Satellite navigation provides the position and the velocity of a flight.
Satellite navigation: Description and current status
Satellite navigation systems have been a key element in our civilisation for providing CNS/ATM services. The global navigation satellite system is a worldwide system to determine positioning and time, which includes one or more satellite constellations, aircraft receivers and monitoring system integrity, with augnmentation as needed, to provide navigation for future operations.
Since man first needed to orient himself on voyages over land and sea, we have used the term navigation to designate the most appropriate way of determining our trajectory. In the beginning man oriented himself with the stars, then he used a compass as a guide and in the 19th Century, Marconi's radio was crucial to developing new forms of orientation.
Satellite navigation is the latest step for mankind in its quest to determine its position. Today's satellite navigation systems allow the user to find his position, speed and time with high precision and reliability, with at least four satellites measuring his distance.
Global Navigation Satellite System (GNSS)
In the 80s, the term Global Positioning System began to be used more widely as spatial systems emerged. The first were the American GPS (Global Positioning System) and the Russian GLONASS (Global Navigation Satellite System).
In the 90s the capabilities of these systems began to be revealed, in aviation as well as many other completely different fields. They offered accuracy and ease of use with low price receivers.
The ICAO and other organisations launched different working groups to standardise the aeronautical use of satellite positioning systems. This defines the term GNSS (Global Navigation Satellite System), a generic, integrating term for the different systems.
GNSS systems offer major operational benefits in civil aviation:
- World-wide coverage and the ability to operate around the planet using just one receiver device.
- Greater efficiency in the use of airspace and airports.
- Improved operational safety and access to aerodromes.
- Reduced environmental impact.
- Possible rationalisation of conventional radio aid infrastructure (reduced costs, greater economic efficiency).
- Increased capacity and improved predictability.
- Improved possibilities for airports with no equipment.
GNSS currently includes two operational constellations: the US GPS and the Russian GLONASS.
The United States is modernising the GPS constellation with satellites that transmit signals on two frequencies (L1 A/C and L5), which will enable duly equipped aircraft to reduce ionospheric errors, have better performance and greater robustness.
The Russian Federation's GLONASS system is operative with a stable constellation of 24 GLONASS-M satellites and additional reserve satellites. The Russian Federation is also developing a new generation of satellites called GLONASS-K.
Both GPS and GLONASS have been offered to the international community for use free of charge, in line with the ICAO recommendation that basic GNSS services must be offered with no direct cost for users. Both were designed in the 1970s for military purposes: to provide positioning information on their respective armies anywhere on the planet where they were located. Both GPS and GLONASS include a civilian service (Standard Positioning Service) which is universally accessible.
China is deploying the BeiDou constellation (COMPASS) which already offers a positioning, navigation and timing service (PNT) in the area of China, and will later offer a global service. According to the plan, global deployment will be completed in 2020.
The European Union is developing the Galileo constellation. 18 satellites are expected to be operational in 2015, offering initial services in combination with GPS and other constellations.
GNSS augmentation systems
In addition to the satellite constellations mentioned above, GNSS includes GNSS augmentation systems. These systems arose to provide improved accuracy, integrity, continuity and availability, all necessary for air navigation.
The term "augmentation" was coined to refer to the technical solutions proposed so that the GNSS system could be used in the different flight stages. The three types of augmentation are based on the aircraft (ABAS), space (SBAS) or ground stations (GBAS).
- ABAS is an avionics solution that processes GNSS signals to meet performance requirements in the route to approach flight stages.
- SBAS: the service for critical applications (Safety-of-Life, SoL) by EGNOS (as GPS augmentation) has been operational in Europe since 2011 and is valid for all flight stages in civil aviation. There are another three satellite based augmentation systems in addition to EGNOS in Europe: WAAS in the United States, MSAS in Japan and GAGAN in India.
- GBAS: GBAS augmentation (of the GPS system) is based on transmitting complementary information to the aircraft from the ground station (located in an aerodrome).
Multi-constellation / Multi-frequency GNSS
As new constellations are deployed and existing ones improved, signals from multiple constellations are available on several frequency bands. These developments lead to improved features, offering potentially important operational benefits but at the same time they pose new technical and regulatory challenges.
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