Use this:

to access this:

GG2170: GPS and GIS
Spring 2003; January 18/25
Dr. Rudnicki

This list contains words that directly relate to the Global Positioning System (GPS) and Global Positioning System radio signal receivers. These terms are differentiated from related terminology, as, say, navigational terms or surveying terms or geodesy terms or cartographic terms or geographical information (GIS) terms. Separate listings exist for those areas of human endeavor.

2D Mode:
- On GPS receivers, a two-dimensional position fix that includes only horizontal co-ordinates (longitude (x-coordinate) and latitude (y-coordinate) but not elevation (z-coordinate)). 2D mode requires a minimum of three visible satellites.

2DRMS Twice-distance Root Mean Square:
- A horizontal measure of accuracy representing the radius of a circle within which the true value lies at least 95% of the time. (see Confidence Region)

3D Operating Mode:
- On GPS receivers, a three-dimensional position fix that includes horizontal co-ordinates (longitude (X-coordinate), latitude (Y-coordinate)), plus elevation (Z-coordinate). 3D mode requires a minimum of four visible satellites.

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).

Accuracy:
- The degree of conformance between the estimated or measured position, time, and/or velocity of a GPS receiver and its true time, position, and/or velocity as compared with a constant standard. Radionavigation system accuracy is usually presented as a statistical measure of system error and is characterized as follows:
Predictable: The accuracy of a radio navigation system's position solution with respect to the charted solution. Both the position solution and the chart must be based upon the same geodetic datum.
Repeatable: The accuracy with which a user can return to a position whose co-ordinates have been measured at a previous time with the same navigation system
Relative: The accuracy with which a user can measure position relative to that of another use of the same navigation system at the same time

Acquisition Time:
- In general, the time it takes a GPS receiver to acquire satellite signals and determine the initial position.
More specifically, the amount of time required for a GPS unit to lock onto 3 satellites to provide a 2D fix of present position.

ACTIVE ANTENNA:
- An antenna assembly with active electronics such as a low noise amplifier or radio frequency filter that attempt to improve the GPS signal before sending it to the receiver.

Almanac:
- A data file that contains orbit and state (health) of the entire constellation, clock corrections, and atmospheric delay parameters. When received by a GPS receiver, the file allows the GPS receiver to rapidly acquire satellites as soon as it is turned on. Almanac data must be acquired before GPS navigation can begin. Each GPS satellite message contains and transmits the almanac data for all GPS satellites. (see Ephemeris) Information transmitted by each satellite contains not just orbit data but also the state (health) of the entire constellation. Almanac data allow the GPS receiver to rapidly acquire satellites as soon as it is turned on. Almanac data are available on the web from the U.S. Navy as well as the Coast Guard. The almanacs are published in two formats: sem and yuma.

AMBIGUITIES:
- The initial bias in a carrier-phase observation of an arbitrary number of cycles; the uncertainty of the number of complete cycles a receiver is attempting to count. If wavelength is known, the distance to a satellite can be computed once the total number of cycles is established via carrier-phase processing. For GPS signals, the wavelength of L1 is 190.3 mm, L2 is 244.2 mm.

ANALOG SIGNAL:
- A type of transmission characterized by continuous, variable waveforms representing information. In contrast, digital signals consist of values measured at discrete intervals. A standard clock with moving hands is an analog device. The human voice and audible sounds are analog. Modern computers are invariably digital, but when they communicate over telephone lines, their signals must be converted to analog using a modem (a modulator/demodulator). The analog signal is converted back into a digital from before delivering it to a destination. GPS satellites transmit data messages in analog form.

ANTENNA:
- Device to receive radio waves of a certain frequency or frequency range and convert them to an electric current.

Antenna Height:
- The distance of the antenna phase center above the ground or object being measured.

ANTI-SPOOFING:
- Encryption of the P-code to protect the P-signals from being "spoofed" and thus misleading authorized users through the transmission of false GPS signals by an adversary.

Anywhere Fix:
- The ability of a receiver to start position calculations without being given an approximation of location and time.

Atomic Clock:
- A very precise clock that operates using the elements Cesium or Rubidium. A Cesium clock has an error of one second per million years. GPS satellites contain multiple Cesium and Rubidium clocks.

Atomic Time:
- Time defined by the precisely known oscillations (vibration) of the atoms of a particular element accepted as an international standard.

Attenuation:
- The reduction of signal strength (loss in power) between a transmitter and receiver. Usually due to obstruction, reflection, or absorption of the GPS signal

Automatic Vehicle Location (AVL):
- A type of system using any sort of technology (such as GPS) to track or locate a vehicle.

Availability:
- In a navigation system, the proportion of time that navigation information is available (within defined accuracy limits); 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. Availability is a function of both the physical characteristics of the operational environment and the technical capabilities of the transmitter facilities.

Averaged Point:
- Point feature with coordinates derived from averaging a number of estimates of the position.

Bandwidth:
- The range of frequencies in a signal.

Base Station:
- Also called a reference station. A GPS receiver 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.

Baseline:
- In general, a line whose length has been calculated. The line's end points are a pair of stations for which simultaneous GPS data have been collected. More specifically, a three-dimensional vector or coordinate difference between two GPS antennas computed during carrier-phase GPS surveying. Code GPS surveying typically refers to the "baseline distance" between the base station and rover as "separation distance".

BEACON:
- Stationary transmitter that emits signals in all directions; also called a non-directional beacon. In DGPS, the beacon transmitter also broadcasts pseudorange correction data to nearby GPS receivers for greater accuracy.

Binary:
- A digital signal with only two states, usually represented by 0 and 1.

Binary Phase Shift Keying (BPSK):
- A form of modulation used by GPS satellites to superimpose information (PRN codes and message) on the GPS carrier wave.

Bit:
- A single unit of digital information.

Bit Rate:
- The speed of a digital transmission (measured in bits per second or bps).

Block I, II, IIR, IIF satellites:
- The various generations of GPS satellites: Block I were prototype satellites that were launched between Feb. 1978 & Oct. 1985, SVN 1-11, in circular orbits inclined 63 degrees to the equator and contained 1 cesium and 1 rubidium clock;
Block II, first generation of production GPS satellites (Feb. 1989-Oct. 1990), SVN 13-21 in 55 degree orbits.
Block IIA, second generation of production satellites (Nov. 1990-Nov. 1997), SVN 22-40. Block IIA satellites have some capability of keeping their orbits stable without input from the Ground Control Segment.
Block IIR are replenishment satellites and are third generation production satellites (Jan. 1997-Jan. 2001), SVN 41-62. Block IIR satellites have a capability for ranging between them for increased orbit accuracy and autonomous operation. This group of satellites contains 3 Rubidium clocks. Block II and IIA satellites contained 2 cesium and 2 rubidium clocks.
Block IIF refers to the follow-on (4th) generation. Block IIF satellites are still in the design and procurement stage.

Carrier:
- A radio wave supplying the energy upon which information may be carried. The information (Data) is transported by modifying the carrier. That is by changing its frequency, amplitude, phase or by interrupting it. This changing of the carrier signal is known as modulation.

Carrier-aided tracking:
- A signal processing strategy that uses the GPS carrier wave to achieve an exact lock on the pseudo random code. More accurate than standard approach.

CARRIER FREQUENCY:
- The frequency of an unmodulated output of a radio transmitter. The GPS L1 carrier frequency is 1575.42 MHz. The L2 carrier frequency is 1227.6 MHz.

Carrier Phase:
- GPS measurements based on the L1 or L2 carrier signal. A fundamental GPS observable. The fractional phase angle of the incoming carrier wave can be measured very accurately (few mm) although the absolute distance to the satellite cannot be measured.

Carrier phase GPS:
- GPS measurements based on the L1 or L2 carrier signal.

Carrier Phase Surveying:
- Surveying technique using phase observations on the GPS carrier wave as well as ambiguity resolution to achieve millimeter-level accuracy.

Carrier Signal:
- The smooth, stable frequency on which the ranging codes (C/A-code and P-code) is superimposed.

Carrier Smoothing:
- Using the precise carrier phase observable to smooth out noise in the coarse pseudorange observable. This technique is used in most high-end code phase receivers.

Carrier to Noise Ratio (C/N):
- The ratio of received carrier power to noise power over a given bandwidth.

Cartesian (3D):
- In geodesy, a three-dimensional coordinate system with orthogonal axes and a single origin and scale. The basis for the Earth Centered Earth Fixed (ECEF) coordinates reported by GPS receivers and utilizing the WGS84 ellipsoid and datum.

CARTOGRAPHY:
- The art(style and technique), science, and technology of making maps or charts. Many GPS receivers have detailed mapping, or cartography, capabilities.

Channel:
- A channel of a GPS receiver consists of the circuitry necessary to receive the signal from a single GPS satellite; Section of a GPS receiver containing the signal identification and tracking electronics for individual satellite signals.

Chip:
- A single computer bit of a PRN signal; the length of time to transmit either a "0" or "1" in a binary pulse code; the transition time for individual bits in the pseudo-random sequence.

Chip Length:
- The repeat length, in computer bits "chips" of a PRN signal. C/A code has a chip length of 1023 bits.

Chip Rate:
- Number of chips per second. For example, C/A code = 1.023 MHz (a one-millisecond repeat rate).

Choke Ring:
- Series of concentric rings mounted below an antenna to reduce multipath from low elevation angle signals.

Circular Error Probable (CEP):
- A statistical measure of horizontal accuracy representing the radius of a circle within which the true value lies 50% of the time. Thus, half the data points are within a 1D CEP circle and half are outside the circle. (see SEP, Confidence Region)

CLOCK BIAS:
- The difference between the indicated clock time in the GPS receiver (GPS satellite time) and true universal time (UTC).

CLOCK OFFSET:
- A constant difference in the time reading between two clocks, normally used to indicate a difference between two time zones. Also, the unknown error of a receiver clock which is eliminated by using a fourth pseudorange observation and solving for the unknown.

(C/A) CODE:
- The coarse acquisition (also Clear/Acquisition) code; the standard GPS code; also known as the 'civilian code' or S-code. (see P-code); is modulated onto the GPS L1 carrier signal. This code is a sequence of 1023 pseudorandom binary biphase modulations on the GPS carrier at a chipping rate of 1.023 MHz, thus having a code repetition period of 1 millisecond. The code was selected to provide good acquisition properties. Also known as the 'civilian code' because it's the standard positioning signal the GPS satellite transmits to the civilian user. The C/A code contains the information the GPS receiver uses to fix its position and time. Accurate to 100 meters or better with Selective Availability (SA) turned on and accurate to 10 meters or better with SA turned off. Also, a spread spectrum direct sequence code that is used primarily by commercial GPS receivers to determine the range to the transmitting GPS satellite. C/A code has a wavelength of 293 mm (verify) and repeats every millisecond. It is modulated on the L1 frequency only and is the ranging code used for most civilian purposes. C/A code is relatively easy to "lock on" to so it is used by military receivers to acquire the P-code (precise code).

CODE DIVISION MULTIPLE ACCESS (CDMA):
- A method whereby many radios use the same frequency but each one has a unique code. GPS uses CDMA techniques with Gold's codes for their unique cross-correlation properties.

Code Phase:
- A fundamental GPS observable. The fractional phase of the incoming PRN code signal is observed. It is added to the known number of full-wavelengths to the satellite to get a range to the satellite.

Code phase GPS:
- GPS measurements based on the pseudo random code (C/A or P) as opposed to the carrier of that code.

COLD START:
- The power-on sequence where the GPS receiver downloads almanac data before establishing a position fix. The process of powering up a new GPS receiver for the first time and having it search out and lock onto the satellites by itself, without the benefit of initialization data. This procedure is slower and may require up to 8 minutes for initial satellite acquisition only.

Confidence Region:
- A rigorous measure of accuracy representing the region within which the true value should be found a certain percentage of the time. Usually the 95% figure is used. Confidence regions can be 1-D (confidence limits), 2-D (confidence ellipse), or 3-D (confidence ellipsoid).

Constellation:
- The full set of GPS satellites or else a subset of GPS satellites being used by a receiver. The GPS constellation comprises 28 satellites (as of Jan. 2002) in 12-hour circular orbits at an altitude of 10,900 nautical miles (20,200 kilometers). In the nominal constellation, four satellites are spaced in each of six orbital planes inclined 55 degrees to the equator. The constellation was designed to provide a very high probability of satellite coverage even in the event of satellite outages.

Continuously Operating Reference Stations (CORS):
- CORS organized by the U.S. Geodetic Survey (NGS) is a network of real-time (24/7/365) Differential GPS stations operating with free public access via the wwweb. Currently CORS sites are accurate to within 3 centimeters horizontally and 5 centimeters vertically relative to each other, but NGS expects to have this down to 1 centimeter horizontally and 2 centimeters vertically.

CONTROL SEGMENT:
- A worldwide chain of five (as of 2000) ground-based monitoring and control stations that ensure the accuracy of the GPS satellite constellation's positions and their clocks. The chief control station is in Colorado at Schriever (formerly Falcon) Air Force Base. (see Space Segment and User Segment)

Conventional Terrestrial System:
- A cartesian coordinate system defined with origin at the center of the earth (geocenter), Z-axis parallel to the conventional North axis, X-axis through the Greenwich meridian, and Y-axis through the meridian 90º E to form a "right-handed" system. (see ECEF.) The fundamental way GPS receivers compute location, then translated to appear as lat/long/elev on the GPS screen.

COORDINATED UNIVERSAL TIME (UTC):
- Replaced Greenwich Mean Time (GMT) as the world standard for time in 1986. It is based on atomic measurements rather than the rotation of the earth. Greenwich Mean Time (GMT) is still the standard time zone for the prime meridian (zero longitude). GPS receivers display a form of UTC.

Correction Age:
- In real-time GPS, the age (in seconds) of a correction being used. This includes the latency as well as any local delay in using the signal.

Cycle Slip:
- A momentary loss of "lock" (tracking) on the GPS carrier signal caused by signal obstruction or ionospheric effects. Also referred to as a discontinuity in the measured carrier beat phase resulting from a temporary loss-of-lock in the carrier tracking loop of a GPS receiver.

Data message:
- A 1500 bit message included in the GPS signal which reports the satellite's location, clock corrections, and health. Included is rough information about the other satellites in the constellation (see Almanac).

DATUM:
- A mathematically derived reference point where the surface of the earth meets the ellipsoid model of the earth. Used to compute lat/long coordinates for a part of the surface of the earth and have those coordinates be close to the coordinates that would be generated if the ellipsoid had been used to compute them. Latitude and longitude lines on a paper map, therefore, are referenced to a specific map datum. The map datum selected on a GPS receiver needs to match the datum listed on the corresponding paper map in order for the position readings to match. GPS receivers by default generate coordinates referenced to the WGS84 datum.

Decibel (dB):
- A standard unit expressing the ratio of two power levels. dBW is the ratio of a signal's power to one Watt. A way to measure the signal to noise ratio (SNR) of a GPS satellite's signal at the receiver's antenna.

Differencing:
- Using the difference between two or more GPS observations to correct for various random and systematic errors in the observations.

DIFFERENTIAL GPS (DGPS):
- A technique used to improve positioning or navigation accuracy by determining the positioning error at a known location and incorporating that correction factor (by real-time transmission of corrections or by post-processing) to correct for observations made by other receivers operating in the same general geographical area and simultaneously tracking the same satellites. This technique (DGPS) improves GPS-reported position accuracy by using pseudorange errors measured at a known location to improve the measurements made by other GPS receivers. The term is usually reserved for code-phase, single differences (see double-difference), often using real-time corrections.
In coastal areas, DGPS, as an extension of the GPS system, uses land-based radio beacons established by the U.S. Coast Guard to transmit position corrections to GPS receivers in the vicinity (see WAAS). DGPS reduces the effect of Selective Availability (SA), propagation delay, etc. and can improve position accuracy to better than 10 meters especially for real-time GPS positioning. Closeness to the radio beacon (base-line length) also improves position accuracy. With the shutting off of SA (May, 2000), positions recorded by GPS receivers operating in stand-alone mode now approach the position accuracy that used to only be attainable using the Coast Guard beacon signals.

DIFFERENTIAL POSITIONING:
- Accurate measurement of the relative positions of two receivers tracking the same GPS signals.

DILUTION OF PRECISION (DOP):
- The multiplicative factor that modifies ranging error. It is caused solely by the geometry between the user and his set of satellites. Consequently, a scalar number has been developed describing the way the geometry between the satellites and the users affects the precision of the computed position, given a certain precision of the satellite ranges.
A measure of the GPS receiver-GPS satellite constellation geometry; A description of the purely geometrical contribution to the uncertainty in a position fix. A low DOP value indicates better relative geometry and correspondingly higher accuracy. Standard terms for the GPS application are: The DOP indicators are GDOP (geometric DOP), PDOP (position DOP), HDOP (horizontal DOP), VDOP (vertical DOP), and TDOP (time clock offset).

Distance Root Mean Square (drms):
- The root-mean-square value of the distances from the true location point of the position fixes in a collection of measurements. As typically used in GPS positioning, 2 drms is the radius of a circle that contains at least 95 percent of all possible fixes that can be obtained with a system at only one place. Actually, the percentage of fixes contained within 2 drms varies between approximately 95.5% and 98.2%, depending on the degree of ellipticity of the error distribution.

Dithering:
- The process of adding artificial "noise" to a digital signal. This is the process the DoD uses to add inaccuracy to GPS signals to induce Selective Availability.

DoD:
- The U.S. Department of Defense. The DOD manages and controls the Global Positioning System primarily through Schriever (formerly Falcon) Air Force Base in Colorado.

Doppler:
- A fundamental GPS observable. The apparent frequency change caused by relative motion between the receiver and the transmitter (satellite).

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

Doppler shift:
- The apparent change in the frequency of a signal caused by the relative motion of the transmitter and/or the receiver.

Double Difference:
- The arithmetic differencing of carrier phases measured simultaneously by a pair of receivers tracking the same pair of satellites. Single differences are obtained by each receiver from each satellite; these differences are then differenced in turn, which essentially deletes all satellite and receiver clock errors. (uses four observations). (see single difference)

DOWNLINK:
- A transmission path for the communication of signals and data from a communications satellite or other space vehicle to the earth.

Dynamic positioning:
- See Kinematic Positioning.

Dynamic Traverse:
- Lineal feature defined by moving the GPS antenna over a feature while logging (saving) data at a specified interval.

Earth Centered, Earth Fixed (ECEF):
- A phrase often used in the United States for the Conventional Terrestrial coordinate system (see above).

Elevation:
- Height above mean sea level. also, vertical distance above the geoid.

Elevation Angle:
- The angle above the horizon of an object (e.g. a GPS satellite).

Elevation Mask Angle:
- The elevation angle below, which GPS satellites will not be used in a position computation. Normally set to 15 degrees to avoid interference problems caused by buildings, trees, and/or multipath errors.

ELLIPSOID:
- In geodesy, a mathematical figure formed by revolving an ellipse about its minor axis. Two quantities define an ellipsoid, the length of the semimajor axis, a, and the flattening, f=(a-b)/a, where b is the length of the semiminor axis. All of the plane sections of this geometric surface are either ellipses or circles. The earth closely approximates a biaxial ellipsoid and so this mathematical model is used for the computation of lat/long coordinates. Many ellipsoids have been defined to represent the earth over time and at different locations. GPS receivers contain the definitions of a number of ellipsoids and can thus compute the appropriate lat/long values referenced to the specified ellipsoid. GPS receivers have programmed into them the WGS84 earth model which is based on the GRS80 ellipsoid. NAD83 uses GRS80, which is nearly identical to WGS84.

Ellipsoid Height (h):
- The height above the ellipsoid. This vertical distance is a geometrical, not a physical parameter, thus is not the same as elevation above sea level. Ellipsoid height is what is given by GPS techniques. GPS receivers output position fix height relative to the WGS-84 datum (ellipsoid).

EPHEMERIS:
- A list of accurate positions or locations of a celestial object as a function of time. For GPS satellites, available as "broadcast ephemeris " or as post-processed "precise ephemeris." The former, exact within 20 meters, is transmitted as part of the satellite data message from each satellite and is valid for several hours.
"Precise" (within 7 centimeters) ephemerides are also available from tracking networks, typically several days after the date of observation.

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

EPOCH:
- An event of measuring and storing GPS observations; Measurement interval or data frequency, as in making observations every 15 seconds. "Loading data using 30-second epochs" means loading every other measurement.

Error Ellipse:
- Two-dimensional confidence region and often used in describing the scatter of GPS fixes at a point.

Federal Radionavigation Plan (FRP):
- Congressionally mandated, joint Department of Defense (DoD) and 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 U.S. government-provided radionavigation services, including GPS.

Fix:
- A single position with latitude, longitude (or grid position), elevation (or altitude if airborne), time, and date.

FREQUENCY:
- Generally, the number of repetitions per unit time of a complete waveform, as of a radio wave, thus when applied to GPS satellite transmissions, the number of full cycles of a radio wave which occur per second. Measured in cycles per second, or Hertz (Hz). (see L1 and L2 frequencies).

Frequency band:
- A particular range of frequencies, such as the L-band in which GPS signals are found.

Frequency Division Multiple Access (FDMA):
- Scheme where different transmitters all use the same code and different frequencies to distinguish the signals. Russian GLONASS radionavigation satellites use FDMA.

GEOCACHING:
- A high-tech version of hide-and-seek. Geocachers seek out hidden treasures utilizing GPS coordinates posted on the Internet by those hiding the cache.

GEO-DASHING:
- The latest (2001) GPS based game.

GEODETIC DATUM:
- A math model representing the size and shape of the earth (or a portion of it). GPS receivers typically contain parameters for numerous datums, so the receiver can give the correct position coordinate readout anywhere in the world.

Geoid:
- An equipotential surface corresponding approximately with mean sea level over the oceans and beneath the land masses.

Geoid Height (N):
- The difference between the geoid and the ellipsoid at a certain location. This value is essential in converting between ellipsoidal (GPS-derived) and orthometric (Mean Sea Level) elevations. In British Columbia, the geoid height varies between -20 and +10 meters.

Geometric Dilution of Precision (GDOP):
- See Dilution of Precision.

GEOSYNCHRONOUS ORBIT:
- A specific circular orbit (22,000 (statute?) miles above earth) where a satellite revolves around the earth at the same rotational speed as the earth. A satellite revolving in geosynchronous orbit appears to remain stationary when viewed from a point on or near the equator. It is also referred to as a geostationary orbit. The GPS constellation of satellites orbits at a height 1/2 the geostationary altitude or 10,900 nautical miles (20,000km), and therefore complete 2 orbits around the earth per day.

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):
- The U.S. Department of Defense Global Positioning System. This global navigation system currently (January 2002) consists of 28 satellites (as of Jan. 2002) in circular earth orbits at an altitude of 10,900 nautical miles (20,000km). GPS satellites transmit radio signals that allow one to determine, with great accuracy, the locations of GPS receivers. The receivers can be fixed on the Earth, in moving vehicles, aircraft, or in low-Earth orbiting satellites. GPS is used in air, land and sea navigation, mapping, surveying and other applications where precise positioning is necessary. The system provides very precise, worldwide positioning and navigation information 24 hours a day, in any weather. Also called the NAVSTAR system.

GPS RECEIVER:
- Receives the signals from the satellites, and computes position, course, and speed. After power is turned on, it takes anywhere from a few seconds to a few minutes to find the GPS signals, and compute the first position fix. The readings from a GPS receiver will have the following typical errors: Position 50 meters (164 feet), Speed 0.5 knots. Errors may sometimes be much larger, when the satellite positions in the sky are not favorable, or when the lines of sight from the satellites to the antenna are obstructed by masts, boat structure, trees, buildings, or bridges.

Ground Plane:
- Grounding and signal gathering device mounted below an antenna's phase-center. Also can reduce multipath from below the antenna.

HEALTHY:
- A term used when an orbiting GPS satellite is suitable for use. Health Information is broadcast in the satellite message to indicate whether or not a satellite is performing to specification.

I/O (INPUT/OUTPUT):
- The two-way transfer of GPS information with another device, such as a nav plotter, autopilot, or another GPS unit.

INITIALIZATION:
- The first time a GPS receiver orients itself to its current location and collects almanac data. After initialization has occurred, the receiver remembers its location and acquires a position more quickly because it knows which satellites to look for.

Integer Ambiguity:
- The integer (whole) number of carrier wave cycles from the currently observed carrier cycle (see carrier phase) to the GPS satellite.

IONOSPHERE:
- A layer of earth's atmosphere where ionization caused by incoming solar radiation affects the transmission of GPS (and other) radio waves. This region of charged particles (ions) extends from a height of 80 kilometers (45 nautical miles) to 400 kilometers (220 nautical miles) above the surface and represent a non-homogeneous and dispersive medium for radio signals.

Ionospheric Delay:
- A wave propagating through the ionosphere experiences delay. Phase delay depends on electron content and affects carrier signals. Group delay depends on dispersion in the ionosphere as well and affects signal modulation (codes). The phase and group delays are of the same magnitude but opposite sign.

Ionospheric Refraction:
- The change in the propagation speed of a signal as it passes through the ionosphere.

Kalman Filter:
- A numerical method used to track a time-varying signal in the presence of noise, as is the case with GPS signals, whose 50 watt signal traverses over 12,000 nautical miles (20,000km) through a dynamic atmosphere. Kalman filters are often used to integrate different measuring systems such as GPS and Inertial Navigation Systems (INS).

Julian Day (JD):
- The serial day of the year, beginning January 01. Role in GPS orbit time keeping.

Kinematic positioning:
- Kinematic positioning refers to applications in which the position of a non-stationary object (automobile, ship, bicycle) is determined. Also called mobile mode. This is the way to collect data when surveying in new streets using GPS for addition to a GIS street data layer.

L-band:
- The group of radio frequencies extending from 390 MHz to 1550 MHz (or from ~1.0GHz to ~2.0GHz). The GPS carrier frequencies (1227.6 MHz and 1575.42 MHz) are in the L-band.

L1 FREQUENCY:
- The primary L-band frequency of the two radio frequencies transmitted by the GPS satellites. This frequency is modulated with the Coarse Acquisition Code (C/A code), the encrypted P-Code, and the navigation message and is transmitted on a carrier frequency of 1575.42 MHz.

L2 FREQUENCY:
- 1227.6 MHz GPS carrier frequency which contains only the encrypted P-Code, used primarily to calculate signal delays caused by the ionosphere. One of the two radio frequencies transmitted by the GPS satellites.

Leap Second:
- A second is occasionally added or subtracted to the day in order to account for the Earth's changing rotation speed. These changes are made to UTC, not GPS clocks. Therefore, currently (January, 2002), the difference between earth-bound and GPS satellite clocks is about 13 seconds.

Line of Sight (LOS):
- Radio signals of VHF (30 MHz) or higher frequencies (such as GPS signals at 1.5GHz) behave like a light beam. The receiving antenna must be able to "see" the transmitting antenna.

LINE OF SIGHT PROPAGATION:
- Of an electromagnetic wave, propagation in which the direct transmission path from the transmitter to the receiver is unobstructed. The need for LOS propagation is most critical at GPS frequencies.

LIQUID CRYSTAL DISPLAY (LCD):
- A display circuit characterized by a liquid crystal element sandwiched between two glass panels. Characters are produced by applying an electric field to liquid crystal molecules and arranging them to act as light filters. GPS receivers with LCD screens are subject to wash-out when left or used in the sun. The heat from the sun causes the contrast to drop between the light and dark areas making the screen difficult to read. The condition disappears when the unit returns to normal operating temperatures.

LITHIUM BATTERY:
- A soft, silvery, highly reactive metallic element that is used in batteries where weight and cold weather conditions are concerns. Also used as back-up batteries in most GPS receivers.  Currently being replaced in GPS receivers with flash memory.

LOCAL AREA AUGMENTATION SYSTEM (LAAS):
- The implementation of ground-based DGPS to support aircraft landings in a local area (20 mile range).

MAP DISPLAY:
- A graphic representation of a geographic area and the features in it as viewed on a GPS receiver screen.

Microstrip Antenna:
- Low profile antenna using a small microstrip "patch". Microstrip antennas usually require a low-noise amplifier because of their low gain.

Monitor Stations:
- One of the worldwide group of stations used in the GPSD control segment to track satellite clock and orbital parameters. Data collected at monitor stations are linked to a master control station at which corrections are calculated and from which correction data is uploaded to the satellites as needed.

Multichannel receiver:
- A receiver containing multiple independent channels, each of which tracks one satellite continuously, so that position solutions are derived from simultaneous calculations of pseudoranges. Most receivers nowadays (January 2002), including the Garmin 12xl, are 12 channel receivers.

Multipath:
- The phenomenon where a GPS satellite's signal arrives at the antenna by different paths, some reflected. Multipath distorts both code and carrier information, making it difficult for the receiver to accurately measure the range and causes errors in the computed position.

MULTIPATH ERROR:
- An error caused when a satellite signal reaches the GPS receiver antenna by more than one path. Usually caused by one or more paths being bounced or reflected. Signals traveling longer paths (i.e. bounced off nearby structures or other reflective surfaces) produce higher (erroneous) pseudorange estimates and consequently, positioning errors. The TV equivalent of multipath is "ghosting."

MULTIPATH RESISTANT ANTENNA:
- A type of GPS antenna in which the antenna is positioned on a ground plane plate. This circular metal plate helps serve as a shield keeping bounced or reflected GPS satellite signals from reaching the antenna.

MULTIPLEXING RECEIVER:
- A GPS receiver that switches at a very rapid rate between satellites being tracked. 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. This type of receiver was typical in the late 1980s and early 1990s. Not common anymore (January 2002).

Nautical Mile:
- A unit of length used in sea and air navigation, based on the length on one minutes of arc of a great circle, especially an international and U.S. unit equal to 1,852 meters (about 6,076 feet).

NAVIGATION (NAV) MESSAGE:
- The 1500-bit navigation message broadcast by each GPS satellite at 50 bps on the L1 and L2 signals. This message contains system time, clock correction parameters, ionospheric delay model parameters, and the vehicle's ephemeris and health. This information is used to process GPS signals to give user time, position, and velocity. Also known as the data message.

NAVSTAR:
- The official U.S. Government name given to the GPS satellite system. NAVSTAR is an acronym for NAVigation Satellite Timing and Ranging.

NMEA (NATIONAL MARINE ELECTRONICS ASSOCIATION):
- A 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.

NMEA 0183:
- A standard data communication protocol used by GPS receivers and other types of navigation and marine electronics.

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

Observable:
- A quantity which can be measured. GPS observables include pseudoranges, carrier phase, and doppler frequency shift.

Observation:
- A single measurement of an observable. Also, the period of time over which GPS data is collected simultaneously by two or more receivers.

Observation Time:
- The precise time that GPS observations are made. Usually stored as a Time of Week (TOW) and GPS Week Number.

On-the-Fly (OTF):
- A term used to describe the technique of resolving differential carrier-phase integer ambiguities without requiring a GPS receiver to remain stationary.

Outlier:
- An observation which does not fit the expected distribution of error of a set of observations. An outlier probably contains a systematic or a gross error. Outliers often occur when using GPS receivers in mobile mode and passing under a tree or under a bridge where signal loss occurs. Upon reacquiring signals from GPS satellites, elevation values tend to be grossly incorrect, gradually returning to acceptable errors. These GPS observations are often removed from the data set.

PARALLEL CHANNEL RECEIVER:
- A continuous tracking receiver using multiple receiver circuits to track more than one satellite simultaneously. (same as multichannel receiver?)

PATCH ANTENNA:
- A type of GPS antenna that exhibits a very low physical profile. Found on some Garmin GPS units, such as the 12XL.

P-CODE:
- The precise (or protected) code of the GPS signal (with a wavelength of 2.93 meters) available typically only to the U.S. military. A very long sequence of pseudo-random binary biphase modulations onto both the L1 and L2 carrier frequencies 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 encrypted (Anti-Spoofing) and reset every seven days to prevent use by unauthorized persons.

Position dilution of precision (PDOP):
- A unit less figure of merit expressing the relationship between the error in user position and the error in satellite position, which is a function of the configuration of satellites from which signals are derived in positioning. Geometrically, PDOP is proportional to 1 divided by the volume of the pyramid formed by lines running from the receiver to four observed satellites. Small values, such as "3", are good for positioning while higher produce less accurate position solutions. Small PDOP is associated with separated satellites. The spacing of satellites in orbit is arranged so that a minimum of five satellites will be in view to users worldwide, with a position dilution of precision (PDOP) of six or less.

Period:
- The time taken for a complete cycle of something. GPS satellites have an orbit period of just less than 12 hours.

Phase Center:
- The apparent electrical center of an antenna. In GPS, ranges are measured to and positions are computed for the phase center. Reference is made to antenna phase center when dealing with centimeter accuracy GPS position determination and when reading about CORS base station antenna set-ups.

Phase Lock:
- The technique whereby the phase of an oscillator signal is made to follow exactly the phase of a reference signal. The receiver first compares the phases of the two signals, then uses the resulting phase difference signal to adjust the reference oscillator frequency. This eliminates phase difference when the two signals are next compared.

PIXEL:
- A single display element on an LCD screen. The more pixels, the higher the resolution and definition.

Point Positioning:
- A geographic position produced using a single GPS receiver to compute uncorrected positions. Also known as autonomous positioning or stand-alone mode. Point positioning was most subject to S/A errors.

POSITION:
- An exact, unique location based on a geographic coordinate system.

Position Corrections:
- A method for correcting GPS positions where the observed error in the position of a reference station at any time is applied as a correction to rover positions at the same time.

POSITION FIX:
- A single determination of a position. GPS uses pseudoranges to four or more satellites to compute a position fix.

POSITION FORMAT:
- The way in which the GPS receiver's position will be displayed on the screen. Commonly displayed as latitude/longitude in degrees and minutes, with options for degrees, minutes and seconds (DMS), decimal degrees (DD), or one of several grid formats.

Precise Code:
- see P-code.

Precise Positioning Service (PPS):
- The most accurate dynamic positioning possible with standard GPS, based on the dual frequency P-code and no S/A.

PSEUDO-RANDOM CODE:
- A signal with random noise-like properties. Signals appear as noise to users or user equipment without specific knowledge of the signal structure. A sequence of digital 1's and 0's that appear to be randomly distributed like noise but that can be reproduced exactly. Their most important property is a low autocorrelation value for all delays or lags except when they coincide exactly. Each GPS satellite has unique C/A and P pseudo-random noise codes.
- The identifying signature signal transmitted by each GPS satellite and mirrored by the GPS receiver in order to separate and retrieve the signal from background noise.

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. The term "pseudorange" is used since at this time the observable is only an estimate of the true range.

Pseudorange Corrections:
- A method of differentially correcting GPS data by finding the errors in each pseudorange

QUADRIFILAR HELIX ANTENNA:
- A type of GPS antenna in which four spiraling elements form the receiving surface of the antenna. For GPS use, quadrifilar antennas are typically half-wavelength or quarter-wavelength size and encased in a plastic cylinder for durability. Common antenna type among Garmin and Magellan GPS receiver models.

Quality Assurance(QA):
- Procedures for verifying (assuring) the quality of GPS data produced by another organization before acceptance.

Quality Control(QC):
- Procedures for checking and controlling the quality of GPS data produced by an organization, by that organization.

Radio Technical Commission for Maritime Services:
- A commission established for the purposes of establishing standards and guidance for interfacing between radiobeacon-based data links and GPS receiver, and to provide standards for ground based differential GPS stations.

RADIO TECHNICAL COMMISSION FOR MARITIME SERVICES (RTCM) SPECIAL COMMITTEE 104:
- A committee established for the purposes of establishing standards and guidance for interfacing between radiobeacon-based data links and GPS receivers, and to provide standards for ground-based differential GPS stations.

Radionavigation:
- The determination of position, or the obtaining of information relative to position, for the purpose of navigation by means of the propagation properties of radio waves. GPS is a method of radionavigation.

Range:
- A distance between two objects (at least one is usually moving), as between a GPS satellite and a GPS receiver.

Range Rate:
- The rate at which a range is changing. The range to a satellite changes due to satellite and observer motions. In real-time GPS measurements, range rates are used to correct for latency. Range rate is determined by measuring the doppler shift of the satellite beacon carrier.

Real-Time:
- Where GPS positions are corrected as the observations are made. The reference receiver sends corrections and/or observations to the roving receiver, usually through some form of radio link. Real-time differential (RT-DGPS) and real-time kinematic (RTK) methods are commonly used in positioning and surveying.

Real Time Kinematic (RTK):
- DGPS procedure whereby carrier phase corrections are transmitted in real time from a reference station at a known location to one or more remote 'rover' receiver(s). Typically a VHF or UHF radio link is used to communicate each new GPS observation correction to the rover(s) (most likely using the RTCM SC-104 standard) (see below). The roving unit requires some sort of 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.

Reference Station:
- GPS receiver observing over a known location in order to correct data from receivers (rovers or field receivers) observing at unknown locations. Also known as Base Station.

RINEX:
- Receiver INdependent EXchange format for GPS data. An ASCII based, receiver-independent data format for the exchange between dissimilar GPS receiver models, post-processing software, or users (organizations). Includes provisions for three fundamental GPS observables: pseudorange (range), carrier-phase (phase), and doppler (time).

Root Mean Square (RMS):
- A one-dimensional measure of accuracy representing the limits (plus or minus) within which the true value lies 68% of the time. (see Confidence Region)

Rover:
- Any mobile GPS receiver collecting data during a field session. The receiver's position can be computed relative to another, stationary GPS receiver.

RS-232:
- A serial input/output standard that allows for compatibility between data communication equipment made by various manufacturers, as for example, between the Garmin 12xl and a computer.

RTCM (SC-104):
- A standard data format for transmitting real-time GPS correction data, regardless of the receiver manufacture. The format includes provision for code corrections and carrier-phase corrections. (see RTK)

Satellite constellation:
- The arrangement in space of a set of satellites. The GPS satellite constellation consists of 28 actively broadcasting satellites arranges in six planes around the earth each tilted 55 degrees to the equator. Each plane has at least four satellites.

Satellite Status Display:
- An information screen that shows technical data about each satellite in view. Information includes receiver channel numbers; actual satellite I.D. numbers; status of satellite tracking (T) or searching (S); satellite elevations and azimuths; signal to noise ratios (SNR) (the higher the number, the better); and dilution of precision ratings (e.g. PDOP, GDOP); the smaller the number, the better potential accuracy).

SEARCH THE SKY:
- A message shown when a GPS receiver is gathering satellite almanac data. This data tells the GPS receiver where to look for each GPS satellite.

Selective Availability (SA):
- A DoD policy controlling the accuracy of pseudorange measurements, degrading the signal available to non-qualified receivers by dithering the time and ephemerides data provided in the navigation message. SA limits civilian accuracy to 100 meters, about 95% of the time. This policy was discontinued as of May 2, 2000 and now SA is turned off.

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.

Sidereal Day:
- A full day defined by the apparent rotation of a point fixed among the stars (or celestial sphere) called the vernal equinox. A sidereal day is shorter than a solar day by approximately 3 minutes 56 seconds. GPS satellites have an orbital period of 12 sidereal hours.

Single Difference:
- The arithmetic "differencing" of carrier phases simultaneously measured by a pair of receivers tracking the same satellite (between-receivers and satellite), or by a single receiver tracking two satellites between-satellite and receivers); the former essentially deletes all satellite clock errors, while the latter essentially deletes all receiver errors.

SPACE SEGMENT:
- The part of the entire GPS system that is in space, i.e. the satellites. The current (January 2002) space segment consist of 28 actively broadcasting satellites.

Spherical Error Probable (SEP):
- A 3-dimensional measure of accuracy representing the radius of a sphere within which the true value lies 50% of the time (i.e., 50% probability of locating a point or being located). SEP is the three-dimensional analogue of CEP. (see CEP, Confidence Region)

SPREAD SPECTRUM:
- A system in which the transmitted signal is spread over a frequency band much wider than the minimum bandwidth needed to transmit the information being sent. This is done by modulating with a pseudo random code, for GPS. The received GPS signal is wide bandwidth and low power (-160 dBW). The L-band signal is modulated with a pseudo random noise (PRN) code to spread the signal energy over a much wider bandwidth than the signal information bandwidth. This provides the ability to receive all satellites unambiguously and to give some resistance to noise and multipath.

Standard Positioning Service (SPS):
- The normal civilian positioning accuracy obtained by using the single frequency C/A code. Under Selective Availability (SA) conditions, guaranteed to be no worse than 100 meters 95 percent of the time (2 drms).

Static positioning:
- Location determination when the receiver's antenna is presumed to be stationary on the Earth. This reportedly allows the use of various averaging techniques that improve accuracy by a factor of over 1000.

SV:
- Satellite vehicle or space vehicle. Each GPS satellite has a unique SV number (SVN). Each satellite weighs approximately 2000 lbs and is 17 feet end to end w/ solar panels extended.

Time of Week (TOW):
- Time, in seconds from 0:00 h UTC, Saturday night (16:00 PST). There are 604,800 seconds in a week (3600 s per hour, 86,400 s per day).

TIME TO FIRST FIX (TTFF):
- If you have not used your GPS unit for several months, the almanac data for the satellites may be out of date. The unit is capable of recollecting this information on its own, but the process can take several minutes. The time it takes after the user first turns on the GPS receiver, when a GPS receiver has lost memory, or has been moved over 300 miles from its last location. The time it takes to find the satellites is called the Time to First Fix (TTFF).

TRILATERATION:
- The form of surveying that locates a point relative to two other points based on the distances between the first point and the other two.

Triple Difference:
- The arithmetic difference of sequential, doubly-differenced carrier-phase observations that are free of integer ambiguities, and therefore useful for determining initial, approximate coordinates of a site in relative GPS positioning, and for detecting cycle slips in carrier-phase data. (See single difference & double difference)

TROPOSPHERE:
- The lowest region of the atmosphere between the surface of the earth and the tropopause, characterized by decreasing temperature with increasing altitude. As GPS signals travel through the troposphere it may be affected particularly by local concentrations of water vapor.

Universal Time Coordinated (UTC):
- An international, highly accurate and stable uniform atomic time system kept very close, by offsets, to the universal time corrected for seasonal variations in the earth's rotation rate. Maintained by the US Naval Observatory. GPS time is directly relatable to UTC: UTC-GPS = seconds. (The changing constant = 5 seconds in 1988 and 15 seconds in 2002.)

UPLINK:
- A transmission path by which radio or other signals are sent from the ground to an aircraft or a communications satellite. The manner in which the ground segment sends corrected time and ephemeris data to the GPS constellation.

User Equivalent Range Error (UERE):
- The effective accuracy of a pseudorange.

User Range Accuracy (URA):
- The predicted accuracy of a pseudorange broadcast by GPS satellites as part of the navigation message.

USER INTERFACE:
- The way in which information is exchanged between the GPS receiver and the user. This takes place through the screen display and buttons on the unit.

USER SEGMENT:
- One of three segments of the complete GPS system. This segment consists of the GPS receiver, its operator/user, and support personnel and/or equipment.

WAVELENGTH:
- The distance between points of corresponding phase of two consecutive cycles of a wave such as light or radio signals.

Week Number:
- Sequential week from 0:00 hours UTC, 1980-01-16 (January 6, 1980).

Week Rollover:
- The GPS week number is broadcast as a 10-bit binary "word". When it reached 1024 on August 22, 1999, it "rolled over" to 0. When the event occurred there was concern that some older receivers misinterpreted the 'roll over' and would not work.

World Geodetic System (WGS):
- A 3-dimensional datum chosen to provide a best fit over the entire Earth, its oceans and lands masses. WGS-84 is the datum and ellipsoid used by GPS since January, 1987. International Terrestrial Reference Frame(ITRF) of 19xx is the datum usually used for precise surveying on a global scale. Conversions between the two datums are maintained by the National Geodetic Survey (NGS).

WIDE AREA AUGMENTATION SYSTEM (WAAS):
- A system of satellites and ground stations that provide GPS signal corrections for better position accuracy. A WAAS-capable receiver can give you a position accuracy of better than three meters, 95 percent of the time. WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal. WAAS is a FAA-funded system of equipment and software that augments GPS. The WAAS provides a satellite signal for WAAS users to support enroute and precision approach aircraft navigation.

WGS-84:
- World Geodetic System-1984. The primary map datum used by GPS. Secondary datums are computed as differences from the WGS 84 standard.

Y-CODE:
- The encrypted P-Code.

terminology compiled from a variety of sources, including:
http://www.icomamerica.com/gps/glossary.html
http://www.garmin.com/aboutGPS/glossary.html
http://home.gdbc.gov.bc.ca/bcacs/gpsglossary.htm
http://www.navtechgps.com/glossary.asp
http://giswww.pok.ibm.com/gps/glossary.html
http://www.gpsw.co.uk/acatalog/Website/glossary.htm
http://www.trimble.com/gps/glossary.html
animated gif at top of document from: http://scign.jpl.nasa.gov/learn/gps2.htm