Satellite navigation

Satellite navigation (satnav) or satellite positioning is the use of 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 QZSS, India's GAGAN, and the European EGNOS, all of them based on GPS. Satellite navigation devices determine their location (longitude, latitude, and altitude/elevation) to high precision (within a few centimeters to meters) using time signals transmitted along a line of sight by radio from satellites. The system can be used for providing position, navigation or for tracking the position of something fitted with a receiver (satellite tracking). The signals also allow the electronic receiver to calculate the current local time to a high precision, which allows time synchronisation. These uses are collectively known as Positioning, Navigation and Timing (PNT). Satnav systems operate independently of any telephonic or internet reception, though these technologies can enhance the usefulness of the positioning information generated. Global coverage for each system is generally achieved by a satellite constellation of 18–30 medium Earth orbit (MEO) satellites spread between several orbital planes. The actual systems vary, but all use orbital inclinations of >50° and orbital periods of roughly twelve hours (at an altitude of about 20,000 kilometres or 12,000 miles).

Classification

GNSS systems that provide enhanced accuracy and integrity monitoring usable for civil navigation are classified as follows:

GNSS-1 is the first generation system and is the combination of existing satellite navigation systems (GPS and GLONASS), with satellite-based augmentation systems (SBAS) or ground-based augmentation systems (GBAS). In the United States, the satellite-based component is the Wide Area Augmentation System (WAAS); in Europe, it is the European Geostationary Navigation Overlay Service (EGNOS); in Japan, it is the Multi-Functional Satellite Augmentation System (MSAS); and in India, it is the GPS-aided GEO augmented navigation (GAGAN). Ground-based augmentation is provided by systems like the local-area augmentation system (LAAS). GNSS-2 is the second generation of systems that independently provide a full civilian satellite navigation system, exemplified by the European Galileo positioning system. These systems will provide the accuracy and integrity monitoring necessary for civil navigation; including aircraft. Initially, this system consisted of only Upper L Band frequency sets (L1 for GPS, E1 for Galileo, and G1 for GLONASS). In recent years, GNSS systems have begun activating Lower L Band frequency sets (L2 and L5 for GPS, E5a and E5b for Galileo, and G3 for GLONASS) for civilian use; they feature higher aggregate accuracy and fewer problems with signal reflection. As of late 2018, a few consumer-grade GNSS devices are being sold that use both. They are typically called "Dual-band GNSS" or "Dual-band GPS" devices. By their roles in the navigation system, systems can be classified as:

There are four global satellite navigation systems, currently GPS (United States), GLONASS (Russian Federation), BeiDou (China) and Galileo (European Union). Satellite-based augmentation systems (SBAS) such as OmniSTAR and StarFire. Regional SBAS, including WAAS (US), EGNOS (EU), MSAS (Japan), GAGAN (India) and SDCM (Russia). Regional navigation satellite systems (RNSS) such as India's NAVIC and Japan's QZSS. Continental-scale Ground-Based Augmentation Systems (GBAS); for example, the Australian GRAS and the joint US Coast Guard, Canadian Coast Guard, US Army Corps of Engineers and US Department of Transportation National Differential GPS (DGPS) service. Regional-scale GBAS such as CORS networks. Local GBAS typified by a single GPS reference station operating Real Time Kinematic (RTK) corrections. As many of the global GNSS systems (and augmentation systems) use similar frequencies and signals around L1, many "Multi-GNSS" receivers capable of using multiple systems have been produced. While some systems strive to interoperate with GPS as well as possible by providing the same clock, others do not.

History

Ground-based radio navigation is decades old. The DECCA, LORAN, GEE and Omega systems used terrestrial longwave radio transmitters which broadcast a radio pulse from a known "master" location, followed by a pulse repeated from a number of "slave" stations. The delay between the reception of the master signal and the slave signals allowed the receiver to deduce the distance to each of the slaves, providing a fix. The first satellite navigation system was Transit, a system deployed by the US military in the 1960s. Transit's operation was based on the Doppler effect: the satellites travelled on well-known paths and broadcast their signals on a well-known radio frequency. The received frequency will differ slightly from the broadcast frequency because of the movement of the satellite with respect to the receiver. By monitoring this frequency shift over a short time interval, the receiver can determine its location to one side or the other of the satellite, and several such measurements combined with a precise knowledge of the satellite's orbit can fix a particular position. Satellite orbital position errors are caused by radio-wave refraction, gravity field changes (as the Earth's gravitational field is not uniform), and other phenomena. A team, led by Harold L Jury of Pan Am Aerospace Division in Florida from 1970 to 1973, found solutions or corrections for many error sources. Using real-time data and recursive estimation, the systematic and residual errors were narrowed down to accuracy sufficient for navigation.

Principles

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