Global Navigation Satellite Systems. This is the name for a collection of all the satellites used for navigation and positioning.
This involves the charting of the course of an entity (aircraft, vehicle etc.). Though navigation can be achieved without satellites, the employment of satellites in navigation allows for faster charting, real time monitoring and increased accuracy. Now, there are application that allow for realtime navigation using smartphones which have become a "must own" property. Satellite navigation systems have been integrated in cars to acquire their precise locations at any point in time and also provide valuable information as to traffic information, fastest routes and estimated time of arrival (ETA). ( when used in road vehicles.)The application of real time navigation cannot be exaggerated especially when it comes to the safety of life and property. Satellite navigation has helped in search and rescue missions where beacons were used to loacte with precision the location of survivors in remote areas.
In 1958, he first generation of satellite positioning system known as Navy Navigation Satellite System (NNSS) also known as TRANSIT was produced which operated on the Doppler principle (observation of changes in frequency with respect to distance to the satellites). Due to the success of the system, the U.S. Department of Defence began development of NAVigation Satellite Timing and Ranging(NAVSTAR) Global Positioning System(GPS). The first satellite was placed in orbit in 1978 since then, many additional satellites have been launched. It became fully operational in December 1993.
For satellite positioning, precise distances from satellites to the receivers must be determined. This is achieved using the timing and the signal information.
The global positioning system has three segments
Space segment
This is made of 24 satellites operating in six orbital planes spaced at 60°. These orbital planes are inclined to the equator at 55°. The mean altitude of the satellites above the earth is 20200km. This configuration enables 24hours satellite coverage within latitudes 80°N and 80°S. Atomic clocks are employed in the satellites (2 caesium and 2 rubidium)for extremely accurate timing.
Control segment
This segment is made of twelve monitoring stations and a master control station at Schriever Air force Base in Colorado Springs.the satellites transmit from the satellites are monitored and transmitted to the master control station. This information used to make precise near future predictions of satellite orbit and clock correction parameters and thus transmitted back to the satellites to be part of their broadcast messages to receivers.
User segment
This consists of two categories of receivers classified by their access to the two services provided by the system. The services are the Standard Positioning Service (SPS) and the Precise Positioning Service(PPS). The SPS is provided free with accuracies of 100m in horizontal and 156m in vertical positions.it is broadcasted only on the L1 frequency. The PPS is only available to receivers with valid cryptographic keys the accuracies of 18m in horizontal and 28m is vertical are achieved in this mode. It is broadcasted on both L1 and L2 frequencies.
