TITLE:
A Novel Approach to Study Regional Ionospheric Variations Using a Real-Time TEC Model
AUTHORS:
S. C. Chakravarty
KEYWORDS:
GPS; Real Time VTEC Model; GAGAN; Low Latitude TEC
JOURNAL NAME:
Positioning,
Vol.5 No.1,
January
26,
2014
ABSTRACT:
Since IGY (International Geophysical Year),
through coordinated global observations, ionospheric research has been carried
out by many countries. This effort primarily helped in the design and operation
of HF radio wave communication
systems. The Indian region covers a highly
variable part of the equatorial electrojet and EIA (Equatorial Ionisation
Anomaly) phenomena making its predictability difficult. With the advent of
satellite communication and navigation, the need for accurate ionospheric TEC
(Total Electron Content) models at global and regional scales has been
stressed. The GAGAN (GPS Aided Geo Augmented Navigation) project jointly
undertaken by the Indian Space Research Organisation (ISRO) and the Airport Authority
of India (AAI) aims at effectively utilising the Global Navigational Satellite
System (GNSS) to determine position coordinates accurately for aircraft
precision landing applications. For this purpose the range errors are estimated
by using a ground network of TEC stations spread over Indian region. The near simultaneous data collected from these dual frequency GPS stations can be used to generate the geo-referenced TEC values
for various applications. The author has developed necessary algorithm and associated
computer programmes for a real-time vertical TEC (VTEC) model based on TEC data
collected from the GAGAN ground based network stations. The model has been
tested and sample results presented here show that it adequately provides for
the latitudinal resolution of 1° for the entire longitude span and also for two
longitude blocks (73 - 83 & 83 - 93°E)
separately. Cubic spline and bilinear interpolation techniques are used for
filling up temporal and spatial data gaps. The model provides tabulated output
of hourly average VTEC data with latitude for ready use, as well as graphical
displays of VTEC maps and contours for monitoring purpose. The real-time model
and its extensions are also being used for detailed scientific studies;
examples of these show small day to day variability of VTEC without any change
in solar activity and indication of the change in the shape of the VTEC diurnal
curve with season. The present model will be used for further studies to derive
the monthly average variation of the diurnal pattern and the relationship
between VTEC peak amplitudes with changes in solar activity. The new
information generated can be fed back to improve the real-time model so that eventually the dependence of such models on ground
based network stations data can be minimised.