Global Navigation Satellite System (GNSS)

A Global Navigation Satellite System (GNSS) refers to a constellation of satellites that allows the positioning, in real time, of objects, as well as navigation on land or sea. These systems are used in several areas, such as topographic and geodetic mapping, aviation, maritime and land navigation, fleet monitoring, border demarcation, precision agriculture, among other uses.

The performance of these systems is evaluated according to the following criteria:

• accuracy: the difference between the received measurement and the actual position;

• integrity: the system's ability to trigger an alert when it detects an abnormal measurement;

• continuity: the system's ability to work without interruption;

• availability: percentage of the time that the system fulfills the previous requirements (accuracy, integrity and continuity).

The NAVSTAR-GPS system, developed and controlled by the United States Department of Defense - initially for military purposes and, later, open for civilian use -, is still the most used system in the world today. It works with a constellation of 31 satellites, in order to ensure that there are always at least 24 satellites operating, distributed in six orbits, at an altitude of approximately 20,200 km from the Earth's surface.

In addition to the GPS system, other countries have been working on their regional or global positioning systems, seeking to become independent and autonomous in the acquisition of georeferenced data, the main ones being:

• GLONASS: the Russian global satellite positioning system was initially developed for military purposes by the former Soviet Union and, in the middle of the process, it was also opened for civilian use. With the extinction of the USSR, the Russian Federation continued its deployment, becoming fully operational in 2011. The system operates with a constellation of 24 satellites distributed over three orbital planes, at an altitude of approximately 19,100 km.

• Galileo: civil initiative satellite positioning system, developed and operated by the European Community, with a constellation of thirty satellites, including the six extras for replacement, distributed in three orbital planes, at an altitude of approximately 23,222 km. The Galileo system is interoperable with the GPS and GLONASS systems, which allows for more accurate measurements.

• BeiDou/ BDS: also known as Compass, this positioning system was developed by China, and operates regionally at the moment, but with global coverage forecast, with 35 satellites (five geostationary satellites and thirty non-geostationary satellites). Like Galileo, BeiDou is also designed to be interoperable with the other geolocation systems mentioned earlier.

Stations for post-processing of GNSS signals. Credit: IBGE (Brazil).

­In Brazil, the Brazilian Institute of Geography and Statistics (IBGE) operates a network of permanent stations, the Brazilian Network for Continuous Monitoring of GNSS Systems (RBMC), consisting of 146 stations. These stations are a support tool for the use of this technology in Brazil and the main link with international reference systems.

Currently, humanity would no longer be able to live without the use of GNSS systems, any more than it would be able to do without a telephone or electricity. Technology is increasingly part of our lives.

(Adapted from an article published on the IBGE website, available at www.ibge.gov.br).