Throughout time people have developed a variety of ways to figure out their position on earth and to navigate from one place to another. Early mariners relied on angular measurements to celestial bodies like sun and stars to calculate their location. The 1920s witnessed the introduction of more advanced technique-radio navigation-based at first on radios that allowed navigators to locate the direction of shore-based transmitters when in range. Later development of artificial satellites made possible the transmission of more precise, line of sight radio navigation signals and sparked a new era in navigation technology. Satellites are first used in position finding in a simple but reliable 2D Navy system called Transit. This laid the groundwork for a system that would later revolutionize navigation for ever-the Global Positioning System.
The Global Positioning System (GPS) is a satellite based navigation system. The concept of GPS was introduced by the United States Department of Defense (DoD). It is in the year 1994 that the GPS was completely developed. The GPS is developed to provide continuous, highly precise positions, velocity and time information to the land, sea, air and space based users. The intent of system is to use a combination of ground stations, orbiting satellites and special receivers to provide
navigation capabilities to virtually everyone, at any time, anywhere in the world, regardless of weather conditions.
THE GPS SEGMENTS
The Space Segment
The space segments, also known as satellite segment, consist of 24 operational satellites revolving around earth in 6 orbital planes approximately 600 GPS satellites are not geosynchronous. First satellite was launched in the year 1978. The satellites take approximately 12 hours to orbit Earth. These satellites revolve the earth in a circular pattern with an inclined orbit. Out of the 24 satellites 21 are working satellites and the remaining 3 satellites will be in standby. In the event of a satellite failure, one of the spare space vehicles can be moved in to its place using modern propulsion and guidance system. Each satellite circles the Earth twice every day at an altitude of 20,200 kilometers. At a time 5 to 8 satellites can be viewed by the user, there by ensuring worldwide coverage. The information from three satellites is needed to calculate a navigational unit’s horizontal location on Earth’s surface (2D-Reporting), but information from four satellites enables a receiver to determine its altitude (3D-Reporting). Each satellite contains a Cesium Atomic Clock and all these clocks will be synchronized and are accurate within a few nanoseconds.
The Control Segment
The GPS control segment (CS), called the Operational Control System (OCS), includes all the fixed locations ground-based monitor stations located throughout the world, a Master Control Station (MCS) and the up-link transmitters. The monitor stations are simply GPS receivers that track the satellites as they pass overhead and accumulate ranging and ephemeris data from them. This information is relayed to the Master Control Station. These ground stations around the world are responsible for monitoring the flight paths of the GPS satellites and synchronizing the satellite’s onboard atomic clocks. This information is relayed to MCS where it is processed and compares the actual satellite position with the GPS computed position. The MCS receives data from the monitor stations in real time 24hrs a day, and uses that information to determine if any satellite are experiencing clock or ephemeris change and to detect malfunctions. Corrections are done and then it is uploaded to the satellites twice per day by the uplink antennae.
The User Segment
The GPS user segment consists of all the GPS receivers and the user community. Initially the GPS service was available for military...