Glossary: GNSS Terms and Definitions

Reference guide for terminology and technical concepts

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z




A measure of the correctness of a GNSS positioning solution. Accuracy is the difference between the actual position of an object and its location as measured by the GNSS positioning solution.


Advanced Driver Assistance Systems. These are technologies that assist drivers with the safe operation of a vehicle, leveraging various sensors to detect hazardous conditions or driver errors and provide alerts or interventions.

Alert Limit

A parameter of the integrity concept. The alert limit is the largest positioning error allowable to maintain safe operations. If a positioning system cannot ensure accuracy within the alert limit, it is considered unavailable and must provide a warning.


Automotive Safety Integrity Level. ASIL is a risk classification system defined by the ISO 26262 standard for the functional safety of road vehicles.


Automotive Software Performance Improvement and Capability dEtermination. ASPICE is a maturity model that is used to assess the development processes for suppliers of electronic and software-based systems.


BeiDou Navigation Satellite System (BDS)

A satellite navigation system owned and operated by the China National Space Administration.



A collection of satellites that make up a global navigation satellite system. Constellations are owned and operated by individual countries or regions. The constellations are: GPS (United States), BeiDou (China), Galileo (EU), GLONASS (Russia), and QZSS (Japan).


Continuously Operating Reference Station. A CORS is a reference station that provides continuous and permanent real-time positioning information for a specific area.



A reference frame that enables the location of latitudes and longitudes to be identified onto the surface of the earth.


Differential GNSS. DGNSS corrections leverage a base station in a precisely known location to calculate the position of a rover. DGNSS is similar in methodology to RTK, but DGNSS leverages code phase, resulting in lower accuracy but also a longer baseline.

Dead Reckoning

Dead reckoning complements GNSS positioning in environments where GNSS signals are unreliable. Dead reckoning leverages inertial sensors such as accelerometers as well as wheel ticks to measure the movement of a vehicle or robot and calculate its position.


Earth-Centered, Earth-Fixed (ECEF)

Earth-centered, Earth-fixed is a three-dimensional Cartesian coordinate system with it’s origin at the Earth’s center of mass. It represents positions in meters in X,Y,Z. These coordinates can be converted to geodetic coordinates (latitude and longitude).


Fixed RTK

RTK uses a complicated mathematical algorithm to calculate the exact number of radio wavelengths between the satellites and the base station antenna (known as ambiguity resolution) and yield either a fixed or float solution. In a fixed solution, the number of wavelengths is a whole number, or integer, and the algorithm is constrained to yield a whole number. Fixed RTK is the most accurate solution. A low number of visible satellites, poor satellite constellation geometry and a poor radio link between the base station and the rover may prevent a fixed solution.

Float RTK

In a float solution, the algorithm does not yield an acceptable fixed solution, so the ambiguity is allowed to be a decimal or floating point number. Float RTK is less accurate than fixed RTK. If a float solution is the only solution available, it may be possible to reinitialize an RTK system, or simply wait, for a more precise fixed solution. However, if poor satellite visibility is to blame, a fixed solution may be unavailable.

Functional Safety

Functional safety requirements are intended to avoid unreasonable risks derived from hazards caused by a malfunctioning of a system (usually a vehicle). Functional safety standards for the automotive industry are defined in ISO 26262.


Galileo (GAL)

A satellite navigation system owned and operated by the European Space Agency and the European Union Agency for the Space Programme.


A Russian satellite navigation system.


Global Navigation Satellite System. GNSS refers to a system that uses satellites to provide signals from space that transmit positioning and timing data with global coverage.


A satellite navigation system owned and operated by the United States. GPS is the oldest GNSS system.


Hardware-in-the-Loop Testing

A technique where recorded signals from a receiver are connected to a test system that runs simulations of potential real-world scenarios. Hardware-in-the-loop testing is used when a large number of conditions need to be tested to ensure the robustness and safety of a system, making it challenging or impossible to test each condition sufficiently in real-world test environments.


Inertial Measurement Unit (IMU)

An inertial measurement unit is a sensor that measures gravity and angular rate of an object. The IMU is a component of a positioning system within a vehicle or device.


The measure of trust that can be placed in the correctness of the information supplied by a GNSS system. Integrity is expressed in terms of protection levels (the accuracy that is guaranteed with exceedingly high confidence) and integrity risk (the probability of providing a position outside the protection level).

Integrity Risk

A parameter of the integrity concept. The integrity risk is the probability of providing a position outside the protection level.

ISO 21434

Standard for cybersecurity engineering in road vehicles defined by the International Organization for Standardization.

ISO 26262

12-part standard that defines functional safety in road vehicles defined by the International Organization for Standardization.

ISO 21448

Standard that defines safety of the intended functionality (SOTIF) in road vehicles defined by the International Organization for Standardization.


International Terrestrial Reference Frame. ITRF is used to measure position on or near the Earth's surface. New ITRF solutions are produced every few years leveraging the latest mathematical and surveying techniques to maximize accuracy.





Measurement Engine (ME)

The measurement engine is responsible for turning raw radio signals into observations. The ME is a component of a positioning system within a vehicle or device.


Network RTK

An RTK configuration where multiple reference stations are used to calculate corrections for a rover, rather than a single local base station. Network RTK is valuable for use cases where a device may move from one location to another and cannot be tied down to a physical base station.


Networked Transfer of RTCM over Internet Protocol. NTRIP is a common messaging protocol for streaming GNSS data over the internet.



Measurements made by a GPS receiver of the pseudorange, carrier phase, and Doppler shift of each satellite. The pseudorange and carrier phase are (noisy) measurements of the distance to the satellite, and the Doppler shift is a measurement of how these values, specifically the carrier phase, change. These important values form the core data used to produce accurate position solutions.


Observation Space Representation. OSR is a data format in which GNSS errors are all added together into a single value per satellite and signal, along the line-of-sight vector between the satellite and the reference station location.


Positioning Engine (PE)

The positioning engine takes inputs from multiple sensors, such as GNSS observations and corrections, inertial measurement, and wheel ticks, and calculates the precise position of a vehicle or device.

Protection Level

A parameter of the integrity concept. The protection level is the accuracy that can be achieved within the defined target integrity risk (TIR). A positioning system is considered available if the protection level is smaller than the alert limit.


Precise Point Positioning. PPP is a navigation technique that leverages satellites and a global network of reference stations to compute the position of a receiver anywhere in the world.


PPP-RTK is a hybrid navigation technique combining precise point positioning (PPP) and real-time kinematics (RTK). PPP-RTK combines the global coverage of PPP with the high accuracy of RTK to deliver high-precision positioning on a global scale.



A satellite navigation system owned and operated by Japan.


Reference Frame

A framework for coordinates of points located on the Earth's surface. There are multiple internationally defined reference frames that are commonly used in GNSS systems, ITRF being one of the most popular.


Receiver Independent Exchange Format. RINEX is a format for raw satellite navigation system data.


A rover is a component of a GNSS system. The rover refers to the device in the field, whether it is a vehicle, a drone, a robot, or a handheld receiver. It is the object whose position needs to be determined.


Radio Technical Commission for Maritime Services. A messaging standard for exchanging differential corrections. 


Real-Time Kinematic. RTK satellite navigation is a technique used to enhance the precision of position data derived from satellite-based positioning systems. RTK systems leverage a base station at a precisely known location to transmit GNSS corrections to rovers to enable them to calculate their position with centimeter accuracy.



Satellite-Based Augmentation System. A network of fixed, ground-based reference stations that enhance the precision of satellite positioning systems. Like the satellite constellations they enhance, SBAS systems are run by countries or government agencies.

Single Base RTK

An RTK configuration where the rover is receiving corrections from a single physical base station. This is in contrast to network RTK, where the information from multiple reference stations may be incorporated into the corrections.

Software-in-the-Loop Testing

A method of testing and validating code in a simulation environment in order to quickly and cost-effectively catch bugs and improve the quality of the code. Software-in-the-loop testing is commonly used in the automotive industry as OEMs are increasingly building software-defined vehicles.


Safety of the Intended Functionality. SOTIF requirements are intended to avoid unreasonable risks due to potentially hazardous behaviors related to functional insufficiencies or deficiencies. SOTIF standards for the automotive industry are defined in ISO 21448.


State Space Representation. SSR is a data format in which GNSS errors are disaggregated and a receiver is sent a set of corrections that separately describe each individual type of GNSS error.


A GNSS receiver that is installed in a fixed, precisely known position on the surface of the Earth. These stations are used either individually in an RTK solution or as a network to provide corrections to a larger region.


Target Integrity Risk (TIR)

A parameter of the integrity concept. The target integrity risk is the desired probability of providing a position outside the protection level. The safety requirements of the use case will inform how small the target integrity risk must be.


Time to First Fix. A measure of the time required for a GNSS system to acquire satellite signals and calculate a position solution.




Vehicle-to-Everything. A system where a vehicle can share information from its sensors, cameras, and internal systems with other vehicles, nearby pedestrians, and infrastructure.


Virtual Reference Station. A VRS is an element of a network RTK solution in which a software-defined base station is created near a rover. The rover cannot distinguish between a physical base station and a VRS.


Wheel Odometry

A sensor that leverages wheel ticks to measure the distance a vehicle has traveled. Wheel odometry is used in conjunction with other sensors such as inertial measurement units and GNSS systems.