Satellite navigation

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 generic term for the use of existing and future global satellite systems such as

• GPS ( Global Positioning System) in the United States of America
• GLONASS (Global Navigation Satellite System) of the Russian Federation
• Galileo of the European Union
• Compass (China)

And various complementary systems of Europe, the USA, Japan and India. GPS is fully functional since 1995, Glonass still limited. The full expansion of Compass and Galileo is expected around 2020.

Operation

The satellites of the GNSS satellite constellation share about radio codes with their exact location and time. To determine the position, a receiver needs to receive the same signals from at least four satellites. In the receiver, the pseudo - signal transit times are measured ( from the satellites to the receiver antenna including clock error of the receiver) and to calculate the current position (including the amount ) and the clock error.

At an altitude of about 25,000 km is an constellation of 24-30 satellites is used. This is to ensure that the receivers - can receive at the same time whenever possible signals from at least four satellites ( the GPS system, there are 6 to 12 satellites) - even if not perfectly clear view of the horizon.

By stationary receiving stations, the position accuracy can be improved. You transmit correction signals (DGPS ) to the users. Of the land survey offices SAPOS the German system is operated. SAPOS provides three different signal services available which can achieve an accuracy of up to less than 1 cm.

Satellite-based auxiliary systems, Eng. Satellite-Based Augmentation Systems ( SBAS ), are the European EGNOS, WAAS, the U.S., the Japanese MSAS and the Indian GAGAN which radiate the correction signals via geostationary satellites. The Chinese system Compass is still under construction, the Indian system IRNSS still in planning.

Details of the technique used in the GPS can also be found in Articles GPS technology and hyperbolic; the other systems mentioned above differ therefrom in varying degrees.

Measuring practice

The satellite location is constantly changing (for GPS by 6.1 km / s) and with it the removal of the satellite at any given point on the earth. However, the user of the information contained in the satellite signals orbital data (ephemeris ) to calculate the satellite locations for each time point. This path data ( for GPS and Galileo is Kepler orbital elements at GLONASS to coordinate velocity and acceleration vectors) are regularly adjusted by the ground stations (with GPS about every 2 hours).

The distance from the satellite to the observer is derived from the signal propagation time. Every satellite continually from his individual code and its individual trajectory data. The code is repeated for GPS and GLONASS every millisecond. The receiver generates the same satellite codes and compares them via an appropriate time and frequency shift of the satellite signals being received (PLL, delay and Doppler effects).

The time thus measured shift would correspond with exactly synchronized clocks in the satellites and receivers of the term of the satellite signals. Multiplying these runtime with the signal velocity ( speed of light ) gives the distance from the satellite to the receiver.

For a distance of 3 meters accuracy maturities must be determined with an accuracy of 10 nanoseconds. But instead equip the receiver with a correspondingly high precision atomic clock, the error of the receiver's clock is determined and taken into account in the position calculation. To determine the four unknowns ( three spatial coordinates and receiver clock errors ) you need four satellites. This leads to four equations with four unknowns.

The determined coordinates refer to the coordinate system of each navigation system; for example, GPS to WGS84. Also the determined time is defined by the navigation system; deviates eg the GPS time by a few seconds of Universal Time UTC, since leap seconds in the GPS system time will not be considered. These are added approximately every two years since 1980, so that the deviation at the time (August 2012) is 16 seconds.

From the spatial coordinates, the longitude, latitude and altitude can be calculated using the defined reference ellipsoid. Note, however, that the coordinate systems used by other common coordinate systems may differ, so that the determined position of the position in many, especially older maps may differ up to several hundred meters. The determined by GNSS height and the height " above the sea " can vary by several meters from the actual value ( geoid ).

Measurement error

As with the triangulation should the volume of the tetrahedron, which span the satellite with the observer at the top, be as large as possible; otherwise reduces the achievable position accuracy ( Dilution of Precision DOP ). Are the satellite to the receiver in a plane that is seen by the observer apparently on a line, no localization is possible. Such a situation, however, there is practically never.

The atmosphere changes, the signal propagation time. Unlike the effect of the ionosphere, the troposphere is frequency dependent. It can be corrected in part if the receiver evaluates signals sent by the satellite at different frequencies. For the currently (2011) available in the leisure market GPS receiver is only a signal.

The variation in the number of free electrons in the ionosphere causes a local error of up to 30 m. To reduce it to less than 10 m, transmit GPS satellite 6 parameters that describe the current Ionosphärenzustand. Short-term scintillations can thus not correct.

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

The satellite- related errors, ie satellite position and timing, are in English as a signal in space - called User Range Error ( SIS URE ), the error in the path propagation User Equipment Range Error ( UERE ).

The accuracy increases when more than 4 satellites can be received. This measurement is referred to as " on certain location ". The error can be reduced to a few centimeters later by comparison with reference measurements. This differential GPS (DGPS ) is also possible in real time, if the reference data are available online.

Also one evaluates nor the phases of satellite signals, also dynamically relative accuracies of a few centimeters can be achieved.

Systems

The military systems NAVSTAR GPS ( GPS for short ) of the USA and the Russian GLONASS is called the first generation systems. After upgrading with new satellites, the GPS of the second generation is expected from 2012. It will be comparable to Galileo, which will also include the second generation. In ESA parlance is GNSS -1 for the original GPS and GLONASS, GNSS -2 for Galileo and second generation systems.

The term GPS III, the complete revision of all system components is called. This new concept will take to the final design of the second generation and have quality improvements in many areas result.

End of 2011, the quasi - zenith satellite system ( QZSS ) Japan go into operation and improve the location in Japan canyons. From Chinese Compass system 20 satellites are already in circulation. In India, support at least one satellite ( GSAT -8) of ISRO since mid-2011 GAGAN (GPS Aided Geo Augmented Navigation ).

Other applications

GNSS satellites send not only a radio signal, but also the exact position of the transmitter. From the localization of the signal source and a comparison with the known position, there are indications on the nature of the propagation medium.

Using radio - occultation can be personalized with GPS signals perform observations of the atmosphere and GNSS -R observations about the nature of water surfaces.