GNSS ( Global Navigation Satellite Systems ) is a general term that encompassing different types satellite-based positioning, navigation, and timing ( PNT ) system used globally.
GNSS use constellations of satellites and work based on the concept of trilateration. GNSS worked in collaboration with GPS systems to provide precise location positioning anywhere on earth. They work together but the main difference between them is that GNSS equipment are compatible with various navigational systems from other networks beyond the GPS system, more satellite means the receiver accuracy and reliability are increased. All GNSS receivers are compatible with GPS, but there is no necessity for GPS receivers to be compatible with GNSS. Initially these satellites would all have been from one GNSS, but multi-GNSS receivers are now commonplace.
Just like GPS, GNSS also consist of three major segments: the space segment (satellites), the ground segment (ground control stations), and the user segment (GNSS or GPS receivers). Satellites are constantly sending radio signals toward earth, which are picked up by GNSS or GPS receivers. The ground control stations that monitor the Global Navigation Satellite System continuously track satellites, update the positions of each and enable information on earth to be transmitted to the satellites.
The GPS itself is one of the Global Navigation Satellite System (GNSS) because it’s a satellite navigation system with global coverage. Currently there are 4 main satellite navigation system that considered as Global Navigation Satellite System ( GNSS ).
The 4 Main Global Navigational Satellite System
- GPS (NAVSTAR) Navigational Signal Timing and Ranging owned by U.S Government. GPS is the oldest and the pioneer of GNSS system. GPS operates in a frequency band referred to as the L-Band, a portion of the radio spectrum between 1 and 2 GHz. The system consists of 32 satellites known as the best and most widely-utilised satellite system in the world.
- GLONASS (Globalnaya Navigazionnaya Sputnikovaya Sistema, or Global Navigation Satellite System ) is a global GNSS owned and operated by the Russian Federation. The system development began in the 1976. GLONASS operates at 1.602 GHz (L1 signal). The system consists of 24 satellites with 3 satellites as spares or testing.
- BDS ( BeiDou Navigation Satellite System ) owned and operated by the People’s Republic of China. Previously BDS was called Compass. The navigation system consists of two separate satellite constellations, BeiDou-1 and BeiDou-2 ( and soon to be BeiDou-3 ). The operational satellite system consists of 35 satellites.
- Galileo owned and operated by the European Union. In 2016 Galileo made the first initial services and became fully operated in the end of 2021. The Galileo system is compatible with GPS and GLONASS. The operational satellite system consists of 22 satellites. The Galileo system has an accuracy of less than 1 metre. It is the better accuracy than the GPS.
Regional and Augmentation System
- Besides the global system, there are also several regional and augmentation system.
IRNSS (Indian Regional Navigation Satellite System ) or NavIC ( Navigation Indian Constellation is a regional GNSS that owned and operated by the Government of India. IRNSS is designed to provide accurate position information services to users in the Indian region and 1500 km around the Indian mainland. The operational satellite system consists of 7 satellites. - QZSS ( Quasi-Zenith Satellite System ) is a regional GNSS owned by the Government of Japan and operated by QZS System Service Inc. (QSS). QZSS complements GPS to improve coverage in East Asia and Oceania. Japan initiated the services in 2018 with 4 operational satellites, and plans to expand the constellation to 7 satellites by 2023 for autonomous capability.
- SBAS ( Satellite-based augmentation systems ) such as the Wide Area Augmentation System (WAAS) for North America and the European Geostationary Navigation Overlay Service (EGNOS) have long been established as a method to provide a positioning service for critical applications such as commercial aviation. EGNOS improves the accuracy and reliability of GPS information by correcting signal measurement errors and by providing information about the integrity of its signals.
Satellite-Based Augmentation Systems (SBAS) have long known provide global error corrections to improved accuracy in GNSS applications. Many countries manage their own SBAS systems and these are usually considered separate from traditional GNSS constellations.
The main reason for all 6 satellite constellations is availability and redundancy. If one system fails, another GNSS constellation can help take over even though system failures don’t happen often. All of these satellite systems use radio frequencies in the L-Band to transmit their signals, however each satellite system may choose different frequencies and corresponding labels for these signals.
GNSS positioning equipment typically receives at least two frequencies, which more specialty equipment able to receiver additional L-Band signals. Despite the difference, all the GNSS constellation have something in common which is the need for accurate timing and the accuracy. This is typically achieved by high-performance rubidium atomic clocks or LEO Satellite GPSDOs.