Small-scale Ionospheric Delay Variation Associated with Plasma Bubbles Studied with GNSS and Optical Measurements and its Impact on GBAS

S. Saito, T. Yoshihara, Y. Otsuka

Abstract:	For differential GNSS systems such as the ground-based augmentation system (GBAS), spatial variation (or spatial gradient) in ionospheric delay is a critical issue. In designing safe and available systems, so-called ionospheric threat models that describe possible ranges of ionospheric variability need to be defined. Characteristics of the ionosphere varies with the magnetic latitude. In the mid-latitude region, the largest ionospheric spatial variation has been observed associated with ionospheric density enhancement associated with a severe magnetic storm. In the low latitude region, the dominant ionospheric disturbance is the plasma bubble. Plasma bubbles are local ionospheric depletions that often occur after sunset most of which occur without magnetic disturbances. They accompany ionospheric irregularities with various scale sizes from 100 km down to a meter. However, the characteristics of the low latitude ionospheric variations in terms of a specific threat to GBAS has not been well studied yet. A recent study shows that ionospheric delay variations of more than 500 mm/km at the L1 frequency was reported at a 1.6 km distance[1] which may exceed the range of the threat model proposed for the Catetory-III GBAS with GPS L1 frequency signals (GAST-D) [2]. For ionospheric threat model development, not only the gradient in the ionospheric delay but also the total ionospheric delay change associated with the gradient, or in other words the product of the gradient and the horizontal scale size, is important. It is also important to know the spatial correspondences of the ionospheric delay variation to the large-scale structure of the plasma bubble. For instance, if steep ionospheric gradiens are concentrated at a certain portion of a plasma bubble, the probability of satellites experiencing steep gradients could be reduced. To study the ionospheric delay variation associated with the plasma bubble, Electronic Navigation Research Institute (ENRI) has installed five sets of dual-frequency GPS receiver and GPS scintillation receiver with mutual distances of 0.4 to 1.6 km at Ishigaki(24.3N, 124.2E), Japan since 2008. Precise ionospheric delay gradients were estimated by a single-frequency carrier-based and code-aided method developed by ENRI [3,4]. Total delay variations were estimated by the dual-frequency measurements as well as by using the drift velocity estimated from the correlation of temporal variation of the ionospheric delay observed at the spatially distributed receivers. The large-scale plasma bubble structures were photographed with airglow emissions at the wavelengths of 630.0 and 777.4 nm by an all-sky imager installed at Yonaguni (24.5N, 123.0E), Japan near Ishigaki. The locations of large ionospheric delay variations at the ionospheric height were compared with the plasma bubble images projected on a horizontal plane at the ionospheric height. We analyze data the ionospheric delay variation observed at Ishigaki and the large-scale structure of the plasma bubble Yonaguni from 2008. The preliminary results shows that the extreme delay gradient more than 500 mm/km was observed and its total ionospheric delay change was estimated to be about 4.8m on 3 April 2008. As the increase in the solar activity, the number of plasma bubble events are increasing. At the meeting, results of analysis of ionospheric delay variations and the large-scale plasma bubble structure for more plasma bubble events will be presented. The spatial correspondences of the ionospheric delay variation to the large-scale structure of the plasma bubble will be discussed. The impact of the ionospheric variation associated with the plasma bubble on GBAS will also be discussed. References [1] Saito, S., S. Fujita, and T. Yoshihara, Precise measurements of ionospheric delay gradient at short baselines associated with low latitude ionospheric disturbances, Proc. ION ITM 2012, 2012. [2] GBAS CAT II/III Development Baseline SARPs, 2010. [3] Fujita, S., T. Yoshihara, and S. Saito, Determination of ionospheric gradients in short baselines by using single frequency measurements, J. Aero. Astro. Avi., A-42, 269-275, 2010. [4] Saito, S., T. Yoshihara, and S. Fujita, Absolute gradient monitoring for GAST-D with a single-frequency carrier-phase based and code-aided technique, Proc. ION GNSS 2012, 2012.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	1869 - 1874
Cite this article:	Saito, S., Yoshihara, T., Otsuka, Y., "Small-scale Ionospheric Delay Variation Associated with Plasma Bubbles Studied with GNSS and Optical Measurements and its Impact on GBAS," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1869-1874.
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