An Improved Ionospheric Disturbance Detection Method for GNSS with Time-Resolved FFT Averaging Ratio

The ionosphere exhibits rapid spatial and temporal variability, particularly during Geomagnetic storms and Equatorial Plasma Bubble (EPB) events which can severely degrade the performance of Global Navigation Satellite System (GNSS). Although, the Fast Fourier Transform (FFT) Averaging Ratio (FAR) algorithm effectively detects ionospheric irregularities, it is computationally intensive, lacks precise temporal resolution and may miss ionospheric disturbance events when most of the satellite PRNs are simultaneously corrupted. To overcome this limitation, the present study proposes a Time Resolved-FAR (TR-FAR) technique that operates on time averaged Vertical Total Electron Content (VTEC) at every epoch. The TR-FAR algorithm applies FFT over consecutive non-overlapping 30-min windows, enabling almost real-time detection of medium- and large-scale ionospheric disturbances. A disturbance is detected when the ratio of the mean to average Power Spectral Density within a window exceeds a predetermined Threshold. GNSS data from the low-latitude stations, IISC (13.02117° N, 77.57038° E) and HYDE (17.41728° N, 78.55088° E), for the year 2023 were analyzed. Case studies of severe geomagnetic storms on 23–24 April, demonstrate that TR-FAR successfully captures short-time ionospheric irregularities that are underestimated by daily FAR indices. Comparative analysis shows that TR-FAR detects medium- and large-scale ionospheric disturbances with significantly higher sensitivity than the Rate of TEC Index (ROTI) by suppressing link-dependent ionospheric variability. The ionospheric climatological analysis from TR-FAR algorithm reveals pronounced seasonal dependance, with number of disturbances during March equinox exceeding those of the September equinox by approximately 8%, indicating clear equinoctial asymmetry. During solstitial period, the occurrence of ionospheric disturbances is suppressed with number of ionospheric disturbances reduced by approximately 24% compared to equinoctial periods. However, localized enhancements during December solstice reflect winter anomaly. These results establish TR-FAR as a computationally efficient tool for GNSS based ionospheric disturbance monitoring.