4. Filtering and Normalization
In order to filter the ionospheric perturbations associated with earthquake and tsunami, the estimated dTEC, rTEC and gROT are filtered using zero-phase bidirectional band-pass (butterworth) filter with a pass band of 1.67 to 16 mHz (1-10 minute period) (Catherine et al, 2015) and 0.5 to 5 mHz (3.3 – 33.3 minute) (Galvan et al, 2011), respectively. The filtered dTEC, rTEC and gROT are normalized using Z-score normalization for inter-comparison as the three parameters are measured in different units.
- (8)
Where Z is the normalized dTEC or rTEC or gROT, x is filtered dTEC or rTEC or gROT, is mean of x , and is the standard deviation. The mean () and standard deviation () of dTEC, rTEC, and gROT are computed for each satellite-PRN time series.
In the following sections, we show that adopting the SPLA removes aliasing and artifacts from TIPs and CIPs using the normalized dTEC, rTEC and gROT.