Satellite navigation

Satellite navigation, also called satnav, is the use of satellites for finding places or figuring out where you are. A global navigation satellite system, or GNSS, can help anyone on Earth, whether you're on land, in the air, or at sea. There are four main 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. There are also some regional systems, like Japan's Quasi-Zenith Satellite System (QZSS) and India's Indian Regional Navigation Satellite System (IRNSS, also called NavIC).

Satellite navigation devices can find their location—how far east or west, north or south, and how high up—very accurately, sometimes within just a few centimeters. They do this by listening to time signals sent from satellites. These devices can be used for many things, like helping you drive to a new place, tracking where something is, or even keeping time very precisely. They work even when you don't have phone service or the internet, though those can make the information even better.

Each system uses a group of satellites moving around Earth in special paths called orbits. These satellites are about 20,000 kilometers or 12,000 miles above us and they circle Earth about every twelve hours. This way, they can cover almost the whole planet and help anyone who needs to know where they are.

Classification

Further information: GNSS augmentation

There are different types of satellite navigation systems. GNSS-1 includes older systems like GPS and GLONASS, along with extra tools to make them more accurate, such as satellite-based augmentation systems and ground-based augmentation systems. Examples include the Wide Area Augmentation System in the United States and the European Geostationary Navigation Overlay Service in Europe.

GNSS-2 refers to newer, independent systems like the European Galileo system. These provide better accuracy for things like airplane navigation. They started using a specific frequency called Upper L Band, but now also use Lower L Band frequencies for even better results. Some devices can use both and are called "Dual-band GNSS" devices.

There are four main global satellite navigation systems: GPS from the United States, GLONASS from Russia, BeiDou from China, and Galileo from the European Union. There are also smaller regional systems and tools to improve accuracy, like NAVIC in India and QZSS in Japan. Many devices can use more than one system at the same time.

History

Further information: GPS § History, GLONASS § History, GALILEO#History, and BeiDou § History

Long before satellites, people used radio signals from the ground to find their way. Systems like DECCA, LORAN, GEE, and Omega sent out radio pulses from known places. By measuring the time between these pulses, people could figure out their position.

The first system that used satellites for navigation was called Transit, created by the US military in the 1960s. Transit worked by tracking how the frequency of signals from satellites changed as they moved. This change, known as the Doppler effect, helped receivers determine their location relative to the satellites. Even though there were some challenges like changes in radio signals caused by Earth's atmosphere, scientists found ways to improve the system's accuracy for navigation.

Principles

Further information: GPS § Principles, and GPS § Navigation equations

Satellite navigation uses signals from orbiting satellites to find a user's location. Each satellite sends out information about where it is and the exact time it sent the signal. Your device can then figure out how far away each satellite is by measuring how long it took the signal to arrive. By looking at several satellites at once, the device can pinpoint your exact spot on Earth.

These systems need very accurate clocks, and they use special timekeeping devices on the satellites to stay in sync. Scientists even use theories about how time changes due to gravity to keep these clocks perfect. Modern devices can also use signals from many different satellite systems together to get even better results.

Applications

Main article: GNSS applications

Further information: Automotive navigation system

Satellite navigation was first created for military use. It helps guide weapons accurately to their targets, making them more effective while avoiding unintended harm.

Today, systems like Galileo help people find their location and the location of others or objects anywhere in the world. Satellite navigation has many uses in the future, including science, transport, and agriculture.

Global navigation satellite systems

GPS (1978)

Main article: Global Positioning System

The United States' Global Positioning System (GPS) uses up to 32 satellites to help people find their way. These satellites orbit Earth in six different paths. GPS has been helping people since 1978 and is available all over the world since 1994. It is the most widely used system for finding locations.

GLONASS (1982)

Main article: GLONASS

GLONASS is a satellite system used mainly in Russia. It has been providing service since 1995 and uses 24 active satellites to cover the whole world.

BeiDou (2000)

Main article: BeiDou Navigation Satellite System

BeiDou began as a system for Asia and the Pacific. It expanded over time, and by the end of 2018, it could serve the entire world. The latest part of the system was finished in June 2020 after the last satellite was launched.

Galileo (2011)

Main article: Galileo (satellite navigation)

The European Union and the European Space Agency created Galileo as an alternative to GPS. It became fully operational in December 2016. Galileo uses 30 satellites and works together with GPS to give even more accurate location information. The last satellite was launched in December 2021.

Regional navigation satellite systems

NavIC

Main article: NavIC

NavIC, or Navigation with Indian Constellation, is a regional satellite navigation system made by the Indian Space Research Organisation. The Indian government started this project in May 2006. It uses seven satellites—three in geostationary orbit (GEO) and four in geosynchronous orbit (GSO). This helps cover a larger area with fewer satellites. NavIC can give very accurate positions, better than 7.6 metres, across India and a surrounding region about 1,500 km wide. There is also a larger service area stretching even further.

By 2018, NavIC was ready for public use. It offers two types of service: one open for everyone and another special, encrypted service for authorized users. India plans to grow NavIC by adding more satellites, eventually making it a global system called Global NavIC, free for everyone worldwide.

Early BeiDou

Main articles: BeiDou-1 and BeiDou-2

China's BeiDou system had its first two versions made to work well in certain areas, not everywhere.

Augmentation

GNSS augmentation helps make navigation systems more exact and reliable by adding extra information. Examples include the Wide Area Augmentation System, the European Geostationary Navigation Overlay Service, the Multi-functional Satellite Augmentation System, Differential GPS, GPS-aided GEO augmented navigation (GAGAN), and inertial navigation systems.

QZSS

Main article: Quasi-Zenith Satellite System

The Quasi-Zenith Satellite System (QZSS) is a regional system to improve GPS in Japan and the Asia-Oceania areas. It began trial services in January 2018 and official services in November 2018. The first satellite was launched in September 2010, and a full system with 7 satellites is planned for 2023.

Comparison

See also: GPS signals, GLONASS signals, Galileo signals, and BeiDou signals

Using more than one satellite system helps find your location faster and more accurately. In 2019, the errors in measuring position were smallest for Galileo at just over an inch, followed by GPS at about two inches. The other systems had larger errors, but using updates for the satellites' positions helped make them more accurate too. The newest part of the BeiDou system was still being set up, so its measurements were not as precise as the others yet.

Related techniques

Further information: Satellite geodesy § Radio techniques

DORIS

Main article: DORIS (satellite system)

DORIS is a French system that helps with very exact navigation. Instead of satellites sending signals, it uses special stations on Earth to send signals up to satellites. This helps scientists know exactly where the satellites are. This system can also be used on the ground, though it does not cover as much area as other systems. When used together with other navigation systems, it can help find positions even more accurately.

LEO satellites

The two current operational low Earth orbit (LEO) satellite phone networks can follow special devices and know where they are within a few kilometres. They do this by measuring how the satellite’s signal changes. The information can be shown on a screen or used to control certain phone features based on location.

International regulation

The International Telecommunication Union (ITU) defines a radionavigation-satellite service (RNSS) as a special kind of service used for navigation from space. This service is very important for safety and must be protected.

There are special types of this service for use on airplanes and ships. These help make sure that planes and ships can find their way accurately using satellites.

Examples of these satellite navigation systems include BeiDou Navigation Satellite System, GALILEO, Global Positioning System, and GLONASS.

Alternatives

Alternative Positioning, Navigation and Timing (AltPNT) is an idea for ways to find your location that are not the same as using satellites. Some of these ways include:

• Inertial navigation systems (INS)
• eLORAN
• Terrain-based navigation (TBN)
• Visual Positioning Systems (VPS)
• LiDAR