**2.2 TEC measurements**

Up to 11 GPS satellites are in view and provide outputs in 22 receiver channels [35]. The ionosphere has an effect on the signal of GPS satellite. TEC is measured along the path from the GPS satellite to a receiver. The TEC is defined by the integral of electron density in TEC unit (TECU), where 1 *TEC unit* = 10<sup>16</sup> *electrons*/*m*<sup>2</sup> column along the signal transmission path. The dual frequency GPS receivers are used to measure the TEC, which is one of the most important methods to investigate the dynamics of ionosphere. Several research groups are showing interest in the equatorial ionospheric research using GPS data. Dow et al. [36] did an analysis of the importance of GPS data in support of the terrestrial reference frame, earth observations and research, positioning, navigation and timing as well as other applications that benefit the society. The slant TEC is the measure of the total number of free electrons in a column of unit cross section along the path of the electromagnetic wave between the satellite and the receiver [37].

A dual frequency (L1 = 1575.42 MHz and L2 = 1227.60 MHz) GPS receiver (LEICA GRX1200GGPRO GNSS) is operating at Hyderabad (17.37°N, 78.48°E) [3]. It is a unique station to study the ionospheric irregularities because it is located at the northern crest of the equatorial ionization anomaly (EIA). A dualfrequency GPS receiver can measure the difference in ionospheric delays between the L1 and L2 of the GPS frequencies, which are generally assumed to travel along the same path through the ionosphere. Thus, the group delay can be obtained as

*Geographic Information Systems in Geospatial Intelligence*

$$
\Delta \left( \delta t \right) = \delta t\_{L1} - \delta t\_{L2} \tag{1}
$$

Here, ∆(*t*) is a time delay in the pseudo-range (*tL*1) at L1 and pseudo-range (*tL*1) at L2. The resulting equation is (Jain et al., 2011),

$$
\Delta(\delta t) = 40.3 \times TEC \times \frac{\left(f\_{L1}^2 - f\_{L2}^2\right)}{c \times f\_{L1}^2 \times f\_{L2}^2} \tag{2}
$$

where *fL*1 and *fL*2 are the group path lengths corresponding to the high and low GPS frequencies *fL*<sup>1</sup> = 1575.42 *MHz* and *fL*<sup>2</sup> = 1227.60 *MHz*, respectively and "*c*" is speed of light in vacuum. The TEC can be obtained by rewrite above equation as,

$$\text{TEC} = \frac{1}{40.3} \times \frac{c \times f\_{L1}^2 \times f\_{L2}^2}{\left(f\_{L1}^2 - f\_{L2}^2\right)} \times \Delta \text{(\\$t\text{)}\tag{3}$$

The signal from different GPS satellites, at random elevation angles, recorded as a TEC measurements. These different satellites are identified by a pseudo-random number (PRN). The portions of the ionosphere cross by GPS signal depend on the elevation angle of GPS satellite. Therefore, in the present work the TEC data of only those GPS satellites, having elevation angles above 30° to avoid the multipath effect of signals, are considered. The maximum elevation angle over Hyderabad station is 60°. The STEC is measured at every 30 s by the GPS receiver.
