  Ionospheric irregularity studies are important aspects for understanding
ionospheric physics and related processes, especially near the low-latitude
regions. However, simultaneous measurements (utilizing the L-band signals of
NavIC and GPS) of irregularity scale sizes over the Indian longitude sector,
has not been addressed extensively. To address this problem, the paper presents
simultaneous characterization of low-latitude ionospheric irregularities over a
location near the northern EIA crest (Indore: 22.52$^\circ$N, 75.92$^\circ$E
geographic and magnetic dip of 32.23$^\circ$N) and a location (Hyderabad:
17.41$^\circ$N, 78.55$^\circ$E geographic and magnetic dip of 21.69$^\circ$N)
between the crest and the magnetic equator, utilizing the Indian navigation
system, NavIC and GPS L5 signal C/N$_o$ variations to determine the range of
the ionospheric irregularity scale sizes using Power Spectral Density (PSD)
analysis. The study period spans from September 2017- September 2019, covering
both disturbed and quiet-time conditions in the declining phase of solar cycle
24. Observations show that the irregularity scale size ranges from about 500 m
to 6 km. This study for the first time, shows the nature of the temporal PSD
for ionospheric scintillation during varying solar and geophysical conditions,
by measuring the irregularity scale sizes utilizing simultaneous observations
from NavIC and GPS from locations near the northern crest of the EIA and in
between crest and the magnetic equator, ensuring proper characterization of
ionosphere over the geosensitive Indian subcontinent.
