# What is GNSS? Satellite navigation and Galileo explained GNSS (Global Navigation Satellite System) is the generic term for satellite systems that provide positioning, navigation and timing with global coverage, such as GPS, GLONASS, Galileo and BeiDou. A receiver measures how long signals from at least four satellites take to arrive and computes its position and the time. Augmentation services such as EGNOS improve accuracy and add integrity information. Satellite navigation tells a device where it is and what time it is. It runs in phones, vehicles, machines, ships, aircraft, power grids and telecom networks. This page explains how GNSS works, how GPS, Galileo and EGNOS relate to each other, how accurate a position is and where the limits are. **Definition: GNSS (Global Navigation Satellite System).** GNSS is the generic term for satellite navigation systems that provide positioning with global coverage. It includes GPS, GLONASS, Galileo and BeiDou, as well as regional systems. ## Positioning, navigation and timing GNSS delivers three things, often abbreviated as PNT [2]: - **Positioning:** where a receiver is, as coordinates. - **Navigation:** how it moves, derived from its position over time, and guidance along a route. - **Timing:** a precise time signal, because every receiver computes the time as well as its position. GPS is not another word for GNSS. It is one system, owned by the United States [2]. The other global systems are GLONASS, continued by the Russian Federation, the Chinese BeiDou system [3] and Galileo, Europe's own system under civilian control [6]. Many receivers use several systems at once. EUSPA notes that Galileo improves accuracy in urban canyons when used in multi-constellation receivers, because the additional satellites increase visibility and measurement redundancy [19]. ## How satellite positioning works Each satellite carries atomic clocks and transmits navigation data from which the receiver computes where the satellite is. The receiver measures how long the signal took to arrive and multiplies this travel time by the speed of light. The result is its distance to the satellite [4]. Receivers have inexpensive quartz clocks, so every measured distance contains the same unknown clock error. The receiver therefore solves for four unknowns: three coordinates and its own clock offset. This is why it needs signals from at least four satellites [4]. Because it solves for the clock offset, the receiver also knows the time precisely, which makes GNSS a time source as well as a position source. The position of the satellites in the sky matters too. When they are bunched together, the same measurement error causes a larger position error, an effect called dilution of precision [4]. A navigation system has three parts: the satellites (space segment), the control centres and sensor stations on the ground (ground segment), and the receivers (user segment). EUSPA describes Galileo in these three segments [5]. ## Galileo and EGNOS: Europe's satellite navigation Galileo and EGNOS are two of the five components of the EU Space Programme, established by Regulation (EU) 2021/696 [7]. **Galileo** is the EU's global navigation satellite system. Unlike the other global systems, it is a civilian system. The EU owns the programme and the European Commission is its programme manager. EUSPA is responsible for the operational management of the services, and ESA is entrusted with the design and evolution of the system [5]. Galileo started its initial services on 15 December 2016 [6]. For businesses, the question is which Galileo services they can use and on what terms: **Galileo services and who can use them** | What it provides | Who can use it | | ------------------------------ | ------------------------------------------------------------------------------------------------------- | | Open Service | Positioning, navigation and timing, as used in smartphones and vehicle navigation | | High Accuracy Service (HAS) | Corrections for higher accuracy than the Open Service, typically below 25 cm horizontally | | OSNMA | Authentication of the navigation message, so that a receiver can check that the data comes from Galileo | | Public Regulated Service (PRS) | Encrypted navigation with high continuity, more robust against malicious interference | | Search and Rescue Service | Relays signals from distress beacons to rescue services | Status: October 2026. The services are described by EUSPA [8]. HAS went live in January 2023 as an initial service [9], and the full service is still in preparation [10]. OSNMA has been operational since 24 July 2025 [11]. It authenticates the navigation message, not the ranging signal itself. EUSPA lists three further services under development: a Signal Authentication Service, which will add authentication of the ranging signal on E6, an Emergency Warning Satellite Service and a Timing Service [8]. **EGNOS** is Europe's regional satellite-based augmentation system (SBAS). Ground stations monitor the GPS signals and compute corrections and integrity messages, which three geostationary satellites broadcast to users [12]. Its free Open Service improves GPS accuracy to 1 to 2 metres and needs no authorisation [13]. Since its certification for civil aviation in 2011, EGNOS has increased the safety and accessibility of airports, in particular small and regional airports and airfields. Its coverage is limited to Europe and some neighbouring countries. The next generation, EGNOS V3, is in preparation and will augment Galileo as well as GPS [12]. ## How accurate is GNSS? There is no single answer: accuracy depends on the method and on the conditions. Typical values from the operators and from ESA's GNSS reference: **Figure:** Typical accuracy by method on a logarithmic scale: around 5 m with GPS alone, 1 to 2 m with EGNOS, below 25 cm with Galileo HAS and a few centimetres with RTK. Values under nominal conditions, from the sources named below. **Typical accuracy by positioning method** | Typical accuracy | What it needs | | ------------------------------- | ------------------------- | | GPS without augmentation | Around 5 m in open sky | | EGNOS Open Service | 1 to 2 m | | Galileo HAS | Below 25 cm horizontally | | PPP (precise point positioning) | Centimetres to decimetres | | RTK (real-time kinematic) | A few centimetres | Values under nominal conditions as stated by the sources. Sources: EUSPA for GPS without augmentation and EGNOS [13] and for HAS [9]; ESA Navipedia for PPP [14] and RTK [15]. How construction companies choose between these corrections is covered under [GNSS machine guidance and surveying](https://leto.space/use-cases/gnss-machine-guidance/). Accuracy is only one requirement. **Integrity** describes whether a system warns its user in time when a position should not be trusted. EGNOS adds an integrity message for this purpose [12], and its Safety of Life Service is used mainly in civil aviation [13]. For critical national infrastructure, the UK Blackett review of 2018 recommends that procurers specify requirements for accuracy, integrity, availability and continuity [17]. ## What GNSS is used for GNSS applications in industry rely on position, on time, or on both. EUSPA describes, among others, centimetre-level surveying and machine guidance with RTK networks, the time synchronisation of phasor measurement units in power grids and the alignment of time and frequency between the base stations of mobile networks [19]. The use cases in our library that rely on satellite navigation: [Browse all use cases](https://leto.space/use-cases/) ## Limits **Limits** - **Weak signals can be jammed or spoofed:** GNSS signals reach the ground well below the level of ambient noise, and more powerful transmissions in the GNSS band can overwhelm a receiver \[17]. Jamming blocks the signals, spoofing feeds false ones \[18]. Our use case on GNSS jamming and spoofing explains how to protect operations. - **No signal indoors or underground:** GNSS is not available indoors or underground \[17]. Near buildings, signals reflected by walls reach the antenna by several paths and distort the measurement \[16]. - **Published accuracy is conditional:** The figures above apply under nominal conditions. Satellite geometry \[4], the antenna and reflecting structures nearby \[16] change the result, so accuracy needs testing where it matters. - **Dependence is often hidden:** Systems that look independent of GNSS, such as some network time services, can take their time from GNSS receivers. Even careful operators may not know how much they rely on it \[17]. ## Key terms - **PNT:** positioning, navigation and timing, the three services GNSS provides. - **Constellation:** the set of satellites that make up a navigation system. - **Pseudorange:** the distance to a satellite computed from the signal travel time, still containing the receiver's clock error. - **SBAS:** satellite-based augmentation system, such as EGNOS, which broadcasts corrections and integrity information via geostationary satellites. - **RTK:** real-time kinematic positioning, which uses corrections from a nearby base station to reach a few centimetres. - **PPP:** precise point positioning, which uses precise satellite orbits and clocks to position a single receiver without a nearby base station. - **Integrity:** the ability of a system to warn its users in time when it should not be used. - **Multipath:** signals that reach the antenna by reflection as well as directly, which distorts the measurement. - **OSNMA:** Galileo Open Service Navigation Message Authentication, which lets receivers check that navigation data is genuine. - **Jamming and spoofing:** deliberate interference that blocks GNSS signals, or counterfeit signals that make a receiver compute a wrong position or time. ## How we help We help organisations decide whether satellite positioning or timing fits a task, which accuracy, integrity and continuity it needs, and how to specify and procure a solution that delivers it. If you are new to the field, start with the definition of [space-enabled solutions](https://leto.space/knowledge/space-enabled-solutions/), which places satellite navigation next to Earth observation, satellite communication and AI. **Next steps** - [Our consulting](https://leto.space/consulting/): From the use case to requirements, provider selection and introduction. - [Satellite communication](https://leto.space/knowledge/satellite-communication/): How satellite links work and when they pay off. - [GNSS jamming and spoofing](https://leto.space/use-cases/gnss-resilience/): How to protect operations that depend on satellite position or time. ## Frequently asked questions ### What is the difference between GPS and GNSS? GNSS is the generic term for satellite navigation systems with global coverage [1]. GPS is one of them, owned by the United States [2]. GLONASS, Galileo and BeiDou are the others. Many receivers combine several systems, which gives them more satellites to work with. ### How accurate is GNSS? It depends on the method. A GPS receiver without augmentation typically reaches around 5 metres in open sky, and the EGNOS Open Service improves this to 1 to 2 metres [13]. The free Galileo High Accuracy Service typically reaches below 25 centimetres horizontally [9], and RTK corrections from a nearby base station reach a few centimetres [15]. ### What is the Galileo High Accuracy Service? The High Accuracy Service (HAS) is a free Galileo service that sends precise point positioning corrections worldwide, through the Galileo signal and over the internet. Under nominal conditions it typically reaches an accuracy below 25 centimetres horizontally. It serves professional applications such as surveying, precision agriculture and civil engineering [9]. Its initial service started in January 2023, and the full service is still in preparation [10]. ### What are jamming and spoofing? Jamming is radio interference that stops a receiver from locking onto the satellite signals. Spoofing broadcasts counterfeit signals, so the receiver computes a wrong position, navigation or time [18]. How organisations protect their operations is described in our use case on [GNSS jamming and spoofing](https://leto.space/use-cases/gnss-resilience/). ### Is Galileo free to use? The Open Service, the High Accuracy Service and the authentication service OSNMA are free of charge. The encrypted Public Regulated Service is reserved for government-authorised users [8]. ## Sources 1. [Navipedia: GNSS (glossary entry), European Space Agency (ESA), Navipedia, accessed 2026-10-06](https://gssc.esa.int/navipedia/index.php/GNSS) 2. [GPS.gov: What is GPS?, U.S. government, GPS.gov, accessed 2026-10-06](https://www.gps.gov/) 3. [Navipedia: GLONASS General Introduction, European Space Agency (ESA), Navipedia, accessed 2026-10-06](https://gssc.esa.int/navipedia/index.php/GLONASS_General_Introduction) 4. [Navipedia: An intuitive approach to the GNSS positioning, European Space Agency (ESA), Navipedia, accessed 2026-10-06](https://gssc.esa.int/navipedia/index.php/An_intuitive_approach_to_the_GNSS_positioning) 5. [Galileo, EU Agency for the Space Programme (EUSPA), accessed 2026-10-06](https://www.euspa.europa.eu/eu-space-programme/galileo) 6. [Navipedia: Galileo General Introduction, European Space Agency (ESA), Navipedia, accessed 2026-10-06](https://gssc.esa.int/navipedia/index.php/Galileo_General_Introduction) 7. [Regulation (EU) 2021/696 of 28 April 2021 establishing the Union Space Programme and the European Union Agency for the Space Programme, European Union, EUR-Lex, 2021-05-12](https://eur-lex.europa.eu/eli/reg/2021/696/oj/eng) 8. [Galileo services, EU Agency for the Space Programme (EUSPA), 2026-09-17](https://www.euspa.europa.eu/eu-space-programme/galileo/services) 9. [Galileo High Accuracy Service goes live!, EU Agency for the Space Programme (EUSPA), 2023-01-24](https://www.euspa.europa.eu/newsroom-events/news/galileo-high-accuracy-service-goes-live) 10. [Galileo High Accuracy Service (HAS), European GNSS Service Centre (GSC), operated for EUSPA, accessed 2026-10-07](https://www.gsc-europa.eu/galileo/services/galileo-high-accuracy-service-has) 11. [From Testing to Operations: Galileo OSNMA Service Now Available to Users, EU Agency for the Space Programme (EUSPA), 2025-09-05](https://www.euspa.europa.eu/newsroom-events/news/testing-operations-galileo-osnma-service-now-available-users) 12. [EGNOS, EU Agency for the Space Programme (EUSPA), accessed 2026-10-06](https://www.euspa.europa.eu/eu-space-programme/egnos) 13. [EGNOS services, EU Agency for the Space Programme (EUSPA), 2025-11-19](https://www.euspa.europa.eu/eu-space-programme/egnos/services) 14. [Navipedia: Precise Point Positioning, European Space Agency (ESA), Navipedia, accessed 2026-10-06](https://gssc.esa.int/navipedia/index.php/Precise_Point_Positioning) 15. [Navipedia: RTK Fundamentals, European Space Agency (ESA), Navipedia, accessed 2026-10-06](https://gssc.esa.int/navipedia/index.php/RTK_Fundamentals) 16. [Navipedia: Multipath, European Space Agency (ESA), Navipedia, 2011](https://gssc.esa.int/navipedia/index.php/Multipath) 17. [Satellite-derived time and position: a study of critical dependencies (Blackett review), UK Government Office for Science, 2018-01-30](https://www.gov.uk/government/publications/satellite-derived-time-and-position-blackett-review) 18. [Global Navigation Satellite System (GNSS) outages and alterations, European Union Aviation Safety Agency (EASA), 2026-07-03](https://www.easa.europa.eu/en/domains/air-operations/global-navigation-satellite-system-outages-and-alterations) 19. [EU Space Market Report 2026, Issue 1, EU Agency for the Space Programme (EUSPA), 2026-05-26](https://www.euspa.europa.eu/sites/default/files/documents/EU_Space_Market_Report_2026.pdf) Page: