7.1 Impact on phase
Phase and time of occurrence of the ionospheric perturbations are the
required parameters to determine velocity of the perturbations. Velocity
of a ionospheric perturbation is one of the important characteristics as
it directly affects the ray tracing which eventually lead to error in
locating the source of the perturbation. To quantify the impact of
aliases and artifacts on the phase, we carried out a cross-correlation
analysis and identified the difference in phase and time of occurrence
between the perturbations obtained using the three methods. The gROT of
each receiver-PRN pair was correlated with rTEC and dTEC, and the phase
differences were converted into time differences. The results show that
occurrence time of TIP in rTEC is different from gROT. Among the 44 TIP
time series, in 16 cases the time of occurrence of TIP in rTEC is
delayed compared to gROT, in 17 cases rTEC detects TIPs prior to gROT
and at 11 cases rTEC did not identify the TIPs (Table 1). The delay
varies from 2.5 to 6.0 minutes and the advancement is ranging from 2.0
to 5.5 minutes (Fig. 14 and Table 1). The difference in time of
occurrence between rTEC and gROT may be due to either aliasing or the
subjective selection of the high-order polynomial to represent the trend
of GPS TEC. Such differences in time of occurrence will consequently
affect the characteristic velocity estimates of the tsunami induced
ionospheric perturbations. However, there is no phase difference found
between dTEC and gROT.
Similar to tsunami case, the difference in time of occurrence between
rTEC and gROT of CIPs are varying from 7.64 minutes of advancement to
2.55 minutes of delay (Fig.14,). Further, rTEC failed to detect 50% of
CIPs (18 cases out of 36) compared to dTEC and gROT (Table 2). This
difference in time of occurrence show that the signal detection using
residual method by employing uniform order of polynomial across all
station-PRN pair mislead the detection. These results emphasize the
importance of considering such methodology related differences while
studying the physics of ionospheric perturbations.