Ionospheric Limitations and Specifications in the Auroral Zone

S. Skone, M.E. Cannon

Abstract:	Dual frequency GPS receivers enable the estimation of absolute ionospheric delay and total electron content (TEC) along the signal path. By using a number of reference stations, each equipped with a dual frequency receiver, it is possible to estimate values of the vertical ionospheric delay at a set of designated grid points. This type of ionosphere delay modeling is employed in Wide Area Differential GPS (WADGPS) Networks, where delay values at grid points are determined in an adjustment incorporating all receiver-satellite line-of-sight observations. The grid accuracies, and estimates of the grid ionosphere vertical error (GIVE), generally depend on the estimated temporal/spatial correlations of TEC. These models can suffer degraded performance in regions, such as the auroral zone, where spatial gradients and temporal variations of electron density may differ significantly from assumptions. The auroral zone is characterized by particle precipitation events (i.e. auroral E-ionization and magnetospheric substorms) which cause significant variations in both the vertical and horizontal electron density gradients. The auroral oval is generally located between 65 and 72 degrees magnetic latitude, and can extend several degrees southward under significantly disturbed levels of ionospheric activity. Variations of auroral zone TEC are therefore significant concerns for WADGPS systems operating in Canada, Europe, and Alaska. In order to determine the effects of auroral activity on WADGPS grid performance, ionospheric disturbances must be identified and correlated with variations in ionosphere electron densities or TEC. In the past, performance evaluations of wide area grid algorithms have relied on planetary indices to identify disturbed ionospheric conditions. The use of such indices (i.e. Kp, AE) to infer levels of ionospheric activity is ambiguous, however, since these indices are based on magnetometer measurements made at various stations distributed around the globe. A large planetary index may not necessarily indicate enhanced ionospheric activity in the wide area network region. In this paper, an alternative approach, not previously investigated, is employed to identify the level and magnitude of local ionospheric activity. Magnetic field measurements from twelve Canadian auroral zone magnetometer stations (the CANOPUS MARIA array) are used to identify the magnitude and location of localised particle precipitation events, during the period April - October 1996. Corresponding TEC values are then calculated, using simultaneous GPS data from ten stations in the Natural Resources Canada (NRCan) wide area network. Statistics representing the spatial correlation of TEC, under varying levels of localised ionospheric activity, are determined for the NRCan data set. The corresponding temporal variations of TEC, and the spatial extent of ionospheric disturbances, are also presented. Implications, with respect to wide area ionosphere grid models, are then analysed. In particular, the following considerations are discussed: grid spacing requirement for adequate resolution of localised activity, modeling of temporal correlations, and accuracies of wide area corrections.
Published in:	Proceedings of the 10th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1997) September 16 - 19, 1997 Kansas City, MO
Pages:	187 - 197
Cite this article:	Skone, S., Cannon, M.E., "Ionospheric Limitations and Specifications in the Auroral Zone," Proceedings of the 10th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1997), Kansas City, MO, September 1997, pp. 187-197.
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