1. NAVSTAR system (Known as the GPS), owned by the US and managed by Department of Defense,
2. GLONASS, owned by Russian Federation.
3. Galileo, a European GPS system, expected to be operational by the year 2008.
Space segment: The satellites that are used for GPS are constitute the space segment. The satellites transmit timing information, and location information to the GPS receiver.
User Segment: A GPS receiver is used to receive the transmissions from the satellite. The GPS receiver is a specialized micro controller system that calculates the location of the user based on the satellite signals. It is important to know that the user segment does not transmit anything to the satellite (space segment) and the satellite (space segment) does not know the existence of the user. Since GPS is a broadcast type of server, the only important factor to use GPS server is the satellite coverage of the area. Otherwise, there is no limit to the number of users that can be using the system at any given time. The satellites are controlled and monitored from ground control stations (the Control Segment). The control stations monitor and correct the satellite clock so that the GPS signals are as accurate as possible. The control stations also predict the orbital positions based on the past orbits, and make necessary changes to the GPS data broadcast to the users.
Normally, the GPS systems provide a better accuracy for military use, and the public are provided with lower accuracy signals. However, it is going to be different with Galileo, because Galileo is expected to deliver consistent commercial quality GPS signals. Another major difference between GPS, GLONASS, and Galileo is that the first two does not guarantee service, whereas Galileo comes with service guarantees.
GNSS and its operation:
What is GNSS?
GNSS stands for Global Navigation Satellite System. These systems will use NAVSTAR. GLONASS, or Galileo to provide positioning, along with other components to improve accuracy and provide fast warning of problems.
A GNSS uses GPS signals to provide users with accurate and timely navigation information. The GPS satellites should be able to provide real time information at sufficiently high intervals of time. For example, if navigation is required for flying an aircraft, the GPS signals must be accurate enough to guide the aircraft. On the other hand, for boat, the GNSS signals could be slow. The GNSS system should be able to provide accurate and fast information signals to satisfy different kinds of user requirements.