6.5 Detecting aliasing and artifact free CIPs
Time-distance plots given in Fig. 10 show that the aliasing is prominent
in CIPs obtained using the differential method (dTEC). There are
significant differences in amplitude of gROT and dTEC in 61% of the
cases when the inter-IPP distances are more than 4 km (Fig. 10), which
indicate that unaccounted inter-IPP distances in differential method
aliased as ionospheric perturbations in dTEC.
The scenario is completely different when using the residual method
(rTEC). The rTEC values show strong signals in many cases at different
times compared to dTEC and gROT (Fig. 10). Upon comparing these values
with the corresponding time series (Fig. 11), one can easily identify
that most of the signals are artifacts associated with data gaps. Other
than artifacts, notable CIPs occur prior to the earthquake, for example
PYUT16 (Fig. 10). The corresponding time series given in Fig. 11 reveals
that these are multiple artifacts occurred in a short time which caused
this signal aliasing.
Apart from the artifacts and aliasing, time series of receiver-PRN pairs
show that residual method (rTEC) completely failed to detect CIPs in
50% of the cases compared to the differential method and SPLA (Fig. 11
and Table 2). Upon exploring, we found that the poor fits of high order
polynomials used to represent the trend of vTEC time series lead to
failure in detecting the CIPs (Fig. 12). For example, in the case of
TPLJ03 (Fig. 12) it is shown that the polynomial fits well with vTEC
only from 4 UT to 5:45 UT. However, the CIP detected by dTEC (Fig. 12c)
and gROT (Fig. 12d) at 6:15UT falls under the misfit region in the case
of rTEC (Fig. 12b) which leads to failure in detecting the CIP by
residual method. Further inspections revealed that the filtering process
of obtaining CIPs from rTEC filtered out the high residual appear from
6:15UT to 8:45UT (Fig. 12b) and this lead to the failure of detecting
the sharply varying CIP with a frequency range of 1.6 to 16 mHz by the
residual method. However, such sharp variations were successfully
detected by SPLA.