Glossary of GNSS Terms and Abbreviations

A brief glossary aimed primarily at users of PyGPSClient and u-blox GNSS RTK receivers.

Term	Definition	Comments
AGNSS	Assisted GNSS	A term denoting various techniques to improve the TTFF. In essence, these aim to transmit the necessary ephemerides and almanac data to the receiver via alternate faster communications channels e.g. mobile radio or internet.
ARP	Antenna Reference Point	The identifier for a specific RTK data source.
baseline		In the context of GNSS RTK techniques, the distance between the fixed-base reference station antenna and the user's receiver antenna.
Beidou		China's GNSS.
bias	Code or Phase bias	In the context of GNSS, the error in PRN code or carrier wave phase pseudorange calculation arising from imperfections and/or physical limitations in the satellite's electronic hardware, which results in small delays between events which should ideally be simultaneous.
BNC	Bayonet Neill-Concelman	A standard bayonet antenna connector type.
C/A	Coarse/Acquisition (sometimes Civilian Access)	A 1023-chip PRN code transmitted at 1.023 Mchips/sec.
CLAS	Centimeter-level Augmentation Service	A QZSS GNSS centimeter-level augmentation service only available on the L6 band.
CN₀	Carrier to noise ratio	A measure of GNSS signal strength.
CORS	Continuously Operating Reference Station	A term denoting an (N)RTK correction source (e.g. NTRIP caster) which runs 24/7/365 or thereabouts.
CRS	Coordinate Reference System	A framework used to precisely measure locations on the surface of Earth as coordinates. CRS can be broadly categorised as spherical (e.g. geodetic, aka geographic) or cartesian (e.g. geocentric, aka ECEF - the internal CRS used by GNSS - or projected, aka planar or grid e.g. Mercator).
DGPS/DGNSS	Differential GPS/GNSS	A high resolution positioning technique that enhances the positional data available from GNSS systems by reference to a network of fixed position, ground-based (or "virtual") reference stations. See also OSR.
DOP	Dilution of Precision	A measure of the effect of the geometric distribution of visible GNSS satellites on positional accuracy.
ECEF	Earth Centered, Earth Fixed	A cartesian spatial reference system, also known as the 'geocentric coordinate system', that represents locations in the vicinity of the Earth (including its surface, interior, atmosphere, and surrounding outer space) as X, Y, and Z measurements from its center of mass.
ENU	East North Up	A spatial reference system based on the tangent plane defined by the local vertical direction and the Earth's axis of rotation, also known as 'local geodetic coordinate system'.
ellipsoid		An approximation of the earth's surface as an oblate spheroid ('squashed sphere') - the core datum against which GNSS navigation solutions are calculated. See also 'geoid'.
ephemerides		A set of parameters defining a GNSS satellite's orbital position at any given time, which along with the pseudorange Rp provides the means by which GNSS establishes a navigation solution.
epoch		The time period within which a given GNSS navigation solution (or set of GNSS measurements) applies.
Galileo		Europe's GNSS.
geoid		A more advanced reference surface than the ellipsoid, often used to approximate the shape of the Earth and to measure elevations. Any point on the geoid is subject to the same level of gravity and the earth's geoid is set at the mean sea level.
GLONASS	Global Navigation Satellite System	Russia's GNSS.
GNSS	Global Navigation Satellite System	A global satellite navigation system that uses satellites to provide autonomous geopositioning.
GPS	Global Positioning System	The USA's GNSS, often used as a generic term for any GNSS.
HACC	Horizontal Accuracy	A statistical estimate of horizontal positional accuracy based on a number of factors.
HAE	Height Above Ellipsoid	Vertical height above the nominal ellipsoidal approximation of the earth's surface.
HAS	High Accuracy Service	A Galileo PPP premium accuracy service only available on the L6 band.
HMSL	Height Above Mean Sea Level	Vertical height above the geoid, aka 'Orthometric Height'. The difference between HAE and HMSL is often referred to as the 'separation'.
IODC	Issue of Data, Clock	A unique identifier assigned to each GNSS satellite's clock correction coefficients that is broadcast by the satellite as part of the NAV message.
IODE	Issue of Data, Ephemeris	A unique identifier assigned to each GNSS satellite's ephemeris data set that is broadcast by the satellite as part of the NAV message.
IODS	Issue of Data, Satellite	A unique identifier assigned to each GNSS satellite's orbital accuracy and health data that is broadcast by the satellite as part of the NAV message.
IRNSS	Indian Regional Navigation Satellite System	India's regional satellite navigation system, also known as NavIC.
ITRF	International Terrestrial Reference Frame	A reference frame which takes into account tectonic plate movement and gravitational/magnetic displacements, necessary for mm level accuracy. The “earth-fixed” ITRF is typically not regarded as a “datum” – rather it is the international standard reference framework to which national geocentric datums are aligned.
L1	1575.42 MHz	Original GPS C/A carrier frequency band.
L1c	1575.42 Mhz	A new carrier signal, transmitting on the same frequency as L1, which is intended to provide improved unification between different GNSS constellations. Currently in deployment.
L2	1227.60 MHz	Original GPS P(Y) carrier frequency band.
L5	1176.45 MHz	A more modern GNSS carrier frequency band, currently used by GPS, Galileo, QZSS and NavIC (GLONASS plans to use it from 2025). The use of L5 signals provides the capability of delivering 10x higher precision than a legacy L1 GNSS receiver in an open environment, as well as very noticeable benefits in multipath environments.
L6	1278.75 MHz	A more modern GNSS carrier frequency band, used for Galileo HAS and QZSS CLAS.
L-Band		A radio frequency band covering the spectrum 1 GHz to 2 GHz.
LLH	Latitude, Longitude, Height	The familiar spatial reference system.
LNA	Low-noise Amplifier	A signal booster built into some active GNSS antennae.
MCX	Micro-Coaxial Connector	A common miniature snap-on antenna connector type, similar to SMB.
NAV	Navigation Message	A message broadcast by each GNSS satellite containing empheris, almanac, satellite and atmospheric data as a series of sub-frames.
NavIC	Navigation with Indian Constellation	The operational name of India's IRNSS.
NED	North East Down	A spatial reference system based on the tangent plane defined by the local vertical direction and the Earth's axis of rotation, also known as 'local geodetic coordinate system'.
NMEA	National Marine Electronics Association	The organisation which publishes the proprietary text-based NMEA 0183 protocol - an electrical and data specification widely used for GNSS and maritime control systems. It is slowly being superseded by the NMEA 2000 protocol.
NRTK	Networked Real-time Kinematics	As RTK but using a network of physical or virtual reference stations (CORS) rather than a single reference station.
NTRIP	Networked Transport of RTCM via Internet Protocol	A proprietary RTK DGPS protocol published by the RTCM. At time of writing the latest iteration is 10410.1 aka "NTRIP ver 2.0".
observable		In the context of GNSS, a parameter which may be directly observed and measured by the GNSS receiver (such as PRN code, carrier phase or doppler shift), as opposed to data which must be provided to the receiver (such as ephemerides and almanac).
OSR	Observation Space Representation	In the context of GNSS, a range of traditional techniques (including DGPS, RTK and NRTK) for achieving cm-level precision based on the use of a single physical or virtual fixed-base reference station. The reference station provides 'corrections' to the raw observation data provided by the user's receiver and returns the corrected raw data and pseudoranges. This method makes it possible to provide personalized corrections to each user through two-way communication over the Internet. Contrast with SSR.
PPK	Post-processing kinematics	A high resolution surveying technique, similar in principal to RTK but with corrections being applied retrospectively to a set of captured RAW GNSS navigation and observation data, typically converted to RINEX format.
PPP	Precise Point Positioning	An high precision GNSS positioning technique which utilises a wide area or global network of reference stations to provide "state space" data (e.g. satellite clock, orbit and biases and possibly atmospheric measurements) to GNSS receivers within their coverage area. See also SSR.
PPP-RTK	Precise Point Positioning with Real Time Kinematics	A combination of PPP and RTK techniques which utilises global PPP "state space" data in conjunction with local RTK data to achieve faster accuracy convergence than would be available via PPP alone.
PRN	Pseudo-random Noise	A unique, non-repeating binary code which uniquely identifies each satellite in a GNSS constellation and is used in the determination of pseudorange. Traditionally provided in twoversions denoted C/A and P(Y).
pseudorange		An estimate of the distance between GNSS satellite and receiver antennae, which along with the satellite's ephemerides provides the means by which GNSS establishes a navigation solution. Various mathematical techniques are available to estimate pseudorange using available 'observable' information e.g. PRN codes and/or carrier wave phase and doppler-shift frequency.
P(Y)	Precision (sometimes Protected)	An encrypted 6x10^12-chip PRN code transmitted at 10.23 Mchips/sec (10 times faster than the C/A code). It was originally reserved for military use.
QZSS	Quasi-Zenith Satellite System	Japan's regional navigation satellite system, developed to enhance GPS in the Asia-Oceania regions.
RA	Right Ascension	The angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point in question above the Earth.
RAW		In the context of GNSS, this generally refers to the raw observational data (pseudorange, Doppler shift, carrier phase, phase lock and signal quality) captured by a GNSS receiver prior to conversion to an ECEF or LLH navigation solution. It is used as a precursor to PPK analyses.
residual(s)		The difference between the estimated distance from the receiver to each satellite (range) and the actual range measurements after adjusting them to remove errors.
Rp		See pseudorange.
RF	Radio Frequency	In the context of GNSS this generally refers to the L-Band.
RF240		A higher quality solid-core RF coaxial cable specification which offers reduced attenuation over longer distances than RG58.
RG58		A common specification of stranded-core RF coaxial cable suitable for short cable runs.
RINEX	Receiver Independent Exchange Format	A public domain vendor-agnostic GNSS data protocol published by the RTCM and commonly used for PPK.
RTCM	Radio Technical Commission for Maritime Services	The not-for-profit body which publishes the NTRIP, RTCM3 and RINEX protocols.
RTCM3	RTCM version 10403.n	The latest generation of the proprietary RTCM differential navigation service protocol, as used by most NTRIP service providers. At time of writing the latest iteration is 10403.4.
RTK	Real-time kinematics	A real-time high resolution surveying technique which corrects for common errors in GNSS positional data using measurements of the phase of the signal's carrier wave in addition to the information content of the signal. It relies on a single reference station or interpolated virtual station to provide real-time corrections, providing up to centimeter-level accuracy. See also OSR.
RTN	Real-time Network	A real-time high resolution surveying technique, similar in principle to RTK but using a computed or "virtual" reference station rather than a physical reference station.
SBAS	Satellite-based Augmentation System	These support wide-area or regional augmentation of GNSS positioning through the use of additional satellite-broadcast messages.
SIP	Satellites in Position	The number of satellites actually used in the receiver's navigation solution.
SIV	Satellites in View	The number of satellites the receiver can see.
SMA	Subminiature version A	A standard miniature threaded antenna connector type.
SMB	Subminiature version B	A common miniature snap-on antenna connector type.
SPARTN	Secure Position Augmentation for Real Time Navigation	A public domain RTK DGPS protocol published by u-blox. At time of writing the latest iteration is 2.02.
SSR	State Space Representation	In the context of GNSS, a range of techniques (including PPP) for achieving cm-level precision based on the use of a network of reference stations covering a large geographical area. The network transmits data (including direct observables and ephemerides) to all users in the coverage area via a one-way Internet and/or satellite link, allowing each receiver to model GNSS errors locally and apply corrections to its own observations. Contrast with OSR.
TEC	Total Electronic Content	A measure of the level of ionisation in the Ionosphere - one of the main potential sources of error in GNSS pseudorange determination. See also VTEC.
TNC	Threaded Neill-Concelman	A standard threaded antenna connector type commonly used in surveying equipment.
TOA	Time Of Arrival	The precise time a GNSS signal arrives at the receiver's antenna.
TOF	Time Of Flight	The precise time it takes the phase center of a GNSS signal to travel from satellite to receiver i.e. TOA - TOT.
TOT	Time Of Transmission	The precise time a GNSS signal leaves the satellite's antenna.
TTFF	Time to First Fix	The time it takes a GNSS receiver to establish a navigation solution from startup.
UERE	User equivalent range errors	Collective term for a range of errors which must be compensated for in GNSS pseudorange calculations, sometimes referred to as pseudorange 'residuals'.
UBX		A proprietary (though largely public domain) GNSS binary data protocol published by u-blox. UBX messages tend to provide more comprehensive GNSS information in a more concise format than NMEA.
U.FL	Ultra-fine Fluorinated	An ultra-miniature antenna connector type common in hobbyist equipment.
URA	User range error	See UERE.
VACC	Vertical Accuracy	A statistical estimate of vertical positional accuracy based on a number of factors.
VTEC	Vertical Total Electronic Content	A measure of the level of ionisation in the Ionosphere as a function of altitude, based on a model which assumes that all the ions are concentrated in a layer of inﬁnitesimal thickness located at the specified altitude.