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GPS (Global Positioning System)
Global Positioning System (GPS) is navigation system that was developed by United States (US) in 1970. It was major project of department of defence to provide state of the art technology to fulfil the need of US forces. GPS provides continuous timing and positioning information anywhere in the world in all circumstances.
Presently GPS navigation system consist of 24 operational satellites and five ground station providing 24 hours instantaneous, precise and three dimensional navigation data via one way wireless communication to properly equipped military and civilian users. The altitude of satellites is 11000 miles from the earth. These 24 satellite constellation arranged in 6 orbital planes with 4 satellites per plane. The health and status of the satellite is monitored by worldwide ground stations. The network uploads the navigation and other data to satellites. The GPS satellite communication is referenced to highly precise atomic clock frequencies which exist on board.
The satellites use two frequencies to broadcast the ranging codes and navigation data by using the technique called Code Division Multiple Access(CDMA). The frequencies known as L1 (1575,42 MHz) and L2 (1227.6 MHz.
In above discussion our focus was the American GSP system that is based on NAVSTAR satellites manufactured by Rockwell International present the other international powers also developed their own systems that is mentioned below with short detail.
GLONASS is Russian global navigation satellite system. The GLONASS satellite signal identifies the satellite and provides position, velocity and acceleration vectors at a reference to compute satellite locations. It also provides the synchronization bits, data age and satellite health. This system is using the Frequency Division Multiple Access (FDMA) technique for communication.
Galileo is global satellite navigation system currently built by European Union (EU) and European Space Agency. This system will be fully operational in 2013. This navigation system consists on 28 satellites. The satellites will be in 3 orbital planes, 56° inclination, ascending nodes separated by 120° longitude. There are four kind of different services are expected from this navigation system that are Open Service (OS), Commercial Service (CS),Public regulated Service (PRS) and Safety of Life Service (SoL).
Quasi-Zenith Satellite System (QZSS) is basically Japanese communication satellite system but also designed for navigation purposes. The major target of QZSS was mobile applications to provide communication based services (Audio, video and data) and positioning information. Without going into further detail about constellation that is not yet fully defined, it is planned that QZSS satellite transmit navigation signals with same characteristics as current and future GPS L1, L2 and L5 signals including the GPS future LIC signal. These signals will allow QZSS satellites to be used for ranging purposed.
The Network Architecture
The GPS navigation system network architecture consist on three segments. The Space Segment, Control Segment and User Segment. Now each segments will be discussed in detail to cover network architecture. This architecture can be assumed by following figure 2.1
The Space Segment consists on the constellation of satellite through witch user receives ranging measurements. The space segment also include all onboard equipments This constellation transmits the precision code towards ground through which the ranging measurements are possible. The length of P-Code PRN for each satellite is 6.1871 × 1012 bits long (6,187,100,000,000 bits) and this code repeats once in a week. This particular code increases correlation gain due to its long length and eliminates any range ambiguity within the Solar System.
If the basic scenario is to be considered the signal that is being transmitted from the satellite can be divided number of components. Which are the two sine waves are known as career frequency, two digital codes and a navigation message.
Control segment is also responsible to continuously maintain the satellites and their health.
This includes maintaining the satellites in their proper orbital positions (called stationkeeping) and monitoring satellite subsystem health and status. The CS also monitors the satellite solar arrays, battery power levels, and propellant levels used for manoeuvres. Furthermore, the CS activates spare satellites (if available) to maintain system availability.