTM

Universal Transverse Mercator. UTM is a map projection based on the Transverse Mercator projection which can be used anywhere around the globe. To use the UTM projection the globe is divided in 60 zones. These zones are 6┬░ wide and are stepped along the equator such that each zone corresponds to a north-south strip of the earth.

Velocity-Made-Good (VMG)

The speed you are closing in on a destination along a desired course. A navigation term.

VRS

Virtual Reference System

WAAS

Wide Area Augmentation System. WAAS is an enhancement to GPS in North America that can allow specially equipped GPS receivers to more accurately calculate their position.

WAAS uses a network of ground-based stations that compare their precisely known location with locations calculated from GPS satellite signals. Any differences found can be used to create correction data that's broadcast from WAAS satellites.

WASS can help correct for error caused by distortion of GPS signals as they pass through the ionosphere as well as clock and orbital variations associated with individual GPS satellites.

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.

Y-Code

The term used to refer to the encrypted P-Code, generated within the satellites and transmitted on both the L1 and L2 carrier signals under the policy of "Anti-Spoofing".

Civilian GPS receivers use proprietary signal processing techniques to make measurements of pseudo-range and carrier phase on both L-Band frequencies.

Zero Baseline

A Zero Baseline test can be used to study the precision of receiver measurements (and hence its correct operation), as well as the data processing software. The experimental setup, as the name implies, involves connecting two GPS receivers to the same antenna.

When two receivers share the same antenna, biases such as those which are satellite (clock and ephemeris) and atmospheric path (troposphere and ionosphere) dependent, as well as errors such as multipath CANCEL during data processing.

The quality of the resulting "zero baseline" is therefore a function of random observation error (or noise), and the propagation of any receiver biases that do not cancel in double-differencing.