subject: Understanding The Concept And Evolution Of The Differential GPS System [print this page] The Global Positioning System meant for the civilian use was freely available throughout the world. The security establishments in the US suspected its use to be done by enemy countries to launch precision guided weapons against it. To thwart any such danger arising out of the potential misuse of the GPS technology, the frequency of the coarse acquisition signal (C/A) was deliberately degraded by the US military so that now an error of 100 meters was put into the satellite calculations. It was famously called as Selective Availability (or, S/A). Though this was considered sufficient from the security point of view, it severely affected its use in the civilian applications. An error of 100 meters was too much for delivering quality GPS services.
One of the main areas which got affected was the Geographical information system which was based on the GPS technology. To come round to this problem, the differential GPS was devised. The S/A signal did not change very fast. This meant that if the offset was applied to the error, it would hold over a large area. So, if this difference is signalled to the devices having the Global Positioning System, better location accuracy can be achieved to the extent of 15 meters. This level of accuracy could be further improved if the errors resulting from the transmission delays in the ionosphere are taken into account in the GPS enabled devices. The differential GPS corrections can be applied to achieve the accuracy for 5 meters. This met the requirement of most of the civilian applications and uses. There are two ways to apply these corrections to the GPS calculations. The first way is to adjust these in the real time and the second way is to apply the corrections in in the post-processing stage.
This initial differential GPS system was based on the ground receiving stations. However, one of its main limitations was that the as the distance of coverage increased, the accuracy of differentials got reduced because most of the bigger stations were close to the main population centres. This limitation led to the development of Wide Area Augmentation System (WAAS). This concept did away with the need to develop the ground networks. Rather, the use was to be made of the geostationary communication satellites orbiting the earth. The levels of accuracy were same as that of the ground based differential GPS networks but the reach increased to the whole hemisphere. At the same time, with increasing criticism of the S/A and defeat of its intended purpose due to the evolution of DGPS, the S/A was rolled back.
So, with the original GPS accuracy levels restored, did it make the differential global positioning system a thing of the past? The DGPS had, by this time, evolved to such an extent that it offered better accuracy levels, devoid of many errors, which were even not given by the non S/A GPS services. Most of the current applications of the GPS also make use of the differential GPS services for enhanced mapping and locating services.
Understanding The Concept And Evolution Of The Differential GPS System
