How does GNSS work?
The Navstar Global Positioning System (GPS) operated by the USA was the world’s first truly global positioning system. As Navstar (GPS) was a first, the term GPS became the defacto label for all Satellite Positioning systems. Now that there are multiple operational global navigation systems the correct term that describes this is GNSS.
GNSS uses multiple constellations of satellites which provide signals from space that transmit positioning and timing data to GNSS receivers. The GNSS receivers then use this data to determine location. Principal GNSS systems are listed below, click on the link for information provided by the system operators:
|Global Navigation System||Common||Owner / Operator|
|Navstar||GPS||United States of America|
|Global'naya Navigatsionnaya Sputnikovaya Sistema||GLONASS||Russian Federation|
|BeiDou||BeiDou||People Republic of China|
The foundation of all GNSS systems is timing. All GNSS satellites have precise atomic clocks onboard and each system is precisely synchronised. On power-up a GNSS receiver will synchronise with the orbiting atomic clocks. The receiver decodes the orbital information of each satellite. The orbital data allows the receiver to calculate the location of each satellite. Each satellite transmits a unique signal, the receiver decodes these unique signals and calculates the time it has taken to transit from space to the receiving antennae. The distance (range) from the orbiting satellite to the receiver’s antennae is calculated using the transit time and the formulae:
Distance (Range) = Time x Velocity of Light
The principle of every GNSS system is trilateration. This is the three-dimensional version of triangulation a more widely known map reading technique.
To trilaterate a GNSS receiver measures multiple ranges from antennae to the satellites. Because the receiver has calculated the satellites location and has measured ranges the receiver can the calculate the point at which the range intersect. The intersection point is the location of the GNSS antenna upon the earth’s surface.
All measurements have errors, Differential GNSS is technique that compliments GNSS. By monitoring GNSS systems the majority of the errors can be measured. These errors are broadcast in real-time over satellites to DGNSS receivers. The systematic errors can then be corrected by the GNSS receiver. The result is a position with improved accuracy.
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