The present working GNS system is the 'Global Positioning System (GPS)' developed by USA. The use of GPS is mainly for military purposes and controlled by the military. Europe is going to launch its own GNSS under the name of 'Galileo'. Galileo is Europe's first GNSS which is intended to be used by various civil sectors such as transport, social service, judicial system, public works, rescue teams, etc. Galileo is set to be fully operational by 2008.
The fully deployed system consists of 30 satellites (27 operational + 3 active spares), positioned in three circular Medium Earth Orbit (MEO) planes in 23616 km altitude above the earth, and at an inclination of the orbital planes of 56 degrees with reference to the equatorial plane (ESA, 2005). As a large number of satellites are working together with three active spares, therefore the dysfunction of one of the satellites does not have much effect on the overall functioning of the system.
Each satellite of a GNSS is provided with an atomic clock which measures time very accurately. The satellites continuously emit signals simultaneously indicating the time of leaving of the signals from the satellite. A receiver on the surface of the earth has the exact details of the orbits of all the satellites in its memory. By reading the incoming signal it can thus recognise the particular satellite, determine the time taken by the signal to arrive and calculate the distance from the satellite (EUROPA, 2006). The ground receiver receives the signals from about four different satellites simultaneously. Once it does so it can calculate the exact position of itself.
The control of the satellites and performance of navigation management is carried out by 'Ground Control Centres (GCC)' located at different places on the earth. The control centres contact with the satellites through 'Up-link stations' and carry out the operations such as time synchronisation.
The first experimental satellite, part of the so called 'Galileo System Test Bed (GSTB)' will be launched in the second semester of 2005 (ESA, 2005). This satellite is intended to test the technologies still under development. Then in the second stage about four operational satellites are scheduled to be launched in 2006 for In Orbit Validation (IOV). After this the remaining satellites will be launched to make it fully operational in 2008.
GNSS has to provide four basic services as follows.
Position, Velocity and Time Service (PVT)
Accuracy and Integrating Services (AI)
Ranging and Timing Services (RT)
High Integrity Services (HI)
There is an opposition to the European 'Galileo' GNSS mainly from GPS who want to be the sole satellite navigation system. The US is continuously making some announcements regarding GPS in order to derail Galileo. These sorts of announcements, however, do not have any effective impact on the development of Galileo. The objections from US side are that Galileo will be purely used for civil navigation without the control of military over it.
Instead of this opposition from the US, development of Galileo will surely go ahead. This is a big step forward because it is for the benefit and welfare of the common people. It will directly benefit various sectors of the society and indirectly almost whole