Ionospheric Index I95

Correction models for the distance dependent GPS errors in the form of area correction parameters (Flächenkorrekturparameter, FKP) are an intermediate product of the computation of observations of virtual reference stations. Ionospheric FKP give full details of ionospheric effects on cm-level relative positioning. The ionospheric index I95 merges the ionospheric FKP to one single value per hour. Large values of the ionospheric index I95 indicate that difficulties may be encountered in GPS baseline processing (delayed ambiguity resolution, reduced positioning accuracy). Often these difficulties can be mitigated by longer observation periods. Experiences from the last solar maximum show that even the technique of virtual reference stations may be subject to complications if large I95 values are observed. Under such circumstances FKP and virtual observations tend to be incomplete and the quality of the ionospheric modelling may be reduced. The state survey agencies of Saxony-Anhalt in Magdeburg and of Lower-Saxony (LGLN) in Hannover have computed hourly I95 values based on SAPOS subnetwork observations for many years. The figure below shows I95 weekly averages in a date and local time coordinate system. In addition, the figure includes the sunspot number (SSN, source: SIDC) to indicate solar activity. It illustrates the dependence of ionospheric difficulties on the eleven-year solar activity cycle. In Central Europe, during years of strong solar activity (1999 - 2002, 2012 - 2015, 2023 - 2026), large I95 values are observed mainly during winter daylight hours. Current I95 values are available on the LGLN Ionospheric Index page.

Additional information on the I95 index can be found in: Wanninger, L. (2023): Die befleckte Sonne - GNSS-Vermessung unter erschwerten Bedingungen. FORUM (BDVI), 2/2023, 40-42 (PDF-file) Wanninger, L. (2004): Ionospheric Disturbance Indices for RTK and Network RTK Positioning, Proc. of ION GNSS 2004, Long Beach, CA, 2849-2854 (PDF-file)

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