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 department of Saxony-Anhalt has computed hourly I95 values for some years using the former WaSoft/Virtuell or WaV2 based on the observations of three of its GPS reference stations. The figure below shows I95 weekly mean values in a coordinate system of date and local time. It illustrates the dependence of ionospherically conditioned difficulties from the eleven year long solar activity cycle. During years of strong solar activity (1999 - 2002, 2012 - 2015, 2023 - 2026) large I95 values are observed in central Europe during daylight hours in winter time.

The state survey department of Lower Saxony (LGLN, Hannover) is another department to compute I95 index values. Hourly values are produced from the observations of four GPS reference stations using WaV2. Recent values and archived data are available from the LGLN Ionospheric Index page. Additional information on the I95 index can be found in: Wanninger, L. (2004): Ionospheric Disturbance Indices for RTK and Network RTK Positioning, Proc. of ION GNSS 2004, Long Beach, CA, 2849-2854 (PDF-file)

WaSoft.de, 2022