Statistics of Occurrence of Ionospheric Scintillation and TEC Depletions Over Indian Region and its Effect on Satellite Navigation

S.M. Regar, S. Sunda, M.R. Sivaraman, K. Bandyopadhyay

Abstract:	Indian Space Research Organization (ISRO) in collaboration with Airports Authority of India (AAI) is planning to demonstrate Wide Area Differential GPS (WADGPS), over Indian Airspace. In this project named SATNAV TDS, also called GAGAN (GPS Aided Geo Augmented Navigation), with minimum ground and satellite segment, accuracy, integrity, continuity and availability of GAGAN that can be achieved, will be demonstrated. The Equatorial Anomaly in the ionosphere over Indian Airspace poses a challenge to implement GAGAN, as it is very difficult to develop suitable Ionospheric models and use. In addition, GPS Receivers used in Indian regions can suffer from rapid amplitude and phase fluctuations known as scintillation, causing frequent fading of signals. Under GAGAN Project, a network of GPS Dual Frequency receivers (GISTM) are installed at 18 airports each in 50 by 50 Grid to cover all Indian airspace, to collect Total Electron Content (TEC) and L Band Scintillation Index (S4 and sf) data. We have estimated the percentage occurrence of L Band scintillation exceeding 3, 6, 10 and 15 dBs for all 18 stations for last two years. We observe that in all stations, large ( 10dB) percentage of scintillations occur in Equinoctial months in 2004 and 2005 (Low sun spot activity years). Maximum scintillation can be seen in the geographic latitude belt of 15o-25° north of the equator in India, which is referred to as the equatorial anomaly region, causing frequent loss of lock on L1 and L2 frequencies. The major source of error in GPS that affects the aircraft positioning accuracy is the Ionospheric Propagation delay. A Near Real Time Grid Based Ionospheric Model is planned to be implemented in GAGAN, to correct the Ionospheric effects to an accuracy better than 0.5m. This model will be sufficient, if Ionospheric gradients are smooth. Sometimes, over Indian region, highly localized Ionospheric gradients (known more commonly as Ionospheric Bubbles) are observed and the Grid Based model will not be sufficient. The maximum number of bubbles and loss of lock occur at stations near the Equatorial Anomaly peak region in India and they are observed only after sunset and up to midnight. As the Ionospheric bubbles are much localized, sometimes it can be seen only by user aircrafts but not by any Reference station which can cause the failure of the conventional Grid based Model. Also the bubble can cause strong scintillations, causing loss of lock of GPS signals. They affect accuracy, integrity, availability and continuity of GAGAN service over Indian region.
Published in:	Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006) September 26 - 29, 2006 Fort Worth Convention Center Fort Worth, TX
Pages:	1336 - 1340
Cite this article:	Regar, S.M., Sunda, S., Sivaraman, M.R., Bandyopadhyay, K., "Statistics of Occurrence of Ionospheric Scintillation and TEC Depletions Over Indian Region and its Effect on Satellite Navigation," Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006), Fort Worth, TX, September 2006, pp. 1336-1340.
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