The start of the Galileo system is a direct competition of the GPS and GLONASS systems. Without a doubt, it has a positive influence on the modernisation of the GPS system and the further development of the GLONASS system. Multiple navigation systems operating independently help increase the awareness and accuracy of the real time positioning and navigation. Undoubtedly, a global navigation satellite system of the future is a combined GNSS system which uses the GPS, GLONASS and Galileo systems together. A constellation of about 75 satellites of the three systems greatly increases the visibility of the satellites especially in critical areas such as urban canyons.
The times and coordinate systems used in the GPS, GLONAS and Galileo systems are different due to the system independency. The three time systems are all based on the UTC and the three coordinate systems are all Cartesian systems; therefore, their relationships can be determined and any system can be transformed from one to another. The origins of the GPS and GLONASS coordinates are meters apart from each other. The origins of GPS and Galileo coordinates have differences of a few centimetres. Several carrier frequencies are used in each system for the removal of the effects of the ionosphere. The frequency differences within the GLONASS system and between the GPS, GLONASS and Galileo systems are generally not a serious problem if the carrier phase observables are considered in a distance survey by multiplying the wavelength.
In the present edition of this topic, the theory and algorithms of a global positioning system will be discussed in a more general aspect in order to take the differences of the GPS, GLONASS and Galileo systems into account.
