GPS Glossary

A - B C - D E - F G - I J - M N - P Q - S T - Z

2D Operating Mode

A two-dimensional GPS position calculation based only on horizontal coordinates (no GPS elevation). Requires a minimum of three visible satellites.

3D Operating Mode

A three-dimensional GPS position calculation based on horizontal coordinates and elevation. It requires a minimum of four visible satellites.

A - B

Accuracy

A measurement of how precise a GPS location is relative to the actual location.

Active Antenna

The antenna that amplifies the radio signal before sending the signal to the receiver.

Almanac Data

Satellite positions data. As each satellite broadcasts the position information for every satellite in the system, the receiver stores the information so it can determine it's own location. After the GPS unit is turned on, almanac data allows the receiver to quickly obtain satellite signals.

Altimeter

An instrument for determining elevation distances above sea level. Atmospheric pressure decreases as elevation increase; most GPS receivers measure pressure changes and relate this information to height above sea level.

Analog Signal

Analog signals are continuous where as digital signals consist of values measured at separate intervals.

Anti-Spoofing

The GPS unit can confuse real satellite signals with imitated transmitted signals. Anti-spoofing encrypts the P-code to protect the P-signals so that only authorized users can recognize it.

Azimuth

The horizontal direction of travel from one point on the earth to another in reference to true/magnetic north. The azimuth is measured clockwise in degrees (0-360) from a north or south reference line. See Bearing.

Base map

Some GPS units come equipped with a permanant installed basemaps. Coverage includes water bodies, urban centers, road systems, airports and political boundaries. Depending on the user's requirements, basemaps are available for many world wide regions.

Bearing

Direction of travel or direction between two points. A bearing is measured from the cardinal directions of north or south in reference to true/magnetic north. However, the bearing measurement is never over 90 degrees. Bearing usually refers to the direction to a waypoint in a GPS receiver. See Azimuth.

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C - D

Cartography

The art or science of map or chart making. Many GPS receivers have detailed cartography/mapping capabilities.

Coarse/Acquisition Code (C/A Code)

The standard code used by civilian units to determine position. CA codes are transmitted only on one frequency, therefore, the delay through the ionosphere is impossible to detect with civilian units. The CA code has the information the GPS receiver requires to fix its position and time, and is accurate to approximately 100 meters. The accuracy present with CA codes is called the Standard Positioning Service (SPS).

Cold Start

A cold start occurs when a receiver has to download almanac information before it's own position can be calculated. See Time to First Fix.

Control Segment

Control stations that monitor and manage GPS satellites.

Coordinates

A set of numbers (and/or letters) that describes your location within a spatial reference system. Coordinates are typically based on latitude/longitude lines or global/regional grid projections such as UTM, MGRS, and Maidenhead.

Course

Path between two points or the direction from a waypoint to the next waypoint in the route segment. This is measured in degrees, radians, or mils.

Course Deviation Indicator (CDI)

A function of the GPS unit that indicates the amount and direction of CrossTrack Error (XTE).

Course Made Good (CMG)

The bearing from your starting position to your present position.

Course Over Ground (COG)

The direction of movement relative to the ground position.

Crosstrack Error (XTE/XTK)

The distance you are off the desired course in either direction.

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Datum

The vertical or horizontal reference system associated with particular coordinates and elevations. Consists of a model of the earth (ellipsoid) and a physical location chooses as the origin point. Latitude and longitude lines are referenced to a specific map datum. The GPS receiver map datum must match the datum on the corresponding paper maps in order for position readings to match.

Desired Track (DTK)

The compass course between the "from" and "to" waypoints.

Differential GPS (DGPS)

A method using land-based radio signals broadcast from ground stations to transmit positions corrections to GPS receivers to improve position accuracy. DGPS overcomes Selective Availability.

Dilution Of Precision (DOP)

Analysis of satellite geometry and the impact on accuracy. The receiver estimates the amount of position calculation error using measurements based on several factors that reduce accuracy. Better relative geometry and higher corresponding accuracy result in a low DOP (values between 1-3). A high DOP (4 and above) indicates an increasing more error in the position indicated. The DOP indicators are GDOP (geometric DOP), PDOP (position DOP), HDOP (horizontal DOP), VDOP (vertical DOP), and TDOP (time clock offset). See Position Dilution of Precision.

Distance

The length measurement in feet, meters, miles, etc.) between two waypoints (e.g. current position to a destination waypoint). This distance is measured in straight-line (rhumb line) or great-circle (over the earth) terms. GPS normally uses great circle calculations for distance and desired track.

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E - F

Easting

The distance of a point east or west of a zone meridian. Used in several grid coordinate systems: UTS, OSBG, MGRS, etc.

Elevation

The measurement of height (expressed in feet or meters), above or below mean sea level.

Ellipsoid

A geometric surface, all of whose plane sections are either ellipses or circles. Used to describe the surface of the earth at sea level.

Ephemeris Data

The current satellite position and timing data transmitted as part of the satellite data message. A set of ephemeris data is valid for several hours.

Estimated Position Error (EPE)

The amount of horizontal error that may be present in position calculation. Based upon DOP and satellite geometry.

Estimated Time Enroute (ETE)

The time remaining to reach your arrival destinate from your current position, speed and course. Calculated in hours/minutes or minutes/seconds.

Estimated Time Of Arrival (ETA)

The estimated time you will arrive at a destination waypoint. Based on current position, speed and course.

Frequency

The number of repetitions per unit time of a complete waveform, as of a radio wave.

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G - I

Geodetic Datum

A math model depicting the shape and size of the earth or portions of the earth.

Geographic Information System (GIS)

A class of computer programs and applications used to organize, analyze and display spatial (geographic) data.

Global Positioning System (GPS)

A global satellite-based navigation system used to calculate position and navigation information for any location on earth. Also called the NAVSTAR system.

Glonass

The Global Orbiting Navigational Satellite System. The Russian equivalent to the United States’ GPS system.

GoTo

A route comprised of one leg, with the receivers' present position being the starting point of the route and a single waypoint as the destination.

Goto Function

A mode of operation where the receiver guides the unit to a previously stored destination. The receiver will display a screen that directs the user to the destination.

Greenwich Mean Time (GMT)

Time measured from Greenwich, England or 0 degrees longitude (Prime Meridian). Used as the basis for calculating standard time throughout most of the earth.

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Heading

The direction you are travelling usually expressed as magnetic or true north. For air and sea operations, this may differ from actual Course Over Ground (COG) due to winds, currents, etc.

Initialization

The power-on process the GPS goes through to to collect almanac data and establish its current location. This occurs when the receiver has lost memory, moved more than a few hundred kilometers, lost the correct UTC time or has not been in use for more than half a year.

Ionosphere

A area of the earth's atmosphere where ionization caused by incoming solar radiation affects the transmission of GPS radio waves. The ionosphere extends from a height of 50 kilometers (30 miles) to 400 kilometers (250 miles) above the surface.

Invert Route

Used to display and navigate a route in reverse from the destination to the beginning point.

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J - M

L1 Frequency

One of the two radio frequencies transmitted by the GPS satellites. The Coarse Acquisition Code (C/A code) is carried on this frequency (1575.42 MHz).

L2 Frequency

One of the two radio frequencies transmitted by the GPS satellites. The P-Code is carried on this frequency (1227.6 MHz).

L Band

The radio frequencies between 390 MHz to 1550 MHz. The GPS carrier frequencies are in the L band (L1 at 1575.42 MHz and L2 at 1227.6 MHz).

Latitude

An angular distance measured in degrees (0 to 90) north or south of the equator. One minute of latitude equals one nautical mile.

Leg (Route)

A portion of a route consisting of one waypoint within the original route to another waypoint in the route. For example, a route made from waypoints A, B, C, and D would contain three legs. The route legs would be from A to B, from B to C, and from C to D.

Local Area Augmentation System (LAAS)

The implementation of ground-based DGPS to support aircraft landings in a local area (20-mile range).

Longitude

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

LORAN

LOng RAnge Navigation. Comprised of a grid of radio waves in many areas of the globe that allows accurate position plotting. LORAN transmitting stations around the globe continually transmit 100 kHz radio signals. Shipboard LORAN receivers interpret the signals and provide readings that correspond to a grid overprinted on nautical charts. By comparing signals from two different stations, the mariner can use the grid to determine vessel position.

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Magnetic North

The direction to which a compass points. Represents the direction of the north magnetic pole from an observers' location.

Magnetic Variation

The horizontal angle (or difference) between true north and magnetic north. Magnetic variation is measured east or west of true north.

Map Datum

The reference point to which a map is drawn, for example, NAD 27 and WGS 84.

Military Grid Reference System (MGRS)

A metric based coordinate system that is subset to the UTM system. Used by the US military.

Multipath Error/Interference

An error caused when a satellite signal reaches the GPS receiver antenna by more than one path. The satellite signal is bounced off a reflective surface before reaching the receiver.

Multiplexing Receiver

A GPS receiver that switches at a very rapid rate between tracked satellites. Typically, multiplexing receivers require more time for satellite acquisition and are not as accurate as parallel channel receivers. Multiplexing receivers are also more prone to lose a satellite fix in dense woods than parallel channel GPS receivers.

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N - P

NAD 27

North American Datum of 1927. The datum used on most large scale USGS topographic maps.

National Marine Electronics Association (NMEA)

A U.S. standards organization that defines data message structure, contents, and protocols to allow the GPS receiver to communicate with other pieces of electronic equipment using the same NMEA.

Navigation

The process of establishing the course or heading of movement for all means of travel.

Navigation Message/Data Message

The message transmitted from each GPS satellite containing system time, clock correction parameters, ionospheric delay model parameters, and satellite ephemeris data and health. The information is used to process GPS signals to give the user time, position, and velocity.

NAVSTAR

NAVigation Satellite Timing and Ranging. The official U.S. Government name given to the GPS satellite system. Now referred to as Global Positioning System.

NMEA Standard

A NMEA standard defines an electrical interface and data protocol for communications between marine instrumentation.

Northing

The distance of a point north or south of a fixed reference point. The UTM used the equator as the reference point.

North Up Orientation

Fixes the GPS receiver map display so north is always displayed at the top of the screen.

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P-Code

A second signal sent by satellites over the L2 channel. This code is usually only used by the U.S. Military and is encrypted and reset every seven days to prevent use from unauthorized persons.

Position Dilution of Precision (PDOP)

A summary DOP indicator that shows horizontal, vertical and time related error all in one figure. See Dilution of Precision.

Position Fix

The GPS receiver's calculated position coordinates based on three satellite signals.

Position Format

The manner in which the position of a GPS receiver will be displayed on the screen. Most often displayed as latitude and longitude in degrees and minutes, with options for degrees, minutes and seconds, degrees only, or one of several grid formats.

Precise Positioning Service PPS

Full accuracy GPS services obtained by a receiver that has both C/A-codes and P-codes.

Prime Meridian

The line of latitude assigned a zero value; used as a reference line from which longitude east and west is measured. The Prime Meridian passes through Greenwich, England.

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Q - S

Quadrifilar Helix Antenna

A type of circular polarized antenna used in some GPS receivers. For GPS use, quadrifilar antennas are typically half-wavelength or quarter-wavelength size and encased in a plastic cylinder for durability.

Ranging

A method used in a GPS receiver to measure the receiver's distance to a satellite.

Re-acquisition

When a GPS receiver temporarily loses one or more satellite signals and must re-establish a satellite lock.

Route

A set of waypoints entered/stored into the GPS receiver in the sequence for desired navigation.

Route Function

A sequential list of waypoints used by the GPS receiver to guide you from each waypoint on the list to your destination. See Goto Function.

RS-232

A serial input/output standard that allows for communication between a receiver and a computer.

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Selective Availability (SA)

The random error, which the government can intentionally add to GPS signals, causes decreased position accuracy in civilian GPS receivers . SA is not currently in use.

Serial Communication

The sequential transmission of the signal elements of a group representing a character or other entity of data. The characters are transmitted in a sequence over a single line, rather than simultaneously over two or more lines, as in parallel transmission. The sequential elements may be transmitted with or without interruption.

Space Segment

The satellite portion of the complete GPS system.

Speed Over Ground (SOG)

The actual speed the GPS is travelling regardless of direction.

Spoofing

Method of rendering a GPS unit useless. The attacker transmits fake radio signal on the same frequency as satellite signals causing the unit to calculate an incorrect position. See Anti-Spoofing.

Straight-Line Navigation

The method of travelling from one waypoint to anther waypoint in the most direct line and with no turns.

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T - Z

Time To First Fix (TTFF)

The time a GPS receiver takes to find and receive satellite information (almanac data) used to determine the units location. Time to First Fix will occur if the receiver has been turned off for several months, lost memory or has been moved more than 480 km (300 miles). See Cold Start.

Track (TRK)

Your current direction of travel relative to a ground position. See Course Over Ground.

Triangulation

Using the laws of plane trigonometry the location of an unknown point can be determined. Used in GPS navigation.

True North

The direction of the north pole from your current position. The north pole is not the magnetic north. The variation between true north and magnetic north is called declination. GPS receiver can display headings referenced to true north or magnetic north.

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Universal Transverse Mercator (UTM)

A grid system that splits the earth into 60 sections each of which is 6 degrees wide. Coordinates, called eastings and northings, are relative to the equator and a zone meridian. UTM is the primary coordinate system used on U.S. Geological Survey topographic maps.

Uplink

A transmission course by which signals are sent from the ground to an aircraft or a communications satellite.

User Interface

The method through which information is exchanged between the GPS receiver and the user. This is done through the screen display and the GPS receiver's buttons.

User Segment

The segment of the complete GPS system that includes the GPS receiver and operator.

Velocity Made Good (VMG)

The rate at which a GPS receiver is closing in on a destination; based on current speed and course.

WAAS (Wide Area Augmentation System)

System of satellites and ground stations that provide GPS corrections. Increases position accuracy to within 3 meters approximately 95 percent of the time. The system comprises of 25 ground towers across the United States that monitor GPS signals. Master stations located on each coast create GPS corrections from data collected from the ground towers. Currently this system is still under development and is not available in all locations.

Waterproof

A GPS receiver is only listed as waterproof if it can remain submerged under water without being ruined. An IPX8 designation is for continuous underwater use.

Water Resistant

A water resistant GPS receiver can be used in a damp environment, however, the unit was not designed for complete submersion in water. An IPX7 designation means the GPS case can withstand accidental immersion in one meter of water for up to 30 minutes.

Waypoint

Coordinates of a location stored in a GPS receivers memory. Waypoints may be your present position or coordinated stored prior to your journey. These points may be check points along a route, land marks, ground features or any location you may later want to return to. Waypoint can be defined and manually stored into a GPS receiver using coordinates from a map or other form of reference. Waypoints can also be stored directly by taking a reading at the desired location of the unit and then saving the coordinated in the receiver. A previously stored waypoint may also be referenced to store a new waypoint.

WGS-84

World Geodetic System 1984. The primary internal datum of the NAVSTAR GPS system. Secondary datums are computed as differences from the WGS 84 standard.

Y-Code

The encrypted version of P-Code. See Anti-Spoofing

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