TEC Gradients and Fluctuations at Low, Latitudes Measured With High Data Rate GPS Receivers

Charles S. Carrano and Keith M. Groves

Abstract:	Large scale gradients in TEC can degrade the performance of Space Based Augmentation Systems (SBAS) that supply differential corrections of ionospheric delay to GPS users to improve positioning accuracy. These gradients can also complicate the analysis of GPS occultation data, the interpretation of verticalized TEC measurements, and the determination of interfrequency receiver biases. Small scale TEC fluctuations are commonly associated with the scintillation of transionospheric signals that can impact a variety of communication and navigation systems including GPS. Nevertheless, the spatiotemporal morphology of large scale gradients and small scale fluctuations in TEC have yet to be characterized as functions of local time, magnetic latitude, and solar flux. In this paper, we characterize large scale gradients and small scale fluctuations in TEC using high data rate dualfrequency GPS receivers maintained by the Air Force Research Laboratory (AFRL). These receivers are distributed throughout the American, Indian, and Asian longitude sectors and primarily in the low latitude regions of the globe. The intent of this work is not to quantify the morphology of TEC variations but instead to present representative environments at low latitudes that may be useful for system design and impacts assessments. A statistical analysis of large scale TEC gradients as a function of magnetic latitude and local time is presented. Small scale TEC fluctuations are quantified in terms of the 60second standard deviation of the rate of change of TEC, sampled at 1050 Hz. The high sampling rates make this quantity sensitive to small scale ionospheric structure (roughly 525 meter scalelengths), depending on the ionospheric projection of the satellite velocity vector and the ExB drift. Dominant mechanisms for the generation of large scale TEC gradients at low latitudes include the Appleton anomaly, geomagnetic storms, and electron density depletions associated with Equatorial Spread F (ESF). Small scale TEC fluctuations are closely associated with ionospheric structures responsible for the scintillation of UHF and GPS satellite signals.
Published in:	Proceedings of the 63rd Annual Meeting of The Institute of Navigation (2007) April 23 - 25, 2007 Royal Sonesta Hotel Cambridge, MA
Pages:	156 - 163
Cite this article:	Carrano, Charles S., Groves, Keith M., "TEC Gradients and Fluctuations at Low, Latitudes Measured With High Data Rate GPS Receivers," Proceedings of the 63rd Annual Meeting of The Institute of Navigation (2007), Cambridge, MA, April 2007, pp. 156-163.
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