A. K. Gwal

Effect of TEC Variation on GPS Precise Point at Low Latitude

The ionosphere is a dispersive medium of charged particles between the satellite and the user on Earth. These dispersive ionized media play a vital role in the various applications of GPS (Global Positioning Systems) because the ionosphere directly influences transionospheric radio waves propagating from the satellite to the receiver. Solar flares af- fect the ionization state of the ionosphere with their high intensity. Sometimes the intensity is so severe that it accelerates the rate of ionization, resulting in ionospheric storms; during the ionospheric storms the concentration of charged particles varies. Among the various phenomena in the ionosphere, TEC (Total Electron Content) is responsible for range error which produces a time delay in the radio signal. The rate of change of TEC with respect to time is abbreviated as ROT. It is one of the parameters that express the ionospheric irregularities with respect to time. This work investigates the effect of ROT fluctuation on the precise positioning of GPS receivers during low solar activity periods in the equatorial anomaly region. Good geometry and a sufficient number of locked satellites provide more accuracy within the centimeter level, but the case may be different when there are any ionospheric storms. Even a few satellite signals passing through the iono- spheric irregularities can cause a significant error in positioning. Thus, it is important to understand the ionospheric ir- regularities observed by GPS receivers in order to correct them. The ROT (TEC/Minute) parameter is used here to study the occurrence of TEC fluctuation and its potential effect on GPS, such as a horizontal positional error or the satellite ge- ometry of the GPS receiver. This investigation is based on the analysis of a one-year observation of a fixed GPS receiver installed at Bhopal (23.202 0 N, 77.452 0 E), India during low solar active period in 2005. The GPS receiver used here is a GISTM-based dual frequency NovAtel OEM4 GPS receiver.

Adaptive modeling of equatorial ionization anomaly crest parameters by using multistation ionosonde data over the Indian region

The paper explores the new technique to study the temporal and spatial variation of equatorial ionization anomaly (EIA) by using the limited number of ground based observing stations. The technique involved the use of the Gaussian fitting over the latitudinal distribution of foF2. The multi-station F2-region critical frequency (foF2) data observed by ionosonde system during the 19th (1954–1964) solar cycle is used for the present study. Results obtained by this analysis method suggest that the EIA crest exhibits the feature of latitudinal shifting and expansion in coverage area with increasing solar activity. The present results have been discussed in the light of relative contribution from transequatorial interhemispheric neutral wind and the strength of the equatorial fountain process. The study is further extended to derive the relationship between location and foF2 of the EIA crest by using the regression analysis which has been performed for the first time over the Indian sector. The results obtained by this analysis can be used to predict the latitudinal position and foF2 of the EIA crest for any given smoothed sunspot number. Results show that the predicted values by using this new approach is better as compared to the International Reference Ionosphere model-2001 predictions and hence the technique can be used for the adaptive modeling of EIA.