: Analysis of Ionospheric Scintillations using

The disturbances and irregularities in the ionosphere are the primarily recognized ramifications of space weather called scintillations. Irregularities in the electron densities are the source of the ionospheric scintillations. This article investigates the ionospheric scintillations, which are predominant in the trans-equatorial and equatorial regions. Based on the data from a multi-constellation Global Navigation Satellite Systems (GNSS) receiver at the Chaitanya Bharathi Institute of Technology Hyderabad, the relationship between the amplitude scintillation index S 4 and the rate of change of total electron content (ROTI) is examined. The correlation coefficient between S 4 and ROTI is demonstrated in this article. The outcome validates the usefulness of the ROTI in identifying the scintillations.


INTRODUCTION
The advancement of the Global Navigation Satellite System (GNSS), which encompasses GLONASS, GPS, BeiDO, Galileo, and numerous regional navigation systems, directed advanced steps in the earth's atmosphere research [1]. The space-based investigation of the GNSS signals is one of best enforced ways to investigate the ionosphere [2]. The space weather trans-ionospheric signals can undergo expeditious variations and disturbances both in amplitude and phase when they pass through indiscriminate electron density irregularities which are present in the ionosphere [3][4]. This anomaly is generally referred to as ionospheric scintillations. The temporal and spatial status of the ionospheric scintillations depends on geographical and geophysical locations, the time of the day, and season [5][6]. The ionospheric scintillations are the major sources of degradation of the VHF band and C band signals, immensely effecting navigation and communication systems [7]. Hence, it is vital to investigate and analyze them. The GNSS signals are considered a robust tool to investigate the Total Electron Content (TEC) and the existence of ionospheric scintillations [8]. Phase and amplitude scintillations are both ionosphere irregularities.

II. GNSS DATA COLLECTION
For this analysis, the data of the multi constellation GNSS receiver of Chaitanya Bharati Institute of Technology, which is located in Hyderabad, during the period from 2020-06-07 to 2020-06-13 were taken into account. The considered parameters are time, PRN (Pseudo Random Number) of the satellite, carrier to noise values of the received signal, and the ionospheric delay of the received signal. To reduce complexity, only the results of 2020-06-07 are shown in this paper.

III. METHODOLY
The ionospheric irregularities and scintillations can be investigated in two ways when using GNSS signals. The first of these methods uses carrier to noise ( ⁄ ) ratio to calculate the amplitude scintillation index S [9] and the second one uses ionospheric delay to calculate the rate of change of TEC index (ROTI) which is the replacement index of S [10]. The current work is based on the multi-step interpretation and processing of the L (1575.42MHz), L (1176.45MHz), and S (2492.028MHz) GNSS signals corresponding to GPS and NavIC satellite constellations.
In the first method, the amplitude scintillation index S is calculated as the ratio of the standard deviation to the mean value of the averaged ⁄ [9]. The effectiveness of the amplitude scintillations index is divided into 3 categories which are week scintillations when S is less than 0.3, moderate scintillations when S ranges between 0.3 and 0.6, and strong scintillations when S is above 0.6 [11]. The amplitude scintillations are calculated as follows:

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where SI is the intensity of the received trans-ionospheric signal. The S value is computed using the signals received on the L (1575.42MHz), L (1176.45MHz) and S (2492.028MHz) frequencies for a week-long data from 2020-06-07 to 2020-06-13.
The second method is to calculate the ROTI using the delay due to ionosphere in the received signals [12]. The ionospheric delay is computed by: where TEC indicates the Total Electron Content on the frequency of the received trans-ionospheric signal and is the ionospheric delay of the received signal. TEC is used to describe all the electrons that are present along the path from the satellite to the receiver [12][13]. The TEC is expressed below and is measured in TECU: The rate of change of TEC (ROT) is calculated at regular intervals by (5): where b specifies the epoch time and a specifies the noticeable satellite.
ROTI is calculated by the standard deviation of ROT: The effectiveness of ROTI is divided into three categories: It is fragile when the ROTI index ranges between 0.25 and 0.5, modest when the ROTI index ranges between 0.5 and 1, and vigorous when the ROTI index is above 1 [14][15]. ROTI was calculated for signals received on L , L , and S frequencies for a weak long data from 2020-06-07 to 2020-06-13. To observe the correlation between ROTI and S , the covariance and standard deviation of S and ROTI are computed [16] by: The covariance of S and ROTI is: where A B is the mean of and ROTI ;;;;;;; is the mean of ROTI.
The correlation coefficient is a measure of the correlation between ROTI and S methods which is expressed by C 6 ' 5 , and it is given as: where Ϲ1 6 ' 5 specifies the covariance of S and ROTI .
The flowchart of the followed methodology is shown in Figure 1. IV. RESULTS AND DISCUSSION The pseudo range measurements data of multi-constellation GNSS receiver, which were sampled every 1s, are considered for computation and analysis of amplitude scintillations, ROT, and ROTI. Figure 2 specifies S for all the available satellites on 2020-06-07 for the L 1 frequency. The highest scintillations are observed from the satellite PRN 9. Therefore, the values of S and ROTI are compared to the PRN 9 values. S index on 2020-06-07 for the L1 frequency. Figure 3 shows the comparison plot for S and ROTI for the PRN-9, which exhibits maximum scintillations. Figure 4 shows the correlation between ROTI and S . The x axis specifies the epoch time for every hour and the y axis the correlation coefficient between S and ROTI. The highest correlation coefficient is 1 and the minimum is higher than the cutoff level. As a result, the correlation coefficient is robust due to the persistent link between ROTI and S . Figure 5 depicts the comparison of S for all the available NavIC satellites on 2020-06-07 for the L 5 frequency. The maximum observed S index is 0.0205 for IR-02, which is an indication of no scintillations on 2020-06-07 for the L 5 frequency. The highest S values are observed in the IR-02 satellite. Therefore, the values of S and ROTI are compared to the IR-02 values.    In Figure 6, the maximum S index value of 0.0078 and the maximum ROTI index value of 0.1911 are observed. From these, it can be concluded that no scintillations are observed on the L 5 frequency for IR-02. Figure 7 shows the correlation between S and ROTI. The maximum correlation of 1 is observed many times, which is an indicative of the similarity between S and ROTI. Figure 8 shows the S variations of all visible NavIC satellites on 2020-06-07 for the frequency. The maximum observed S index is 0.0294 for IR-02, which is an indication of no scintillations there on 2020-06-07 for the S frequency. The highest S 4 values are observed in the IR-02 satellite. Therefore, all S and ROTI values are compared with the ones of the IR-02 satellite. Correlation of S and ROTI on 2020-06-07 for IR-02 (L5 frequency). S4 amplitute variations on 2020-06-07 for S1 frequency. Comparison of S4 and ROTI on 2020-06-07 for IR-02 (S1 frequency). Figure 9 shows that the maximum S index value is 0.0078 and the maximum ROTI index value is 0.1911. From this it can

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be concluded that no scintillations are observed on the S frequency for IR-02. Figure 10 shows the correlation between S 4 and ROTI for NavIC satellite IR-02 on 2020-06-07 on the S frequency. Maximum correlation is observed many times, which indicates that ROTI can be considered as an alternative metric for the S index. Figure 11 shows the skyplot of all the visible GPS and NavIC satellites at 15:00hrs of 2020-06-07 for the IGS (International GNSS Service) station which is located at Hyderabad (Lat/Long:17.417 0 N/78.551 0 E). Fig. 10. Correlation of S4 and ROTI on 2020-06-07 for IR-02 (S1 frequency).

V. CONCLUSIONS
In this article, the analysis of GNSS trans-ionospheric signals for a typical day (2020-06-07) is presented. In order to identify the ionospheric perturbations, the amplitude of the scintillations S , the rate of the change of total electron content (ROT), and its index (ROTI) are computed on L 1 , L 5 , and S frequencies using GPS and NavIC combined constellation. Based on the correlation coefficient, maximum similarity between the S and ROTI is observed, and, thus, ROTI can be considered as an alternative index for the investigation of the amplitude scintillations. It is also observed that the ionospheric scintillations are robust only on L 1 frequency of the GNSS trans-ionospheric signals and no ionospheric scintillations are observed on the L 5 and S frequencies. As the L 5 and S are the frequencies for NavIC constellation, it is concluded that the ionospheric scintillation effects are minimum on the NavIC signal. Hence, it is concluded that the ionosphere scintillation effects can be minimized with higher frequencies, like L 5 and S 1 .