Galileo (satellite navigation) in the context of "EGNOS"

Time dilation is the difference in elapsed time as measured by two clocks, either because of a relative velocity between them (special relativity), or a difference in gravitational potential between their locations (general relativity). When unspecified, "time dilation" usually refers to the effect due to velocity. The dilation compares "wristwatch" clock readings between events measured in different inertial frames and is not observed by visual comparison of clocks across moving frames.

These predictions of the theory of relativity have been repeatedly confirmed by experiment, and they are of practical concern, for instance in the operation of satellite navigation systems such as GPS and Galileo.

Satellite navigation (satnav) or satellite positioning is the use of artificial satellites for navigation or geopositioning. A global navigation satellite system (GNSS) provides coverage for any user on Earth, including air, land, and sea. There are four operational GNSS systems: the United States Global Positioning System (GPS), Russia's Global Navigation Satellite System (GLONASS), China's BeiDou Navigation Satellite System (BDS) and the European Union's Galileo. Furthermore, there are two regional navigation satellite systems (RNSS) in the form of Japan's Quasi-Zenith Satellite System (QZSS), and the Indian Regional Navigation Satellite System (IRNSS, also known as NavIC).

A satellite-based augmentation system (SBAS) is a system that is designed to enhance the accuracy of the global GNSS systems. The SBAS systems include Japan's Quasi-Zenith Satellite System (QZSS), India's GAGAN, and the European EGNOS, all of them based on GPS.

A satellite navigation device, also called a satnav device or GPS device, uses satellites of the Global Positioning System (GPS) or similar global navigation satellite systems (GNSS) to determine the user's geographic coordinates. It may also display the user's position on a map and offer routing directions (as in turn-by-turn navigation).

As of 2023, four GNSS systems are operational: the original United States' GPS, the European Union's Galileo, Russia's GLONASS, and China's BeiDou Navigation Satellite System. The Indian Regional Navigation Satellite System (IRNSS) will follow and Japan's Quasi-Zenith Satellite System (QZSS) scheduled for 2023 will augment the accuracy of a number of GNSS.

Thales Alenia Space (/ˈθɑːlɛz/) is a European spaceflight services joint venture between the French technology corporation Thales Group (67%) and Italian defense conglomerate Leonardo (33%). It forms one half of the Space Alliance along with Telespazio which is also owned by Leonardo (67%) and Thales (33%).

It provides space-based systems, including satellites and ground segments, used for telecommunications, navigation, earth observation, space exploration and scientific purposes. The company is a major industrial participant in the International Space Station (ISS), having produced numerous pressurized modules including the Cupola, the Harmony and Tranquility nodes, the Columbus laboratory and for the Cygnus cargo spacecraft. It is a key contributor to Galileo, a European global satellite navigation system, being responsible for the ground segment in particular. In 2021, the company was also awarded a contract by the European Space Agency (ESA) and the European Commission to build 6 of the 12 new Galileo Second Generation satellites. The company is also an important industrial partner towards development of the Lunar Gateway.

The BeiDou Navigation Satellite System (BDS; Chinese: 北斗卫星导航系统; pinyin: běidǒu wèixīng dǎoháng xìtǒng) is a satellite-based radio navigation system owned and operated by the China National Space Administration. It provides geolocation and time information to a BDS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more BDS satellites. It does not require the user to transmit any data and operates independently of any telephonic or Internet reception, though these technologies can enhance the usefulness of the BDS positioning information.

The current service, BeiDou-3 (third-generation BeiDou), provides full global coverage for timing and navigation, along with Russia's GLONASS, the European Galileo, and the US's GPS. It comprises satellites in three types of orbits: 24 in medium Earth orbit (global coverage), 3 in inclined geosynchronous orbit (Asia–Pacific coverage), and 3 in geostationary orbit (China coverage). The BeiDou-3 system was fully operational in July 2020. In 2016, BeiDou-3 reached millimeter-level accuracy with post-processing.