What are the different GNSS Constellations?

Global Navigation Satellite System (GNSS) constellations refer to a collection of Earth-orbiting satellites that provide positioning, navigation and timing data to GNSS receivers on Earth. There are various GNSS constellations in use around the world which include GPS (Global Positioning System) developed by the United States, GLONASS (GLObal NAvigation Satellite System) developed by Russia, GALILEO (Europe's Satellite Navigation System), China's COMPASS/Bei-Dou, India's Regional Navigation Satellite System (IRNSS or NavIC) and Japan's Quasi-Zenith Satellite System (QZSS).

GPS

The United States’ Global Positioning System (GPS), operated by the US Space Force, is the oldest GNSS system that became operational in 1978 and was made available for global use in 1994. As it was originally created as an independent military navigation system by the Department of Defense (DoD), significant effort was put in to provide high accuracy and also to make it secure against jamming and spoofing attempts before being made public later. GPS operates in the L-Band frequency from 1176.45 to 1575.42 MHz. GPS can provide a real-time positioning accuracy in the centimetre range and a long-term accuracy in the millimeter range. It has 31 satellites in orbit and the latest generation of GPS satellites use rubidium clocks that are accurate up to ±5 parts in 1011. The clocks are synchronized by even more accurate ground-based cesium clocks.

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GLONASS

Russia’s Global Navigation Satellite System (GLONASS) is the second navigational system in operation with global coverage and of comparable precision to GPS. It was developed by Roscosmos and operates in the frequency from 1202.026 to 1600.995 MHz and provides positional accuracy in between 2 to 8 meters. GLONASS has 24 satellites in orbit and does not have any significant advantages over GPS. In standalone applications, GLONASS doesn't provide as strong of signal compared to GPS but is a great alternative/backup for GPS.

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GALILEO

GALILEO is Europe's GNSS system that is compatible with GPS and GLONASS. It was created by the European Union through the European Space Agency and started providing service in December 2016. GALILEO has 24 satellites, and its receivers track the satellite constellation's position in the "GALILEO Reference System" using satellite technology and triangulation principles. GALILEO operates at a frequency from 1176.45 to 1575.42 MHz and is intended to provide horizontal and vertical position measurements within 1–3-meter precision (for general applications) with positioning services at higher latitudes than other positioning systems. GALILEO also provides a new global search and rescue (SAR) function as part of the MEOSAR system.

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BeiDou

BeiDou is the Chinese satellite navigation system, operated by the Chinese National Space Administration (CNSA), that is currently in its 3rd generation of development which intends to provide global coverage. The first generation was called BeiDou-1 and was mainly used by China and neighboring regions. It was decommissioned at the end of 2012. The 2nd generation called the BeiDou-2 has been in use since 2012 and initially consisted of 10 satellites only. It can serve users in the entire Asia-Pacific region. The latest generation called BeiDou-3 has 35 satellites with the last one being launched on 23 June 2020 and will serve users all over the world. In total 55 satellites have been launched for BeiDou. BeiDou acts as another alternative/back-up to GPS and GLONASS and is said to reach millimeter-level accuracy with post-processing after its development is completed.

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IRNSS / NavIC

The Indian Regional Navigation Satellite System (IRNSS), with an operational name of NavIC (NAVigation with Indian Constellation), is an independent regional navigation satellite system being developed by Indian Space Research Organisation (ISRO). It is designed to provide accurate position information service to users in India as well as the region extending up to 1500 km from its boundary, which is its primary service area. IRNSS will provide two types of services, namely, Standard Positioning Service (SPS) which is provided to all the users (civilian use) and Restricted Service (RS), which is an encrypted service provided only to the authorized users (military purposes). The IRNSS System is expected to provide a position accuracy of better than 20 m in the primary service area. It consists of 7 satellites that have been in orbit since 2018 and operates at a frequency from 1176.45 to 2492.028 MHz.

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QZSS

The Quasi-Zenith Satellite System (QZSS), operated by Japan Aerospace Exploration Agency (JAXA), is the regional satellite system from Japan and is sometimes referred to as "Japanese GPS". It uses one geostationary satellite and three satellites in the QZO orbit (highly inclined, slightly elliptical, geosynchronous orbit). QZSS uses a timekeeping system (TKS) based on the Rubidium clock hence eliminating the requirement of onboard atomic clocks. TKS technology is also used in GPS, GLONASS and Galileo systems and allows optimal operation when satellites are in direct contact with the ground station. QZSS operates in the frequency from 1176.45 to 1575.42 MHz. It is compatible with GPS and ensures a sufficient number of satellites for stable, high-precision positioning in the Asia-Oceania region.

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