A Glossary of GPS Terms

[A - C] D E F G H [I - Q] [R - Z]

- D -

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.

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.

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-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.

Doppler Shift

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

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

- E -

Ephemeris (plural: Ephemerides)

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.

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.

- F -

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.

- G -

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.

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.

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.

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.

Since the late 1980's considerable attention has been paid to the first three points, as they were considered to be unnecessarily restrictive for typical GPS surveying applications. As a result of vigorous R&D, new GPS surveying methodologies have been developed, which complement the "conventional static" technique. These modern GPS Surveying techniques are given a variety of names but the following are considered generic: (a) rapid static positioning techniques, (b) "stop & go" techniques, and (c) "on-the-fly" positioning techniques.

Each of the techniques represents a technological solution to the problem of obtaining high productivity (measure as many baselines in as short a period of time as possible) and/or versatility (for example, the ability to obtain results even while the receiver is in motion) without sacrificing very much in terms of accuracy and reliability. None of these techniques is as accurate or reliable as conventional static GPS surveying, and each of these techniques has its special strengths and weaknesses. They represent the state-of-the-art in precision carrier phase-based GPS positioning, and are a direct outcome of considerable innovation by instrument manufacturers seeking to address surveying and non-surveying applications.

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.

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.

- H -

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).