RTK & GNSS Glossary
Definitions of key terms in RTK correction services and GNSS technology.
- Almanac
- Coarse orbital data for all satellites, used by a receiver to know which satellites should be visible.
- Ambiguity Resolution
- The process of determining the whole number of carrier wavelengths between satellite and receiver, required for an RTK fix.
- Antenna Phase Center
- The electrical point where an antenna actually receives signals; its offset must be accounted for in precise work.
- ARP
- Antenna Reference Point — a physical point on the antenna from which height measurements are taken.
- Base Station
- A stationary GNSS receiver at a known location that generates corrections for nearby rovers.
- Baseline
- The vector between a base station and a rover; longer baselines reduce RTK accuracy.
- BeiDou
- China's global navigation satellite system.
- Carrier Phase
- Measurement of the satellite signal's carrier wave, enabling centimeter-level positioning.
- Caster
- A server that distributes NTRIP correction streams from one or many base stations to rovers.
- Code Measurement
- Pseudorange measurement based on the satellite's code signal; less precise than carrier phase.
- Convergence Time
- The time a PPP or PPP-RTK solution takes to reach its stated accuracy.
- CORS
- Continuously Operating Reference Station — a permanent GNSS base that streams or logs data continuously.
- Cycle Slip
- A sudden break in carrier-phase tracking, often caused by obstructions, that can cause loss of fix.
- Datum
- A reference model that defines how coordinates relate to the Earth.
- DGNSS
- Differential GNSS — code-based corrections giving sub-metre accuracy, less precise than RTK.
- DOP
- Dilution of Precision — a measure of satellite geometry quality; lower is better.
- Dual-Frequency
- A receiver that tracks two signal frequencies per constellation, allowing ionospheric error removal.
- ECEF
- Earth-Centered, Earth-Fixed — a Cartesian coordinate system with its origin at Earth's centre.
- EGNOS
- European satellite-based augmentation system providing metre-level corrections.
- Elevation Mask
- A minimum satellite elevation angle below which signals are ignored to reduce errors.
- Ellipsoid
- A mathematical surface approximating Earth's shape, used as the reference for GNSS heights.
- Ellipsoidal Height
- Height above the reference ellipsoid, as measured directly by GNSS.
- Ephemeris
- Precise orbital data broadcast by each satellite to compute its position.
- ETRS89
- European Terrestrial Reference System 1989, the standard reference frame for Europe.
- Fix
- A state where the rover has resolved carrier-phase ambiguities and is delivering full RTK accuracy.
- Float
- A degraded RTK state with unresolved ambiguities; accuracy is decimeter-level rather than centimeter.
- Galileo
- The European Union's global navigation satellite system.
- Geoid
- A model of mean sea level used to convert ellipsoidal heights to orthometric heights.
- GGA
- An NMEA sentence reporting position, fix quality and satellite count; sent to VRS casters.
- GLONASS
- Russia's global navigation satellite system.
- GNSS
- Global Navigation Satellite System — a generic term covering GPS, GLONASS, Galileo, BeiDou, and others.
- GPS
- Global Positioning System — the United States' satellite navigation system.
- Ground Control Point
- A surveyed marker on the ground used to georeference and verify mapping data.
- HDOP
- Horizontal Dilution of Precision — geometric quality of horizontal position.
- IMU
- Inertial Measurement Unit — sensors used with GNSS for tilt compensation and dead reckoning.
- Initialization
- The process of a receiver reaching an RTK fix after startup or signal loss.
- Ionosphere
- Upper atmosphere layer whose charged particles delay GNSS signals; a major error source.
- ITRF
- International Terrestrial Reference Frame, the global standard reference frame.
- L-Band
- Radio frequency band used to deliver satellite-based corrections without internet.
- L1 / L2 / L5
- GPS signal frequencies; modern receivers track several for better accuracy and speed.
- Localization
- Fitting GNSS coordinates to a local site grid using known control points.
- MAC
- Master-Auxiliary Concept — a network RTK method using a master and auxiliary stations.
- Machine Guidance
- Using RTK positioning to guide construction or agricultural machinery.
- Mountpoint
- A named correction stream on an NTRIP caster.
- MSM
- Multiple Signal Messages — the modern RTCM 3 message family for multi-constellation data.
- Multi-Constellation
- Tracking satellites from several systems (GPS, Galileo, GLONASS, BeiDou) at once.
- Multipath
- GNSS signal reflections off nearby surfaces that degrade positioning accuracy.
- Network RTK
- RTK using a network of reference stations to model errors over a wide area.
- NMEA 0183
- A standard text format for receivers to output position and status data.
- NTRIP
- Networked Transport of RTCM via Internet Protocol — used to stream RTK corrections over the internet.
- Orthometric Height
- Height above the geoid, approximately height above mean sea level.
- OSR
- Observation Space Representation — conventional RTK corrections expressed as observations.
- PDOP
- Position Dilution of Precision — combined geometric quality for 3D position.
- PPK
- Post-Processed Kinematic — applying corrections after data collection rather than in real time.
- PPP
- Precise Point Positioning — uses precise satellite orbit and clock data to position a single receiver without a local base.
- PPP-RTK
- A hybrid technique combining PPP corrections with RTK ambiguity resolution for fast cm-level positioning.
- Projection
- A method of representing Earth's curved surface on a flat map grid, such as UTM.
- Pseudorange
- The approximate distance from receiver to satellite, derived from signal travel time.
- QZSS
- Japan's regional satellite navigation system.
- Rapid Static
- A survey method recording data for several minutes per point for high accuracy.
- Reference Frame
- A realisation of a coordinate system defined by a set of known station positions.
- RINEX
- Receiver Independent Exchange format — the standard file format for raw GNSS data.
- RMS
- Root Mean Square — a statistical measure of positioning accuracy.
- Rover
- A mobile GNSS receiver that uses corrections from a base or network to compute its precise position.
- RTCM
- Radio Technical Commission for Maritime Services — the standard format for GNSS correction messages.
- RTK
- Real-Time Kinematic — a satellite navigation technique used to enhance GNSS accuracy to the centimeter level.
- RTK Extend
- A receiver feature that maintains near-RTK accuracy briefly after correction loss.
- SBAS
- Satellite-Based Augmentation System, such as WAAS or EGNOS, giving metre-level corrections.
- Signal-to-Noise Ratio
- A measure of signal strength relative to background noise.
- Single-Base RTK
- RTK using corrections from one nearby base station.
- SSR
- State Space Representation — corrections describing individual error sources, used in PPP-RTK.
- Static Survey
- Long-duration GNSS observation for the highest accuracy, typically post-processed.
- Tilt Compensation
- Using an IMU to correct for a survey pole that is not held vertically.
- Time to First Fix
- The time a receiver takes to calculate its first position after startup.
- Troposphere
- Lower atmosphere layer that delays GNSS signals depending on temperature, pressure and humidity.
- UHF Radio
- A radio link commonly used to send corrections from a local base to a rover.
- VDOP
- Vertical Dilution of Precision — geometric quality of vertical position.
- VRS
- Virtual Reference Station — synthetic correction stream tailored to a rover's approximate location.
- WAAS
- Wide Area Augmentation System — North America's SBAS.
- WGS84
- World Geodetic System 1984, the reference frame used by GPS.