GNSS

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Satellite-based Positioning has become part of everyday life. The following chapters will give you an insight about this fascinating technology.

The Satellite Systems


The term Global Navigation Satellite System (GNSS) unites

  • the US-American Global Positioning System (GPS) and
  • the Russian GLObalnaya NAvigatsioannaya Sputnikovaya Sistema (GLONASS)
  • as well as the future European GALILEO and
  • the Chinese COMPASS

in a global satellite navigation system.

Global Positioning via Satellite

Satellite-based positioning is carried out by measuring signal delays. Every satellite sends out a signal. This signal is provided with a timestamp and the current satellite position in the orbit. The GNSS receiver determines the delay between the time of sending and receiving the signal and computes the distance to the satellite.

At least four satellite signals have to be received in order to determine a three-dimensional position. Along with the three vectors for the spatial coordinate, time is an unknown factor due to an imprecise clock. The required coordinate set is determined by a spatial intersection of arcs.

Accuracy of Positioning

The accuracy of satellite-based positioning depends on the following factors:

  • locally observable satellite constellation
  • satellite systems in use
  • interferences in the ionosphere and troposphere
  • multi-path effects (signal reflection, e.g. on buildings)
  • available frequencies (single, dual or triple frequency receivers)
  • hardware features (signal processing, processing speed, algorithm)

With a GNSS receiver only, you are usually able to achieve an accuracy of 3-10 metres. The accuracy depends very much on external and internal factors as mentioned above. Due to solar activities - the periodically changing ionospheric interferences can account for deviations of up to 5 metres.

Deviations

On the assumption that there is an open, clear view of the sky and an absence of multi-path effects, deviations can be calculated as follows:

  • interferences in ionosphere: up to 5 metres
  • interferences in troposphere: up to 2 metres
  • errors of the satellite orbit: up to 1 metre
  • errors of the satellite clock: up to 1 metre
  • receiver hardware: up to 1 metre

Some of these deviations can be corrected, if they are known locally. This approach is used by DGNSS and RTK.

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