Glossary of GPS Terms

Our GPS glossary can be helpful whether you are new to GPS or just looking to brush up on some terminology. The jargon can be confusing, but this should make most of what you see and hear clearer.

A

Absolute Positioning

Positioning mode in which a position is identified with respect to a well-defined coordinate system, commonly a geocentric system (i.e., a system whose point of origin coincides with the center of mass of the Earth).

Almanac

A data file that contains orbit information on all satellites, clock corrections, and atmospheric delay parameters. It is transmitted by a GPS satellite to a GPS receiver, where it facilitates rapid satellite vehicle acquisition within GPS receivers. Almanac data must be acquired before GPS navigation can begin.

Anywhere Fix

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

Attribute

A characteristic that describes a feature. Attributes can be thought of as questions that are asked about the feature.

Availability

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

B 

Baseline

A baseline consists of a pair of stations for which simultaneous GPS data has been collected.

Base Station

Also called a reference station. A receiver is set up on a known location specifically to collect data for differentially correcting rover files. The base station calculates the error for each satellite and, through differential correction, improves the accuracy of GPS positions collected at unknown locations by a roving GPS receiver.

Bearing

The compass direction from position to a destination.

 

C

 C/A Code

The standard (Clear/Acquisition) GPS code, also known as the “civilian code” or S-code.

Carrier

A radio wave that has at least one characteristic (e.g., frequency, amplitude, phase) that can be varied from a known reference value by modulation.

Carrier Phase

GPS measurements based on the L1 or L2 carrier signal.

Carrier-Aided Tracking

A signal processing strategy that uses the GPS carrier signal to achieve an exact lock on the pseudorandom code. More accurate than the standard approach.

Circular Error Probable (CEP)

A statistical measure of horizontal precision. The CEP value is defined as a circle of a specified radius that encloses 50% of the data points. Thus, half the data points are within a 2D CEP circle, and half are outside the circle.

Clock Bias

The difference between the clock’s indicated time and true universal time.

Coarse Acquisition (C/A) Code

A group of pseudorandom noise codes that GPS satellites transmit. There are 32 codes in the group, and every satellite is assigned one. It comprises a 1023-bit pattern called a Gold Code that modulates the L1 signal.

Code Phase

GPS measurements based on the C/A code.

Constellation

Refers to either the specific set of satellites used in calculating positions or all the satellites visible to a GPS receiver at one time.

Control Point

Also called a control station. A monumented point to which coordinates have been or are being assigned by surveying observations. The National Geodetic Survey maintains a nationwide set of control points.

Control Segment 

A worldwide network of GPS monitoring and control stations that ensure the accuracy of satellite positions and their clocks.

Course Made Good (CMG)

The bearing from your starting point to your present position.

Crosstrack Error (XTE)

The distance you are off a desired course in either direction

Cycle Slip

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

D

Data Message

A 1500-bit message included in the GPS signal which reports the satellite’s location, clock corrections, and health.

Datalogger

Also called a data recorder. A handheld, lightweight data entry computer. It can be used to store additional data obtained by a GPS receiver.

Differential GPS (DGPS)

A technique to improve GPS accuracy that uses pseudo-range errors measured at a known location to improve the measurements made by other GPS receivers within the same general geographic area.

Differential Positioning

Precise measurement of the relative positions of two receivers tracking the same GPS signals.

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 a lower DOP.

Dithering

The introduction of digital noise. This is the process the Department of Defense (DoD) uses to add inaccuracy to GPS signals to induce Selective Availability.

Doppler-Aiding

A signal processing strategy that uses a measured Doppler shift to help the receiver smoothly track the GPS signal. This allows more precise velocity and position measurement.

Doppler-Shift

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

Dynamic Positioning

See Kinematic Positioning.

E

Ephemeris

The predictions of the current satellite position that are transmitted to the user in the data message.

Ephemeris Errors

Errors that originate in the ephemeris data transmitted by a GPS satellite. Ephemeris errors are removed by differential correction.

Estimated Time of Arrival (ETA)

The time of day of your arrival at your destination.

Estimated Time Enroute (ETE)

The time left to your destination at your present speed.

F

Federal Radionavigation Plan (FRP)

Congressionally mandated, joint Department of Defense (DoD) and Department of Transportation (DOT) efforts to reduce the proliferation and overlap of federally funded radio navigation systems. The FRP is designed to delineate policies and plans for U.S. government-provided radio navigation services.

Fix

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

G

Geodetic Surveys

Global surveys done to establish control networks (comprised of reference or control points) as a basis for accurate land mapping.

Geometric Dilution of Precision

See Dilution of Precision.

GIS (Geographic Information System)

A computer-based system capable of collecting, managing, and analyzing geographic spatial data. This capability includes storing and utilizing maps, displaying the results of data queries, and conducting spatial analysis.

Global Navigation Satellite System (GLONASS)

This is the Russian counterpart to GPS. GLONASS provides worldwide coverage. However, its accuracy performance is optimized for the northern latitudes and is specified as identical to that of GPS SPS.

Global Positioning System (GPS)

A system for providing precise locations which are based on data transmitted from a constellation of 24 satellites

Grid

Also, “Coordinate System”. A coordinate system that projects the Earth on a flat surface, using square zones for position measurements.

Ground Speed

The velocity you are traveling relative to a ground position.

I

I/O

Abbreviation for Input/Output.

K

Kinematic Positioning

Kinematic positioning refers to applications in which the position of a non-stationary object (automobile, ship, bicycle) is determined.

 

L

L1 Frequency

1575.42 MHz GPS carrier frequency which contains only encrypted P-code. It is used primarily to calculate signal delays caused by the ionosphere.

Latitude

A north/south measurement of position perpendicular to the Earth’s polar axis.

L-Band

The group of radio frequencies extending from 390 MHz to 1550 MHz. The GPS carrier frequencies L1 (15735 MHz) and L2 (1227.6 MHz) are in the L-band.

Longitude

An east/west measurement of position in relation to the Prime Meridian, an imaginary circle that passes through the north and south poles.

 

M

Mask Angle

The minimum acceptable satellite elevation above the horizon to avoid blockage of line-of-sight.

Multi-Channel Receiver

A GPS receiver that can simultaneously track more than one satellite signal.

Multipath

Interference is caused by reflected GPS signals arriving at the receiver, typically due to nearby structures or other reflective surfaces.

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.

Multiplexing Channel

A channel of a GPS receiver that can be sequenced through several satellite signals.

N

NAVSTAR

The name given to GPS satellites. NAVSTAR is an acronym for NAVigation Satellite Timing and Ranging.

O

OEM

Original equipment manufacturer.

P

P-Code

The Precise or Protected code. A very long sequence of pseudorandom binary biphase modulations on the GPS carrier at a chip rate of 10.23 MHz, which repeats about every 267 days. Each 1-week segment of this code is unique to one GPS satellite and is reset each week.

Position

The latitude, longitude, and altitude of a point. An estimate of error is often associated with a position.

Positional Dilution of Precision (PDOP)

Measure of the geometrical strength of the GPS satellite configuration.

Post-Processed Differential GPS

In post-processed differential GPS, the base and roving receivers have no active data link between them. Instead, each records the satellite observations that will allow differential correction at a later time. Differential correction software combines and processes the data collected from these receivers.

Precise Positioning Service (PPS)

The most accurate dynamic positioning possible with GPS, based on the dual frequency P-code.

Pseudo-Lite

A ground-based differential GPS receiver that transmits a signal like that of an actual GPS satellite and can be used for ranging.

Pseudo-Random Noise (PRN)

A signal with random noise-like properties. It is a complicated but repeated pattern of 1’s and 0’s.

Pseudo-Range

A distance measurement based on the correlation of a satellite transmitted code and the local receiver’s reference code that has not been corrected for errors in synchronization between the transmitter’s clock and the receiver’s clock.

R

Range

A fixed distance between two points, such as between a starting and an ending waypoint or a satellite and a GPS receiver.

Real-Time Kinematic (RTK)

The DGPS procedure whereby carrier phase corrections are transmitted in real-time from a reference station to the user’s roving receiver

Real-Time Differential GPS

A base station that computes, formats, and transmits corrections, usually through a data link (e.g., VHF radio or cellular telephone) with each new GPS observation. The roving unit requires some data link receiving equipment to receive the transmitted GPS corrections and get them into the GPS receiver so they can be applied to its current observations.

Relative Positioning

The determination of relative positions between two or more receivers simultaneously tracking the same GPS signals.

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 software package. The format includes definitions for three fundamental GOS observables: time, phase, and range.

Rover

Any mobile GPS receiver collecting data during a field session. The receiver’s position can be computed relative to another stationary GPS receiver.

 

S

Satellite Constellation

The arrangement in space of a set of satellites.

Selective Availability (S/A)

Intentional degradation of the performance capabilities of the NAVSTAR satellite system for civilian use by the U.S. military, accomplished by artificially creating a significant clock error in the satellites.

Space Segment

The space-based component of the GPS system (i.e., the satellites).

Standard Positioning Service (SPS)

The normal civilian positioning accuracy obtained by using the single-frequency C/A code.

Static Positioning

Location determination when the receiver’s antenna is presumed to be stationary on the Earth. This allows the use of various averaging techniques that improve the accuracy by factors of over 1000. 

T

Track (TRK)

The direction of movement relative to a ground position.

U

UTC

Universal Coordinated Time, formerly GMT or Greenwich Mean Time.

Universal Transverse Mercator (UTM)

A grid coordinate system that projects global sections onto a flat surface to measure position in specific zones.

User Segment

The component of the GPS system that includes the receivers.

V

Velocity Made Good (VMG)

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

 

W

WAAS

Wide Area Augmentation System is an FAA-funded system of equipment and software that augments GPS. The WAAS provides a satellite signal for WAAS users to support en route and precision approach aircraft navigation.

X

XTE

See “Crosstrack Error”