Validation of ROTI for Ionospheric Amplitude Scintillation Measurements in a Low-Latitude Region Over Africa

Measurement of the effect of ionospheric irregularities on Global Navigation Satellite Services has drawn great interest due to the adverse effects of such irregularities on precise position estimation. Specialized receivers are currently being used to measure the effects of ionospheric irregularities on the satellite signals. These effects are characterized by the amplitude scintillation index S-4 and the phase scintillation index sigma (sigma phi). In order to compute these scintillation indices, one requires a receiver that measures data at a high sampling rate. The intensity of ionospheric scintillation is an important parameter in the modeling of ionospheric scintillation. Therefore, an index is required that accurately reflects the amplitude scintillation as conventionally measured by means of the S-4 index. To infer the characteristics of ionospheric irregularities from ionospheric scintillation related to geophysical processes, the choice of sampling rate is very important, especially for small-scale phenomena. In this work, we used a Global Positioning System receiver with high sampling rate, sampling at 20Hz to compute both the amplitude scintillation index S-4 and the rate of change of total electron content index (ROTI) based on relative total electron content. The results show that by scaling the ROTI values using an appropriate scaling factor, a scintillation index can be calculated that correlates well with the amplitude scintillation index S-4, for values of S-4<0.4. For strong scintillation events S-4>0.5, the scaled ROTI values are more sensitive for events that lead to loss-of-lock (as evidenced by jumps in the L1-phase values) than the S-4 index. It is also shown that the choice of sampling interval may affect the computed ROTI at a particular location.