Investigation of GPS Signal Fading Due to Scintillation & Multipath and its Effects on SBAS

Surendra Sunda, S.M. Regar, P.V. Khekale, K. Bandyopadhyay, and B.M. Vyas

Abstract:	Indian Space Research Organization is planning to demonstrate Satellite Based Augmentation System (SBAS) named GAGAN (GPS Aided Geo Augmented Navigation) in collaboration with Airports Authority of India. As ionosphere is biggest hurdle in implementing the SBAS over Indian region which lies in equatorial anomaly region, wide network of GPS Ionospheric Scintillation and TEC Monitors were set-up all over India to study the ionospheric effects on GPS signal. These receivers provide Total Electron Content (TEC), S4 index and óÄö (scintillation parameters), C/N0, code-carrier divergence and its variance (óccd ) etc. The GPS signal experiences fading due to small-scale irregularities in the electron density known as scintillation. It has already been established that scintillation occurs after sunset till midnight and causes loss of lock during strong scintillation activity (high S4 value). But very high S4 values have been observed during day time which needs to be investigated thoroughly as it is not considered a normal phenomenon. It may either be due to genuine scintillation activity and/or Multipath or GPS receiver malfunction. Strong scintillation and Multipath can cause a serious challenge to accuracy, availability and continuity of SBAS/GPS signal. We have analysed the effects of both activities- scintillation and Multipath on receiver’s available constellation for positioning. Geometric Dilution of Precision (GDOP) was computed and analysed with respect to scintillation and Multipath activity at many stations covering all over India. It has been observed that at some stations during high scintillation coupled with poor geometry of GPS satellites, GDOP value goes beyond 10 which severely affect the accuracy in positioning. The same has been observed during day time which is considered as scintillation free time but due to multipath and other factors. In this paper we have investigated the factors responsible for GPS signal fading i.e. scintillation and multipath or receiver malfunction and their characteristics. We present the observation of scintillation/multipath at number of stations and their impact on Indian SBAS-GAGAN. We have tried to investigate how the number of visible satellites reduces due to severe scintillation/multipath and affects the Geometrical Dilution of Precision (GDOP).
Published in:	Proceedings of the 2008 National Technical Meeting of The Institute of Navigation January 28 - 30, 2008 The Catamaran Resort Hotel San Diego, CA
Pages:	903 - 910
Cite this article:	Sunda, Surendra, Regar, S.M., Khekale, P.V., Bandyopadhyay, K., Vyas, B.M., "Investigation of GPS Signal Fading Due to Scintillation & Multipath and its Effects on SBAS," Proceedings of the 2008 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2008, pp. 903-910.
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