Stand-alone real-time GPS positioning without SA

GPS is a satellite based navigation system developed by the United States Department of Defense (DoD). This method has been densely used for determining 3D positioning for surveying, scientific or other applications because of the advantages, such as being independent of weather conditions, having no requirement of surveying points seeing each other, being able to manage surveying during the day and night. There are two types of service in GPS available for civil and military users. The first one for civilian users is called standard positioning service (SPS) and the second for only authorized users is the precise positioning service (PPS). The SPS is available to all users worldwide and there are no restrictions on the SPS usage. The main difference of these services is there navigation accuracy. The accuracy of the GPS SPS was degraded deliberately with stand-alone accuracy by the DoD performing through manipulation of the broadcast ephemeris data and dithering of the satellite clocks. The former President Clinton decided that the U.S. government would stop the SA a few minutes past midnight Eastern Daylight Time (EDT) after the end of May 1, 2000. The DoD has released the new performance standard for GPS SPS in October 2001, taking into account the new levels of performance achievable without SA. DoD, as operator of the GPS, now provides civil users a horizontal positioning accuracy of 36 meters for single frequency users, compared to 100-meters accuracy in the previous (second) edition of the standard, which was published in 1995. In terms of accuracy, it can be said that the stand-alone GPS accuracy of the SPS has improved significantly after the termination of SA and point positioning could be realized up to 10 times more accurately than before May 1, 2000. The removal of SA has not only enhanced the performance of stand-alone GPS but also created some new application areas that have not been used until the removal of SA. On the other hand, some applications such as in marine geodesy, hydrography and precise navigation require better accuracy. For these applications, the DGPS is still a useful tool in determining position accurately and reliably. However, in case of requirement an accuracy level of better than sub-decimeter (e.g. for the high accuracy marine positioning), kinematic positioning technique by using carrier phase observations has to be used.