Global navigation satellite system

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Frequencies of the different GNSS

A global navigation satellite system ( English global navigation satellite system ) or GNSS is a system for positioning and navigation on the ground and in the air by the reception of signals from navigation satellites and pseudolites .

GNSS is a collective term for the use of existing and future global satellite systems such as

and various supplementary systems in Europe, the USA, Japan and India . NAVSTAR GPS has been fully functional since 1995, GLONASS since 1996, but then lost almost half of the satellites due to age in the following years. GLONASS has been fully operational again since 2011. The full expansion of Beidou and Galileo is expected around 2020.

Working method

Intersection of three spherical surfaces

The satellites of the GNSS satellite constellation communicate their exact position and time via radio codes. To determine position, a receiver must receive signals from at least four satellites at the same time. The pseudo signal propagation times are measured in the receiving device (from the satellites to the receiving antenna, including the receiver's clock errors) and the current position (including altitude) and the clock errors are determined from this.

In an orbit of approximately 25,000 km, a constellation of 24 to 30 satellites is used. This is to ensure that the receiving devices - even if the view of the horizon is not completely free - can always receive signals from at least four satellites at the same time (with the GPS system there are 6 to 12 satellites).

The position accuracy can be improved by stationary receiving stations. They transmit correction signals ( DGPS ) to the users. The German SAPOS system is operated by the state survey offices . SAPOS provides three different signal services that achieve an accuracy of less than 1 cm.

Satellite-based additional systems , engl. Satellite-Based Augmentation Systems (SBAS) are the European EGNOS , the US WAAS , the Japanese MSAS and the Indian GAGAN , which transmit the correction signals via geostationary satellites. The Chinese Beidou system is still being set up, the Indian IRNSS system is still being planned.

Details of the technology used in GPS can also be found in the articles GPS technology and hyperbola navigation ; the other systems mentioned above differ from this in varying degrees.

Measurement practice

The satellite location changes constantly (with GPS by almost 3.9 km / s) and with it the distance of the satellite to a certain point on earth. However, the user can calculate the satellite locations for each point in time from the orbit data ( ephemeris ) contained in the satellite signals . These orbital data (GPS and Galileo are Kepler orbital elements , GLONASS are coordinate, velocity and acceleration vectors) are regularly compared by the ground stations (about every two hours with GPS).

The distance from the satellite to the observer results from the signal propagation time. Each satellite continuously transmits its individual code, the current time and its individual orbit data. With GPS and GLONASS, this sequence is repeated every millisecond. The receiver uses a phase-locked loop to deal with time and frequency shifts caused by delay and Doppler effects.

With precisely synchronized clocks in the satellite and receiver, the time shift measured in this way would correspond to the transit time of the satellite signals. The multiplication of this transit time with the signal speed (approximately the speed of light ) results in the distance from the satellite to the receiver.

For a route accuracy of three meters, the transit times must be determined with an accuracy of ten nanoseconds. Instead of equipping the receiver with a correspondingly highly accurate atomic clock , the error in the receiver clock is determined and taken into account in the position calculation. To determine the four unknowns (three spatial coordinates and receiver clock errors), four satellites are required. This leads to four equations with four unknowns.

The determined coordinates relate to the coordinate system of the respective navigation system; with GPS for example on WGS84 . The determined time is also defined by the navigation system; so z. B. the GPS time from the universal time UTC by a few seconds , since leap seconds are not taken into account in the GPS system time. These have been added about every two years since 1980, so that the deviation is currently (as of January 2017) 18 seconds.

The geographical longitude, geographical latitude and height above the defined reference ellipsoid can be calculated from the spatial coordinates . However, it should be noted that the coordinate systems used can deviate from other common coordinate systems, so that the determined position can deviate from the position in many, especially older, maps by up to a few hundred meters. The height determined by GNSS and the height "above sea level" can also deviate from the actual value ( geoid ) by several meters.

Measurement errors

Grinding cut
Runtime error due to refraction
Runtime errors due to angles

As with triangulation, the volume of the tetrahedron spanned by the satellites with the observer at the tip should be as large as possible; otherwise the achievable position accuracy ( Dilution of Precision , DOP) is reduced . If the satellites are in the same plane as the receiver, i.e. apparently in a line as seen by the observer, no position determination is possible. However, such a constellation practically never occurs.

The atmosphere changes the signal propagation time. In contrast to the troposphere, the influence of the ionosphere is frequency-dependent. It can be partially corrected if the receiver evaluates signals that the satellite sends on different frequencies ( dual-frequency receiver). Only one signal is available for the GPS receivers currently (2020) common in the leisure market.

The fluctuation range of the number of free electrons in the ionosphere causes a spatial error of up to 30 m. To reduce it to less than 10 m, GPS satellites transmit six parameters that describe the current state of the ionosphere. However, short-term scintillations cannot be corrected with it.

Position accuracy with uncorrected measured values ​​( User Range Error , URE):

source Time error Location error
Satellite position 6-60 ns 1-10 m
Time drift 0-9 ns 0-1.5 m
ionosphere 0-180 ns 0-30 m
Troposphere 0-60 ns 0-10 m
Multi-way effect 0-6 ns 0-1 m

The satellite- related errors, i.e. satellite position and time measurement, are referred to in English as Signal in Space - User Range Error (SIS-URE), the errors in the path propagation User Equivalent Range Error (UERE).

The accuracy increases when more than four satellites can be received. This measurement is then called "overdetermined location". The errors can subsequently be reduced to a few centimeters by comparing them with reference measurements. This type of correction is known as the Differential Global Navigation Satellite System (DGNSS). She finds the differential GPS (DGPS) in real-time instead, if the reference data online are available.

If you also evaluate the phases of the satellite signals, dynamic relative accuracies of a few centimeters can also be achieved.

Systems

Number of GNSS satellites launched from 1978 to 2014

The US military systems NAVSTAR-GPS ( GPS for short) and the Russian GLONASS are called first-generation systems . After upgrading with new satellites, the second generation GPS is available. It will be comparable to Galileo , which will also belong to the second generation . In ESA parlance, GNSS-1 stands for the original systems GPS and GLONASS, GNSS-2 for Galileo and systems of the second generation. The term GPS III describes the complete revision of all system components. This redesign will last until the second generation is finally built and result in quality improvements in many areas.

The Japanese quasi-zenith satellite system (QZSS) is intended to improve positioning in Japan's urban canyons. Twenty satellites from the Chinese Beidou system are already in orbit. In India , at least one satellite (GSAT-8) from ISRO has been supporting GAGAN ( GPS Aided Geo Augmented Navigation ) since mid-2011 .

Other uses

GNSS satellites not only send a radio signal, but also the exact position of the transmitter. The location of the signal source and a comparison with the known position provide information about the nature of the propagation medium.

Using radio occultation , observations of the earth's atmosphere can be carried out with GNSS signals and observations of the reflectivity of water surfaces with GNSS-R .

See also

literature

  • Manfred Bauer: Surveying and positioning with satellites. Global Navigation Systems (GNSS) and other satellite navigation systems. 6th edition. Wichmann, Berlin 2011, ISBN 978-3-87907-482-2 .
  • Werner Mansfeld: Satellite positioning and navigation. Basics, modes of operation and application of global satellite navigation systems. 3. Edition. Vieweg, Wiesbaden 2010, ISBN 978-3-8348-0611-6 .
  • Hans Dodel, Dieter Häupler: Satellite navigation. 2nd Edition. Springer, Berlin 2010, ISBN 978-3-540-79443-1 .

Web links

Wiktionary: Satellite navigation  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ Performance Analysis of the QZSS SIS-URE and user Positioning Accuracy with GPS and QZSS ( Memento of December 30, 2011 in the Internet Archive )
  2. ↑ List of navigation satellites
  3. ↑ Geo-Stationary Satellite: GSAT-8. ( Memento from October 13, 2011 in the Internet Archive )