A Glossary of GPS Terms

[A - C] [D - H] [I - Q] R S T U V W X Y Z

- R -

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 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.

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 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.

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.

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.

- S -

Satellite Constellation

See Constellation, or Space Segment.

Selective Availability (SA)

Intentional degradation of the Absolute Positioning performance capabilities of the NAVSTAR satellite system for civilian use (the Standard Positioning Service) by the U.S. military, accomplished by artificially "dithering" the clock error in the satellites. Has generally been mitigated through the use of Relative Positioning techniques. SA was activated on 25 March 1990, and was removed on the 1st May 2000 (midnight Washington D.C. time).

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.

- T -

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). 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.

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.

- U -

UTC (Coordinated Universal Time)

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

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° wide and are stepped along the equator such that each zone corresponds to a north-south strip of the earth.

User Segment

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

- V -

Velocity-Made-Good (VMG)

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

- W -

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.

- X -

XTE

See Crosstrack Error.

- Y -

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.

- Z -

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.