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INTRODUCTION
There is a need forever to define location of citizenry, topographic point and objects on the surface of the earth. Over the period, we have progressed from crudely marked paths and trails to developing detailed maps using sophisticated systems. The Positioning, Navigation, and Timing (PNT) systems are in use for long. USA has been leading the development of satellite navigation with GPS for more than two decennaries, besides slow emergence of its Russian equivalent, GLONASS and other systems worldwide. Today, space-borne (SB) positioning systems (PS) can determine the direction, velocity and location i.e. latitude, longitude and altitude almost instantaneously. High Precision SBPS technology is becoming a dominant instrument for a diverse surveying and geodetic applications. Beginning with post-processing solutions in 1980s, better accuracy Real-Time Kinematics (RTK) Positioning with GPS is one of todays most widely used surveying techniques. The developing concept of Virtual Reference Stations offers a new opportunity vis-à-vis traditional control networks. Countries in Europe, Asia and America have already formulated a strong base for the GPS based virtual reference station across their territories. These networks are regularly employed for providing services to post-processing RTK surveys besides helping in delineating the datum, cadastre, construction, and national mapping applications. These solutions today make use of interoperability of SBPS with classical surveying instruments and unexampled technology for wireless communication like GPRS and Bluetooth to provide productive field tools for surveying activities.

HISTORICAL FACTS
Origins of positioning and navigation memorializes back to 1920s with the radio-navigation beckon (Early WW II LORAN) developed by the MIT Radiation Laboratory. Much later in 1959, first operational satellite-based navigation system was developed by ‘John Hopkins Applied Physics Laboratory’ (APL) under project code named ‘Transit’ which was meant to support SBPS needs of U.S. Navy’s submarine fleet. This technology predates the foundation of ‘Global Positioning System’ (GPS) as known today. Application of artificial satellites in navigation and positioning are in continuum observe since then. Later in the same year, in response to an United States Air Force (USAF) requirement for accurate ICBM space borne controls, Raytheon Corporation suggested the first three-dimensional (longitude, latitude, altitude) SBPN system for guidance system to be used with the proposed ICBM that would achieve mobility by traveling on a railroad system. The navigation system named MOSAIC (Mobile System for Accurate ICBM Control) never saw light and the Mobile Minuteman plan was cancelled.



The concept of GPS originated from the Aerospace Corporation’s study on using a space borne system for a navigation system of vehicles. The USAF supported the Aerospace study and named it ‘System 621B’. The concept involved measuring the time of arrival of radio signals transmitted from satellites whose positions were precisely known. This gave the distances to the known satellite positions, which in turn, established the user’s position. Substantiating this concept by 1972, operation of a new type of satellite-ranging signal based on ‘Pseudo-Random Noise’ (PRN) was established in an experiment where balloons carrying transmitters simulating as satellites were used, and accuracies of a hundredth of a mile were demonstrated. Based on these experiments in Dec 1973, a new concept was approved, now known as the NAVSTAR GPS, marking the start of Phase I of the GPS programme. The very first NAVSTAR satellite was launched on 14 Jul 1974, designated as ‘Navigation Technology Satellite (NTS) No 1’ and the second (and last) of the NTS series was launched in 1977. These satellites were used for concept validation purposes only and carried the first atomic clocks ever launched into space. After the successful testing of the system, the approved system configuration subsequently consisted of 24 satellites placed in 12-hour inclined orbits, A total of 11 Block-I satellites were launched between 1978 and 1985 on the Atlas-Centaur, as developmental prototypes. The Blocks were used for system testing purposes that were later on modified as a full system.

By Apr 1980, the GPS satellite technology was developed to the limits where fully functional ‘Integrated Operational Nuclear Detonation Detection System’ (IONDS) sensors were carried by the satellite as a secondary essential payload. In order to cut 30 percent from the budget in1982, decision to reduce the GPS satellite constellation from 24 to 18 satellites was approved by DoD.

Following the Soviet downing of Korean Air flight 007 on16 Sep 1983, US President Reagan made GPS available for civilian aircraft free of charge. This marked the beginning of the use of GPS technology from military to civilian aircraft. Persian Gulf War demonstrated the military use of the technology first time under combat conditions. By this time collimate SBPNS by countries like European Union, Russia, India had started conceptualizing project with limited synergism.

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