Error analysis of satellite-based global navigation system over the low-latitude region

The effect of changes in the environmental conditions surrounding an airport on ground-based navigational aids, such as Doppler very high frequency omni range, distance measuring equipment and instrument landing system course structure, is a major issue that affects the performance of these equipment. Due to the limitations of the ground-based navigational aids, the International Civil Aviation Organization has established a special committee on the Future Air Navigation System (FANS) to look into the development of satellite-aided communication, navigation and surveillance system. The FANS committee concluded that satellite technology was the solution to overcome the shortcomings of ground-based systems and to meet the future needs of the international civil aviation community. Global Positioning System (GPS) is one such system, which is an all-weather satellite-based positioning system funded by the Department of Defence, USA and is available to users anywhere on the globe for various applications, including civil aviation. However, the performance of stand-alone GPS does not meet the Cat-I PA requirement due to several errors. Therefore, for aviation users in the Indian subcontinent, augmentation of GPS is planned through a regional satellite-based augmentation system called GPS-aided geo augmentation system. However, over the Indian subcontinent the ionospheric effects are severe and erratic due to equatorial anomaly. Total electron content variations have been analysed for several years in the low-latitude region. GPS signal structure, various errors affecting the positional accuracy of GPS and the techniques used to mitigate their affects are presented in this article.