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. |
| Beidou | China's GNSS. | |
| 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 RTK correction source (e.g. NTRIP caster) which runs 24/7/365 or thereabouts. |
| 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. |
| 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 GNSS 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. |
| 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". |
| 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 GNSS positioning technique offering centimeter-level accuracy with near-consumer-grade hardware without the need for external fixed base reference stations (unlike RTK). |
| 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 two versions 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. |
| 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. |
| 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. |
| 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. |