Glossary of Terms and Abbreviations for GPS
Accuracy – An indication of how closely a measurement is to the true value.
ACP – Active control pointAlmanac – Data transmitted by a GPS satellite which includes orbit information on all the satellites, clock correction, and atmospheric delay parameters. The almanac is used to facilitate rapid SV acquisition. The orbit information is a subset of the ephemeris data with reduced precision.
AS – Antispoofing AST – Apparent sidereal time AST1 – Apparent sidereal time corrected for polar motion.Attribute – Characteristics of features in a Geographic Information System (GIS) or Coordinate Geometry (COGO) package. Every identifiable feature has attributes.
Autonomous Positioning – The least precise form of positioning that a GPS receiver can produce. The position fix is calculated in real-time from satellite data along. Autonomous positions are generally accurate to within 100 meters.
Base Station, Base Receiver, Base Unit – In GPS surveying practice, one observes and computes baseline, that is the position of one receiver relative to another. The base station (sometimes called a reference station) acts as the position from which all other unknown positions are derived. A base station is an antenna and receiver set up on a known location specifically to collect data to be used in differentially correcting rover files.
BIPM – International Bureau of Weights and Measures. BPSK – Binary phase shift keyingCarrier Phase – The difference between the carrier signal generated by the internal oscillator of a receiver and the carrier signal coming from the satellite.
CBN – Cooperative Base Network CDMA – Code division multiple accessCMR – Compact Measurement Record, type 2. A correction message which is broadcast by the base receiver and used in RTK surveys to calculate an accurate vector from the base to the rover.
Coarse/Acquistion (C/A) Code – a pseudo-random noise code (PRN) modulated onto a L1 signal. This code helps the receiver computer the distance from the satellite. The standard (Coarse/Acquisition) GPS code - a sequence of 1023 pseudo-random, binary, biphase modulations on the GPS carrier at a chip rate of 1.023 MHz. Also known as the "civilian code".
Control Segment - A world-wide network of GPS monitor and control stations that ensure the accuracy of satellite positions and their clocks.
Cycle Slip - A discontinuity in the measured carrier beat phase resulting from a temporary loss of lock in the carrier tracking loop of a GPS receiver.
DOP (Dilution Of Precision) - An indicator of the quality of a GPS position which takes acount of each satellite’s location relative to the other satellites in the constellation and their geometry in relation to the GPS receiver. A low DOP value indicates a higher probability of accuracy.
Standards DOP’s for GPS applications are:
 | PDOP – Position Dilution of Precision. A unit-less figure of merit expressing the relationship between the error in user position and the error in satellite position. Geometrically, PDOP is proportional to 1 divided by the volume of the pyramid formed by lines running from the receiver to four satellites observed. Values considered good for positioning are small.(such as 3) Value greater than 7 are considered poor. |
| RDOP – Relative Dilution of Precision. It is usually in units of meters/cycle. Multiplying RDOP by the uncertainty of a double-difference measurement yields the spherical relative-position error. RDOP is used as a guide to the adequacy of receiver observation during real-time surveying measurements in static mode. |
| HDOP – Horizontal Dilution of Precision. |
| VDOP – Vertical Dilution of Precision. |
| TDOP – Time Dilution of Precision. |
| GDOP – Geometric Dilution of Precision. The relationship between errors in user position and time and in satellite range. |
Duel-Frequency – a type of receiver that uses both L1 and L2 signals from GPS satellites. A duel Frequency receiver can compute more precise position fixes over longer distances and under more adverse conditions by compensating for ionospheric delays.
ECEF – Earth Centered Earth Fixed, A cartesian coordinate system used by the WGS-84 reference frame. The center of the system is at the earth’s center of mass. The z axis is coincident with the mean rotational axis of the earth, the x axis passes through 0Xn and 0Xe, the y axis is perpendicular to the plane of the x and z axes.
Ellipsoid – A mathematical model of the earth formed by rotating an ellipse around its minor axis. For ellipsoids which model the earth, the minor axis is the polar axis, and the major axis is the equatorial axis. An ellipsoid is completely defined by specifying the lengths of both axes, or by specifying the length of the major axis and the flattening.
Epoch - The measurement interval of a GPS receiver.
Ephemeris – The current satellite position predictions that are transmitted in the NAVDATA message.
Geodetic Datum – A mathematical model designed to fit part or all of the geoid, defined by the relationship between an ellipsoid and a point on the topographic surface established as the origin of a datum. World geodetic datums are typically defined by the size and shape of an ellipsoid and the location of the center of the ellipsion with respect to the center of the earth.
Geoid – The surface of gravitational equipotential which closely approximates mean sea level.
Initialization – The technique performed in either real-time or postprocessed GPS surveying to resolve satellite interger wavelength ambiguities, therefore enabling centimeter-level positioning.
Integer ambiguity – The whole number cycles in a pseudo range between the GPS satellite and the GPS receiver.
Ionospheric Delay – A wave propagating through the ionosphere experiences delay. The delay depends on electron content and affects carrier signals. Group delay depends on dispersion in the ionosphere as well, and affects signal modulation. The phase and group delay are of the same magnitude but opposite sign.
L1 – The primary L-band carrier used by GPS satellites to transmit satellite data. The frequency is 1575.42 MHz. It is modulated by C/A Code, P-Code and a 50 bit/second navigation message.
L2 – The secondary L-band carrier used by GPS Satellites to transmit satellite data. The frequency is 1227.6MHz. It is modulated by P-Code and a 50 bit/second navigation message.
Multipath – Interference similar to ghosts on a television screen which occurs when GPS signals arrive at an antenna after traversing different paths. The signal traversing the longer path will yield a larger pseudo range estimate and increase the error. Multipath paths may arise from reflections from structures near the antenna.
NAVDATA – The navigation message broadcast by each GPS satellite on both the L1 and L2 beacons. This message contains system time, clock correction parameters, ionospheric delay model parameters, and the vehicle’s ephemeris and health. A GPS receiver can use this information to process GPS signals and thus obtain user position and velocity.
NAVSTAR – The name given to GPS satellites. NAVSTAR is an acronym formed from NAVigation Satellite Timing and Ranging.
P-Code – The precise code transmitted by the GPS satellites. Each satellite has a unique code that is modulated on to both the L1 and L2 carrier waves. The P-code is replacyed by a Y-code when Anti-Spoofing is active.
Pseudorandom Noise or Number (PRN) – A signal that carries a code that appears to be randomly distributed like noise, but can be exactly reproducted. PRN codes have a low auto-correlation value for all delays or lags, except when they are exactly coincident. Each NAVSTAR satellite has its own unique PRN code.
QA/QC – Quality Assurance/Quality Control – A receiver option for precise positioning applications that processes broadcast corrections and satellite data in real time to provide position accuracy statistics.
Ratio – During initialization, the receiver determines the interger number of wavelenghts for each satellite. For a particular set of integers, the receiver works out the probability that it is the correct set. Ratio is the ratio of probability of correctness of the currently best set of integers to the probability of correctness of the next-best set. Thus a high ratio indicates that the best set of integers is much better than any other set, which gives us confidence that it is correct. The ratio must be above 5 for New Point and On-the-Fly (OTF) initialization.
RMS – Root Mean Square. An expression the accuracy of point measurement. It is the radius of the error circle, within which approximately 70% of position fixes are to be found. It can be expressed in distance units or in wavelength cycles.
RTCM – Radio Technical Commission for Maritime Services. A commission established to define a differential data link for real-time differential correction of roving GPS receivers. There are two types of RTCM differential correction messages.
SA (Selective Availability) - Artificial degradation of the satellite signal by the US DOD. The error in position caused by SA can be up to 100 meters.
SNR – Signal-to-noise ratio. A measure of the strength of a satellite signal. SNR ranges from 0 to around 35. Higher-elevation satellites have SNR’s in the high teens to low 20’s. SNR’s lower than 5 are considered unusable.
SV – Satellite Vehicle or Space Vehicle
Tropospheric Delay – A wave propagating through the ionosphere experiences delay. The delay depends on electron content and affects carrier signals. Group Delay depends on dispersion in the troposphere as well, and affects signal modulation (codes). The phase and group delay are of the same magnitude but opposite sign.
UTC – Universal Time Coordinate. Local solar mean time at Greenwich Meridian. A uniform atomic time system maintained by the US Naval Observatory and kept very clost to UT2 by offsets.
WGS-84 – World Geodetic System (1984). The mathematical ellipsoid used by GPS since 1984.
